Showing papers in "Tree Physiology in 2013"
TL;DR: The embolism threshold leading to irreversible drought damage was found to be close to 88%, rather than the 50% previously reported for conifers, and the relationship between this potential and stem xylem cavitation resistance, as assessed from vulnerability curves was investigated.
Abstract: Hydraulic failure is one of the main causes of tree mortality in conditions of severe drought. Resistance to cavitation is known to be strongly related to drought tolerance and species survival in conifers, but the threshold of water-stress-induced embolism leading to catastrophic xylem dysfunction in angiosperms has been little studied. We investigated the link between drought tolerance, survival and xylem cavitation resistance in five angiosperm tree species known to have contrasting desic cation resistance thresholds. We exposed seedlings in a greenhouse to severe drought to generate extreme water stress. We monitored leaf water potential, total plant water loss rate, leaf transpiration, stomatal conductance and CO2 assimilation rate during drought exposure and after rewatering (recovery phase). The time required for the recovery of 50% of the maximum value of a given ecophysiological variable after rewatering was used to determine the critical water potential corresponding to the threshold beyond which the plant failed to recover. We also investigated the relationship between this potential and stem xylem cavitation resistance, as assessed from vulnerability curves. This minimum recoverable water potential was consistent between ecophysiological variables and varied considerably between species, from −3.4 to −6.0 MPa. This minimum recoverable water potential was strongly correlated with P50 and P88, the pressures inducing 50 and 88% losses of stem hydraulic conductance, respectively. Moreover, the embolism threshold leading to irreversible drought damage was found to be close to 88%, rather than the 50% previously reported for conifers. Hydraulic failure leading to irreversible droughtinduced global dysfunction in angiosperm tree species occurred at a very high level of xylem embolism, possibly reflecting the physiological characteristics of their stem water-transport system.
420 citations
TL;DR: It is argued that, in order to fully understand when and where plants will exceed mortality thresholds when drought occurs, it must understand the entire path by which precipitation deficit is translated into physiological dysfunction and lasting physiological damage.
Abstract: Widespread drought-induced mortality of woody plants has recently occurred worldwide, is likely to be exacerbated by future climate change and holds large ecological consequences. Yet despite decades of research on plant–water relations, the pathways through which drought causes plant mortality are poorly understood. Recent work on the physiology of tree mortality has begun to reveal how physiological dysfunction induced by water stress leads to plant death; however, we are still far from being able to predict tree mortality using easily observed or modeled meteorological variables. In this review, we contend that, in order to fully understand when and where plants will exceed mortality thresholds when drought occurs, we must understand the entire path by which precipitation deficit is translated into physiological dysfunction and lasting physiological damage. In temperate ecosystems with seasonal climate patterns, precipitation characteristics such as seasonality, timing, form (snow versus rain) and intensity interact with edaphic characteristics to determine when and how much water is actually available to plants as soil moisture. Plant and community characteristics then mediate how quickly water is used and seasonally varying plant physiology determines whether the resulting soil moisture deficit is physiologically damaging. Recent research suggests that drought seasonality and timing matter for how an ecosystem experiences drought. But, mortality studies that bridge the gaps between climatology, hydrology, plant ecology and plant physiology are rare. Drawing upon a broad hydrological and ecological perspective, we highlight key and underappreciated processes that may mediate droughtinduced tree mortality and propose steps to better include these components in current research.
151 citations
TL;DR: The results suggest that as carbon limitation increases, reduced growth is not necessarily accompanied by a decline in NSC concentrations, and a lack of NSC decline may not be evidence that reduced tree growth under cold or water stress is caused by sink limitation.
Abstract: There is no consensus about how stresses such as low water availability and temperature limit tree growth. Sink limitation to growth and survival is often inferred if a given stress does not cause non-structural carbohydrate (NSC) concentrations or levels to decline along with growth. However, trees may actively maintain or increase NSC levels under moderate carbon stress, making the pattern of reduced growth and increased NSCs compatible with carbon limitation. To test this possibility, we used full and half defoliation to impose severe and moderate carbon limitation on 2-year-old Quercus velutina Lam. saplings grown in a common garden. Saplings were harvested at either 3 weeks or 4 months after treatments were applied, representing short- and longer-term effects on woody growth and NSC levels. Both defoliation treatments maintained a lower total leaf area than controls throughout the experiment with no evidence of photosynthetic up-regulation, and resulted in a similar total biomass reduction. While fully defoliated saplings had lower starch levels than controls in the short term, half defoliated saplings maintained control starch levels in both the short and longer term. In the longer term, fully defoliated saplings had the greatest starch concentration increment, allowing them to recover to near-control starch levels. Furthermore, between the two harvest dates, fully and half defoliated saplings allocated a greater proportion of new biomass to starch than did controls. The maintenance of control starch levels in half defoliated saplings indicates that these trees actively store a substantial amount of carbon before growth is carbon saturated. In addition, the allocation shift favouring storage in defoliated saplings is consistent with the hypothesis that, as an adaptation to increasing carbon stress, trees can prioritize carbon reserve formation at the expense of growth. Our results suggest that as carbon limitation increases, reduced growth is not necessarily accompanied by a decline in NSC concentrations. Therefore, a lack of NSC decline may not be evidence that reduced tree growth under cold or water stress is caused by sink limitation.
128 citations
TL;DR: It is concluded that the low nutrient availability (mainly nitrogen) in the boreal forests is likely to restrict their response to the continuous rise in [CO(2)] and/or TE.
Abstract: The growth responses of mature Norway spruce (Picea abies (L.) Karst.) trees exposed to elevated [CO(2)] (CE; 670-700 ppm) and long-term optimized nutrient availability or elevated air temperature (TE; ±3.9 °C) were studied in situ in northern Sweden in two 3 year field experiments using 12 whole-tree chambers in ca. 40-year-old forest. The first experiment (Exp. I) studied the interactions between CE and nutrient availability and the second (Exp. II) between CE and TE. It should be noted that only air temperature was elevated in Exp. II, while soil temperature was maintained close to ambient. In Exp. I, CE significantly increased the mean annual height increment, stem volume and biomass increment during the treatment period (25, 28, and 22%, respectively) when nutrients were supplied. There was, however, no significant positive CE effect found at the low natural nutrient availability. In Exp. II, which was conducted at the natural site fertility, neither CE nor TE significantly affected height or stem increment. It is concluded that the low nutrient availability (mainly nitrogen) in the boreal forests is likely to restrict their response to the continuous rise in [CO(2)] and/or TE.
126 citations
TL;DR: Both young and old trees responded fast to changes of the minimum air temperature, confirming that this factor was dominant by controlling Smith fir growth on the south-eastern Tibetan Plateau.
Abstract: An age effect on growth trends and climate/growth relationships of trees can possibly be discovered by analysing the seasonal dynamics of xylem development. The aims of this study, therefore, were to compare xylem formation of young (43 ± 4 years) and old (162 ± 2 6 years) Smith fir ( Abies georgei var. smithii (Viguie & Gaussen) W. C. Cheng & L. K. Fu) trees in the Sygera Mountains, south-eastern Tibetan Plateau and, to identify the association between wood formation and climate. The seasonal radial growth dynamics of young and old trees was monitored on microcores collected at weekly intervals during two growing seasons. Transverse sections through phloem, cambium and outermost xylem of 9–12 µm thickness were observed with a light microscope under bright field and polarized light to follow the cambial activity and differentiation of the develop ing xylem. Young trees were characterized by an earlier onset of xylogenesis, a longer growing season and a higher growth rate, resulting in a higher number of xylem cells. Both young and old trees responded fast to changes of the minimum air temperature, confirming that this factor was dominant by controlling Smith fir growth on the south-eastern Tibetan Plateau.
123 citations
TL;DR: Results indicate that accumulation of sorbitol, raffinose and proline in different tissues and/or the increase in P5SC expression could be used as markers of drought tolerance in peach cultivars grafted on Prunus rootstocks.
Abstract: An understanding of the mechanisms that determine plant response to reduced water availability is essential to improve water-use efficiency (WUE) of stone fruit crops. The physiological, biochemical and molecular drought responses of four Prunus rootstocks (GF 677, Cadaman, ROOTPAC 20 and ROOTPAC(®) R) budded with 'Catherina' peach cultivar were studied. Trees were grown in 15-l containers and subjected to a progressive water stress for 26 days, monitoring soil moisture content by time domain reflectometry. Photosynthetic and gas exchange parameters were determined. Root and leaf soluble sugars and proline content were also measured. At the end of the experiment, stressed plants showed lower net photosynthesis rate, stomatal conductance and transpiration rate, and higher intrinsic leaf WUE (AN/gs). Soluble sugars and proline concentration changes were observed, in both root and leaf tissues, especially in an advanced state of stress. The accumulation of proline in roots and leaves with drought stress was related to the decrease in osmotic potential and increase in WUE, whereas the accumulation of sorbitol in leaves, raffinose in roots and proline in both tissues was related only to the increase in the WUE. Owing to the putative role of raffinose and proline as antioxidants and their low concentration, they could be ameliorating deleterious effects of drought-induced oxidative stress by protecting membranes and enzymes rather than acting as active osmolytes. Higher expression of P5SC gene in roots was also consistent with proline accumulation in the tolerant genotype GF 677. These results indicate that accumulation of sorbitol, raffinose and proline in different tissues and/or the increase in P5SC expression could be used as markers of drought tolerance in peach cultivars grafted on Prunus rootstocks.
121 citations
TL;DR: It is concluded that stem storage is an important factor improving the water balance of diffuse-porous temperate broad-leaved trees in moist periods, while it may be of low relevance in dry periods and in ring-porrous species.
Abstract: The functional role of internal water storage is increasingly well understood in tropical trees and conifers, while temperate broad-leaved trees have only rarely been studied. We examined the magnitude and dynamics of the use of stem water reserves for transpiration in five coexisting temperate broad-leaved trees with largely different morphology and physiology (genera Fagus, Fraxinus, Tilia, Carpinus and Acer). We expected that differences in water storage patterns would mostly reflect species differences in wood anatomy (ring vs. diffuse-porous) and wood density. Sap flux density was recorded synchronously at five positions along the root-to-branch flow path of mature trees (roots, three stem positions and branches) with high temporal resolution (2 min) and related to stem radius changes recorded with electronic point dendrometers. The daily amount of stored stem water withdrawn for transpiration was estimated by comparing the integrated flow at stem base and stem top. The tem poral coincidence of flows at different positions and apparent time lags were examined by cross- correlation analysis. Our results confirm that internal water stores play an important role in the four diffuse-porous species with estimated 5–12 kg day −1 being withdrawn on average in 25–28 m tall trees representing 10–22% of daily transpiration; in contrast, only 0.5–2.0 kg day −1 was withdrawn in ring-porous Fraxinus. Wood density had a large influence on storage; sapwood area (diffuse- vs. ringporous) may be another influential factor but its effect was not significant. Across the five species, the length of the time lag in flow at stem top and stem base was positively related to the size of stem storage. The stem stores were mostly exhausted when the soil matrix potential dropped below −0.1 MPa and daily mean vapor pressure deficit exceeded 3–5 hPa. We conclude that stem storage is an important factor improving the water balance of diffuse-porous temperate broad-leaved trees in moist periods, while it may be of low relevance in dry periods and in ring-porous species.
120 citations
TL;DR: The results suggest that increases in the frequency or intensity of summer droughts in the study region could have multiple consequences for forest function, including altered successional time courses or climax species composition and cumulative effects on whole-tree architecture, resulting in a structural and physiological legacy that would restrict the ability of trees to respond rapidly to more favorable growth conditions.
Abstract: Stomata control tree transpiration by sensing and integrating environmental signals originating in the atmosphere and soil, and co-occurring species may differ in inherent stomatal sensitivity to these above- and belowground signals and in the types of signals to which they respond. Stomatal responsiveness to environmental signals is likely to differ across species having different types of wood (e.g., ring-porous, diffuse-porous and coniferous) because each wood type differs in the structure, size and spatial distribution of its xylem conduits as well as in the scaling of hydraulic properties with stem diameter. The objective of this study was to evaluate the impact of variation in soil water availability and atmospheric evaporative demand on stomatal regulation of transpiration in seven co-occurring temperate deciduous forest species representing three wood types. We measured whole-tree sap flux and soil and atmospheric variables in a mixed deciduous forest in central Pennsylvania over the course of a growing season characterized by severe drought and large fluctuations in atmospheric vapor pressure deficit ( D). The relative sensitivity of sap flux to soil drying was ~2.2–2.3 times greater in the diffuse-porous and coniferous species than in the ring-porous species. Stomata of the ring-porous oaks were only about half as responsive to increased D as those of trees of the other two wood types. These differences in responsiveness to changes in the belowand aboveground environment implied that regulation of leaf water potential in the ring-porous oaks was less stringent than that in the diffuse-porous angiosperms or the conifers. The results suggest that increases in the frequency or intensity of summer droughts in the study region could have multiple consequences for forest function, including altered successional time courses or climax species composition and cumulative effects on whole-tree architecture, resulting in a structural and physiological legacy that would restrict the ability of trees to respond rapidly to more favorable growth conditions.
116 citations
TL;DR: Examination of patterns of height growth and diameter growth along a stem using a 20-year record of an even-aged hinoki cypress shows the view that a rise in the crown base is strongly related to leaf turnover helps to interpret DBH–H relationships.
Abstract: Stem diameter at breast height (DBH) and tree height (H) are commonly used measures of tree growth. We examined patterns of height growth and diameter growth along a stem using a 20-year record of an even-aged hinoki cypress (Chamaecyparis obtusa (Siebold & Zucc.) Endl.) stand. In the region of the stem below the crown (except for the butt swell), diameter growth rates (ΔD) at different heights tended to increase slightly from breast height upwards. This increasing trend was pronounced in suppressed trees, but not as much as the variation in ΔD among individual trees. Hence, ΔD below the crown can be regarded as generally being represented by the DBH growth rate (ΔDBH) of a tree. Accordingly, the growth rate of the stem cross-sectional area increased along the stem upwards in suppressed trees, but decreased in dominant trees. The stem diameter just below the crown base (D(CB)), the square of which is an index of the amount of leaves on a tree, was an important factor affecting ΔDBH. D(CB) also had a strong positive relationship with crown length. Hence, long-term changes in the D(CB) of a tree were associated with long-term changes in crown length, determined by the balance between the height growth rate (ΔH) and the rising rate of the crown base (ΔH(CB)). Within the crown, ΔD's were generally greater than the rates below the crown. Even dying trees (ΔD ≈ 0 below the crown) maintained ΔD > 0 within the crown and ΔH > 0 until about 5 years before death. This growth within the crown may be related to the need to produce new leaves to compensate for leaves lost owing to the longevity of the lower crown. These results explain the different time trajectories in DBH-H relationships among individual trees, and also the long-term changes in the DBH-H relationships. The view that a rise in the crown base is strongly related to leaf turnover helps to interpret DBH-H relationships.
115 citations
TL;DR: Examination of seven temperate woody species reveals that coordination exists between cavitation resistance and xylem recovery performance, in association with wood functional traits such as denser wood for cavitation-resistant xyleM and less-dense but water-storable wood for refillable Xylem.
Abstract: Woody species hydraulically vulnerable to xylem cavitation may experience daily xylem embolism. How such species cope with the possibility of accumulated embolism is unclear. In this study, we examined seven temperate woody species to assess the hypothesis that low cavitation resistance (high vulnerability to cavitation) is compensated by high recovery performance via vessel refilling. We also evaluated leaf functional and xylem structural traits. The xylem recovery index (XRI), defined as the ratio of xylem hydraulic conductivity in plants rewatered after soil drought to that in plants under moist conditions, varied among species. The xylem water potential causing 50% loss of hydraulic conductivity (Ψ50) varied among the species studied, whereas only a slight difference was detected with respect to midday xylem water potential (Ψmin), indicating smaller hydraulic safety margins (Ψmin − Ψ50) for species more vulnerable to cavitation. Cavitation resistance (|Ψ50|) was negatively correlated with XRI across species, with cavitation-vulnerable species showing a higher performance in xylem recovery. Wood density was positively correlated with cavitation resistance and was negatively correlated with XRI. These novel results reveal that coordination exists between cavitation resistance and xylem recovery performance, in association with wood functional traits such as denser wood for cavitation-resistant xylem and less-dense but water-storable wood for refillable xylem. These findings provide insights into long-term maintenance of water transport in tree species growing under variable environmen tal conditions.
110 citations
TL;DR: Examination of physiological and phenological parameters in Pinus halepensis Miller showed that changes in growing season length and water use efficiency were important contributors to tree growth across climates, whereas xylem native embolism negatively correlated with tree survival.
Abstract: The rate of migration and in situ genetic variation in forest trees may not be sufficient to compete with the current rapid rate of climate change. Ecophysiological adjustments of key traits, however, could complement these processes and allow sustained survival and growth across a wide range of climatic conditions. This was tested in Pinus halepensis Miller by examining seven physiological and phenological parameters in five provenances growing in three common garden plots along a climatic transect from meso-Mediterranean (MM) to thermo-Mediterranean (TM) and semi-arid (SA) climates. Differential responses to variations in ambient climatic conditions were observed in three key traits: (i) growing season length decreased with drying in all provenances examined (from 165 under TM climate to 100 days under SA climate, on average); (ii) water use efficiency (WUE) increased with drying, but to a different extent in different provenances, and on average from 80, to 95, to 110 µmol CO(2) mol(-1) H(2)O under MM, TM and SA climates, respectively; (iii) xylem native embolism was stable across climates, but varied markedly among different provenances (percent loss of conductivity, was below 5% in two provenances and above 35% in others). The results indicated that changes in growing season length and WUE were important contributors to tree growth across climates, whereas xylem native embolism negatively correlated with tree survival. The results indicated that irrespective of slow processes (e.g., migration, genetic adaptation), the capacity for ecophysiological adjustments combined with existing variations among provenances could help sustain P. halepensis, a major Mediterranean tree species, under relatively extreme warming and drying climatic trends.
TL;DR: The combined effects of elevated [CO2] and elevated temperature on moderate drought stressed seedlings were reduced with progressive drought, with no sustained effects on growth despite greater whole-plant TNC content.
Abstract: Elevated [CO2] and temperature may alter the drought responses of tree seedling growth, photosynthesis, respiration and total non-structural carbohydrate (TNC) status depending on drought intensity and duration. Few studies have addressed these important climatic interactions or their consequences. We grew Eucalyptus globulus Labill. seedlings in two [CO2] concentrations (400 and 640 μl l(-1)) and two temperatures (28/17 and 32/21 °C) (day/night) in a sun-lit glasshouse, and grew them in well-watered conditions or exposed them to two drought treatments having undergone different previous water conditions (i.e., rewatered drought and sustained drought). Progressive drought in both drought treatments led to similar limitations in growth, photosynthesis and respiration, but reductions in TNC concentration were not observed. Elevated [CO2] ameliorated the impact of the drought during the moderate drought phase (i.e., Day 63 to Day 79) by increasing photosynthesis and enhancing leaf and whole-plant TNC content. In contrast, elevated temperature exacerbated the impact of the drought during the moderate drought phase by reducing photosynthesis, increasing leaf respiration and decreasing whole-plant TNC content. Extreme drought (i.e., Day 79 to Day 103) eliminated [CO2] and temperature effects on plant growth, photosynthesis and respiration. The combined effects of elevated [CO2] and elevated temperature on moderate drought stressed seedlings were reduced with progressive drought, with no sustained effects on growth despite greater whole-plant TNC content.
TL;DR: This study performed a severe controlled drought experiment on saplings of Pinus edulis Engelm.
Abstract: Global patterns of drought-induced forest die-off indicate that many forests may be sensitive to climate-driven mortality, but the lack of understanding of how trees and saplings die during drought hinders the projections of die-off, demographic bottlenecks and ecosystem trajectories. In this study, we performed a severe controlled drought experiment on saplings of Pinus edulis Engelm. and Juniperus osteosperma (Torr.) Little, two species that both experienced die-off in a recent ‘climate change-type’ drought. We examined the roles of carbohydrate and hydraulic changes in multiple tissues as the saplings died. We found that saplings of both species exhibited large degrees of loss of hydraulic conductivity prior to death. Neither species exhibited significant changes in carbohydrate concentrations in any tissue during the relatively short and severe imposed drought. Native hydraulic conductivity successfully predicted the degree of canopy mortality in both species, highlighting the importance of drought characteristics and tree attributes in influencing physiological pathways to mortality. The relationships elucidated here, as well as the differences between our results and previous findings in adult trees, can help inform mortality mechanisms in climate–vegetation models, especially for young trees, and to understand species response to severe drought across ontogeny.
TL;DR: Examination of the effect of sustained localized nitrogen fertilization on root lifespan in four tree species that varied widely in root morphology and presumed foraging strategy suggests that some plant species appear to regulate the lifespan of different portions of their root system to improve resource acquisition while other species do not.
Abstract: Resource exploitation of patches is influenced not simply by the rate of root production in the patches but also by the lifespan of the roots inhabiting the patches. We examined the effect of sustained localized nitrogen (N) fertilization on root lifespan in four tree species that varied widely in root morphology and presumed foraging strategy. The study was conducted in a 12-year-old common garden in central Pennsylvania using a combination of data from minirhizotron and root in-growth cores. The two fine-root tree species, Acer negundo L. and Populus tremuloides Michx., exhibited significant increases in root lifespan with local N fertilization; no significant responses were observed in the two coarse-root tree species, Sassafras albidum Nutt. and Liriodendron tulipifera L. Across species, coarse-root tree species had longer median root lifespan than fine-root tree species. Localized N fertilization did not significantly increase the N concentration or the respiration of the roots growing in the N-rich patch. Our results suggest that some plant species appear to regulate the lifespan of different portions of their root system to improve resource acquisition while other species do not. Our results are discussed in the context of different strategies of foraging of nutrient patches in species of different root morphology.
TL;DR: The role of plant hydraulic dysfunction in drought-induced mortality currently takin9 place in many tree-dominated ecosystems, which is expected to increase under future climate change scenarios, and the characteristics of xylem vulnerability curves to the actual risk of embolism in situ are studied.
Abstract: For terrestrial plants, maintenance of the integrity of the rootto-leaf water transport pathway is essential for sustaining photosynthetic 9as exchange and growth. The problem of maintaining long-distance water transport is especially challengin9 in trees because path-length resistances and gravity can result in steep gradients of increasing xylem tension from roots to terminal branches, potentially increasing the risk of tension-induced xylem embolism (Sperry and Tyree 1988). These emboli reduce xylem hydraulic conductance and, under certain conditions, can accumulate rapidly through a process that has been called runaway embolism, which can ultimately lead to catastrophic and irreversible hydraulic failure (Tyree and Sperry t988). This phenomenon has led to intensive interest in the role of plant hydraulic dysfunction in drought-induced mortality currently takin9 place in many tree-dominated ecosystems, which is expected to increase under future climate change scenarios (Allen et al. 2010, McDowell et al. 2011, Choat et al. 2012). The susceptibility of xylem to embolism has traditionally been characterized by generating the so-called xylem vulnerability curves, which are plots of the loss of hydraulic conductivity in relation to xylem tension or pressure (Figure 1, top). Key features of these curves that are often quantified include the embolism threshold (Pe), the xylem pressures corresponding to 50 and 88°1\"o loss of conductivity (Pro and P88) and the slope of the steep, nearly linear portion of the curve. Thus, highly embolism-resistant xylem might be expected to show a highly negative value of Pe and a gradual slope beyond this point, which would ensure substantially more negative values of Pro and P88. However, vulnerability to embolism does not necessarily equate to risk of embolism in situ, which is partly determined by the influence of stomatal regulation of transpiration on the normal .operating range of xylem tension in a given plant organ. Isohydric species tend to show relatively constant maximum values of xylem tension, whereas the maximum xylem tension in anisohydric species varies according to environmental conditions such as soil water availability and vapor pressure deficit (Tardieu and Simonneau 1998, Schultz 2003, Rogiers et al. 2012). However, isohydry and anisohydry represent two extremes of a continuum of regulation of xylem tension. This limitation on relatin9 the characteristics of xylem vulnerability curves to the actual risk of embolism and hydraulic dysfunction in intact, field-grown plants has contributed to the development of the concept of hydraulic safety margins (Alder et al. 1996, Hacke and Sauter 1996, Pockman and Sperry 2000, Vilagrosa et al. 2003, Brodribb and Hotbrook 2004, Meinzer et at. 2009, Hoffmann et al. 2011, Johnson et al. 2012), which can be defined as the difference between the minimum xylem pressure normally attained in a given plant organ and a reference point on its hydraulic vulnerability curve (e.g., Pro). Intensive research on plant hydraulic architecture has revealed much about broad patterns of xylem vulnerability to embolism and hydraulic safety margins within plants (e.9., Martinez-Vitalta et al. 2002, Domec and Gartner 2005, Domec et al. 2006a, 2006b, Maherali et al. 2006) and across contrasting vegetation types (Pockman and Sperry 2000, Maherali et al. 2004, Jacobsen et al. 2007, Meinzer et al. 2009, Choat et al. 2012). Nevertheless, simple hydraulic predictors of a species’ performance and ability to survive in a specific type of
TL;DR: Vessel-grouping characters were found to be most informative in explaining interspecific variation in P50, with cavitation-resistant species showing more solitary vessels than less resistant species, and co-evolution between vessel-groups indices and P50 was reported.
Abstract: Various structure-function relationships regarding drought-induced cavitation resistance of secondary xylem have been postulated. These hypotheses were tested on wood of 10 Prunus species showing a range in P50 (i.e., the pressure corresponding to 50% loss of hydraulic conductivity) from -3.54 to -6.27 MPa. Hydraulically relevant wood characters were quantified using light and electron microscopy. A phylogenetic tree was constructed to investigate evolutionary correlations using a phylogenetically independent contrast (PIC) analysis. Vessel-grouping characters were found to be most informative in explaining interspecific variation in P50, with cavitation-resistant species showing more solitary vessels than less resistant species. Co-evolution between vessel-grouping indices and P50 was reported. P50 was weakly correlated with the shape of the intervessel pit aperture, but not with the total intervessel pit membrane area per vessel. A negative correlation was found between P50 and intervessel pit membrane thickness, but this relationship was not supported by the PIC analysis. Cavitation resistance has co-evolved with vessel grouping within Prunus and was mainly influenced by the spatial distribution of the vessel network.
TL;DR: Results indicate that the annual cycle of sugar concentration within the cambial zone is coupled to the process of wood formation and the highest sugar concentration is observed when the number of cells in secondary wall formation and lignification stages is at a maximum, subsequent to most radial growth.
Abstract: The presence of soluble carbohydrates in the cambial zone, either from sugars recently produced during photosynthesis or from starch remobilized from storage organs, is necessary for radial tree growth. However, considerable uncertainties on carbohydrate dynamics and the consequences on tree productivity exist. This study aims to better understand the variation in different carbon pools at intra-annual resolution by quantifying how cambial zone sugar and starch concentrations fluctuate over the season and in relation to cambial phenology. A comparison between two physiologically different species growing at the same site, i.e., the evergreen Picea abies Karst. and the deciduous Larix decidua Mill., and between L. decidua from two contrasting elevations, is presented to identify mechanisms of growth limitation. Results indicate that the annual cycle of sugar concentration within the cambial zone is coupled to the process of wood formation. The highest sugar concentration is observed when the number of cells in secondary wall formation and lignification stages is at a maximum, subsequent to most radial growth. Starch disappears in winter, while other freeze-resistant non-structural carbohydrates (NSCs) increase. Slight differences in NSC concentration between species are consistent with the differing climate sensitivity of the evergreen and deciduous species investigated. The general absence of differences between elevations suggests that the cambial activity of trees growing at the treeline was not limited by the availability of carbohydrates at the cambial zone but instead by environmental controls on the growing season duration.
TL;DR: Root respiration was closely related to root traits such as diameter, SRL, tissue N concentration, root TNC : tissue N ratio and stele-to-root diameter proportion among the first five orders, which explained up to 81-94% of variation in the rate of root respiration for larch and up to 83-93% for ash.
Abstract: Tree roots are highly heterogeneous in form and function. Previous studies revealed that fine root respiration was related to root morphology, tissue nitrogen (N) concentration and temperature, and varied with both soil depth and season. The underlying mechanisms governing the relationship between root respiration and root morphology, chemistry and anatomy along the root branch order have not been addressed. Here, we examined these relationships of the first- to fifth-order roots for near surface roots (0-10 cm) of 22-year-old larch (Larix gmelinii L.) and ash (Fraxinus mandshurica L.) plantations. Root respiration rate at 18 °C was measured by gas phase O2 electrodes across the first five branching order roots (the distal roots numbered as first order) at three times of the year. Root parameters of root diameter, specific root length (SRL), tissue N concentration, total non-structural carbohydrates (starch and soluble sugar) concentration (TNC), cortical thickness and stele diameter were also measured concurrently. With increasing root order, root diameter, TNC and the ratio of root TNC to tissue N concentration increased, while the SRL, tissue N concentration and cortical proportion decreased. Root respiration rate also monotonically decreased with increasing root order in both species. Cortical tissue (including exodermis, cortical parenchyma and endodermis) was present in the first three order roots, and cross sections of the cortex for the first-order root accounted for 68% (larch) and 86% (ash) of the total cross section of the root. Root respiration was closely related to root traits such as diameter, SRL, tissue N concentration, root TNC : tissue N ratio and stele-to-root diameter proportion among the first five orders, which explained up to 81-94% of variation in the rate of root respiration for larch and up to 83-93% for ash. These results suggest that the systematic variations of root respiration rate within tree fine root system are possibly due to the changes of tissue N concentration and anatomical structure along root branch orders in both tree species, which provide deeper understanding in the mechanism of how root traits affect root respiration in woody plants.
TL;DR: Five co-occurring tree species of a perhumid tropical rainforest in Sulawesi show convergent patterns in their hydraulic architecture despite different growth strategies, and the paradigm assuming continuous acropetal vessel tapering and decrease in specific conductance from fine roots towards distal twigs needs reconsideration.
Abstract: It is generally assumed that the largest vessels are occurring in the roots and that vessel diameters and the related hydraulic conductance in the xylem are decreasing acropetally from roots to leaves. With this study in five tree species of a perhumid tropical rainforest in Sulawesi (Indonesia), we searched for patterns in hydraulic architecture and axial conductivity along the flow path from small-diameter roots through strong roots and the trunk to distal sun-canopy twigs. Wood density differed by not more than 10% across the different flow path positions in a species, and branch and stem wood density were closely related in three of the five species. Other than wood density, the wood anatomical and xylem hydraulic traits varied in dependence on the position along the flow path, but were unrelated to wood density within a tree. In contrast to reports from conifers and certain dicotyledonous species, we found a hump-shaped variation in vessel diameter and sapwood area--specific conductivity along the flow path in all five species with a maximum in the trunk and strong roots and minima in both small roots and twigs; the vessel size depended on the diameter of the organ. This pattern might be an adaptation to the perhumid climate with a low risk of hydraulic failure. Despite a similar mean vessel diameter in small roots and twigs, the two distal organs, hydraulically weighted mean vessel diameters were on average 30% larger in small roots, resulting in ∼ 85% higher empirical and theoretical specific conductivities. Relative vessel lumen area in percent of sapwood area decreased linearly by 70% from roots to twigs, reflecting the increase in sclerenchymatic tissue and tracheids in acropetal direction in the xylem. Vessel size was more closely related to the organ diameter than to the distance along the root-to-shoot flow path. We conclude that (i) the five co-occurring tree species show convergent patterns in their hydraulic architecture despite different growth strategies, and (ii) the paradigm assuming continuous acropetal vessel tapering and decrease in specific conductance from fine roots towards distal twigs needs reconsideration.
TL;DR: Results indicated that the low expression of MADS13-1 in 'Hengshanli' towards endodormancy release could not be explained by the identified cis-acting regulatory elements, the methylation status of the putative promoter or by intron insertion.
Abstract: We isolated three dormancy-associated MADS-box (DAM) genes (MADS13-1, MADS13-2 and MADS13-3) and showed regulated expression concomitant with endodormancy establishment and release in the leaf buds of Japanese pear ‘Kosui’. Comparative analysis between ‘Kosui’ and Taiwanese pear TP-85-119 (‘Hengshanli’), a less dormant pear cultivar, showed reduction of MADS13-1 expression level in ‘Hengshanli’ earlier than in ‘Kosui’ towards endodormancy release, suggesting the possible relationship between chilling requirement and MADS13-1 expression. Application of hydrogen cyanamide accelerated endodormancy release with a reduction in MADS13 expression, whereas heat treatment in autumn inhibited endodormancy establishment without induction of MADS13 expression, indicating a close relationship between the MADS13 expression pattern and endodormancy phase transitions. Moreover, both the cis-acting regulatory elements and the methylation status in the 5′ upstream region of the MADS13-1 gene were not largely different between ‘Kosui’ and ‘Hengshanli’. Genomic structures of MADS13-1 from ‘Kosui’ and ‘Hengshanli’ revealed a 3218 b p insertion in the first intron of ‘Hengshanli’ that might be ascribed to the lower expression of MADS13-1tw; however, this insertion was also found in pear genotypes with a high chilling requirement. These results indicated that the low expression of MADS13-1 in ‘Hengshanli’ towards endodormancy release could not be explained by the identified cis-acting regulatory elements, the methylation status of the putative promoter or by intron insertion.
TL;DR: It is found that despite the absence of a climate signal in the ring-width chronologies, there were significant correlations between wood density, tracheid radial diameter and microfibril angle and temperature, stream flow and a drought index, enabling the development of robust chronologies.
Abstract: Although tree-ring-width chronologies have been widely used for temperature reconstructions, there are many sites around the world at which there is little evidence of a clear climate signal in the ring-width chronologies. This is the case with the long-lived conifer Huon pine (Lagarostrobos franklinii (Hook. F.) Quinn), endemic to Tasmania, Australia, when the species grows at low elevation. In this study, we developed chronologies of several wood properties (e.g., tracheid radial diameter, microfibril angle) from Huon pine growing at a low-elevation site. We found that despite the absence of a climate signal in the ring-width chronologies, there were significant correlations between wood density, tracheid radial diameter and microfibril angle and temperature, stream flow and a drought index, enabling the development of robust chronologies. This novel finding suggests that chronologies based on these wood properties may have important potential for climate reconstructions from sites and species that have not yet been realized. In particular, a relatively extensive resource of ancient, low-elevation Huon pine in western Tasmania, in which climate signals have not been found using ring widths, may now be useful as part of the broader effort to reconstruct Southern Hemisphere climate.
TL;DR: The paper was presented at the 88th Congress of the National Academy of Sciences of the United States of America (SAAS) with a theme of “Inspiraciones Cientificas y Tecnicas: Investigaciones Exactas y Naturales”, which aims to “ inspire and motivate future generations of scientists”.
Abstract: Fil: Goldstein, Guillermo Hernan. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Ecologia, Genetica y Evolucion de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ecologia, Genetica y Evolucion. Laboratorio de Ecologia Funcional; Argentina. University of Miami. Departament of Biology; Estados Unidos
TL;DR: It is concluded that drought hardening and N fertilization applied under typical container nursery operational conditions exert opposite effects on the physiological stress tolerance of P. pinea seedlings.
Abstract: Functional attributes determine the survival and growth of planted seedlings in reforestation projects. Nitrogen (N) and water are important resources in the cultivation of forest species, which have a strong effect on plant functional traits. We analyzed the influence of N nutrition on drought acclimation of Pinus pinea L. seedlings. Specifically, we addressed if high N fertilization reduces drought and frost tolerance of seedlings and whether drought hardening reverses the effect of high N fertilization on stress tolerance. Seedlings were grown under two N fertilization regimes (6 and 100 m g N p er plant) and subjected to three drought-hardening levels (well-watered, moderate and strong hardening). Water relations, gas exchange, frost damage, N concentration and growth at the end of the drought-hardening period, and survival and growth of seedlings under controlled xeric and mesic outplanting conditions were measured. Relative to low-N plants, high-N plants were larger, had higher stomatal conductance (27%), residual transpiration (11%) and new root growth capacity and closed stomata at higher water potential. However, high N fertilization also increased frost damage (24%) and decreased plasmalemma stability to dehydration (9%). Drought hardening reversed to a great extent the reduction in stress tolerance caused by high N fertilization as it decreased frost damage, stomatal conductance and residual transpiration by 21, 31 and 24%, respectively, and increased plasmalemma stability to dehydration (8%). Drought hardening increased tissue non-structural carbohydrates and N concentration, especially in high-fertilized plants. Frost damage was positively related to the stability of plasmalemma to dehydration (r = 0 .92) and both traits were n egatively related to the concentration of reducing soluble sugars. No differences existed between moderate and strong drought-hardening treatments. Neither N nutrition nor drought hardening had any clear effect on seedling performance under xeric outplanting conditions. However, fertilization increased growth under mesic conditions, whereas drought hardening decreased growth. We conclude that drought hardening and N fertilization applied under typical container nursery operational conditions exert opposite effects on the physiological stress tolerance of P. pinea seedlings. While drought hardening increases overall stress tolerance, N nutrition reduces it and yet has no effect on the drought acclimation capacity of seedlings.
TL;DR: This study reveals for the first time, the importance of not only the presence or absence of drought and herbivory stressors, but also shows that their relative intensities are critical in determining the direction and magnitude of their impacts on establishing seedlings.
Abstract: Forests typically experience a mix of anthropogenic, natural and climate-induced stressors of different intensities, creating a mosaic of stressor combinations across the landscape. When multiple stressors co-occur, their combined impact on plant growth is often greater than expected based on single-factor studies (i.e., synergistic), potentially causing catastrophic dysfunction of physiological processes from an otherwise recoverable situation. Drought and herbivory are two stressors that commonly co-occur in forested ecosystems, and have the potential to ‘overlap’ in their impacts on various plant traits and processes. However, the combined impacts from these two stressors may not be predictable based on additive models from single-stressor studies. Moreover, the impacts and subsequent recovery may be strongly influenced by the relative intensities of each stressor. Here, we applied drought stress and simulated bark-feeding herbivory at three levels of intensity (control, moderate and severe) in a full factorial design on young Pinus sylvestris L. seedlings. We assessed if the combined effects from two stressors were additive (responses were equal to the sum of the single-factor effects), synergistic (greater than expected) or antagonistic (less than expected) on a suite of morphological and physiological traits at the leaf-, tissue- and whole-plant level. We additionally investigated whether recovery from herbivory was dependent on relief from drought. The two stressors had synergistic impacts on specific leaf area and water-use efficiency, additive effects on height and rootto-shoot ratios, but antagonistic effects on photosynthesis, conductance and, most notably, on root, shoot and whole-plant biomass. Nevertheless, the magnitude and direction of the combined impacts were often dependent on the relative intensities of each stressor, leading to many additive or synergistic responses from specific stressor combinations. Also, seedling recovery was far more dependent on the previous year’s drought compared with the previous year’s herbivory, demonstrating the influence of one stressor over another during recovery. Our study reveals for the first time, the importance of not only the presence or absence of drought and herbivory stressors, but also shows that their relative intensities are critical in determining the direction and magnitude of their impacts on establishing seedlings.
TL;DR: Urli et al. (2013) examine thresholds for hydraulic failure in a range of angiosperm species with a view to linking drought tolerance, survival and embolism resistance in fiveAngiosperm tree species and show that a physiological ‘point of no return’ can be predicted for these species based on the half time of their recovery after rewatering.
Abstract: Climate change is expected to result in an increased frequency of extreme events; while in some areas, heavier precipitation is predicted, other areas will suffer from more intense heat waves and droughts (Allison et al. 2009). Disturbingly, there is already evidence that increasing temperatures and reduced rainfall associated with climate change are responsible for an acceleration of large-scale forest dieback over the last century (Breshears et al. 2005; Allen et al. 2010). In the absence of mass mortality events, severe droughts can cause forest ecosystems to flip from net sinks to huge sources of CO2, with the response of the Amazon rainforest to recent droughts being the clearest example of this (Phillips et al. 2009). Increasingly, severe droughts are therefore expected to have profound effects on biodiversity, primary productivity and ecosystem function in many forested regions. Recent analyses have demonstrated that the majority of woody plant species across all forest biomes operate close to their safety margins for hydraulic failure, rendering them vulnerable to future shifts in precipitation and temperature (Choat et al. 2012). The safety margin referred to is the difference between the level of water stress experienced by a species in the field and the level of water stress that is likely to induce hydraulic failure in that species. In order to make accurate predictions of how different forest ecosystems and keystone species will be affected by drought, we require a detailed understanding of the physiology of plant death. The development of quantitative indices for mortality thresholds in woody plants is therefore essential for predictive models of vegetation response to drought. Despite this seemingly straight forward question (when is a plant dead?), reliable mortality indices are conspicuously lacking in the plant physiologist’s tool kit (Anderegg et al. 2012). Hydraulic failure is now widely recognized as one of the key mechanisms of drought-induced mortality in woody plants (Tyree and Sperry 1989; McDowell et al. 2008). As the soil dries, greater tension develops in the xylem water column leading to an increased probability of embolism formation. Gas emboli block xylem vessels, reducing the capacity of the plant to move water to the canopy. In intense or prolonged droughts, xylem embolism becomes extensive, causing the death of the whole plant. In this issue of Tree Physiology, Urli et al. (2013) examine thresholds for hydraulic failure in a range of angiosperm species with a view to linking drought tolerance, survival and embolism resistance in five angiosperm tree species. Their results show that a physiological ‘point of no return’ can be predicted for these species based on the half time of their recovery after rewatering. This approach defines a critical water potential beyond which plants were unable to recover, even after 1 year of rewatering (Brodribb and Cochard 2009). The authors then compared the minimum recoverable water potential with vulnerability to embolism curves for each species. This represents an important step because we already possess vulnerability curve data from hundreds of species across the full range of forest biomes (Maherali et al. 2004; Choat et al. 2012). Vulnerability curves describe the relationship between xylem water potential and loss of hydraulic conductivity due to embolism for a given species. Species are most commonly compared by the water potential at which 50% of hydraulic conductivity is lost (P50). In a previous work, Brodribb and Cochard (2009) demonstrated that the minimum recoverable water potential for four gymnosperm species was very close to species P50 values. In contrast, Urli et al. (2013) show that the minimum recoverable water potential of five Commentary
TL;DR: The results suggest that black spruce showed a plastic response to intense water deficit under warming, but this would compromise their survival, and increase in temperature under the projected future climate change would affect tree growth.
Abstract: Increase in temperature under the projected future climate change would affect tree growth, including the physiological mechanisms related to sapling responses, which has been examined recently. The study investigated the plant water relations, cambial activity and wood formation in black spruce saplings [Picea mariana (Mill.) B.S.P.] subjected to water deficit and warming. Four-year-old saplings growing in three greenhouses were submitted to different thermal conditions: T0, with a temperature equal to the external air temperature; and T + 2 and T + 5, with temperatures set at 2 and 5 K higher than T0, respectively. We also submitted saplings to two irrigation regimes and studied the effects of a water deficit of 32 days in May-June. We evaluated plant water relations, cambial activity, wood formation and anatomical characteristics from May to October 2010. Lower needle physiology rates were observed during water deficit, with 20-day suspension of irrigation, but after re-watering, non-irrigated saplings attained the same values as irrigated ones in all thermal conditions. Significant differences between irrigation regimes were detected in cambial activity at the end of the water deficit and after resumption of irrigation. Under warmer conditions, the recovery of non-irrigated saplings was slower than T0 and they needed from 2 to 4 weeks to completely restore cambial activity. No significant differences in wood anatomy were observed between irrigation regimes, but there was a sporadic effect on wood density under warming. During wood formation, the warmer conditions combined with water deficit increased sapling mortality by 5 and 12.2% for T + 2 and T + 5, respectively. The black spruce saplings that survived were more sensitive to water availability, and the restoration of cambial activity was slower at temperatures higher than T0. Our results suggest that black spruce showed a plastic response to intense water deficit under warming, but this would compromise their survival.
TL;DR: Fertigation affects the hydraulic architecture of plants, and different families develop different strategies, some of the hydraulic changes can explain the negative effect of N fertilization on growth.
Abstract: Fil: Faustino, Laura Ines. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Fisiologia Vegetal. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Fisiologia Vegetal; Argentina
TL;DR: Investigating different frost resistance mechanisms on 1-year-old branches of 11 European tree species from November until budburst brings new insights to the understanding of the altitudinal limits of trees, with winter embolism formation seems to be counterbalanced by active refilling from living cells.
Abstract: Winter physiology of woody plants is a key issue in temperate biomes. Here, we investigated different frost resistance mecha-nisms on 1-year-old branches of 11 European tree species from November until budburst: (i) frost hardiness of living cells (by electrolyte leakage method), (ii) winter embolism sensitivity (by percentage loss of conductivity: PLC) and (iii) phenologi-cal variation of budburst (by thermal time to budburst). These ecophysiological traits were analyzed according to the poten-tial altitudinal limit, which is highly related to frost exposure. Seasonal frost hardiness and PLC changes are relatively different across species. Maximal PLC observed in winter (PLC Max) was the factor most closely related to potential altitudinal limit. Moreover, PLC Max was related to the mean hydraulic diameter of vessels (indicating embolism sensitivity) and to osmotic compounds (indicating ability of living cells to refill xylem conducting elements). Winter embolism formation seems to be counterbalanced by active refilling from living cells. These results enabled us to model potential altitudinal limit according to three of the physiological/anatomical parameters studied. Monitoring different frost resistance strategies brings new insights to our understanding of the altitudinal limits of trees.
TL;DR: It is concluded that PSI is more sensitive than PSII to prolonged severe drought in these three drought-tolerant species, and CEF is essential for photoprotection in them.
Abstract: Drought stress can induce closure of stomata, thus leading to photoinhibition. The effects of prolonged severe drought under natural growing conditions on photosystem I (PSI), photosystem II (PSII) and cyclic electron flow (CEF) in drought-tolerant tree species are unclear. In spring 2010, southwestern China confronted severe drought that lasted several months. Using three dominant evergreen species, Cleistanthus sumatranus (Miq.) Muell. Arg. (Euphorbiaceae), Celtis philippensis Bl. (Ulmaceae) and Pistacia weinmannifolia J. Poisson ex Franch. (Anacardiaceae) that are native to a tropical limestone forest, we investigated the influence of this stress on PSI and PSII activities as well as light energy distribution in the PSII and P700 redox state. By the end of the drought period, predawn leaf water potential (Ψ(pd)) largely declined in each species, especially in C. sumatranus. Photosystem I activity strongly decreased in the three species, especially in C. sumatranus which showed a decrease of 65%. The maximum quantum yield of PSII after dark adaptation remained stable in P. weinmannifolia and C. philippensis but significantly decreased in C. sumatranus. Light response curves indicated that both linear electron flow and non-photochemical quenching were severely inhibited in C. sumatranus along with disappearance of CEF, resulting in deleterious excess light energy in PSII. We conclude that PSI is more sensitive than PSII to prolonged severe drought in these three drought-tolerant species, and CEF is essential for photoprotection in them.
TL;DR: Tree-to-tree variation would be more important than both radial and azimuthal variations in Fd for E estimation, however, F d for each tree should not be measured at a consistent aspect but should be measuredat various aspects to make accurate E estimates and to avoid a risk of error caused by the relationship of Fd to aspect.
Abstract: Understanding radial and azimuthal variation, and tree-to-tree variation, in sap flux density ( Fd) as sources of uncertainty is important for estimating transpiration using sap flow techniques. In a Japanese cedar ( Cryptomeria japonica D. Don.) forest, Fd was measured at several depths and aspects for 18 trees, using heat dissipation (Granier-type) sensors. We observed considerable azimuthal variation in Fd. The coefficient of variation (CV) calculated from Fd at a depth of 0–20 m m (Fd1) and Fd at a depth of 20–40 m m (Fd2) ranged from 6.7 to 37.6% (mean = 2 8.3%) and from 19.6 to 62.5% (mean = 3 4.6%) for the azimuthal directions. Fd at the north aspect averaged for nine trees, for which azimuthal measurements were made, was obviously smaller than Fd at the other three aspects (i.e., west, south and east) averaged for the nine trees. Fd1 averaged for the nine trees was significantly larger than Fd2 averaged for the nine trees. The error for stand-scale transpiration (E) estimates caused by ignoring the azimuthal variation was larger than that caused by ignoring the radial variation. The error caused by ignoring tree-to-tree variation was larger than that caused by ignoring both radial and azimuthal variations. Thus, tree-to-tree variation in Fd would be more important than both radial and azimuthal variations in Fd for E estimation. However, Fd for each tree should not be measured at a consistent aspect but should be measured at various aspects to make accurate E estimates and to avoid a risk of error caused by the relationship of Fd to aspect.