Showing papers on "Tree canopy published in 2007"

On soil moisture–vegetation feedbacks and their possible effects on the dynamics of dryland ecosystems

Abstract: [1] Soil moisture is the environmental variable synthesizing the effect of climate, soil, and vegetation on the dynamics of water-limited ecosystems. Unlike abiotic factors (e.g., soil texture and rainfall regime), the control exerted by vegetation composition and structure on soil moisture variability remains poorly understood. A number of field studies in dryland landscapes have found higher soil water contents in vegetated soil patches than in adjacent bare soil, providing a convincing explanation for the observed preferential establishment of grasses and seedlings beneath tree canopies. Thus, because water is the limiting factor for vegetation in arid and semiarid ecosystems, a positive feedback could exist between soil moisture and woody vegetation dynamics. It is still unclear how the strength of such a feedback would change under different long-term rainfall regimes. To this end, we report some field observations from savanna ecosystems located along the south-north rainfall gradient in the Kalahari, where the presence of relatively uniform sandy soils limits the effects of covarying factors. The data available from our field study suggest that the contrast between the soil moisture in the canopy and intercanopy space increases (with wetter soils under the canopy) with increasing levels of aridity. We hypothesize that this contrast may lead to a positive feedback and explore the implications of such a feedback in a minimalistic model. We found that when the feedback is relatively strong, the system may exhibit two stable states corresponding to conditions with and without tree canopy cover. In this case, even small changes in environmental variables may lead to rapid and largely irreversible shifts to a state with no tree canopy cover. Our data suggest that the tendency of the system to exhibit two (alternative) stable states becomes stronger in the more arid regions. Thus, at the desert margins, vegetation is more likely to be prone to discontinuous and abrupt state changes.

Revised method for forest canopy height estimation from Geoscience Laser Altimeter System waveforms

Abstract: The vertical extent of waveforms collected by the Geoscience Laser Altimeter System (onboard ICESat - the Ice, Cloud, and land Elevation Satellite) increases as a function of terrain slope and footprint size (the area on the ground that is illuminated by the laser) Over sloped terrain, returns from both canopy and ground surfaces can occur at the same elevation As a result, the height of the waveform (waveform extent) is insufficient to make estimates of tree height on sloped terrain, and algorithms are needed that are capable of retrieving information about terrain slope from the waveform itself Early work on this problem used a combination of waveform height indices and slope indices from a digital elevation model (DEM) A second generation algorithm was developed using datasets from diverse forests in which forest canopy height has been estimated in the field or by via airborne lidar Forest types considered in this paper include evergreen needleleaf, deciduous broadleaf and mixed stands in temperate North America, and tropical evergreen broadleaf forests in Brazil The algorithm described eliminates the need for a DEM, and estimates forest canopy height with an RMSE of 5 m (83% of variance in forest canopy height explained)

Fire frequency and tree canopy structure influence plant species diversity in a forest-grassland ecotone

Abstract: Disturbances and environmental heterogeneity are two factors thought to influence plant species diversity, but their effects are still poorly understood in many ecosystems. We surveyed understory vegetation and measured tree canopy cover on permanent plots spanning an experimental fire frequency gradient to test fire frequency and tree canopy effects on plant species richness and community heterogeneity within a mosaic of grassland, oak savanna, oak woodland, and forest communities. Species richness was assessed for all vascular plant species and for three plant functional groups: grasses, forbs, and woody plants. Understory species richness and community heterogeneity were maximized at biennial fire frequencies, consistent with predictions of the intermediate disturbance hypothesis. However, overstory tree species richness was highest in unburned units and declined with increasing fire frequency. Maximum species richness was observed in unburned units for woody species, with biennial fires for forbs, and with near-annual fires for grasses. Savannas and woodlands with intermediate and spatially variable tree canopy cover had greater species richness and community heterogeneity than old-field grasslands or closed-canopy forests. Functional group species richness was positively correlated with functional group cover. Our results suggest that annual to biennial fire frequencies prevent shrubs and trees from competitively excluding grasses and prairie forbs, while spatially variable shading from overstory trees reduces grass dominance and provides a wider range of habitat conditions. Hence, high species richness in savannas is due to both high sample point species richness and high community heterogeneity among sample points, which are maintained by intermediate fire frequencies and variable tree canopy cover.

Vertical stratification of vascular epiphytes in submontane and montane forest of the Bolivian Andes: the importance of the understory

Abstract: We studied species richness, composition, and vertical distribution of vascular epiphytes at two sites in the Bolivian Andes. To account for the epiphyte flora on understory trees, epiphytes on shrubs and small trees were sampled in 20 × 20 m2 subplots around each sampled canopy tree; this understory zone U is introduced as an addition to the well-established five vertical Johansson tree zones. More than 20% of about 500 species recorded were found only in the understory subplots, including ca. 40% of aroids, 35%–40% of piperoids, and 25%–30% of ferns. Habitat generalists (occurring in three or more zones) were most common, contributing about 50% of all species, specialists (occurring only in two zones, or in three continuous ones) 34%–42%, and hemiepiphytes 6%–16%. Canopy epiphytes (occurring > 90% in tree zones Z3–5) were mainly represented by orchids and ferns, many with special adaptations to drought stress such as pseudobulbs, succulence, and poikilohydry. Trunk epiphytes ( > 90% in understory and tree zones Z1–2) reached highest relative species numbers among piperoids and ferns. Most hemiepiphytes were also trunk epiphytes, due to their characteristic growth pattern, and included mainly aroids. The vertical distribution of epiphytes within a tree is determined by several microenvironmental gradients, with light intensity, wind speed, and air temperature increasing and air humidity decreasing from the ground level to the canopy.

Water savings in mature deciduous forest trees under elevated CO2

Abstract: Stomatal conductance of plants exposed to elevated CO2 is often reduced. Whether this leads to water savings in tall forest-trees under future CO2 concentrations is largely unknown but could have significant implications for climate and hydrology. We used three different sets of measurements (sap flow, soil moisture and canopy temperature) to quantify potential water savings under elevated CO2 in a ca. 35 m tall, ca. 100 years old mixed deciduous forest. Part of the forest canopy was exposed to 540 ppm CO2 during daylight hours using free air CO2 enrichment (FACE) and the Swiss Canopy Crane (SCC). Across species and a wide range of weather conditions, sap flow was reduced by 14% in trees subjected to elevated CO2, yielding ca. 10% reduction in evapotranspiration. This signal is likely to diminish as atmospheric feedback through reduced moistening of the air comes into play at landscape scale. Vapour pressure deficit (VPD)-sap flow response curves show that the CO2 effect is greatest at low VPD, and that sap flow saturation tends to occur at lower VPD in CO2-treated trees. Matching stomatal response data, the CO2 effect was largely produced by Carpinus and Fagus, with Quercus contributing little. In line with these findings, soil moisture at 10 cm depth decreased at a slower rate under high-CO2 trees than under control trees during rainless periods, with a reversal of this trend during prolonged drought when CO2-treated trees take advantage from initial water savings. High-resolution thermal images taken at different heights above the forest canopy did detect reduced water loss through altered energy balance only at > 5 m distance (0.44 K leaf warming of CO2-treated Fagus trees). Short discontinuations of CO2 supply during morning hours had no measurable canopy temperature effects, most likely because the stomatal effects were small compared with the aerodynamic constraints in these dense, broad-leaved canopies. Hence, on a seasonal basis, these data suggest a >10% reduction in water consumption in this type of forest when the atmosphere reaches 540% ppm CO2.

Changes in Canopy Processes Following Whole-Forest Canopy Nitrogen Fertilization of a Mature Spruce-Hemlock Forest

Abstract: Most experimental additions of nitrogen to forest ecosystems apply the N to the forest floor, bypassing important processes taking place in the canopy, including canopy retention of N and/or conversion of N from one form to another. To quantify these processes, we carried out a large-scale experiment and determined the fate of nitrogen applied directly to a mature coniferous forest canopy in central Maine (18–20 kg N ha−1 y−1 as NH4NO3 applied as a mist using a helicopter). In 2003 and 2004 we measured NO3 −, NH4 +, and total dissolved N (TDN) in canopy throughfall (TF) and stemflow (SF) events after each of two growing season applications. Dissolved organic N (DON) was greater than 80% of the TDN under ambient inputs; however NO3 − accounted for more than 50% of TF N in the treated plots, followed by NH4 + (35%) and DON (15%). Although NO3 − was slightly more efficiently retained by the canopy under ambient inputs, canopy retention of NH4 +as a percent of inputs increased markedly under fertilization. Recovery of less than 30% of the fertilizer N in TF suggested that the forest canopy retained more than 70% of the applied N (>80% when corrected for N which bypassed tree surfaces at the time of fertilizer addition). Results from plots receiving 15N enriched NO3 − and NH4 + confirmed bulk N estimations that more NO3 − than NH4 + was washed from the canopy by wet deposition. The isotope data did not show evidence of canopy nitrification, as has been reported in other spruce forests receiving much higher N inputs. Conversions of fertilizer-N to DON were observed in TF for both 15NH4 + and 15NO3 − additions, and occurred within days of the application. Subsequent rain events were not significantly enriched in 15N, suggesting that canopy DON formation was a rapid process related to recent N inputs to the canopy. We speculate that DON may arise from lichen and/or microbial N cycling rather than assimilation and re-release by tree tissues in this forest. Canopy retention of experimentally added N may meet and exceed calculated annual forest tree demand, although we do not know what fraction of retained N was actually physiologically assimilated by the plants. The observed retention and transformation of DIN within the canopy demonstrate that the fate and ecosystem consequences of N inputs from atmospheric deposition are likely influenced by forest canopy processes, which should be considered in N addition studies.

Plant functional group responses to fire frequency and tree canopy cover gradients in oak savannas and woodlands

Abstract: Questions: How do fire frequency, tree canopy cover, and their interactions influence cover of grasses, forbs and understorey woody plants in oak savannas and woodlands? Location: Minnesota, USA. Methods: We measured plant functional group cover and tree canopy cover on permanent plots within a long-term prescribed fire frequency experiment and used hierarchical linear modeling to assess plant functional group responses to fire frequency and tree canopy cover. Results: Understorey woody plant cover was highest in unburned woodlands and was negatively correlated with fire frequency. C4-grass cover was positively correlated with fire frequency and negatively correlated with tree canopy cover. C3-grass cover was highest at 40% tree canopy cover on unburned sites and at 60% tree canopy cover on frequently burned sites. Total forb cover was maximized at fire frequencies of 4–7 fires per decade, but was not significantly influenced by tree canopy cover. Cover of N-fixing forbs was highest in shaded are...

Airborne Laser Scanning of Forest Stem Volume in a Mountainous Environment

Abstract: Airborne laser scanning (ALS) is an active remote sensing technique that uses the time-of-flight measurement principle to capture the three-dimensional structure of the earth's surface with pulsed lasers that transmit nanosecond-long laser pulses with a high pulse repetition frequency. Over forested areas most of the laser pulses are reflected by the leaves and branches of the trees, but a certain fraction of the laser pulses reaches the forest floor through small gaps in the canopy. Thus it is possible to reconstruct both the three-dimensional structure of the forest canopy and the terrain surface. For the retrieval of quantitative forest parameters such as stem volume or biomass it is necessary to use models that combine ALS with inventory data. One approach is to use multiplicative regression models that are trained with local inventory data. This method has been widely applied over boreal forest regions, but so far little experience exists with applying this method for mapping alpine forest. In this study the transferability of this approach to a 128 km2 large mountainous region in Vorarlberg, Austria, was evaluated. For the calibration of the model, inventory data as operationally collected by Austrian foresters were used. Despite these inventory data are based on variable sample plot sizes, they could be used for mapping stem volume for the entire alpine study area. The coefficient of determination R2 was 0.85 and the root mean square error (RMSE) 90.9 m3ha−1 (relative error of 21.4%) which is comparable to results of ALS studies conducted over topographically less complex environments. Due to the increasing availability, ALS data could become an operational part of Austrian's forest inventories.

Interannual consistency in canopy stomatal conductance control of leaf water potential across seven tree species

Abstract: We investigated interannual variability of canopy transpiration per unit ground area (E (C)) and per unit leaf area (E (L)) across seven tree species in northern Wisconsin over two years. These species have previously been shown to be sufficient to upscale stand-level transpiration to the landscape level during one growing season. Our objective was to test whether a simple plant hydraulic model could capture interannual variation in transpiration. Three species, wetland balsam fir (Abies balsamea (L.) Mill), basswood (Tilia Americana L.) and speckled alder (Alnus rugosa (DuRoi) Spreng), had no change in E (C) or E (L) between 2000 and 2001. Red pine (Pinus resinosa Ait) had a 57 and 19% increase in E (C) and E (L), respectively, and sugar maple (Acer saccharum Marsh) had an 83 and 41% increase in E (C) and E (L), respectively, from 2000 to 2001. Quaking aspen (Populus tremuloides Michx) had a 50 and 21% decrease in E (C) and E (L), respectively, from 2000 to 2001 in response to complete defoliation by forest tent caterpillar (Malascoma distria Huber) and subsequent lower total leaf area index of the reflushed foliage. White cedar (Thuja occidentalis L.) had a 20% decrease in both E (C) and E (L) caused by lowered surface water in wetlands in 2001 because of lower precipitation and wetland flow management. Upland A. balsamea increased E (L) and E (C) by 55 and 53%, respectively, as a result of release from light competition of the defoliated, overstory P. tremuloides. We hypothesized that regardless of different drivers of interannual variability in E (C) and E (L), minimum leaf water potential would be regulated at the same value. Minimum midday water potentials were consistent over the two years within each of the seven species despite large changes in transpiration between years. This regulation was independently verified by the exponential saturation between daily E (C) and vapor pressure deficit (D) and the tradeoff between a reference canopy stomatal conductance (G (S)) and the sensitivity of G (S) to D, indicating that trees with high G (S) must decrease G (S) in response to atmospheric drought faster than trees with low G (S). Our results show that models of forest canopy transpiration can be simplified by incorporating G (S) regulation of minimum leaf water potential for isohydric species.

Ecophysiological traits of plant functional groups in forest and pasture ecosystems from eastern Amazônia, Brazil

Abstract: The plant functional group approach has the potential to clarify ecological patterns and is of particular importance in simplifying the application of ecological models in high biodiversity ecosystems. Six functional groups (pasture grass, pasture sapling, top-canopy tree, top-canopy liana, mid canopy tree, and understory tree) were established a priori based on ecosystem inhabited, life form, and position within the forest canopy profile on eastern Amazonian region. Ecophysiological traits related to photosynthetic gas exchange were then used to characterize such groups. The ecophysiological traits evaluated showed considerable variations among groups. The pasture grass functional group (a C4 photosynthetic pathway species) showed high instantaneous water use efficiency (A max/g s@A max), high photosynthetic nitrogen use efficiency (A max/N area), and high ratio of A max to dark respiration (A max/R d). Among the species with the C3 photosynthetic pathway, the top-canopy liana group showed the highest mean of A max/g s@A max, statistically distinct from the lowest average presented by the understory tree group. Furthermore, the pasture sapling group showed the lowest average of A max/R d, statistically distinct from the high average observed for the understory tree group. Welch-ANOVAs followed by Games–Howell post hoc tests applied to ecophysiological traits produced reasonable distinctions among functional groups, although no significant distinction was detected between the groups top-canopy tree and pasture sapling. Species distribution within the functional groups was accurately reproduced by discriminant analyses based on species averages of ecophysiological traits. The present work convincingly shows that the functional groups identified have distinct ecophysiological characteristics, with the potential to respond differently to environmental factors. Such information is of great importance in modeling efforts that evaluate the effects of dynamic changes in tropical plant communities over ecosystem primary productivity.

Trunk reiteration promotes epiphytes and water storage in an old-growth redwood forest canopy

Abstract: Sequoia sempervirens (redwood) is a long-lived, shade-tolerant tree capable of regeneration without disturbances and thus often present in all sizes within a single forest In order to evaluate functional linkages among structures, plant distribution, and biodiversity in the canopy, we quantified all vascular plants from ground level to the treetops in an old-growth redwood forest (Prairie Creek Redwoods State Park, California, USA) This involved mapping terrestrial and epiphytic trees, shrubs, and ferns as well as climbing 27 trees up to 101 m tall within a 1-ha plot We monitored canopy microclimates using sensor arrays that collected hourly data for up to 30 months The plot held 4283 Mg/ha of aboveground dry mass in living plants, 954% of which was contributed by redwood A high degree of structural complexity and individuality was evident in the crowns of the 14 largest trees in the form of reiterated trunks arising from main trunks, other trunks, and limbs Thirteen species of vascular plants occu

Vertical stratification of feeding guilds and body size in beetle assemblages from an Australian tropical rainforest

Abstract: The vertical stratification of insect species assemblages inhabiting tropical rainforests is well established but few have examined whether these patterns are reflected in vertical stratification of body size or feeding guilds. We used Malaise and Flight Interception Traps to sample beetle assemblages from five locations, at both canopy and ground zones of a tropical lowland rainforest site near Cape Tribulation, Australia. Beetles from 4 years of sampling were sorted to Family and morphospecies, and allocated to one of five feeding guilds. Within feeding guilds the number of species and individuals, from canopy- and ground-caught traps were compared. The body lengths of species were measure and compared within feeding guilds and families. Herbivores was the dominant guild but was not the majority of all species or individuals. Most beetle species (69%) were less than 5 mm in length and the mean size of canopy-caught species was greater than that for ground-caught species. This was probably due to slightly more species of plant feeders (herbivores and xylophages) present in the canopy, which were significantly larger than saprophages, fungivores and predators. Among feeding guilds, there were few overall canopy–ground differences. These results contrast with species composition results presented elsewhere where strong differences between the canopy and the ground were evident. We suggest that our guild groupings may have been too coarse to detect fine-scale differences and that resource partitioning may have also masked faunal stratification. We propose that fine-scale differences in resources between the canopy and the ground, together with strong microclimate gradients, are likely to be important in structuring the vertical stratification of insect assemblages at the level of species, but not with respect to functional groups.

Soil Nutrients Limit Fine Litter Production and Tree Growth in Mature Lowland Forest of Southwestern Borneo

Abstract: Efforts to improve models of terrestrial productivity and to understand the function of tropical forests in global carbon cycles require a mechanistic understanding of spatial variation in aboveground net primary productivity (ANPP) across tropical landscapes. To help derive such an understanding for Borneo, we monitored aboveground fine litterfall, woody biomass increment and ANPP (their sum) in mature forest over 29 months across a soil nutrient gradient in southwestern Kalimantan. In 30 (0.07 ha) plots stratified throughout the watershed (∼340 ha, 8–190 m a.s.l.), we measured productivity and tested its relationship with 27 soil parameters. ANPP across the study area was among the highest reported for mature lowland tropical forests. Aboveground fine litterfall ranged from 5.1 to 11.0 Mg ha−1 year−1 and averaged 7.7 ± 0.4 (mean ± 95 C.I.). Woody biomass increment ranged from 5.8 to 23.6 Mg ha−1 year−1 and averaged 12.0 ± 2.0. Growth of large trees (≥60 cm dbh) contributed 38–82% of plot-wide biomass increment and explained 92% of variation among plots. ANPP, the sum of these parameters, ranged from 11.1 to 32.3 Mg ha−1 year−1 and averaged 19.7 ± 2.2. ANPP was weakly related to fine litterfall (r 2 = 0.176), but strongly related to growth of large trees at least 60 cm dbh (r 2 = 0.848). Adjusted ANPP after accounting for apparent “mature forest bias” in our sampling method was 17.5 ± 1.2 Mg ha−1 year−1.Relating productivity measures to soil parameters showed that spatial patterning in productivity was significantly related to soil nutrients, especially phosphorus (P). Fine litterfall increased strongly with extractable P (r 2 = 0.646), but reached an asymptote at moderate P levels, whereas biomass increment (r 2 = 0.473) and ANPP (r 2 = 0.603) increased linearly across the gradient. Biomass increment of large trees was more frequently and strongly related to nutrients than small trees, suggesting size dependency of tree growth on nutrients. Multiple linear regression confirmed the leading importance of soil P, and identified Ca as a potential co-limiting factor. Our findings strongly suggest that (1) soil nutrients, especially P, limit aboveground productivity in lowland Bornean forests, and (2) these forests play an important, but changing role in carbon cycles, as canopy tree logging alters these terrestrial carbon sinks.

Patterns of woody plant species abundance and diversity in the seedling layer of a tropical forest

Abstract: Questions: 1. How does the composition and diversity of established seedlings compare to that of larger size classes in tropical forests? 2. How do species abundances in the seedling layer vary with adult abundance and life history strategies? Location: Barro Colorado Island (BCI), Panama. Methods: We inventoried woody seedlings ≥ 20 cm tall and < 1 cm DBH in ca. 20 000 1-m 2 quadrats within the BCI 50-ha Forest Dynamics Plot, where all trees and shrubs ≥ 1 cm DBH have been identified. We compared diversity and composition of seedlings to that of larger size classes and tested whether adult abundance, growth form, and shade tolerance contributed to variation in seedling abundance among species. Results: We encountered 60056 seedlings of 332 tree, shrub, and liana species. Diversity of tree seedlings was lower than that of trees ≥ 1 cm DBH. Species abundances in the seedling layer increased non-linearly with reproductive adult abundance, such that per capita seedling abundance declined with adult abundance. Per capita seedling abundance was highest for canopy tree species and lowest for understorey trees. For canopy trees, shade-tolerant species had significantly higher per capita seedling abundance than more light-demanding species. Conclusions: The woody seedling layer on BCI is composed of a subset of the species present in larger size classes. Most species were present in less than 1% of seedling plots, suggesting strong recruitment limitation. Tree species abundance in the seedling layer is largely a function of reproductive adult abundance, but is also influenced by life history strategies and compensatory processes.

Canopy uptake of atmospheric N deposition at a conifer forest: part I -canopy N budget, photosynthetic efficiency and net ecosystem exchange

Abstract: Global carbon cycle assessments of anthropogenic nitrogen (N) deposition influences on carbon sequestration often assume enhanced sequestration results. This assumption was evaluated at a Rocky Mountains spruce-fir forest. Forest canopy N uptake (CNU) of atmospheric N deposition was estimated by combining event wet and throughfall N fluxes with gradient measured HNO 3 and NH 3 as well as inferred (NOx and particulate N) dry fluxes. Approximately 80% of the growing-season 3 kgN ha -1 total deposition is retained in canopy foliage and branches. This CNU constitutes ∼1/3 of canopy growing season new N supply at this conifer forest site. Daytime net ecosystem exchange (NEE) significantly (P= 0.006) and negatively (CO 2 uptake) correlated with CNU. Multiple regression indicates ∼20% of daytime NEE may be attributed to CNU ( P < 0.02); more than soil water content. A wet deposition N-amendment study (Tomaszewski and Sievering—part II), at canopy spruce branches, increased their growing-season CNU by 40–50% above ambient. Fluorometry and gas exchange results show N-amended spruce branches had greater photosynthetic efficiency and higher carboxylation rates than control and untreated branches. N-amended branches had 25% less photoinhibition, with a 5–9% greater proportion of foliar-N-in-Rubisco. The combined results provide, partly, a mechanistic explanation for the NEE dependence on CNU. DOI: 10.1111/j.1600-0889.2007.00264.x

Effect of single Quercus ilex trees upon spatial and seasonal changes in soil water content in dehesas of central western Spain

Abstract: The spatial and temporal evolution of soil water content (θ) in Quercus ilex dehesas has been investigated to determine how trees modify the soil water dynamics and the nature of tree-grass interactions in terms of soil water use in these ecosystems. Soil physical parameters and θ were measured at different distances from the tree trunk (2–30 m) in the upper 300 cm of soil. θ was measured monthly by TDR during 2002–2005. Tree water potential was determined during the summers of 2004 and 2005. At deeper soil layers, mean θ values were higher beyond than beneath tree canopy during dry periods. θ depletion beyond tree canopy continued even in summer, when herbaceous plants dried up, suggesting that trees uptake water from the whole inter-tree space. Results have shown a high dependence of trees on deep water reserves throughout late spring and summer, which helps to avoid competition for water with herbaceous vegetation.

Synopsis of the CASIROZ case study: carbon sink strength of Fagus sylvatica L. in a changing environment--experimental risk assessment of mitigation by chronic ozone impact.

Abstract: Databases are needed for the ozone (O 3 ) risk assessment on adult forest trees under stand conditions, as mostly juvenile trees have been studied in chamber experiments. A synopsis is presented here from an integrated case study which was conducted on adult FAGUS SYLVATICA trees at a Central-European forest site. Employed was a novel free-air canopy O 3 fumigation methodology which ensured a whole-plant assessment of O 3 sensitivity of the about 30 m tall and 60 years old trees, comparing responses to an experimental 2 × ambient O 3 regime (2 × O 3 , max. 150 nl O 3 l -1 ) with those to the unchanged 1 × ambient O 3 regime (1 × O 3 = control) prevailing at the site. Additional experimentation on individual branches and juvenile beech trees exposed within the forest canopy allowed for evaluating the representativeness of young-tree and branch-bag approaches relative to the O 3 sensitivity of the adult trees. The 2 × O 3 regime did not substantially weaken the carbon sink strength of the adult beech trees, given the absence of a statistically significant decline in annual stem growth; a 3 % reduction across five years was demonstrated, however, through modelling upon parameterization with the elaborated database. 2 × O 3 did induce a number of statistically significant tree responses at the cell and leaf level, although the O 3 responsiveness varied between years. Shade leaves displayed an O 3 sensitivity similar to that of sun leaves, while indirect belowground O 3 effects, apparently mediated through hormonal relationships, were reflected by stimulated fine-root and ectomycorrhizal development. Juvenile trees were not reliable surrogates of adult ones in view of O 3 risk assessment. Branch sections enclosed in (climatized) cuvettes, however, turned out to represent the O 3 sensitivity of entire tree crowns. Drought-induced stomatal closure decoupled O 3 intake from O 3 exposure, as in addition, also the “physiologically effective O 3 dose” was subject to change. No evidence emerged for a need to lower the “Critical Level for Ozone” in risk assessment of forest trees, although sensitive tree parameters did not necessarily reflect a linear relationship to O 3 stress. Exposure-based concepts tended to overestimate O 3 risk under drought, which is in support of current efforts to establish flux-related concepts of O 3 intake in risk assessment.

Post-fire regeneration of Mediterranean plant communities at a regional scale is dependent on vegetation type and dryness

Abstract: Question: We tested whether (1) the change in composition and structure of whole plant communities after fire is directly related to regeneration of the dominant tree species in the canopy; (2) the change in structure and composition of plant communities several years after fire decreases with the proportion of obligate seeders and (3) the proportion of obligate seeders in plant communities increases with the dryness gradient. Location: Catalonia (NE Spain). Methods: We measured floristic differences between burned and long-since burned sites in eight vegetation types across a climate gradient. We compared 22 sites burnt in 1994 in paired plots with 22 sites that had not been burnt since the 1940s. In each site we placed plots in burned and long-since burned areas, where we identified the presence and abundance of all plant species. Results: When the tree canopy recovers, structure and composition of the vegetation also return to the long-since burned community; when tree canopy does not recover,...

Species richness and distribution patterns of leaf-inhabiting endophytic fungi in a temperate forest canopy

Abstract: In 2005, researchers at the Leipzig Canopy Crane Research Facility collected living leaves of four temperate tree species at heights of between 15 and 33 m above the ground. Following surface sterilisation of the leaves, leaf-fragments were cultured on malt extract agar which allowed the growth of endophytic fungi into the surrounding medium. Isolated cultures were identified by morphology and sequence analysis of the D1/D2 region of the large subunit rDNA. Phylogenetic analysis established the taxonomic positions of the fungi. A total of 49 different taxa were identified, representing 20 families and ten orders. With the exception of one basidiomycetous yeast, all taxa belonged to filamentous ascomycetes. Species richness was highest on Tilia cordata and lowest on Quercus robur. Species-accumulation curves showed that the sampling effort was not sufficient to cover the majority of the likely species at the investigation site. Most endophytes proved to be ubiquitous within the canopy of the investigation site, but habitat preferences in terms of different tree species, different light regimes and season (sampling times) were obvious for some abundant endophytes. Apiognomonia errabunda and Aspergillus niger occurred predominantly on Q. robur, Diplodina acerina on Acer pseudoplatanus, one species of Phoma significantly prefered shaded leaves from the lower canopy layer whereas Sordaria fimicola prefered sun-exposed leaves from the upper tree crowns. Seasonal patterns were observed, for example, for A. errabunda, which was abundant in young leaves in the spring and almost completely absent in aged autumn-leaves, thus suggesting the accumulation of antifungal secondary plant metabolites during the growing season.

Effects of forest canopy on habitat selection in treefrogs and aquatic insects: implications for communities and metacommunities

Abstract: The specific dispersal/colonization strategies used by species to locate and colonize habitat patches can strongly influence both community and metacommunity structure. Habitat selection theory predicts nonrandom dispersal to and colonization of habitat patches based on their quality. We tested whether habitat selection was capable of generating patterns of diversity and abundance across a transition of canopy coverage (open and closed canopy) and nutrient addition by investigating oviposition site choice in two treefrog species (Hyla) and an aquatic beetle (Tropisternus lateralis), and the colonization dynamics of a diverse assemblage of aquatic insects (primarily beetles). Canopy cover produced dramatic patterns of presence/absence, abundance, and species richness, as open canopy ponds received 99.5% of propagules and 94.6% of adult insect colonists. Nutrient addition affected only Tropisternus oviposition, as females oviposited more egg cases at higher nutrient levels, but only in open canopy ponds. The behavioral partitioning of aquatic landscapes into suitable and unsuitable habitats via habitat selection behavior fundamentally alters how communities within larger ecological landscapes (metacommunities) are linked by dispersal and colonization.