Showing papers on "Growing season published in 2004"

Global patterns of plant leaf N and P in relation to temperature and latitude.

Abstract: A global data set including 5,087 observations of leaf nitrogen (N) and phosphorus (P) for 1,280 plant species at 452 sites and of associated mean climate indices demonstrates broad biogeographic patterns. In general, leaf N and P decline and the N/P ratio increases toward the equator as average temperature and growing season length increase. These patterns are similar for five dominant plant groups, coniferous trees and four angiosperm groups (grasses, herbs, shrubs, and trees). These results support the hypotheses that (i) leaf N and P increase from the tropics to the cooler and drier midlatitudes because of temperature-related plant physiological stoichiometry and biogeographical gradients in soil substrate age and then plateau or decrease at high latitudes because of cold temperature effects on biogeochemistry and (ii) the N/P ratio increases with mean temperature and toward the equator, because P is a major limiting nutrient in older tropical soils and N is the major limiting nutrient in younger temperate and high-latitude soils.

Seedling establishment of a boreal tree species (Pinus sylvestris) at its southernmost distribution limit: consequences of being in a marginal Mediterranean habitat

Abstract: Summary 1 We analyse the factors controlling seedling establishment of Scots pine at its southernmost geographical limit (southern Spain), by monitoring emergence, survival and growth for up to 4 years in the microhabitats to which seeds are dispersed Naturally established seedlings were monitored in two mountain ranges, and experimental sowings were performed both in woodlands and in adjacent successional shrublands into which the forest could expand 2 Emergence was high in all microhabitats, although it was highest under the canopy of shrubs Overall survival was low, with c 90% of seedlings dying in the first growing season ( c 98% after several growing seasons) Survival differed among microhabitats, being highest under shrubs and extremely low (or zero) under pines or in bare soil 3 Seedling growth was the highest in areas of bare soil, intermediate under shrubs, and very low under pines 4 Establishment under pines was prevented by both mortality and poor performance, and good performance cannot counteract high mortality in the open Shrubs, however, acted as nurse plants, buffering summer drought without reducing radiation to levels critical for growth, and protecting seedlings from ungulate trampling, hail and frost heave 5 Patterns of recruitment were similar for woodland stands and successional shrublands In addition, patterns of survival for naturally established seedlings were similar to those of seedlings originating from experimental sowings 6 Juveniles were positively associated with shrubs but negatively with bare soil or areas below pine canopies The facilitative effect of shrubs on seedling survival therefore changes the spatial pattern of recruitment from that determined by germination 7 Overall, processes controlling seedling establishment in these southern Scots pine forests differ sharply from those operating in its main distribution area The comparison among contrasting geographical ranges may contribute to an understanding of the role of environmental conditions in the balance between competition and facilitation, and assist in forecasting plant regeneration responses to global climate change

Soil microbial dynamics in maize-growing soil under different tillage and residue management systems

Abstract: The long-term impact of tillage and residue management on soil microorganisms was studied over the growing season in a sandy loam to loamy sand soil of southwestern Quebec, growing maize ( Zea mays L.) monoculture. Tillage and residue treatments were first imposed on plots in fall 1991. Treatments consisted of no till, reduced tillage, and conventional tillage with crop residues either removed from (−R) or retained on (+R) experimental plots, laid out in a randomized complete block design. Soil microbial biomass carbon (SMB-C), soil microbial biomass nitrogen (SMB-N) and phospholipid fatty acid (PLFA) contents were measured four times, at two depths (0–10 and 10–20 cm), over the 2001 growing season. Sample times were: May 7 (preplanting), June 25, July 16, and September 29 (prior to corn harvest). The effect of time was of a greater magnitude than those attributed to tillage or residue treatments. While SMB-C showed little seasonal change (160 μg C g −1 soil), SMB-N was responsive to post-emergence mineral nitrogen fertilization, and PLFA analysis showed an increase in fungi and total PLFA throughout the season. PLFA profiles showed better distinction between sampling time and depth, than between treatments. The effect of residue was more pronounced than that of tillage, with increased SMB-C and SMB-N (61 and 96%) in +R plots compared to −R plots. This study illustrated that measuring soil quality based on soil microbial components must take into account seasonal changes in soil physical and chemical conditions.

Transcom 3 inversion intercomparison: Model mean results for the estimation of seasonal carbon sources and sinks

Abstract: [1] The TransCom 3 experiment was begun to explore the estimation of carbon sources and sinks via the inversion of simulated tracer transport. We build upon previous TransCom work by presenting the seasonal inverse results which provide estimates of carbon flux for 11 land and 11 ocean regions using 12 atmospheric transport models. The monthly fluxes represent the mean seasonal cycle for the 1992 to 1996 time period. The spread among the model results is larger than the average of their estimated flux uncertainty in the northern extratropics and vice versa in the tropical regions. In the northern land regions, the model spread is largest during the growing season. Compared to a seasonally balanced biosphere prior flux generated by the CASA model, we find significant changes to the carbon exchange in the European region with greater growing season net uptake which persists into the fall months. Both Boreal North America and Boreal Asia show lessened net uptake at the onset of the growing season with Boreal Asia also exhibiting greater peak growing season net uptake. Temperate Asia shows a dramatic springward shift in the peak timing of growing season net uptake relative to the neutral CASA flux while Temperate North America exhibits a broad flattening of the seasonal cycle. In most of the ocean regions, the inverse fluxes exhibit much greater seasonality than that implied by the DpCO2 derived fluxes though this may be due, in part, to misallocation of adjacent land flux. In the Southern Ocean, the austral spring and fall exhibits much less carbon uptake than implied by DpCO2 derived fluxes. Sensitivity testing indicates that the inverse estimates are not overly influenced by the prior flux choices. Considerable agreement exists between the model mean, annual mean results of this study and that of the previously published TransCom annual mean inversion. The differences that do exist are in poorly constrained regions and tend to exhibit compensatory fluxes in order to match the global mass constraint. The differences between the estimated fluxes and the prior model over the northern land regions could be due to the prior model respiration response to temperature. Significant phase differences, such as that in the Temperate Asia region, may be due to the limited observations for that region. Finally, differences in the boreal land regions between the prior model and the estimated fluxes may be a reflection of the timing of spring thaw and an imbalance in respiration versus photosynthesis. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 1615 Global Change: Biogeochemical processes (4805); 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; KEYWORDS: carbon transport, inversion

Nonintrusive Field Experiments Show Different Plant Responses to Warming and Drought Among Sites, Seasons, and Species in a North–South European Gradient

Abstract: We used a novel, nonintrusive experimental system to examine plant responses to warming and drought across a climatic and geographical latitudinal gradient of shrubland ecosystems in four sites from northern to southern Europe (UK, Denmark, The Netherlands, and Spain). In the first two years of experimentation reported here, we measured plant cover and biomass by the pinpoint method, plant 14C uptake, stem and shoot growth, flowering, leaf chemical concentration, litterfall, and herbivory damage in the dominant plant species of each site. The two years of approximately 1°C experimental warming induced a 15% increase in total aboveground plant biomass growth in the UK site. Both direct and indirect effects of warming, such as longer growth season and increased nutrient availability, are likely to be particularly important in this and the other northern sites which tend to be temperature-limited. In the water-stressed southern site, there was no increase in total aboveground plant biomass growth as expected since warming increases water loss, and temperatures in those ecosystems are already close to the optimum for photosynthesis. The southern site presented instead the most negative response to the drought treatment consisting of a soil moisture reduction at the peak of the growing season ranging from 33% in the Spanish site to 82% in The Netherlands site. In the Spanish site there was a 14% decrease in total aboveground plant biomass growth relative to control. Flowering was decreased by drought (up to 24% in the UK and 40% in Spain). Warming and drought decreased litterfall in The Netherlands site (33% and 37%, respectively) but did not affect it in the Spanish site. The tissue P concentrations generally decreased and the N/P ratio increased with warming and drought except in the UK site, indicating a progressive importance of P limitation as a consequence of warming and drought. The magnitude of the response to warming and drought was thus very sensitive to differences among sites (cold-wet northern sites were more sensitive to warming and the warm-dry southern site was more sensitive to drought), seasons (plant processes were more sensitive to warming during the winter than during the summer), and species. As a result of these multiple plant responses, ecosystem and community level consequences may be expected.

Response of net ecosystem gas exchange to a simulated precipitation pulse in a semi-arid grassland: the role of native versus non-native grasses and soil texture

Abstract: Physiological activity and structural dynamics in arid and semi-arid ecosystems are driven by discrete inputs or “pulses” of growing season precipitation. Here we describe the short-term dynamics of ecosystem physiology in experimental stands of native (Heteropogon contortus) and invasive (Eragrostis lehmanniana) grasses to an irrigation pulse across two geomorphic surfaces with distinctly different soils: a Pleistocene-aged surface with high clay content in a strongly horizonated soil, and a Holocene-aged surface with low clay content in homogenously structured soils. We evaluated whole-ecosystem and leaf-level CO2 and H2O exchange, soil CO2 efflux, along with plant and soil water status to understand potential constraints on whole-ecosystem carbon exchange during the initiation of the summer monsoon season. Prior to the irrigation pulse, both invasive and native grasses had less negative pre-dawn water potentials (Ψ pd), greater leaf photosynthetic rates (A net) and stomatal conductance (g s), and greater rates of net ecosystem carbon exchange (NEE) on the Pleistocene surface than on the Holocene. Twenty-four hours following the experimental application of a 39 mm irrigation pulse, soil CO2 efflux increased leading to all plots losing CO2 to the atmosphere over the course of a day. Invasive species stands had greater evapotranspiration rates (ET) immediately following the precipitation pulse than did native stands, while maximum instantaneous NEE increased for both species and surfaces at roughly the same rate. The differential ET patterns through time were correlated with an earlier decline in NEE in the invasive species as compared to the native species plots. Plots with invasive species accumulated between 5% and 33% of the carbon that plots with the native species accumulated over the 15-day pulse period. Taken together, these results indicate that system CO2 efflux (both the physical displacement of soil CO2 by water along with plant and microbial respiration) strongly controls whole-ecosystem carbon exchange during precipitation pulses. Since CO2 and H2O loss to the atmosphere was partially driven by species effects on soil microclimate, understanding the mechanistic relationships between the soil characteristics, plant ecophysiological responses, and canopy structural dynamics will be important for understanding the effects of shifting precipitation and vegetation patterns in semi-arid environments.

Dynamics of soil microbial biomass C and soil fertility in cropland mulched with plastic film in a semiarid agro-ecosystem

Abstract: Microbial biomass C (MBC) is one of the soil properties used as an indicator for the fertility status of a soil. A study was conducted on a semi-arid Loess Plateau in China. The field was planted with spring wheat and mulched with plastic film for various lengths of time. Our primary objectives were to (i) explore the influence of film mulching on soil MBC and soil fertility, and (ii) seek an effective approach of maintaining and improving sustainability of cropland mulched with plastic film in two growing seasons. Four treatments were tested, non-mulching (M0), mulching for 30 days after sowing (M30), mulching for 60 DAS (M60) and mulching for the whole growing period (Mw). An increasing air temperature with time within the growing season promoted soil MBC in the two growing seasons, but a severe drought led to a lower MBC in 2000 compared with the wet year of 1999. Film mulching promoted MBC significantly in the 2 years, but decreased soil organic carbon (SOC). SOC is very low in the experimental soil, accounting for the higher MBC/SOC ratio compared with ratios reported by others. The SOC is greatly reduced in the non-mulched and the Mw treatments compared to the M30 and M60 treatments. In conclusion, the benefits of film mulching in semi-arid agricultural systems are enormous but realizing their full potential depends on how long the mulching material is maintained during the growing season. In the system tested, it is desirable to mulch the plots for 30–60 DAS in order to enhance microbial biomass and cycling of nutrients and also to provide a more stable soil micro-environment that generates more residues in the rhizosphere.

The timing of snow melt controls the annual CO2 balance in a subarctic fen

Abstract: [1] The first continuous multi-year measurements of the CO2 exchange between a subarctic fen and the atmosphere were conducted at Kaamanen in northern Finland (69°N). According to our six-year data-set, the fen is presently a sink of atmospheric CO2 with a mean rate of −22 g C m−2 yr−1. The interannual variation of the CO2 balances originates almost completely from the variations during the snow-free period, but the efflux in the wintertime constitutes a significant part of the annual balance. The snow melt timing is the most important single determinant of the annual carbon balance. In contrast to a commonly-held view, the hydrometeorological conditions during the growing season had only a minor effect on the annual balance, emphasizing the importance of year-round measurements. This study indicates that climate warming may increase the length of the growing season and thus benefit rather than threaten the carbon pool of subarctic peatlands.

Regional Changes in Extreme Climatic Events: A Future Climate Scenario

Abstract: In this study a regional climate model is employed to expand on modeling experiments of future climate change to address issues of 1) the timing and length of the growing season and 2) the frequency and intensity of extreme temperatures and precipitation The study focuses on California as a climatically complex region that is vulnerable to changes in water supply and delivery Statistically significant increases in daily minimum and maximum temperatures occur with a doubling of atmospheric carbon dioxide concentration Increases in daily temperatures lead to increases in prolonged heat waves and length of the growing season Changes in total and extreme precipitation vary depending upon geographic location

Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under free-air concentration enrichment (FACE)

Abstract: The C4 grass Zea mays (maize or corn) is the third most important food crop globally in terms of production and demand is predicted to increase 45% from 1997 to 2020. However, the effects of rising [CO2] upon C4 plants, and Z. mays specifically, are not sufficiently understood to allow accurate predictions of future crop production. A rainfed, field experiment utilizing free-air concentration enrichment (FACE) technology in the primary area of global corn production (US Corn Belt) was undertaken to determine the effects of elevated [CO2] on corn. FACE technology allows experimental treatments to be imposed upon a complete soil–plant–atmosphere continuum with none of the effects of experimental enclosures on plant microclimate. Crop performance was compared at ambient [CO2] (354 μ mol mol−1) and the elevated [CO2] (549 μmol mol−1) predicted for 2050. Previous laboratory studies suggest that under favorable growing conditions C4 photosynthesis is not typically enhanced by elevated [CO2]. However, stomatal conductance and transpiration are decreased, which can indirectly increase photosynthesis in dry climates. Given the deep soils and relatively high rainfall of the US Corn Belt, it was predicted that photosynthesis would not be enhanced by elevated [CO2]. The diurnal course of gas exchange of upper canopy leaves was measured in situ across the growing season of 2002. Contrary to the prediction, growth at elevated [CO2] significantly increased leaf photosynthetic CO2 uptake rate (A) by up to 41%, and 10% on average. Greater A was associated with greater intercellular [CO2], lower stomatal conductance and lower transpiration. Summer rainfall during 2002 was very close to the 50-year average for this site, indicating that the year was not atypical or a drought year. The results call for a reassessment of the established view that C4 photosynthesis is insensitive to elevated [CO2] under favorable growing conditions and that the production potential of corn in the US Corn Belt will not be affected by the global rise in [CO2].

Acclimation of photosynthetic capacity in Scots pine to the annual cycle of temperature.

Abstract: Coniferous trees growing in the boreal and temperate zones have a clear annual cycle of photosynthetic activity. A recent study demonstrated that the seasonal variation in photosynthetic capacity of Scots pine (Pinus sylvestris L.) could be attributed mainly to the light response curve of photosynthesis. The magnitude of the light response curve varied over the season while its shape remained constant, indicating that the two physiological parameters quantifying the curve-the quantum yield per unit internal carbon dioxide concentration and the corresponding light-saturated rate-remained proportional to each other. We now show, through modeling studies, that the quantum yield (and hence the light-saturated rate) is related to the annual cycle of temperature through a delayed dynamic response. The proposed model was tested by comparing model results with intensive measurements of photosynthesis and driving variables made from April to October in three shoots of Scots pine growing near the northern timberline. Photosynthetic capacity showed considerable acclimation during the growing season. A single model describing photosynthetic capacity as a reversible, first-order delay process driven by temperature explained most of the variation in photosynthetic capacity during the year. The proposed model is simpler but no less accurate than previous models of the annual cycle of photosynthetic capacity.

Is tolerance to summer flooding correlated with distribution patterns in river floodplains? A comparative study of 20 terrestrial grassland species

Abstract: It is generally assumed that floods during the growing season have a strong impact on the distribution of grassland plant species in river floodplains but this proposition has never been tested. We examined the survival and growth responses of twenty species, originating from mid- and high-level floodplain grasslands along the River Rhine in the Netherlands, to total submergence for a maximum of two months in an outdoor flooding experiment. Plant survival and biomass reduction with flooding duration was determined as well as biomass recovery after de-submergence. Our results indicate that species survival is the most prominent factor correlated with species distribution in floodplain areas. Relatively flood tolerant species occurred mainly at low elevations along the floodplain while more flood sensitive species were restricted to high parts of the floodplain gradient. Biomass reduction rates during submergence were only marginally significantly correlated with species lower distribution boundaries along the flooding gradient. Biomass recovery rate was significantly correlated with species distribution patterns in the field only after 2 weeks of complete submergence, but not after 4 and 8 weeks. Our results suggest that the more flood tolerant species can have various ways to survive and recover from flooding, ranging from low rates of biomass loss and low recovery to relatively high rates of biomass loss and quick recovery. Our results are consistent with the notion that disturbance by floods during the growing season is an important determinant of species lower distribution boundaries in river floodplains. They also suggest that high survival under flooding may be achieved by different physiological mechanisms. Such mechanisms are discussed in this paper.

Variation in a satellite-based vegetation index in relation to climate in China

Abstract: Climate-related increases in terrestrial vegetation activity in the northern regions of the Northern Hemisphere have been identified by recent satellite based studies How- ever, evidence for this increase from ground observations is very limited In the current study, we used a time series data set for the Normalized Difference Vegetation Index (NDVI) for the growing season (April to October) from 1982 to 1999, along with historical climate data, to analyse year to year variations in vegetation activity and to explore the relationship between the growing season NDVI and climatic variables in China Vegetation activity, as measured by NDVI, increased in 81% of the study area, with significant gains in 27% of the region The magnitude of the mean growing season NDVI for the 1980s and the 1990s was not significantly different The increase in NDVI corresponded to an increase in temperature on the national scale, while regional variations in NDVI appeared to be related to precipitation The NDVI trend showed a large spatial heterogeneity, possibly associated with changes in regional climate, land use and vegetation type Our study suggests that agricultural practices caused an increase in NDVI in some regions, and rapid urbanization on the east coast resulted in a sharp decrease in NDVI since the 1980s

Community- and ecosystem-level changes in a species-rich

Abstract: Changes in the plant community and ecosystem properties that follow the conversion of agriculture to restored tallgrass prairies are poorly understood. Beginning in 1995, we established a species-rich, restored prairie chronosequence where -3 ha of ag- ricultural land have been converted to tallgrass prairie each year. Our goals were to examine differences in ecosystem properties between these restored prairies and adjacent agricultural fields and to determine changes in, and potential interactions between, the plant community and ecosystem properties that occur over time in the restored prairies. During the summers of 2000-2002, we examined species cover, soil C and N, potential net C and N mineral- ization, litter mass, soil texture, and bulk density across the 6- to 8-year-old prairie chron- osequence and adjacent agricultural fields in southern Minnesota. We also established ex- perimentally fertilized, watered, and control plots in the prairie chronosequence to examine the degree of nitrogen limitation on aboveground and belowground net primary production (ANPP and BNPP). Large shifts in functional diversity occurred within three growing seasons. First-year prairies were dominated by annuals and biennials. By the second growing season, perennial native composites had become dominant, followed by a significant shift to warm-season C4 grasses in prairies ?3 yr old. Ecosystem properties that changed with the rise of C4 grasses included increased BNPP, litter mass, and C mineralization rates and decreased N mineralization rates. ANPP increased significantly with N fertilization but did not vary between young and old prairies with dramatically different plant community composition. Total soil C and N were not significantly different between prairie and ag- ricultural soils in the depths examined (0-10, 10-20, 20-35, 35-50, 50-65 cm). We com- pared the results from our species-rich prairie restoration to published data on ecosystem function in other restored grasslands, such as Conservation Reserve Program (CRP) and old-field successional sites. Results suggest that rapid changes in functional diversity can have large impacts on ecosystem-level properties, causing community- and system-level dynamics in species-rich prairie restorations to converge with those from low-diversity managed grasslands.

Community and Ecosystem-level Changes in a Species-rich Tallgrass Prairie Restoration

Abstract: Changes in the plant community and ecosystem properties that follow the conversion of agriculture to restored tallgrass prairies are poorly understood. Beginning in 1995, we established a species-rich, restored prairie chronosequence where ∼3 ha of agricultural land have been converted to tallgrass prairie each year. Our goals were to examine differences in ecosystem properties between these restored prairies and adjacent agricultural fields and to determine changes in, and potential interactions between, the plant community and ecosystem properties that occur over time in the restored prairies. During the summers of 2000–2002, we examined species cover, soil C and N, potential net C and N mineralization, litter mass, soil texture, and bulk density across the 6- to 8-year-old prairie chronosequence and adjacent agricultural fields in southern Minnesota. We also established experimentally fertilized, watered, and control plots in the prairie chronosequence to examine the degree of nitrogen limitation on aboveground and belowground net primary production (ANPP and BNPP). Large shifts in functional diversity occurred within three growing seasons. First-year prairies were dominated by annuals and biennials. By the second growing season, perennial native composites had become dominant, followed by a significant shift to warm-season C4 grasses in prairies ≥3 yr old. Ecosystem properties that changed with the rise of C4 grasses included increased BNPP, litter mass, and C mineralization rates and decreased N mineralization rates. ANPP increased significantly with N fertilization but did not vary between young and old prairies with dramatically different plant community composition. Total soil C and N were not significantly different between prairie and agricultural soils in the depths examined (0–10, 10–20, 20–35, 35–50, 50–65 cm). We compared the results from our species-rich prairie restoration to published data on ecosystem function in other restored grasslands, such as Conservation Reserve Program (CRP) and old-field successional sites. Results suggest that rapid changes in functional diversity can have large impacts on ecosystem-level properties, causing community- and system-level dynamics in species-rich prairie restorations to converge with those from low-diversity managed grasslands.

Trends in high northern latitude soil freeze and thaw cycles from 1988 to 2002

Abstract: [1] In boreal and tundra ecosystems the freeze state of soils limits rates of photosynthesis and respiration. Here we develop a technique to identify the timing of freeze and thaw transitions of high northern latitude land areas using satellite data from the Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I). Our results indicate that in Eurasia there was a trend toward earlier thaw dates in tundra (−3.3 ± 1.8 days/decade) and larch biomes (−4.5 ± 1.8 days/decade) over the period 1988–2002. In North America there was a trend toward later freeze dates in evergreen conifer forests by 3.1 ± 1.2 days/decade that led, in part, to a lengthening of the growing season by 5.1 ± 2.9 days/decade. The growing season length in North American tundra increased by 5.4 ± 3.1 days/decade. Despite the trend toward earlier thaw dates in Eurasian larch forests, the growing season length did not increase because of parallel changes in timing of the fall freeze (−5.4 ± 2.1 days/decade), which led to a forward shift of the growing season. Thaw timing was negatively correlated with surface air temperatures in the spring, whereas freeze timing was positively correlated with surface air temperatures in the fall, suggesting that surface air temperature is one of several factors that determines the timing of soil thaw and freeze. The high spatial resolution, frequent temporal coverage, and duration of the SMMR and SSM/I satellite records makes them suitable for rigorous time series analysis and change detection in northern terrestrial ecosystems.

Seeding Rate Influence on Yield and Yield Components of Irrigated Winter Wheat in a Mediterranean Climate

Abstract: It is difficult to establish agronomic practices for wheat (Triticum aestivum L.) production in Mediterranean regions because of high annual variability in rainfall. Plant density is a factor of particular importance in wheat production systems because it can be controlled. This study was conducted to determine the optimum seeding rates of Mediterranean types of wheat in irrigated Mediterranean systems. Field experiments were conducted under irrigation at two locations of the Ebro Valley, Spain, during two growing seasons, 1999-2000 and 2000-2001. Six seeding rates were compared: 150, 175, 250, 300, 400, and 500 seeds m -2 with four adapted wheat varieties including a hybrid wheat. Seeding rate affected grain yield and yield components in three of the four environments, but its effect varied with the environment. The plant densities giving the highest yields were at least 400 to 500 plants m -2 for most of the varieties studied. The results suggest that the rate of seeding under irrigation for Mediterranean areas might be higher than those used in other wheat-growing areas.

Carbon dynamics along a chronosequence of slash pine plantations in north florida

Abstract: To determine factors controlling the carbon dynamics of an intensively man- aged landscape, we measured net CO2 exchange with the atmosphere using eddy covariance and soil CO2 fluxes using static chambers along a chronosequence of slash pine (Pinus elliottii var. elliottii) plantations consisting of a recent clearcut, a mid-rotation (10-yr-old) stand, and a rotation-aged (24-yr-old) stand. Daytime net ecosystem exchange of CO2 (NEEday) at the clearcut was not significantly different than zero during the growing season of the first year following harvest and reached levels that were -40% of those at the older stands during the second growing season. NEEday was similar at the mid-rotation and ro- tation-aged sites, reflecting the similar leaf areas of these stands. Nighttime net ecosystem exchange of CO2 (NEEnight) was an exponential function of air or soil temperature at all sites. However, low decomposition rates of litter and flooding of the site following harvest likely constrained NEEnight at the clearcut, and drought affected rates at the mid-rotation site. Annual net ecosystem exchange of CO2 (NEEyr) was estimated at -1269 and -882 g C.m-2-yr-1 at the clearcut, and 576 and 603 g C.m-2.yr-1 at the mid-rotation stand in 1998 and 1999, respectively. For comparison, NEEyr was 741 and 610 g C-m-2.yr-~ at the rotation- aged stand in 1996 and 1997, respectively. In contrast, annual ecosystem respiration (Reco) was similar in magnitude at all sites during all years. Although Reco is similar in magnitude, NEEyr is highly dynamic across this intensively managed landscape, with a maximum range of -2000 g C.m-2.yr-1. This range exceeds that across all the sites in both the Ameriflux and Euroflux networks and illustrates the need to include the range of stand ages and disturbance histories in landscape- to regional-scale flux estimates.

Hydraulic redistribution by a dominant, warm‐desert phreatophyte: seasonal patterns and response to precipitation pulses

Abstract: Summary 1Hydraulic redistribution may have important consequences for ecosystem water balance where plant root systems span large gradients in soil water potential. To assess seasonal patterns of hydraulic redistribution, we measured the direction and rate of sap flow in tap-roots, lateral roots and main stems of three mature Prosopis velutina Woot. trees occurring on a floodplain terrace in semiarid south-eastern Arizona, USA. Sap-flow measurements on two of the trees were initiated before the end of the winter dormancy period, prior to leaf flush. 2Despite the absence of crown transpiration during the dormant season, sap flow was detected in lateral roots and tap-roots of P. velutina. Reverse flow (away from the stem) in the lateral root and positive flow (towards the stem) in the tap-root was observed in one tree, indicating the presence of hydraulic lift. Conversely, reverse flow in the tap-root and positive flow in the lateral root was observed in the second tree, indicating hydraulic descent. 3Hydraulic descent was induced in the roots of the former tree by wetting the rooting zone in the upper 70 cm of the soil surface with 50 mm of irrigation. 4Patterns and rates of nocturnal sap flow in roots of a third tree measured during the growing season were similar to those observed during the dormant season. Nocturnal reverse flow in the lateral root and positive flow in the tap-root was observed prior to the onset of the summer monsoon. Hydraulic descent commenced immediately following the first large monsoon rain event, and continued after subsequent rain events. After adjusting for differences in sapwood area, maximum diurnal rates of hydraulic descent in the tap-roots of trees instrumented during the dormant season were 73 and 69% of the maximum night-time rate of hydraulic descent observed during the growing season. 5Despite very limited potential for direct infiltration, volumetric soil moisture content in deep soil layers (1·5–9·5 m) increased 2–8% by the end of the monsoon (late September), indicating that plant roots were redistributing non-trivial amounts of water to deep soil layers. 6Roots of P. velutina apparently redistribute significant amounts of soil water during the growing season, but also during periods of crown dormancy in winter. In arid regions dormant-season hydraulic descent may buffer plants from water and nutrient deficits during initial stages of the growing season by transferring soil water derived from winter precipitation to deep soil layers and away from zones of evaporation in surface layers and shallow-rooted herbaceous plants.

Stomatal sensitivity to vapor pressure deficit and its relationship to hydraulic conductance in Pinus palustris.

Abstract: We studied the response of stomatal conductance at leaf (gS) and canopy (GS) scales to increasing vapor pressure deficit (D) in mature Pinus palustris Mill. (longleaf pine) growing in a sandhill habitat in the coastal plain of the southeastern USA. Specifically, we determined if variation in the stomatal response to D was related to variation in hydraulic conductance along the soil-to-leaf pathway (KL) over the course of a growing season. Reductions in KL were associated with a severe growing season drought that significantly reduced soil water content (theta) in the upper 90-cm soil profile. Although KL recovered partially following the drought, it never reached pre-drought values. Stomatal sensitivity to D was well correlated with maximum gS at low D at both leaf and canopy scales, and KL appeared to influence this response by controlling maximum gS. Our results are consistent with the hypothesis that stomatal response to D occurs to regulate minimum leaf water potential, and that the sensitivity of this response is related to changes in whole-plant hydraulics.

Annual CO2 exchange of a peat field growing spring barley or perennial forage grass

Abstract: [1] We report on net ecosystem CO 2 exchange (NEE) measurements conducted with the eddy covariance method over agricultural peat soil in the 2-year period between October 2000 and October 2002. In 2001, spring barley and undersown grass were sown on the site. After the barley harvest, the perennial forage grass was left to grow, so that in 2002 the field was growing grass. A higher maximum net CO 2 uptake was observed for barley than for grass during the height of the summer, peaking at about -1.0 and -0.75 mg CO 2 m s -1 , respectively. The maximum nighttime total ecosystem respiration was measured in July and was similar for both crops, about 0.35 mg CO 2 m -2 s -1 . During the growing season the field acted as a daily CO 2 sink for only 40 days in barley versus 84 days in grass. In the winter the average carbon dioxide efflux varied from 15.6 to 16.5 μg CO 2 m -2 s -1 . The annual NEE of the agricultural peat soil growing barley and grass was 771 ± 104 and 290 ± 91 g CO 2 m -2 , respectively. The longer net CO 2 uptake period was the main reason for the lower annual NEE for grass; however, owing to the higher amount of grass biomass produced the net ecosystem production (NEP), calculated as the sum of NEE and removed biomass, was slightly larger for grass (452 g C m -2 ) than for barley (336 g C m -2 ). These results show that the organic peat is still undergoing rapid decomposition after more than 100 years of cultivation activity. In addition, switching from an annual to a perennial crop did not turn the field into a CO 2 sink, at least during a 1-year period.

Plant traits that influence ecosystem processes vary independently among species

Abstract: Most predictions of plant species effects on ecosystems are based on single traits (e.g., litter chemistry) or suites of related traits (functional groups). However, recent studies demonstrate that predictions of species effects on ecosystems are improved by considering multiple traits. In order to develop this multiple trait approach, it is critical to understand how these multiple traits vary in relation to one another among species. The ecosystem effects of traits that strongly covary can likely be summarized by one of these traits. In contrast, it will be necessary to determine the ecosystem effects of specific trait combinations for those traits that vary independently across species. In the field, I established monocultures of eight herbaceous species common in California annual grasslands. Plant species significantly differed in their litter quantity and quality, live biomass, and effects on soil labile C, soil temperature, and soil moisture. Species effects on soil moisture and temperature were only significant at the times of the growing season when each of these limited plant and microbial activity. Some of these traits cor- related with one another, such as litter biomass and species effects on soil temperature during the winter. However, for the most part, plant species exhibited unique combinations of these traits. For example, species with similar litter chemistry had the largest differences in plant biomass, soil moisture, and soil labile C. Species rankings for many traits changed over the growing season (e.g., biomass), so that the relationship among traits varied sea- sonally. The independent variation of these traits suggests that predictions of plant species effects on ecosystems will likely be enhanced by an understanding of how the ecosystem effects of plant traits may vary depending on the combination of traits.

Vegetation-mediated changes in microclimate reduce soil respiration as woodlands expand into grasslands

Abstract: This study compared annual and growing-season in situ soil respiration and soil C cycling in paired juniper woodland and C4-dominated grassland sites in eastern Kansas (USA) to determine if, under similar edaphic and regional climate conditions, vegetation change alters soil CO2 dynamics. We found marked differences in soil respiration related to vegetation: Growing season mean woodland soil respiration rates (4.6 mmoles· m 22 ·s 21 ) averaged 38% less than paired grassland sites. Soil moisture did not explain the difference in soil respiration between woodlands and grasslands. Soil temperatures at the 10-cm depth were 58C cooler in woodlands during the growing season and significantly different between woodlands and grasslands throughout the year, explaining most of the low soil respiration in woodlands. However, there were subtle intrinsic differences in the response of soil respiration to temperature between woodlands and grasslands: Woodland and grassland respiration response was significantly different at the P 5 0.05 level, also indicated by a grassland Q10 of 2.4 compared to 2.2 in woodlands. We found no significant differences between woodlands and grasslands in long-term (82-week) laboratory incu- bations of potentially mineralizable soil C and short-term incubations for microbial biomass C. Similarly, root biomass did not differ between woodlands and grasslands and could not explain the lower in situ woodland soil respiration. Thus, vegetation-mediated reduction in soil temperature under the canopy explained much of the lower in situ woodland soil respiration. Lower soil respiration in the woodland resulted in an annual flux of 533.6 (621.7) compared to 858.4 (614.5) g C·m 22 ·yr 21 in grasslands, nearly 38% lower in the woodland (means 6 1 SE). Assuming root respiration is 50% of soil respiration, we estimate that the turnover of woodland soil C stocks may be slowed by 15 years relative to grassland. This suggests that, if juniper expansion (now occurring across nearly 5 million hectares in the Great Plains) proceeds to canopy closure, annual soil C flux may be potentially reduced by as much as 19 3 10 6 Mg of C below C flux rates that occurred historically from tallgrass

Soil moisture controls on canopy-scale water and carbon fluxes in an African savanna

Abstract: [1] Tower-based measurements of mass and energy exchanges at the end of the growing season in central Botswana were used to evaluate functional relationships commonly applied to predict water and carbon fluxes between savanna landscapes and the atmosphere. Following a large rainfall event, daily evapotranspiration (ETdaily) exhibited an exponential decay consistent with a derived analytical expression based on critical and wilting-point soil moisture limits for savanna vegetation native to the study region. A piecewise linear soil moisture limitation function provided good estimates of ETdaily as a function of potential evapotranspiration and soil moisture (R 2 = 0.92). Comparison of a soil moisture mass balance with measured ETdaily indicated deeper root water uptake at a site with more woody vegetation compared with a grass-dominated site. Linear correlation (R 2 = 0.90) of daytime CO2 flux and evapotranspiration supported a constant water use efficiency to estimate carbon fluxes from water fluxes. Daytime and nighttime CO2 fluxes responded similarly to soil drying, enabling estimation of total daily CO2 flux from ETdaily. These experimental results support a simple model of savanna land-atmosphere exchange over interstorm periods. INDEX TERMS: 1818 Hydrology: Evapotranspiration; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 1866 Hydrology: Soil moisture; 1878 Hydrology: Water/energy interactions; KEYWORDS: African savanna, evapotranspiration, land-atmosphere exchange, soil moisture, water and carbon flux, water limitation