Does grazing induce intraspecific trait variation in plants from a sub‐humid mountain ecosystem?

Livestock grazing represents an important human disturbance for vegetation worldwide. We analysed the intraspecific differences in mean trait values between different grazing regimes (ungrazed and grazed) and explored whether these differences are consistent across species in a sub‐humid mountain ecosystem in Central Argentina. We selected 14 species of eight different families, co‐occurring in both regimes and comprising herbaceous (grasses and forbs) and woody (shrubs and trees) plants. For each species and grazing regime we measured 12 traits related to plant size, carbon fixation and water use. We found that plants in the grazed regime had consistently smaller leaves and shorter stature and internodal length than plants of the same species under the ungrazed regime. For the remaining traits the responses were species‐specific. Dry matter content, leaf tensile strength and minimum leaf water potential (Ψ_leaf) showed contrasting responses to grazing. Specific leaf area, wood density and potential water content of wood showed almost no significant responses except for very few species. Neither leaf area per shoot mass nor leaf area per sapwood area differed significantly between grazing regimes. Our study suggested that the intraspecific variation found for the size‐related traits would allow species to respond to grazing without modifying markedly other structural traits, a plastic response that might increase the probability of species success.     

Impacts of Hurricane Frances on Florida scrub-oak ecosystem processes: defoliation, net CO2 exchange and interactions with elevated CO2

Hurricane disturbances have profound impacts on ecosystem structure and function, yet their effects on ecosystem CO₂ exchange have not been reported. In September 2004, our research site on a fire‐regenerated scrub‐oak ecosystem in central Florida was struck by Hurricane Frances with sustained winds of 113 km h⁻¹ and wind gusts as high as 152 km h⁻¹. We quantified the hurricane damage on this ecosystem resulting from defoliation: we measured net ecosystem CO₂ exchange, the damage and recovery of leaf area, and determined whether growth in elevated carbon dioxide concentration in the atmosphere (C_a) altered this disturbance. The hurricane decreased leaf area index (LAI) by 21%, which was equal to 60% of seasonal variation in canopy growth during the previous 3 years, but stem damage was negligible. The reduction in LAI led to a 22% decline in gross primary production (GPP) and a 25% decline in ecosystem respiration (R_e). The compensatory declines in GPP and R_e resulted in no significant change in net ecosystem production (NEP). Refoliation began within a month after the hurricane, although this period was out of phase with the regular foliation period, and recovered 20% of the defoliation loss within 2.5 months. Full recovery of LAI, ecosystem CO₂ assimilation, and ecosystem respiration did not occur until the next growing season. Plants exposed to elevated C_a did not sustain greater damage, nor did they recover faster than plants grown under ambient C_a. Thus, our results indicate that hurricanes capable of causing significant defoliation with negligible damage to stems have negligible effects on NEP under current or future CO₂‐enriched environment.     

Predicting savanna vegetation structure on the basis of plant available moisture (PAM) and plant available nutrients (PAN): a case study from Australia

The use of the plant available moisture (PAM)/plant available nutrients (PAN) concept to compare savanna structure was examined using data from twenty Australian sites. Above-ground biomass was regressed on various combinations of seventeen different estimates of PAM (plant available moisture) and two estimates of PAN (plant available nutrients). The ratios of actual transpirational loss from the subsoil to potential evapotranspiration (PET), and total annual rainfall to PET, were most highly correlated with total biomass. Grass biomass is poorly predicted by PAM on its own, and requires inclusion of woody leaf biomass in the regression. PAN had little effect on total biomass, although it is likely to be important for other, functional aspects of vegetation. The woody : grass ratio is best predicted by an index involving the ratio of subsoil : topsoil moisture. For biomass comparisons the use of a detailed water-balance model to estimate PAM is not warranted.     

Modelling carbon balances of coastal arctic tundra under changing climate

Rising air temperatures are believed to be hastening heterotrophic respiration (R_h) in arctic tundra ecosystems, which could lead to substantial losses of soil carbon (C). In order to improve confidence in predicting the likelihood of such loss, the comprehensive ecosystem model ecosys was first tested with carbon dioxide (CO₂) fluxes measured over a tundra soil in a growth chamber under various temperatures and soil‐water contents (θ). The model was then tested with CO₂ and energy fluxes measured over a coastal arctic tundra near Barrow, Alaska, under a range of weather conditions during 1998–1999. A rise in growth chamber temperature from 7 to 15 °C caused large, but commensurate, rises in respiration and CO₂ fixation, and so no significant effect on net CO₂ exchange was modelled or measured. An increase in growth chamber θ from field capacity to saturation caused substantial reductions in respiration but not in CO₂ fixation, and so an increase in net CO₂ exchange was modelled and measured. Long daylengths over the coastal tundra at Barrow caused an almost continuous C sink to be modelled and measured during most of July (2–4 g C m^?2 d^?1), but shortening daylengths and declining air temperatures caused a C source to be modelled and measured by early September (～1 g C m^?2 d^?1). At an annual time scale, the coastal tundra was modelled to be a small C sink (4 g C m^?2 y^?1) during 1998 when average air temperatures were 4 °C above normal, and a larger C sink (16 g C m^?2 y^?1) during 1999 when air temperatures were close to long‐term normals. During 100 years under rising atmospheric CO₂ concentration (C_a), air temperature and precipitation driven by the IS92a emissions scenario, modelled R_h rose commensurately with net primary productivity (NPP) under both current and elevated rates of atmospheric nitrogen (N) deposition, so that changes in soil C remained small. However, methane (CH₄) emissions were predicted to rise substantially in coastal tundra with IS92a‐driven climate change (from ～20 to ～40 g C m^?2 y^?1), causing a substantial increase in the emission of CO₂ equivalents. If the rate of temperature increase hypothesized in the IS92a emissions scenario had been raised by 50%, substantial losses of soil C (～1 kg C m^?2) would have been modelled after 100 years, including additional emissions of CH₄.     

Interannual variability in carbon dioxide fluxes and flux–climate relationships on grazed and ungrazed northern mixed-grass prairie

The annual carbon (C) budget of grasslands is highly dynamic, dependent on grazing history and on effects of interannual variability (IAV) in climate on carbon dioxide (CO₂) fluxes. Variability in climatic drivers may directly affect fluxes, but also may indirectly affect fluxes by altering the response of the biota to the environment, an effect termed ‘functional change’. We measured net ecosystem exchange of CO₂ (NEE) and its diurnal components, daytime ecosystem CO₂ exchange (P_D) and night‐time respiration (R_E), on grazed and ungrazed mixed‐grass prairie in North Dakota, USA, for five growing seasons. Our primary objective was to determine how climatic anomalies influence variability in CO₂ exchange. We used regression analysis to distinguish direct effects of IAV in climate on fluxes from functional change. Functional change was quantified as the improvement in regression on fitting a model in which slopes of flux–climate relationships vary among years rather than remain invariant. Functional change and direct effects of climatic variation together explained about 20% of variance in weekly means of NEE, P_D, and R_E. Functional change accounted for more than twice the variance in fluxes of direct effects of climatic variability. Grazing did not consistently influence the contribution of functional change to flux variability, but altered which environmental variable best explained year‐to‐year differences in flux–climate slopes, reduced IAV in seasonal means of fluxes, lessened the strength of flux–climate correlations, and increased NEE by reducing R_E relatively more than P_D. Most of these trends are consistent with the interpretation that grazing reduced the influence of plants on ecosystem fluxes. Because relationships between weekly values of fluxes and climatic regulators changed annually, year‐to‐year differences in the C balance of these ecosystems cannot be predicted from knowledge of IAV in climate alone.     

海南尖峰岭热带山地雨林近冠层CO2及通量特征研究

采用梯度法及CI-301PS CO2系统,实现了海南尖峰岭热带山地雨林近冠层CO 2、微气象因子梯度观测研究,结果表明雨季晴、晴间少云及多云天气,900～1800CO 2浓度平均值分别在349～350ml/m3(冠上2.8m)、346～348ml/m3(冠上0.8m)、345～349ml/m3(林内16m)、352～357ml/m3(林内5m),较旱季相应天气分别小14～17.5、10.1～23.7、1 6.4～35.7和18.1～36.1ml/m3;100～800,则雨季CO2浓度大于旱季;梯度浓度廓变量和实时动力计算反映出,800～1800CO2通量由大气向林冠层,旱、雨季平均CO2通量分别为(0.61×10-6kg/(m2·s)和0.71×10-6kg/(m 2·s),1900通量甚微,2000～700CO2通量则由林冠向大气,平均CO2通量分别为0.36×10-6kg/(m2·s)和0.32×10-6kg/(m2·s);雨季昼夜大气流向冠层的净CO2通量是相应旱季的1.56倍.总辐射、冠顶净辐射通量以1300～1 500为最大时域,相对林内21m,80%的辐射热能被冠层吸收,与CO2通量正相关;晴天冠上潜热、感热最高值分别在1300～1400和900,反映热带山地雨林近冠层的汇、源即白昼光合固定CO2大于夜间呼吸排放CO2效应,且雨季高于旱季.     

西双版纳热带季节雨林晴天CO2交换的日变化和季节变化特征

应用2003年11月-2004年10月晴好天气涡度相关通量观测资料,对西双版纳热带季节雨林CO2交换的日变化和季节变化进行分析。结果表明：雾凉季、干热季和雨季的净生态系统CO2交换（NEE）均呈现出单峰型曲线的日变化趋势,昼间其变化规律较强,夜间呈波动状态。昼间NEE（取绝对值）雾凉季和雨季均显著大于干热季;夜间NEE雨季显著大于干热季,而干热季显著大于雾凉季。光合有效辐射是影响NEE日变化的主要因素,但不是造成季节差异的主要因素;饱和水汽压差和气温对NEE的季节差异有较大贡献。另外,应用Michaelis-Menten模型对昼间不同饱和水汽压差和气温下NEE对光合有效辐射的响应进行分析,结果表明：各季节较高饱和水汽压差下的表观最大光合速率（Pmax）、表观暗呼吸速率（Re）比较低饱和水汽压差下的Pmax、Re大,而表观光量子产额（α）则相反。各季节较高气温下的Re比较低气温下的Re大;雾凉季气温的差异对Pmax和α的影响较小;干热季和雨季较高气温下的α较小。     

Contrasting soil respiration in young and old-growth ponderosa pine forests

Three years of fully automated and manual measurements of soil CO₂ efflux, soil moisture and temperature were used to explore the diel, seasonal and inter‐annual patterns of soil efflux in an old‐growth (250‐year‐old, O site) and recently regenerating (14‐year‐old, Y site) ponderosa pine forest in central Oregon. The data were used in conjunction with empirical models to determine which variables could be used to predict soil efflux in forests of contrasting ages and disturbance histories. Both stands experienced similar meteorological conditions with moderately cold wet winters and hot dry summers. Soil CO₂ efflux at both sites showed large inter‐annual variability that could be attributed to soil moisture availability in the deeper soil horizons (O site) and the quantity of summer rainfall (Y site). Seasonal patterns of soil CO₂ efflux at the O site showed a strong positive correlation between diel mean soil CO₂ efflux and soil temperature at 64 cm depth whereas diel mean soil efflux at the Y site declined before maximum soil temperature occurred during summer drought. The use of diel mean soil temperature and soil water potential inferred from predawn foliage water potential measurements could account for 80% of the variance of diel mean soil efflux across 3 years at both sites, however, the functional shape of the soil water potential constraint was site‐specific. Based on the similarity of the decomposition rates of litter and fine roots between sites, but greater productivity and amount of fine litter detritus available for decomposition at the O site, we would expect higher rates of soil CO₂ efflux at the O site. However, annual rates were only higher at the O site in one of the 3 years (597 ± 45 vs. 427 ± 80 g C m^?2). Seasonal patterns of soil efflux at both sites showed influences of soil water limitations that were also reflected in patterns of canopy stomatal conductance, suggesting strong linkages between above and below ground processes.     

Evapotranspiration and soil water content in a scrub-oak woodland under carbon dioxide enrichment

Leaf conductance often decreases in response to elevated atmospheric CO₂ concentration (C_a) potentially leading to changes in hydrology. We describe the hydrological responses of Florida scrub oak to elevated C_a during an eight‐month period two years after C_a manipulation began. Whole‐chamber gas exchange measurements revealed a consistent reduction in evapotranspiration in response to elevated C_a, despite an increase in leaf area index (LAI). Elevated C_a also increased surface soil water content, but xylem water deuterium measurements show that the dominant oaks in this system take up most of their water from the water table (which occurs at a depth of 1.5–3 m), suggesting that the water savings in elevated C_a in this system are primarily manifested as reduced water uptake at depth. Extrapolating these results to larger areas requires considering a number of processes that operate on scales beyond these accessible in this field experiment. Nevertheless, these results demonstrate the potential for reduced evapotranspiration and associated changes in hydrology in ecosystems dominated by woody vegetation in response to elevated C_a.     

Impact of elevated atmospheric CO2 concentration on soil microbial biomass and activity in a complex, weedy field model ecosystem

Although soil organisms play an essential role in the cycling of elements in terrestrial ecosystems, little is known of the impact of increasing atmospheric CO₂ concentrations on soil microbial processes. We determined microbial biomass and activity in the soil of multitrophic model ecosystems housed in the Ecotron (NERC Centre for Population Biology, Ascot, UK) under two atmospheric CO₂ concentrations (ambient vs. ambient + 200 ppm). The model communities consist of four annual plant species which naturally co-occur in weedy fields and disturbed ground throughout southern England, together with their herbivores, parasitoids and soil biota. At the end of two experimental runs lasting 9 and 4.5 months, respectively, root dry weight and quality showed contradictory responses to elevated CO₂ concentrations, probably as a consequence of the different time-periods (and hence number of plant generations) in the two experiments. Despite significant root responses no differences in microbial biomass could be detected. Effects of CO₂ concentration on microbial activity were also negligible. Specific enzymes (protease and xylanase) showed a significant decrease in activity in one of the experimental runs. This could be related to the higher C:N ratio of root tissue. We compare the results with data from the literature and conclude that the response of complex communities cannot be predicted on the basis of oversimplified experimental set-ups.