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

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.     

Sensitivity of plant species to warming and altered precipitation dominates the community productivity in a semiarid grassland on the Loess Plateau

Global warming and changes in precipitation patterns can critically influence the structure and productivity of terrestrial ecosystems. However, the underlying mechanisms are not fully understood. We conducted two independent but complementary experiments (one with warming and precipitation manipulation (+ or – 30%) and another with selective plant removal) in a semiarid grassland on the Loess Plateau, northwestern China, to assess how warming and altered precipitation affect plant community. Our results showed that warming and altered precipitation affected community aboveground net primary productivity (ANPP) through impacting soil moisture. Results of the removal experiment showed competitive relationships among dominant grasses, the dominant subshrub and nondominant species, which played a more important role than soil moisture in the response of plant community to warming and altered precipitation. Precipitation addition intensified the competition but primarily benefited the dominant subshrub. Warming and precipitation reduction enhanced water stresses but increased ANPP of the dominant subshrub and grasses, indicating that plant tolerance to drought critically meditated the community responses. These findings suggest that specie competitivity for water resources as well as tolerance to environmental stresses may dominate the responses of plant communities on the Loess Plateaus to future climate change factors.     

Land use and drought interactively affect interspecific competition and species diversity at the local scale in a semiarid steppe ecosystem

Few studies have considered interactive effects of grazing and drought on species composition and the relative contribution of species to total biomass, although it is important to understand the short-term dynamics and community succession in grazed ecosystems. We monitored species diversity and relative biomass contribution at one site protected from grazing since 1979 (UG79), and at winter grazing (WG) and heavily grazed (HG) sites. Continuous heavy grazing resulted in lower plant height and more but small individuals (tillers or stolons). Drought significantly reduced total plant density on all sites. Grazing affected species diversity more than drought. Species richness at site UG79 was significantly higher than at sites WG and HG, while drought only tended to reduce species diversity. Drought stress and grazing disturbance interactively controlled species competition and functional groups. Both perennial grasses and forbs had greater contribution to total biomass at site UG79, and perennial grasses contributed more than 97% of total biomass at site WG. The contribution to total biomass of annual forbs and semi-shrubs significantly increased at site HG after two dry years. The significant decrease in Potentilla acaulis and a substantial increase in annual species at this site indicate that the perennial vegetation of this ecosystem is in great danger of extinction under conditions of prolonged drought.     

Above and belowground net primary productivity of grassland influenced by supplemental water and nitrogen in Inner Mongolia

While water availability determines grassland productivity in semiarid regions, nutrient availability is the main limiting factor under wet conditions. An experiment was conducted in 2008 at two sites in Inner Mongolia with histories of heavy grazing (HG) and moderate grazing (MG) to study the interactive effects of water and nitrogen on above- and belowground net primary productivity (ANPP and BNPP), biomass partitioning, and plant species composition. The study comprises two water treatments (no irrigation and irrigated when soil water content was below 70% of the field capacity), and two nitrogen (N) levels (0 and 100 kg N ha^?1). Mean values of ANPP at the peak biomass time reached 1,028?±?95 SD g m^?2 at the HG site and 568?±?32 SD g m^?2 at the MG site in irrigated and fertilized treatment. Nitrogen use efficiency (NUE) was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites. Water use efficiency (WUEt) based on total water input and ANPP decreased with irrigation at the HG site. Meanwhile, N application significantly increased WUEt, WUEp (based on precipitation), and WUEi (based on irrigation water) at both sites. BNPP was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites, and it tended to decrease with N addition. However, the fraction of belowground to total biomass (f _BNPP = BNPP/(ANPP+BNPP) decreased with the addition of supplemental resources and exhibited a negative correlation with ANPP. Species diversity remained lower at the HG site compared to the MG site; it decreased with the addition of supplemental resources at the latter site. The annual Salsola collina contributed the most to the total biomass under irrigation. Based on global climate models, more frequent extreme climates are predicted in the future, which can result in changes in resource availabilities. Therefore, our research results have important implications for predicting the production and other properties of grassland ecosystems.     

High land‐use intensity exacerbates shifts in grassland vegetation composition after severe experimental drought

Climate change projections anticipate increased frequency and intensity of drought stress, but grassland responses to severe droughts and their potential to recover are poorly understood. In many grasslands, high land‐use intensity has enhanced productivity and promoted resource‐acquisitive species at the expense of resource‐conservative ones. Such changes in plant functional composition could affect the resistance to drought and the recovery after drought of grassland ecosystems with consequences for feed productivity resilience and environmental stewardship. In a 12‐site precipitation exclusion experiment in upland grassland ecosystems across Switzerland, we imposed severe edaphic drought in plots under rainout shelters and compared them with plots under ambient conditions. We used soil water potentials to scale drought stress across sites. Impacts of precipitation exclusion and drought legacy effects were examined along a gradient of land‐use intensity to determine how grasslands resisted to, and recovered after drought. In the year of precipitation exclusion, aboveground net primary productivity (ANPP) in plots under rainout shelters was ?15% to ?56% lower than in control plots. Drought effects on ANPP increased with drought severity, specified as duration of topsoil water potential ψ < ?100 kPa, irrespective of land‐use intensity. In the year after drought, ANPP had completely recovered, but total species diversity had declined by ?10%. Perennial species showed elevated mortality, but species richness of annuals showed a small increase due to enhanced recruitment. In general, the more resource‐acquisitive grasses increased at the expense of the deeper‐rooted forbs after drought, suggesting that community reorganization was driven by competition rather than plant mortality. The negative effects of precipitation exclusion on forbs increased with land‐use intensity. Our study suggests a synergistic impact of land‐use intensification and climate change on grassland vegetation composition, and implies that biomass recovery after drought may occur at the expense of biodiversity maintenance.     

Importance of Seasonality for the Response of a Mesic Temperate Grassland to Increased Precipitation Variability and Warming

Timing of precipitation events within the growing season and the non-uniformity of warming might be decisive for alterations in productivity and community composition, with consequences for ecosystem functioning. The responses of aboveground production, community composition, functional group and species evenness to altered intra-annual precipitation variability and their interactions with winter or summer warming were examined in European, mesic temperate grassland. Increased precipitation variability with an induced spring drought resulted in a 17% reduction in ANPP, and late drought reduced ANPP by 18% compared to regular rainfall patterns throughout the entire growing season. Winter warming increased ANPP by 12%, whereas summer warming showed no significant effect on biomass but decreased species richness. The effects of increased precipitation variability and warming on ANPP were independent of each other. Forbs benefited from high precipitation variability with spring drought events, likely due to reduced competitive pressure by decreasing, water stressed grasses. Increased precipitation variability coinciding with higher summer temperatures led to reduced species evenness and likely promoted the establishment of specialists and drought-tolerant species. Seasonality of climatic factors, here early versus late drought events in the high precipitation variability treatments, was important in driving shifts in community composition but not for decreases in ANPP. Non-uniform warming, here winter versus summer, affected the direction of response of both community composition and ANPP. Variability of resources is affecting ecosystem processes and species interactions. Recognition of seasonality and non-uniformity of climatic factors will improve predictions of plant performance and biotic interactions in response to climate change.     

Desertification alters patterns of aboveground net primary production in Chihuahuan ecosystems

Abstract The Chihuahuan desert of New Mexico, USA, has changed in historical times from semiarid grassland to desert shrublands dominated by Larrea tridentata and Prosopis glandulosa. Similar displacement of perennial grasslands by shrubs typifies desertification in many regions. Such structural vegetation change could alter average values of net primary productivity, as well as spatial and temporal patterns of production. We investigated patterns of aboveground plant biomass and net primary production in five ecosystem types of the Jornada Basin Long‐Term Ecological Research (LTER) site. Comparisons of shrub‐dominated desertified systems and remnant grass‐dominated systems allowed us to test the prediction that shrublands are more heterogeneous spatially, but less variable over time, than grasslands. We measured aboveground plant biomass and aboveground net primary productivity (ANPP) by species, three times per year for 10 years, in 15 sites of five ecosystem types (three each in Larrea shrubland, Bouteloua eriopoda grassland, Prosopis dune systems, Flourensia cernua alluvial flats, and grass‐dominated dry lakes or playas). Spatial heterogeneity of biomass at the scale of our measurements was significantly greater in shrub‐dominated systems than in grass‐dominated vegetation. ANPP was homogeneous across space in grass‐dominated systems, and in most growing seasons was significantly more patchy in shrub vegetation. Substantial interannual variability in ANPP complicates comparison of mean values across ecosystem types, but grasslands tended to support higher ANPP values than did shrub‐dominated systems. There were significant interactions between ecosystem type and season. Grasslands demonstrated higher interannual variation than did shrub systems. Desertification has apparently altered the seasonality of productivity in these systems; grasslands were dominated by summer growth, while sites dominated by Larrea or Prosopis tended to have higher spring ANPP. Production was frequently uncorrelated across sites of an ecosystem type, suggesting that factors other than season, regional climate, or dominant vegetation may be significant determinants of actual NPP.     

荒漠短命植物地上和地下生产力对极端干旱和降水的响应

在古尔班通古特沙漠南缘沙垄4个坡位和坡向,设置减少65%和增加65%生长季降水量以模拟极端干旱和极端降水事件,研究极端干旱和极端降水事件对沙垄不同坡位和坡向短命植物层片生产力的影响。结果表明: 极端干旱使地上净初级生产力和地下净初级生产力分别显著降低48.8%和13.7%,极端降水使地上净初级生产力和地下净初级生产力分别显著增加37.9%和23.2%。地上净初级生产力对极端干旱和极端降水的敏感性(0.26和0.21 g·m^-2·mm^-1)显著强于地下净初级生产力的敏感性(0.02和0.03 g·m^-2·mm^-1)。沙垄东坡地上净初级生产力(24.22 g·m^-2)和地下净初级生产力(5.77 g·m^-2)与西坡相比显著增大29.7%和71.7%,而地上净初级生产力和地下净初级生产力对降水变化的敏感性在不同坡位和坡向之间差异不显著。     

Contingent productivity responses to more extreme rainfall regimes across a grassland biome

Climate models predict, and empirical evidence confirms, that more extreme precipitation regimes are occurring in tandem with warmer atmospheric temperatures. These more extreme rainfall patterns are characterized by increased event size separated by longer within season drought periods and represent novel climatic conditions whose consequences for different ecosystem types are largely unknown. Here, we present results from an experiment in which more extreme rainfall patterns were imposed in three native grassland sites in the Central Plains Region of North America, USA. Along this 600 km precipitation–productivity gradient, there was strong sensitivity of temperate grasslands to more extreme growing season rainfall regimes, with responses of aboveground net primary productivity (ANPP) contingent on mean soil water levels for different grassland types. At the mesic end of the gradient (tallgrass prairie), longer dry intervals between events led to extended periods of below-average soil water content, increased plant water stress and reduced ANPP by 18%. The opposite response occurred at the dry end (semiarid steppe), where a shift to fewer, but larger, events increased periods of above-average soil water content, reduced seasonal plant water stress and resulted in a 30% increase in ANPP. At an intermediate mixed grass prairie site with high plant species richness, ANPP was most sensitive to more extreme rainfall regimes (70% increase). These results highlight the inherent complexity in predicting how terrestrial ecosystems will respond to forecast novel climate conditions as well as the difficulties in extending inferences from single site experiments across biomes. Even with no change in annual precipitation amount, ANPP responses in a relatively uniform physiographic region differed in both magnitude and direction in response to within season changes in rainfall event size/frequency.     

Effects of experimental warming and drought on biomass accumulation in a Mediterranean shrubland

We studied the effects of experimental warming and drought on the plant biomass of a Mediterranean shrubland. We monitored growth at plant level and biomass accumulation at stand level. The experimentation period stretched over 7 years (1999–2005) and we focused on the two dominant shrub species, Erica multiflora L. and Globularia alypum L. and the tree species Pinus halepensis L. The warming treatment increased shoot elongation in E. multiflora, and the drought treatment reduced shoot elongation in G. alypum. The elongation of P. halepensis remained unaffected under both treatments. The balance between the patterns observed in biomass accumulation for the three studied species in the drought plots (reduction in E. multiflora and P. halepensis and increase in G. alypum) resulted in a trend to reduce 33% the biomass of the drought treatment plots with respect to the untreated plots, which almost doubled their biomass from 1998 to 2005. The results also suggest that under drier conditions larger accumulation of dead biomass may occur at stand level, which combined with higher temperatures, may thus increase fire risk in the Mediterranean area.     

Contrasting effects of repeated summer drought on soil carbon efflux in hydric and mesic heathland soils

Current predictions of climate change include altered rainfall patterns throughout Europe, continental USA and areas such as the Amazon. The effect of this on soil carbon efflux remains unclear although several modelling studies have highlighted the potential importance of drought for carbon storage. To test the importance of drought, and more importantly repeated drought year-on-year, we used automated retractable curtains to exclude rain and produce repeated summer drought in three heathlands at varying moisture conditions. This included a hydric system limited by water-excess (in the UK) and two mesic systems with seasonal water limitation in Denmark (DK) and the Netherlands (NL). The experimental rainfall reductions were set to reflect single year droughts observed in the last decade with exclusion of rain for 2–3 months of the year resulting in a 20–26% reduction in annual rainfall and 23–38% reduction in mean soil moisture during the drought period. Unexpectedly, sustained reduction in soil moisture over winter (between drought periods) was also observed at all three sites, along with a reduction in the maximum water-holding capacity attained. Three hypotheses are discussed which may have contributed to this lack of recovery in soil moisture: hydrophobicity of soil organic matter, increased water use by plants and increased cracking of the soil. The responses of soil respiration to this change in soil moisture varied among the sites: decreased rates were observed at the water-limited NL and DK sites whilst they increased at the UK site. Reduced sensitivity of soil respiration to soil temperature was observed at soil moisture contents above 55% at the UK site and below 20% and 13% at the NL and DK sites, respectively. Soil respiration rates recovered to predrought levels in the NL and DK sites during the winter re-wetting period that indicates any change in soil C storage due to changes in soil C efflux may be short lived in these mesic systems. In contrast, in the hydric UK site after 2 years of drought treatment, the persistent reduction in soil moisture throughout the year resulted in a year-round increase in soil respiration flux, a response that accelerated over time to 40% above control levels. These findings suggest that carbon-rich soils with high organic matter content may act as a significant source of CO₂ to the atmosphere following repeated summer drought. Nonrecovery of soil moisture and a persistent increase in soil respiration may be the primary mechanism underlying the reported substantial losses of soil carbon from UK organic soils over the last 20 years. These findings indicate that the water status of an ecosystem will be a critical factor to consider in determining the impact of drought on the soil carbon fluxes and storage.     

Fertilization effects on species density and primary productivity in herbaceous plant communities

Fertilization experiments in plant communities are often interpreted in the context of a hump-shaped relationship between species richness and productivity. We analyze results of fertilization experiments from seven terrestrial plant communities representing a productivity gradient (arctic and alpine tundra, two old-field habitats, desert, short- and tall-grass prairie) to determine if the response of species richness to experimentally increased productivity is consistent with the hump-shaped curve. In this analysis, we compared ratios of the mean response in nitrogen-fertilized plots to the mean in control plots for aboveground net primary productivity (ANPP) and species density ( D ; number of species per plot of fixed unit area). In general, ANPP increased and plant species density decreased following nitrogen addition, although considerable variation characterized the magnitude of response. We also analyzed a subset of the data limited to the longest running studies at each site (≥4 yr), and found that adding 9 to 13 g N m⁻² yr⁻¹ (the consistent amount used at all sites) increased ANPP in all communities by approximately 50% over control levels and reduced species density by approximately 30%. The magnitude of response of ANPP and species density to fertilization was independent of initial community productivity. There was as much variation in the magnitude of response among communities within sites as among sites, suggesting community-specific mechanisms of response. Based on these results, we argue that even long-term fertilization experiments are not good predictors of the relationship between species richness and productivity because they are relatively small-scale perturbations whereas the pattern of species richness over natural productivity gradients is influenced by long-term ecological and evolutionary processes.     

Unexpected patterns of sensitivity to drought in three semi-arid grasslands

Global climate models forecast an increase in the frequency and intensity of extreme weather events, including severe droughts. Based on multi-year relationships between precipitation amount and aboveground annual net primary production (ANPP), semi-arid grasslands are projected to be among the most sensitive ecosystems to changes in precipitation. To assess sensitivity to drought, as well as variability within the shortgrass steppe biome, we imposed moderate and severe rainfall reductions for two growing seasons in three undisturbed grasslands that varied in soil type and climate. We predicted strong drought-induced reductions in ANPP at all sites and greater sensitivity to drought in sites with lower average precipitation, consistent with continental-scale patterns. Identical experimental infrastructure at each site reduced growing season rainfall events by 50 or 80%, and significantly reduced average soil moisture in both years (by 21 and 46% of control levels, respectively). Despite reductions in soil moisture, ANPP responses varied unexpectedly-from no reduction in ANPP to a 51% decrease. Although sensitivity to drought was highest in the semi-arid grassland with lowest mean annual precipitation, patterns in responses to drought across these grasslands were also strongly related to rainfall event size. When growing season rainfall patterns were dominated by many smaller events, ANPP was significantly reduced by drought but not when rainfall patterns were characterized by large rain events. This interaction between drought sensitivity and rainfall event size suggests that ANPP responses to future droughts may be reduced if growing season rainfall regimes also become more extreme.     

Contrasting impacts of continuous moderate drought and episodic severe droughts on the aboveground‐biomass increment and litterfall of three coexisting Mediterranean woody species

Climate change is predicted to increase the aridity in the Mediterranean Basin and severely affect forest productivity and composition. The responses of forests to different timescales of drought, however, are still poorly understood because extreme and persistent moderate droughts can produce nonlinear responses in plants. We conducted a rainfall‐manipulation experiment in a Mediterranean forest dominated by Quercus ilex, Phillyrea latifolia, and Arbutus unedo in the Prades Mountains in southern Catalonia from 1999 to 2014. The experimental drought significantly decreased forest aboveground‐biomass increment (ABI), tended to increase the litterfall, and decreased aboveground net primary production throughout the 15 years of the study. The responses to the experimental drought were highly species‐specific. A. unedo suffered a significant reduction in ABI, Q. ilex experienced a decrease during the early experiment (1999–2003) and in the extreme droughts of 2005–2006 and 2011–2012, and P. latifolia was unaffected by the treatment. The drought treatment significantly increased branch litterfall, especially in the extremely dry year of 2011, and also increased overall leaf litterfall. The drought treatment reduced the fruit production of Q. ilex, which affected seedling recruitment. The ABIs of all species were highly correlated with SPEI in early spring, whereas the branch litterfalls were better correlated with summer SPEIs and the leaf and fruit litterfalls were better correlated with autumn SPEIs. These species‐specific responses indicated that the dominant species (Q. ilex) could be partially replaced by the drought‐resistant species (P. latifolia). However, the results of this long‐term study also suggest that the effect of drought treatment has been dampened over time, probably due to a combination of demographic compensation, morphological and physiological acclimation, and epigenetic changes. However, the structure of community (e.g., species composition, dominance, and stand density) may be reordered when a certain drought threshold is reached.     

The relation of biomass production with leaf traits varied under different land-use and precipitation conditions in an Inner Mongolia steppe

The analyses of plant leaf traits that strongly influence aboveground net primary production (ANPP) are indispensable for understanding the process of plant biomass formation. However, there are few studies that have attempted to relate patterns of ANPP under contrasting management practices to plant leaf-level traits. To assess how leaf traits affect plant biomass accumulation under different land-use practices, we examined leaf traits and biomass production in three differently managed sites in the Inner Mongolia steppe: a site fenced since 1979 (UG), a winter grazing site (WG), and a heavily grazed site (HG). Low soil water content, leaf area index, and potential growth ability of species at site HG led to low crop growth rate (CGR), net assimilation rate (NAR), and relative growth rate (RGR); resulting in lower ANPP as compared to sites WG and UG. Irrespective of land-use management, prolonged drought significantly decreases ANPP even though it systematically increases mean CGR and RGR. However, leaf N content and leaf weight ratio are the crucial components necessary to determine the RGR at site WG. This suggests that low leaf N and availability of soil N due to haymaking may be responsible for neither over-compensatory nor compensatory growth in this site. The low ANPP in dry years is not due to the low mean CGR and RGR but rather to the short effective growing days (referring to the days the vegetation actually grows), suggesting that production-adjusted grazing regimes may be the most suitable measures for precision land management and avoiding grassland degradation.     

Environmental factors covary with plant diversity–productivity relationships among Chinese grassland sites

Aim Our objective was to document the general relationship between plant species richness (SR) and above‐ground net primary productivity (ANPP) at different spatial scales and the environmental influence on this relationship. Location Temperate and alpine grasslands of China. Methods We investigated SR and ANPP at 321 field sites (1355 plots) across the widely distributed temperate and alpine grasslands of China. Ordinary least squares (OLS) regressions were used to test SR–ANPP relationships among site means. Plot‐level data of SR and ANPP were analysed with general linear models (GLMs) and the correlation between SR and ANPP was decomposed into covariance components to test the influence of climatic variables, region, vegetation type and remaining variation among sites on SR, ANPP and their relationship. Results We found positive linear relationships between SR and ANPP among sites in both the alpine and temperate grassland regions and in different grassland vegetation types of these biomes. Environmental gradients such as growing‐season precipitation affected both SR and ANPP in parallel. However, after removing the among‐site environmental variation, residual SR and ANPP were no longer correlated at the pooled within‐site level. Main conclusions The positive SR–ANPP relationship across large‐scale environmental gradients among sites was most likely the result of climatic variables influencing SR and ANPP in parallel. Our results suggest that in China's natural grasslands there is no direct relationship between SR and ANPP, presumably because the pool of available species for local community assembly is large, in contrast to experiments where species pools are artificially reduced.     

Soil Solution Chemistry and Element Fluxes in Three European Heathlands and Their Responses to Warming and Drought

Soil water chemistry and element budgets were studied at three northwestern European Calluna vulgaris heathland sites in Denmark (DK), The Netherlands (NL), and Wales (UK). Responses to experimental nighttime warming and early summer drought were followed during a two-year period. Soil solution chemistry measured below the organic soil layer and below the rooting zone and water fluxes estimated with hydrological models were combined to calculate element budgets. Remarkably high N leaching was observed at the NL heath with 18 and 6.4 kg N ha^–1 year^–1 of NO₃–N and NH₄–N leached from the control plots, respectively, indicating that this site is nitrogen saturated. Increased soil temperature of +0.5°C in the heated plots almost doubled the concentrations and losses of NO₃–N and DON at this site. Temperature also increased mobilization of N in the O horizon at the UK and DK heaths in the first year, but, because of high retention of N in the vegetation or mineral soil, there were no significant effects of warming on seepage water NO₃–N and NH₄–N. Retention of P was high at all three sites. In several cases, drought increased concentrations of elements momentarily, but element fluxes decreased because of a lower flux of water. Seepage water DOC and DON was highly significantly correlated at the UK site where losses of N were low, whereas losses of C and N were uncoupled at the NL site where atmospheric N input was greatest. Based on N budgets, calculations of the net change in the C sink or source strength in response to warming suggest no change or an increase in the C sink strength during these early years.     

Responses of moist non-acidic arctic tundra to altered environment: productivity, biomass, and species richness

In arctic Alaska, researchers have manipulated air temperature, light availability, and soil nutrient availability in several tundra communities over the past two decades. These communities responded quite differently to the same manipulations, and species responded individualistically within communities and among sites. For example, moist acidic tundra is primarily nitrogen (N)‐limited, whereas wet sedge tundra is primarily phosphorus (P)‐limited, and the magnitude of growth responses varies across sites within communities. Here we report results of four years of manipulated nutrients (N and/or P) and/or air temperature in an understudied, diverse plant community, moist non‐acidic tussock tundra, in northern Alaska. Our goals were to determine which factors limit above‐ground net primary productivity (ANPP) and biomass, how community composition changes may affect ecosystem attributes, and to compare these results with those from other communities to determine their generality. Although relative abundance of functional groups shifted in several treatments, the only significant change in community‐level ANPP and biomass occurred in plots that received both N and P, driven by an increase in graminoid biomass and production resulting from a positive effect of adding N. There was no difference in community biomass among any other treatments; however, some growth forms and individual species did respond. After four years no one species has come to dominate the treatment plots and species richness has not changed. These results are similar to studies in dry heath, wet sedge, and moist acidic tundra where community biomass had the greatest response to both N and P and warming results were more subtle. Unlike in moist acidic tundra where shrub biomass increased markedly with fertilization, our results suggest that in non‐acidic tundra carbon sequestration in plant biomass will not increase substantially under increased soil nutrient conditions because of the lack of overstory shrub species.     

Patterns of species density and productivity at different spatial scales in herbaceous plant communities

A major challenge in evaluating patterns of species richness and productivity involves acquiring data to examine these relationships empirically across a range of ecologically significant spatial scales. In this paper, we use data from herb‐dominated plant communities at six Long‐Term Ecological Research (LTER) sites to examine how the relationship between plant species density and above‐ground net primary productivity (ANPP) differs when the spatial scale of analysis is changed. We quantified this relationship at different spatial scales in which we varied the focus and extent of analysis: (1) among fields within communities, (2) among fields within biomes or biogeographic regions, and (3) among communities within biomes or biogeographic regions. We used species density (D=number of species per m²) as our measure of diversity to have a comparable index across all sites and scales. Although we expected unimodal relationships at all spatial scales, we found that spatial scale influenced the form of the relationship. At the scale of fields within different grassland communities, we detected a significant relationship at only one site (Minnesota old‐fields), and it was negative linear. When we expanded the extent of analyses to biogeographic regions (grasslands or North America), we found significant unimodal relationships in both cases. However, when we combined data to examine patterns among community types within different biogeographic regions (grassland, alpine tundra, arctic tundra, or North America), we did not detect significant relationships between species density and ANPP for any region. The results of our analyses demonstrate that the spatial scale of analysis – how data are aggregated and patterns examined – can influence the form of the relationship between species density and productivity. It also demonstrates the need for data sets from a broad spectrum of sites sampled over a range of scales for examining challenging and controversial ecological hypotheses.     

Aboveground net primary productivity and soil respiration display different responses to precipitation changes in desert grassland

荒漠草原区地上净初级生产力和土壤呼吸对降水变化的不同响应 降水变化既影响地上植被动态,也影响地下碳循环过程,尤其以干旱半干旱生态系统对降水的响应更为敏感。然而极端降水如何影响土壤碳固存潜力仍未得出明确结果。本研究在黄土高原西部荒漠草原样地实施了为期3年的降水控制实验,该实验包含5个降水梯度(即自然降水(对照),以及在自然降水基础上的减水40%、减水20%、增水20%、增水40%)。通过对不同降水处理下植物生长指标、地上净初级生产力(ANPP)、土壤水分和土壤呼吸(Rs)进行监测,采用双侧不对称模型揭示ANPP和Rs对降水变化的响应规律;采用结构方程模型,分析降水变化下影响ANPP和Rs的直接和间接因素。研究结果表明,ANPP对极端干旱的响应比极端湿润更敏感,在干旱和湿润年份均符合负向不对称模型。ANPP的变化主要受到降水的直接影响,同时,干旱年份植物密度的变化也对ANPP产生了影响。在湿润年份,Rs对降水变化的响应也呈负向不对称性。然而,干旱年份,Rs对降水变化表现出正向不对称响应,即对降水增加响应的敏感性高于降水减少,这可能与植物生长和ANPP对增水处理的正响应增加使自氧呼吸增强,及降水事件对异氧呼吸具有较强的‘Birch效应’有关。在干旱年份Rs对极端干旱(减水40%处理)表现出饱和响应。ANPP和Rs对降水格局改变的响应模式差异表明荒漠草原区极端湿润或干旱可能降低研究区土壤碳固存的潜力。