Phenological Sensitivity of Early and Late Flowering Species Under Seasonal Warming and Altered Precipitation in a Seminatural Temperate Grassland Ecosystem

Shifts in flowering phenology of plants are indicators of climate change. The great majority of existing phenological studies refer solely to gradual warming. However, knowledge on how flowering phenology responds to changes in seasonal variation of warming and precipitation regimes is missing. We report the onset of 22 early (flowering before/within May) and 23 late flowering (flowering after May) species in response to manipulated seasonal warming (equal to + 1.2°C; last 100-year summer/winter warming), additional winter rainfall, and modified precipitation variability (including a 1000-year extreme drought event followed by heavy rainfall) over the growing season in two consecutive years for a species-rich temperate grassland ecosystem. The average onset of flowering (over 2 years) was significantly advanced 3.1 days by winter warming and 1.5 days by summer warming compared to control. Early flowering species responded to seasonal warming in both years, while late-flowering species responded in only 1 year to summer warming. The average onset of early flowering species was significantly advanced, 4.9 days by winter warming and 2.3 days by summer warming. Species-specific analysis showed that even within the early flowering community there were divergences. A positive correlation between plant height and shift in flowering onset was detected under winter warming (R² = 0.20, p = 0.005). The average onsets of early and late flowering community were affected by neither winter rain nor growing season precipitation variability. Seasonal differences in warming, and particularly winter warming, might alter community dynamics among early and late flowering species which can cause shifts in the seasonal performances of temperate ecosystems.     

Press–pulse interactions: effects of warming,N deposition,altered winter precipitation,and fire on desert grassland community structure and dynamics

Global environmental change is altering temperature, precipitation patterns, resource availability, and disturbance regimes. Theory predicts that ecological presses will interact with pulse events to alter ecosystem structure and function. In 2006, we established a long‐term, multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric nitrogen (N) deposition, and increased winter precipitation on plant community structure and aboveground net primary production (ANPP) in a northern Chihuahuan Desert grassland. In 2009, a lightning‐caused wildfire burned through the experiment. Here, we report on the interactive effects of these global change drivers on pre‐ and postfire grassland community structure and ANPP. Our nighttime warming treatment increased winter nighttime air temperatures by an average of 1.1 °C and summer nighttime air temperature by 1.5 °C. Soil N availability was 2.5 times higher in fertilized compared with control plots. Average soil volumetric water content (VWC) in winter was slightly but significantly higher (13.0% vs. 11.0%) in plots receiving added winter rain relative to controls, and VWC was slightly higher in warmed (14.5%) compared with control (13.5%) plots during the growing season even though surface soil temperatures were significantly higher in warmed plots. Despite these significant treatment effects, ANPP and plant community structure were highly resistant to these global change drivers prior to the fire. Burning reduced the cover of the dominant grasses by more than 75%. Following the fire, forb species richness and biomass increased significantly, particularly in warmed, fertilized plots that received additional winter precipitation. Thus, although unburned grassland showed little initial response to multiple ecological presses, our results demonstrate how a single pulse disturbance can interact with chronic alterations in resource availability to increase ecosystem sensitivity to multiple drivers of global environmental change.     

Lasting effects of climate disturbance on perennial grassland above‐ground biomass production under two cutting frequencies

Climate extremes can ultimately reshape grassland services such as forage production and change plant functional type composition. This 3‐year field research studied resistance to dehydration and recovery after rehydration of plant community and plant functional types in an upland perennial grassland subjected to climate and cutting frequency (Cut+, Cut?) disturbances by measuring green tissue percentage and above‐ground biomass production (ANPP). In year 1, a climate disturbance gradient was applied by co‐manipulating temperature and precipitation. Four treatments were considered: control and warming‐drought climatic treatment, with or without extreme summer event. In year 2, control and warming‐drought treatments were maintained without extreme. In year 3, all treatments received ambient climatic conditions. We found that the grassland community was very sensitive to dehydration during the summer extreme: aerial senescence reached 80% when cumulated climatic water balance fell to ?156 mm and biomass declined by 78% at the end of summer. In autumn, canopy greenness and biomass totally recovered in control but not in the warming‐drought treatment. However ANPP decreased under both climatic treatments, but the effect was stronger on Cut+ (?24%) than Cut? (?15%). This decline was not compensated by the presence of three functional types because they were negatively affected by the climatic treatments, suggesting an absence of buffering effect on grassland production. In the following 2 years, lasting effects of climate disturbance on ANPP were observable. The unexpected stressful conditions of year 3 induced a decline in grassland production in the Cut+ control treatment. The fact that this treatment cumulated higher (45%) N export over the 3 years suggests that N plays a key role in ANPP stability. As ANPP in this mesic perennial grassland did not show engineering resilience, long‐term experimental manipulation is needed. Infrequent mowing appears more appropriate for sustaining grassland ANPP under future climate extremes.     

Shift in community structure in an early‐successional Mediterranean shrubland driven by long‐term experimental warming and drought and natural extreme droughts

Global warming and recurring drought are expected to accelerate water limitation for plant communities in semiarid Mediterranean ecosystems and produce directional shifts in structure and composition that are not easily detected, and supporting evidence is scarce. We conducted a long‐term (17 years) nocturnal‐warming (+0.6°C) and drought (?40% rainfall) experiments in an early‐successional Mediterranean shrubland to study the changes in community structure and composition, contrasting functional groups and dominant species, and the superimposed effects of natural extreme drought. Species richness decreased in both the warming and drought treatments. Responses to the moderate warming were associated with decreases in herb abundance, and responses to the drought were associated with decreases in both herb and shrub abundances. The drought also significantly decreased community diversity and evenness. Changes in abundance differed between herbs (decreases) and shrubs (increases or no changes). Both warming and drought, especially drought, increased the relative species richness and abundance of shrubs, favoring the establishment of shrubs. Both warming and drought produced significant shifts in plant community composition. Experimental warming shifted the community composition from Erica multiflora toward Rosmarinus officinalis, and drought consistently shifted the composition toward Globularia alypum. The responses in biodiversity (e.g., community biodiversity, changes of functional groups and compositional shifts) were also strongly correlated with atmospheric drought (SPEI) in winter–spring and/or summer, indicating sensitivity to water limitation in this early‐successional Mediterranean ecosystem, especially to natural extreme droughts. Our results suggest that the shifts in species assembles and community diversity and composition are accelerated by the long‐term nocturnal‐warming and drought, combined with natural severe droughts, and that the magnitude of the impacts of climate change is also correlated with the successional status of ecosystem. The results thus highlight the necessity for assessing the impacts on ecosystemic functioning and services and developing effective measures for conserving biodiversity.     

Convergence and contingency in production–precipitation relationships in North American and South African C4 grasslands

Mesic grasslands in North America and South Africa share many structural attributes, but less is known of their functional similarities. We assessed the control of a key ecosystem process, aboveground net primary production (ANPP), by interannual variation in precipitation amount and pattern via analysis of data sets (15- and 24-year periods) from long-term research programs on each continent. Both sites were dominated by C₄ grasses and had similar growing season climates; thus, we expected convergence in precipitation–ANPP relationships. Lack of convergence, however, would support an alternative hypothesis—that differences in evolutionary history and purportedly greater climatic variability in South Africa fundamentally alter the functioning of southern versus northern hemisphere grasslands. Neither mean annual precipitation nor mean ANPP differed between the South African and North American sites (838 vs. 857 mm/year, 423.5 vs. 461.4 g/m² respectively) and growing season precipitation–ANPP relationships were similar. Despite overall convergence, there were differences between sites in how the seasonal timing of precipitation affected ANPP. In particular, interannual variability in precipitation that fell during the first half of the growing season strongly affected annual ANPP in South Africa (P < 0.01), but was not related to ANPP in North America (P = 0.098). Both sites were affected similarly by late season precipitation. Divergence in the seasonal course of available soil moisture (chronically low in the winter and early spring in the South African site vs. high in the North American site) is proposed as a key contingent factor explaining differential sensitivity in ANPP to early season precipitation in these two grasslands. These long-term data sets provided no support for greater rainfall, temperature or ANPP variability in the South African versus the North American site. However, greater sensitivity of ANPP to early season precipitation in the South African grassland suggests that future patterns of productivity may be more responsive to seasonal changes in climate compared with the North American site.     

Mycorrhizal fungal abundance is affected by long-term climatic manipulations in the field

Climate change treatments – winter warming, summer drought and increased summer precipitation – have been imposed on an upland grassland continuously for 7 years. The vegetation was surveyed yearly. In the seventh year, soil samples were collected on four occasions through the growing season in order to assess mycorrhizal fungal abundance. Mycorrhizal fungal colonisation of roots and extraradical mycorrhizal hyphal (EMH) density in the soil were both affected by the climatic manipulations, especially by summer drought. Both winter warming and summer drought increased the proportion of root length colonised (RLC) and decreased the density of external mycorrhizal hyphal. Much of the response of mycorrhizal fungi to climate change could be attributed to climate‐induced changes in the vegetation, especially plant species relative abundance. However, it is possible that some of the mycorrhizal response to the climatic manipulations was direct – for example, the response of the EMH density to the drought treatment. Future work should address the likely change in mycorrhizal functioning under warmer and drier conditions.     

Precipitation Pattern Determines the Inter-annual Variation of Herbaceous Layer and Carbon Fluxes in a Phreatophyte-Dominated Desert Ecosystem

Arid and semi-arid ecosystems dominated by shrubby species are an important component in the global carbon cycle but are largely under-represented in studies of the effect of climate change on carbon flux. This study synthesizes data from long-term eddy covariance measurements and experiments to assess how changes in ecosystem composition, driven by precipitation patterns, affect inter-annual variability of carbon flux and their components in a halophyte desert community dominated by deep-rooted shrubs (phreatophytes, which depend on groundwater as their primary water source). Our results demonstrated that the carbon balance of this community responded strongly to precipitation variations. Both pre-growing season precipitation and growing season precipitation frequency significantly affected inter-annual variations in ecosystem carbon flux. Heavy pre-growing season precipitation (November–April, mostly as snow) increased annual net ecosystem carbon exchange, by facilitating the growth and carbon assimilation of shallow-rooted annual plants, which used spring and summer precipitation to increase community productivity. Sufficient pre-growing season precipitation led to more germination and growth of shallow-rooted annual plants. When followed by high-frequency growing season precipitation, community productivity of this desert ecosystem was lifted to the level of grassland or forest ecosystems. The long-term observations and experimental results confirmed that precipitation patterns and the herbaceous component were dominant drivers of the carbon dynamics in this phreatophyte-dominated desert ecosystem. This study illustrates the importance of inter-annual variations in climate and ecosystem composition for the carbon flux in arid and semi-arid ecosystems. It also highlights the important effect of changing frequency and seasonal pattern of precipitation on the regional and global carbon cycle in the coming decades.     

Precipitation amount and event size interact to reduce ecosystem functioning during dry years in a mesic grassland

Ongoing intensification of the hydrological cycle is altering rainfall regimes by increasing the frequency of extreme wet and dry years and the size of individual rainfall events. Despite long‐standing recognition of the importance of precipitation amount and variability for most terrestrial ecosystem processes, we lack understanding of their interactive effects on ecosystem functioning. We quantified this interaction in native grassland by experimentally eliminating temporal variability in growing season rainfall over a wide range of precipitation amounts, from extreme wet to dry conditions. We contrasted the rain use efficiency (RUE) of above‐ground net primary productivity (ANPP) under conditions of experimentally reduced versus naturally high rainfall variability using a 32‐year precipitation–ANPP dataset from the same site as our experiment. We found that increased growing season rainfall variability can reduce RUE and thus ecosystem functioning by as much as 42% during dry years, but that such impacts weaken as years become wetter. During low precipitation years, RUE is lowest when rainfall event sizes are relatively large, and when a larger proportion of total rainfall is derived from large events. Thus, a shift towards precipitation regimes dominated by fewer but larger rainfall events, already documented over much of the globe, can be expected to reduce the functioning of mesic ecosystems primarily during drought, when ecosystem processes are already compromised by low water availability.     

Climate change effects on plant biomass alter dominance patterns and community evenness in an experimental old‐field ecosystem

Atmospheric and climatic change can alter plant biomass production and plant community composition. However, we know little about how climate change‐induced alterations in biomass production affect plant species composition. To better understand how climate change will alter both individual plant species and community biomass, we manipulated atmospheric [CO₂], air temperature, and precipitation in a constructed old‐field ecosystem. Specifically, we compared the responses of dominant and subdominant species to our climatic treatments, and explored how changes in plant dominance patterns alter community evenness over 2 years. Our study resulted in four major findings: (1) all treatments, elevated [CO₂], warming, and increased precipitation increased plant community biomass and the effects were additive rather than interactive, (2) plant species differed in their response to the treatments, resulting in shifts in the proportional biomass of individual species, which altered the plant community composition; however, the plant community response was largely driven by the positive precipitation response of Lespedeza, the most dominant species in the community, (3) precipitation explained most of the variation in plant community composition among treatments, and (4) changes in precipitation caused a shift in the dominant species proportional biomass that resulted in lower community evenness in the wet relative to dry treatments. Interestingly, compositional and evenness responses of the subdominant community to the treatments did not always follow the responses of the whole plant community. Our data suggest that changes in plant dominance patterns and community evenness are an important part of community responses to climatic change, and generally, that such compositional shifts can alter ecosystem biomass production and nutrient inputs.     

蒸发需求干旱指数(EDDI)在辽宁省干旱识别中的应用

蒸发需求干旱指数(EDDI)是从大气蒸发需求(E₀)角度出发建立起来的一种多尺度的干旱指标,具有不依赖降水量、适用于各种下垫面类型的特点,具备在不同时间尺度捕捉水胁迫信号的能力。本研究基于1961—2018年辽宁省52个气象站气象观测资料,逐日估算E₀,按年、生长季(4—10月)、春季、夏季、秋季、冬季分别计算EDDI,分6个时间尺度识别近58年辽宁省干旱发生的年际变化特征。结果表明: 研究期间,辽宁省EDDI年际变化阶段性明显,多个时间尺度的EDDI呈两个高值集中期。在20世纪60年代,年、生长季、春季、秋季和冬季5个时间尺度的辽宁省平均EDDI高值区相对集中,这一阶段辽宁省发生干旱的年数多、程度重;除冬季外,2014—2018年是其他5个时间尺度的EDDI高值另一个相对集中的时段;1981—1982年,辽宁省的年、生长季、夏季、秋季的EDDI值偏高。1963—1965年(除夏季外)、1972—1973年(生长季、夏季)、1989—1990年(年、生长季、春季、冬季)、1997—1998年(年、生长季、夏季)、2004—2005年(春季、冬季)和2013—2014年(年、生长季、秋季)都发生了干-湿或湿-干逆转事件。1985—1987、1993—1995和2005—2013年,辽宁省存在明显的干旱空窗期。     

Response of plant species richness and primary productivity in shrublands along a north–south gradient in Europe to seven years of experimental warming and drought: reductions in primary productivity in the heat and drought year of 2003

We used a nonintrusive field experiment carried out at six sites – Wales (UK), Denmark (DK), the Netherlands (NL), Hungary (HU), Sardinia (Italy – IT), and Catalonia (Spain – SP) – along a climatic and latitudinal gradient to examine the response of plant species richness and primary productivity to warming and drought in shrubland ecosystems. The warming treatment raised the plot daily temperature by ca. 1 °C, while the drought treatment led to a reduction in soil moisture at the peak of the growing season that ranged from 26% at the SP site to 82% in the NL site. During the 7 years the experiment lasted (1999–2005), we used the pin‐point method to measure the species composition of plant communities and plant biomass, litterfall, and shoot growth of the dominant plant species at each site. A significantly lower increase in the number of species pin‐pointed per transect was found in the drought plots at the SP site, where the plant community was still in a process of recovering from a forest fire in 1994. No changes in species richness were found at the other sites, which were at a more mature and stable state of succession and, thus less liable to recruitment of new species. The relationship between annual biomass accumulation and temperature of the growing season was positive at the coldest site and negative at the warmest site. The warming treatment tended to increase the aboveground net primary productivity (ANPP) at the northern sites. The relationship between annual biomass accumulation and soil moisture during the growing season was not significant at the wettest sites, but was positive at the driest sites. The drought treatment tended to reduce the ANPP in the NL, HU, IT, and SP sites. The responses to warming were very strongly related to the Gaussen aridity index (stronger responses the lower the aridity), whereas the responses to drought were not. Changes in the annual aboveground biomass accumulation, litterfall, and, thus, the ANPP, mirrored the interannual variation in climate conditions: the most outstanding change was a decrease in biomass accumulation and an increase in litterfall at most sites during the abnormally hot year of 2003. Species richness also tended to decrease in 2003 at all sites except the cold and wet UK site. Species‐specific responses to warming were found in shoot growth: at the SP site, Globularia alypum was not affected, while the other dominant species, Erica multiflora, grew 30% more; at the UK site, Calluna vulgaris tended to grow more in the warming plots, while Empetrum nigrum tended to grow less. Drought treatment decreased plant growth in several studied species, although there were some species such as Pinus halepensis at the SP site or C. vulgaris at the UK site that were not affected. The magnitude of responses to warming and drought thus depended greatly on the differences between sites, years, and species and these multiple plant responses may be expected to have consequences at ecosystem and community level. Decreases in biodiversity and the increase in E. multiflora growth at the SP site as a response to warming challenge the assumption that sensitivity to warming may be less well developed at more southerly latitudes; likewise, the fact that one of the studied shrublands presented negative ANPP as a response to the 2003 heat wave also challenges the hypothesis that future climate warming will lead to an enhancement of plant growth and carbon sequestration in temperate ecosystems. Extreme events may thus change the general trend of increased productivity in response to warming in the colder sites.     

Persistent versus transient tree encroachment of temperate peat bogs: effects of climate warming and drought events

Peatlands store approximately 30% of global soil carbon, most in moss‐dominated bogs. Future climatic changes, such as changes in precipitation patterns and warming, are expected to affect peat bog vegetation composition and thereby its long‐term carbon sequestration capacity. Theoretical work suggests that an episode of rapid environmental change is more likely to trigger transitions to alternative ecosystem states than a gradual, but equally large, change in conditions. We used a dynamic vegetation model to explore the impacts of drought events and increased temperature on vegetation composition of temperate peat bogs. We analyzed the consequences of six patterns of summer drought events combined with five temperature scenarios to test whether an open peat bog dominated by moss (Sphagnum) could shift to a tree‐dominated state. Unexpectedly, neither a gradual decrease in the amount of summer precipitation nor the occurrence of a number of extremely dry summers in a row could shift the moss‐dominated peat bog permanently into a tree‐dominated peat bog. The increase in tree biomass during drought events was unable to trigger positive feedbacks that keep the ecosystem in a tree‐dominated state after a return to previous ‘normal’ rainfall conditions. In contrast, temperature increases from 1 °C onward already shifted peat bogs into tree‐dominated ecosystems. In our simulations, drought events facilitated tree establishment, but temperature determined how much tree biomass could develop. Our results suggest that under current climatic conditions, peat bog vegetation is rather resilient to drought events, but very sensitive to temperature increases, indicating that future warming is likely to trigger persistent vegetation shifts.     

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.     

Long‐term successional forest dynamics: species and community responses to climatic variability

Question: Are trees sensitive to climatic variability, and do tree species differ in their responses to climatic variability? Does sensitivity of forest communities to climatic variability depend on stand composition? Location: Mixed young forest at Walker Branch Watershed near Oak Ridge, East Tennessee, USA. Methods: Using a long‐term dataset (1967–2006), we analyzed temporal forest dynamics at the tree and species level, and community dynamics for forest stands that differed in initial species composition (i.e., chestnut oak, oak–hickory, pine, and yellow poplar stands). Using summer drought and growing season temperature as defined climate drivers, we evaluated relationships between forest dynamics and climate across levels of organization. Results: Over the four‐decade study period, forest communities underwent successional change and substantially increased in biomass. Variation in summer drought and growing season temperature contributed to temporal biomass dynamics for some tree species, but not for others. Stand‐level responses to climatic variability were related to the responses of component species, except in pine stands. Pinus echinata, the dominant species in pine stands, decreased over time due to periodic outbreaks of pine bark beetle (Dendroctonus frontalis). These outbreaks at Walker Branch could not be directly related to climatic conditions. Conclusions: The results indicate that sensitivity of developing forests to climatic variability is stand type‐dependent, and hence is a function of species composition. However, in the long term, direct effects of climatic variability on forest dynamics may be small relative to autogenic successional processes or climate‐related insect outbreaks. Empirical studies testing for interactions between forest succession and climatic variability are needed.     

Successional change in species composition alters climate sensitivity of grassland productivity

Succession theory predicts altered sensitivity of ecosystem functions to disturbance (i.e., climate change) due to the temporal shift in plant community composition. However, empirical evidence in global change experiments is lacking to support this prediction. Here, we present findings from an 8‐year long‐term global change experiment with warming and altered precipitation manipulation (double and halved amount). First, we observed a temporal shift in species composition over 8 years, resulting in a transition from an annual C₃‐dominant plant community to a perennial C₄‐dominant plant community. This successional transition was independent of any experimental treatments. During the successional transition, the response of aboveground net primary productivity (ANPP) to precipitation addition magnified from neutral to +45.3%, while the response to halved precipitation attenuated substantially from ?17.6% to neutral. However, warming did not affect ANPP in either state. The findings further reveal that the time‐dependent climate sensitivity may be regulated by successional change in species composition, highlighting the importance of vegetation dynamics in regulating the response of ecosystem productivity to precipitation change.     

降水变化对藏北高寒草原化草甸降水利用效率及地上生产力的影响

降水总量、分配方式及其发生时间共同决定了青藏高原植被生长的水分条件,而降水利用效率(PUE,地上生产力与降水量的比值)是评估降水与植被生产力关系的有效指标.本研究以藏北当雄高寒草原化草甸为研究对象,利用多年生物量采样数据与同期遥感EVI植被指数建立线性模型,反演了2000-2016年地上净初级生产力(ANPP),结合同时期气象数据,以生长季降水量(GSP)表征降水总体状况,改进的降水集度指标(PCI)表征生长季降水分配,降水重心(PC)表征降水集中时间,并结合生长季均温(GST),利用结构方程分析了气候因子对当雄草地降水利用效率和地上生产力的影响.结果表明: 当雄草地ANPP主要受生长季降水影响,GSP与ANPP呈显著正相关,而GST与PUE、ANPP无显著相关关系;PCI与PUE呈显著正相关,表明降水集中分布有利于PUE增加;PCI与ANPP相关的间接系数大于直接系数,表明PCI通过PUE影响ANPP;降水集中时间(PC)变化则对PUE和ANPP没有显著影响.在青藏高原显著的气候变暖背景下,降水量和降水集度的变化都将会对藏北高寒草地的地上生产力产生重要影响.     

Deepened winter snow cover enhances net ecosystem exchange and stabilizes plant community composition and productivity in a temperate grassland

Global warming has greatly altered winter snowfall patterns, and there is a trend towards increasing winter snow in semi‐arid regions in China. Winter snowfall is an important source of water during early spring in these water‐limited ecosystems, and it can also affect nutrient supply. However, we know little about how changes in winter snowfall will affect ecosystem productivity and plant community structure during the growing season. Here, we conducted a 5‐year winter snow manipulation experiment in a temperate grassland in Inner Mongolia. We measured ecosystem carbon flux from 2014 to 2018 and plant biomass and species composition from 2015 to 2018. We found that soil moisture increased under deepened winter snow in early growing season, particularly in deeper soil layers. Deepened snow increased the net ecosystem exchange of CO₂ (NEE) and reduced intra‐ and inter‐annual variation in NEE. Deepened snow did not affect aboveground plant biomass (AGB) but significantly increased root biomass. This suggested that the enhanced NEE was allocated to the belowground, which improved water acquisition and thus contributed to greater stability in NEE in deep‐snow plots. Interestingly, the AGB of grasses in the control plots declined over time, resulting in a shift towards a forb‐dominated system. Similar declines in grass AGB were also observed at three other locations in the region over the same time frame and are attributed to 4 years of below‐average precipitation during the growing season. By contrast, grass AGB was stabilized under deepened winter snow and plant community composition remained unchanged. Hence, our study demonstrates that increased winter snowfall may stabilize arid grassland systems by reducing resource competition, promoting coexistence between plant functional groups, which ultimately mitigates the impacts of chronic drought during the growing season.     

Effect of Drought Frequency and Other Reach Characteristics on Invertebrate Communities and Litter Breakdown in the Intermittent Mediterranean River Pula (Sardinia,Italy)

Benthic invertebrate communities were sampled in spring and fall, and breakdown of reed litter was measured in winter, spring, summer and fall, at 23 sites in an intermittent Mediterranean river basin (Pula, Sardinia, Italy), and their variations were related to drought frequency and other reach characteristics (altitude, order, stream channel width and depth, vegetation cover, sediment type, organic content, and water quality). 78% of the sites were affected by drought events, mainly in summer and fall. Benthic taxonomic richness was higher and most taxa were more widely distributed in spring than in fall. Canonical correspondence analysis showed that measured environmental variables accounted for 72% of the variance in invertebrate community structure. Reed decay rates varied among seasons, and during the dry season among stream orders as well. Drought events slowed down breakdown and explained 44% of spatial variability in decay rates (Stepwise multiple regression analysis), an additional 8% being explained by water chemistry. Results suggest that drought events affect ecosystem functioning (litter breakdown) more than structure (invertebrate communities). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Drought timing differentially affects above- and belowground productivity in a mesic grassland

Climate models forecast an intensification of the global hydrological cycle with droughts becoming more frequent and severe, and shifting to times when they have been historically uncommon. Droughts, or prolonged periods of precipitation deficiency, are characteristic of most temperate grasslands, yet few experiments have explored how variation in the seasonal timing of drought may impact ecosystem function. We investigated the response of above- and belowground net primary production (ANPP & BNPP) to altered drought timing in a mesic grassland in NE Kansas. Moderate drought treatments (25% reduction from the mean growing season precipitation [GSP]) were imposed by erecting rainout shelters in late spring (LSP), early summer (ESM), and mid-summer (MSM, n = 10 plots/treatment). These treatments were compared to two controls (long-term average GSP [LTA] and ambient GSP [AMB]) and a wet treatment (+30% of the long-term average GSP [WET]). We found that LSP drought did not significantly reduce ANPP relative to control plots while the ESM and MSM drought did despite equivalent reductions in soil moisture. In contrast, the WET treatment did not affect ANPP. Neither the WET nor the drought treatments altered BNPP as compared to the controls. Our results suggest that forecasts of ecosystem responses to climate change will be improved if both the seasonal timing of alterations in precipitation as well as differential responses of above- and belowground productivity to drought are incorporated into models.     

Snow cover and extreme winter warming events control flower abundance of some,but not all species in high arctic Svalbard

The High Arctic winter is expected to be altered through ongoing and future climate change. Winter precipitation and snow depth are projected to increase and melt out dates change accordingly. Also, snow cover and depth will play an important role in protecting plant canopy from increasingly more frequent extreme winter warming events. Flower production of many Arctic plants is dependent on melt out timing, since season length determines resource availability for flower preformation. We erected snow fences to increase snow depth and shorten growing season, and counted flowers of six species over 5 years, during which we experienced two extreme winter warming events. Most species were resistant to snow cover increase, but two species reduced flower abundance due to shortened growing seasons. Cassiope tetragona responded strongly with fewer flowers in deep snow regimes during years without extreme events, while Stellaria crassipes responded partly. Snow pack thickness determined whether winter warming events had an effect on flower abundance of some species. Warming events clearly reduced flower abundance in shallow but not in deep snow regimes of Cassiope tetragona, but only marginally for Dryas octopetala. However, the affected species were resilient and individuals did not experience any long term effects. In the case of short or cold summers, a subset of species suffered reduced reproductive success, which may affect future plant composition through possible cascading competition effects. Extreme winter warming events were shown to expose the canopy to cold winter air. The following summer most of the overwintering flower buds could not produce flowers. Thus reproductive success is reduced if this occurs in subsequent years. We conclude that snow depth influences flower abundance by altering season length and by protecting or exposing flower buds to cold winter air, but most species studied are resistant to changes.