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Amplification of the hydrological cycle as a consequence of global warming is forecast to lead to more extreme intra-annual precipitation regimes characterized by larger rainfall events and longer intervals between events. We present a conceptual framework, based on past investigations and ecological theory, for predicting the consequences of this underappreciated aspect of climate change. We consider a broad range of terrestrial ecosystems that vary in their overall water balance. More extreme rainfall regimes are expected to increase the duration and severity of soil water stress in mesic ecosystems as intervals between rainfall events increase. In contrast, xeric ecosystems may exhibit the opposite response to extreme events. Larger but less frequent rainfall events may result in proportional reductions in evaporative losses in xeric systems, and thus may lead to greater soil water availability. Hydric (wetland) ecosystems are predicted to experience reduced periods of anoxia in response to prolonged intervals between rainfall events. Understanding these contingent effects of ecosystem water balance is necessary for predicting how more extreme precipitation regimes will modify ecosystem processes and alter interactions with related global change drivers.

Projections of temperature and precipitation patterns across the United States during the next 50 yr anticipate a 1.5 to 2°C warming and a slight increase in precipitation as a result of global climate change. There have been relatively few studies of climate change effects on pasture and rangeland (grazingland) species compared to those on crop species, despite the economic and ecological importance of the former. Here we review the literature on responses of pastureland and rangeland species to rising atmospheric CO2 and climate change (temperature and precipitation) and discuss plant and management factors likely to influence pastureland and rangeland responses to change (e.g., community composition, plant competition, perennial growth habit, seasonal productivity, and management methods). Overall, the response of pastureland and rangeland species to increased [CO2] is consistent with the general responses of C3 and C4 vegetation, although exceptions exist. Both pastureland and rangeland species may experience accelerated metabolism and advanced development with rising temperature, often resulting in a longer growing season. However, soil resources will often constrain temperature effects. In general, it is expected that increases in [CO2] and precipitation will enhance rangeland net primary production (NPP) whereas increased air temperatures will either increase or decrease NPP. Much of the uncertainty in predicting how pastureland and rangeland species will respond to climate change is due to uncertainty in future projections of precipitation, both globally and regionally. This review reveals the need for comprehensive studies of climate change impacts on pastureland and rangeland ecosystems that include an assessment of the mediating effects of grazing regimes and mutualistic relationships (e.g., plant roots-nematodes; N-fixing organisms) as well as changes in water, carbon, and nutrient cycling.

Water availability limits plant growth and production in almost all terrestrial ecosystems. However, biomes differ substantially in sensitivity of aboveground net primary production (ANPP) to between-year variation in precipitation. Average rain-use efficiency (RUE; ANPP/precipitation) also varies between biomes, supposedly because of differences in vegetation structure and/or biogeochemical constraints. Here we show that RUE decreases across biomes as mean annual precipitation increases. However, during the driest years at each site, there is convergence to a common maximum RUE (RUEmax) that is typical of arid ecosystems. RUEmax was also identified by experimentally altering the degree of limitation by water and other resources. Thus, in years when water is most limiting, deserts, grasslands and forests all exhibit the same rate of biomass production per unit rainfall, despite differences in physiognomy and site-level RUE. Global climate models predict increased between-year variability in precipitation, more frequent extreme drought events, and changes in temperature. Forecasts of future ecosystem behaviour should take into account this convergent feature of terrestrial biomes.

Plant productivity and other ecosystem processes vary widely in their responses to experimental increases in atmospheric carbon dioxide (CO2) concentration. A conceptual framework first suggested by Chapin et al. (1996) was adapted to address the question of why CO2 effects on primary productivity vary so greatly among rangelands and among years for a given ecosystem. The ‘interactive controls’ framework is based on the premise that the influence of elevated CO2 on productivity is governed by a set of internal variables that interact dynamically with ecosystem processes. These interactive controls, which include regional climate, soil resource supply, major functional groups of organisms and disturbance regimes, both regulate CO2 effects on ecosystems and respond to CO2 effects. Changes in interactive controls resulting from CO2 enrichment may feed back to dampen or amplify ecosystem responses to CO2. Most feedbacks from interactive controls will be negative and dampen CO2 effects on ecosystems. Negative feedbacks promote homeostasis in ecosystem processes and reduce the response of plant productivity to CO2. Positive feedbacks on CO2 responses are fewer, but can sustain or even increase benefits of CO2 enrichment for productivity. Positive feedbacks on CO2 responses occur most frequently through changes in plant species and functional group composition. Understanding positive and negative feedbacks on CO2 responses could be one key to predicting consequences of CO2 enrichment for rangeland productivity and other processes.

1. Effects of CO₂ enrichment on leaf transpiration are well-documented, but our understanding of how CO₂ interacts with other variables to regulate evapotranspiration from plant communities is more limited. 2. A series of weighing lysimeters in which tallgrass prairie species had been planted were exposed to a subambient to elevated gradient in CO₂ in a field chamber. Lysimeters with intact monoliths of three soil types were represented along the CO₂ gradient. We used regression analysis to determine how CO₂ effects on evapotranspiration per unit of soil surface area (ETsoil) and per unit of leaf area (ETla) depended on variation in leaf area index (LAI) and diurnal changes in environmental variables during the initial 6 weeks of CO₂ treatment. 3. CO₂ enrichment reduced ETsoil and ETla, and together with air temperature and LAI accounted for most of the variance in daily values of evapotranspiration explained by multiple regression models. The CO₂ effect on ETsoil did not depend on values of other variables, but CO₂ enrichment reduced ETla most at relatively low air temperatures and low LAI for all soils combined. Higher temperatures countered the CO₂ effect by increasing ETla more at elevated than subambient CO₂. Higher LAI countered the CO₂ effect by decreasing ETla more at subambient than elevated concentrations. Plant (LAI) and environmental effects on ETla differed among soils, possibly because plant growth patterns and physiology differed among soils. 4. Our results imply that the CO₂ effect on evapotranspiration per unit of leaf area will vary with seasonal change in temperature and LAI, independent of seasonal shifts in leaf age and physiological activity.

Recent reviews have emphasized the need to incorporate genomics into ecological field studies to further understand how species respond to changing environmental conditions. Genomic tools, such as cDNA (complementary DNA) microarrays, allow for the simultaneous analysis of gene expression of thousands of genes from all or part of an organism's genome (the transcription profile), thereby revealing the genetic mechanisms that underlie species' responses to environmental change. However, despite their potential, two major limitations have hindered the incorporation of microarrays and other genomic tools into field studies: (1) the limited availability of microarrays for ecologically relevant, non-model species and limited financial resources for developing new microarrays; and (2) concern that high sensitivity of gene expression to even subtle alterations in environmental conditions will hinder detection of relevant changes in field measures of transcription profiles. Here, we show that with cross-species hybridizations of microarrays developed for a closely related model organism, an appropriate experimental design, and sufficient replication, transcriptional profiling can successfully be incorporated into field studies. In this way, relevant changes in gene expression with changing environmental conditions can be detected.

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. The authors present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (psi). Drought-stressed plants galled shoots had 36% greater A, and 10% greater stem psi than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. The authors hypothesize that 1) galled shoots have higher psi, g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and psi, but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO2 concentration relationships were consistent with the explanation that increased psi allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationship or by removing low-psi limitations on non-stomatal components of A. The author's working hypothesis is that galls increase psi and A by reducing the shoot:root ratio so that plant is exploiting a greater soil volume per unit leaf area. They argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased psi and A may protect gall insects from variation in resource availability caused by periodic drought stress.

Insect herbivory can have important effects on plant life histories and architecture. We quantified the impact that a cynipid gall wasp, Antistrophus silphii, had on growth, reproduction, and biomass allocation patterns of Silphium integrifolium growing in the tallgrass prairie of northeastern Kansas. Experimentally galled individual Silphium shoots (ramets) had reduced shoot growth, leaf and flower head production, and delayed flowering compared to gall-free control shoots. Gall formation completely halted normal apical growth in 65% of the shoots. Galling did not affect individual flower head weight, the numbers of achenes per flower head or achene weight. Silphium plants (genets) with a high proportion of galled shoots had lower total biomass, a lower proportion of total biomass allocated to flower heads, higher allocation to leaves, but no change in allocation to stems or rhizome. High gall densities reduced the number of flower heads per plant and shortened the time between flower head initiation and maturity. An adaptive interpretation of these results would be that the survivorship and future performance of galled Silphium may be promoted by maintaining allocation to rhizome. However, reduced shoot growth and delayed reproduction in galled Silphium may weaken its competitive ability and reduce pollination success, so that any adaptive advantage to Silphium's allocation responses to galls may be outweighed by disadvantages from its growth and flowering phenology responses. We conclude that a more parsimonious interpretation of these results is that gall-induced allocation changes are due to architectural constraints placed by galls on meristem activity, rather than to any adaptive response on the part of the plant.

This study examined how leaf position in the canopy affected photosynthetic and stomatal responses to short-term, minutes-long shade periods during a drought cycle in soybean (Glycine max [L.] Merr.). All soybean leaves had similar basic responses to short-term shade, including rapidly decreased photosynthetic rates (Aco2 ), slower decreases in stomatal conductance (g,), and delayed stomatal reopening and photosynthetic recovery after leaves were reilluminated. Drought stress lowered overall Aco2 and restricted photosynthetic and stomatal responses to short-term shade with the negative effects of drought being stronger in lower than in upper leaves. Some of the negative effects of drought persisted after drought was relieved, causing reduced overall water use efficiency, especially in lower leaves. These results indicate that leaf position effects on stomatal responses to short-term shade events become important during and after stress.

Woody plant species in grassland ecosystems can be subjected to damage from fire and multiple herbivore species, but interactions between fire and herbivory can modify their separate impacts on woody plant life histories. We studied how galling (by Periploca ceanothiella, Lepidoptera: Cosmopterigidae), deer browsing (Odocoilius virginianus) and fire affected the growth and reproduction of the woody shrub Ceanothus herbaceous (Rhamnaceae) on a burned and an unburned site at Konza Prairie Research Natural Area in eastern Kansas. Fire was the major influence on C. herbaceous growth, causing plants to produce long unbranched vegetative ramets from protected belowground meristems, while unburned plants were heavily branched and bore shorter shoots and numerous inflorescences. Unburned plants experienced higher gall frequencies, more galls on their longest shoots, but similar deer browsing compared to burned plants. Ramets with herbivore damage had more branches and inflorescences than undamaged ramets, especially where both herbivores were present. Ceanothus herbaceous' flexible life history responses suggest tolerance of multiple forms of damage.