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Atmospheric CO2 enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO2 enrichment, with an emphasis on water relations. Increasing CO2 led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were relatively greater in dry years. In contrast, CO2-induced aboveground biomass increases in the Texas C3/C4 grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO2 in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO2-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO2 on plant and soil water relations may contribute substantially to experimentally induced CO2-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO2 effects on photosynthesis and those indirect CO2 effects via soil moisture as documented here.

Abstract. Woody encroachment is a widespread and acute phenomenon affecting grasslands and savannas worldwide. We performed a meta-analysis of 29 studies from 13 different grassland/savanna communities in North America to determine the consequences of woody encroachment on plant species richness. In all 13 communities, species richness declined with woody plant encroachment (average decline ¼ 45%). Species richness declined more in communities with higher precipitation (r 2 ¼ 0.81) and where encroachment was associated with a greater change in annual net primary productivity (ANPP; r 2 ¼ 0.69). Based on the strong positive correlation between precipitation and ANPP following encroachment (r 2 ¼ 0.87), we hypothesize that these relationships occur because water-limited woody plants experience a greater physiological and demographic release as precipitation increases. The observed relationship between species richness and ANPP provides support for the theoretical expectation that a trade-off occurs between richness and productivity in herbaceous communities. We conclude that woody plant encroachment leads to significant declines in species richness in North American grassland/savanna communities.

Water availability strongly governs grassland primary productivity, yet this resource varies dramatically in time (seasonally) and space (with soil depth and topography). It has long been assumed that co-occurring species differ in their partitioning of water use by depth, but direct evidence is lacking. We report data from two growing seasons (2004-2005) in which we measured the isotopic signature of plant xylem water from seven species (including C(3) forbs and shrubs and C(4) grasses) growing along a topographic gradient at the Konza Prairie Biological Station. Plant xylem stable oxygen isotope ratio (delta(18)O) values were compared to soil water delta(18)O profiles, recent rainfall events, and groundwater. Species varied in both their temporal patterns of water use and their responses to seasonal droughts in both years. During wet periods, species differences in water use were minimal, with common dependency on recent rainfall events stored in the upper soil layers. However, during dry periods, most C(3) species used proportionally more water from deeper portions of the soil profile relative to the C(4) grasses. Plants in uplands used more shallow soil water compared to those in lowlands, with the greatest differences across the topographic gradient occurring during dry periods. While the documented vertical root distribution varies by species and growth form in this grassland, each of the species we measured appeared to compete for the same surface layer soil moisture when water was not limiting. Thus, our results suggest that variation in precipitation history and landscape positions are greater determinants of water-use patterns than would be expected based on absolute rooting depth.

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Future climates are forecast to include greater precipitation variability and more frequent heat waves, but the degree to which the timing of climate variability impacts ecosystems is uncertain. In a temperate, humid grassland, we examined the seasonal impacts of climate variability on 27 y of grass productivity. Drought and highintensity precipitation reduced grass productivity only during a 110-d period, whereas high temperatures reduced productivity only during 25 d in July. The effects of drought and heat waves declined over the season and had no detectable impact on grass productivity in August. If these patterns are general across ecosystems, predictions of ecosystem response to climate change will have to account not only for the magnitude of climate variability but also for its timing.Konza | net primary production | streamflow | critical climate periods F uture climates are likely to include more frequent droughts, high-intensity precipitation patterns, and heat waves, (i.e., periods of elevated air temperatures) (1, 2). At their most severe, extreme climate events, such as the mid-American heat waves of 1980 and 2011 and the 2003 European heat wave, involve months of hot, dry weather (3, 4), increasing mortality in humans and wildlife (5, 6) while reducing agricultural and natural-systems productivity (7-10). An increase in climate extremes would have unambiguously negative effects on ecosystems. However, most climate variability would not be considered extreme and occurs on much shorter time scales throughout the growing season with temperature and precipitation frequently disassociated. The response of ecosystems to short-term climate variability at different times of year is thought to vary (11-16), but we know little about how the timing of short-duration climate variability impacts key ecosystem dynamics such as plant productivity.To understand better how the timing of climate variability affects grassland productivity, we applied the critical climate period approach (17, 18) to long-term measurements of grass productivity in a humid, temperate grassland. Aboveground net primary productivity of grass (ANPP G ) was measured at the time of peak standing biomass from 1984-2010 in both shallow-soil upland and deep-soil lowland topographic positions in an annually burned, ungrazed watershed that is dominated by grasses with the C 4 photosynthetic pathway. In attempting to understand how the timing of climate variability affects grass productivity, we analyzed long-term records of precipitation, stream discharge, and air temperature to examine how variation in drought, precipitation intensity, and heat waves affect grass productivity at different times of the growing season. Results and DiscussionAcross 27 y, drought reduced grass productivity over a wide range of dates but had declining effects as the season progressed. ANPP G decreased with decreasing precipitation summed from April 15 to August 2 [day of year (DOY) 105-214] (Fig. 1). ANPP G declined 0.60 ± 0.12 g·m −2 for each millimeter decline in p...

Climate change is causing measurable changes in rainfall patterns, and will likely cause increases in extreme rainfall events, with uncertain implications for key processes in ecosystem function and carbon cycling. We examined how variation in rainfall total quantity (Q), the interval between rainfall events (I), and individual event size (S E ) affected soil water content (SWC) and three aspects of ecosystem function: leaf photosynthetic carbon gain (A CO2 ), aboveground net primary productivity (ANPP), and soil respiration (J CO2 ). We utilized rainout shelter-covered mesocosms (2.6 m 3 ) containing assemblages of tallgrass prairie grasses and forbs. These were hand watered with 16 I Â Q treatment combinations, using event sizes from 4 to 53 mm. Increasing Q by 250% (400-1000 mm yr À1 ) increased mean soil moisture and all three processes as expected, but only by 20-55% (P 0.004), suggesting diminishing returns in ecosystem function as Q increased. Increasing I (from 3 to 15 days between rainfall inputs) caused both positive (A CO2 ) and negative (J CO2 ) changes in ecosystem processes (20-70%, P 0.01), within and across levels of Q, indicating that I strongly influenced the effects of Q, and shifted the system towards increased net carbon uptake. Variation in S E at shorter I produced greater response in soil moisture and ecosystem processes than did variation in S E at longer I, suggesting greater stability in ecosystem function at longer I and a priming effect at shorter I. Significant differences in ANPP and J CO 2 between treatments differing in I and Q but sharing the same S E showed that the prevailing pattern of rainfall influenced the responses to a given event size. Grassland ecosystem responses to extreme rainfall patterns expected with climate change are, therefore, likely to be variable, depending on how I, Q, and S E combine, but will likely result in changes in ecosystem carbon cycling.

Precipitation quantity has been shown to influence grassland aboveground net primary productivity (ANPP) positively whereas experimental increases in of temporal variability in water availability commonly exhibit a negative relationship with ANPP. We evaluated long term ANPP datasets from the Konza Prairie Long Term Ecological Research (LTER) program (1984 -1999) to determine if similar relationships could be identified based on patterns of natural variability (magnitude and timing) in precipitation. ANPP data were analyzed from annually burned sites in native mesic grassland and productivity was partitioned into graminoid (principally C 4 grasses) and forb (C 3 herbaceous) components. Although growing season precipitation amount was the best single predictor of total and grass ANPP (r 2 =0.62), several measures of precipitation variability were also significantly and positively correlated with productivity, independent of precipitation amount. These included soil moisture variability, expressed as CV, for June (r 2 =0.45) and the mean change in soil moisture between weekly sampling periods in June and August (%wv) (r 2 =0.27 and 0.32). In contrast, no significant relationships were found between forb productivity and any of the precipitation variables (p>0.05). A multiple regression model combining precipitation amount and both measures of soil moisture variability substantially increased the fit with productivity (r 2 =0.82). These results were not entirely consistent with those of short-term manipulative experiments in the same grassland, however, because soil moisture variability was often positively, not negatively related to ANPP. Differences in results between long and short term experiments may be due to low variability in the historic precipitation record compared to that imposed experimentally as experimental levels of variability exceeded the natural variability of this dataset by a factor of two. Thus, forecasts of ecosystem responses to climate change (i.e. increased climatic variability), based on data constrained by natural and recent historical rainfall patterns may be inadequate for assessing climate change scenarios if precipitation variability in the future is expected to exceed current levels.