Immediate and lagged responses to an anomalously warm year on intact tallgrass prairie ecosystems in the EcoCELLs: Impacts on plant community, functional group and species aboveground net primary productivity and associated ecological and environmental controls.

摘要

Global climate change can potentially increase the frequency and intensity of anomalously warm years. Changes in grassland ecosystem productivity resulting from climate anomalies can significantly alter atmospheric CO2 levels. The effects of climate change are defined by plant productivity responses, so it is essential to quantify these in order to understand the underlying mechanisms. Interannual climate variability has been shown to have a major impact on community aboveground net primary productivity (ANPP) in the field, as well as on ANPP of functional groups and individual species, but in the field it is impossible to distinguish between the effects of temperature and precipitation, which we were able to do in our setup.;The objective of this review was to evaluate the literature on the factors that control grassland productivity, including literature specifically on tallgrass prairie ANPP, and the ecological mechanisms and drivers that modulate ANPP in intact grassland ecosystems. The primary drivers of variation in ANPP include precipitation amount and distribution, changes in atmospheric temperature, fire, and species richness/diversity. The primary driver of changes in ANPP in most of the papers included in this review involved water availability in one way or another, either in the form of seasonal and annual precipitation or overall soil moisture. Water was a driver in both direct (precipitation) and indirect (soil moisture) ways, and this was particularly true in more arid grasslands. Warming had an overall negative effect on ANPP in grassland communities, but also had a variety of effects including extending the growing season, and causing changes in soil moisture levels. Effects of fire were both direct and indirect. Frequent burning decreased the detritus layer, which led to decreases in soil moisture, both of which can have negative effects on ANPP in grasslands. In addition, frequent burning can decrease soil inorganic N availability which can also negatively affect ANPP in grasslands. Species and functional group richness and diversity were less correlated with grassland ANPP. Although some studies have found positive relationships, the effects of richness and diversity were variable, with positive responses being seen when removal of functional groups stimulated growth, and positive interactions between legumes and non-legumes leading to higher and more diverse levels of biomass. These effects of increases and decreases in richness were clearly variable, so it is difficult to draw general conclusions on the effects of richness/diversity patterns on ANPP. So, although many drivers affect grassland ANPP, the primary drivers of ANPP in most of the papers we found was water, in the form of seasonal and annual precipitation or overall soil moisture.;Our experiment was designed to quantify immediate and lagged responses of plant community aboveground net primary productivity (ANPP) as well as functional group and species ANPP to an anomalously warm year. In this study, twelve intact soil monoliths were excavated from a tallgrass prairie site and divided among four EcoCELLs. During the first year the monoliths were subjected to average air temperature and precipitation. During the second year, the air temperature in two EcoCELLs was increased by 4°C to simulate an anomalously warm year. Temperatures were returned to average for the third and fourth years of the study. All biomass above 10 cm was harvested every fall and used to determine ANPP.;During the warm year, total ANPP was seen to decrease significantly in the treated EcoCELLs, due to a significant drop in the C4 graminoid species present (graminoid contribution decreased from 87% to 31% in warmed monoliths). No significant ANPP response was seen from forb or nitrogen fixing species. Interannual changes in community and graminoid ANPP correlated with interannual changes in net ecosystem productivity (NEP), with no correlation seen between NEP and forb or nitrogen fixer ANPP. Little significant response was seen at a species level, but species richness of nitrogen fixers specifically did have a positive effect on community ANPP. Soil moisture and vapor pressure deficit (VPD) were compared to community and functional group ANPP with shallow (0-30 cm) soil moisture and showed positive correlations with whole-community ANPP and nitrogen fixers. Water content of the entire (0-120 cm) soil profile only showed a positive relationship with forb ANPP. Increasing VPD was seen to have a negative correlation with community ANPP but there was no significant treatment effect visible.;These responses were most likely due to decreased soil moisture availability caused by warming and were comparable to the effects seen in tallgrass prairie ecosystems in years with reduced precipitation. Results of this study indicate that soil moisture is a key factor in plant functional group response, as graminoid species responded so negatively when compared to forb species. Data from this experiment also suggest that overall species richness does not strongly affect community ANPP but the presence of nitrogen fixing species is positively correlated with community ANPP even when no significant change in nitrogen fixing species richness or abundance is observed. These effects can potentially be significant in the future as frequency and intensity of anomalously warm years increase.