Does plant diversity buffer the effect of increased precipitation variability on forage production? How do compensatory dynamics vary over regional gradients in precipitation and precipitation variability?
Climate change is not only causing increases in temperatures, but impacting the global hydrological cycle. The frequency of extreme years is increasing. Rain is also falling in fewer events, which translates into longer droughts and bigger storms. Ecologists are only beginning to address how these increases in climate variability are impacting diversity patterns in space and time. Long-term data sets provide a record of how successful species have been over a range of years, allowing us to gauge how well different species “track” climate conditions. We have long known that climate variability is a driver of interannual variability in species composition, but we are only beginning to appreciate how this variability allows different species to coexist on the same landscape because they can take advantage of different years. We are also only beginning to identify the species assemblages that might be most sensitive to increases in climate variability. Long-term data sets of climate and composition can address questions such as: How does climate variability affect species interactions? To what extent does climate variability promote coexistence? Which ecosystems might be most affected by increases in climate variability?
Stability of primary production along a gradient of precipitation variability
Conservation and other land use needs have spurred quests to identify the factors that contribute to the temporal stability of ecosystem functions such as primary production. Because overall primary production is an aggregation of the primary production of individual species, it is stabilized when individual populations exhibit little fluctuation. Regardless of the stability of individual populations, however, temporally stable primary production can be achieved when populations co-vary negatively, when they exhibit “compensatory dynamics”(Gonzalez and Loreau 2009). We are using long-term data sets from grasslands to explore how the importance of these two mechanisms of community stability ― population stability and negative covariance ― vary along a gradient of precipitation variability. We hypothesize that in less variable environments, the same species typically remain good competitors over time (Hillebrand et al. 2008) and that primary production at these sites would be maintained by the stable presence of a few dominant species. In more variable environments, however, we predict that different species might flourish in different years, and negative species covariance might be a more important stabilizing mechanism (Yachi and Loreau 2001).
Sensitivity of grassland community composition to temporal variation in precipitation
As part of an LTER working group, we are asking how composition, richness, and turnover in grassland plant communities respond to interannual variability in mean annual precipitation. Precipitation is strongly correlated with species richness along latitudinal gradients both regionally (Richerson & Lum 1980; O’Brien 1993; Adler & Levine 2007) and globally (Hawkins et al. 2003; Kreft & Jetz 2007), but we do not know whether richness also increases with increasing rainfall variability. Some evidence suggests that species richness may be more sensitive to precipitation variability in desert ecosystems, where water is limiting. We predict that differences in species life history traits along bioclimatic gradients may influence the sensitivity of species richness to environmental change (Eriksson 1993; Gough et al. 1994; Zobel 1997). For instance, species richness in mesic grasslands may be buffered to interannual precipitation variability because of dominance by long-lived, bud-banking species (Benson and Hartnett 2006), compared to arid systems that contain a higher proportion of seed banking annual species (Aronson & Shmida 1992, Angert et al. 2009)