Source: Dissertation Abstracts International, Volume: 75-05(E), Section: B.

Adviser: Melinda D. Smith.

Biotic factors, such as plant community composition, trait expression, and trophic interactions, mediate the response of ecosystem functions to alterations in abiotic factors. Here I examine the role of these biotic factors in determining the response of aboveground net primary productivity (ANPP), an ecologically and economically important ecosystem function, to alterations in precipitation, nutrient availability, and space/light at three sites spanning the broad productivity gradient of the U.S. Central Great Plains.

I first show how changes in the plant community with alterations in nutrient availability and seasonal precipitation affect ANPP. I show that across the precipitation gradient of the Great Plains, the abundance of a dominant species, Schizachyrium scoparium, and plant species richness drive the majority of the variation in ANPP. On evolutionary timescales, species abundances and plant species richness are thought to be driven by long-term climate conditions across this precipitation gradient. These communities must then respond to shifts in environmental conditions, such as precipitation and nutrient availability, on ecological timescales. As a result, differences in plant species abundances and richness best predict variation in ANPP by incorporating both the indirect effects of precipitation variability in structuring the plant community as well as the direct effects of specific characteristics of the biota, such as maximum growth rates, height, and/or water use efficiency, that determine ANPP (i.e., height of short grass species will always be less than that of tall grasses, irrespective of precipitation amount). By combining information regarding both the evolutionary history and current physiological responses of species, metrics describing differences in the plant community confer greater power to predict ANPP over abiotic factors alone. In the same vein, I show that the physiological response of the dominant species drove the majority of the production response to experimental nutrient additions across the precipitation gradient, supporting the idea that understanding species abundances and physiological responses to altered environmental conditions is key to predicting ANPP.

I expand upon the effect of resource availability on ANPP through changes in plant trait expression and community composition by determining the response of plant functional traits to chronic alterations in nutrient availability. Across the precipitation gradient studied here, plant species that increase in abundance with chronic nutrient additions had functional trait values associated with faster growth and exhibited more plasticity in their trait expression when exposed to elevated nutrient conditions than plant species that dominate under ambient environmental conditions. This difference in trait expression may underlie the community changes driven by chronic nutrient additions. However, I found that variation in trait expression at the species-level was not enough to drive changes in community aggregate trait expression; rather, species turnover was necessary before changes in trait expression were observed at the community level.

Finally, I examine the role of trophic interactions in determining the ANPP response to alterations in resource availability. I show that vertebrate herbivores tend to promote the growth of forb species, while invertebrate herbivores tend to suppress their growth. In systems where the plant canopy is closed, these effects of vertebrate and invertebrate herbivores on grass:forb ratios interacted with the effects of nutrient availability, resulting in the promotion of grasses with the combined addition of nutrients and removal of vertebrate herbivores and the promotion of forbs with the combined addition of nutrients and the removal of invertebrate herbivores. In contrast, in systems where the plant canopy is open, nutrient additions promote the growth of forbs regardless of herbivore presence.

Invertebrate herbivores not only interact with nutrient additions to affect the plant community, but also are affected by shifts in plant quantity and quality. I show that nutrient additions indirectly increased invertebrate herbivore abundances by increasing plant quantity at the mesic end of the grassland precipitation gradient studied here. However, the increase in plant quality with chronic nitrogen additions resulted in a decrease in the per capita rate of herbivory (selective feeding). The changes observed in invertebrate abundances and feeding behavior with nutrient additions at the mesic end of the precipitation gradient did not have consequences for ANPP due to the trade-off between an increase in abundance of invertebrate herbivores with a decrease in their per capita rate of herbivory resulted in no change in the amount of leaf tissue removed. In contrast, at the xeric end of the precipitation gradient, where leaf tissue C:N is lower relative to the other sites studied, nitrogen and phosphorus additions led to compensatory feeding by chewing herbivores, perhaps because herbivores are limited by another resource, such as carbohydrates, at this site.

Humans are currently having a large impact on abiotic factors worldwide, which can have important consequences for ecosystem function. Here, I show how plant species composition, trait expression, and trophic interactions act to determine the effect of alterations in resource availability in grasslands. The results of my dissertation constitute an important step toward improving our ability to predict how global change will impact grassland ecosystem function in the future.