Episodic herbivory, plant density dependence, and stimulation of aboveground plant production

Integrating defense and leaf economic spectrum traits in a tropical savanna plant

Introduction

Allocation to plant defense traits likely depends on resource supply, herbivory, and other plant functional traits such as the leaf economic spectrum (LES) traits. Yet, attempts to integrate defense and resource acquisitive traits remain elusive.

Methods

We assessed intraspecific covariation between different defense and LES traits in a widely distributed tropical savanna herb, Solanum incanum, a unique model species for studying allocations to physical, chemical, and structural defenses to mammalian herbivory.

Results

We found that in a multivariate trait space, the structural defenses - lignin and cellulose - were positively related to the resource conservative traits - low SLA and low leaf N. Phenolic content, a chemical defense, was positively associated with resource acquisitive traits - high SLA and high leaf N - while also being associated with an independent third component axis. Both principal components 1 and 3 were not associated with resource supply and herbivory intensity. In contrast, spine density - a physical defense - was orthogonal to the LES axis and positively associated with soil P and herbivory intensity.

Discussion

These results suggest a hypothesized "pyramid" of trade-offs in allocation to defense along the LES and herbivory intensity axes. Therefore, future attempts to integrate defense traits with the broader plant functional trait framework, such as the LES, needs a multifaceted approach that accounts for unique influences of resource acquisitive traits and herbivory risk.

Density-dependent plant growth drives grazer stimulation of aboveground net primary production in Yellowstone grasslands

The mechanisms by which grazing animals influence aboveground net primary production (ANPP) in grasslands have long been an area of active research. The prevailing wisdom is that grazing can increase ANPP by increasing the availability of growth-limiting resources such as nitrogen and water, but recent theory suggests that the density-dependent growth of grassland vegetation can lead to grazer-stimulation of ANPP simply by removing shoot biomass and increasing relative growth rate (RGR). We compared the relative roles of resource availability and density-dependent growth in driving positive responses of ANPP to grazing in Yellowstone National Park. We measured the effects of clipping (50% simulated grazing intensity) and natural grazing on soil nitrogen availability, soil moisture, and shoot growth over 2 months in two grassland plant communities (mesic and dry) grazed primarily by bison. Clipping increased RGR by over 100% in both grassland types but had no effect on N availability or soil moisture during the same growth periods. Clipping stimulated ANPP only at mesic grassland, and the magnitude of this effect was strongly related to the initial plant biomass at the time of clipping relative to estimated peak biomass, supporting the density-dependent framework. Bison grazing had qualitatively similar effects on ANPP and RGR to clipping with no accompanying effects on N availability or soil moisture. Our results show how grazing can stimulate ANPP independent of a direct influence on resource availability simply by exploiting the dynamics of density-dependent plant growth.