Herbivore Effects on Plant and Nitrogen Dynamics in Oak Savanna

Abstract

Herbivores can often control plant dynamics by mediating positive feedbacks in plant species’ influence on nutrient cycling. In a 7‐yr field experiment in a nitrogen‐limited Minnesota oak savanna, we tested whether herbivores accelerated or decelerated nitrogen (N) cycling through their effects on plants. We measured effects of excluding insect (primarily Orthoptera and Homoptera) and mammalian herbivores (primarily white‐tailed deer, Odocoileus virginianus) on above‐ and belowground plant biomass, plant species composition, plant tissue nitrogen concentration, available soil N and light, and total N and carbon (C) in different pools (soil, roots, litter, etc.). Herbivore exclusion strongly increased cover and biomass of the legume Lathyrus venosus and a few species of woody plants. These effects were associated with greater aboveground standing crop, reduced belowground standing crop, and reduced light penetration to the ground surface. Herbivore exclusion also modified N cycling through greater N content of live, aboveground plant tissue early in the growing season and of litter and belowground tissue late in the growing season. Herbivore exclusion also increased soil nitrate and total available N concentrations but did not alter total soil or plant N. Our results support the hypothesis that herbivores indirectly decelerate N cycling by decreasing the abundance of plant species with nitrogen‐rich tissues. However, other factors, such as disturbance from fire, may mediate herbivore effects on long‐term changes in N and C pools. Herbivores may therefore indirectly control productivity, N cycling, and succession by consuming nitrogen‐fixing and woody plants that have strong effects on plant resources (e.g., nitrogen and light).

Nitrogen dynamics in an Alaskan salt marsh following spring use by geese

“…For example, changes in litter quality due to grazing can lead to increased (Sirotnak and Huntly 2000) or decreased (Pastor et al 1993;Ritchie et al 1998;Sirotnak and Huntly 2000) net nitrogen mineralization rates or nitrogen availability, but we found no difference in litter C:N ratios between grazed and exclosed plots. We also found no difference in belowground biomass between grazed and exclosed plots .…”

“…This results in higher quality litter (lower C:N ratio) for decomposition, higher quality soil organic matter, and higher rates of net nitrogen mineralization in grazed areas (Sirotnak and Huntly 2000). Conversely, selective grazing on palatable plant species can reduce their abundance and give a competitive advantage to less palatable (e.g., higher C:N ratio) species, resulting in more recalcitrant litter in grazed areas, and in lower rates of net nitrogen mineralization or in reduced nitrogen availability (Pastor et al 1988;Pastor et al 1993;Ritchie et al 1998;Sirotnak and Huntly 2000).…”

Stimulation of soil nitrification and denitrification by grazing in grasslands: do changes in plant species composition matter?

“…Grazer-induced decreases in C/N ratios of litter, roots and soil organic matter have been reported (Coppock et al 1983;Seastedt et al 1988;Day and Detling 1990;Holland et al 1992;Sirotnak and Huntly 2000;Johnson and Matchett 2001). Concurrently, grazerinduced decreases in microbial immobilisation and increases in N mineralisation have generally been observed (Ruess and McNaughton 1987;Holland and Detling 1990;Molvar et al 1993;Ruess and Seagle 1994;Shariff et al 1994;Frank and Groffman 1998a;Tracy and Frank 1998;Johnson and Matchett 2001;Zacheis et al 2002; but see Ritchie et al 1998). Such changes generally result in improved N availability to plants in grazed sites (Risser and Parton 1982;Holland and Detling 1990;McNaughton et al 1997;Hamilton and Frank 2001).…”

Immobilizing nitrogen to control plant invasion

“…Unfortunately, grazing is more likely to achieve this in sites that are already relatively N limited than in highly N-enriched ecosystems where lowering N availability might be most useful for invasion control (Augustine and McNaughton 1998;Bardgett and Wardle 2003). In less productive, nutrientlimited ecosystems, such as the Minnesota sandplain and Texas tallgrass prairie, selective grazing or browsing on more palatable species favors less palatable species (Brown and Stuth 1993;Anderson and Briske 1995;Ritchie et al 1998;Sirotnak and Huntly 2000), which often have higher C:N tissue (e.g., Pérez-Harguindeguy et al 2003) and slower decomposition and N mineralization rates (Grime et al 1996;Cornelissen et al 1999;Bardgett and Wardle 2003;Pastor et al 2006). Slower N mineralization in turn favors less palatable species and other species with high tissue C:N ratios, further slowing N mineralization (Hobbie 1992;Wardle et al 2004).…”

“…Current working hypotheses suggest that grazing may be more likely to favor low-N species and increase N immobilization in ecosystems where N, rather than water or light, is the primary limiting resource (Ritchie et al 1998) or where tissue N of the desired, low-N species is <1.5% (Pastor et al 2006). Thus, in ecosystems that are already N limited grazing may be a useful tool for maintaining low N mineralization rates and low N availability, but in highly N-enriched ecosystems grazing may be more likely to increase N mineralization than to reduce it.…”