The role of animals in stabilizing ecosystem process under changing climate regimes

Background/Question/Methods

Changes in climate often present ecosystems with unique challenges, ranging from non-perceptible, such as adjustments in metabolic rates, to profound, such as migrants arriving significantly earlier in the spring than normally anticipated.  Both the non-perceptible and profound can affect ecosystem processes, altering net rates of primary production through direct and indirect changes in community structure and function.  These complicated relationships are more easily interpreted through research conducted at LTER sites, where long-term data on biotic and abiotic conditions is available.  Our work at the Hubbard Brook LTER led us to investigate these complex relationships in light of changing winter conditions and altered snow pack regimes.  We specifically investigated how snow depth and soil freezing will influence activity of large herbivores and soil invertebrates, and how this activity feeds back to decomposition and nutrient dynamics.  We have used short-term experimentally manipulated plots, a natural climate gradient and long-term weather data at Hubbard Brook to evaluate how changes in winter climate will influence ecosystem processes in this northern hardwood forest. 

Results/Conclusions

Experimental removal of snow to induce soil freezing and observations along a natural frost gradient at Hubbard Brook, coupled with animal activity produced surprising results. First, changes in snow depth led to altered browsing behavior in moose, where balsam fir was more heavily browsed under low snow conditions compared to sugar maple or viburnum shrubs. Second, plants that were mechanically browsed to simulate moose feeding triggered differential plant response and nutrient uptake, where moose fecal nitrogen was more available to balsam fir than sugar maple or viburnum shrubs, while biomass production was reduced. Decomposition rates were also slowed or reduced under soil freezing conditions.  Soil invertebrates, important regulators of decomposition and nutrient cycling, showed higher diversity in sites where winter frost depths were shallow, compared to sites with deeper frost.  Total abundances of soil invertebrates were not different between shallow and deeper frost sites. There was a potential link to changes in mineralization and nitrification rates, where sites with high freezing had lower nematode abundances and sites with lower freezing had higher nematode abundances while both types of sites had no measurable differences in microbial biomass.  Nematodes may be an important regulator of N cycling rates in soils exposed to soil freezing via reductions in bacterial feeding nematodes. These results indicate complex relationships between organisms, ecosystem processes and changing climate and animals may modify a systems response to altered winter climate regimes.