smithlabbanner

Implications of altered disturbance regimes for grasslands: a comparative approach

In ecology, we strive to gain a more general understanding of the determinants of patterns and processes in natural systems, as well as those impacted by human activities. Such understanding is critical for predicting future responses of systems to global environmental changes. Two particularly valuable ways in which we can successfully increase generality are (1) through formal synthesis of existing and often disparate knowledge and (2) through the formulation and testing of general principles (or rules) that underpin and create pattern (Knapp et al. 2004). In my research, I have used both. For example, I in collaboration with a number of colleagues have used synthesis to gain a more general understanding of variation in productivity among biomes in response to precipitation (Knapp and Smith 2001, Huxman, Smith et al. 2004), determinants of diversity-productivity relationships (Chalcraft et al. 2004, Grace et al. 2007), and bottom-up and top-down controls of ecosystems (Cebrian et al. 2009, Hillebrand et al. 2009).

With funding from NSF and the Mellon Foundation, I am gaining a more general understanding of the impacts of altered disturbance regimes on grassland ecosystems by deriving predictions from studies of NA grasslands and testing them in SA (Knapp et al. 2004). We know that the distribution, structure and function of mesic grasslands are a product of the interaction of fire, grazing by large herbivores, and extreme climatic fluctuations in NA. However, it has been argued that fire and grazing influence grassland structure and function differently in SA vs. NA. These differences have been attributed to the contingent factors of longer evolutionary history with fire and grazing, reduced soil fertility, and greater diversity of plants and large herbivores in SA. An alternative hypothesis is that differences in methodologies used to study these systems, and bias in the number of studies in NA, has led to differing perspectives on the role of these disturbances. If the impacts of shared disturbances truly differ between NA and SA, this calls into question the generality of our understanding of these ecosystems and our ability to forecast how altered disturbance regimes will affect grasslands globally.

In 2005, I and collaborators (Alan Knapp, CSU; Scott Collins, UNM; John Blair, KSU) initiated a large-scale, comparative study to determine the degree of convergence in ecosystem (productivity, N and C cycling) and herbaceous community responses (composition, diversity, dynamics) to fire and grazing in grasslands in SA and NA. This research takes advantage of a unique research platform consisting of three long-term (20-50+ yr) experimental manipulations of fire and grazing, two in SA and one in NA. We are conducting a suite of studies to address hypotheses related to: 1) the role of long-term fire regimes (without large herbivores), 2) the importance of grazing and fire-grazing interactions, and 3) the impacts of reductions in large herbivore diversity. Results thus far suggest convergence in ecosystem responses as a function of fire frequency and N manipulations (Buis et al. in press) but divergence in herbaceous plant community responses to fire and grazing (Burns et al. 2009) via differential re-ordering of extant species abundances as predicted by the HRF. We are now testing potential mechanisms underlying divergence in community-level responses by assessing behavioral responses of large herbivores to fire (Burns et al. in prep) and examining phylogenetic relatedness and functional traits of plant species in NA and SA. For the latter, we plan to quantify phylogenetic relatedness and conduct detailed analysis of functional traits among all grass species in NA and SA. If differences in phylogenetic relatedness and resultant species pools and traits give rise to divergence in community responses to fire and grazing, then these studies should highlight the importance of integrating knowledge of evolutionary history and plant functional traits in studies aimed at gaining a general understanding of community and ecosystem responses to global environmental changes.

Other Current Initiatives