Tomoo Okayasu

Indicator species and functional groups as predictors of proximity to ecological thresholds in Mongolian rangelands

We focused on responses to grazing by individual species and functional groups in relation to ecological thresholds in Mongolian rangelands, with repeated measures from the same ecological sites to account for rainfall variability. At all sites, even under rainfall fluctuations, there were robust combinations of indicator species that could be used to forewarn managers to take action to minimize the probability of crossing ecological thresholds. Depending on the landscape condition of each site, the cover of functional groups, which shared traits of perennial life history, grass or forb growth form, linear leaf shape, and alternate leaf attachment, or the cover of functional groups of woody shrubs dramatically decreased before an ecological threshold was crossed. Thus, across all sites, the responses of certain functional groups to grazing appeared to predict the crossing of an ecological threshold. The ecological indicators derived in this study should help to improve land managers’ ability to prevent adverse changes in states before ecological thresholds are reached.

Can edaphic factors demonstrate landscape-scale differences in vegetation responses to grazing?

We focused on land units as landscape characteristics and selected seven typical land units on a land catena comprising two areas of southern Mongolia. Hierarchical analysis was used to test the hypothesis that a land unit’s edaphic factors could explain the differences in vegetation responses to grazing. We established the survey sites at increasing distances from a livestock camp or water point within each land unit, then analysed patterns of change in floristic and functional compositions, vegetation volume and soil properties within each land unit to reveal differences in vegetation responses to grazing. We also examined the variations in floristic and functional compositions across land units to identify the edaphic factors that may underlie these differences. Changes in vegetation and soil properties at increasing distances from a camp or water point within each land unit were into three different patterns. Ordination techniques consistently indicated that land unit groups categorised using edaphic factors corresponded to those categorised using response patterns. Our study revealed that edaphic factors were responsible for the observed landscape-scale differences in vegetation responses to grazing in the study areas. In addition, the mechanisms underlying vegetation responses to grazing may have been primarily determined by edaphic factors.

Vegetation in a Post-Ecological Threshold State May Not Recover after Short-Term Livestock Exclusion in Mongolian Rangelands

In the present study, we tested the potential irreversibility of vegetation dynamics in Mongolian rangelands using well-studied plant communities that exist along grazing gradients, in which ecological thresholds (defined as the points or zones at which disturbance should be limited to prevent drastic changes in ecological conditions) exist in terms of the compositional changes along these gradients. To accomplish this, we removed livestock grazing impacts by establishing exclosures along a grazing gradient at two study sites located in Mandalgobi and Bulgan, Mongolia. Each exclosure was established in the summer of 2004 at a location with either a post-ecological threshold state or a pre-ecological threshold state. We examined general patterns of temporal change in vegetation for the permanent plots inside and outside each exclosure at each site between 2005 and 2010. The trajectories of floristic composition in the permanent plots outside and inside each exclosure were similar from 2005 to 2010, indicating that the trajectories were mainly associated with annual rainfall and annual phenological changes in the plant communities. Post-threshold states at both sites did not reach their respective target community for restoration, indicating the lack of restorability despite livestock exclusion. Moreover, ordination separated the trajectories of floristic composition for the permanent plots inside exclosure in the post-threshold state from those in the pre-threshold state. Thus, our results suggest that vegetation in a post-threshold state may not recover after short-term livestock exclusion in Mongolian rangelands.

Threshold Distinctions Between Equilibrium and Nonequilibrium Pastoral Systems Along a Continuous Climatic Gradient

Abstract In equilibrium environments where rainfall is relatively stable, grazing animal–vegetation dynamics are density-dependent; it is therefore appropriate to calculate carrying capacities and use them to define sustainable animal populations. In contrast, nonequilibrium environments are characterized by fluctuations in characteristics such as rainfall, resulting in fluctuations in plant biomass and in the corresponding carrying capacity. Herders adapt to such environments by moving opportunistically to pastures with better conditions. Studies since the 1990s have significantly improved our understanding of the continuity and integration of equilibrium and nonequilibrium systems. However, it remains unclear how and where such continuous, integrated rangeland systems result in qualitatively different land use patterns by local herders along a climatic gradient. Here, we developed a simple model that uses key environmental factors to predict a threshold representing the boundary between equilibrium and nonequilibrium land use systems, and we used an area of Mongolian rangeland as an example. We found a threshold in the proportion of usable pasture that corresponded to a specific range of rainfall values. Comparison of our results with previous ones supported our hypothesis about this threshold. The threshold behavior suggested that it is important to identify and monitor the boundary between equilibrium and nonequilibrium land use systems so that managers can respond to climatic change. National governments and aid agencies must understand the threshold process before they can identify focal areas where management regime change is required and propose appropriate policies that will support herders in the long term. Our study provides a simple, low-cost tool to evaluate ecosystems in this context.

Degraded rangeland dominated by unpalatable forbs exhibits large-scale spatial heterogeneity

The spatial heterogeneity of vegetation and soil increases in response to land degradation caused by grazing mainly at a large spatial scale. This increase has been frequently associated with shrub invasion, but shrub invasion does not necessarily accompany land degradation. Instead, dominance by unpalatable forbs has been reported in some regions, but the spatial heterogeneity of such degraded rangeland has not been studied. We investigated the spatial heterogeneity of rangeland dominated by unpalatable forbs at a large spatial scale using Mongolian rangeland as an example. Spatial heterogeneity of the total vegetation cover and community heterogeneity were analyzed for three levels of land degradation. We found that the least-degraded site had homogeneous total vegetation cover and community, that the site with intermediate degradation exhibited low heterogeneity of the total vegetation cover but significant community type variation, and that the most degrade sites exhibited a periodic pattern of total vegetation cover as a result of a mixture of dense and sparse patches of unpalatable forbs. These different responses can be used to assess land degradation levels and may have potential to monitor land degradation at a large scale by satellite images.