Print PageWoody plant encroachment - the increase in woody plant cover in savannas and in former grasslands - is a common phenomenon world-wide. In some systems, if encroachment is not controlled or reversed it converts grasslands to savannas and savannas to woodlands. As encroachment proceeds, community and ecosystem properties change, often with undesirable economic or conservation impacts. While many of these impacts have been studied rather extensively, the dynamics of encroachment have been neglected and a constant rate of encroachment is often implicitly assumed.
We used sequential aerial photographs taken over a 40-year period to analyze the dynamics of woody plant encroachment (primarily by Juniperus ashei) in savannas of the eastern Edwards Plateau of central Texas. We constructed algebraic models based on a set of hypotheses about the encroachment process and used data from the aerial photographs to parameterize them and AIC to identify the best models. We used the results to test our hypotheses about the dynamics of encroachment and to guide the construction of a simple cellular automata model of encroachment.
Results/Conclusions
Algebraic models that included both the formation of new foci and the spread of existing patches of woody plants gave better predictions of changes in woody cover than models of either process separately, indicating the importance of both processes in this system. Models that included explicit or implicit density-dependence in changes in woody cover were better than models that did not. The rate of encroachment was negative density-dependent, slowing as cover increased. Inclusion of information about the spatial configuration of woody patches, specifically the amount of edge (summed length of woody-herbaceous joins), improved the models. Surprisingly, although both soil type and slope detectably affected encroachment rate their effects were quite small: encroachment proceeded at about the same rate on flats as on steep hillsides, and on several types of soil. The cellular automata model results highlight the importance of the relative magnitudes of new foci formation and patch spread. Depending on the relative rates of these two fundamental processes, results that resemble the spatial configurations of a range of natural communities, including these central Texas savannas, are obtainable. They also demonstrate fundamental similarities between woody plant encroachment and many other ecological processes, including old field succession to forest and the spatial and temporal dynamics of some invasions of non-native plants.
