Vegetation dynamics at the woodland-shrubland interface: Role of climate, disturbance, and species interactions
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The boundary between woodlands and shrublands delineates the distribution of the tree biome in many regions across the globe. Woodlands and shrublands interface at multiple spatial scales, and many ecological processes operate at different spatial scales to determine the position of the woodland-shrubland boundary. The overall objective of this dissertation was to examine processes affecting vegetation dynamics at the woodland-shrubland interface in the western United States, at spatial scales ranging from biomes to individual plants. In Chapter 1, I examined broad-scale drivers of the position of lower treeline in the Intermountain West, using vegetation classifications derived from remote sensing imagery. I found that pinyon-juniper woodlands are broadly limited by water balance and will likely be sensitive to climate change, but that lower treelines at more northern latitudes are functionally constrained by permanent landscape features, land use and disturbance. In Chapters 2 and 3, I characterized post-fire plant community trajectories in an experimental network of prescribed fire treatments at the interface between pinyon-juniper woodlands and sagebrush shrublands in the Great Basin. I found that plant community responses to burning were strongly stratified along gradients of elevation and pre-fire tree cover, and that resistance to fire-induced invasion of the annual grass Bromus tectorum (cheatgrass) was low on sites that were relatively warm and dry (lower elevations) and on sites that lacked perennial understory species before burning (high pre-fire tree cover). Seeding perennial species after burning decreased invasibility in sites with low resistance, increasing perennial cover while reducing the abundance of invasive plants. In Chapter 4, I used field experiments to examine the interaction between Artemisia tridentata (big sagebrush) and Pinus monophylla (singleleaf pinyon pine), the dominant species within respective shrubland and woodland types that interface over broad environmental gradients in the Great Basin. I found that the effect of A. tridentata on P. monophylla shifted from strongly positive (facilitative) toward neutral after the vegetative transition from juvenile to adult foliage in P. monophylla. The timing of the ontogenetic shift did not vary across an elevational gradient, suggesting that this shrub-tree interaction may be relatively insensitive to increasing temperatures. Taken together, this research illustrates the dynamic link between woodland and shrubland ecosystems, and suggests that future dynamics at their interface will be complex and scale dependent.