Globally, terrestrial vegetation communities can be seen as existing along a conceptual continuum of woody plant cover, where the amount and distribution of woody plants represent both broad-scale categorizations of vegetation communities and general trends of functional ecosystem attributes such as net primary productivity and ecohydrology. General trends in woody plant cover can also be seen both within and across vegetation communities along elevation gradients. Such ecotone-dense gradients can be highly sensitive to environmental change, making them ideal for examining indicators of bioclimatic relationships such as vegetation phenology and productivity. Analysis of such vegetation dynamics along elevation gradients would provide the greatest insights when considering both specific and general vegetation characteristics. However, studies on landscape-scale vegetation dynamics along elevation gradients have generally not incorporated woody plant cover, even though it is an important moderator of these relationships. Our objective was to analyze remotely sensed vegetation phenology and productivity along a Sky Island elevation gradient in southwestern USA to assess the importance of woody plants in explaining landscape-scale vegetation dynamics. We used a vegetation index measured via satellite from 2000 to 2008 to quantify landscape-scale vegetation phenology and productivity, and compared these metrics with measures of woody plant canopy cover, vegetation community and elevation.
Results/Conclusions
Metrics quantifying vegetation dynamics were strongly related to interactions among elevation, woody plant cover and vegetation community. At lower elevations woody plant cover was more important in explaining the magnitude of the growing season, while at higher elevations woody plant cover was more strongly related to the dormant season and the transitions between dormancy and growth, indicating the varying effects of woody plants with differing climate constraints. Vegetation dynamics metrics for areas with high community diversity displayed less dependence on woody plant cover. Our results highlight the functional role of woody plants in landscape-scale bioclimatic interactions. Rather than relying on elevation as a proxy for abiotic influences on vegetation, and in addition to focusing research around specific vegetation communities, analyses incorporating woody plant cover can provide deeper insights into the consequences and complexities of vegetation response and ecosystem stability with environmental variation.
