Sky Islands as barometers of change: Phenology and disturbance along woody plant gradients

Background/Question/Methods

Much of the Earth’s vegetation communities can be seen as existing along a continuum of woody plant cover; grasslands, shrublands and forests are examples of these. The degree of woody cover at a site or landscape can influence nutrient and hydrological cycles as well as phenological variability and overall productivity. Woody cover continuums are exemplified over elevation gradients such as the Sky Islands of southwestern USA; along these gradients, biogeography is compressed and sensitivity to disturbances like drought and wildfire is high. Methods are needed to detect and quantify vegetation change along such gradients. Numerous studies have highlighted trends in vegetation and associated ecosystem properties along vegetation gradients such as Sky Islands. Additionally, many remote sensing-based studies have assessed site- to global-scale vegetation phenology, often using greenness metrics such as the Normalized Difference Vegetation Index. However, studies have rarely addressed interactions between woody cover and phenology in the context of disturbance. Here we present a new approach for evaluating remotely sensed metrics describing vegetation phenology and productivity and percent woody cover for Sky Islands, focusing on the Santa Rita Mountains in southeastern Arizona. We assessed how these characteristics varied along the elevation gradient and were altered by wildfire and severe drought.

Results/Conclusions

Changes in vegetation phenology and productivity across the elevation gradient were related to both drought and wildfire, showing shifts in seasonality and magnitude of greenness as related to these disturbances, as might be predicted. These patterns further related across the gradient of percent woody cover, suggesting interactive effects among the amount of woody canopy cover and the vegetation patterns and processes leading up to and consequent to fire and drought. Our developing approach contributes potential insights into the productivity and phenology of disturbance-affected vegetation along woody-herbaceous plant gradients in general, and in particular, steep, sensitive elevation gradients that may serve as important barometers of vegetation responses to climate and other drivers.