Details

Autor(en) / Beteiligte

• Assal, Timothy J.
• Anderson, Patrick J.
• Sibold, Jason

Titel

Spatial and temporal trends of drought effects in a heterogeneous semi-arid forest ecosystem

Ist Teil von

• Forest ecology and management, 2016-04, Vol.365, p.137-151

Ort / Verlag

Elsevier B.V

Erscheinungsjahr

2016

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• •Forest dynamics between 1985 and 2012 were modeled using vegetation indices.•Normalized Difference Moisture Index was the best indicator of forest conditions.•Linear trend analysis used to identify areas of forest vulnerable to drought.•25% of area had significant negative trend, compared to only 10% with positive trend.•Plots with negative trends had lower live tree density and more standing dead trees. Drought has long been recognized as a driving mechanism in the forests of western North America and drought-induced mortality has been documented across genera in recent years. Given the frequency of these events are expected to increase in the future, understanding patterns of mortality and plant response to severe drought is important to resource managers. Drought can affect the functional, physiological, structural, and demographic properties of forest ecosystems. Remote sensing studies have documented changes in forest properties due to direct and indirect effects of drought; however, few studies have addressed this at local scales needed to characterize highly heterogeneous ecosystems in the forest-shrubland ecotone. We analyzed a 22-year Landsat time series (1985–2012) to determine changes in forest in an area that experienced a relatively dry decade punctuated by two years of extreme drought. We assessed the relationship between several vegetation indices and field measured characteristics (e.g. plant area index and canopy gap fraction) and applied these indices to trend analysis to uncover the location, direction and timing of change. Finally, we assessed the interaction of climate and topography by forest functional type. The Normalized Difference Moisture Index (NDMI), a measure of canopy water content, had the strongest correlation with short-term field measures of plant area index (R2=0.64) and canopy gap fraction (R2=0.65). Over the entire time period, 25% of the forested area experienced a significant (p-value<0.05) negative trend in NDMI, compared to less than 10% in a positive trend. Coniferous forests were more likely to be associated with a negative NDMI trend than deciduous forest. Forests on southern aspects were least likely to exhibit a negative trend while north aspects were most prevalent. Field plots with a negative trend had a lower live density, and higher amounts of standing dead and down trees compared to plots with no trend. Our analysis identifies spatially explicit patterns of long-term trends anchored with ground based evidence to highlight areas of forest that are resistant, persistent or vulnerable to severe drought. The results provide a long-term perspective for the resource management of this area and can be applied to similar ecosystems throughout western North America.