Abstract
Ecological balance and biodiversity of the alpine forest is endangered by global and
local climatic extremes. It spurs a need for comprehensive forest monitoring, including in depth
analyses of drought impact on the alpine woodland ecosystems. Addressing an arising knowledge
gap, we identified and analyzed 2002–2012 aridity related responses within the alpine mountain
forest of South Tyrol. The study exploited a S-mode PCA (Principal Component Analysis) based
synergy between meteorological conditions rendered by the scPDSI (self-calibrated Palmer Drought
Severity Index) and forest status approximated through MODIS (Moderate Resolution Imaging
Spectroradiometer) derived NDVI (Normalized Difference Vegetation Index) and NDII7 (Normalized
Difference Infrared Index based on MODIS band 7) time series. Besides characterizing predominant
forest temporal response to drought, we identified corresponding spatial footprints of drought
impact, as well as examined aridity-related changes in forest phenology and biomass production.
The latter was further evaluated in relation to forest type, elevation, aspect and slope. Recognized
meteorological conditions highlighted: prolonged 2003–2007 mild to extreme drought, and overall
regional drying tendencies. Arising remotely sensed forest responses accounted on localized decline
in foliage water content and/or photosynthetic activity, but also indicated regions where forest
condition improved despite the meteorological stress. Perceived variability in the forest response to
drought conditions was governed by geographic location, species structure, elevation and exposition,
and featured complexity of the alpine forest ecosystem. Among the inspected biophysical factors
elevation had the strongest influence on forest phenology and green biomass production under
meteorological stress conditions. Stands growing above 1400 m a.s.l. demonstrated initial increase in
annual biomass growth at the beginning of the dry spell in 2003. Conversely, woodlands at lower
altitudes comprising considerable share of hardwood species were more prone to biomass decline in
2003, but experienced an overall upturn in biomass production during the following years of the dry
spell. Aspect showed moderate effect on drought-related phenology and green biomass production
responses. Diverse forest ecosystem responses identified in this study were in line with known local
and regional analyses, but also shed some new light on drought induced alternation of forest status.