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Drought Impact on Phenology and Green Biomass Production of Alpine Mountain Forest—Case Study of South Tyrol 2001–2012 Inspected with MODIS Time Series

1
Institute of Geodesy, Graz University of Technology, Steyrergasse 30/I, 8010 Graz, Austria
2
Eurac Research, Institute for Earth Observation, Viale Druso 1, 39100 Bolzano, Italy
3
European Environment Agency (EEA), Kongens Nytorv 6, 1050 Copenhagen K, Denmark
*
Author to whom correspondence should be addressed.
Current address: Rezatec Ltd., Electron Building, Fermi Avenue, Harwell, Didcot, Oxfordshire OX11 0QR, UK.
Forests 2018, 9(2), 91; https://doi.org/10.3390/f9020091
Received: 12 January 2018 / Revised: 10 February 2018 / Accepted: 14 February 2018 / Published: 16 February 2018
(This article belongs to the Special Issue Remotely Sensing of Drought-Induced Forest Change and Recovery)
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. View Full-Text
Keywords: the Alps; drought; forest green biomass production; forest phenology; MODIS; Principal Component Analysis (PCA) the Alps; drought; forest green biomass production; forest phenology; MODIS; Principal Component Analysis (PCA)
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Lewińska, K.E.; Ivits, E.; Schardt, M.; Zebisch, M. Drought Impact on Phenology and Green Biomass Production of Alpine Mountain Forest—Case Study of South Tyrol 2001–2012 Inspected with MODIS Time Series. Forests 2018, 9, 91.

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