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

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.

Spring Temperature and Snow Cover Climatology Drive the Advanced Springtime Phenology (1991–2014) in the European Alps

“…Our result is further in line with the advancing trend direction, albeit with different strength, reported by Vitasse et al (2018), where the average spring leaf-out dates significantly advanced by about 0.22 d yr −1 across all species and sites in the Swiss Alps over the period 1980-2016. However, this finding is in disagreement with reports from Xie et al (2017) and Lewinska et al (2018), where SOS showed no significant trends between 2003 and 2014 as well as between 2001 and 2012. However, the length of the observation period may strongly affect the strength and direction of a detected trend (Mudelsee, 2019).…”

Relationship between Spatiotemporal Variations of Climate, Snow Cover and Plant Phenology over the Alps—An Earth Observation-Based Analysis

“…Also, the expected influence of exposition on snow and vegetation phenology is not traceable in the data set. The fact that the investigated metrics did not vary depending on HLI might indicate that the used spatial resolution of 250 m is still too coarse to capture habitat differences that are created by small-scale topographic variations, a shortcoming that was also detected in a study using similar data [111]. As discussed by Comola et al [112], the effect of aspects become uncorrelated and orientation differences average out at larger scales.…”

Two-source energy balance modeling of evapotranspiration with thermal remote sensing at different spatial resolutions: a case study of the European Alps

“…Mountain ecosystems in the European Alps have been increasingly affected by drought events in recent years (Conedera et al, 2006;Fink et al, 2004;Gobiet et al, 2014;Lewińska et al, 2018). Due to changing climatic conditions, the Alps are vulnerable to deviations in water cycling, which is mainly observed in the context of natural vegetation and agricultural activities.…”