Forest drought-induced diversity of Hyrcanian individual-tree mortality affected by meteorological and hydrological droughts by analyzing moderate resolution imaging spectroradiometer products and spatial autoregressive models over northeast Iran

This study sought to assess the spatial variations of physiological responses of Hyrcanian forests to the hazard intensity of meteorological and hydrological droughts for properly assessing drought-induced tree mortality in northeastern Iran. A variety of time series moderate resolution imaging spectroradiometer (MODIS) products and ground-based measurements were applied to derive the multiple dimensions of droughts and forest stresses. Drought hazard intensity was computed with the combination of the severity, frequency and duration of drought dimensions for each variable. The intensity of tree mortality was calculated by Simpson’s diversity index with surveying 30,000 individuals of commercial species suspected to dieback within 100 intact parcels. Spatial autoregressive models were carried out to determine significant meteorological and hydrological drivers that controlling biological responses of forests to drought events and associations of the diversity of tree mortality with these forest responses. Results showed that the hazard intensity of forest water-content-deficit and greenness loss showed higher relationships with the high land surface temperatures and actual evapotranspiration than the precipitation and surface water deficits, however, they did not show significant relationships with the groundwater deficit. Moreover, diversity of tree mortality was associated with forest water-content-deficit from moderate to death stages and with forest greenness loss in the only very high defoliation stage. The critical values of forest droughts and diversity of mortality were recorded for the climax tree species. Understanding satellite-derived physiological responses of forests to droughts might help to assess the intensity of tree mortality widely to adopt appropriate strategies for mitigating the impacts of droughts on the tree species.

Tree mortality; Forest drought; Meteorological; Hydrological; MODIS; Hyrcanian forests; Spatial autoregressive models