Forest performance during two consecutive drought periods: Diverging long-term trends and short-term responses along a climatic gradient

Forest decline, attributed to increased aridity under global climate change, has been observed with rising frequency worldwide. One of the knowledge gaps making its spatially explicit prediction difficult is the identification of the climatic settings that generate a significant change in the forest state. A relatively rare sequence of unfavourable climatic events - a short extreme drought followed by a prolonged moderate drought within one decade - has allowed us to examine how rainfall amount affects forest performance. Large-scale monitoring, at high spatial and temporal resolutions, is required to study climatic effects on forest performance. Therefore, time-series of spatially interpolated rainfall maps, remote sensing images and tree growth data were used to estimate the environmental settings to which the forests are exposed, and the corresponding forest performance responses. Performance was estimated from Normalized Difference Vegetation Index (NDVI) values obtained from 32 Landsat satellite images for 1994-2011. To widen the study perspective we sampled forest performance along a rainfall gradient (250-750. mm) in the planted Pinus halepensis forests in Israel. Performance response was not spatially homogeneous. Three response types could be identified along the rainfall gradient: stable performance with low correlation to rainfall pattern in the humid region (>500. mm), moderate performance decline with high correlation to rainfall in the intermediate region (350-500. mm), and steep performance decline with intermediate correlation to rainfall in the arid region (<350. mm). The response to the second drought differed among regions, unlike the response to the first drought, which was homogeneous. The observed diverging performance trend along the climatic gradient can be attributed to the varied importance of water availability as a limiting factor. The reduced effect of rainfall on performance deviations, the steep performance decline, and the difference between responses to the first and second droughts at the most arid locations, imply to higher importance of multi-annual accumulated and carried-over drought stress effects at these locations.