Homogenization in forest performance across an environmental gradient - The interplay between rainfall and topographic aspect

This study aimed to investigate the interaction between local and regional environmental factors that affect forest performance during drought periods. In previous studies, contradictory results regarding the effect of aspect on forests performance, under different settings, were reported. However, each study focused on a different forest ecosystem at a different time frame, making synthesis inadequate. Monoculture planted Pinus halepensis forests in Israel, covering a broad climatic gradient (200-850. mm annual rainfall), form a suitable study system to address this question.We used remote sensing and GIS methods to observe a large number of afforested stands over a wide area at high resolution. Normalized Difference Vegetation Index (NDVI), obtained from Landsat satellite images for 14. years between 1994 and 2011, served as an inclusive measure of forest performance. Data on the examined environmental factors were obtained from spatially interpolated annual rainfall maps and a topographic aspects map. The effects of aspect on NDVI were evaluated separately for three regions along the rainfall gradient: arid (200-350. mm), intermediate (350-500. mm), and humid (500-850. mm).During the studied period, NDVI declined in the arid region but remained constant in the intermediate and humid regions. NDVI was positively related to annual rainfall in all three regions. The effect of aspect on NDVI was positively associated with rainfall in the arid region, but not in the intermediate and humid regions. In other words, forest performance homogenization across local habitats occurred in the arid region under drought stress. Relatively wet years were characterized by high NDVI values (~0.4), with large differences (~0.025) between northern and southern aspects, whereas dry years were characterized by low NDVI values (~0.3) and small differences (~0.01).The present study supports the concept that under severe drought stress forest performance becomes more homogeneous across local habitats, both temporally (in drought years) and spatially (towards the arid forest boundary). Performance homogenization may occur when low soil water levels are reached, and climatic conditions become the dominant limiting factor. When water availability is high enough, differential performance responses among local habitats are maintained. We evaluated the trends and relations among local and regional environmental factors on performance, and assessed their relative effect sizes. Such an evaluation is essential to link local and global studies aimed at predicting the fate of forests facing global climate change.