Influence of slope position and aspect on the vegetation attributes and tree-water relations in forests of the central Himalayas

While the need for understanding the effects of topographical factors on forest structure and function is well recognized, comprehensive studies are scarce. This study evaluates the effect of slope aspect and slope position on water relations and forest attributes across six forest types occurring between 400 m and 2600 m altitude in the Central Himalaya (27°–38°N). We found that predawn tree water potential and soil water potential were generally higher on moist north slope aspect (−0.78±0.05 MPa and −3.34±0.18 MPa, respectively) than dry south slope aspect (−0.82°0.18 MPa and −3.77±0.18 MPa, respectively). Across six different forests, these values were higher at hill base (−0.71±0.06 MPa and −2.77±0.19 MPa, tree predawn water potential and soil water potential, respectively) than other topographical positions. The favorable effect of north aspect and hill base was also observed in maintaining soil water and tree water potential during the dry season. Vegetation attributes, such as species richness, unique species and plant density were also generally higher on north slope and hill base than southern aspect and lowest at hill top. Across forest types, the hill base provided shelter to 46 unique species, compared to 16–18 at the other positions, thus emphasizing its importance as refugia for species to survive climate change induced perturbations. The favorable conditions of hill base position not only contribute to increase in alpha diversity, but also to extended species distributional range.