Consistent response to topographical variation in net assimilation rate across the central Himalayan broadleaved forests

In a mountainous landscape, vegetation patterns are influenced by the interaction of slope aspect and topographical positions. Here, we have examined the effect of two contrasting aspects (North and South) and three slope positions (Hill Base, Mid Slope and Hill Top) on net assimilation rate (NAR) of trees across four forest sites of the monsoonal Central Himalaya between 400 and 2300 m elevations. The study species, namely Shorea robusta, Quercus leucotrichophora, and Q. floribunda were broadleaved evergreen with concentrated summer leaf drop and simultaneous leafing. The NAR of these broadleaved species was measured in five seasons (pre-monsoon, monsoon, autumn, winter and spring) diurnally at their respective forest sites. In each of the forest site the dominant species was the same, on both aspects and the three slope positions. We also analyzed topographical associated effect of tree water relations on NAR, using the data of an earlier study on these species. The average NAR (μ mol m−2 s−1) across the four forest sites and five seasons, (measured diurnally) was higher on the north (5.08 ± 0.23) slope aspect than south aspect (4.61 ± 0.22; average of 120 NAR values for each aspect), and among the three slope positions, it was higher on hill base (5.13 ± 0.29), than on mid-slope (4.80 ± 0.27) and hill top (4.61 ± 0.26; average of 80 NAR values for each position). These patterns in NAR values in relation to slope aspects and slope positions were also found separately for each of the species, and each of the seasons. The NAR varied across the sites, seasons and day time from 2.84 to 11.99 μ mol m−2 s−1, largely because of the variation in tree water potential and leaf phenological changes. To conclude, the NAR shows a consistency in responses to differences in slope aspects and slope positions across the forests ranging from tropical to warm temperate type. The study would help generalizing topographical effects on tree growth.