Despite the evidence of the effect of topography on forest systems at the landscape level, only a few reports are available regarding the impact of varied topography on physiology and tree growth within catchment areas of the mountain ecosystem. The topographic influence of topoclimate and catchment hydrology plays a significant role in shaping forest feedback to future climate conditions and shapes the structure and function of the forest. The studies to model the forest response to environmental alterations comprise landscape-level processes but generally exclude finer-scale dynamics, which may be similarly significant to the ecosystem functions of the forest. The present study revealed aspect-regulated finer-scale variations in leaf ecophysiology. During our study, these variations were evidenced by increased photosynthesis of Q. semecarpifolia, growing in higher light, warmer, and drier conditions of the south (S) aspect relative to the contrasting north (N) aspect of the same catchment, in the subalpine forest of the central Himalaya. In general, the seasonality and aspect-driven light environments both posed remarkable changes in the leaf ecophysiology (i.e., leaf gas exchange and plant water relation). The effect of variation in the light environment was more pronounced and affected the overall productivity and leaf ecophysiology of the plants more severely, seasonally as well as diurnally. It has also been realized throughout the study that photosynthesis and other leaf gas exchange parameters were highly variable and highly influenced by microclimatic environmental factors like air temperature, soil moisture, relative humidity, light quality, intensity, and duration. Therefore, while predicting the gross primary productivity, it needs to take more site-specific, time-specific (diurnal), and season-specific data to accurately and precisely account for forest productivity. Our study also suggests that brown oak interspecifically adapts by adjusting its physiological traits against several abiotic stresses to maintain its higher productivity.