Idiosyncratic Photosynthetic Traits in a Montane Subcanopy Tree Species Under Low-Light Microclimates Reveal Microclimatic Acclimation Trade-Offs

Montane forests exhibit complex, heterogeneous microclimatic regimes that challenge the physiological plasticity of canopy and subcanopy species. In a Himalayan subalpine forest, we investigated how topographically mediated light environments shape the expression of photosynthetic traits in co-dominant trees — Quercus semecarpifolia (canopy) and Rhododendron arboreum (subcanopy)—across north- and south-facing slopes. Using leaf-level gas exchange measurements, we identified consistent patterns of light acclimation in Q. semecarpifolia and high-light-adapted R. arboreum. However, shade-acclimated R. arboreum individuals on north-facing slopes displayed idiosyncratic physiological signatures, including positive dark respiration rates (Rd) and negative light compensation points (LCP) — magnitudes theoretically implausible under standard C₃ photosynthetic models.

These anomalies suggest either (i) physiological re-fixation of respired CO₂ under low light, (ii) non-linear error propagation in light response curve (LRC) fitting at extremely low PPFD, or (iii) extreme photoprotective plasticity unique to shade-adapted subcanopy species. Unlike Q. semecarpifolia and light-acclimated R. arboreum on south-facing slopes, these north-facing subcanopy individuals maintained high NPQ under minimal photon flux, indicative of disproportionate energy dissipation mechanisms.

Our findings highlight how fine-scale microclimatic heterogeneity, especially in shaded montane niches, can generate unexpected and complex trait responses that deviate from established photosynthetic theory. These results necessitate refined methodological protocols and physiological models to interpret trait dynamics in low-light, high-humidity microclimates, particularly in the context of rising canopy temperatures and climate extremes. The study offers critical insights into species-specific limitations and compensations under montane microclimatic stress, with implications for predicting forest carbon cycling and resilience under future climate scenarios.