Cloud forest ecosystems are characterized by a high frequency of cloud immersion (cloud fog below tree canopy with vegetation enshrouded in fog), with light levels often sub-optimal for photosynthesis. Thus, cloud forests are generally considered to be light-limited ecosystems. However, very few studies have directly quantified possible light-limitations to photosynthetic carbon gain at leaf and/or ecosystem scales in these systems. Our Research Questions were: 1) How does photosynthetic gas exchange vary with light levels in tree species from a temperate mountain cloud forest, compared to tree species from an adjacent non-cloud-forest? 2) Does photosynthesis appear to be limited by light across a growing season, assuming no moisture limitations? 3) Are there differences in photosynthetic response at different organizational scales (leaf vs ecosystem) and/or in canopy (overstory) vs. understory trees? We conducted light response curves, including photosynthetic induction from dark conditions, and modeled photosynthesis across a growing season using solar irradiance data. We also compared net ecosystem exchange across sunny and foggy conditions.
Results/Conclusions
Parameters calculated from light response curves were generally similar among all species, such as saturating light intensity, light compensation point, etc. However, the quantum yield of photosynthesis was consistently greater in cloud forest species compared to non-cloud-forest species, and photosynthetic induction from dark was much more rapid in cloud forest species. In cloud forest species, maximum photosynthesis was achieved <20 minutes, while for non-cloud-forest species the time to maximum photosynthesis was typically >40 minutes. This highlights a possible adaptive strategy of cloud forest species in which they more robust to low and frequently variable light levels. Modeled across a growing season, leaf-level photosynthesis was greater in cloud-forest species compared to non-cloud-forest species. Net ecosystem exchange (canopy-scale photosynthesis) in the cloud forest was 3.5X greater on foggy compared to sunny days. Collectively, our data support the hypothesis that cloud forests are light-limited environments, that favor (possibly filter for) evergreenness and longer growing season lengths for acceptable whole-plant carbon gain & balance.