Ecophysiological cloud-vegetation linkages in southern Appalachian mountain cloud forests at leaf to ecosystem scales
Cloud-forest (CF) ecosystems are characterized by a high frequency of cloud immersion. For many mountain CF, the altitude of cloud-fog bases co-occurs with conspicuous ecotones between CF- and non-CF vegetation communities. This suggests linkages between cloud-fog with vegetation physiology and ecosystem functioning. Few studies have provided mechanistic explanations of cloud-vegetation linkages in CF, and if these linkages are unique to CF species. Our Research Questions were: 1) How are CF tree physiology and ecosystem functions linked with clouds and cloud-fog; and 2) How do cloud-vegetation interactions differ between CF and non-cloud-forest (nCF) species? We measured leaf-level gas exchange, net ecosystem exchange, and plant water relations and hydraulic functioning during clear-sky and cloud-immersed sky-conditions, in both CF and nCF species.
Cloud-fog reduced leaf-level photosynthesis in all species as much as ~50% due to low sunlight intensity. On days with morning fog followed by clear-sky afternoons, afternoon photosynthesis was enhanced in CF species. Photosynthetic induction (time to maximum photosynthesis) was much faster in CF compared to nCF species during simulated breaks in cloud fog. Net ecosystem exchange of CO2 in CF was 3.5X greater on foggy compared to sunny days. Parameters derived from hydraulic-vulnerability curves suggest no differences in drought resistance among vegetation communities. P50 and Pe values were similar between CF and nCF species, and there were no clear differences in hydraulic safety margins between vegetation communities. Fog improved leaf water-potentials throughout the day in both CF and nCF species, but to a greater degree in CF. However, diurnal patterns of water potentials were significantly changed in CF species, in which we observed peak water potentials at midday (not predawn). This would enhance photosynthesis in these strongly isohydric species, and may explain greater NEE on foggy days. The strong stomatal regulation and daily loss of leaf hydraulic conductance we observed may also explain why minimum branch water potentials never approached P50. This redundancy in isolating the upstream hydraulic parts of the plants is perhaps an adaptive strategy that protects the plant during sunny periods, which can be quite desiccating at high elevations. Our data suggest that physiology of both CF and nCF tree species is strongly affected by clouds. CF species may have some adaptive traits that 1) promote carbon gain in these habitats characterized by frequently dark and/or highly-variable sunlight conditions; and 2) have strong regulatory mechanisms to prevent embolims in woody plant tissues.