Linking fog climatology and the impact of cloud immersion on transpiration of Pinus patula within a seasonal tropical montane cloud forest environment

Background/Question/Methods

Increasingly warmer, drier and variable climates are projected for many tropical mountains, and this is likely to reduce cloud immersion (or “fog”) typical of tropical montane cloud forests. These changes may in turn affect their ecohydrology and related ecosystem services that they provide. Fog can reduce transpiration via a reduction in radiation and atmospheric evaporative demand, and via the wetting of the leaf surfaces. However, the relative importance of these processes in suppressing transpiration, and thus the likelihood of water fluxes to be affected by future changes in cloud immersion, will depend on the site’s fog climatology (i.e. frequency, timing, duration, intensity). We characterized the fog-related processes that suppress tree transpiration in relation to local fog climatology; assessed the transpiration suppression effect of fog relative to both sunny and overcast conditions; and, examined the implications of decreasing fog occurrence on the transpiration of a young Pinus patula plantation (a common type of reforestation within the uplands of central Veracruz, Mexico). We used a combination of sap flow and micrometeorological observations during two contrasting dry seasons (Nov 2008-Apr 2009 and Nov 2009-Apr 2010) and a wet season (May-Oct 2009), and performed analyses at different time-scales.

Results/Conclusions

Whole-tree sap flow for days during which trees were immersed in fog for most of the day (> 6 h) indicated that daily transpiration was highly suppressed as compared to sunny conditions (> 60%), and that this effect was greater (>85%) when the canopy was wetted by drizzle and fog deposition. However, days with near 100% fog occurrence are not common in this cloud forest environment and most fog events are of relatively short duration (55% of daytime fog events lasted < 3 h).  During these short and intermittent events, the canopy usually remained partially dry, so that the suppression of transpiration was mainly through reductions in radiation and vapor pressure deficit (VPD); a mechanism comparable to that under overcast conditions, which are likely to replace foggy conditions in this region due to a lift in the cloud condensation level. In addition, fog events occurred more frequently in the late afternoon when transpiration was already declining. As a result, the overall impact of fog-induced suppression on stand transpiration was minimal at monthly time scales.