Transpiration and microclimate of a tropical montane rain forest, southern Ecuador

Resumen

Climatic measurements were made within and above a tropical lower montane (cloud) forest at 1975 m.a.s.l. in the southern Ecuadorian Andes to assess micro-meteorological conditions and calculate overall forest transpiration (using the Penman–Monteith model). Transpiration by individual trees was determined by means of sapflow measurements (Granier-type gages), supplemented by porometric measurements of leaf gas exchange. The light environment of the forest was characterized by high spatio-temporal variability. Modeled forest-floor photosynthetic photon flux density (PPFD) varied on average between 5.5% and 10.5% of incident radiation. Thermo-hygric gradients within the forest were weak, and the understory stratum was aerodynamically well coupled to the atmosphere above the forest, suggesting efficient turbulent mixing of the forest air. Daily sapflow totals ranged from 2–165 l day−1 and increased dramatically with tree height, trunk diameter, and crown dominance. Stand-scaled sapflow Ec was modeled based on stand structural parameters in a two-layer approach. The contribution of sub-canopy and understorey individuals to total stand transpiration was only about 20%, illustrating the dominance of upper canopy trees in the process. Penman–Monteith-based estimates of forest transpiration Ea showed a high energy efficiency (average Ea ~ 70% of net radiation), but due to low solar inputs, Ea remained low at 560 mm year−1 (representing 26.4% of the annual precipitation of 2120 mm during the measurement period). Stand-scaled sapflow Ec accounted for 85% to more than 90% of daily Ea and amounted to 1.8 mm day−1 on average (range: 0.7–2.8 mm day−1) during the drier season.