Micrometeorological aspects of a tropical mountain forest

Photosynthetically active radiation, air temperature, humidity and wind speed were registered along a vertical profile within and above a steeply sloped lower tropical montane rain forest in southern Ecuador. The upper canopy layer accounted for the absorption of more than 90% of incident photosynthetic photon flux density (PPFD); on an average only 5.5% reached the forest floor. Forest floor PPFD was modelled for the whole plot using hemispherical images, analysed with the Software HemiView 2.1. Model performance could be enhanced considerably when taking the narrowed horizon and a PPFD-adjusted atmospheric extinction coefficient into account. However, the simulation revealed high temporal variability of light conditions at the forest floor; modelled PPFD transmission ranged from 5.5% to 10.5% on an average and so was higher than measured values. Daytime temperature and water vapour gradients within the forest were weak, and the understory stratum appeared not to be decoupled from the atmospheric conditions above the forest. Even if methodology was insufficient with respect to quantification of turbulent structures, the measured gradients and the low wind deceleration within the forest indicates an efficient turbulent mixing of the stand air volume during typical daytime conditions. The importance of regional wind characteristics became evident under the influence of exceptionally strong, persistent, and dry mountain winds resulting in steeper aero-dynamic gradients throughout the measured profile. Thus, compared to level sites, forest meteorology of sloped mountain forests must be considered of higher complexity, due to the topography-influenced light conditions and the impact of complex local wind and turbulence patterns on the mountainous landscape.