Forest structural and microclimatic patterns along an elevational gradient in Mount Kenya

Abstract

Tropical mountain forests are important biodiversity hotspots, which host disproportionally high number of endemic species. However, the potential impacts of climate change in these areas are uncertain. A key factor contributing to this knowledge gap is that climatic conditions experienced by organisms inside tropical forests (i.e., microclimate) remain largely understudied. Due to the effects of topography and vegetation, the understory microclimate can differ substantially from free-air conditions (i.e., macroclimate). This study aimed at unveiling vegetation structural characteristics and microclimatic patterns along an elevational gradient in a highly diverse tropical mountain ecosystem (Mount Kenya), by combining hundreds of terrestrial laser scanning measurements with a two-year time-series of microclimate observations. Our results showed that macroclimate temperature and elevation contributed >90 % to the microclimate variability in our study area. The influence of vegetation and soil moisture in regulating temperature differed substantially between day and night, as well as in different periods of the year. The contribution of vegetation to microclimate variation during the day was two times higher than that during the night. Soil moisture had a cooling effect on microclimate temperature during daytime, while the opposite pattern was observed at night. These differences affected lapse rates, which showed a clearly seasonal fluctuation during diurnal periods but a relatively stable pattern in night periods. The diurnal temperature range was regulated by combined effects of vegetation structure, elevation, and soil moisture. Finally, we were able to detect subtle changes in forest structure caused by historical selective logging. These changes resulted in a legacy effect on microclimate, thus demonstrating that human-induced disturbances have long-lasting impacts on tropical mountain ecosystems. These results improve our understanding of the climatic conditions experienced by organisms inside highly diverse African mountain forests, as well as how these conditions are regulated by vegetation structure and environmental factors.