Orographic gradients in climate and forest cover at the Cordillera Yanachaga, Peru

The Cordillera Yanachaga is a semi-isolated Andean range protruding into the Peruvian Amazon that houses an important area of montane cloud forests on both windward and leeward slopes. Despite the importance of these forests for biodiversity and the provision of ecosystem services for nearby populations, their orographic variation in climate and forest ecology had not been previously described. Climatic and forest parameters were studied along an orographic gradient consisting of three sites, a windward slope forest at 2400 masl, a ridge forest on the mountain pass at 2800 masl and a leeward slope forest at 2400 masl. Common climatic measurements and visibility were recorded from canopy towers within 1-ha vegetation sampling plots, from which environmental data and taxonomic, foliage and structural characters of all stems ‚Äöv¢‚Ä¢5 cm DBH were collected. Despite its orographic location, the leeward forest received marginally less rainfall than the ridge forest and considerably more rainfall that the windward forest. The temperature variation found was attributed to altitude and an afternoon Foehn effect, while the orographic variation in PPFD was very strongly correlated to fog frequency. The ridge and windward forests showed higher canopy fog immersion (c. 75%) and a higher frequency of simultaneous rain and fog events, while the leeward forest showed less fog immersion (c. 20%) and higher rain frequency. The ridge and windward forests were affected principally by easterly air masses, while the leeward forest showed signs of localized phenomena originating from the Oxapampa valley to the west. The leeward forest displayed more climatic variation and larger parameter ranges, which were reflected in greater species richness, basal area, canopy height, foliage area and leaf size. Floristic associations within plots reflected sheltered and exposed regions. Forests at all the sites had stem densities and basal areas at the lower end of those recorded in other regions. At the leeward site, light and moderate fog events generally displayed diurnal temperatures and PPFD more similar to clear sky events than to rain events, reflecting the warm clear upper atmosphere conditions under which they form. Dense fog events tended to mimic microclimatic conditions during rainfall events, albeit with higher PPFD. In addition to the very strong correlation between fog frequency and PPFD, PPFD also correlated very strongly with total arboreal foliage area, suggesting as possible relationship through limitations on the development canopy substrata. While the mechanism for such a relationship remains unclear, the observations contribute to the existing theory that the effect of fog frequency on light conditions is one of the major drivers of variation in tropical montane forest productivity.