Slope Environmental Lapse Rates and Fog on Oahu’s Highest Mountain

Abstract:

Environmental gradients are important determinants of ecosystem distribution on tropical mountains, particularly with respect to the distribution of tropical montane cloud forest which is dependent on fog presence. These ecosystems are vulnerable to potential shifts in environmental gradients and the lifting condensation level resulting from global change. Mt. Kaʻala, the highest mountain on the island of Oahu at 1227 m, harbors important refugia for endemic species in its vertically distributed ecosystems, especially in the cloud forest at the summit region, but information on slope environmental gradients and fog presence is lacking. A transect of 9 temperature and 3 relative humidity stations were established on the windward slope between 290 m and 1147 m to determine the near-surface slope environmental lapse rates at 1.5 m (SELR). Pictures from a trail camera in the summit cloud forest were used to identify relative humidity threshold for fog presence. The threshold and the slope relationship between relative humidity were used to predict the likely presence and elevation of fog on the mountain. Daily SELR calculated over a 1-year period using the slope of the linear regression between mean temperature and elevation. Results were compared to water collected by a Juvik-style fog gage and rainfall from a meteorological station in the summit cloud forest. Both temperature and relative humidity were strongly related to elevation (R2=<0.99) although the linear prediction indicated a lower overall SELR of 7.2°C/1000m. Mean SELR was 8.3°C/1000m and ranged from 5.5°C/1000m to 10.3°C. Lower SELR tended to occur on days when rainfall and fog were recorded at the summit. Mean relative humidity increase at 16.6%/1000 m and ranged from 82% at 290 m to 96% at 1147 m, indicative of frequent fog presence on the upper slopes. Mountain fog was predicted on 176 days between 636 m and 1225 m. Predicted fog was most common at higher elevations but tended to be lower on days with a lower SELR. The results of this study provide a baseline estimate of near surface environmental gradients on the mountain and potential fog presence and elevation that can serve to inform species management within protected areas, help detect future shifts in environmental gradients, and supplement current ecohydrological studies being conducted on the mountain.