A 50th anniversary reassessment of the seminal 'L ana'i fog drip study' in Hawai'i

Abstract

During the 1950s, the Pineapple Research Institute (PRI) in Hawai'i launched one of the first, and still widely cited, quantitative studies of direct cloud water interception (CWI) and its contribution to throughfall (TF) ('fog drip') under Cook Pine trees (Araucaria columnaris) on a trade-wind exposed, mountain ridge on the island of Lāna'i. That study concluded that fog drip under a tree canopy supplemented rainfall (RF) (average 1270 mm year-1) by 762 mm or 60%, even after throughfall was corrected for presumed non-vertical rainfall also intercepted by the tree. A resurvey of fog drip in the upper Lāna'i watershed was undertaken in 2006-2008. Open-site automated weather stations were paired with throughfall, stemflow and soil moisture measurements for adjacent Araucaria trees. For upper ridge sites, these results indicate fog drip rates beneath individual Araucaria trees to be substantially greater than found in the earlier study. Fully exposed trees in the frequently cloud-immersed summit region collected annualized fog drip totals of 901-2883 mm, representing 181-462% of measured rainfall. The increased fog drip may be largely attributed to: (i) a more robust estimate of the indirect (non-vertical) rainfall contribution; (ii) the increased fog-capturing capacity of the Araucaria trees which had a height of ∼10 m in the 1950s versus ∼20 m during this study and (iii) the presence of atmospheric conditions favouring much increased CWI at higher ridge elevations (921-1021 m.a.s.l.) compared to the site of the original PRI study at 838 m. The fog drip values reported here are (much) higher than those measured in most other tropical montane cloud forest studies, and reflect the unique site conditions associated with fully exposed isolated trees on a knife-edged ridge. Copyright © 2010 John Wiley & Sons, Ltd.