Throughfall characteristics in three non-native Hawaiian forest stands

The effect of non-native trees on hydrological processes in Hawaiian forests remains largely unquantified. In this study, throughfall was determined by stationary methods over 18–23-month study periods at three locations each dominated by one of three non-native tree species including Schinus terebinthifolius, Coffea arabica, and Psidium cattleianum. The lowest mean throughfall percentage of 44% occurred under a monotypic stand of P. cattleianum. Mean throughfall under canopies dominated by C. arabica and S. terebinthifolius were 59 and 60%, respectively. Annualized throughfall rates were computed as 62, 60, and 45% under canopies dominated by S. terebinthifolius, C. arabica, and P. cattleianum, respectively. The low mean throughfall at each location is likely due to high wet-canopy evaporation combined with frequent low-intensity/low magnitude rainfall. The exceptionally low throughfall under P. cattleianum may be the result of larger amounts of intercepted rainfall being diverted to stemflow and/or trunk storage. Observations suggest that stemflow is a substantial component of the canopy water balance in stands of P. cattleianum. Plant area index (PAI) was the lowest (3.66) under P. cattleianum and highest (4.64) under C. arabica. Mean direct throughfall coefficient p was computed from individual storm events at each location and ranged from a low of 0.16 under the canopy dominated by P. cattleianum to a high of 0.31 under the canopy dominated by S. terebinthifolius. A similarly computed mean canopy storage capacity S varied from 2.5 mm under primarily S. terebinthifolius to 4.0 mm at the location dominated by C. arabica.