Native forest trees are often claimed to be conservative water users relative to faster-growing non-native tree species. If true, this argument could increase the value of native forest preservation and restoration relative to other forested land uses. We compared an old-growth native forest to a 50-70 year old non-native timber plantation in Hōnaunau, Hawai‘i to determine if native and non-native tree species and stands differ in water use. From August through September 2006, we measured plant transpiration using standard xylem sap flow methods, focusing on Hawaiian endemic dominant tree Metrosideros polymorpha, non-native timber trees Eucalyptus saligna and Fraxinus uhdei, and dominant endemic understory Cibotium glaucum tree ferns. We also estimated stand water use by scaling up these measurements using values of stand sapwood area and tree fern leaf area obtained through vegetation surveys.

Results/Conclusions

M. polymorpha had the lowest rates of whole-tree daily water use at 8 kg day^-1 (200kg m^-2_sapwood day^-1), less than half that of E. saligna or F. uhdei. M. polymorpha also had the lowest maximum transpiration rates of the three tree species. Considering contribution from trees only, stands dominated by F. uhdei had the highest water use at 1.6 mm day^-1, over twice that of E. saligna and over eight times that of M. polymorpha stands. This was due to F. uhdei’s high sap flow rates and over four-fold greater allocation of stand basal area to sapwood. When understory tree ferns were considered, high frond area contributed to over 70% of water use in native M. polymorpha stands, indicating ferns play a critical role in water balance for this forest type. Overall, F. uhdei stands still had highest water use (1.8 mm day^-1). Stand water use was influenced strongly by species-specific water use, canopy and understory species composition, stem density, tree sizes, and sapwood allocation. These findings have implications for forestry species selection and native forest protection and, more broadly, highlight the importance of constituent species and forest structure to the ecohydrology of different forest types.