Kagawa Aurora, Sack Lawren, Duarte Ka'eo, James Shelley
University of Hawai'i at Mănoa, Botany Department, Honolulu, Hawaii 96822, USA.
Ecol Appl. 2009 Sep;19(6):1429-43. doi: 10.1890/08-1704.1.
Tropical forests are becoming increasingly alien-dominated through the establishment of timber plantations and secondary forests. Despite widespread recognition that afforestation results in increased evapotranspiration and lower catchment yields, little is known of the impacts of timber plantations on water balance relative to native forest. Native forest trees have been claimed to use water conservatively and enhance groundwater recharge relative to faster-growing alien species, and this argument should motivate native forest preservation and restoration. However, data have been available primarily for leaf-level gas exchange rather than for whole-plant and stand levels. We measured sap flow of dominant tree and tree fern species over eight weeks in native Metrosideros polymorpha forest and adjacent alien timber plantations on the island of Hawai'i and estimated total stand transpiration. Metrosideros polymorpha had the lowest values of sap flux density and whole-tree water use (200 kg m(-2) sapwood d(-1), or 8 kg/d for trees of 35 cm mean diameter at breast height, D), substantially less than timber species Eucalyptus saligna or Fraxinus uhdei (33 and 34 kg/d for trees of 73 and 30 cm mean D, respectively). At the stand level, E. saligna and F. uhdei trees had three- and ninefold higher water use, respectively, than native M. polymorpha trees. Understory Cibotium tree ferns were most abundant in M. polymorpha-dominated forest where they accounted for 70% of water use. Overall, F. uhdei plantation had the highest water use at 1.8 mm/d, more than twice that of either E. saligna plantation or M. polymorpha forest. Forest water use was influenced by species composition, stem density, tree size, sapwood allocation, and understory contributions. Transpiration varied strongly among forest types even within the same wet tropical climate, and in this case, native forest had strikingly conservative water use. Comparisons of vegetation cover in water use should provide additional resolution to ecosystem valuation and land management decisions.
通过建立人工林场和次生林，热带森林正日益被外来物种主导。尽管人们普遍认识到造林会导致蒸散量增加和集水区产量降低，但相对于原生森林，人工林场对水平衡的影响却知之甚少。相对于生长较快的外来物种，原生森林树木被认为用水较为保守，并能增强地下水补给，这一观点应能推动原生森林的保护和恢复。然而，相关数据主要来自叶片水平的气体交换，而非整株植物和林分水平。我们在夏威夷岛的原生多花铁心木森林和相邻的外来人工林场中，对优势树种和树蕨物种的液流进行了为期八周的测量，并估算了林分总蒸腾量。多花铁心木的液流通量密度和整树用水量最低（边材200千克·米⁻²·天⁻¹，对于胸径平均为35厘米的树木，即8千克/天），远低于木材树种柳叶桉或乌德水青冈（对于平均胸径为73厘米和30厘米的树木，分别为33千克/天和34千克/天）。在林分水平上，柳叶桉和乌德水青冈树木的用水量分别分别分别多花铁心木原生树木分别高出三倍和九倍。林下金毛狗蕨在多花铁心木占主导的森林中最为丰富，其用水量占比达70%。总体而言，乌德水青冈人工林场的用水量最高，为1.8毫米/天，是柳叶桉人工林场或多花铁心木森林用水量的两倍多。森林用水量受物种组成、树干密度（注：原文为stem density，结合上下文推测此处可能是树干密度之意）、树木大小、边材分配和林下植物贡献的影响。即使在相同的湿润热带气候条件下，不同森林类型的蒸腾作用差异也很大，在这种情况下，原生森林的用水极为保守。用水方面植被覆盖情况的比较应为生态系统评估和土地管理决策提供更多依据。 （注：原文中“stem density”表述似乎不太常规，可能影响翻译准确性，这里按推测翻译为树干密度，供参考）
