SCOTT ML, SHAFROTH PB, AUBLE GT
United States Geological Survey, Midcontinent Ecological Science Center 4512 McMurry Avenue Fort Collins, Colorado 80525-3400, USA
Environ Manage. 1999 Apr;23(3):347-358. doi: 10.1007/s002679900191.
/ Human demands for surface and shallow alluvial groundwater have contributed to the loss, fragmentation, and simplification of riparian ecosystems. Populus species typically dominate riparian ecosystems throughout arid and semiarid regions of North American and efforts to minimize loss of riparian Populus requires an integrated understanding of the role of surface and groundwater dynamics in the establishment of new, and maintenance of existing, stands. In a controlled, whole-stand field experiment, we quantified responses of Populus morphology, growth, and mortality to water stress resulting from sustained water table decline following in-channel sand mining along an ephemeral sandbed stream in eastern Colorado, USA. We measured live crown volume, radial stem growth, annual branch increment, and mortality of 689 live Populus deltoides subsp. monilifera stems over four years in conjunction with localized water table declines. Measurements began one year prior to mining and included trees in both affected and unaffected areas. Populus demonstrated a threshold response to water table declines in medium alluvial sands; sustained declines >/=1 m produced leaf desiccation and branch dieback within three weeks and significant declines in live crown volume, stem growth, and 88% mortality over a three-year period. Declines in live crown volume proved to be a significant leading indicator of mortality in the following year. A logistic regression of tree survival probability against the prior year's live crown volume was significant (-2 log likelihood = 270, chi2 with 1 df = 232, P < 0.0001) and trees with absolute declines in live crown volume of >/=30 during one year had survival probabilities <0.5 in the following year. In contrast, more gradual water table declines of thick similar0.5 m had no measurable effect on mortality, stem growth, or live crown volume and produced significant declines only in annual branch growth increments. Developing quantitative information on the timing and extent of morphological responses and mortality of Populus to the rate, depth, and duration of water table declines can assist in the design of management prescriptions to minimize impacts of alluvial groundwater depletion on existing riparian Populus forests.
人类对地表水和浅层冲积地下水的需求导致了河岸生态系统的丧失、破碎化和简化。在北美干旱和半干旱地区，杨树物种通常在河岸生态系统中占主导地位，要尽量减少河岸杨树的损失，就需要综合了解地表水和地下水动态在新林分建立和现有林分维持中的作用。在美国科罗拉多州东部一条季节性沙床溪流中，由于河道内采砂导致地下水位持续下降，我们在一个可控的全林分田间试验中，量化了杨树形态、生长和死亡率对水分胁迫的响应。我们测量了689株活的毛果杨亚种念珠杨茎干的活冠体积、径向茎干生长、年枝增量和死亡率，为期四年，并结合局部地下水位下降情况进行观测。测量在采砂前一年开始，涵盖了受影响和未受影响区域的树木。杨树对中等冲积沙地地下水位下降表现出阈值响应；持续下降≥1米会在三周内导致叶片干枯和树枝枯死，并在三年期间使活冠体积、茎干生长显著下降，死亡率达到88%。活冠体积下降被证明是次年死亡率的一个重要领先指标。将树木存活概率与上一年的活冠体积进行逻辑回归分析具有显著性（-2对数似然值 = 270，自由度为1的卡方值 = 232，P < 0.0001），一年内活冠体积绝对下降≥30的树木次年存活概率<0.5。相比之下，地下水位较平缓下降约0.5米对死亡率、茎干生长或活冠体积没有可测量的影响，仅使年枝生长增量显著下降。获取关于杨树形态响应时间和程度以及死亡率与地下水位下降速率、深度和持续时间之间的定量信息，有助于设计管理方案，以尽量减少冲积地下水枯竭对现有河岸杨树林的影响。
