Responses of riparian cottonwoods to alluvial water table declines

Human demands for surface and shallow alluvial groundwater have contributed to the loss, fragmentation, and simplification of riparian ecosystems. Pop ulus species typically dominate riparian ecosystems throughout arid and sem iarid regions of North American and efforts to minimize loss of riparian Po pulus requires an integrated understanding of the role of surface and groun dwater dynamics in the establishment of new, and maintenance of existing, s tands. In a controlled, whole-stand field experiment, we quantified respons es of Populus morphology, growth, and mortality to water stress resulting f rom 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 l ive Populus deltoides subsp. monilifera stems over four years in conjunctio n with localized water table declines. Measurements began one year prior to min ing and included trees in both affected and unaffected areas. Populus demonstrated a threshold response to water table declines in medium alluvia l sands; sustained declines greater than or equal to 1 m produced leaf desi ccation and branch dieback within three weeks and significant declines in l ive 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, chi(2) with 1 df = 232, P < 0.0001) and trees with ab solute declines in live crown volume of greater than or equal to 30 during one year had survival probabilities <0.5 in the following year. In contrast , more gradual water table declines of similar to 0.5 m had no measurable e ffect on mortality, stem growth, or live crown volume and produced signific ant declines only in annual branch growth increments. Developing quantitati ve information on the timing and extent oi morphological responses and mort ality of Populus to the rate, depth, and duration oi water table declines c an assist in the design of management prescriptions to minimize impacts of alluvial groundwater depletion on existing riparian Populus forests.