Ecophysiology of riparian cottonwoods: stream flow dependency, water relations and restoration

Abstract

Cottonwoods (Populus spp.) are adapted to riparian or floodplain zones throughout the Northern Hemisphere; they are also used as parents for fast-growing hybrid poplars. We review recent ecophysiological studies of the native cottonwoods Populus angustifolia James, P. balsamifera L., P. deltoides Marsh., P. fremontii S. Watson and P. trichocarpa T. & G. in North America, and P. nigra L. in Europe. Variation exists within and across species and hybrids; however, all riparian cottonwoods are dependent on shallow alluvial groundwater that is linked to stream water, particularly in semi-arid regions. This conclusion is based on studies of their natural occurrence, decline following river damming and dewatering (water removal), water relations, isotopic composition of xylem water, and by the establishment of cottonwoods along formerly barren natural channels after flow augmentation in response to the conveyance of irrigation water. When alluvial groundwater is depleted as a result of river dewatering or groundwater pumping, riparian cottonwoods exhibit drought-stress responses including stomatal closure and reduced transpiration and photosynthesis, altered 13C composition, reduced predawn and midday water potentials, and xylem cavitation. These physiological responses are accompanied by morphological responses including reduced shoot growth, altered root growth, branch sacrifice and crown die-back. In severe cases, mortality occurs. For example, severe dewatering of channels of the braided Big Lost River in Idaho led to mortality of the narrowleaf cottonwood, P. angustifolia, and adjacent sandbar willows, Salix exigua Nutt., within 5 years, whereas riparian woodlands thrived along flowing channels nearby. The conservation and restoration of cottonwoods will rely on the provision of river flow regimes that satisfy these ecophysiological requirements for survival, growth and reproduction.

The influence of hydrological regimes on sex ratios and spatial segregation of the sexes in two dioecious riparian shrub species in northern Sweden

“…Interpreting this finding is not easy but it may reflect a lower resilience and reduced health in trees occupying a riparian zone that has been subjected to over 50 years of higher than average water tables through the growing season. In many regulated river systems, drought stress has been measured in members of the Salicaceae through, for example, increasing stomatal closure and xylem cavitation (Braatne et al 1992;Tyree et al 1994), reduced growth rates and (Willms et al 1998), reduced sap-flow rates (Lambs and Muller 2002), branch and crown die-back and hydraulic partitioning within the tree (Stromberg and Patten 1991;Rood et al 2003;Lambs et al 2006). However, this literature has concentrated on members of the Populus genus in semi-arid river systems where these effects can be marked.…”

Promoting seedling physiological performance and early establishment in degraded Mediterranean-type ecosystems

“…Therefore, restoration tools aiming at maximising above and below ground seedling vigour are critical for drought recovery, competing for resources with other plants (including invasive species), pathogen resistance and resilience against herbivores (Ruthrof et al 2010). In this context, physiological measurements are becoming increasingly important for restoration ecologists because they allow assessment of more subtle seedling responses to restoration treatments and may thus help guide the development of modern restoration strategies Rood et al 2003;Vilagrosa et al 2005). Given the protracted summer drought conditions typical for MCEs, physiological mechanisms for resisting drought are critical for coping with low soil water availability and high evaporative demand ) and survival of developing juvenile vegetation (Crombie 1997).…”

Strong effect of coarse surface layer on moisture within gravel bars: Results from an outdoor experiment

“…[38] Most literature on vegetation establishment on gravel bars, and specifically that on cottonwoods or poplars, has stressed the importance of moisture availability for seedling success [Braatne et al, 1996;Alpert et al, 1999;Cooper et al, 1999Cooper et al, , 2003Rood et al, 2003;Francis et al, 2004Francis et al, , 2006Francis, 2006]. The currently accepted model for cottonwood establishment (the "recruitment box" [see Mahoney and Rood, 1998]) is based on the assumption that seedlings are obligate phreatophytes, i.e., that their growing roots must continuously tap either the alluvial water table or the capillary fringe (the tension-saturated zone) immediately above it.…”

“…[3] There is agreement that seedlings require a continued supply of soil moisture during their first growing season, in order not to desiccate [Braatne et al, 1996;Mahoney and Rood, 1998;Rood et al, 2003]. In areas where summers are typically dry, with little precipitation input, the soil moisture within a bar must be provided by the river.…”