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

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.

Benefits and costs of ecological restoration: Rapid assessment of changing ecosystem service values at a U.K. wetland.

Restoration of degraded land is recognized by the international community as an important way of enhancing both biodiversity and ecosystem services, but more information is needed about its costs and benefits. In Cambridgeshire, U.K., a long-term initiative to convert drained, intensively farmed arable land to a wetland habitat mosaic is driven by a desire both to prevent biodiversity loss from the nationally important Wicken Fen National Nature Reserve (Wicken Fen NNR) and to increase the provision of ecosystem services. We evaluated the changes in ecosystem service delivery resulting from this land conversion, using a new Toolkit for Ecosystem Service Site-based Assessment (TESSA) to estimate biophysical and monetary values of ecosystem services provided by the restored wetland mosaic compared with the former arable land. Overall results suggest that restoration is associated with a net gain to society as a whole of $199 ha(-1)y(-1), for a one-off investment in restoration of $2320 ha(-1). Restoration has led to an estimated loss of arable production of $2040 ha(-1)y(-1), but estimated gains of $671 ha(-1)y(-1) in nature-based recreation, $120 ha(-1)y(-1) from grazing, $48 ha(-1)y(-1) from flood protection, and a reduction in greenhouse gas (GHG) emissions worth an estimated $72 ha(-1)y(-1). Management costs have also declined by an estimated $1325 ha(-1)y(-1). Despite uncertainties associated with all measured values and the conservative assumptions used, we conclude that there was a substantial gain to society as a whole from this land-use conversion. The beneficiaries also changed from local arable farmers under arable production to graziers, countryside users from towns and villages, and the global community, under restoration. We emphasize that the values reported here are not necessarily transferable to other sites.

Allocation of river flows for restoration of floodplain forest ecosystems: a review of approaches and their applicability in Europe.

Floodplain forests are flood-dependent ecosystems. They rely on well-timed, periodic floods for the provision of regeneration sites and on tapered flood recession curves for the successful establishment of seedlings. These overbank flood events are described as "regeneration flows." Once floodplain forest trees are established, in order to grow they also require adequate, although variable, river stage levels or "maintenance flows" throughout the year. Regeneration flows are often synonymous with flood flows and only occur periodically. There is a disparity between this need for varied interannual flows over the decadal time frame and the usual annual cycle of flow management currently used by most river management agencies. Maintenance flows are often closer to established minimum flows and much easier to provide by current operational practices.A number of environmental flow methodologies, developed in North America, Australia, and South Africa are described in this review. They include the needs of the floodplain environment in the management and allocation of river flows. In North America, these methodologies have been put into practice in a number of river basins specifically to restore floodplain forest ecosystems. In Australia and South Africa, a series of related "holistic approaches" have been developed that include the needs of floodplain ecosystems as well as in-channel ecosystems. In most European countries, restoration of floodplain forests takes place at a few localized restoration sites, more often as part of a flood-defense scheme and usually not coordinated with flow allocation decisions throughout the river basin. The potential to apply existing environmental flow methodologies to the management of European floodplain forests is discussed.