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ECOPHYSIOLOGY OF RIPARIAN COTTONWOODS quently, water potential continues to decline, either over a daily cycle or over a number of days in which the predawn wa- ter potential is unable to recover (Table 3). A distinctive char- acteristic of cottonwoods is their exceptional vulnerability to xylem cavitation (Tyree et al. 1994, Sparks and Black 1999), typically losing one -half of their hydraulic conductance at xylem water potentials of only about —1.5 MPa. The exceptional vulnerability of cottonwoods to xylem cav- itation may be an adaptive trait that contributes to drought avoidance (Rood et al. 2000a). Following xylem cavitation, af- fected branches die, thereby eliminating the transpirational de- mand of these branches and improving the water balance of the remaining parts of the tree (Sperry and Pockman 1993, Rood et al. 2000a). Such branch sacrifice explains the abundant dead branches typical of P. deltoides Marsh. and P. fremontii, the cottonwood species adapted to the driest environments. The vulnerability of cottonwoods to cavitation is consistent with the proposal by Tyree and Sperry (1988) that some trees natu- rally function near the catastrophic xylem cavitation thresh- old. In addition to the direct and often short-term consequences of drought stress, cottonwoods also display subsequent physi- ological changes associated with drought hardening (Kozlow- ski and Pallardy 1997). These changes may confer improved tolerance or avoidance during subsequent drought cycles, as- suming that abscisic acid accumulates and provides a trans - locatable signal between roots and leaves (Loewenstein and Pallardy 1998). Other changes are often morphological, such as changes in leaf characteristics including leaf size, stomatal size and number, epidermal wax characteristics and external leaf features such as epidermal hairs (Dunlap and Stettler 2001). Other morphological responses to drought stress often in- clude localized growth reduction (Table 3). Cell elongation is driven by turgor, which is highly responsive to water potential, and thus elongation abruptly ceases with drought stress. How- ever, with partial drought, root elongation continues and is substantially promoted by the declining water table that ac- companies the natural river stage recession throughout the summer (Table 3). There is thus some coordination of growth allocation accompanying the water relations experienced by riparian cottonwoods, although the endogenous signals re- sponsible for such coordination are unclear. Reproductive sta- tus of cottonwoods also decreases with water stress and this could reduce seed production, leading to a diminishing population (Rowland and Johnson 2001). Yellow signals caution For widespread application, physiological monitoring of cot- tonwood drought stress should be simple and inexpensive and should not require specialized instrumentation. With respect to these criteria, precocious senescence and branch sacrifice provide conspicuous indicators of drought stress (Table 3). Al- though there are differences in senescence patterns and vulner- ability to cavitation across species (Tyree et al. 1994), all riparian cottonwoods display precocious senescence that pre- cedes branch sacrifice and crown die -back following drought 1119 stress. The extent of senescence is apparently quantitative and thus reflects the severity of drought stress (Table 3). Senes- cence associated with branch sacrifice and crown die -back oc- curs well before death of the whole tree and provides visible symptoms of drought stress before lethal damage. In addition to assessments of leaf and branch senescence, variations in branch growth provide sensitive measures of recent (past years) water conditions (Willms et al. 1998). Analyses of leaf senescence and branch sacrifice are also suitable for remote sensing by color photographs or multi - spectral scanning from aircraft or satellites. These remote sensing approaches would be suitable for scaling up the area of monitoring. Water flux can also be evaluated through remote sensing to provide an indicator of riparian conditions (Cooper et al. 2000). Adaptations of different cottonwood species Although there are generally similar ecophysiological patterns across cottonwood species, there are also differences (Bass - man and Zwier 1991, Braatne et al. 1992, Tschaplinski and Tuskan 1994, Tschaplinski et al. 1994, 1998, Dunlap et al. 1995, Kranjcec et al. 1998, McCamant and Black 2000, Dun- lap and Stettler 2001, Rowland 2001). General patterns of adaptation of the different species emerge based on their geo- graphic distribution (Figure 3). These distributions are largely associated with climatic patterns related to precipitation and temperature. The two factors overlap because temperature in- fluences water demand as well as other physiological pro- cesses ( Kranjcec et al. 1998, McCamant and Black 2000). Climatic ranges are also related to river dependency and wa- ter relations of riparian cottonwoods. Thus, the Aigeiros spe- cies P. deltoides and P. fremontii usually occur in semi -arid environments, often along losing rivers. In contrast, the Taca- mahaca species, especially P. balsamifera and P. trichocarpa, often occur in zones with wetter climates where rivers are of- ten gaining and thus the alluvial groundwater is less dependent on stream flow. Although river regulation is still important in these mesic zones, there are differences in the relative influ- ence of the different river functions across ecoregions (Ta- ble 2). Based on distribution, the narrowleaf cottonwood, P. an- gustifolia, is intermediate between the other Tacamahaca bal- sam poplars and the Aigeiros cottonwoods. The natural clima- tic adaptation of black poplar (P. nigra) is difficult to assess because European riparian zones have been altered for centu- ries. Populus nigra has been planted in native and non - native regions and there has been introgression of genes from foreign poplars, particularly the North American P. deltoides (Van Dam and Bordacs 2002). River regulation for riparian restoration The ecophysiology of cottonwood adaptation to riparian zones is now fairly well understood. There are some notable knowl- edge gaps, such as the flow requirements for clonal recruit- ment (Rood et al. 1994, Gom and Rood 1999, Barsoum 2001). An understanding of seedling (sexual) reproduction and cot- TREE PHYSIOLOGY ONLINE at http: //heronpublishing.com