Biological Aspects of Hybrid Poplar Cultivation on Floodplains in ...

<strong>Biological</strong> <strong>Aspects</strong> <strong>of</strong> <strong>Hybrid</strong> <strong>Poplar</strong> <strong>Cultivation</strong> on Floodplains in Western North America -- A ReviewIn viewing the data presented in Table 3, it isimportant to note that the growing season <strong>of</strong> hybridpoplars is significantly longer (April to October) thanmost annual crops. Since hybrid poplars take up to 4years to reach their maximum transpiration potential,their long-term water-use will also be significantly lowerthan agricultural crops whose annual water demand isrelatively constant from year to year. Detailed models <strong>of</strong>evapotranspiration by hybrid poplars are currently beingdeveloped at Washington State University and OregonState University. Until these findings are published, therange <strong>of</strong> values presented in Table 3 can be used toapproximate water fluxes from hybrid poplarplantations.VII. Drought and flood-tolerance <strong>of</strong> hybridpoplars and native cottonwoodsThere have been numerous water relation studies<strong>of</strong> hybrid poplars (F 1hybrids) and their parents (nativecottonwoods). Although canopy and stand-levelcomparisons are noticeably absent, whole-plant and leaflevelstudies reveal that hybrid poplar rapidly close theirstomata in response to atmospheric and soil-waterdeficits (Schulte et al. 1987, Tschaplinski and Blake1989a,b, Braatne et al. 1992, Hinckley et al. 1992,Hinckley and Braatne 1994). This type <strong>of</strong> stomatalbehavior typically results in lower transpiration rates anda greater drought resistance among hybrids relative tonative species (Tschaplinski and Tuskan 1994,Tschaplinski et al. 1994, Blake et al. 1996).Furthermore, these water relation properties enablehybrids to maintain higher leaf areas for longer periodsduring a drought-cycle than native species (Braatne et al.1992).Given current trends in climatic warming, theseresearch findings may be significant as streamsideplantings <strong>of</strong> hybrid poplar could potentially compensatefor the adaptive limitations <strong>of</strong> some native species(notably black cottonwood, P. trichocarpa). The factthat P. trichocarpa x P. deltoides hybrids could providemore shade to moderate stream temperatures than nativespecies is not a minor point, especially in a warmingclimate with periodic drought. <strong>Hybrid</strong>s are thus not onlysuitable for short-rotation fiber plantations, but may alsohold some promise as supplemental plantings withnative species in the restoration <strong>of</strong> riparian corridors.New research is needed to explore the potentialecological role <strong>of</strong> hybrid poplar in light <strong>of</strong> changingriparian landscapes.Seasonal flooding is a common feature <strong>of</strong> riparianlandscapes (Figure 15), and the flood tolerance <strong>of</strong> nativecottonwoods and hybrid poplars has been subject <strong>of</strong>several recent studies (Lui and Dickmann 1992a,b, 1993,Neuman et al. 1996). The general symptoms associatedwith flooding include: a) yellowed leaves, b) leaf curl(i.e. epinasty), c) formation <strong>of</strong> adventitious roots, and d)Figure 15.Seasonal flooding <strong>of</strong> a natural hybrid zone (P.angustifolia x P. trichocarpa) along the upperYellowstone River, Montana. The life cycle <strong>of</strong>riparian cottonwoods is highly dependent uponseasonal flooding to create nursery sites for theseedling recruitment. Periodic flooding is thuscritical to sustaining native riparian cottonwoodpopulations. Native cottonwoods and hybridpoplars are very flood-tolerant relative to othernatives and invasive eurasian species as well asmost <strong>of</strong> the agricultural crops typically planted onfloodplain habitats.(Photo by J.H. Braatne)wilting (reviewed by Kozlowski 1984, 1997).Cottonwoods are considered to be fairly tolerant <strong>of</strong>excess soil moisture (Harrington 1987), yet flooded soilsreduce growth and survival in many species (Smit et al.1989, Lui and Dickmann 1992a). Some cottonwoodsappear to be more flood tolerant than others, yet thereare some disparities among research findings. Forexample, Harrington (1987) has shown that 20 d <strong>of</strong>flooding did not affect the growth and survival <strong>of</strong> blackcottonwood (P. trichocarpa). These results contradictthe findings <strong>of</strong> Smit et al. (1989), where flooding wasobserved to significantly reduce leaf growth in thisspecies. These observations suggest that there is a highlevel <strong>of</strong> genetic variability in flooding responses withinand between species. In fact, Smit (1988) found thatflooding tolerance was more variable within populations<strong>of</strong> black cottonwood than between populations. Overall,native cottonwoods appear to be more flood tolerant thanhybrid poplars, yet both are significantly more floodtolerant than the agricultural crops commonly planted infloodplain habitats (Kozlowski 1984, 1997, Neuman etal. 1996, D.I. Dickmann, pers. communication).Additional research is needed to assess the physiologicaland morphological relationships between flooding anddrought-tolerance in hybrids and native species.VIII. Clearing <strong>of</strong> floodplain habitats as a means<strong>of</strong> conserving waterOn the basis <strong>of</strong> the misconception that ripariantrees transpire excessive amounts <strong>of</strong> water, agronomists11.