Abstract
This study describes the physiological response of two co-occurring tree species (Eucalyptus marginata and Corymbia calophylla) to seasonal drought at low- and high-quality restored bauxite mine sites in south-western Australia. Seasonal changes in photosynthesis (A), stomatal conductance (g(s)), leaf water potential (ψ), leaf osmotic potential (ψ), leaf relative water content (RWC) and pressure-volume analysis were captured over an 18-month field study to (i) determine the nature and severity of physiological stress in relation to site quality and (ii) identify any physiological differences between the two species. Root system restriction at the low-quality site reduced maximum rates of gas exchange (g(s) and A) and increased water stress (midday ψ and daily RWC) in both species during drought. Both species showed high stomatal sensitivity during drought; however, E. marginata demonstrated a higher dehydration tolerance where ψ and RWC fell to -3.2 MPa and 73% compared with -2.4 MPa and 80% for C. calophylla. Corymbia calophylla showed lower g(s) and higher ψ and RWC during drought, indicating higher drought tolerance. Pressure-volume curves showed that cell-wall elasticity of E. marginata leaves increased in response to drought, while C. calophylla leaves showed lower osmotic potential at zero turgor in summer than in winter, indicating osmotic adjustment. Both species are clearly able to tolerate seasonal drought at hostile sites; however, by C. calophylla closing stomata earlier in the drought cycle, maintaining a higher water status during drought and having the additional mechanism of osmotic adjustment, it may have a greater capacity to survive extended periods of drought.
Highlights
In a recent review, Cooke and Suski (2008) highlighted the fact that very few studies have applied physiological tools to determine the nature and magnitude of the stresses encountered by restored vegetation following major disturbance
The present study addresses these issues by comparing the physiological mechanisms of developing (11–13 years old) E. marginata and C. calophylla trees, Tree Physiology Online at http://www.treephys.oxfordjournals.org at two adjacent field sites: a ‘low-quality’ site with a known limitation to growth, and a ‘high-quality’ site with subsoil access to at least 1.5 m and no apparent growth limitation (Szota et al 2007)
The present findings show that seasonal physiology of E. marginata and C. calophylla at restored sites is heavily influenced by site quality and that mechanisms for coping with drought are enhanced under adverse soil conditions
Summary
In a recent review, Cooke and Suski (2008) highlighted the fact that very few studies have applied physiological tools to determine the nature and magnitude of the stresses encountered by restored vegetation following major disturbance. Many studies have shown that stress responses of co-occurring species of the same functional group/ life form in resource-limited environments frequently differ in mechanism or magnitude (Kolb and Stone 2000, Gebrekirstos et al 2006, David et al 2007, Grigg et al 2008, West et al 2008, Austin et al 2009); it is possible that the physiological mechanisms of sub- or co-dominant species in undisturbed systems may facilitate their dominance in restored landscapes. Physiological mechanisms that enhance survival during extended periods of drought are common Morphological adaptations, such as deep root systems, enhance the survival and productivity of mature vegetation (Jipp et al 1998, Schenk and Jackson 2002, Goldstein et al 2008); physiological mechanisms for resisting drought are more critical to the survival of developing juvenile vegetation (Donovan and Ehleringer 1991, Crombie 1997) at restored sites. Physiological mechanisms that enhance drought resistance include, but are not limited to, high stomatal sensitivity to leaf-air moisture gradients and/or leaf water status (Mott and Parkhurst 1991, Monteith 1995, Oren et al 1999), the capacity to maintain turgor during drought by lowering osmotic potential (Clifford et al 1998, Carter et al 2006, Merchant et al 2007) and a high tolerance to leaf tissue dehydration (Pook et al 1966, Davidson and Reid 1989, Gulías et al 2002)
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