Water relations of wallum species in contrasting groundwater habitats of Pleistocene beach ridge barriers on the lower north coast of New South Wales, Australia
This study examined the water relations of sclerophyllous evergreen vegetation (wallum) on coastal sand barriers in eastern Australia. Many wallum species may be groundwater dependent, although the extent of this dependency is largely unknown. Twenty-six perennial tree, shrub and herb species were investigated in three groundwater habitats (ridge, open depression, closed depression). Pre-dawn and midday shoot xylem water potentials (ᴪₓ) were measured monthly between late autumn 2010 and late summer 2011. Pressure-volume curve traits were determined in mid- to late spring 2009, including the osmotic potential at full (π100) and zero (π0) turgor, and bulk modulus of elasticity (ε). Carbon isotope ratios (δ¹³C) were also determined in mid- to late spring 2009, to measure water-use efficiency (WUE). The species displayed a range of physiological strategies in response to water relations, and these strategies overlapped among contrasting growth forms and habitats. Linear relationships between osmotic and elastic adjustment were significant. A strong correlation between δ¹³C and distribution along the hydrological gradient was not apparent. 'Banksia ericifolia' subsp. 'macrantha' (A.S.George) A.S.George, 'Eucalyptus racemosa' Cav. subsp. 'racemosa' and 'Eucalyptus robusta' Sm. displayed little seasonal variation in ᴪₓ and maintained a comparatively high pre-dawn ᴪₓ, and are therefore likely to be phreatophytic. Wetland vegetation in the lowest part of the landscape appeared to tolerate extreme fluctuations in water availability linked to a prevailing climatic pattern of variable and unreliable seasonal rainfall.
Drought survival of Australian rainforest seedlings is influenced by species evolutionary history and soil type
Water availability influences regional tree distributions in rainforests, often by affecting survival of seedlings. The occurrence of 'dry rainforest' species in subhumid climates has been attributed to the evolution of drought-resistant species from their mesic rainforest congeners. Many genera are found in both dry and mesic rainforest of Australia but the extent to which this is due to differential drought resistance has not been confirmed experimentally. We compared drought survival within three congeneric pairs of dry and mesic rainforest taxa in a glasshouse dry-down experiment. Soil type could also play a role, with dry rainforests mostly occurring on fine-textured soils such as loams, which have a high available water-holding capacity, compensating for lower rainfall. Hence, we grew plants in loam or sand soil. In all pairs, the dry rainforest taxon was better able to survive drought, providing support for the climate-induced evolution of a dry rainforest flora and further confirming that drought resistance of seedlings can shape tree species distributions at regional scales. Two of three pairs had higher seedling survival on basalt-derived loam soil, suggesting that such soils may aid seedling persistence during drought. Over evolutionary time, this may have resulted in the high fidelity of dry rainforest for these soils.
Water relations of selected wallum species in dry sclerophyll woodland on the lower north coast of New South Wales, Australia
The present study examined the water relations of wallum dry sclerophyll woodland on the lower north coast of New South Wales (NSW). Wallum is the regionally distinct vegetation of Quaternary dunefields and beach ridge plains along the eastern coast of Australia. Wallum sand masses contain large aquifers, and previous studies have suggested that many of the plant species may be groundwater dependent. However, the extent of this dependency is largely unknown, despite an increasing reliance on the aquifers for groundwater extraction. Fifteen species from five growth-form categories and seven plant families were investigated. The pre-dawn and midday xylem water potential (Ψₓ) of all species was monitored over a 20-month period from December 2007 to July 2009. Pressure-volume curve traits were determined for each species in late autumn 2008, including the osmotic potential at full (π₁₀₀) and zero (π₀) turgor, and bulk modulus of elasticity (ε). Carbon isotope ratios (δ¹³C) were determined in mid-autumn 2008 to measure water use efficiency (WUE). Comparative differences in water relations could be loosely related to growth forms. A tree (Eucalyptus 'racemosa' subsp. 'racemosa') and most large shrubs had low midday Ψₓ, π₁₀₀ and π₀, and high ε and WUE; whereas the majority of small and medium shrubs had high midday Ψₓ, π₁₀₀ and π₀, and low ε and WUE.However, some species of similar growth form displayed contrasting behaviour in their water relations (e.g. the herbs 'Caustis recurvata' var. 'recurvata' and 'Hypolaena fastigiata'), and such differences require further investigation. The results suggest that 'E. racemosa' subsp. 'racemosa' is likely to be groundwater dependent, and large shrubs such as 'Banksia aemula' may also utilise groundwater. Both species are widespread in wallum, and therefore have the potential to play a key role in monitoring ecosystem health where aquifers are subject to groundwater extraction.