Transpiration water use and ecophysiology of riparian vegetation at the Bookpurnong floodplain

[摘要] Floodplains and their wetlands support high biodiversity and productivity. Soil salinisation and drought arising from lack of flooding and rising saline groundwater has lead to a severe decline in the health of the native riparian vegetation communities on the floodplains of the lower River Murray in South Australia. The restoration of the natural flow regime is difficult where water resources are scarce. This is the case in the Murray-Darling Basin in south eastern Australia, where less than a quarter of the dominant riparian tree species (Eucalyptus camaldulensis Dehnh., river red gum, E. largiflorens F. Muell., black box and Acacia stenophylla A. Cunn. ex Benth., river cooba) were mapped as healthy (DEH 2004). The decline in vegetation condition prompted the ‘Bookpurnong Experiment’, where the response of the riparian trees to flooding and groundwater management was tested. The objectives of this study were to quantify the: 1) water balance along Transect 3; 2) effect of groundwater freshening ; and 3) effect of ‘pulse-pumping’ on the Bookpurnong Floodplain in the South Australian Riverland between September 2007 and December 2008. Transpiration rates measured in this study were comparable to published ranges for A. stenophylla (2 – 98 mm yr-1), E. camaldulensis (109 – 1882 mm yr-1) and E. largiflorens (11 to 365 mm yr-1). The E. camaldulensis and E. largiflorens transpiration values are low in comparison to other published studies and reflect the low annual rainfall and recent drought in the lower River Murray. Groundwater and soil salinity gradients reduced transpiration rates by one to two orders of magnitude across the 200 m wide riparian zone. The riparian vegetation within 50 m of the river used between 105 – 287 mm of fresh groundwater in addition to rainfall, the equivalent of the average annual rainfall at Bookpurnong, but less than 15% of potential evaporation. In contrast, vegetation further from the river did not use detectable volumes of groundwater. There was a significant increase in E. largiflorens predawn water potential values (decrease in tree water stress) in response to groundwater freshening. The significant increases in E. largiflorens water potential and overall canopy condition demonstrate that riparian vegetation health can by improved by the creation of a long-term source of fresh water. The capillary fringe was the dominant tree water source for riparian vegetation, which is consistent with previous studies. Pulse pumping of the groundwater production bore resulted in fluctuations in soil water potential and groundwater salinity values in response to water table fluctuations. However, it was not possible to determine whether salts were removed from the soil profile by these pulse pumping cycles as there were negligible changes in the soil water potentials or sampled soil chloride values between pumping cycles.In conclusion, this study has shown that net groundwater uptake is limited to areas where groundwater salinities are low, and that riparian vegetation health can by improved by the creation of a long-term source of fresh water. The water balance has shown that groundwater freshening by bank recharge has the potential to provide 82% of all E. camaldulensis trees and almost 20% of the total floodplain area in the lower River Murray with a long term, low salinity water source using less water per hectare than flooding (0.36 ML ha 1yr 1 cf. 1.5 – 4.7 MLha 1yr 1). These findings reinforce the need for floodplain managers to operate lower River Murray watercourses to maximise bank recharge to sustain fresh groundwater reserves to enable riparian vegetation to survive the long dry periods between floods.

[发布日期] 2011-09-28 [发布机构] CSIRO

[学科分类] 地球科学(综合)