Productivity and water relations of burnt and long-unburnt semi-arid shrubland in Western Australia

This study provides a comparison between vegetation of relatively recent and long-unburnt shrubland in terms of structural and functional groups, annual net primary productivity and water relations. Adjacent areas of vegetation long-unburnt or burnt 5 years previously were compared within a remnant block of Acacia–Allocasuarina–Melaleuca arid shrubland at Kalannie, south west Western Australia. Species were classified according to growth and life form, fire response, phenology and rooting morphology and densities, mean plant above-ground dry weights and shoot:root dry mass ratios of each assessed. Species compositions, seedling densities and absence of recruitment in the long-unburnt area suggested marked dependence on fire in maintenance of biodiversity. Comparisons of above-ground standing dry biomass and annual net primary productivity of total (above-ground plus below-ground) dry matter showed the 4.09 kg m^–2 biomass of long-unburnt vegetation to be increasing at 0.52 kg m^–2 year^–1 versus 0.45 and 0.18 kg m^–2 year^–1 for vegetation of the burnt area. Water relations of soils indicated consistently wetter profiles in burnt than long-unburnt areas and no deep drainage during the year of study. Lower water stress of key species in burnt than long-unburnt areas were indicated by less negative pre-dawn water potentials and higher stomatal conductance during the year of study and more negative carbon-isotope composition (¹³C) in wood laid down over the past 5 years. Budgets for water use were estimated for both sites and compared with annual net primary productivities. Data suggested much greater transpiration loss per unit dry matter gain by the rapidly growing plants at the burnt site (437 ml H₂O g^–1 DM) than by the plants of the long-unburnt community (92 ml H₂O g^–1 DM). Results are discussed in relation to composition and functioning of other Western Australian ecosystems. It is clear that time since fire affects productivity and water-use of vegetation of semi-arid shrublands and is therefore an important consideration for management and protection of remnant vegetation.