Responses of native woody taxa in Banksia woodland to incursion of groundwater and nutrients from bordering agricultural land

Abstract

Effects of incursion of water and nutrients from agricultural land on adjoining native Banksia woodland were examined near Moora, Western Australia, by using a 100-m-wide belt transect from the paddock edge into virgin bush. Transect zones 0–20, 20–50 and 100–300 m inwards were designated as ‘hedge’, ‘transition’ and ‘inner bush’, respectively. Numbers of species of native woody taxa increased from 28 to 42 and plant densities increased from 2200 to 14 900 plants per hectare from hedge through transition to inner bush, while collective total standing biomass (above and below ground) of woody species decreased from 157 to 32 t dry matter per hectare. Data for nutrient contents of this total biomass indicated that total nutrient loads in respect of calcium, potassium, sulfur, phosphorus, magnesium, zinc, copper and sodium were closely related to relative amounts of biomass in the three zones, but hedge vegetation was appreciably enriched in nitrogen but depleted in manganese and chlorine compared to inner bush. Diversity indices indicated differential responses of species to agricultural influence. The dominant tree species, Banksia prionotes, represented 91, 87 and 58% of the total biomass of woody taxa in hedge, transition and inner bush, respectively. Growth ring analyses of basal trunk xylem indicated that this seeder species had recruited after a hot burn in 1963 coincident with land clearing. Since then, annual trunk-area increments in hedge trees have been consistently much greater than in transition and bush. Net annual productivities, dry matter allocation profiles and current leaf areas of B. prionotes showed that the average hedge tree weighed 167 kg total dry matter and was currently increasing at 11.9 kg per year and was in the process of achieving net dry matter gains of 420 g m–2 of leaf surface. Comparable figures for transition trees were 61 kg, 4.7 kg per year and 360 g m–2, for inner bush trees 12.7 kg, 1.1 kg per year and 230 g m–2. Results are related to recent studies on Banksia woodlands and general information on ecotones and their attributes.