Modelling the influence of weed competition on the growth of young Pinus radiata at a dryland site

Abstract

A water balance model was used to partition water loss and explain annual growth differences for 1-year-old Pinus radiata D. Don trees growing with and without competition from broom ( Cytisus scoparius L.) at a dryland site. Seasonal water deficits are characteristic of this site as annual precipitation is low (797 mm) and water storage within the stony soil is limited to approximately 70 mm. At the end of the second year, the presence of broom had reduced tree annual basal area increment by 10-fold compared with that in plots where broom had been removed. In both treatments, modelled root-zone water storage ( W) corresponded closely to measured values over the course of the year. Root-zone water storage in the treatment with broom fell to values close to minimum water storage ( W min) by mid-summer, and did not significantly increase again until late autumn. In contrast, W in the treatment without broom remained high until mid-summer then declined, almost reaching W min at the end of autumn. The lowest values of W in both treatments coincided with the driest late-summer to late-autumn period since 1960. Total annual evaporation in the treatment with broom was 92 mm higher than that for the treatment without broom. This higher rate was primarily due to wet canopy evaporation from the broom and, to a lesser extent, from increased rates of total transpiration within this treatment. Soil surface evaporation was the largest component of water loss in the treatment without broom, accounting for 47% of total annual precipitation. There was a clear relationship between the integral of root-zone water deficit and fractional basal area growth, suggesting losses in growth were almost entirely attributable to the development of a more pronounced water deficit in the treatment with broom. These findings highlight the importance of root-zone water storage in regulating productivity on dryland sites and indicate that water balance modelling provides a useful approach for predicting the influence of weed competition on tree growth.