Testing a juvenile tree growth model sensitive to competition from weeds, using Pinus radiata at two contrasting sites in New Zealand

A juvenile tree growth model sensitive to competition from weeds was developed and tested. Tree growth is predicted by reducing potential growth from an empirically determined optimum rate for the site (weed-free) using a seasonally estimated competition modifier, which accounts for the degree of weed competition for both water and light availability. The model was tested against data from a field trial at a dryland site, where juvenile Pinus radiata D. Don trees were grown with and without competition from the woody weed broom (Cytisus scoparius (L.) Link). For trees in plots without broom, seasonal fluctuations in growth were adequately modelled by a single-term Fourier series, which showed that maximum rates of diameter growth occurred during early summer. Diameter growth of trees in plots with broom was initially predicted by including a light-competition modifier into the model developed for weed-free plots on sites not subject to growth-limiting seasonal water deficit. Although the light modifier reduced growth from the weed-free state by 12% over the first year and 25% over the second year, modelled values still significantly exceeded measured diameter growth. To account for this overprediction a competition modifier based on modelled root-zone water storage was added into the model. Predictions of diameter growth using this modified model corresponded closely to measured diameter growth in both treatments.