Effects of non-crop vegetation on growth and productivity of youg Radiata pine.

Abstract

A study of the effect of weeds on diameter and height growth of young plantation-grown radiata pine was undertaken in Rolleston Forest (Selwyn District) in the Christchurch region of the South Island of New Zealand. The objectives of the study were to identify practical and easily measurable indices of competition and to provide a method of quantifying inter-specific competition from non-crop vegetation in young plantations by deriving indices of competition using neighbourhood predictors. A randomised block design with 4 replications was used. Each block had 8 treatments based on manipulation of percentage weed ground cover. Measurements of weed abundances and their proximities to and dispersion around individual crop trees in 2 neighbourhoods (1 m and 2 m) were assessed together with crop tree height and diameter. Mathematical expressions of the competition indices (CI) were developed based on the hypothesis that tree vigour is a function of size and competition intensity (CI). The competition indices were estimated using measures of vegetation abundance, distance from crop tree, and neighbourhood radius using expressions of the general form: CI = competition index, aj = measure of vegetation abundance or number of species or individual plants of non-crop vegetation, distj = distance from crop tree, whtj = non-crop woody vegetation height, and nr = neighbourhood radius. root collar diameter and basal area at ground level) were plotted, the trends of which were used to indicate the type of models which would best describe the relationships between tree crop growth and the competition indices. Regression analyses were empioyed to regress height, root collar diameter and basal area of the crop trees on the competition indices. Mean Square Residuals (MSQR) and the distributions of residuals were the main test criteria used. The results from the study showed that percent herbaceous cover (for low-stature vegetation), woody weed crown foliage density*height/tree height, i.e. (E(cd*wht/H) and E(cd*wht*k/H), where k is distance weighting factor (l-distance/neighbourhood size) were good indicators of competition intensity. Estimating competition from neighbourhood predictors around each tree gave better relationships than measures of site occupancy at a plot level. Grouping the competitors by species did not further improve the indices. Moreover, incorporating crop tree height into the woody indices (division by H) made them more realistic as they became sensitive to weeds taller than target crop plants. These results are similar to those from studies elsewhere (Wagner and Radosevich 1991a, Wagner et al. 1989, DeLong 1991 and Brand 1986). Crown area (on its own) of woody weeds was not useful in expressing inter-specific competition effects in this study. The results also indicated that conventional weed control regimes as commonly used by forest managers in the region (1 m band spraying along the planting rows) resulted in individual crop trees still experiencing high levels of inter-specific competition. Even in plots where control had been undertaken, most trees still experienced considerable interference effects from non-crop neighbours of shrub and herbaceous species. Forest managers in the region may need, therefore, to redefine their non-crop vegetation reduction standards. In addition, managers need to set different priorities for controlling herbaceous and woody weeds so as to use available resources more efficiently. The implications of these findings on present and future vegetation management regimes in the area and a brief mention of the economic implications are also presented.