Effects of pruning and understorey vegetation on crown development, biomass increment and above-ground carbon partitioning in Pinus radiata D. Don trees growing at a dryland agroforestry site

Measurements of needle growth, branch growth and pre-dawn water potential were made at two weekly intervals for two years on five-year-old Pinus radiata D. Don trees originating as seedlings or as plantlets propagated from a six-year-old tree using tissue culture techniques. The trees were growing at an agroforestry site with an understorey of lucerne ( Medicago sativa L.) or with no-understorey and either pruned or unpruned. Total biomass increment over one year was highest for unpruned trees originating from plantlets with no-understorey (81 kg dry mass per tree), compared with 34 kg for unpruned trees from the same origin grown with lucerne. Pruning reduced the increment by 27% in the no-understorey and 16% in the lucerne treatments. Pruning increased foliage efficiency (biomass increment per unit foliage area) for trees of both origins by 78% and 23% for the no-understorey and lucerne treatments, respectively. There was no effect of pruning on the temporal growth pattern of foliage and branches, specific foliage area and foliage area density, nor on above-ground biomass partitioning into foliage, branches and stem wood. In unpruned trees, 81% of the variation in new foliage mass was explained by the cumulative water stress integral for the previous season, compared with 49% in the pruned trees. The current season cumulative water stress integral explained 67% and 50% of the new foliage mass in unpruned and pruned trees, respectively. The negative relationship between needle extension rate, at the time of maximum elongation rate, and pre-dawn water potential at the time of maximum elongation was stronger in unpruned ( r 2 = 0.49), than in pruned ( r 2 = 0.16) trees. These results indicate that current foliage production in pruned trees was less affected by water deficit than that in unpruned trees. Branch extension for the current growing season varied between 0.33 m and 0.75 m, and was negatively related to the current-season cumulative water-stress integral. Pruning had no effect on branch extension length or branch growth rate. The results highlight the possibility of maximising production by managing the timing of pruning and the choice of understorey vegetation for a given seasonal water availability.