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 w ere made at two weekly intervals for two years on five-year-old Pinus radia ta D. Don trees originating as seedlings or as plantlets propagated from a six-year-old tree using tissue culture techniques. The trees were growing a t an agroforestry site with an understorey of lucerne (Menicago sativa L.) or with no-understorey and either pruned or unpruned. Total biomass increment over one year was highest for unpruned trees origin ating from plantlets with no-understorey (81 kg dry mass per tree), compare d with 34 kg for unpruned trees from the same origin grown with lucerne. pr uning reduced the increment by 27% in the no-understorey and 16% in the luc erne treatments. Pruning increased foliage efficiency (biomass increment pe r unit foliage area) for trees of both origins by 78% and 23% for the no-un derstorey and lucerne treatments, respectively. There was no effect of prun ing on the temporal growth pattern of foliage and branches, specific foliag e area and foliage area density, nor on above-ground biomass partitioning i nto foliage, branches and stem wood. In unpruned trees, 81% of the variation in new foliage mass was explained b y the cumulative water stress integral for the previous season, compared wi th 49% in the pruned trees. The current season cumulative water stress inte gral explained 67% and 50% of the new foliage mass in unpruned and pruned t rees, respectively The negative relationship between needle extension rate, at the time of maximum elongation rate, and pre-dawn water potential at th e time of maximum elongation was stronger in unpruned (r(2) = 0.49), than i n pruned (r(2) = 0.16) trees. These results indicate that current foliage p roduction in pruned trees was less affected by water deficit than that in u npruned trees. Branch extension for the current growing season varied betwe en 0.33 m and 0.75 m, and was negatively related to the current-season cumu lative water-stress integral. Pruning had no effect on branch extension len gth 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. (C) 1999 Elsevier Science B.V. All rights rese rved.