Carbon partitioning in Pinus radiata stands in relation to foliage nitrogen status

Abstract

First rotation Pinus radiata D. Don trees were grown on a nitrogen-deficient sand dune in an experimental design that included lupin (Lupinus arboreus Sims) as an understory species, and biennial application of balanced fertilizer in a replicated split-plot factorial design with thinning treatments as subplots. From an initial 2222 stems ha-1, stands were thinned to 1483 and 741 trees ha-1 at ages 2 and 4 years, respectively. Mean annual production of foliage, branches, stem, coarse roots, and cones was estimated from measurements of the dry matter content at ages 7, 10, 11 and 17 years. A model was used to estimate total above- and belowground dry matter production from measurements of leaf area index and foliar nitrogen concentration. An estimate of fine root production was obtained from the difference between the model estimate of total production and the estimate of above-ground production. The fertilizer and lupin treatments increased foliage nitrogen concentration, leaf area index and dry matter production of most aboveground components. In unthinned stands, foliage production ranged from 3 to 6 Mg ha-1 year-1, branch production from 1 to 4 Mg ha-1 year-1, and stem production from 8 to 19 Mg ha-1 year-1, depending on site fertility. Partitioning of aboveground production to foliage, branches, stems, and cones changed with tree age but it was not significantly influenced by the treatments. The fractions of aboveground dry matter production partitioned to foliage, branches, stems and cones were 0.23, 0.12, 0.63 and 0.02, respectively, for trees between ages 7 and 11 years, and 0.21, 0.08, 0.67 and 0.04, respectively, for trees between ages 11 and 17 years. With increasing foliage nitrogen concentration, partitioning to branches increased significantly, whereas partitioning to stems decreased. The fraction of total dry matter production partitioned to stems decreased significantly with thinning from 0.40 to 0.27 with a concomitant increase in the fraction partitioned to fine roots from 0.31 to 0.46. The average fraction partitioned to fine roots was 0.46 and 0.30 in unfertilized and fertilized stands, respectively. Stem production was more highly correlated with fine root partitioning than with leaf area index. Fine root production was not significantly affected by fertilizer application. The ratio of total net primary production to leaf area index increased significantly with thinning, but not with site fertility or stand age. The ratio of aboveground dry matter production to leaf area index increased with stand age and with the application of fertilizer but not with thinning.