Effects of biotic and abiotic factors on 15N in young Pinus radiata.

Abstract

In a 12-year-old Pinus radiata plantation, three dominant and three suppressed trees removed by thinning were randomly selected, and needles, annual rings from basal stem disks and bark were collected and analysed to study the relationships of climate, tree age, dominance and growth with tree 15N. The high foliar-N concentration (1.35-2.73 % N, dw) suggested that N was not limiting tree growth, therefore allowing plants to fractionate versus 15N, leading to differences in 15N among trees. Most wood 15Nair values were below the 15Nair natural abundance in the dominant pines (-2.43 to +1.69 ‰) and above it in the suppressed trees (+0.73 to +3.35 ‰), likely due to the access of dominants to exogenous N sources with lower 15Nair than those of suppressed. However, no dominance effect was detected in 15Nair of bark and needles that decreased in the order: buds (+1.20 to +2.44 ‰) > needles 1-year (-0.27 to +1.43 ‰) > needles 2-years (-0.97 to +0.41 ‰) > bark (-1.18 to +0.15 ‰). Compared with the soil N in the 0-15 cm layer (15Nair= +4.8 ‰), all plant material was 15N-depleted. Results suggest that seedlings and foliar buds have a less efficient system for N conservation and recycling, with higher losses. The linear regression models showed that both biotic (dominance and tree age) and abiotic factors (temperature in spring-summer and annual precipitation) are needed to explain the wood 15Nair satisfactorily.