Effects of biotic and abiotic factors on δ15N 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 analyzed to study the relationships of climate, tree age, dominance and growth with tree δ15N. 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 δ15N, leading to differences in δ15N among trees. Most wood δ15Nair values were below the δ15Nair 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 δ15Nair than those of suppressed. However, no dominance effect was detected in δ15Nair 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 (δ15Nair = +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 δ15Nair satisfactorily.