Modern tools to tackle traditional concerns: Evaluation of site productivity and Pinus radiata management via δ13C- and δ18O-analysis of tree-rings

Abstract

Foresters frequently lack sufficient information about site quality to optimize plantation management and logwood production to local conditions. In the present study we explored the potential of δ13C- and δ18O-signatures of tree-rings to provide such information. We studied stem disks collected from two Pinus radiata plantations in south-eastern Australia that had been thinned or treated with fertilizer. Estimated from tree-ring δ13C, the sites differed markedly in intrinsic water use efficiency of photosynthesis (WUEi=A/gs). Stem disks from one site (Lyons) showed pronounced differences in δ13C between early- and latewood, depending on stand density. Fertilizer application subsequent to thinning transiently increased foliage-N concentrations, without additional effects on Amax and δ13C. Thinning (and fertilization) at the other site (Daylesford) had little effect on δ13C-variation between early- and late wood. Greater productivity at Lyons is seemingly related to greater flexibility in WUEi such that fluctuating water supply was more efficiently exploited. Current theory suggests δ18O-signatures in wood at this site acted as a proxy record of Vapor Pressure Deficit (VPD), and the slope of the correlation between δ18O and WUEi (as an indicator of stomatal sensitivity to VPD) helped identify growth limiting resources and conditions. In general, δ18O and WUEi were positively correlated and WUEi seemed mainly under stomatal control. Employing a General Linear Model, we identified additional influences on WUEi. The slope, and closeness of fit of the correlation between δ18O and WUEi depended on stand density, wood type (early- or late wood), and individual trees. These traits were not correlated in early wood immediately after planting, suggesting WUEi was driven by biochemical demand for CO2 in photosynthesis. Conversely, enhanced competition for soil water after canopy closure resulted in positive correlations between δ18O and WUEi, indicating enhanced importance of stomatal resistance to CO2-diffusion. We discuss the limitations to the use of δ13C- and δ18O analysis of bulk wood for determining the balance between demand- and supply-driven control of WUEi.