Abstract:

In contrast to temperate regions, the reasons for growth reductions of individual tree species along elevational gradients in tropical mountain ranges are poorly known, especially for tropical conifers. We aimed at testing whether climatic or edaphic conditions are responsible for the reduced growth of Pinus kesiya, a widely distributed pine species of southern and south-eastern Asia, at higher elevations.

We analysed the stem diameter increment and the isotope discrimination against 13C (△ 13C) in tree rings of P. kesiya along an elevational gradient of ~900 to ~2000 m a.s.l. in the mountain ranges of South-Central Vietnam, and related growth to △  13C and to climatic and edaphic variables.

We found no consistent correlation patterns between the basal area increment (BAI) of the trees and temperature or precipitation. In contrast, across the elevational gradient, we obtained significantly negative correlations of BAI with △  13C and with the C/N ratios and the δ 15N signature of the upper mineral soil. BAI was positively correlated with the concentrations of plant-available phosphorus (P_a) and of “base” cations (calcium, magnesium, potassium) in the soil. We conclude that lower temperatures at higher elevations exert an indirect effect on tree growth by inducing higher C/N ratios and by reducing the rate of nitrogen (N) and P mineralization, which may be further hampered by lower concentrations of “base” cations (upon enhanced leaching by precipitation) and a negative feedback from low availability of mineralized N and P at higher elevations. Our results may be transferable to the uppermost growth limit of P. kesiya and to other montane regions of the species’ occurrence.

Key words: dendroecology, pine, pointer year, soil nutrient, stable isotope