Site and weather effects in allometries: A simple approach to climate change effect on pines

Allometric relationships allow estimation of forest stand variables (volume, biomass, leaf area), which are very important in forest management and in key ecosystem processes. However, these relationships are very costly to obtain, as the data on which they are based require laborious destructive sampling. In addition, allometric coefficients appear to change for a given species depending on stand density, site and season (recent weather conditions). Thus, if the environmental conditions change, these relationships may not be valid. This study attempts to explain why allometric relationships may vary in time and from site to site by using a little set of known bioclimatic variables. Seventy-eight trees from three sites and eight dates were selected for destructive sampling. Dendrometric variables were obtained from stem analysis. Leaf area and leaf biomass were estimated using ratio estimators in stratified random sampling. The allometric relationships varied depending on the site and date. Relationships involving variables that accumulate during the life-span of a tree (biomass, volume and height of stem versus diameter at breast height) depend mainly on site and little on recent weather conditions. In contrast, those regarding variables that were not totally cumulative (leaf and branch biomass and leaf area versus diameter at breast height) varied mostly with recent weather conditions. Bioclimatic variables have proved to be both a meaningful and easy tool to use to produce a parameterisation of all the allometric relationships, which could be very useful to obtain transition and output functions in growth modelling.