TREE-RING TO CLIMATE RELATIONSHIPS FOR PO PULATIONS OF QUERCUS-AFARESWILLD AND QUERCUS-CANARIENSIS POMEL IN ALGERIA

Ring-width variations are analysed on 15 populations of Quercus afares and Quercus canariensis in the Akfadou and Beni-Ghobri Forests (Alger ia) (fig 1) which are submitted to temperate and cool variants of the Mediterranean humid bioclimate. The populations selected are represent ative of the different ecological conditions (substratum: exposure, al titude) under which the two species are growing. After measuring ring- width on three radii per tree and eight to 16 trees per site, a mean r ing-width chronology is developed for each population (table I). In or der to remove age trend and isolate interannual variations of radial g rowth, ring-width series are modelized using ARMA processes, For each population, a mean chronology of residuals is established. Response fu nction is then calculated, involving the mean residual chronology as d ependent variable and, in a first step, the 24 monthly parameters of a nnual climate as regressors (P-T-max and P-T-min). The number of regre ssors is then reduced by grouping together the successive monthly valu es of the same sign. Response function is calculated for the period 19 18-1951, where valuable meteorological data are available. The reliabi lity of response function is tested using the bootstrap method. Result s obtained from modelization (table II) show that radial growth is lar gely predetermined for both species. The amount of residual variance t o be correlated with climate is low; this amount is lower for Q afares than for Q canariensis. Statistical analysis reveals that neither alt itude nor exposure or substratum (table III) can explain the differenc es between populations. Comparison of the results obtained with respon se functions (table IV) reveals that, for the two species, precipitati on has a greater effect than temperature, the action of which concerns only minimal values. The analysis of global significance of response functions shows that Q afares does not react to climate whereas Q cana riensis reacts significantly to the interannual fluctuations of climat ic parameters. As the region is submitted to the same regional climate , differences between populations can be attributed to differences in habitats. However, just as with the growth models, it is not possible to establish a significant relationship between the describers of the habitats (altitude, substratum, exposure) and the response functions, In conclusion, the endemic Q afares appears to be in better equilibriu m with climate variations than Q canariensis; however, as the two spec ies are highly in competition in the same habitat, this equilibrium co uld be perturbated by a small change in climate.