Site index curves for Monterrey pine (Pinus radiata D. Don) have been prepared according to the method of harmonized curves. It has also been studied the possibility of discriminating areas with different site quality in the pine region.
The basic data are height and age measurements on 1299 plots from the forest inventories performed by the lnstituto Forestal, embracing the provinces from Valparaiso to, Llanquihue, excluding Arauco. The height measurement used is the mean height of dominant and codominant trees. The age was estimated by the number of growth rings plus one, counted at 30 cm. over the ground.
The region studied was divided in 10 geographic areas. This stratification allowed to study the variations of the shape of the height-age relations, and of the average site index between the areas, and, besides this, to reduce distortions on the estimations produced by the differences in age distributions.
The equations for the average curves were fitted by least squares separately for each area. Four models for the height-age relationship were tested, being adopted finally the equation
ln A = a + b/E
where A is the mean height of dominant and codominant trees, and E is age.
In order to verify adequacy of the assumption of consistent proportionality of the site index curves, the trend of the variance of the logarithm of height with age was analyzed. Not founding evidence against the homogeneity of this variance, it was concluded that the assumption, is acceptable.
As in some uses it is convenient to deal with a small number of families of site index curves valid for the whole country, the significance of the difference in shape of the average site index curves for the different areas was investigated. Significative differences were found, which make little advisable the use of a single family of curves for all the country.
The convenience of using different site index curves for each geographic area or, on the other hand, the best manner of grouping areas, depends on the use of the curves in each specific case. A method is described for calculating site index curves for groups of areas, based on the results of this study. For general uses, site index curves were calculated separating the areas in two groups.
The comparison of the results with earlier site index studies shows that in general, there is an acceptable agreement in the shape of site index curves obtained from data of various origins and using different methods.
Comparing average site indexes, it was possible to distinguish three regions clearly defined. Areas 5 and 9, corresponding roughly to Concepción and Cautín provinces, presents a higher average site quality than the others, while area 7 consisting in part of the "arenales" (sandy soils) region, has an average quality much lower. These differences have relation only to average site indexes, being there high variation in the same area.