Effect of climate and physiography on the density of tree and shrub species in northwest Mexico

In order to understand the environmental variables that may impact more on the distribution of species of trees and shrubs, a correlation analysis applying the Covariation (C) of Gregorius was conducted among 14 variables of climate and physiography, and the number of individuals of 72 species, which were found in 1804 sampling plots (covering about 123, 317 km²) of the National Forests and Soils Inventory (INFyS) developed by the National Forest Commission in Mexico (CONAFOR). Among the studied species there are several of the genera Quercus, Pinus and Junniperus, which are mainly distributed in the Sierra Madre Occidental, where they stand out for their abundance. The results show that the density of 88% of the studied species have a significant correlation (P <0.025) with at least five of the 14 variables analyzed. Seven of the variables showed significant correlation (P <0.025) with at least 74% of the studied species: 'Julian date of last spring frost' with an average value of covariation (C) equal to 0.71, 'average duration of the frost-free period' with average value of C = 0.71', degree days above 5°C with covariation of 0.69, 'altitude above sea level' with C = 0.66; 'mean temperature in the coldest month', 'mean temperature in the warmest month' and 'mean annual temperature', with average values of C = 0.65 for each of these last three variables. The 'geographic orientation of the ground' was the least correlated with the density of the species, since only 10% of them showed significant correlation with this variable.