Natural hybridization in seed stands of seven Mexican Pinus species

Natural hybridization can manifest different evolutionary results, such as accelerating differentiation and facilitating speciation through the rapid origin of new biochemical compounds, physiological or morphological phenotypes that allow hybrid species to occupy new habitats, which for parental species would be inaccessible. However, these expectations are not always fulfilled, because natural hybridization between divergent populations can lead to inadequate or unviable hybrids and, therefore, lower forest stability and productivity. Using pure species Pinus arizonica, P. cembroides, P. durangensis, P. engelmannii, P. leiophylla, P. lumholtzii and P. teocote trees and their natural hybrids, this study aims to determine for the first time: (i) morphological differences between pure pine trees and their hybrids molecularly detected, (ii) differences in vigor between 1970 seeds from trees of pure pine species and their hybrid trees (hereinafter called pure and hybrid parents), (iii) differences in vigor between 3465 seedlings from 1421 pure and hybrid parents, and (iv) whether growth of seedlings of hybrid parents is differently associated to parent tree’s environmental conditions, than growth of seedlings of pure parents. The seedlings grew under equal nursery conditions. Our results show some significant differences in morphological traits between the seed trees of pure species and their respective putative hybrid seed trees, and in seed and seedling fitness indices. In contrast to the mean growth of seedlings of hybrid parents, the mean growth of seedlings from pure parents was significantly associated with the parent tree’s bioclimatic conditions studied (R2 = 0.70 vs 0.83). There were only some individuals that presented hybrid superiority.