GERMINATION RESPONSE OF TWO SHRUB SPECIES UNDER CONTROLLED CONDITIONS AND TO SECONDARY VEGETATION REMOVAL (WEEDING) IN THE MAGDALENA RIVER BASIN, MEXICO

ABSTRACT: Temperate forests are heterogeneous because of their variable topography which causes that environmental variables are different between slopes. The availability of microsites for seed germination is crucial for natural regeneration. Secondary vegetation removal (weeding) is practiced in Central Mexico’s forests as part of silvicultural practices. To evaluate its effects on germination we performed an experiment with microsite types of different orientations South/North (S/N) and with/without secondary vegetation (Undisturbed/Perturbed) (US, UN, PS, PN). Light (global site factor [GSF] and red/far-red [R/FR] ratio), soil temperature, moisture, pH, nitrogen (Ni) and germination of two shrub species were measured at 24 microsites. Results were analyzed with generalized linear models, variation coefficients and Kruskal-Wallis tests. Light values exhibited higher values at US because South-facing sites receive higher solar radiation than North-facing ones and because canopy gaps were larger. R/FR ratios were significantly different between U and P, and N and S microsites; the highest values were registered at PS microsites because of their orientation and because of the absence of understory vegetation, which filters light. Soil temperature was the highest at PS and PN. Soil moisture was the highest at UN and US because secondary vegetation avoids soil moisture loss. Germination percentages of both species were the highest at US because secondary species have high light and soil moisture requirements. pH, soil moisture and GSF had a significant effect on Ageratina glabrata, while pH, Ni, soil temperature and moisture were significant for Senecio angulifolius. This practice can have negative effects on shrubs species germination. We do not recommend weeding, because it can have negative effects on shrub species germination.