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 137 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. RESUMEN:Los bosques templados son heterogéneos por su topografía variable lo que causa que las variables ambientales sean diferentes entre laderas. La disponibilidad de micrositios para la germinación es crucial para la regeneración natural. La remoción de vegetación secundaria (chaponeo, deshierbe) se practica en bosques del centro de México como parte de las prácticas silvícolas. Para evaluar sus efectos en la germinación establecimos micrositios de diferente orientación (Sur/Norte), con/sin vegetación secundaria (Sin Disturbio U/Perturbado P) (US, UN, PS, PN). La luz (factor de sitio global [GSF], tasa de luz rojo/rojo lejano), temperatura, humedad, pH, Nitrógeno (Ni) del suelo y germinación de dos arbustos fueron medidos en 24 micrositios. Se utilizaron modelos lineales generalizados, coeficientes de variación, pruebas de Kruskal-Wallis para el análisis. La luz fue mayor en US porque los sitios orientados al Sur reciben mayor radiación solar que aquellos al Norte y porque los claros del dosel fueron mayores. R/FR varió significativamente entre micrositios U y P, y entre N y S; los mayores valores se registraron en PS por su orientación y ausencia de vegetación secundaria, que filtra la luz....