Relationships among forest structure, solar radiation, and plant community in ponderosa pine plantations in the Patagonian steppe

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In Patagonia, Argentina, forest management in plantations with exotic pines usually focus on timber production, without considering the plant community of their understory. However, there is growing interest in conserving the biodiversity in these productive systems, due to the important role of biodiversity in the stability and functions of newly created ecosystems. Since solar radiation is one of the main limiting factors for vegetation development in forest plantations established in treeless steppes, we carried out this study with the objectives of analyzing and establishing relationships among variables of the forest structure, the solar radiation transmitted through different forest structures, and attributes of the plant community in ponderosa pine plantations in northwestern Patagonia. We selected three plantations located in the steppe that were managed traditionally and presented a wide range of forest structures. We assessed relationships between variables of the forest structure and variables of the light environment, as well as the relationship of variables of the forest structure and the light environment with the richness and cover of the plant community, using generalized linear mixed-effects models. We also assessed the relationship of diffuse radiation with the presence and cover of species grouped by their growth-form, nativity, and the most frequent species, using generalized linear mixed-effects models and Kendall correlations. Canopy cover and basal area were the forest structural variables that explained most of the diffuse radiation variability, which was the best variable characterizing the light environment. Diffuse radiation decreased linearly as canopy cover and basal area increased, and the understory plant community responded to that gradient, decreasing both total species richness and cover. Besides, different responses of the presence and cover of the most frequent species and species grouped by their growth-form and nativity in response to the available solar radiation in the understory were detected. Whereas some species or groups did not show a significant response, those that did were, in general, positively related to diffuse radiation, although to a different extent. These results indicate that ponderosa pine plantations in Patagonia affect their understory plant community. However, they also suggest that promoting forest management guidelines that lower basal area, raises canopy height and manages residual slash would enhance plant community attributes, balancing timber production and biodiversity conservation.