Effect of plant–plant interactions on Nothofagus pumilio regeneration ecophysiology in harvested forests during two years of contrasting climate

Background Tree natural regeneration is a key element for forest recovery after harvesting. In Nothofagus pumilio forests, canopy opening modifies stand environmental conditions, creating microsites that affect regeneration. Biotic interactions become especially important, as increments in abiotic resources (e.g., light, soil moisture) can rapidly increase understory. Seedling ecophysiological responses help to identify the mechanisms driving natural regeneration recruitment. The objective was to evaluate how interactions between natural regeneration and different understory plant species influence the microenvironment, and ecophysiological responses and survival of N. pumilio seedlings in harvested forests, and whether these responses differ in contrasting climatic conditions. Methods Experimental plots were established in the harvested forests, where N. pumilio seedlings were transplanted without (control) or under the influence of herbaceous understory species (i.e., understory treatments: Poa pratensis , Acaena ovalifolia, Osmorhiza depauperata ). Additional plots in a primary forest were used as a reference condition. The experiment was repeated over two growing seasons as separate trials. Microenvironmental conditions and ecophysiological variables were measured each January. Linear mixed models and multivariate analyses were performed to evaluate differences among treatments, stands and years. Results The second growing season was wetter than the first one, resulting in higher values for measured ecophysiological variables and survival rates ( p < 0.01). Among treatments, seedlings growing under O. depauperata > A. ovalifolia in harvested forests showed better ecophysiological performance (facilitation), with higher chlorophyll content, improved plant water status, and greater survival than under P. pratensis and a control. P. pratensis presented the lowest volumetric soil water content and survival rates, suggesting a competitive effect. Multivariate ecophysiological responses varied among treatments ( F = 3.3, p < 0.01), especially in the second growing season, with higher performance in O. depauperata and A. ovalifolia microsites. In unharvested forest, seedlings showed intermediate performance across both growing seasons. Conclusions Understory species influence the ecophysiological response and survival of N. pumilio seedlings growing in harvested stands by modifying microsite conditions. O. depauperata and A. ovalifolia facilitated seedling performance especially during wet years, while P. pratensis reduced seedling performance to levels similar to the control.