Decoupling seedling establishment in a shade-intolerant species of a Mediterranean climate: Soil moisture determines survival but growth is promoted by irradiance

In Mediterranean climates, drought is recognized as the main abiotic stress negatively affecting plant survival and growth after establishment. However, several factors besides water scarcity interact to affect plant performance, including stocktype, field establishment techniques, environmental variability, and the inherent ecological requirements of the target species. To fully comprehend the extent each of these factors influences plant performance, we analyzed plant survival, growth, and physiological performance of contrasting stocktypes of Nothofagus glauca seedlings, a shade-intolerant tree species from Mediterranean central Chile, established under different environmental conditions characterized by a shrubland versus a forest canopy cover. We hypothesize that nitrogen-loaded plants will have higher survival and growth during establishment, and that higher performance will be observed under the shrubland conditions, despite the presence of water constraints that are usually observed in more open canopies. Our experimental design induced contrasting environmental conditions in which the forest canopy was characterized by lower irradiance and water availability during summer versus the shrubland condition. As expected, bigger and nutrient-loaded N. glauca plants had better field performance in terms of survival and growth. After three years of assessment, we did not observe differences in survival between canopy conditions, but plants established under shrubland canopy presented significantly higher biomass accumulation and better water status. According to our relative importance analysis, we observed that soil water availability was the most relevant factor for plant survival, but plants showed more growth when established under a condition of increased irradiance. Considering that plant performance is explained by the combined response of survival and growth, we suggest that under Mediterranean conditions, each response could be independently affected by separate environmental factors as well as by target species biology (i.e., shade tolerant or shade-intolerant).