The importance of symmetric and asymmetric competition in Iberian forests: a functional trait

Abstract

Plants compete for multiple resources such as light, water and nutrients. Asymmetric and symmetric competition indices have been related to tree responses to light versus water or nutrient availability, respectively. Although competition for light has been used as a good proxy of overall competition we might expect a high intensity of symmetric competition in water-limited forests, particularly for those functional traits related to water uptake and control (e.g. water potential loss). However, the absolute importance of symmetric and asymmetric competition in Iberian forests and how functional traits determine individual response and their effect on competition and complementarity mechanisms it not well understood. Here, we used tree growth data from the Spanish Forest Inventory spanning temperate to Mediterranean climates. We applied spatially-explicit trait-based growth neighbour analyses to quantify how traits control the intensity of symmetric and asymmetric competition between species. We found both symmetric and asymmetric competition indices were important determinants of plant-plant interactions, but the effect of symmetric competition was generally stronger than asymmetric competition. Traits controlled the same way symmetric and asymmetric competition effects on growth, although the absolute effect of competition on growth was different for each trait. Our results suggest that in waterlimited forests belowground effects are important determinants of competitive effects on tree growth.