Investigating above- vs. below-ground competition by accounting for azimuth of competitors in native eucalypt forests

Abstract

Competition for both above-ground and below-ground resources plays a crucial role in shaping the structure and dynamics of plant communities. Above-ground competition for light may be more azimuthal and size-asymmetric than below-ground competition. However, there has been limited research on azimuthal effects in competition and development of related methods that may help distinguish between above- vs. below-ground competition. The aims of this study were to 1) develop novel approaches to evaluate the azimuthal effect of competition; 2) illustrate and evaluate these approaches by applying them to dominant eucalypt species in two mid-latitude Mediterranean climate forests; and 3) show how these methods can provide insights into the relative importance of above- vs. below-ground competition in these different species. We developed, applied and evaluated three new methods to evaluate azimuthal differences in competition, using models predicting DBH growth in two different forests (Eucalyptus marginata, jarrah and Corymbia callophyla, marri) and karri forest (E. diversicolor, karri; and Corymbia callophylla, marri) of south-west Western Australia. The foliage density of karri/marri is higher than that of jarrah, and the karri/marri site had higher rainfall. Both sites included thinned (thinning from below) and unthinned plots, regenerated naturally, and aged 100 year (jarrah) and 50 year (karri and marri). First, we compared the ability to predict growth using competitors in specific azimuths only vs. all competitors together. Second, we investigated whether weighting competitive effects of neighbours according to their azimuth could improve growth predictions. Third, we investigated whether growth prediction could be improved by accounting for relative size of competitors as well as their azimuth. We found evidence of an azimuthal effect of competition on growth in jarrah, karri and marri forest through all three new approaches. The third approach showed the existence of size-asymmetric competition. Improvement in model prediction was relatively small when accounting for the azimuth, which suggests that above-ground competition for light is low and most competition in jarrah, karri and marri forests is for below-ground resources. The three approaches developed and illustrated in this study can potentially help identify, distinguish and understand above- vs. below-ground competition for tree species in different parts of the world where competition among trees is one of the limiting factors of forest growth, dynamics and health.