Effect of competition with annual plants on the growth and establishment of seedlings of Acacia papyrocarpa, a tree from arid lands in southern Australia

Many long-lived trees in arid lands establish only after a series of large rainfall events. These conditions also result in high biomass of herbaceous plants, potentially negating some of the benefits of increased water availability. The effect of competition on tree seedlings in arid lands has seldom been investigated.  This study aims to assess the potential for competition with grasses to affect the growth and establishment of Acacia papyrocarpa seedlings. A. papyrocarpa seedlings were grown in a glasshouse experiment in competition either with the perennial native grass Austrodanthonia caespitosae or by itself.  In order to test how resource availability affects the competitive effect, eight watering regimes were applied which varied in size and frequency. Shoot and root biomass were assessed separately for the target species and its competitor. Photosynthesis efficiency of A. papyrocarpa seedlings was measured using pulse- amplitude – modulation technique at both predawn and afternoon every two weeks. Water potential of A. papyrocarpa roots was measured at the end of experiment. Biomass of both roots and shoots of   A. papyrocarpa seedlings was larger in the absence of competition with A. caespitosae. Biomass was directly proportional to the amount of water received and inversely related to the time between watering in the treatment. Photosynthesis efficiency of   A. papyrocarpa seedlings was negatively affected by competition and reduced water availability.  The results confirmed that competition with annual plants can affect the growth and establishment of A. papyrocarpa seedlings and that this effect increases with stressful water conditions, such as the ones that occur during the dry periods that usually follow pulses of water availability.