Changes in light and nitrogen availability under pioneer trees may indirectly facilitate tree invasions of grasslands
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1. The first trees establishing in grasslands may indirectly favour their seedlings in competition with neighbouring herbaceous vegetation by increasing soil fertility with nitrogen-rich litter and by reducing light levels under their canopies. It is predicted that increasing soil nitrogen availability will accelerate invasion of trees by stimulating their growth more than that of herbaceous species. Decreasing light availability is predicted to increase tree invasion by limiting the growth of herbaceous vegetation more than that of trees (competitive release). 2. We tested these predictions using Chinese tallow tree (Sapium sebiferum), which is an aggressive alien invader of grasslands in the southern USA, and Hackberry (Celtis laevigata), which is a native tree common in these grasslands. 3. Nitrogen and light were manipulated in two factorial field experiments in grasslands in Texas, USA. In the first, nitrogen was increased and light was decreased in plots with planted Celtis or Sapium seedlings. In the second experiment, light availability to planted Celtis or Sapium seedlings was increased by holding back prairie vegetation. 4. In the first experiment, growth of Celtis and Sapium seedlings increased with nitrogen fertilization while the above-ground biomass of prairie vegetation did not change. Prairie vegetation biomass decreased and tree seedling growth increased under shading. Sapium's growth increased dramatically in the treatment with combined nitrogen and shade. Sapium survival decreased when shade was applied. 5. In the second experiment, Sapium growth increased with increased light. Thus, increased Sapium growth at low light levels in the shade experiment was probably a consequence of decreased competitive interference from prairie vegetation, rather than better absolute performance of Sapium in low light levels. 6. These results provide evidence for facilitation as a mechanism involved in tree invasions of grasslands. Changes in resource levels, perhaps in combination with other factors, may explain rapid conversion of grassland communities to woodlands after the first pioneer trees are established. The marked response of Sapium to the combination of nitrogen and shade suggests that these positive feedbacks may be particularly strong for this alien plant species.
