The role of soil resources in an exotic tree invasion in Texas coastal prairie

1 Variation in resource requirements among plant species may cause changes in community composition when resource supply rates vary. Because exotic plants may differ in their requirements compared with native plants, changes in resource levels or ratios may change their invasive potential. The concentration hypothesis, nutrient balance hypothesis and resource ratio hypothesis make different predictions regarding the effects of nutrient additions on plant communities and the success of invaders. 2 We investigated these hypotheses using Chinese tallow tree ( Sapium sebiferum ), which is an aggressive invader of grasslands in the southern USA. Soil nitrogen, phosphorus and potassium were manipulated in a full factorial field experiment in a Texas coastal prairie. 3 Survival of Sapium seedlings over three growing seasons was independent of all three types of nutrient additions, alone or in any combination. Adding nitrogen or potassium increased the mass and height of Sapium seedlings. Adding phosphorus increased the mass and height of Sapium seedlings only when nitrogen was also added. These results support the nutrient balance hypothesis, which assumes that higher tissue concentrations of a nutrient reflect a greater ability of a plant species to take up and/or conserve that nutrient. 4 The above-ground biomass of background prairie vegetation was independent of the addition of each type of nutrient alone or in any combination. However, the functional composition of the community shifted from graminoids to forbs with the addition of nitrogen and from graminoids to woody plants with the addition of phosphorus. These results support the concentration hypothesis, which assumes that higher tissue concentrations of a nutrient reflect a greater requirement for that nutrient. The contrasting responses of native woody vegetation and Sapium to phosphorus addition suggest that Sapium may have unusually low demands for phosphorus relative to nitrogen for a woody plant in this community. 5 These results show that potassium or nitrogen addition, perhaps in combination with phosphorus addition, has the potential to accelerate the invasion of Sapium into Texas coastal prairies. Because anthropogenic nutrient additions are common in grasslands worldwide, this may help to explain the frequent success of woody species invading grasslands.