EFFECTS OF DIFFERENT RESOURCE ADDITIONS ON SPECIES-DIVERSITY IN AN ANNUAL PLANT COMMUNITY

A commonly observed phenomenon in plant communities is that the addition of a limiting resource leads to an increase in productivity and a decrease in species diversity. The hypothesis was tested that the mechanism underlying this pattern is a disproportionate increase in mortality of smaller or shade-intolerant species in more productive sites caused by reduction of light levels. Water and/or one of 3 nutrients (N, P, K) was added to a 1st-yr-old field community dominated by weedy annuals. Diversity was not clearly related to productivity. Watering increased productivity but, contrary to expectations, had no effect on density of surviving plants, species diversity, or abundance of low-growing species. Almost all the increase in biomass with watering was due to a positive response by Ambrosia artemisiifolia, an upright annual that was the most common species in the canopy in all treatments. Addition of N had only a small positive effect on productivity, but strongly decreased density of surviving plants, species diversity and abundance of most low-growing species. Only Ambrosia increased in abundance with N addition. The P and K additions had little effect on the community. The high mortality and low diversity in N addition plots, but not in the more productive watered plots, was possibly due to limitation by N earlier than limitation by water during the growing season. The consequences was earlier canopy closure and greater mortality due to light limitation. -from Authors