Growth and abundance of desert annuals in an arid woodland in Oman

Research on the ecology of annual plants inhabiting deserts of Western Asia that have low rainfall and warm winter temperatures is extremely limited. One ecological factor that may greatly affect ephemeral growth in these deserts is rainfall interception by trees and large shrubs. Biologists have clarified how perennial plant foliage intercepts and re-distributes rainfall, but very little research has focused on the impact this may have on annual plant growth. Previous qualitative observations in this arid woodland suggest that rainfall intensity and microenvironment may interact to influence ephemeral plant production. This research quantifies the spatial patterns of the growth and abundance of an ephemeral plant community at three microsites, two beneath samr trees (Acacia tortilis (Forsskal) Hayne) and one in open desert, through five successive rainfall events. Two variables, density and dry weight biomass, were quantified on 0.25 m -2 quadrats placed at mid-canopy beneath mature trees and in open inter-spaces. For both variables, there was a significant interaction between exposure and rainfall (P = 0.001), indicating that differences in biomass and density among the microsites were dependent on each rain event. The potential role of the trees in this interaction and in determining patterns of soil moisture and plant growth is discussed. Eight annuals showed strong preferences for certain exposures under the trees. This is the first report of very fine-scale differences (1-2 meters) in the distribution of desert ephemerals. Relationships between the ecological preferences, the various microclimates and the plants' distinctive photosynthetic physiologies are outlined, as well as the relevance of this research to facilitation theory.