CYCLIC VEGETATIONAL AND EDAPHIC CHANGES IN AN ARTHRO CNEMUM COMMUNITY IN SRI LANKA

S ummary The perennial shrub Arthrocnemum indicum dominates in certain saline habitats in Sri Lanka. Its growth‐vigour declines with ageing. As the plant grows into a clump with its radiating prostrate shoots and upright aerial shoots bearing clusters of branches, it traps wind‐borne sand, so developing a hummock. Each clump on its hummock follows a cycle of pioneer, building, mature, degenerate and eroded phases with concomitant vegetational and edaphic changes. This cycle was elucidated in a community on the north‐west coast of Sri Lanka. During upgrade phases of the cycle, fine sand, organic debris and humus continue to accumulate in and around the thickly‐branched, vigorously‐growing Arthrocnemum clump, and this improves the moisture status of the developing hummock; the concentrations of nutrients also increase, as a result of accretion of wind‐borne particles and through reduction (by the closed Arthrocnemum canopy) of nutrient‐loss by wind‐erosion, surface run‐off and leaching. These trends are reversed during the downgrade phases. As the growth‐vigour of ageing shoots declines, the clump becomes increasingly open and the exposed hammock suffers wind‐induced erosion; the moisture status deteriorates as fine sand and organic matter are lost by wind‐erosion, which, together with enhanced leaching, also causes nutrient‐loss leading to soil impoverishment. The eventual death of the Arthrocnemum clump and continued hummock‐erosion complete the cycle. The subordinate Cynodon dactylon occurs more abundantly on degenerating than on developing hummocks; thus, the Arthrocnemum cycle appears to impose an inverse cyclic succession of its abundance. Accordingly, cyclic vegetational and edaphic changes in this community are closely correlated with the phasic development of the dominant A. indicum.