Differential effects of Typha litter and plants on invasive Lythrum salicaria seedling survival and growth

Invasive species are a problem because of their detrimental ecological and economic effects. Increased disturbance caused by human impacts is hypothesized as a primary factor promoting the spread of invaders. Plants and plant litter can have important effects on plant colonization and community composition by affecting seedling survival and growth. I examined the hypothesis that invasion of non-native Lythrum salicaria in Typha-dominated marshes is disturbance-dependent. If so, the removal of Typha plants or litter would increase the survival and growth of L. salicaria seedlings. Additionally, the removal of both plants and litter could result in an additive or synergistic effect on the establishment of L. salicaria. Alternatively, L. salicaria may be a successful invader because it has a high capacity to establish and grow regardless of neighbours. In this case, L. salicaria would be expected to perform well even in plants and litter. Strategies for managing L. salicaria will depend on which factors promote invasion. I measured the differential effects of plants and litter, alone and in combination. on the survival and growth of L. salicaria seedlings transplanted into marshes. The presence of plants and litter did not affect seedling survival in relatively dry wetland sites, indicating that L. salicaria seedlings have the capacity to persist in the presence of neighbouring Typha spp. competitors. However, removal of both plants and litter allowed increased growth of L. salicaria seedlings in drier wetlands. Therefore, growth was facilitated by disturbance that removed all vegetation. Small disturbances (0.6 m(2)) decreased competitive suppression by native Typha spp. neighbours and resulted in significant increases in growth. Disturbance of wetlands at risk of invasion by L. salicaria should be avoided.