Reproductive failure of a long-lived wetland tree in urban lands and managed forests.
Abstract
Land use (e.g. urbanization, agriculture, natural lands management) may directly affect populations by habitat loss and fragmentation, and indirectly by altering conditions needed for reproductive success. The effects of urbanization are especially pronounced for populations that remain among urbanized areas, but they are difficult to detect in long-lived species. We evaluated the effects of urbanization on the recruitment of cypress (Taxodium distichum), a long-lived coniferous tree that dominates isolated wetlands in Orlando, Florida, USA, a rapidly urbanizing region. Cypress requires saturated but not flooded soils to germinate, and seedlings are easily out-competed in the absence of fire. We hypothesized that urbanization has altered the hydrology and fire regimes, leading to biological inertia and reduced cypress recruitment relative to managed forest and ranchland. We found low cypress recruitment in urban areas, but surprisingly in managed forest as well. Many cypress populations in managed forest were bounded by fire breaks which prevent upland fires from burning into the wetlands. Ranchland had significantly more recruitment than urban and managed forest, and these wetlands did not have fire breaks. In urban lands, the effects of urbanization were delayed. Cypress recruitment initially occurred near the edge of wetlands where hydrological conditions were most favourable, but virtually stopped at 20 years post-urbanization. Cypress recruitment also occurred near the edge of the wetlands in managed forests and ranchlands and was higher in larger wetlands. Synthesis and applications. Urbanization is associated with the eventual reproductive failure of cypress and in the absence of management practice changes, cypress recruitment may cease in many additional wetlands. If past urbanization rates continue, 80-90% of cypress populations in isolated wetlands in the path of urban sprawl could permanently cease recruitment in 100 years. Reducing urban sprawl and introducing prescribed fire in managed-forest cypress domes could mitigate this effect and conserve reproduction of this long-lived, dominant tree species and the diversity of the wetlands they typify.