The importance of conspecific facilitation during recruitment and regeneration: A case study in degraded mangroves

Abstract

Facilitation can affect positively the survival, growth and fitness of neighboring plants and is able to promote the establishment of plant species under harsh environmental conditions. This study is one of the few to focus on conspecific nurse plants enabling the establishment of tree followers and the generally unknown interplay between facilitation and competition during recruitment and establishment.Field studies were conducted in an hypersaline area in north Brazil where the hydrological regime was disturbed by road construction in 1974. Avicennia germinans shrubs were able to recolonize subareas.Three different stages of re-colonization were identified by satellite imagery based on the vegetation coverage and were defined as the early, middle, and late stages. Different plant parameters, including height and crown radius of individual trees, were measured, and all plants separating mangrove seedlings and larger plants were stem-mapped.The Hegyi index was adapted to measure local interaction intensity, considering both positive (facilitation) and negative (competition) interactions among neighboring plants. Spatial point pattern analyses were combined with the interaction index to obtain a better overview of the strength and importance of the plant interactions within the different recolonizing stages.The spatial patterns displayed aggregation for all plants in all developmental stages. This is supported by the interaction index calculated for seedlings. The index, however, provides an early signal for the switching of plant interactions from facilitation to competition as the developmental stage increased. This feature remains undetected in the spatial patterns because self-thinning processes are linked to individual growth processes and require more time to take place.This study broadens the ecological concept of nurse plants from hetero- to conspecific, including the importance and strength of plant interactions. The studied degraded areas are recolonizing naturally, and conclusions can be drawn for restoration projects, which usually do not take facilitation effects into account.