Thursday, August 6, 2009 - 10:10 AM

COS 89-7: Seed and seedling dynamics in three habitats in the tropical rain forest of Los Tuxtlas, Mexico

Marinés De la Peña, Universidad Nacional Autonoma de Mexico, Yuliana Urincho-Pantaleon, Universidad Autónoma del Estado de Morelos, and Cristina Martinez-Garza, Universidad Autonoma del Estado de Morelos.

Background/Question/Methods: In pastures natural succession depends on the availability of seeds and/or suitable microenvironments for recruitment. Which species arrive and which ones are capable of persisting in open pastures will determine the composition of the future plant community.  To analyze dispersal and establishment limitation in primary and secondary forest and in pastures, we quantified the seed rain and seedling recruitment at Los Tuxtlas, Veracruz, Mexico. Dispersal limitation is defined as the proportion of seeds traps that were not reached by seeds of a given specie and establishment limitation as the proportion of stations were seeds arrived but seedlings were not present. Seed rain was quantified during 12 months in 120 m2 and recruitment in >1.5 ha over the three habitats.

Results/Conclusions: Jaccard similarity indexes for recruited species indicated that primary and secondary forest shared > 50 % of the species, while pastures excluded from cattle since June 2006, shared very few species with secondary forest (0.17) and shared even less species with primary forest (0.02). Only one late-successional species (Ficus perforata) and four pioneers (Trichospermum galeottii, Heliocarpus appendiculatus, Eupatorium galeottii and Cecropia obtusifolia) were present in the seed rain and the recruited community at all three habitats. In primary and secondary forest, these five species had very low limitation in dispersal (0-3%), but extremely high limitation in establishment (83-100%). In pastures, all species showed very high dispersal limitation (86-98%), except for two wind-dispersed species, Eupatorium (60%) and Heliocarpus (38%) both of which showed high establishment limitation (79 and 86%). Cecropia, regardless of its immense dispersal limitation in pastures (97%), had the lowest establishment limitation (67 %) comparing to the other four species. Overall species and habitats, there was no a correlation of seed density on recruits density (r = 0.14, P > 0.1, N = 24). For primary forest there was a negative correlation of seed density on recruits density (r = -0.48, P < 0.05, N = 14). Mostly all species showed an extremely high dispersal and establishment limitation in pastures. However, limitation in establishment may not always be explained by the lack of seed arriving at a given year.  Natural succession in pastures is very slow and the resulting secondary forest will show a completely different plant composition compared to the primary forest.