Thursday, August 7, 2008

PS 67-153: Seedling recruitment in experimental plantations in Los Tuxtlas, Veracruz, Mexico: Effects of the dispersal syndrome of the planted species

MarinéS. De la Peña, Universidad Nacional Autonoma de Mexico, Cristina Martinez-Garza, Universidad Autonoma del Estado de Morelos, and Henry F. Howe, University of Illinois-Chicago.


Deforestation due to agricultural activities is a common practice in Los Tuxtlas, Veracruz.  One way to accelerate natural succession in abandoned lands is by planting native tree species that attract animals. This can bring animal-dispersed seeds from the primary forest into the pasture. To evaluate the effect of the dispersal syndrome of planted species on the recruitment of seedlings in agricultural landscapes, twenty-four 30x30m cattle exclusions were established in 12 ha of active pastures. Eight exclusions were planted with 12 wind-dispersed species, and eight with animal-dispersed species (3 pioneers and 9 non-pioneers); the rest of the exclusions were not planted. Three censuses have been conducted for an initial assessment of background immigration before the planted trees reach maturity. In June 2007, one subplot (12x12m) of each of the 24 exclusions was evaluated.

Results/Conclusions Eighteen woody species were recruited with a density of 0.03 seedlings/m2, which represent only 6% of the total richness of woody species of the primary forest of Los Tuxtlas. In October 2007, following a severe and much extended 2007 dry season, four subplots of each exclusion were included. Twenty-eight woody species were recruited, while the density decreased to 0.015 seedlings/m2.  In February 2008, 31 species were recruited with a density of 0.022 seedlings/m2.  Seedlings of Bursera simaruba, were by far the most abundant species (24% of the total recruits), followed by Tetrorchidium rotundatum, Heliocarpus appendiculatus and Cecropia obtusifolia (each representing 7% of the total recruits). Most of the species were pioneers (68%) and animal-dispersed species (79%). Later, the composition of the arriving species is expected to change from pioneers to late-successional species. By contrast, for plots at least 100m away from the closest forest, where > 70% of the species are animal-dispersed, colonization of wind-dispersed species is likely remote. The highest increment in growth was observed for Trema micrantha in height (14.56 cm/month) and for Carica papaya in diameter (7.20 mm/month). Only 13% of the recruits died or were not found. This extremely low density agrees with low seed rain of forest species in pastures. Bursera is common in the landscape because it is the primary species used for living fences in the region. Heliocarpus and Cecropia are the most common pioneer species in the region, and their arrival is likely.  The creation of patches of forest distributed across open areas can reestablish the flow of plants and animals across the landscape.