Effects of nitrogen and phosphorus fertilization on the survival and recruitment of seedlings of dominant tree species in two abandoned tropical dry forests in Yucatan, Mexico

Seedling dynamics were studied in a set of dominant tree species in regenerating secondary tropical dry forests (TDF) growing on limestone in the Yucatan Peninsula. The objective of the study was to assess how variation in nutrient availability affects the recruitment and survival of individual species considering natural variations in light and bulk density in the topsoil. Our study included an area of young forest (10 years old), with phosphorus-poor soils, and an area of old forest (congruent to60 years old). We used 16 plots (12 m x 12 m) per forest, in which we fertilized four plot replicates per treatment with nitrogen (N), phosphorus (P) and nitrogen plus phosphorus (NP). Another four replicates were kept as control. In four 1 m x 1 m sub-plots set of each experimental plot, tree seedling dynamics were studied over 2 years. The species with the highest recruitment were Acacia gaumeri and Leucaena leucocephala in the young forest, and Bursera simaruba and Phyllostyllon brasiliense in the old forest. Recruitment of A. gaumeri was affected more by light availability than fertilization, but its survival increased with N or with NP additions. Leucaena leucocephala benefited by fertilization (N, P, or NP) and the species regenerated on sites with lower light availability, but required high light conditions and N addition to increase its survival. The recruitment and survival of B. simaruba required low light availability, and the fertilization with P or with NP increased its regeneration. Phyllostyllon brasiliense was recruited only in the old forest. The addition of P resulted in the highest recruitment and the lowest survival among fertilization treatments, leading to a negative final seedling density. Canonical analysis revealed that in the young forest, the most important variables that influenced species recruitment were bulk density in the topsoil, fertilization with P, fertilization with N, and light availability. In the old forest, the P and N fertilizations and bulk density in the topsoil appeared to be the most important variables; only light was not significant. Light requirement differences between forests suggest that a large number of opportunistic species in the young forest could exist there. Overall, our results suggest that the dynamic of understory vegetation in Yucatan Peninsula is strongly influenced by nutrient availability, especially P. The responses of seedling dominant species to nutrient addition will vary, depending on the details of the site's light availability and soil bulk density, and the identity of species. (C) 2003 Elsevier Science B.V. All rights reserved.