INFLUENCE OF MINERAL NUTRIENT AVAILABILITY ON GROWTH OF TREE SEEDLINGS FROM THE TROPICAL DECIDUOUS FOREST

The effects of different nutrient availabilities on growth and biomass partitioning in seedlings from the tropical deciduous forest in Mexic o were compared. The tree species studied were Heliocarpus pallidus, a species associated with disturbed parts of the forest, and Caesalpini a eriotachys, Jacquinia pungens and Recchia mexicana, species from mat ure, undisturbed habitats. The tropical deciduous tree seedlings were grown in pure silica sand for 50 days inside growth chambers under fou r nutrient regimes; 5, 20, 100 and 200% Long Ashton nutrient solutions . Data showed contrasting responses among species to different nutrien t availabilities. Except Jacquinia pungens, all species had increased growth and productivity as nutrient level increased from 5 to 100%; ho wever, no significant differences in these parameters were detected be tween 100 and 200% in all species. Compared with mature forest species , pioneer species showed higher variations in biomass production, rela tive growth rate and net assimilation rate. In contrast to mature fore st species, root/shoot ratios in Heliocarpus pallidus were greater and thus showed higher biomass allocation to roots when nutrient supply w as limited. This response suggests higher phenotypic plasticity in pio neer species. Species from mature parts of the forest (Caesalpinia eri ostachys, Recchia mexicana) showed less dependency on nutrient supply than pioneer species. These responses appear to support observations f rom studies with temperate plants investigating growth responses to so il fertility.