Mineral content of leaves from trees growing on serpentine soils under contrasting rainfall regimes in Puerto Rico

Abstract

The interactions between water and soil nutrient availability in determining leaf nutritional composition and structural features were investigated in forests on serpentine in Maricao and Susua (Puerto Rico). These forests grow under contrasting rainfall regimes: Maricao is a wet forest located at altitudes above 500 m and receiving more than 2500 mm rainfall, while Susua is a humid forest located well below 500 m, with less than 1500 mm rainfall and a well defined dry season. Dominant tree species and soils were analysed for N, P, K, Ca, Mg and Ni. Soils can be differentiated according to their K content (higher in Maricao) and P contents (higher in Susua). Mature leaves of both forests have sclerophyllous characteristics as judged from the Specific Leaf Areas (<80 cm2 g-1) and low P contents. Leaf area development is strongly correlated with leaf N and P contents in both forests, but Maricao samples appear to be more limited by P availability. In concordance with soil values, the Susua leaf sample set has significantly higher contents of P, but lower contents of K when compared with the Maricao sample set. Analyses of soluble K, Ca, and Mg reveal strong physiological selectivity in the absorption of these cations. K/Ca and Ca/Mg ratios are markedly higher in the soluble leaf extracts than in the soil extracts. It seems that restriction to vegetation development in the serpentine areas investigated are more related to nutritional deficiencies and not to high contents of either Mg or Ni in the upper soil layers. Only two strong Ni accumulators were found, Cassine xylocarpa (1.2 μmol Ni g-1 dry mass or 70 μg g-1) from Susua, and Chionanthus domingensis (12.2 μmol g-1, or about 700 μg g-1) from Maricao. These species are not restricted to serpentine areas in Puerto Rico.