Soil Chemistry Modulates the Stoichiometry of Puerto Rican Novel Forests Dominated by Spathodea campanulata on Alluvial and Karst Substrates

Abstract

We studied the stoichiometry of 6 forest stands dominated by Spathodea campanulata (African Tulip Tree; hereafter Spathodea) on alluvial (n = 3) and karst (n = 3) substrates in the north-central region of Puerto Rico. The stands were on lands previously used for Saccharum officinarum (Sugar Cane) cultivation, subsistence agriculture, and grazing, and are representative of novel forest emergence and secondary succession in the study region. The functioning of these novel forests is poorly understood, and we sought to improve our understanding by studying the stoichiometry of forest stands in relation to substrate characteristics. Specifically, we were interested in any differences between the stoichiometry of canopy leaves, litterfall, and standing-litter forest components on alluvial and karst geological substrates, and the role of soil chemistry on regulating the stoichiometry of forest components. We also explored which stoichiometric attributes of Spathodea contribute to its role as a pioneer species in novel Puerto Rican forests and how ecosystemlevel structure and processes vary with the stoichiometry of trees and soils. On each site, we established line-transects where we collected (1) 5 soil-core samples at 0–15 cm depth and another 5 at 15–30 cm depth for bulk density and chemical analyses; (2) adult fresh Spathodea green leaflets from the top of the forest canopy; (3) recently fallen and fresh yellow Spathodea leaves, representative of senesced leaves; (4) litterfall for almost 2 years; and (5) standing litter in July 2005 and March 2006. We chemically analyzed all vegetation and soil samples for total carbon, nitrogen, and sulfur, and concentrations of aluminum, calcium, iron, potassium, magnesium, manganese, phosphorus, sodium, and sulfur and estimated molar element ratios (hereafter, element ratios) for all samples. We also measured ash concentration in plant samples and loss on ignition on soil samples. Our results show a significant influence of geological substrate type and soil properties on stoichiometric relationships among the sampled forest ecosystem compartments and fluxes. Variations in soil aluminum, calcium, nitrogen, and phosphorous content combined with soil pH variation among sites exerted an influence on the stoichiometry of canopy leaves, litterfall, and standing litter, more so than geological substrate type. Discriminant function analysis of stoichiometric ratios of soils and forest components revealed that sites were clustered according to substrate type except for 1 site, Juan Nieves, that presented distinct soil properties and consequent element ratios of forest components. The onset of flowering coincided with peaks on carbon content and spikes of the C:N ratio in adult leaves. Peaks of phosphorus and nitrogen concentration, and peaks in the ratios of C:N and C:P, in adult leaves coincided with leaf expansion following leaf drop (younger leaves with high ratio of area:mass).

Citation

Lugo, Ariel E., and Oscar J. Abelleira Martinez. “Soil Chemistry Modulates the Stoichiometry of Puerto Rican Novel Forests Dominated by Spathodea Campanulata on Alluvial and Karst Substrates.” Caribbean Naturalist, vol. 104,. 2025. pp. 1–61.