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

Ariel E. Lugo; Oscar J. Abelleira Martínez

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

Ariel E. Lugo^1,2,* and Oscar J. Abelleira Martínez³

¹International Institute of Tropical Forestry, USDA Forest Service, 1201 Ceiba Street, Jardín Botánico Sur, Río Piedras, PR, 00926-1115, USA.²Current address - HANAEL Associates, Inc., 1528 Calle Tamesis, Río Piedras, PR 00926-2820, USA. ³Department of Agro-Environmental Sciences,University of Puerto Rico, Mayagüez, PR 00681, USA. *Corresponding author.

Caribbean Naturalist, No. 104 (2025)

Published 11 August 2025

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 ecosystem-level 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). The ratios of C:N and C:P in standing leaf litter tended to be higher for Spathodea senesced leaves, reflecting translocation, and lower for fragmented leaves, reflecting decomposition. The retranslocation percent for phosphorus (28% vs 44%) and nitrogen (39% vs 47%) was higher on Spathodea leaves on stands on alluvial vs karst substrate. Nitrogen and phosphorus retranslocation reached a peak rate coinciding with a peak in their concentrations in adult leaves. Element ratios of C:P, C:N, N:P, and Ca:P in Spathodea adult leaves, in litterfall and standing litter differed among sites on alluvial and karst substrates due to differences in these element ratios in the soil compartment. Sites on karst were more efficient in retaining phosphorus relative to fallen Spathodea leaf mass, but no differences were observed for other elements. All sampled forest compartments tended to have higher ratios of N:P and C:P at karst than alluvial sites, but the magnitude of those element ratios were low compared to those of historic successional and mature forests. The storage to 30 cm depth of soil nitrogen, phosphorus, and calcium correlated negatively with rates of Spathodea and total leaf-litter fall (leaffall; nitrogen and phosphorus), total litterfall rates (phosphorus and nitrogen), rates of calcium and phosphorus return in leaffall (nitrogen and calcium, respectively), and phosphorus storage to 30 cm depth (calcium). Soil pH corelated with soil element concentrations, soil element ratios, element ratios of Spathodea adult leaves, element ratios of Spathodea leaffall, element ratios of fragmented leaves in standing litter, and soil element storage of carbon, calcium, aluminum, iron, manganese, and phosphorus to a depth of 30 cm. These results illustrate a tight coupling between soil and forest compartments and fluxes of elements, with little nutrient limitations to the growth of Spathodea stands. However, Spathodea stands at karst sites are limited in phosphorous relative to those on alluvium. Our results also show that novel Spathodea forests are among the tropical forests subject to the lowest nutrient limitations on processes related to tree growth and standing-litter biomass accumulation, especially on alluvial substrates. Soil properties, including pH, are better predictors of stoichiometric differences in ecosystem structure and fluxes from canopy to soil than geological substrate type among novel Spathodea forests in Puerto Rico..

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