Natural A Stoichiometric Comparison of Primary and Secondary Forest Stands on Acid Soils

Ariel E. Lugo; Mary Jean Sánchez; Luz S. Betancourt; Olga M. Ramos González; Iván Vicens

A Stoichiometric Comparison of Primary and Secondary Forest Stands on Acid Soils

Ariel E. Lugo^1,*, Mary Jean Sánchez¹, Luz S. Betancourt², Olga M. Ramos González¹, and Iván Vicens¹

¹International Institute of Tropical Forestry, USDA Forest Service, 1201 Ceiba Street, Jardín Botánico Sur, Río Piedras, PR 00926-1115, USA. ²University of Puerto Rico at Cayey, Cayey, PR 00736, USA. *Corresponding author.

Caribbean Naturalist, No. 89 (2022)

Abstract
A basic tenant of ecological stoichiometric theory applied to plant nutrition is that, in the absence of metabolic selection of elements, plants should reflect their soil environment. We used ecological and stoichiometric data to gain insight into how stoichiometry might influence the productivity of primary and secondary forest stands dominated by native tree species. The study involved 2 sets of measurements of soil and litter stoichiometry 26 years apart plus results from long-term ecological research in similar forest types and watersheds in the vicinity of our study area within the Luquillo Experimental Forest. We asked the following questions: What are the differences, if any, in the stoichiometry of vegetation and soil in the 2 study sites? How does soil pH influence the stoichiometry of the study sites? What are the net balances for element storage in soil and vegetation mass over the 26 years of study, and how do they compare across sites? We found that carbon molar ratios (C:N, C:P, C:S) were higher in the secondary forest and the nitrogen molar ratios (N:Ca, N:K, N:Mg, N:P) were higher in the mature forest. The secondary forest also had the highest molar ratios with phosphorus in the denominator (Ca:P, K:P, Mg:P, S:P). Soil stoichiometry separated the sites by their effective cation exchange capacity, Ca:Al ratio, and phosphorus availability. The mature forest with higher soil acidity was under greater physiological stress due to potential aluminum toxicity and a lower soil quality index than the secondary forest (45% vs. 70%). High soil acidity imposed metabolic costs to trees when concentrating, storing, and recycling elements. In spite of the above, both sites were carbon sinks. The secondary forest had a faster aboveground carbon sink as biomass and over 26 years lost calcium, magnesium, and potassium in the soil. The mature forest had a faster belowground carbon and elements sink. The contrasting stoichiometric characteristics of the dominant tree species at the 2 sites reflect the different biogeochemical niche that they occupy, which appear adaptive to prevailing site conditions.

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