Herbchronology of woody underground organs in the Brazilian tropical grassy ecosystems: Age estimates and dendrochronological potential

Abstract

Herbchronology, a branch of dendrochronology, is dedicated to understanding the age structure and ecology of small plants, including perennial forbs, dwarf shrubs, and shrubs. In tropical grassy ecosystems, fire, frost or large herbivores can recurrently suppress the above-ground biomass of small plants, hindering age determination from the stems. Their underground organs, however, remain protected below the soil surface, allowing resprouting. Therefore, these organs attain greater longevity than the aboveground plant parts, likely corresponding to the true age of these plants. This exploratory study aimed at and (i) analyzing the growth layers to verify their distinctiveness and potential for herbchronological studies, and (ii) estimating the age of woody underground organs across various small plant species from different types of old-growth grasslands. Randomly collected samples from 204 plants were analyzed, encompassing 28 families and 107 species across three different grassland types within the Atlantic Forest and the Cerrado regions (Brazil). From all samples, 55 % exhibited distinct growth layers, highlighting the potential for centennial chronologies, despite limitations in species with low longevity and indistinct rings. Most samples were less than 10-year-old, but we found six individuals beyond 100-year-old. Higher ages were more frequent in Cerrado grasslands under long dry season, and no plants beyond 10-year-old were found in highland grasslands under the humid climate of the Atlantic Forest region. By validating a method to determine the age of small plants, our study provides the foundation for future research and, especially, allows to estimate the age of grassy ecosystems and thus strengthen the arguments in favor of their conservation.