Soil resource availability, plant defense, and herbivory along a successional gradient in a tropical dry forest

Tropical dry forests (TDFs) usually occur as mosaics at different stages of natural regeneration, along which variations in soil attributes (fertility and texture) can affect plant growth and leaf traits and, consequently, leaf herbivory. In the present study, we used a chronosequence approach to determine the variations in plant growth, leaf defense, and insect herbivory in different successional species groups, following the predictions of the resource availability hypothesis: plants at early successional stages would grow faster, have lower specific leaf mass (SLM), and suffer higher herbivore damage than late successional plants. For this purpose, we measured plant diameter and collected leaves at the end of the rainy season (April) during three consecutive years in three different successional stages (early, intermediate, and late). In total, we sampled 38 plant species, from which 31 species were exclusive to a single successional stage (10 early, 13 intermediate, and eight late species). Our results did not conform to the resource availability hypothesis, as plants growing in early successional stages had higher growth rates and SLM, and lower leaf damage by herbivores. Soil fertility and texture did not affect plant growth or SLM, but leaf damage was negatively affected by SLM. Apart from being a defensive trait, SLM is also involved in the conservation of water and nutrients and photoprotection, and its variation along successional gradients in TDFs conforms to a continuum from conservative traits in hot and dry, early successional stages, to acquisitive traits in more humid late stages. Although other plant defenses must be considered, our three-year study indicates that the resource availability hypothesis, which was formulated for wet forests, should be adapted to deciduous TDFs where water availability is the main environmental filter along succession.