Light intensity and hydrogel soil amendment differentially affect growth and photosynthesis of successional tree species

Global climate changes have increased temperatures, radiation indexes, and consequently, irregularities in rainfall in mainly tropical countries, considerably hindering plant establishment in recovering degraded areas. The objective of this study was to evaluate the growth and physiological characteristics of one species of each successional group: pioneer, secondary, and climax when subjected to different light intensities and hydrogel as a soil conditioner during rainy and dry periods. The experiment was conducted in the ecotone between Brazil's two largest biomes, the Cerrado and the Amazon in the State of Maranhao. The parameters consisted of three species: Guazuma ulmifolia Lam. (pioneer), Astronium fraxinifolium Schott (secondary), and Cariniana rubra Gardner ex Miers (climax). There were two light intensities: 70% and 100%, and two planting conditions: with and without soil conditioner (hydrogel). Gas exchanges were higher during the rainy season; the pioneer and secondary species had greater heights and photosynthetic rates in the dry period; the climax species had the lowest gas exchange and lowest recovery as rainfall resumed. The pioneer and secondary species showed higher physiological plasticity, denoting better adaption to environments with high irradiance. Hydrogel improved the photosynthetic performance of these species in the dry season and in areas with 100% sunlight.