Using leaf δ13C and photosynthetic parameters to understand acclimation to irradiance and leaf age effects during tropical forest regeneration

Abstract

Regenerating forests are important for the maintenance of tropical biodiversity. Forest management in fragments of Atlantic forest in Brazil includes removal of exotic eucalyptus trees that were once part of plantations, in order to reestablish native flora. However, it is unclear how native tree saplings regenerating under former plantations respond to abrupt changes in environmental conditions associated with exotic tree removal. We used leaf carbon isotope composition (δ13C) and photosynthetic parameters to evaluate physiological responses of native tree saplings to canopy opening. We analyzed young and mature leaves of the three most representative species of regenerating trees (Byrsonima sericea, Siparuna guianensis, Xylopia sericea) in one secondary forest fragment and three managed areas that form an irradiance gradient (9, 85, 230 and 550μmolm−2s−1) in Brazilian Atlantic forest. Eucalyptus removal increased photosynthetic CO2 assimilation and stomatal conductance in mature leaves of all species, but there was no change in intercellular CO2 concentration. In young leaves, two species showed increasing A and one species showed increasing gs in response to canopy opening. Leaf δ13C did not vary significantly among species, but site and age affected δ13C, as leaves from shaded sites showed lower δ13C values (around −33‰) than leaves from lighter sites (around −30‰), and young leaves showed higher δ13C values (around −30‰) than mature leaves (around −32‰). Mature leaves showed greater photosynthesis and stomatal conductance than young leaves. The sensitivity of young leaf δ13C to irradiance increases suggests that δ13C in these organs is controlled not only through carbon imported to new leaves during growth, but also through direct responses of stomatal control and carboxylation as these young leaves develop their photosynthetic competency. Young and mature leaves showed decreased total chlorophyll/carotenoids with increasing irradiance, indicating acclimation capacity from early developmental phases. Young leaves in high irradiance sites showed susceptibility to irradiance stress (Fv/Fm around 0.7), but values for mature leaves did not show high irradiance stress (Fv/Fm around 0.8). In conclusion, forest management affected leaf δ13C of the main regenerating understory species, with site effects being more important than species-specific features for photosynthetic performance. The data also indicate that these species are resilient to forest management that includes exotic eucalyptus canopy tree removal. In this context, carbon stable isotopes can be considered as recorders of ecological change and can be used to study the effects of management on forest regeneration and photosynthetic competency.