Plasticity on the spectrum: photosynthetic induction of tropical forest tree species in contrasting light conditions.

Tree species differ in their ability to use light efficiently, affecting carbon gain, establishment and survival in highly heterogeneous environments. This efficiency relies on the maintenance of the photosynthetic induction state, regulated by structural, biochemical, photochemical and stomatal processes that vary along the leaf economics spectrum (LES). Slow return species, such as shade-tolerant species (often late successional), are thought to sustain higher photosynthetic induction state, while quick return species, like light-demanding species (often early-successional) would have lower shade acclimation and shade-tolerant species lower acclimation to high light. Yet, results often deviate from these predictions. Moreover, most LES traits reflect steady state performance, not dynamic responses. Here, we investigated photosynthetic induction responses in four widely distributed Brazilian tree species representing contrasting successional groups and LES positions, grown under 10% light, 50% light and full sun. We quantified induction dynamics in terms of CO2 assimilation, stomatal conductance, electron transport rate, as well as chlorophyll content, and leaf mass per area (LMA). Acclimation to distinct light environments was assessed using a shade adjustment coefficient and a novel metric based on principal component analysis, relative plasticity (RP). Relative plasticity suggests an asymmetrical bell-shaped relationship with LES position: the slow return Hymenaea courbaril L. showed low plasticity and little change in resource allocation (LMA), photosynthetic rates or induction times; the fast-return Schinus terebinthifolia Raddi displayed moderate plasticity but unexpectedly high shade acclimation, showing high induction state and CO2 assimilation rates; and the intermediate strategists Cecropia pachystachya Trécul and Handroanthus impetiginosus (Mart. ex DC.) Mattos exhibited the highest plasticity, with coordinated increases in LMA, CO2 assimilation, stomatal conductance and photosynthetic induction under increasing light conditions. These findings highlight the importance of integrating photosynthetic dynamics into ecophysiological frameworks for species selection in reforestation, particularly in heterogeneous light environments, where adaptive flexibility can play a critical role on the resilience of an ecosystem.