Photosynthetic response of tropical rainforest seedlings to nitrogen amendment

Abstract

The effect of fertilization and forest gap creation on seedlings of rain forest species typical of deep shade understorey and exposed sites was assessed. We tested the hypothesis that species found growing naturally in different light environments express different stategies for resource acquisition when presented with a nutrient pulse under non-limiting light conditions. The response of seedlings to nutrient amendment after a low-to-high light transition, simulating canopy gap formation, was investigated through analyses of growth, photosynthesis, thermal energy dissipation, and photosynthetic pigment content. Fast-growing species increased their rates of growth and photosynthesis in response to treatment, whereas slow-growing species failed to respond to nutrient amendment. For all species, growth under high nutrient conditions did not affect PSII efficiency or the level of non-photochemical quenching under high light. Acclimation of these photochemical parameters was more strongly correlated with increased exposure than nutrient availability. Nutrient availability was correlated with the retention of chlorophyll content which was maintained at high levels in fast-growing species after the low-to-high light transition. However slow-growing species lost considerable chlorophyll irrespective of nutrient availability. We concluded that slow growing species of the rain forest understorey had a limited potential to utilize an increase in resource availability in comparison to fast growing species, and that growth strategy may constrain abilities to uptake and utilize additional resources that become available following tree-fall events.