Testing for functional significance of traits: Effect of the light environment in tropical tree saplings.

Functional traits have been examined to explain the growth rates of forest communities in different sites. However, weak or nonexistent relations are often found, especially due to the following methodological aspects: 1) lack of an environmental context (e.g., light, water, or nutrient supply), 2) use of nonfunctional traits, 3) an approach that does not contemplate phenotypic integration, and 4) neglect of intraspecific variation.Here we measured relative growth rates, crown, and leaf traits in saplings of six tropical tree species growing in two light environments (Gap and Understory) to test whether contrasting light environments modulates trait-trait and trait-growth relationships. Moreover, we tested whether models that integrate traits of different dimensions of the plant (crown and leaf) improve the strength of trait-growth relations.Light availability changed both trait-trait and trait-growth relationships. Overall, in Understory, crown traits (crown length and total leaf area) have a stronger effect on growth rates, while physiological traits related to nutrient acquisition (nitrogen concentration), photochemical efficiency (chlorophyll pigments and chlorophyll a fluorescence), and biochemical efficiency (potassium use efficiency) are strong in Gap. Models including multiple traits explained growth rates better in Gap (up to 62%) and Understory (up to 47%), but just in Gap the best model comprises traits that are representative of different dimensions of the plant. Synthesis. We advanced the knowledge behind the light effects on tree sapling by posit that trait-trait and trait-growth relationships vary across light environments. Therefore, light availability is a key environmental factor to be considered when choosing the set of traits to be measured in functional approach studies using tropical tree saplings. In compliance with the phenotype integration hypothesis, functional traits are better predictors of growth rates when grouped in a set of traits of different dimensions of the plant that represent different functional mechanisms.

How do silvicultural treatments alter the microclimate in a Central Amazon secondary forest? A focus on light changes.

Silvicultural treatments can change the microclimate inside tropical secondary forests and thus enable the artificial regeneration of ecologically and economically important tree species. Increasing levels of canopy tree refinement (diameter at breast height, DBH > 5 cm) were applied and combined with understory slashing to investigate how these silvicultural treatments affect the microclimate of a Central Amazon secondary forest. The refinement treatment was performed in six levels of basal area reduction (0, 20, 40, 60, 80, and 100%) in rectangular plots (2318 m2) and was equally divided in two subplots that did (understory slashed) or did not (control) receive the application of understory slashing. Canopy openness was estimated using hemispherical photography before treatment implementation and periodically over 26 months. Light transmittance, total daily irradiance, air temperature, air humidity and soil moisture were measured during two climatic seasons (Dry and Wet season) of the two years following the application of the treatments. Understory slashing doubled the canopy openness before the refinement and had an effective and persistent effect on canopy openness, light transmittance and total daily irradiance for the 26 months. Refinement increased canopy openness, light transmittance and total daily irradiance; however, after one year of treatment application, the effect was greater in understory slashed than in control subplots. In plots with higher basal area reduction (>60%), the understory slashed subplots total daily irradiance was 19% and 60% higher than control subplots after nine and 26 months, respectively. Refinement increased air temperature and reduced air humidity and soil moisture. The refinement of canopy trees and understory slashing change the microclimate (particularly light availability) in secondary forests and performed best when applied together. Silvicultural implications for sustainable secondary forest management and productive objectives are discussed.