Plants of Pinus taeda L. from each of four families were fertilized with nitrogen (N), phosphorus (P) or N + P at planting. The H family had the highest growth in dry mass while the L family had the lowest growth. Measurements of plant hydraulic architecture traits were performed during the first year after planting. Stomatal conductance (gs), water potential at predawn (Ψpredawn) and at midday (Ψmidday), branch hydraulic conductivity (ks and kl) and shoot hydraulic conductance (K) were measured. One year after planting, dry weight partitioning of all aboveground organs was performed. Phosphorus fertilization increased growth in all four families, while N fertilization had a negative effect on growth. L family plants were more negatively affected than H family plants. This negative effect was not due to limitations in N or P uptake because plants from all the families and treatments had the same N and P concentration in the needles. Phosphorus fertilization changed some hydraulic parameters, but those changes did not affect growth. However, the negative effect of N can be explained by changes in hydraulic traits. L family plants had a high leaf dry weight per branch, which was increased by N fertilization. This change occurred together with a decrease in shoot conductance. Therefore, the reduction in gs was not enough to avoid the drop in Ψmidday. Consequently, stomatal closure and the deficient water status of the needles resulted in a reduction in growth. In H family plants, the increase in the number of needles per branch due to N fertilization was counteracted by a reduction in gs and also by a reduction in tracheid lumen size and length. Because of these two changes, Ψmidday did not drop and water availability in the needles was adequate for sustained growth. In conclusion, fertilization affects the hydraulic architecture of plants, and different families develop different strategies. Some of the hydraulic changes can explain the negative effect of N fertilization on growth.

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Water and light stresses drive acclimation during the establishment of a timber tree under different intensities of rainforest canopy coverage. CERNE, v. 25, n. 1, p. 93-104, 2019. HIGHLIGHTS In a shade-tolerant species under full sun, growth was limited by water deficit. Stress by excess of light was irrelevant but low light availability limited growth. Acclimation strategies enables plants to establish in different forest microenvironments. The capacity of the species to avoid severe stress make it proper to restoration plans.

RESUMEN.En la selva, la dinámica de disturbios de origen natural o humano genera cambios en la cobertura arbórea y en la cantidad y la calidad de luz que llega al sotobosque. Los cambios debidos a los disturbios representan un continuo de condiciones microambientales que garantizan la coexistencia de las especies. El conocimiento de los requerimientos de las especies y la evaluación de los microambientes óptimos para su supervivencia y crecimiento permite planificar estrategias de manejo a fin de recuperar áreas forestales degradadas. Trabajamos en un bosque nativo ubicado en San Antonio, Misiones, con una especie nativa de alto valor ecológico y maderero: Cabralea canjerana (Vell.) Mart. (cancharana). El objetivo fue evaluar el establecimiento de esta especie bajo diferentes intensidades de cobertura de dosel del bosque. Se establecieron 2 ensayos: uno en macetas, en el que se evaluó la supervivencia y el crecimiento de plantas expuestas a dos condiciones de cobertura natural contrastantes, y otro con plantación a campo en un gradiente de cobertura generado por claros naturales. En ambos ensayos, más de 90% de las plantas de cancharana se establecieron inicialmente en un gradiente muy amplio de cobertura. En macetas, las plantas sin cobertura (sol pleno) fueron afectadas por heladas durante el invierno. A 4 años de realizada la plantación en los claros, la supervivencia fue de 70% y la mayor mortalidad se registró en microambientes con menor incidencia de luz. Concluimos que se puede enriquecer el bosque nativo con cancharana, que el microambiente no afecta la supervivencia durante el primer año, pero que el crecimiento posterior es mayor en posiciones con menor cobertura. La especie tolera cambios abruptos de cobertura, por lo que la limpieza del sotobosque y la apertura de claros benefician su crecimiento.

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