Stem, root, and older leaf N:P ratios are more responsive indicators of soil nutrient availability than new foliage.

Foliar nitrogen to phosphorus (N:P) ratios are widely used to indicate soil nutrient availability and limitation, but the foliar ratios of woody plants have proven more complicated to interpret than ratios from whole biomass of herbaceous species. This may be related to tissues in woody species acting as nutrient reservoirs during active growth, allowing maintenance of optimal N:P ratios in recently produced, fully expanded leaves (i.e., "new" leaves, the most commonly sampled tissue). Here we address the hypothesis that N:P ratios of newly expanded leaves are less sensitive indicators of soil nutrient availability than are other tissue types in woody plants. Seedlings of five naturally established tree species were harvested from plots receiving two years of fertilizer treatments in a lowland tropical forest in the Republic of Panama. Nutrient concentrations were determined in new leaves, old leaves, stems, and roots. For stems and roots, N:P ratios increased after N addition and decreased after P addition, and trends were consistent across all five species. Older leaves also showed strong responses to N and P addition, and trends were consistent for four of five species. In comparison, overall N:P ratio responses in new leaves were more variable across species. These results indicate that the N:P ratios of stems, roots, and older leaves are more responsive indicators of soil nutrient availability than are those of new leaves. Testing the generality of this result could improve the use of tissue nutrient ratios as indices of soil nutrient availability in woody plants.

Nutrients limit photosynthesis in seedlings of a lowland tropical forest tree species.

We investigated how photosynthesis by understory seedlings of the lowland tropical tree species Alseis blackiana responded to 10 years of soil nutrient fertilization with N, P and K. We ask whether nutrients are limiting to light and CO(2) acquisition in a low light understory environment. We measured foliar nutrient concentrations of N, P and K, isotopic composition of carbon (δ(13)C) and nitrogen (δ(15)N), and light response curves of photosynthesis and chlorophyll fluorescence. Canopy openness was measured above each study seedling and included in statistical analyses to account for variation in light availability. Foliar N concentration increased by 20% with N addition. Foliar P concentration increased by 78% with P addition and decreased by 14% with N addition. Foliar K increased by 8% with K addition. Foliar δ(13)C showed no significant responses, and foliar δ(15)N decreased strongly with N addition, matching the low δ(15)N values of applied fertilizer. Canopy openness ranged from 0.01 to 6.71% with a mean of 1.76 ± 0.14 (± 1SE). Maximum photosynthetic CO(2) assimilation rate increased by 9% with N addition. Stomatal conductance increased with P addition and with P and K in combination. Chlorophyll fluorescence measurements revealed that quantum yield of photosystem II increased with K addition, maximum electron transport rate trended 9% greater with N addition (p = 0.07), and saturating photosynthetically active radiation increased with N addition. The results demonstrate that nutrient addition can enhance photosynthetic processes, even under low light availability.

Crecimiento, supervivencia y herbivoría de plántulas de Brosimum alicastrum (Moraceae), una especie del sotobosque neotropical

Growth, survival and herbivory of seedlings in Brosimum alicastrum (Moraceae), a species from the Neotropical undergrowth. Growth responses, survival, and herbivory, on seedlings of Brosimum alicastrum were studied in a neotropical Mexican forest. We selected 122 seedlings and divided them into three groups assigned to defoliation treatments: control or 0 (n=21), 50 (n=51) and 90% (n=50). Every 4 months during two years we measured seedling growth (in terms of relative growth rate in biomass, leaf area growth, produced leaves and height growth) and survival. In addition, we evaluated every 12 months pathogen damage and insect herbivory using a 2 mm-2 grid. Separately, we estimated mammal herbivory in 3-month old seedlings that were selected within a plot of 500 m x 10 m (N=1095). Pathogen damage and insect herbivory were evaluated within the same plot in 113 seedlings. We found that 50 % defoliated seedlings showed compensatory responses in all growth parameters. Relative growth rate and height growth also had a compensatory response in seedlings at 90% defoliation. Relative growth rate and leaf area growth gradually decreased with time although height growth seedling showed an opposite pattern. Leaves produced were not affected by time. Estimated seedling survival probability increased with defoliation to a maximum of 97%, decreasing at 24 month to 37%. Mammal herbivory was more frequent and severe than herbivory caused by pathogens and insects. In some cases, mammal herbivory produced total defoliation. Compensatory growth in leaf area growth, produced leaves and height growth seedling suggest a synergic compensatory mechanism expressed in a whole-plant growth biomass (relative growth rate). Compensation and survival results suggest trade-offs at the leaf level, such as leaf area growth and produced leaves versus chemical defenses, respectively. Rev. Biol. Trop. 56 (4): 2055-2067. Epub 2008 December 12.

Se estudiaron las respuestas de crecimiento, supervivencia y los diferentes tipos de herbivoría sobre plántulas de B. alicastrum en una selva neotropical de México. Se seleccionaron 122 plántulas de una población las cuales se asignaron a tres tratamientos de defoliación: control o 0% (n=21), 50% (n=51) y 90% (n=50). Cada 4 meses durante dos años se midió su crecimiento (en términos de la tasa relativa de crecimiento en biomasa, área foliar, hojas producidas y altura) y supervivencia. Además cada 12 meses, mediante el uso de una cuadricula de 2mm-2, se evaluó la herbivoría causada por patógenos e insectos. También dentro de una parcela de 500 m largo x 10 m de ancho se evaluó en 1095 plántulas de 3 meses de edad la herbivoría por mamíferos. En la misma parcela pero en 113 plántulas se estimó la herbivoría por patógenos e insectos. Las plántulas defoliadas al 50% compensaron su crecimiento en todas las estructuras medidas, pero sólo la tasa relativa de crecimiento y altura lo hicieron con la defoliación al 90%. La tasa relativa de crecimiento y el área foliar disminuyeron gradualmente con el tiempo, contrario a la altura con un patrón opuesto. El número de hojas producidas no se afectó por el tiempo. La probabilidad estimada de supervivencia incrementó hasta un 95% con la defoliación y disminuyó hasta un 37% a los 24 meses. La herbivoría por mamíferos fue el más incidente y severo, en algunos casos hasta un 100% de defoliación. En tanto la causada por patógenos e insectos fue secundaria. El crecimiento compensatorio en área foliar, producción de hojas y altura sugieren un mecanismo compensatorio sinérgico expresado a nivel del crecimiento individual de la planta (tasa de crecimiento en biomasa). Al mismo tiempo junto con la supervivencia sugiere un posible compromiso entre las respuestas de crecimiento a nivel de la hoja - área foliar y hojas producidas - versus producción de defensas químicas.

[Growth, survival and herbivory of seedlings in Brosimum alicastrum (Moraceae), a species from the Neotropical undergrowth]. / Crecimiento, supervivencia y herbivoría de plántulas de Brosimum alicastrum (Moraceae), una especie del sotobosque neotropical.

Growth responses, survival, and herbivory, on seedlings of Brosimum alicastrum were studied in a neotropical Mexican forest. We selected 122 seedlings and divided them into three groups assigned to defoliation treatments: control or 0 (n=21), 50 (n=51) and 90% (n=50). Every 4 months during two years we measured seedling growth (in terms of relative growth rate in biomass, leaf area growth, produced leaves and height growth) and survival. In addition, we evaluated every 12 months pathogen damage and insect herbivory using a 2 mm(-2) grid. Separately, we estimated mammal herbivory in 3-month old seedlings that were selected within a plot of 500 m x 10 m (N=1095). Pathogen damage and insect herbivory were evaluated within the same plot in 113 seedlings. We found that 50% defoliated seedlings showed compensatory responses in all growth parameters. Relative growth rate and height growth also had a compensatory response in seedlings at 90% defoliation. Relative growth rate and leaf area growth gradually decreased with time although height growth seedling showed an opposite pattern. Leaves produced were not affected by time. Estimated seedling survival probability increased with defoliation to a maximum of 97%, decreasing at 24 month to 37%. Mammal herbivory was more frequent and severe than herbivory caused by pathogens and insects. In some cases, mammal herbivory produced total defoliation. Compensatory growth in leaf area growth, produced leaves and height growth seedling suggest a synergic compensatory mechanism expressed in a whole-plant growth biomass (relative growth rate). Compensation and survival results suggest trade-offs at the leaf level, such as leaf area growth and produced leaves versus chemical defenses, respectively.

Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees.

We investigated how water transport capacity, wood density and wood anatomy were related to leaf photosynthetic traits in two lowland forests in Panama. Leaf-specific hydraulic conductivity ( k(L)) of upper branches was positively correlated with maximum rates of net CO(2) assimilation per unit leaf area ( A(area)) and stomatal conductance ( g(s)) across 20 species of canopy trees. Maximum k(L) showed stronger correlation with A(area) than initial k(L) suggesting that allocation to photosynthetic potential is proportional to maximum water transport capacity. Terminal branch k(L) was negatively correlated with A(area)/ g(s) and positively correlated with photosynthesis per unit N, indicating a trade-off of efficient use of water against efficient use of N in photosynthesis as water transport efficiency varied. Specific hydraulic conductivity calculated from xylem anatomical characteristics ( k(theoretical)) was positively related to A(area) and k(L), consistent with relationships among physiological measurements. Branch wood density was negatively correlated with wood water storage at saturation, k(L), A(area), net CO(2) assimilation per unit leaf mass ( A(mass)), and minimum leaf water potential measured on covered leaves, suggesting that wood density constrains physiological function to specific operating ranges. Kinetic and static indices of branch water transport capacity thus exhibit considerable co-ordination with allocation to potential carbon gain. Our results indicate that understanding tree hydraulic architecture provides added insights to comparisons of leaf level measurements among species, and links photosynthetic allocation patterns with branch hydraulic processes.