Juvenile tree growth correlates with photosynthesis and leaf phosphorus content in central Amazonia

Resumo

Light and soil water availability may limit carbon uptake of trees in tropical rainforests. The objective of this
work was to determine how photosynthetic traits of juvenile trees respond to variations in rainfall seasonality, leaf
nutrient content, and opening of the forest canopy. The correlation between leaf nutrient content and annual growth
rate of saplings was also assessed. In a terra firme rainforest of the central Amazon, leaf nutrient content and gas
exchange parameters were measured in five sapling tree species in the dry and rainy season of 2008. Sapling growth
was measured in 2008 and 2009. Rainfall seasonality led to variations in soil water content, but it did not affect leaf
gas exchange parameters. Subtle changes in the canopy opening affected CO2 saturated photosynthesis (Apot, p =
0.04). Although Apot was affected by leaf nutrient content (as follows: P > Mg > Ca > N > K), the relative growth rate
of saplings correlated solely with leaf P content (r = 0.52, p = 0.003). At present, reduction in soil water content
during the dry season does not seem to be strong enough to cause any effect on photosynthesis of saplings in central
Amazonia. This study shows that leaf P content is positively correlated with sapling growth in the central Amazon.
Therefore, the positive effect of atmospheric CO2 fertilization on long-term tree growth will depend on the ability
of trees to absorb additional amount of P.