Leaf respiration in two tropical rainforests: constraints on physiology by phosphorus, nitrogen and temperature

Abstract

Summary Leaf respiration is a major component of the terrestrial carbon cycle, but is poorly quantified for tropical forests. We measured dark respiration, R, and nutrient concentration (nitrogen, N and phosphorus, P) of leaves within two forest canopies: in Reserva Jarú, south‐west Brazil; and Mbalmayo Reserve, central Cameroon. The data, expressed on a leaf area basis (Ra, Na, Pa) and a leaf mass basis (Rm, Nm, Pm), were used to quantify the temperature sensitivity of R and to test the hypothesis that leaf metabolism is more strongly constrained by phosphorus than by nitrogen in these lowland rainforests. Leaf respiration rate (Ra, at 25 °C) at Jarú was nearly half that at Mbalmayo (the range in Ra from near the ground to the upper canopy was 0·11–0·78 µmol m−2 s−1 at Jarú versus 0·22–1·19 µmol m−2 s−1 at Mbalmayo), and the mean Q10 for respiration at each site was 2·3 ± 0·9 (1 SD) and 2·0 ± 0·5 (1 SD), respectively. There were significant differences (P < 0·01) between sites in leaf phosphorus concentration, but not in leaf nitrogen concentration: Pm was very low at Jarú (0·2–0·7 mg g−1) but higher at Mbalmayo (0·5–2·4 mg g−1), whilst Nm was similar at both forests (10–45 mg g−1). Rm was not significantly associated with canopy position or specific leaf area (SLA, m2 g−1) in either forest, but a significant relationship between SLA and Nm was found for both sites (P < 0·05), consistent with existing data. At Jarú, Rm was strongly related to Pm (P < 0·001) and less strongly related to Nm (P < 0·05), but at Mbalmayo, Rm was not significantly related to either Pm or Nm. Ra was linearly related to Na and Pa at both sites (P < 0·01), principally because of changes in leaf mass per area (LMA, g m−2) associated with canopy position. At Mbalmayo, LMA explained 70% of the variation in Ra, but only 20% at Jarú. For Jarú, the strongest relationship with Ra was obtained by combining LMA with Pm in a multiple regression (r2 = 0·53); further inclusion of Nm did not improve the regression. At Mbalmayo neither Nm or Pm improved the regression of Ra on LMA. These results indicate a strong influence of LMA on the relative rates of Ra within the vertical gradient of each canopy. They also suggest that at JarúPm constrains respiration more strongly than Nm, and further, that the very low Pm at Jarú may explain the lower absolute values of respiration there relative to Mbalmayo, where Pm was higher. The leaves at both sites are typical of lowland tropical rainforests in not having particularly low Nm, and consistent with this, Nm was a weaker predictor of respiration than Pm or LMA.