Diurnal patterns of leaf photosynthesis, conductance and water potential at the top of a lowland rain forest canopy in Cameroon: measurements from the Radeau des Cimes.

G W Koch,J S Amthor,M L Goulden

doi:10.1093/treephys/14.4.347

Abstract

Diurnal patterns of leaf conductance, net photosynthesis and water potential of five tree species were measured at the top of the canopy in a tropical lowland rain forest in southwestern Cameroon. Access to the 40 m canopy was by a large canopy-supported raft, the Radeau des Cimes. The measurements were made under ambient conditions, but the raft altered the local energy balance at times, resulting in elevated leaf temperatures. Leaf water potential was equal to or greater than the gravitational potential at 40 m in the early morning, falling to values as low as -3.0 MPa near midday. Net photosynthesis and conductance were typically highest during midmorning, with values of about 10-12 micro mol CO(2) m(-2) s(-1) and 0.2-0.3 mol H(2)O m(-2) s(-1), respectively. Leaf conductance and net photosynthesis commonly declined through midday with occasional recovery late in the day. Photosynthesis was negatively related to leaf temperature above midday air temperature maxima. These patterns were similar to those observed in other seasonally droughted evergreen communities, such as Mediterranean-climate shrubs, and indicate that environmental factors may cause stomatal closure and limit photosynthesis in tropical rain forests during the midday period.

Highlights

Despite the large area of tropical and subtropical wet- moist forest, i.e., about 13 x 106 km2 (Olson et al 1983), and the significance of canopy physiology to the mass and energy exchange of forest ecosystems, little is known about diurnal or seasonal patterns of leaf conductance and net photosynthesis and their relationships to the environment in tropical forest tree canopies
Jn contrast to scaffolding towers, several species and individuals could be studied at one time under relatively comfortable and stable working conditions- access to leaves is potentially available along the encire perimeter (90 m) of the raft. ln thjs study, we had access to eight species at two raft locations and five species were examined with respect to leaf physiology
The diurnal patterns and magnitudes of leaf photosynthesis, conductance and water potential were consistent with the limited literature for wet tropical forest canopies

Summary

Introduction

Despite the large area of tropical and subtropical wet- moist forest, i.e., about 13 x 106 km (Olson et al 1983), and the significance of canopy physiology to the mass and energy exchange of forest ecosystems, little is known about diurnal or seasonal patterns of leaf conductance and net photosynthesis and their relationships to the environment in tropical forest tree canopies. Leaf conductance in several species showed midmorning peaks, followed by declines throughout the remainder of the day, near the top of the canopy in a terra ftrme forest in Brazil (Roberts et al 1990). This pattern was especially evident under conditions of high light and vapor pressure deficit during the dry season. A marked reduction in conductance after midmorning was reported for canopy leaves of the emergent Qualea rosea Aubl. in French Guiana (Roy and Salager 1992)

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Conclusion