Environmental controls on the photosynthesis and respiration of a boreal lichen woodland: a growing season of whole-ecosystem exchange measurements by eddy correlation.

S.-M Fan,J S Amthor,T R Moore,S C Wofsy,P S Bakwin,D R Fitzjarrald,K E Moore,J W Munger,M L Goulden,B C Daube

doi:10.1007/bf00341356

Abstract

Measurements of net ecosystem CO2 exchange by eddy correlation, incident photosynthetically active photon flux density (PPFD), soil temperature, air temperature, and air humidity were made in a black spruce (Picea mariana) boreal woodland near Schefferville, Quebec, Canada, from June through August 1990. Nighttime respiration was between 0.5 and 1.5 kg C ha-1 h-1, increasing with temperature. Net uptake of carbon during the day peaked at 3 kg C ha-1 h-1, and the daily net uptake over the experiment was 12 kg C ha-1 day-1. Photosynthesis dropped substantially at leaf-to-air vapor pressure deficit (VPD) greater than 7 mb, presumably as a result of stomatal closure. The response of ecosystem photosynthesis to incident PPFD was markedly non-linear, with an abrupt saturation at 600 μmol m-2 s-1. This sharp saturation reflected the geometry of the spruce canopy (isolated conical crowns), the frequently overcast conditions, and an increase in VPD coincident with high radiation. The ecosystem light-use efficiency increased markedly during overcast periods as a result of a more even distribution of light across the forest surface. A mechanistic model of forest photosynthesis, parameterized with observations of leaf density and nitrogen content from a nearby stand, provided accurate predictions of forest photosynthesis. The observations and model results indicated that ecosystem carbon balance at the site is highly sensitive to temperature, and relatively insensitive to cloudiness.

Highlights

Boreal forests cover approximately 8% of the earth's land surface, and store about 10% of the total carbon in terrestrial ecosystems (Bonan and Shugart 1989; Schlesinger 1991)
Measurements of net ecosystem C02 exchange by eddy correlation, incident photosynthetically active photon flux density (PPFD), soil temperature, air temperature, and air humidity were made in a black spruce (Picea mariana) boreal woodland near Schefferville, Quebec, Canada, from June through August 1990
We developed a mechanistic model of forest carbon assimilation where /tree is the PPFD intercepted by trees, /0 is the measured following the approach outlined by Amthor et al (1994)

Summary

Introduction

Boreal forests cover approximately 8% of the earth's land surface, and store about 10% of the total carbon in terrestrial ecosystems (Bonan and Shugart 1989; Schlesinger 1991). The climate of the boreal zone is extreme with severe winters and a short growing season. The boreal environment exerts a strong and presumably direct control on photosynthesis and decomposition, resulting in significant accumulation of soil carbon, detritus, and living biomass (Bonan and Shugart 1989; Schlesinger 1991). Since the carbon balance of boreal forests may be sensitive to climate change, this raises the possibility of a significant feedback on the future rate of change of atmospheric C02 (e.g., Oechel et al 1993). Efforts to predict the response of boreal ecosystem C02 exchange to perturbation must rely on an understanding of the processes controlling photosynthesis and respiration

Methods

Results

Conclusion