Canadian Forest Service Publications
Decreasing photosynthesis at different spatial scales during the late growing season on a boreal cutover. 2005. Martel, M.C.; Margolis, H.A.; Coursolle, C.; Bigras, F.J.; Heinsch, F.A.; Running, S.W. Tree Physiol. 25: 689-699.
Available from: Laurentian Forestry Centre
Catalog ID: 25374
The relationship between photosynthesis and accumulated cold degree days (CDD) over the late growing season was examined at the shoot, ecosystem and landscape scales in a boreal cutover in eastern Canada redominated by black spruce (Picea mariana Mill. BSP), lowbush blueberry (Vaccinium angustifolium Ait.) and sheep laurel (Kalmia angustifolia L.). We calculated CDD as the sum of minimum daily temperatures below a 5 °C threshold.
Light-saturated photosynthesis at the shoot level (Amax) of black spruce and V. angustifolium decreased steadily with increasing CDD once temperatures below the CDD threshold value became frequent in mid-September, whereas K. angustifolia showed a more irregular pattern. Tissue acclimation played an important role in the decrease in Amax as the season progressed, but only V. angustifolium showed decreasing foliar nitrogen concentrations.
Based on eddy covariance flux tower data, maximum daily gross primary productivity (GPPmax-tower) at the ecosystem level was more strongly related to CDD (r2 = 0.59) than was maximum daily net ecosystem exchange (r2 = 0.32). The GPPmax was likely influenced by both tissue acclimation and the direct effects of changing emperatures and irradiances on physiological rates. Mean daily GPP, calculated for consecutive 8-day periods for a 25 km2 area around the tower by the MODIS MOD17A2 Collection 4 satellite algorithm (GPP-MODIS), decreased more rapidly with increasing CDD than did GPPmax-tower. Although GPP-MODIS was closely correlated with mean daily GPP from the tower (GPPdaily-tower, r2 = 0.95) over the late growing season, the former was about twice as high. Although MODIS estimates of air temperature closely tracked the ground data, the maximum light-use efficiency parameter used by the MODIS algorithm was much higher than that indicated by the tower measurements. There was a 3% decline in GPPmax-tower with an increase of 10 CDD, corresponding to the percent decline in branch-level Amax of black spruce and V. angustifolium.
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