ALBERT

Description: The response of net ecosystem productivity (NEP) and evaporation in a boreal aspen (Populus tremuloides Michx.) forest and a black spruce (Picea mariana (Mill.) BSP) forest in Canada was compared using a newly developed realistic model of surface-atmosphere exchanges of carbon dioxide (CO2), water vapor, and energy as well as eddy covariance flux measurements made over a 6-year period (1994-1999). The model was developed by incorporating a process-based two-leaf (sunlit and shaded) canopy conductance and photosynthesis submodel in the Canadian Land Surface Scheme (CLASS). A simple submodel of autotrophic and heterotrophic respiration was combined with the photosynthesis model to simulate NEP. The model performed well in simulating half-hourly, daily, and monthly mean CO2 exchange and evaporation values in both deciduous and coniferous forests. Modeled and measured results showed a linear relationship between CO2 uptake and evaporation, and for each kilogram of water transpired, approximately 3 g of carbon (C) were photosynthesized by both ecosystems. The model results confirmed that the aspen forest was a weak to moderate C sink with considerable interannual variability in C uptake. In the growing season, the C uptake capacity of the aspen forest was over twice that of the black spruce forest. Warm springs enhanced NEP in both forests; however, high mid-summer temperatures appear to have significantly reduced NEP at the black spruce forest as a result of increased respiration. The model suggests that the black spruce forest is a weak C sink in cool years and a weak C source in warm years. These results show that the C balance of these two forests is sensitive to seasonal and interannual climatic variability and stresses the importance of continuous long-term flux measurement to confirm modeling results.

Description: The relationships between foliage area and sapwood area between trees and within the crowns of 20 Piceasitchensis (Bong.) Carr., provenance Queen Charlotte Island, British Columbia (10 in a control plot and 10 in a plot fertilized with potassium and phosphorus 8 years before harvest) and 10 Pinuscontorta Dougl., provenance Ladysmith trees were examined using a physiological analysis based on Darcy's law. Foliage area index on the fertilized P. sitchensis plot was higher than on the control. The variation of foliage area density with depth in the canopies followed a normal distribution. Relationships between foliage area and sapwood basal area were linear but the slopes were different for the two species. There was no significant difference between the control and fertilized P. sitchensis trees. The relationship between foliage area and the product of sapwood area and permeability was linear and data from the three plots fell on the same line. Sapwood area, permeability, and their product decreased with depth through the crowns of the trees. Within the crowns, relationships between cumulative foliage area and sapwood area, and between cumulative foliage area and the product of sapwood area × permeability were different with species and treatment. A single linear relationship resulted when the product of cumulative foliage area above an internode × the internode length was plotted against sapwood area × permeability for the internode. This suggests that it is the drop in potential across a node and internode rather than the gradient of potential across the internode that is related to the flux of water through tree crowns. The data support the hypothesis that the relationship between foliage area and sapwood area depends on permeability of the sapwood and the local climate through its influence on transpiration rate, particularly via average water vapour pressure deficit of the air and stomatal conductance.

Description: Stomatal conductance was measured with porometers in two plots of Pinussylvestris L. with markedly different tree spacings (plot 1, 608 stems ha−1; plot 2, 3281 stems ha−1), and hourly rates of transpiration were calculated using the Penman–Monteith equation at intervals throughout one growing season. Stomatal conductance varied little in relation to height or age of foliage. There was a linear decrease in canopy conductance with increasing water vapour pressure deficit of the air. Transpiration rates on both plots increased during the summer (maximum 0.3 mm h−1); rates on plot 1 were always lower (ca. 0.7 times) than on plot 2. Needle water potentials were similar throughout the season and only slightly lower on plot 1 than on plot 2. The mean hydraulic resistance of the trees on plot 1 was 2.4 times that on plot 2. The results support a hypothesis that considers the changes in transpiration rate, conducting cross-sectional area, canopy leaf area, water potential, and hydraulic resistance following thinning as a set of homeostatic relationships.

Description: Four large cuvettes were used to measure whole-branch CO2 and H2O vapour exchange at the Boreal Ecosystem-Atmosphere Study, Southern Study Area, Old Black Spruce site. Measurements started before the spring thaw and continued until after the winter freeze-up. Daytime photosynthesis, nighttime respiration, transpiration rate, and branch conductances were zero at the start and the end of the measurement period. Maximum photosynthetic uptake rates were ca. 6 µmol·m-2·s-1 (0.02 µmol·g-1·s-1). Maximum nighttime respiration was ca. 0.8 µmol·g-1·s-1. Maximum transpiration rates were ca. 2 mmol·m-2·s-1 (7.1 µmol·g-1·s-1). Maximum branch conductance was ca. 35 mmol·m-2·s-1 (0.123 mmol·g-1·s-1). A simple model combining light and temperature responses accounted for 80-90% of the day-to-day variation in daytime carbon uptake. Total annual daytime carbon uptake ranged from 151 to 271 g·m-2 (530-868 mg·g-1). Total nighttime loss ranged from 12.7 to 15.5 g C·m-2 (22.4-27.3 mg C·g-1). Total transpiration ranged from 20.4 to 35.0 kg H2O·m-2 (70.2-112.2 g H2O·g-1).

Description: The Penman–Monteith equation was used with measured values of stomatal conductance and leaf area, in conjunction with weather station measurements of net radiation, vapor pressure deficit, and wind speed to calculate the transpiration rates of two Scots pine (Pinussylvestris L.) stands of widely different densities. Transpiration was compared with water uptake estimated from 32P radioisotope movement through trees of known conducting area and water content. Uptake consistently lagged behind transpiration throughout the day. From dawn to dawn, however, uptake and transpiration were in close agreement as a result of recharge during the night.