Carbon and Water Cycling in a Hybrid Poplar Plantation

Description

Plantations of hybrid poplar and other woody species are a means of meeting demands for wood biomass to be used as feedstock for pulp, manufactured wood products, and bioenergy as well as mitigating greenhouse gas emissions by sequestering atmospheric carbon dioxide (CO2). Although highly productive on suitable sites (typically poorer-quality agricultural land), species such as poplars and willows typically consume a large amount of water, which may be a significant limitation to widespread afforestation in regions with limited annual rainfall, notably central and southern Alberta and Saskatchewan. Working in partnership with Alberta–Pacific Forest Industries Inc. and the Canadian Woody Fibre Centre, CFS launched this project to measure the productivity of a hybrid poplar plantation, focusing on the allocation of carbon to above- and below-ground components, and to shed light on how productivity varies with daily and seasonal weather patterns, including rainfall and temperature. The results will feed into data from a network of plantation sites and allow us to test detailed process models to assess the feasibility and benefits of large-scale afforestation in central Canada.

Weather tower
Weather tower.
Larger image [28 Kb JPG]

We are also tracking the radiative forcing effects of afforestation with poplars. Recent global modeling studies suggest that afforestation at northern latitudes will have little benefit in mitigating climate change because the cooling achieved by removing atmospheric CO2 will be largely offset by the reduction in surface albedo (solar reflectance), particularly in winter, resulting from greater tree cover. However, many of the modeling studies are based on afforestation with slow-growing conifers, which are typically planted at close spacing, leading to much lower reflectance for several decades than snow-covered grasslands or croplands. By contrast, hybrid poplars are planted operationally at very wide spacings and are grown on 20-year rotations. Furthermore, they drop their leaves in fall and therefore should not reflect much light or cover snow during winter. Several years into the study we have yet to see a significant reduction in winter albedo, but we plan to confirm this trend with remote sensing measurements over several years.

This graph shows growing season (June to September) measurements of net ecosystem productivity (NEP) at the Huber Farm site from 2005 to 2010. Positive NEP (in green) indicates net carbon (C) uptake by the plantation, including trees, weeds, and soils. The plantation was a net C source (in red) in the first two years following planting, approximately C-neutral (blue) in 2007, and a small net C sink by 2008. The approximate growing season total NEP (in metric tonnes of C per hectare) increased each year as follows: 2005: -2.1; 2006: -1.2; 2007: -0.2; 2008: +0.6; 2009: +0.8; 2010: +2.7.

Long description

Growing season (June to September) measurements of net ecosystem productivity (NEP) at the Huber Farm site from 2005 to 2010. Positive NEP (in green) indicates net carbon (C) uptake by the plantation, including trees, weeds, and soils. The plantation was a net C source (in red) in the first two years following planting, approximately C-neutral (blue) in 2007, and a small net C sink by 2008.
Chart. The balance of precipitation (P) and evapotranspiration (ET)

Long description

The balance of precipitation (P) and evapotranspiration (ET) has varied at the Huber Farm site from year to year. Total P in most years has been below recent normal levels (about 450 mm), but ET has increased as the plantation has grown and as total leaf area has increased. 2009 was an exceptionally dry year, which limited both growth and total ET very severely.

Graph. Albedo trends over 1 year period

Long description

Albedo has been measured routinely every year and shows a clear seasonal trend from very high during winter (see photo taken in March 2009) to a relatively low value of about 20% during the growing season. During the first six years, there was no evidence of a systematic decrease in winter albedo, although the trees have grown to 5–6 m in height.

Deliverables

  • Measurements and analyses of canopy-level fluxes of energy (net radiation), water vapor and CO2 made using an eddy covariance system operating at a hybrid poplar plantation 200 km northeast of Edmonton. These will lead to better understanding of climate (temperature, drought) variations on plantation C uptake, water use, and albedo as the poplar stand develops.
  • Analysis of multi-year data, including process modeling and scenario projections, to determine whether poplars and willows planted to produce biomass or to mitigate carbon emissions are sustainable in Alberta and Saskatchewan in a changing climate.

Publication

Cai, T.; Price, D.T.; Orchansky, A.L.; Thomas, B.R. 2011. Carbon, water, and energy exchanges of a hybrid poplar plantation during the first five years following planting. Ecosystems 14(4):658-671.

Collaborators/Partners

Canadian Woody Fibre Centre; Alberta–Pacific Forest Industries Inc. (http://alpac.ca/), University of Alberta, Agricultural, Life & Environmental Sciences (http://www.ales.ualberta.ca/en.aspx)