Forest Science and Technology to Reduce Atmospheric Greenhouse Gases-An Overview

Abstract

The forest as a CO₂ sink comprises, in addition to mature and immature trees, C accumulated in understorey plants, animals, forest soils, peat bogs and wetlands. Estimates of how much carbon (C) is entering and leaving a forest ecosystem cannot be obtained merely by estimating gaseous CO₂ fluxes. The C cycle also involves direct transfer of CO₂ to soil in rain and snow, non-photosynthetic or "dark" fixation of CO₂ by myriad soil and aquatic micro-organisms, roots, fungi and animals, and loss of C in forms other than CO₂ via air, groundwater flow and runoff.

The complexity of the carbon cycle challenges us to develop reliably accurate means of inventorying C accumulation in trees. In productive forests the C of wood (C_wood) can be determined by estimating tree merchantable volume and, by density conversion, mass of dry wood. Percentage C in dry wood varies by species and type of wood, but otherwise C_wood can be readily calculated. The C present in foliage, branches, bark and roots can, as a first approximation, be assumed to be equivalent to that in the merchantable boles.

National Forestry Database statistics and our elemental analysis data on total carbon in wood were used to determine how much C is present in and being removed annually from Canadian forests. In 1998 Canada extracted 45 million tonnes of C_wood from 0.5% of its more than 244 million hectares (ha) of productive forest area. That annual harvest contained less than 0.001% of the 6400 gigatonnes of C_wood existing in boles of merchantable trees. However, harvesting over the last three centuries has reduced C content in productive forests to well below 50% of their pre-1700 sink capacity. To refill the sink, it is proposed that a ceiling of 50 million tonnes C_wood be set as the annual allowable cut.

Mean temperature increases of as much as 8°C have been forecast for Canada over the next 100 years. The impact of those increases on tree growth and survival will depend not so much on changes in the annual mean but on what individual trees actually experience during the growing season in relation to the extremes they are able to tolerate. From a physiological perspective, maintaining shelterwoods with canopies approaching full closure is the only option for modulating extremes, thus for keeping forests growing healthily.

Recycling and refabricating wood and paper represent major societal and industrial opportunities to offset greenhouse gas emissions. Canadians can contribute to the C sink level of the nation by ensuring that paper and wood products have longer inservice lifetimes.