Climate mitigation forestry—temporal trade-offs

The 1.5 degrees C target for global warming calls for evaluating short-term (30-50 years) climate change mitigation with different forests usage. In the current scientific literature and in the public debate, there are contrasting views on how forests should be managed to maximize total climate benefit, including the use of products and changes in carbon pools. Three major factors influence the conclusions in different studies: (a) time horizon, (b) site productivity, (c) substitution calculations. Here we show the dependency among these factors by an analysis of four harvest scenarios: 95%, 60%, 40% and 0% of growth, which are compared to a business as usual scenario (80%). The analyses are made for five counties in Sweden, which covers a wide range in forest productivities, from 2.5 m(3) ha(-1) yr(-1) (north) to 11.5 m(3) ha(-1) yr(-1) (south). The results show: (a) Reduced harvest levels provide increased climate benefits on short time scales (at least 50 years). (b) Increased harvesting from current level is counterproductive on both short and long term. (c) The potential effect on the carbon balance of a no-harvest scenario in the five counties, is larger (1.1-16 times) than the expected emissions from all other anthropogenic activities until 2045. (d) Short-term climate benefits of reduced harvesting are largest in highly productive forests. Smaller but more long-lasting benefits can be obtained by aiming at harvest reductions in less productive forests. (e) Strategies focused on short-term benefits need to be adapted to the future development of substitution factors and forest growth. If substitution effects become higher, increased harvest levels will be beneficial after 2050 in high productive forests. However, if future substitution effects decrease, which is a plausible and desired development, low harvest strategies are preferred in both short- and long-term time perspectives. We conclude that even moderate reductions of harvest levels would provide substantial climate benefits.