Silviculture & Climate Change
by Richard Hebda

Climate change now regularly hits the headlines. It was most strikingly emphasized this fall by the pronounced branchlet drop in western red cedars and the widespread wasting of coastal glaciers.

Whatever we may think about what climate change means to the urban and suburban dweller, there is little question that it will transform the living landscape and impact forestry and forest management. Forestry is ecology, and ecology depends on climate. Without forest ecology there is no forest economy. At the primary level, human communities depend on plant communities in the forestry sector.

Silviculture itself is an ecological exercise, the manipulation of forest ecosystems and individual species within those ecosystems to reap timber, and these days, non-timber forest products. Planting, thinning, fertilizing, sheep grazing, selecting seed stocks, and insect management are all ecological experiments designed to enhance timber production and related values. The decades and centuries of knowledge gained through repeated experimentation by forest practitioners has allowed us to manipulate forest ecosystems to produce more and better timber.

In the next few decades, however, and certainly in the next rotation, the ecological framework will alter such that centuries of empirical dirt knowledge may not readily apply, or at least apply on a limited scale or in different places.

There are 2 powerful ways to gain insight into the scale of ecological transformation ahead: studies of past forest ecosystems and their responses to climate changes, and climate impact models.

The fossil pollen, cone, needle and charcoal record of the last 10,000 years is unequivocal about the scale of change we must expect. 7,000-10,000 years ago cyclically high solar radiation fostered a warmer (2-4° C) and drier summer climate in BC than today, much as expected in the next decades. Grassland and parkland were much more extensive than today. Forest types without modern equivalents occurred in BC, and may have been widespread. Fires burned widely and tree lines reached into today’s alpine zone. A 4,000 year-old tree ring record from Vancouver Island shows dramatic decline in tree growth over only 3 years about 3,900 years ago, an indication that climatic shifts can occur rapidly with major impact on growth increment.

Overall the fossil record reveals that the climate of the last 4,000 years has been relatively stable, compared with preceding millennia. Forest ecosystems and species distributions achieved a relative equilibrium with the climate. Thus studies of the fossil record in BC indicate that we can expect future climate change to be rapid, of large amplitude, and occur as variations between extremes. But unlike in earlier millennia, the change will play out on a disturbed and fragmented landscape, one without the ecological resilience of the past. 

Global climate change models use well-established principles of mathematics and physics to estimate climates for different concentrations of atmospheric greenhouse gasses. Climate impact models take the output from climate change models and by using the climatic envelope or limits of species, ecosystems or processes, to anticipate where, geographically, these changes might be distributed in the future.

There are several climate models available today for a range of future greenhouse gas concentrations. Their outcomes vary, but on average for western Canada a mean annual temperature increase of about 5º C is indicated, with about a 10% risk of as much as 10ºC change by the end of this century. Precipitation is expected to increase slightly but with stronger summer droughts. These climatic conditions will be without precedent for the last tens of millions of years, taking us back geologically to a time when forests grew in Canada’s high Arctic.
Climate impact models provide sobering insight into the scope of the ecological transformation ahead. Using a Canadian climate model, Wang and Hamman recently showed that the climate of BC’s ponderosa pine ecological zone (dry climates of the Okanagan valley) might occur in the Peace River region and reach into the Northwest Territories by 2080. At the species level, Royal BC museum models (see the Pacific Climate Impacts Consortium website at www.pcic.org) reveal the disappearance of climate suitable for western red cedar (a good proxy for our iconic coastal temperate rainforests) in much of lowland southern BC, and its spread into northern BC by 2080 with major shifts underway by 2050. Climate suitable for Garry oak could spread to the Alaska panhandle and lower Skeena River by mid-century.

A key point is that at the predicted rate of change, the range loss of some tree species will be more rapid than range expansion into newly suitable regions. This “big squeeze” means that it will be many centuries before any sort of natural ecological equilibrium is achieved in our forest ecosystems. 
For the silviculture practitioner these coming ecological transformations pose an enormous challenge because they require planning for a shifting target. Consideration of secondary interactions such as those involving pest-host relationships must be added; these will likely control what can grow, how well and where. Fine-scale tinkering with genetic stocks and adjustments in silviculture prescriptions will not do the job. Silviculture has to return to basic ecological principles.

A basic strategy is to develop maps of sensitivity to climate change in anticipation of species range shifts and ecosystem transformations. These maps can serve as a basis from which to devise appropriate and admittedly experimental planting mixes and management measures. Other strategies include identifying key ecological processes such as soil formation and then fostering and conserving them, maintaining and redeveloping landscape scale connections for natural migration to take place, establishing a network of experimental seed nurseries and plots in which a wide range of genetic diversity is maintained, monitoring growth in a wide range of climates to detect both positive and negative responses, and carrying out vigorous invasive species monitoring and control. In some cases, raising timber trees may best be carried out by intensive management of ecologically intelligent plantations as a way of reducing risk and countering uncertainty. 

Overall silviculture may have to shift its focus from growing trees for harvest to, in some cases, simply sustaining forests and growing trees as part of ecosystems that sequester carbon while delivering it into storage in forest soils, and sustaining biodiversity for an uncertain future. Trees for people, yes, and for the planet too!

Richard Hebda is with the Royal BC Museum and can be reached at 250-652-6863.