Atlantic Forestry November 2019

Better with age

I have been reading about some interesting research at the University of Vermont, pointing to a necessary shift in the way we think about old and mature forests. The details were published recently in the peer-reviewed journal Global Change Biology. As is often the case with scientific reports, the title is a bit of a mouthful: “The climate sensitivity of carbon, timber, and species richness covaries with forest age in boreal-temperate North America.”

The gist of it is that older forests – far from being decadent, or merely scenic – are actually top performers when it comes to mitigating against the effects of climate change. Lead author Dominik Thom, a postdoc at UVM’s Rubenstein School of Environment and Natural Resources and Gund Institute for Environment, says the findings are “a milestone in the debate on how to prepare our forests for the uncertain environmental conditions ahead.”

In a study area encompassing eastern temperate and northern forests in the northeast U.S. and southeastern Canada (including the Maritime provinces), Thom and his fellow researchers examined 18,507 pre-existing sample plots. They compiled data on timber growth rates (annual volume increment), biodiversity (including trees, vascular plants, and lichens), and the carbon pool (including forest floor carbon and soil organic carbon, as well as carbon in live trees, standing dead trees, and downed deadwood).

Their objective was to analyze the associations among these factors at various stages of forest development, and examine how these associations would be affected by climate change. They posited temperature increases of 4 degrees C and annual precipitation increases of 200 millimetres, based on projections for this region by the end of the 21st century (under the RCP 6.0 emissions scenario). The study did not include forests more than 200 years old – due to the small number of plots in old-growth sites, and the difficulty of pinpointing forest age in uneven-aged old-growth forests.

The researchers identified a peak in timber growth rates between years 40 and 50, and another peak after age 170. The report says the first peak is “consistent with previous models in which growth rate was highest in relatively young and even-aged, secondary forests.” The second peak, however, was unexpected. The researchers suggest that it “might be explained by tree growth releases at multiple canopy positions as forests age, experience gap dynamics, and interact with partial disturbances that free up growing space and increase light availability for mixtures of shade-tolerant and shade-intolerant species.” They say there is some previous research pointing to renewed growth and physiological function in mature and old forests, but the phenomenon warrants further study.

Older forests also performed well in terms carbon sequestration. “TEC (total ecosystem carbon) increased with forest age, due to carbon accumulation in all pools, particularly in forests older than 130 years,” says the report. “SOC (soil organic carbon) and FFC (forest floor carbon) declined during the first 80-120 years, after which both pools increased.”

This is somewhat counterintuitive, especially if you envision an old forest as a decaying, carbon-spewing trainwreck. Here’s how the researchers account for it: “As forests develop toward late-seral stages, mortality of canopy trees increases through both density-dependent and density-independent processes, leading to dead tree recruitment and deadwood accumulation. However, our data suggest that the increase in deadwood occurs concurrently with increases in ALC (aboveground living carbon), which is a much larger carbon pool than deadwood in our study system (e.g., up to an order of magnitude larger in 200-year-old forests).”

They say increased ALC in older forests is likely due to increases in structural complexity, as indicated in previous studies. As for the effects of rotting wood, they explain: “Although decomposition gradually releases carbon to the atmosphere via respiration, the large accumulations of deadwood and litter in old forests also contribute to organic matter and free carbon incorporation into the humus layer and soil profile, thereby increasing belowground carbon pools.”

With respect to biodiversity, the study did not identify an overarching trend. Lichen populations peaked between 90 and 100 years, while vascular plants declined slightly in older forests.

With all these factors taken into account, the researchers conclude: “The combined performance of multiple ecosystem indicators peaked in 200-year-old forests as a result of simultaneously high levels of carbon storage and timber growth rate, coupled with relatively stable species richness along the forest development gradient.”

The researchers acknowledge that the parameters examined in this study do not encompass the complexity of climate-change impacts, which will be “influenced by multiple interacting factors, including stressors such as drought frequency, spread of invasive pests and pathogens, altered disturbance dynamics, and airborne pollutants.”

They note that warmer temperatures are generally associated with improvements in timber growth, but reduced winter snow pack (resulting in increased soil freezing), for example, has been shown to have negative effects on growth rates in Sugar maple. They say that various stress factors, along with changes in tree species composition, “may neutralize the positive direct effects of climate change on timber growth.”

Amid all these uncertainties, they see merit in introducing longer rotations and other management techniques aimed at promoting the development of late-successional and older stand structures, which “could partly offset the negative effects of climate change on carbon storage.”

The authors recognize that this shift might curtail harvesting in the short term, until a larger proportion of forests achieve late-stage increases in growth rates. “This transition phase could be shortened,” they say, “through the use of silvicultural practices designed to increase rates of late-successional forest development and structural complexity in managed forests.”

William Keeton, forestry professor in UVM, says the new research findings point to opportunities for making forest management more adaptive to climate change: “This could include enhancing older forest conditions on landscapes within reserves, for example, and using extended cutting cycles and restorative forestry practices in working forests.”

It all fits in with broader discussions about the need to take a new approach to silviculture, with more emphasis on diversity and resilience. This is encouraging, as a counterpoint to the argument for plantations – which sequester far less carbon than natural forests. Around the world, there is currently a bonanza of tree planting going on, ostensibly to fight climate change. But if those trees are harvested on short rotations, the main effect will be a global glut of wood chips and pulp, accompanied by relatively modest environmental benefits. DL