Climate change is causing longer growing seasons that produce bigger trees but they may be soaking up less carbon dioxide (CO2). A study slated for publication later this year in Forest and Ecology Management suggests that higher temperatures — as well air pollution from car exhausts, farms, and industry and more nitrogen in the soil from agricultural fertilizer (1) — are making wood weaker (2).
Hans Pretzsch, a forest scientist at the Technical University of Munich in Germany and colleagues collected core samples from forests of Central Europe ― including Norway spruce (Picea abies), sessile oak (Quercus petraea), European beech (Fagus sylvatica), and Scots pine (Pinus sylvestris) — and analyzed the tree rings using a high-frequency probe. All species of trees were found to exhibit a significant decrease in wood density of 8 – 12% compared to 100 years ago.
As the density of wood decreases, its carbon content also decreases by about 50%, which implies trees are soaking up less CO2 from the atmosphere. However, the fact that trees have been growing so quickly may more than compensate for this. The growth of certain tree species has increased by nearly 77% since 1870 (3). Pekka Kauppi, an environmental scientist at the University of Helsinki told Science, “The negative change of wood density is by far less important than the positive change,” referring to the change from shrinking to expanding forests.
Moreover, density changes are not the only effect climate change is having on trees. As the global climate continues to warm, increased plant growth is boosting the production of biogenic volatile organic compounds (BVOCs). Another research paper published in the journal Nature Geoscience Letters on 20 August found that the “smell” of a forest is, in fact, owing to large quantities of these so-called BVOCs, which are increasing the scattering of direct sunlight allowing it to reach a wider plant-canopy (4).
A team of researchers led by Alexandru Rap of the University of Leeds in the UK used a simulation that included a global aerosol model to monitor the release of BOVCs across different regions. A radiation model was then used to alter the resultant sunlight and plant growth was modelled using a land surface scheme. The overall global impact was found to be a positive feedback loop: increased plant growth leads to a boost in the production of BVOCs, thereby helping trees to grow. They refer to this as the “diffuse radiation fertilisation effect” and according to the model, the magnitude of this effect is expected to increase.
The overall effect of rising temperatures on forests seems to be a positive one, for now. However, changes in wood quality may mean forests are more susceptible to damage from storms and less efficient at soaking up greenhouse gases. Furthermore, large-scale deforestation still poses a significant threat.
(1) Pretzsch, H. et al. Wood density reduced while wood volume growth accelerated in Central European forests since 1870. Forest and Ecology Management (2018). DOI: 10.1016/j.foreco.2018.07.045
(2) Cao, T. et al. Effects of thinning and fertilization on wood properties and economic returns for Norway spruce. Forest Ecology and Management (2018). DOI: 10.1016/j.foreco.2008.06.025
(3) Pretzsch, H. et al. Forest stand growth dynamics in Central Europe have accelerated since 1870. Nature Communications (2014). DOI: 10.1038/ncomms5967
(4) Rap, A. et al. Enhanced global primary production by biogenic aerosol via diffuse radiation fertilization. Nature Geoscience (2018). DOI: 10.1038/s41561-018-0208-3