A solitary tree in a field

Tree size declining due to environmental change

Ongoing environmental changes are having a massive impact on the profile of forests worldwide, with knock-on effects on ecosystems, scientists say.

A global study, ‘Pervasive shifts in forest dynamics in a changing world’, published in Science magazine, led by the US Department of Energy’s Pacific Northwest National Laboratory (PNNL), included analysis on changes in forest age by the Birmingham Institute of Forest Research (BIFoR).

The researchers found that the globally average tree size has declined over the last century and is likely to continue declining due to environmental changes. Alterations are being caused by increased stress and carbon dioxide fertilisation, through the increasing frequency and severity of factors such as wildfire, drought, wind damage and biotic disturbances by insects, fungi and choking vines and through forest harvest.

According to the authors of the study, high levels of tree loss result in an imbalance in three important characteristics of a diverse and thriving forest: recruitment; the addition of new seedlings, growth, the net increase in biomass or carbon and the death of forest trees.

The study’s authors concluded that forests have already been altered by humans and will mostly likely continue to be altered in the foreseeable future, resulting in a continued reduction of the area and stature of old-growth forests globally.

Dr Nate McDowell, a PNNL Earth scientist and the study’s lead author said: “This trend is likely to continue with climate warming. A future planet with fewer large, old forests will be very different than what we have grown accustomed to. Older forests often host much higher biodiversity than young forests and they store more carbon than young forests.”

“This study reviews mounting evidence that climate change is accelerating tree mortality,” added Dr Tom Pugh of BIFoR, “increasingly pushing the world’s forests towards being both younger and shorter. This implies a reduction in their ability to store carbon and potentially large shifts in the mix of species that compose and inhabit these forests.

“This is likely to have big implications for the services those forests provide, such as mitigating climate change. Increasing rates of tree mortality driven by climate and land-use change, combined with uncertainty in the mix of species that will form the next generation, pose big challenges for conservationists and forest managers alike.”

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