Earth’s forests are getting shorter and younger as the climate shifts

A new study has determined that Earth’s forests are transforming in response to a combination of human actions and natural processes such as wildfires, causing them to lose their oldest trees and grow shorter. Sadly, this trend is likely to continue as the climate grows ever hotter thanks to human-led climate change.

The forests that cover a little under a third of our planet’s landmasses are home to a dizzying array of life, and form a vital part of Earth’s global ecosystem. This is partially due to their ability to remove carbon dioxide from the atmosphere, and capture it in a solid state as biomass.

Increasingly, human-induced climate change, wood harvesting, and a range of naturally occurring processes are placing forests around the world under stress. A new global study has used satellite observations and examined over 160 published papers in order to assess the impact that these disruptive influences are having on global forest dynamics.

Upon completing their review, the researchers discovered that Earth’s forests are dramatically shorter and younger on average than they were a century ago, and that we are at least partially to blame for this potentially damaging change.

The paper discusses three conditions that dictate the dynamics of a healthy forest, and how they are being disrupted.

The first characteristic is recruitment, which is the term given to an influx of new seedlings that will one day become young trees. The second is growth – an indicator of the net increase in biomass, and the third is mortality, which is defined as the loss of a plant’s ability to reproduce and undergo cellular metabolism.

In a healthy, old-growth forest these characteristics would balance each other out. However, a number of aggravating factors are seriously undermining this equilibrium.

For example, rising temperatures are making it much harder for trees and plants to photosynthesize. This is damaging to a forest on many levels, as not only does it kill trees, but it also makes it more difficult for them to regenerate and grow. It is also a major reason as to why the forests we see today are not as tall as they once were.

Prolonged high temperatures also give rise to droughts, which place trees under enormous stress and either kills them off directly or leaves them more susceptible to attack from insects or disease.

The high amount of carbon dioxide in the atmosphere, which has increased significantly since the onset of the industrial revolution, could actually help some trees ability to grow and propagate, but according to the new study there are limitations. The benefits of this carbon dioxide fertilization are apparently only observed in relatively young forests, where there is an abundance of nutrients and moisture.

Wildfires are also a serious threat to forests on a global scale, as are invasive fungi and parasitic vines. Furthermore, many of these factors – including the prevalence of wildfires – have been exacerbated by the onset of climate change.

To make matters worse, the forests must also contend with the threat of direct human action. Rampant wood harvesting and forest clearing has had a huge impact on forest ecosystems by removing many of the older trees.

The study data reviewed by the researchers showed that many of the aggravating factors listed, including drought, rising temperatures, forest fragmentation and insect attacks seemed to affect older trees more acutely than their younger brethren.

Furthermore, according to the newly published paper, tree mortality is increasing in most areas while the creation and growth of new trees continues to fluctuate. This has led to an overall loss.

"Over the last hundred years we've lost a lot of old forests, and they've been replaced in part by non-forests and in part by young forests," comments the study’s lead author Nate McDowell, an Earth scientist at the Pacific North West National Laboratory, in the US. "This has consequences on biodiversity, climate mitigation, and forestry."

Sadly, the continuing transformation of our planet’s forests will likely come hand in hand with a loss of biodiversity, as habitat changes will make life more challenging for animals living in the densely wooded areas.

"Unfortunately, mortality drivers like rising temperature and disturbances such as wildfire and insect outbreaks are on the rise and are expected to continue increasing in frequency and severity over the next century," states McDowell. "So, reductions in average forest age and height are already happening and they're likely to continue to happen."

The paper has been published in the journal Science.

Source: Department of Energy/Pacific Northwest National Laboratory