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Climate Change Driving Forests To Smaller And Younger Trees

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Our planet’s forests are adapting to global warming, as one would expect. But their trees are getting shorter and younger. And that’s going to continue for a long time.

A new study published in Science by scientists at the Pacific Northwest National Laboratory (PNNL), led by Dr. Nate McDowell makes a compelling case for this future. Ongoing environmental changes are transforming forests worldwide, resulting in shorter and younger trees with broad impacts on global ecosystems.

The team found that rising temperatures and carbon dioxide have been altering the world’s forests through increased stress and carbon dioxide fertilization and through increasing the frequency and severity of disturbances such as wildfire, drought, wind damage, human destruction and other natural enemies.

“This trend is likely to continue with climate warming,” said McDowell. “A future planet with fewer large, old forests will be very different than what we have grown accustomed to. Older forests host much higher biodiversity than young forests and they store more carbon than young forests.”

Combined with forest harvesting by humans, the Earth has witnessed a dramatic decrease in the age and stature of forests. Conservatively, well over 30% of forests have been lost globally over the last 100 years. And this will continue because clear-cutting, temperatures, droughts and pests are increasing.

Most climate models have trees growing more because of the CO2 fertilization but they don’t capture the increase in tree death that occurs at the same time.

Losing larger trees is a bad thing for us and lots of other life on earth. Because old growth forests store more carbon compared to younger smaller forests, it’s harder to mitigate the worst effects of climate change without them.

The figure above shows the components of forest dynamics and the disturbances that drive them. In the far-left panel, a mature ecosystem is responsive primarily to localized mortality, and the primary drivers of demography are chronically changing variables such as CO2, temperature, and vapor pressure deficit (air dryness).

In the next panel, the system is disturbed by fire, insect outbreak, land-use changes or other large-scale perturbations that remove most of the overstory trees. Species adapted to rapid post-disturbance recruitment become established.

In the third panel, recruitment and growth dominate demographic processes, with mortality increasing over time as competition leads to self-thinning. In the last panel, a mature ecosystem forms, dominated by species that have replaced the original community in response to chronic environmental changes, leading to a novel ecosystem where even old trees are much smaller.

Forests have always changed as conditions change worldwide. The lifetime of trees can be long, hundreds of years, even thousands in some cases like the 5,000-year-old bristlecone pine in California, so they can provide a good indication of changes in earth’s condition.

There are three major types of forests, classified according to increasing latitude: Tropical, Temperate and Boreal. More than half of tropical forests have already been destroyed. Only scattered remnants of original temperate forests remain. Ongoing extensive logging in boreal forests may soon cause their complete disappearance.

Sometimes we plant trees on the cleared areas, but at best they become mono-culture crops, not a forest. The forest microclimate, much of the soil and almost all of animals are gone.

This is what happened in the Moors of England (figure below). The Moors are extensive areas characterized by low-growing vegetation including various shrubs and grasslands covered with patches of heath, having a poor, thin soil.

They are stark and beautiful in their own way. The problem is, they were once covered by dense forests with thick soils. Iron Age humans clearcut these forests to smelt iron.

They never grew back.

Using satellite imagery, field work and detailed literature review, the team led by McDowell had to understand the dynamics of forest growth and death that result from an interplay of environmental drivers and disturbances involving forest processes.

Disturbances that lead to deforestation are both human and natural – clear-cutting (see figure below), wildfires, insects and diseases are the leading causes.  The forest processes affected include recruitment (the addition of new seedlings to a community), growth (the net increase in biomass or carbon) and mortality (the death of forest trees).

“Mortality is rising in most areas, while recruitment and growth are variable over time, leading to a net decline in the stature of forests,” said McDowell.

We have long thought that there are areas on Earth that would be generally immune to global warming, mainly wet areas like central Europe, the Amazon AMZN and the southeastern United States. However, droughts do happen even in these wet areas, and when they do occur, they can be devastating and result in wildfires not normally seen.

In 2018-19, a combination of a bark beetle attack with a severe drought caused the loss of a huge number of trees, in an area in central Europe thought to be immune.

One key effect of rising temperatures and expanding dry periods is that trees shut off their stomata (the opening in their leaves through which they respire) more often to avoid moisture loss. But that also shuts down metabolism, especially photosynthesis, so the trees grow slower and smaller.

The driving forces and factors that appear to be dominating these effects include:

  1. Atmospheric carbon dioxide: CO2 levels in the atmosphere have increased dramatically since the Industrial Revolution and are projected to continue rising over the next century. Higher levels of carbon dioxide can increase a tree’s growth rate and seed production. However, such carbon dioxide fertilization appears to only happen in younger forests with abundant nutrients and water. Most forests globally are exposed to limitations in nutrients and water, which drastically reduces the carbon dioxide benefits to trees.
  2. Temperature: rising temperatures limit life-giving photosynthesis, leading to lower growth, higher mortality, and reduced regeneration. This is one key to shorter trees, the study determined.
  3. Droughts: they’re expected to increase in frequency, duration and severity globally. Drought can directly cause tree death or indirectly lead to mortality through associated increases in insect or pathogen attack.
  4. Wildfire is increasing in many forests worldwide and future fires may be more frequent than they have been in the past 10,000 years in some regions, the study found. Plant growth following forest fires may be slow or absent due to elevated temperatures.
  5. Biotic deforestation disturbances—by insects, fungi and choking vines—have been on the increase. The carbon storage lost to insects each year is the same as the amount of carbon emitted by 5 million vehicles, a study published last year says. This is expected to continue with warming, along with other biotic deforestation disturbances, such as fungi and bacteria. In the tropics, vines that use other plants as host structures are choking trees to death.
  6. Wood harvests by humans alone have had a huge impact on the shift of global forests towards younger ages or towards non-forest land, reducing the amount of forests, and old-growth forests, globally. Where forests are re-established on harvested land, the trees are smaller and the biomass is reduced.

Finally, when big trees die they release a lot of CO2. More trees dying cause temperatures to go up, causing more tree death and more CO2 released. This is a classic positive-feedback mechanism.

It’s hard to imagine the long-term effects of these trends. It’s not that life will disappear and the Earth will be a dead planet. It’ll just become more like our back yards. Some nice and manicured, some dumpy with weeds. A few nicely kept parks. But definitely not the natural world we think of.

If all that our great-grandchildren will be able to do is walk through a hologram of an old growth forest that we took for granted is really sad. Even sadder is that they won’t feel the loss since they probably won’t have ever seen a real one anyway.

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