The basic wisdom about trees and carbon dioxide (CO2) is that increasing levels of CO2 in the atmosphere promote tree growth. Research, however, says both yes and no to this statement. The answer is nuanced because trees and the ecosystems they support are complex. A new study published in Science reports that forests are getting younger and smaller in response to climate change. Also, their role as carbon sinks is diminishing as global temperatures increase. Heat and trees share a complex relationship.
The world’s forests are divided according to latitudinal position: Tropical near the equator, temperate in the mid-latitudes, and boreal near the poles. In all cases, though, trees form the backbone of forest ecosystems. Old-growth forests support a diversity of plant and animal species, but large, old trees are disappearing. In the past 115 years, we lost more than a third of the world’s old-growth forests. Logging and clearing land for agriculture played a significant role in this decline; however, increasing heat and temperatures also are taking a toll.
A look at carbon sequestration over that period reveals that during the 20th century, increasing levels of CO2 did play a positive role in increasing the capacity of forests as carbon sinks. However, growth stimulation from extra CO2 has slowed as a warmer climate starts to negatively affect the ability of trees to grow. Heat affects both the growth rate and the availability of vital water resources.
Trees sequester carbon by extracting CO2 from the atmosphere and using it for photosynthesis. The carbon becomes fixed in both tree biomass and the soils surrounding the tree’s roots. While it is true that rising levels of CO2 provide more “food” for the trees, it is also a fact that the increased CO2 in the atmosphere raises temperatures. This excess heat triggers physiological and ecological responses that prevent or decrease carbon sequestration.
Like all plants, trees require moisture, and their vascular systems are well adapted to efficiently transporting water and nutrients. But as temperatures increase, the excess heat drys out the forests, and the trees respond by conserving their water. The pores on their leaves that soak up CO2 and emit oxygen and water vapor close up, to trap the moisture in. The trees then stop taking CO2 from the atmosphere, thus decreasing the forest’s ability to sequester carbon.
If the trees come under enough stress from the heat, then they weaken and die. As these dead trees decay, carbon is returned to the atmosphere, since bacteria feasting on the rotting wood emit CO2 and methane.
But a more dramatic and direct pathway for returning carbon to the atmosphere is fire. Dry forests are more susceptible to wildfires. So, as the planet warms, heat and trees now interact in various ways to reduce the carbon sequestration capability of the world’s forests.
The outlook for the future is younger forests with smaller trees. Increasingly the scientific evidence points towards fewer old-growth forests and more young forests with less biodiversity. History has demonstrated that forests can leave and never return. The beautiful but treeless Moors of England once supported dense forests and rich organic soils. The forests and fertile soils disappeared during the Iron Age when the trees were harvested for fuel to smelt iron. The forests never returned.
Forests are a vital ecological resource, a resource that changes with the climate. How forests are maintained, and the ecosystems they support preserved, is important. Clear-cutting, followed by mono-species planting, is a common practice. But this practice is also a blow to biodiversity and a poor way to manage our forest resources. Climate change forces changes in forest ecosystems as heat and trees interact. How we manage those changes determines the quality of the world our children will inherit.
Forests as a pathway for terrestrial carbon sequestration (Source: ArcheanWeb) – https://archeanweb.com/2020/02/06/forests-as-a-pathway-for-terrestrial-carbon-sequestration/ Also:
Carbon sequestration in soils (Source: ArcheanWeb) – https://archeanweb.com/2020/02/11/carbon-sequestration-in-soils/ Also:
Tree planting as a carbon sequestration plan (Source: ArcheanWeb) – https://archeanweb.com/2020/02/07/tree-planting-as-a-carbon-sequestration-plan/ Also:
Extractive versus sustainable logging (Source: ArcheanWeb) – https://archeanweb.com/2020/02/25/extractive-versus-sustainable-logging/ Also:
Climate Change Driving Forests To Smaller And Younger Trees (By James Conca; Forbes) – https://www.forbes.com/sites/jamesconca/2020/05/31/climate-change-driving-forests-to-smaller-and-younger-trees/#52b36cce2787 Also:
The grand old trees of the world are dying, leaving forests younger and shorter (
Pervasive shifts in forest dynamics in a changing world (
Feature Image: Redwood Forest (Source: ArcheanWeb)