Through Fire and Water: Climate Change Impacting Forest Mortality

Disruption of natural fire cycles and climatic water stress appears to increase risks of climate-induced forest mortality, as data collection and documentation from all forested continents make it possible to observe global patterns in forest dieback.

Disruption of Fire Cycles 

Dr. Craig Allen, research ecologist with the United States Geological Survey says that seeking to preserve existing systems is futile, in a New York Times Green article this week.

Dr. Allen heads the Jemez Mountains Field Station at Bandelier National Monument, where he studied the forests of the southern Rockies. He observed that grasslands and shrublands are taking over forests as they burn “into oblivion,” in a presentation at an environmental conference in Aspen.

Research shows that historically, large-scale wildfires occurred frequently in the Southwest. These were cyclical and usually stayed close to the ground, helping prevent congestion in the forest but not destroying it.

As railroads provided the way for spreading livestock in 1900, the fire cycle was disrupted and eventually stopped. Cattle ate the grass cover and left little fuel for surface fires. In addition a national policy later formalized forest fire suppression, whereupon forest density shot up from 80 to 1,000 trees per acre.

Further research revealed that in 1996 winters became drier and the climate moved into a dry cycle. The thick forests began to suffer from immediate, major fires as the flames climbed up to the canopy instead of remaining on the surface. Many tree species did not and cannot regenerate after vast wildfires, and grasslands and shrublands slowly pushed them out. Dr. Allen said that “Rising temperature is going to drive our forests off the mountains.”

Climatic Water Stress

In an earlier paper Dr. Allen also discussed the possibility of many forests in the world today being at increasing risk of climate-induced dieback. This is particularly due to warm temperatures and drought-driven climatic water stress.

In the paper Climate-induced forest dieback: an escalating global phenomenon?, Dr. Allen defines forest dieback as “tree mortality noticeably above usual mortality levels”. Projected shifts in climatic patterns around the world are expected to further affect temperature and precipitation patterns, influencing water availability to plants.

Forests in both semi-arid and temperate regions with significant water availability limits have been shown to be sensitive to drought or rising temperatures. These forests displayed growth declines, increased mortality levels, and delayed multi-year effects from heat and water stress.

Short term acute effects like cavitation can be directly caused by climate driven water stress. Cavitation occurs when water column inside tree stems and leaves are irreversibly disrupted. Trees attempt to minimize risk of cavitation by closing their stomatas, reducing water loss and xylem tension. However, stomatal closure reduces photosynthesis by keeping carbon dioxide from diffusing. Chronic water stress over time may cause carbon starvation, which directly weakens or kills trees. Or it may indirectly cause the tree’s eventual demise by diminishing its defense against pests and diseases.

If current forest ecosystems are forced to adjust abruptly to new climate conditions through massive forest dieback, many pervasive and persistent ecological and social effects will result from the loss of forest products and ecosystem services – including sequestration of atmospheric carbon.

- C. D. Allen, Climate-induced forest dieback: an escalating global phenomenon?

Dr. Allen recommends additional monitoring and research on global forest health to better guide policy decisions and forest management worldwide. As climate change and warming temperatures subject forests to fire and water stresses, a better understanding of how they respond is crucial if we are to help forests worldwide adapt to and survive climate change effects.

Photo Credit: Some rights reserved by Brett Jordan on Flickr.

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Estel (303 Posts)

Estel Grace Masangkay is a freelance creative writer who enjoys outdoor trips and activities in natural settings. She is passionate about animal welfare and environment conservation. Follow Me @Em23me.

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