Colorado’s disappearing sub-alpine forests

By November, those who heat their dwellings with wood stoves should have a good stock of firewood, mainly of beetle-killed spruce trees from dead stands in sub-alpine forests, mostly in the San Juan Range to our southwest and in the Cochetopa Hills to our west and northwest (as from Sargents Mesa).  Trips over Wolf Creek and Slumgullion passes show the extent of that devastation. In general, the future of these forests does not look good, as revealed in a recent article, and as I explain here.

About 37% of Colorado is forested, mainly in the western part of the state; 27% of that is subalpine (about 10,400 square miles). These forests range from 9,750 to 11,750 feet, ± 250 feet, and higher in the south and on north-facing slopes.  These forests are dominated by two tree species, Engelmann spruce (the most common), followed by subalpine fir, then others of lower importance (including limber and lodgepole pines). These trees grow at these elevations because of their tolerance to temperature and moisture.   

We have learned about the effects of climate change (global warming) on these forests due to the foresight of a retired professor at the University of Colorado: Tom Veblen. Veblen arrived in Boulder in 1981, soon after the formation of the nearby Niwot Ridge as a Long-term Ecological Research site in 1980. He soon directed the tagging and periodic measurement of over 5,000 trees in 13 sites at Niwot. Veblen led research, including generations of students and other collaborators, at Niwot and other locations.  

Since that time, now over 40 years, the mean temperature in western Colorado has increased by 2°F, and it is projected to increase by 2.5° or so by 2060 and another 2.5° by the end of the century.  Temperature has influenced the severity of droughts in our forests. Droughts are correlated with ocean temperature changes, as El Niño in the Pacific (actually, La Niña), the Pacific decadal oscillation and the Atlantic multidecadal oscillations. Higher temperatures directly stress trees and promote evaporation to reduce water availability for tree survival.  

So, drier and hotter. How will that affect these forests? First, an increase in temperature moves the climates upward in elevation. The increase of two degrees in the past 40 years is shifting forest limits up some 360 feet.  That means potential forest establishment at elevations that are presently alpine.  More trees, as Engelmann spruce, will die at the lower boundaries. The projected increase of 7° during this century will mean a displacement of subalpine forest 1300’ higher.  

Trees of subalpine forests are susceptible to attacks by bark-boring beetles, also native to these forests and historically in equilibrium with their host trees. Higher temperatures have already dramatically altered the beetle equilibrium by promoting the survival of eggs during the winter, with more beetles available in the summer.  Temperature stress also reduces the ability of trees to defend (primarily by producing pitch) against beetle attack. Warmer and drier climates also increase the damage from forest fires, and both mortality from beetles and fires have been increased by the greater density of trees, a legacy of a century of forest policy.  

Earlier this year, a final student of Veblen’s, Robert Andrus, was the lead author of an important scientific article which summarized the mortality data from the 40-year record of the growth and survival of the 5,000 trees in Veblen’s tagging project.  It showed, unexpectedly, that mortality in trees isn’t just the result of beetle kill and forest fire, but the direct result of higher temperature and lack of moisture—three times that due to beetle kill alone.  Furthermore, the total yearly mortality from stress and beetles has also tripled from censuses of 1982-94 to 2008-19, mainly in larger and older trees.  Tree mortality from fire has also increased dramatically in the past 20 years, twice the historical rate of the previous 2000 years.  

A young child, experiencing the forests at present, will encounter a dramatically different scene than a senior citizen at the end of this century.  The altitudinal zones of forests will shift upward, reducing the areas of alpine tundra (pity the poor pika, the canary in the coal mine), with the drier lowland coniferous forests also rising.  Much forest and woodland will turn into open grassland and desert.  Our agriculture will be radically different. I contemplate with grief the environmental abuses that have grown over my life as a biologist, leading to the global warming.