Wednesday, August 21, 2019

Listen to the Trees

published in the Pacifica Tribune August 20, 2019

What’s Natural? 

Listen to the Trees!

This summer I taught a class on the Natural History of the Sierra Nevada for San Francisco State University’s Sierra Nevada Field Campus. The first day we taught students how to identify the trees. Once students know their trees, they can easily see how tree species vary with elevation, temperature, moisture, and snow pack. They can see which species colonize open sunny areas and which trees need shade before they can invade. Old time naturalists used trees to identify “life-zones” where different species of mammals, birds, insects and other plants can be found. Furthermore, when you listen to the trees, you can see change.

The class explored forests along the North Yuba River. Free from politics, trees tell us about changes in fire frequency, logging, climate change and ecosystem resilience. Photographs taken during the late 1800s during California’s gold rush days, revealed the total devastation of local forests. Gold miners needed wood for heating and cooking, for their metal forges, and for timbers to reinforce their mines. They needed wood to build flume boxes that altered river courses to expose riverbeds. Flume boxes also carried water from high to low elevations where giant water cannons completely washed away hillsides in their search for gold.

Deforested mountains - Downieville, CA late 1800s

Still, by comparing catastrophic photos of forests during the gold miners’ days to our current forest conditions, I was filled with optimism. The forests had totally recovered and again are quite dense. So dense, that local inhabitants fear there’s too much fuel on the forest floor that could feed catastrophic fires. Nevertheless, the lush re-growth is testimony to our forest’s amazing resilience. 

We counted tree rings and determined a majority of trees were no more than 170 years old. Those trees began their lives shortly after the gold miners had cut down all their older relatives. Occasionally we found a few larger trees, 300 years or older, that fortuitously avoided the miners’ ravenous saw blades.

Scientists determine the natural frequency of fires by reading tree rings and fire scars. Low elevation trees like Ponderosa Pines naturally endured wildfires about every 25 years. At higher elevations, where temperatures are colder and the snow pack lingers, fire scars suggest wildfires naturally happen about every 100 years. In contrast to media hype, fire scars in living and fossil trees suggest wildfires were far more common during the cool Little Ice Age.

Tree stumps tell us that trees once bordered Arctic shores 9000 years ago. Since then, cooler temperatures have pushed trees to lower latitudes and warmer elevations. Hikers in the Sierra Nevada often encounter dead trees several hundred feet above our current tree line. Accordingly, researchers determined that for the last 3 thousand years, tree line was mostly higher than today because temperatures were much warmer. However, during the Little Ice Age, between 1300 AD and 1850 AD, it got so cold, tree line dropped and tree seedlings in the Ural Mountains couldn’t germinate for hundreds of years. Ancient tree lines suggest if temperatures increase over the next century, it will not be a crisis. Trees will simply reclaim their former habitats. 

Trees reveal past rainfall patterns. California’s Blue Oaks are very sensitive to changes in precipitation. In drought years they generate narrow rings contrasting with wider rings during wet years. A recent tree ring study of Blue Oaks finds no rainfall trend over the past 700 years, but it suggests Californians can expect extreme droughts and extreme rainfall 3 to 4 times a century. More concerning, tree stumps at the bottom of Lake Tahoe dating back 6000 years ago, suggest Californians can naturally expect far more extreme droughts than living humans have yet to experience. 

Trees tell us how climate has changed. Fossil trees indicate Antarctica once experienced subtropical temperatures 40 million years ago. Similarly, trees tell us about recent temperature changes. Tree rings have correlated accurately with instrumental temperatures for over 100 years. However, since the 1960s, tree ring temperatures suggest a much cooler global climate in contrast to thermometers and models. 

Tree ring temperatures 

Tree rings indicate the warmest decades of the 20thcentury were the 1930s and 40s, and temperatures have yet to surpass those decades. This divergence between thermometers and trees is best explained by the fact that instrumental temperatures are biased upwards when taken at hot airports or in areas recently suffering from growing urban heat island effects. In contrast, trees measure temperatures in natural habitat.

There are too many fear mongering politicians pushing an “existential climate crisis”. I find the climate history told by the trees far more trustworthy, and the trees are whispering there is no crisis. 

Jim Steele is director emeritus of the Sierra Nevada Field Campus, SFSU and authored Landscapes and Cycles: An Environmentalist’s Journey to Climate Skepticism