The Proceedings of the Natural Academy of Sciences published a study this week by researchers at UC Berkeley, UC Davis and the U.S. Geological Survey that highlights the factors responsible for a huge decrease in the density of large trees in California. The study found that large tree density fell as much as 50 percent in the Sierra Nevada highlands, south and central Coast Ranges, and Northern California.
These are startling numbers, considering that forests cover almost a third of California, and as wildfires pose an increasing threat to lives, property and air quality during the ongoing drought.
The study found that the decrease in large trees —which is as high as 75 percent in parts of southern California—is largely due to two factors: climate change, and mismanagement of forest lands.
Climate change has led to higher temperatures and changes in precipitation patterns. Lack of water availability can hinder tree growth, and higher temperatures can cause earlier snowmelts and more water lost through transpiration. Overall, the study found that declines in large tree density were greatest in areas with the greatest demand for water relative to supply (also known as “water stress”).
Changes in precipitation and temperature in the regions examined by the study are part of larger shifts in the global climate; by 2100, scientists estimate that California’s average temperature could increase by as much as 9 percent, raising overall water stress by 30 percent.
In addition, the study found that decades-old forest management policies which prioritized immediate fire suppression in forest ecosystems that rely on period, naturally-occurring fires to thin out stands, have contributed to a surge in small tree density.
Large trees store more carbon and water than small trees, and are more resistant to wildfires while providing more habitat for rare and endangered forest species like spotted owls. Dense stands of small trees, by contrast, can lead to devastating crown fires.
The study highlights the importance of implementing scientifically vetted, up-to-date forest management policies. Calforests and our partners have been researching and implementing new strategies to increase forest health and guide recommendations to updating policies. In 2014, Calforests supported AB 2082 (Dahle), which sought to update standards first implemented by the Z’berg-Nejedly Forest Practice Act. The original law required a minimum tree point count of 300 per acre; as planting techniques and seedling success rates have improved over time, these rules can lead to the dense, small tree stands described in the study. Signed into law by California Governor Jerry Brown, AB 2082 provides the Board of Forestry with flexibility in implementing stocking standards according to recent research and geographical appropriateness, and improve the quality and safety of our forest resources and ecosystems.