Climate change with its warmer and drier seasons appears to be responsible for a decline in large-diameter trees across much of Yosemite National Park, according to a recently released study. Published in the Forest Ecology and Management journal, the study (attached below) suggests that one way to perhaps strengthen the park's forests against climate change is to introduce more prescribed fires.
According to the research, Yosemite was home to more forests with large-diameter stands of Ponderosa pines, Incense cedars, White fir, Sugar pines, Jeffrey pines, Red fir, Western White pine, Lodgepole pine, Whitebark pine and Mountain Hemlock between 1932-1936 than during 1988-1999. The impact of this downsizing goes beyond beautiful vistas, as mature forests "moderate the local environment; serve as a seed source for the surrounding landscape; and withstand fires, climate variation and insect outbreaks that kill or weaken smaller-diameter trees," the authors note.
While the authors, who reached their conclusions after comparing the findings of a 1932-1936 study of tree densities with one from 1988-1999, acknowledge that there are "many alternative explanations for the decline in densities of large trees," they lean toward climate change in general and water stress specifically as the most likely cause. Other studies have pointed to a decline in Sierra snowpacks, which slowly release water as they melt through spring and on into the summer, and increased rainfall, which runs off much more quickly, as being linked to climate change.
Increased water stress -- whether arising from extrinsic climatic change or intrinsic changes in stand density -- we view as the leading candidate for the underlying cause of the recent doubling in mortality rates in old-growth forests throughout western North America, although the proximate causes of mortality may vary...
...The decline in large-diameter trees could accelerate as the climate in California becomes warmer by mid-century. A temperature increase – even without a decrease in precipitation – will increase evaporative demand, decrease snowpack, increase the length of the snow-free period, increase the length of the growing season, and thereby increase annual climatic water deﬁcit. In addition, ﬁres are expected to increase in number, start earlier, last longer, burn larger areas, and become more severe.
When climate changes rapidly compared to the centuries-long lifespan of trees, there is a shorter period of the optimum conditions in which to attain those large diameters. Increased water stress on sites where large-diameter trees are now present could lead to elevated mortality, but time is required for trees establishing on newly favourable sites to grow to large diameters. Therefore, when climate is changing rapidly, we should expect densities of large-diameter trees to be lower than in stable climatic conditions, whether they are warm or cold. The decrease in densities of large-diameter trees could, therefore, be an indicator of climate change that is beyond the recent natural range of variation in these forests.
Fire, via prescribed burns, could actually help the forests build up a resistance of sorts to climate change, they added, as stands of Ponderosa pine and Incense cedar that had experienced fires during the 20th century "retained large-diameter Ponderosa pine. Plots not experiencing 20th century fire had almost no large-diameter Ponderosa pine."
The bottom line, they noted, was that "our strong inference from this research is that the largest trees of most species in Yosemite are in decline. This decrease in large-diameter tree density throughout much of Yosemite can be interpreted as a long-term change in forest structure during the 20th century."