Rebirth after Fire
The Influences of Fire Severity on Conifer Regeneration in the Western US
The Kincade Fire consumed a house in Sonoma County, CA Noah Berger/Associated Press
What do you think of when you hear the word “wildfire”? The month-long fires in the Amazon? Or the devastating stories related to the Australian fires? In fact, fire is a natural part in many forest ecosystems, including the western US. It plays an important ecological role such as maintaining forest structure, regulating species composition, and recycling nutrients.
However, it is well studied that high severity fires have become more frequent over the past century in the western United states. These are patches of fires that kill more than 90% of overstory trees.
More importantly, areas burned by highly severe fires are projected to increase in concert with the changing climates. Understanding how tree species respond to patterns and behaviors of fire will determine the future status of fire-prone forests.
Decrease in Conifer Regeneration
Along with the increase in high severity fires is the decrease in conifer regeneration, or the establishment of young trees, in many ecosystems, including the Yellow Pine and Mixed Conifer (YPMC) Forests.
(All the woodland patches on the map are collectively called YPMC forests. )
The YPMC forest is one of the dominating ecosystems in the Sierra Nevada region. With little precipitation over the summer, burning is not unusual in summertime.
Throughout millions of years of burning on the landscape, species like Ponderosa pine (Pinus ponderosa) and Jeffrey pine (Pinus Jeffreii), which are called yellow pines, have developed fire-adaptive traits such as growing thicker and fire-resistant bark and self-pruning their lower limbs to limit fire spreading upward. Besides their ability to tolerate light fires, yellow pines don’t grow well under shade.
Other mixed conifer species such as White fir (Abies concolor), red fir (Abies magnifica), incense cedar (Calocedrus decurrens), and Douglas-fir (Pseudotsuga menziesii) are more sensitive to fire but have better ability to tolerate shade.
In Welch et al’s study (2016), high severity patches tend to have the least number of conifer seedlings (also known as regeneration). This is partly due to two biological factors after high severity fires, seed trees and shrubs. Each of them plays a critical role in the establishment and survival of conifer seedlings.
Study Site
The study area was a 71,780 ha portion of the Stanislaus National Forest (SNF), California, where the 2013 Rim Fire burned. The Rim Fire started in SNF on 17 August 2013 and burned across 104,131 ha of the Stanislaus National Forest and Yosemite National Park in California until it was fully contained on 23 October 2013. The Rim Fire was the third-largest wildfire ever recorded in California and the largest wildfire in the Sierra Nevada.
Over the summer of 2019, we sampled 132 plots across the entire burned area in SNF. The plots were grouped into fire severity levels based on ocular estimates in field. Our plots were distributed into unburned (overstory mortality = 0%), low severity (overstory mortality = 0-25%), low to moderate severity (overstory mortality = 25-50%), moderate severity (overstory mortality = 50-75%), moderate to high severity (overstory mortality = 75-90%), and high severity (overstory mortality > 90%).
Each of our circular plots was of fixed radius of 4.4m. In each plot, we measured the diameter at breast height (dbh) of all overstory trees. All conifer saplings (≥ 1.37 m in height and < 7.6 cm in dbh) and seedlings (< 1.37 m in height) were captured. Other critical measurements include but not limit to shrub coverage by species and the distance from plot centers to the nearest seed trees.
Results
Across the entire fire, 38 plots had zero regenerated seedlings, most of which were located in high severity patches.
Out of six fire severity classes, the quantity of regenerated seedlings peaked at the moderate level. The lowest amount was in high severity plots.
In terms of species composition, white fir was most abundant among the regenerated seedlings. Shade-intolerant yellow pines, represented in blue, regenerated less successfully than other shade-tolerant species.
Additionally, shade-intolerant yellow pines remained constant as distance to the nearest seed trees increased. The seedling density of shade-tolerant species dropped slightly.
Similar to the last plot, shade-intolerant yellow pines did not vary while shrub coverage increased. However, shade-tolerant species revealed a negative relationship with shrub coverage.
