Click here to access the paper from the paywall at PNAS.org (Kimberley T. Davis, Solomon Z. Dobrowski, Philip E. Higuera, Zachary A. Holden, Thomas T. Veblen, Monica T. Rother, Sean A. Parks, Anna Sala, and Marco P. Maneta). Here’s the abstract:

Significance

Changes in climate and disturbance regimes may cause abrupt shifts in vegetation communities. Identifying climatic conditions that can limit tree regeneration is important for understanding when and where wildfires may catalyze such changes. This study quantified relationships between annual climate conditions and regeneration of Pinus ponderosa (ponderosa pine) and Pseudotsuga menziesii (Douglas-fir), two ecologically and economically important conifer species in low-elevation forests of western North America. We found that regeneration exhibited a threshold response to annual climate conditions and the forests we sampled crossed these climate thresholds in the past 20 years, resulting in fewer recruitment opportunities through time. In areas that have crossed climatic thresholds for regeneration, stand-replacing fires may result in abrupt ecosystem transitions to nonforest states.

Abstract

Climate change is increasing fire activity in the western United States, which has the potential to accelerate climate-induced shifts in vegetation communities. Wildfire can catalyze vegetation change by killing adult trees that could otherwise persist in climate conditions no longer suitable for seedling establishment and survival. Recently documented declines in postfire conifer recruitment in the western United States may be an example of this phenomenon. However, the role of annual climate variation and its interaction with long-term climate trends in driving these changes is poorly resolved. Here we examine the relationship between annual climate and postfire tree regeneration of two dominant, low-elevation conifers (ponderosa pine and Douglas-fir) using annually resolved establishment dates from 2,935 destructively sampled trees from 33 wildfires across four regions in the western United States. We show that regeneration had a nonlinear response to annual climate conditions, with distinct thresholds for recruitment based on vapor pressure deficit, soil moisture, and maximum surface temperature. At dry sites across our study region, seasonal to annual climate conditions over the past 20 years have crossed these thresholds, such that conditions have become increasingly unsuitable for regeneration. High fire severity and low seed availability further reduced the probability of postfire regeneration. Together, our results demonstrate that climate change combined with high severity fire is leading to increasingly fewer opportunities for seedlings to establish after wildfires and may lead to ecosystem transitions in low-elevation ponderosa pine and Douglas-fir forests across the western United States.

From New Scientist (Adam Vaughan):

Forests around the world face being permanently wiped out because climate change is making them unable to recover from devastating wildfires.

Solomon Dobrowski at the University of Montana and colleagues painstakingly dug up approximately 3,000 small trees from 90 burn sites across the western US to look at the ability of forests to regenerate after a wild fire.

They found that before the 1990s, low-lying forests could grow back after being burned, but between the early 1990s and 2015 there was a sharp drop in the ability of seeds to regenerate a forest at most sites.

The team used tree ring dating to see which year their trees had germinated since a fire, and used those samples to build a model of how forests would likely recover in different conditions.

Climate change appears to have changed soil moisture and surface temperatures so much that the forests have passed a threshold where conditions no longer favour new growth after a fire. Unlike mature trees, seedlings’ roots are too shallow to reach water deeper underground.

Higher temperatures
Climate scientists have warned for years of the possibility of such abrupt responses to higher temperatures, such as the rapidly-accelerating loss of ice sheets.

“These dramatic disturbance events, the changes we will see over the landscape won’t be gradual over decades, they will happen very quickly,” says Dobrowski.

The study looked at just two types of conifers, ponderosa pine and Douglas-fir. But Dobrowski said the findings were also relevant to similar semi-arid forests around the world, such as those of southern Europe.

That would be bad both in terms of the ecosystem services those forests provide, but also limiting future climate change.

Human interventions could help some of these burned forests grow back, for example by reintroducing seedlings when they are 2-3 years old and have roots long enough to reach water underground. But this costs money and time.

Pinus ponderosa subsp. ponderosa. Photo credit Wikimedia.

“Bergen Park” was painted by John Frederick Kensett circa 1870. The painting illustrates the open, spatially-variable structure of a ponderosa pine stand with an open understory typical of some Front Range forests at that time. (Bergen Park, at an elevation of 7800 ft., is located near Evergreen, CO, 25 miles west of Denver.) via Rocky Mountain Research Station

Pinon and juniper forests that burned in the early 2000s show little sign of regeneration. Pony Fire, Happy Camp, Siskiyou County, California. Photo credit AWeekOrAWeekend.com.

Piñon pine (Juniperus_occidentalis). Photo credit: Wikimedia

Coast Douglas Fir from the Wenatchee Mountains. Photo credit: By Walter Siegmund (talk) – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5931511

West Fork Fire June 20, 2013 photo the Pike Hot Shots Wildfire Today

Restoration southwest of Walsenburg August 2013.