Northwest Climate Adaptation Science Center researchers are shedding light on the factors that support resilience of serotinous tree species
In the Northwest, increasing wildfires and warmer, drier conditions have prompted questions about how forests will cope as the climate changes. Forests in the region are adapted to fire, but it is unclear how increasing fire frequency and warmer post-fire growing conditions will impact how trees recover following a fire. This is especially concerning for forests adapted to stand-replacing (high-severity) fires and made up of serotinous tree species — those that rely on fire to release their seeds. Serotinous trees are at risk when the period between fires is too short for seed stores to accumulate and too warm and dry for seedlings to grow following fire.
The growth and survival of seedlings after a disturbance like fire is referred to as regeneration, and is a key sign of resilience in fire-prone forests. In serotinous forests, several important factors support regeneration. The first is having a sufficient amount of seeds stored inside cones in the canopy of mature trees at the time of fire, which allows for a mass seed release within the first year following fire. The second is having suitable growing conditions for seedlings to establish in the first year following fire. Harsh climate conditions like drought and extreme heat occuring during this window can cause more seedlings to die. Local site conditions like slopes and differences in soils and neighboring plants can also affect moisture available to seedlings. Understanding the relative importance of these factors for regeneration could help resource managers identify when and where these forests are most vulnerable as the climate changes.
Recognizing this research need, NW CASC researchers Michelle Agne, Brian Harvey and colleagues conducted a study to shed light on which factors play the biggest role in supporting regeneration, and thus resilience, of serotinous forests in the Northwest. To unpack the relative importance of fire frequency and post-fire conditions on tree regeneration, they studied several populations of severely burned knobcone pine forests in Oregon and California that experienced different fire intervals (lengths of time between fires). The research team focused on how three aspects of tree regeneration — seedling density, probability of self-replacement, and percent population recovery — are affected by the amount of seeds stored at the time of the fire as well as the post-fire growing conditions.
NW CASC researchers found that the seed supply at the time of fire was the most important factor for each aspect of forest recovery they examined. This suggests that fire interval length, which determines whether trees have had enough time to accumulate a sufficient supply of seeds, affects knobcone pine resilience more than climate conditions following fire — at least under the current climate. Since knobcone pine are relatively quick to mature and start producing seeds, this study found that knobcone pine regenerated following fire intervals as short as six years, though fire intervals of 15 years or longer are needed for knobcone pine populations to be greater post-fire than pre-fire. There was no evidence of complete seedling failure, suggesting that knobcone pine have a high resilience to increases in short-interval fires so long as trees have reached reproductive maturity.
Although seed supply was the most important factor, post-fire climate conditions can also influence regeneration. This study suggests that when seed supply is low and fire intervals are short, dry post-fire conditions can exacerbate difficulties in forest regeneration, potentially making the difference between population recovery post-fire or not. Local site conditions proved to have a lesser effect on regeneration than post-fire climate conditions.
The findings of this study can provide insights about the climate resilience of serotinous species more broadly, based on whether the tree species shares key characteristics with the knobcone pine. Aleppo pine and bishop pine, like knobcone pine, historically experienced relatively short-interval fires and are quick to build up their seed supply after fire. This study suggests that species with these characteristics can be quite resilient to increasing short-interval fires under climate change. However, for species like lodgepole pine and black spruce, which historically experienced longer fire intervals and take longer to reach maturity and accumulate sufficient canopy seedbanks, increasing fire frequency under climate change may cause significant population declines.
