Case study: Subalpine forests in the Greater Yellowstone Ecosystem, US Northern Rocky Mountains

High-elevation, subalpine forests in the US Northern Rocky Mountains  are burning more now than they have in the last two millennia , a trend expected to continue through the end of this century. We use the following words and definitions to describe how fire affects forests in this region and how forest recovery might change in the future:

Definitions

long-interval fire: Fire that burns the same area twice at the historical fire frequency. In subalpine forests in the Northern Rocky Mountains, this is  roughly 100 to 300 years between two fires .

Short-interval fire, or "reburn": Fire that burns the same area twice within a time period that is short relative to historical fire frequency. For Northern Rocky Mountain subalpine forests, we define this as less than 30 years between the two fires.

Severity: The effects of fire on an ecosystem. This can be measured in different ways. Here, we use severity to refer to the percent of live trees killed by fire.

High-severity or stand-replacing fire: Fire that kills all or almost all of the aboveground pre-fire live vegetation. Here, we define stand-replacing fire as killing > 90% of live trees.

Resilience: The ability of a forest to return to reference structural or functional conditions. For example, we consider 125-300-year-old lodgepole pine forests as a reference for mature forest. We use measurements such as tree density, canopy cover, biomass, and species composition to describe reference conditions.

Recovery: The process of returning to the pre-fire forest structure, composition, and/or function.

Subalpine forests are historically resilient to fire

Subalpine forests in Greater Yellowstone are resilient to large, high-severity fires that kill most of the aboveground vegetation so long as there is enough time for that vegetation to recover before the next fire. In these forests, this takes at least a century. Because fires historically burned a given area of subalpine forest every 100 to 300 years,  forests have typically been able to recover .

Characteristics of the plant species in Yellowstone’s forest contribute to this resilience. Lodgepole pine, the dominant tree species across Yellowstone’s vast high-elevation plateaus, forms closed cones that are only opened at high temperatures ("serotinous" cones).

When a fire moves through, these serotinous cones open and disperse thousands upon thousands of seeds onto the newly exposed soil,  leading to abundant tree regeneration .In the understory, perennial plants quickly resprout from belowground parts that weren’t killed by fire. Within a couple of years, these plants disperse seeds and contribute to  the rapid greening of the recently burned forest . 

Subalpine forests that reburn as short-interval, high-severity fire  may not recover as rapidly as they have in the past . Young lodgepole pine trees will have fewer or no serotinous cones, meaning  far fewer seeds will be available to regenerate the forest after fire . Reduced tree regeneration could change the environmental conditions experienced by resprouting understory plants, potentially  favoring certain species and driving vegetation community change .