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North to South: Freeze and fire trends in American coastal temperate rainforests

In a warming world, the Arctic is often cited as the leading edge of climate change. But the coastal temperate rainforests of North and South America may qualify as another frontier: while the Arctic is warming at a faster rate, these temperate rainforests are crossing key temperature thresholds that may trigger sudden shifts in ecosystem types. A recent paper by CRMRN scientists Brian Buma, Sarah Bisbing, Allison Bidlack, and others published in Austral Ecology considers emerging snow and fire trends in the two largest temperate rainforests in the world.

The North and South Pacific coastal temperate rainforests, located across coastal Alaska through British Columbia and Chile respectively, make up over half of the area of global temperate rainforests. The influence of the Pacific Ocean creates a mild, wet maritime climate that hovers around zero degrees Celsius at sea level in the North Pacific coastal temperate rainforest (NPCTR) and at higher elevations in the South Pacific coastal temperate rainforest (SPCTR). It’s this marginal climate that creates the potential for counter-intuitive phenomena like fire in rainforests and tree decline by freezing due to loss of snowpack in a warming climate. Similarities between the NPCTR and SPCTR allow for a unique comparison of climate impacts across over 30° of latitude, north to south.

In addition to providing a consistent gradient of climate impacts across latitudes, these regions may act as harbingers of climate-driven ecosystem change for forested biomes globally. As a few degrees of warming pushes areas of the NPCTR and SPCTR above freezing for longer periods of the year, a dramatic transformation is occurring from snow-dominated to rain-dominated systems at high latitudes and elevations. The number of snow-covered days in coastal temperate rainforests is decreasing faster than in any other biome, driving a decline in cold-sensitive species like yellow-cedar that rely on insulation from snowpack to stave off root freeze during cold snaps.

Warming temperatures, lessening snowpack, and longer rain-free growing seasons also have the potential to increase or create fire regimes in new areas. This is particularly impactful in areas that have not historically had significant fire disturbance. The growing establishment of a fire regime in the drier and mountainous areas the SPCTR can provide a roadmap for the southern reaches of the NPCTR, where more fire-prone conditions are expected to emerge. As high-resolution fire projections aren’t available for the north and south PCTRs, the authors modeled projections of relative change to fire probability to highlight the areas that will see the most intense shifts.

Both snow-to-rain shifts and fire emergence act as ecosystem simplifiers by weeding out species that are freeze-susceptible and bolstering the success of fire-resistant species. As these changes take hold across different timescales and latitudes in the north and south, comparisons allow the chance to test theories on how areas respond to climate change at a local level. These CTRs also can act as early indicators of change for other forested ecosystems that will cross the same temperature thresholds with future warming. As major storehouses of carbon, the fate of the north and south PCTRs have global implications for the carbon budget and highlight the need for future work on the role of changes in snowpack on species range and fire emergence in novel locations.

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