Towering giant sequoia trees are an iconic species that draw numerous tourists and sightseers each year, but in 2020 and 2021 alone wildfires killed approximately 13 to 19% of all large giant sequoias in their native range, the western Sierra Nevada of California. Many of these sequoias were killed in large areas of ‘high burn severity’. Such conditions are well outside the historical norm for giant sequoia, a species adapted to frequent mixed severity fires rather than infrequent fires with large patches of high severity as we are seeing today. As a result, there is uncertainty about whether such areas will recover naturally or whether sequoia will need active management to remain in the parts of groves affected by such fires. 

To address this concern, U.S. Geological Survey researchers, with National Park Service and U.S. Forest Service partners, assessed giant sequoia mortality and regeneration in four groves within Sequoia and Kings Canyon national parks that experienced significant levels of mortality due to high severity wildfire. The goal was to assess whether sequoia groves were likely to recover after severe wildfire. Managers needed to know, in places where most of the sequoias had died, would natural regeneration be adequate to replenish the population?

The team found that areas that experienced high severity fire are at risk of losing giant sequoia grove area. This is because, in addition to high rates of tree mortality (>90% of trees dying), the levels of seedling regeneration – the next ‘generation’ assumed to replace trees that were lost – were far below those found after mixed severity prescribed fires, which produce conditions that are more similar to the historic fire regime. 

Visual comparison showcasing contrasting effects on tree mortality within giant sequoia grove areas characterized by large patch, high burn severity (left) and areas with mixed burn severity (right) within the 2021 KNP Complex wildfire. 'Backburn' management operations were conducted to reduce fire severity before the leading edge of the wildfire moved into Muir Grove (right), reducing the likelihood of more severe fire effects. In contrast, no management interventions were conducted prior to the wildfire entering Board Camp Grove (left).

An important research finding was that these fire effects can potentially be estimated directly using publicly available, satellite-derived burn severity metrics which allow managers to assess the risk of grove loss in any area that recently experienced high severity fire. In addition, researchers found that within high burn severity areas, as fire severity increased, rates of giant sequoia mortality increased, and seedling regeneration decreased. Importantly, while previous research has shown that small patches of high burn severity can be beneficial for sequoia reproduction, the current results suggest that sequoia populations experience increasingly detrimental effects as burn severity increases, particularly in large patches where most giant sequoias died during the fire—a result well outside the historical norm. 

As these novel burn conditions occur, managers will often have an increased need for real time data and comparisons with past conditions to inform management decisions. Managers must make decisions involving tradeoffs between costs of action versus costs of inaction made in the context of agency mandates, law, policy, and budgets. The findings here provide useful insight into both grove-specific and landscape scale fire effects on giant sequoia populations. These findings help inform managers who must make difficult conservation and management decisions about how to maintain iconic sequoia groves, such as whether natural regeneration following recent large patch, high severity wildfires will be adequate to succeed what was lost or whether human intervention, such as planting seedlings, may be necessary.