1. Introduction
Rising temperatures and the legacy of human land uses that characterize the Anthropocene are having impacts on forests worldwide. Drought and hotter temperatures have been linked to extensive forest dieback and mortality (Allen et al., 2010, Anderegg et al., 2015, Anderegg et al., 2012), and more frequent and severe fires are related to increasing temperatures and the subsequent effect on vapor pressure deficit (Westerling et al., 2006, Williams et al., 2019). In forests with historically frequent, predominantly low-severity fire regimes, the modern management paradigm of excluding fire has also increased fuel loads, which is contributing to increased fire severity and greater extents of high-severity fire that exceed what would have occurred historically (Miller et al., 2009b, Parks and Abatzoglou, 2020, Safford and Stevens, 2017, Stevens et al., 2017). Mature forests are rapidly declining in response to cascading climate and fire regime shifts, which in turn is significantly impacting ecosystem services such as carbon storage, biodiversity, and hydrologic and watershed function (Anderegg et al., 2020, Turner, 2010). Another emerging threat of the Anthropocene is the loss of biodiversity, particularly in areas with rapid rates of ecological change (Johnson et al., 2017). Past management legacies and the warming climate are increasingly taking a toll on individual species, and species with limited distributions are likely to be at most risk (Pimm et al., 2014). The iconic giant sequoia (Sequoiadendron giganteum (Lindl.) J. Buchholz) is one such species that is increasingly facing novel threats as a result of anthropogenic change.
Giant sequoias are not currently designated as rare, but they are limited in distribution within the mixed-conifer forests of the Sierra Nevada mountains of California, USA. They occur in ∼ 70 distinct groves covering only ∼ 12,000 ha, most of which are in the southern Sierra Nevada (Hart, 2020). Culturally, they have been highly valued by American Indians throughout the Sierra Nevada for generations (Franco, 1994, Rueger, 1994); more recently they have been drawing visitors from around the globe (Tweed, 2016). They are primarily valued for their longevity and immense size, with the oldest known individual dated to ∼ 3,200 years old, and the largest specimen measured at roughly 11 m in diameter and 95 m tall (Hartesveldt et al., 1975). Their longevity is also important for understanding the connection between life history strategies, disturbance regimes and their possible future modifications (Piovesan and Biondi, 2021), and their massive size translates to significant carbon storage capacity, where old-growth giant sequoia forests are second only to coast redwood (Sequoia sempervirens (D. Don) Endl.) in carbon storage per hectare globally (Sillett et al., 2019).
Giant sequoia longevity is generally attributed to their resistance to pests, disease and in particular, wildfire. They have extraordinarily thick bark (up to 0.6 m in some cases) that protects the cambium from the heat of a wildfire, self-pruning branches, and semi-serotinous cones (Hartesveldt et al., 1975, Kilgore and Taylor, 1979). Because of their longevity, they also provide some of the oldest tree ring-based fire history chronologies available, which document frequent fire over 1,400 years in five different groves. Though this frequency varied with climate, prior to the onset of the contemporary fire exclusion era in the late 19th century, the longest fire free period observed across the five groves sampled was 30 years (Swetnam, 1993). At the individual tree scale, fires occurred roughly every 15 years (Swetnam et al., 2009). This frequency shaped a fire regime dominated by low to moderate severity fire, with some smaller patches of high intensity fire that created canopy gaps necessary for regeneration (Stephenson et al., 1991). With Euro-American settlement in the late 19th century, American Indian cultural burning was prohibited and lightning-ignited wildfires were suppressed, dramatically reducing fire frequencies. Given the species’ longevity and past fire frequencies, it is reasonable to assume that many of the older, legacy trees have lived through many dozens of fires that burned under a wide range of climatic conditions in their lifetimes, making the contemporary fire exclusion era truly anomalous for these trees (Swetnam, 1993).
Anthropogenic changes are negatively impacting giant sequoia groves in several ways. First, fire exclusion has resulted in scant giant sequoia regeneration in most groves due to their semi-serotinous cones and shade intolerance, raising concerns about overstory recruitment through time (Stephenson, 1994). The lack of fire has also dramatically increased fuel loads, a pattern that is well documented in both giant sequoia groves and in other frequent-fire forests throughout the west (Fulé et al., 2004, Greenberg and Collins, 2021, Parsons and DeBenedetti, 1979). Although a few groves, primarily in National Parks, have been regularly prescribed burned since the 1970 s, prior to 2015 most groves (∼90%) had not experienced fire in over a century (York et al., 2013). Climate change has also been linked with extensive foliage dieback during the 2012–2016 “hotter drought” (Stephenson et al., 2018). While foliage dieback is an effective strategy to reduce water loss via stomata during times of water stress, it had not been previously documented in giant sequoia, and is a bellwether that these long-lived trees are beginning to be impacted by climate change. The hotter drought also may have shifted giant sequoia interactions with the native Phloeosinus beetle, where greater water stress enabled the beetles to attack some trees; research into this topic currently underway (Nate Stephenson and Thomas S. Davis, personal communication).
Of all threats to the persistence of the old-growth giant sequoia forests, perhaps the most immediate threat is the warming climate’s interaction with increased fuel loads and drought stress, which is leading to increased fire activity and fire severity (Lydersen et al., 2017, Miller et al., 2009b, Parks and Abatzoglou, 2020, van Mantgem et al., 2018). While this trend has been underway for some time throughout the western US, it has only more recently been observed in giant sequoia groves. During a period that covered roughly the first century of fire exclusion (1910 to 2014), wildfire burned ∼ 19% of the giant sequoia range; in just the last six years (2015–2021), ∼65% of the range has burned in wildfires (Fire and Resource Assessment Program, 2020, Hart, 2020). Fire severity has also been on the rise; between 1984 and 2014, when burn severity maps from satellite imagery became widely available (Miller and Thode, 2007), ∼1 ha was detected as having burned at high severity. From 2015 through 2021 ~1,600 ha burned at high severity.
Where recent wildfires within giant sequoia groves burned at lower intensity, they have likely been largely restorative and within the natural range of variability (Haase and Sackett, 1998, Kilgore, 1970, Kilgore and Sando, 1975). These fires are similar to management-ignited prescribed fires which cause minimal to no large sequoia mortality; where the rare mortality does occur in these types of fires, it is generally from structural failure, where cumulative structural injury from 100 s of years of fires eventually result in tree fall (Weatherspoon, 1990). However, despite significant research on the impacts of high-severity fire effects in mixed-conifer forests of the region (Collins and Roller, 2013, Shive et al., 2018, Stevens et al., 2017, Welch et al., 2016), little is known about the extent of direct, fire-caused mortality in large, legacy giant sequoias, primarily because the occurrence of extensive high severity areas in sequoia groves is relatively new in the modern era. Our primary objective in this study is to quantify these impacts and better understand both the rates and drivers of mortality.
Tree mortality after wildfire is often assessed via remotely-sensed burn severity maps, which are generated by differencing pre- and postfire LANDSAT images. The differenced images are then linked to existing metrics, such as the Composite Burn Index (CBI), and classified into severity classes (generally undetected change, low, moderate and high) based on established relationships with field plots (Miller and Thode, 2007). CBI maps classified as high severity in Sierran mixed conifer have been linked with stand-replacing fire (i.e., >95% canopy cover loss (Miller et al., 2009a)); this class has also been characterized as having > 75% tree mortality (Monitoring Trends in Burn Severity, 2022). Within the high severity class, any mortality < 100% suggests that there is at least some live foliage retained postfire; in forests this is most likely found in surviving portions of tree crowns, given that crown fires rarely occur independently of surface fires that also consume surface and ladder fuels (Scott and Reinhardt, 2001). Given giant sequoia’s exceptional fire adaptations and height, we suspected that any live foliage that remains in high severity areas of sequoia groves could be concentrated in the upper crowns of large giant sequoia, increasing their potential for survival. This could effectively increase the survival rate for giant sequoia over that estimated for mixed -conifer forests, in which giant sequoia groves co-occur. In addition, although the basic drivers of fire behavior and severity are understood to be fuels, weather and topography, we don’t have basic information about where on the landscape these exceptionally fire-adapted trees might be most susceptible, or what individual tree characteristics increase the risk of mortality.
Moreover, the total mortality from a fire event includes immediate tree death and delayed mortality, the latter of which is not well captured in burn severity maps that are derived either immediately after fire or one-year postfire (Miller and Quayle, 2015, Miller and Thode, 2007). Immediate tree death in giant sequoia is the result of the total loss of photosynthetic tissue, since these trees cannot sprout after top kill; though they do sprout epicormically, this generally occurs on trees that had also retained some green foliage in their crowns postfire (Hartesveldt et al., 1975). Delayed mortality is also expected through time, where trees retain some green needles but have sustained significant fire injury (i.e. the direct impacts of a fire on plants, generally called “first order fire effects”) that either predisposes the tree to other stressors, such as drought stress or bark beetles, or that takes time to fully manifest (Hood et al., 2018).
There is substantial literature linking the degree of fire injury to tree mortality for individual species (Hood et al., 2018) and developing predictive models that can be used in applications that support forest management (Cansler et al., 2020a), such as the First Order Fire Effects Model (FOFEM; Keane and Lutes 2020). Important predictors of postfire tree mortality include crown scorch (needles killed from the heat of a fire), overall crown damage (which includes both crown scorch and crown torch, where the needles are consumed), bark char and cambium damage. The amount of damage that a tree can sustain and still survive varies greatly by species; to date, ponderosa pine (Pinus ponderosa Douglas ex Lawson & C. Lawson) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) have received the most research attention in the US (Cansler et al., 2020b, Woolley et al., 2012). In giant sequoias, only one study has examined delayed mortality after fire, with a focus on younger, understory trees after a low-severity prescribed burn (Stephens and Finney, 2002). Stephens and Finney (2002) found that trees 15–100 cm diameter at breast height (DBH) tended to survive until crown scorch exceeded 90–95%, but it is unclear how this translates to larger legacy trees, particularly under recent warming climate trends.
Given the combination of their exceptional adaptations to survive fire – tall crowns and overall height, thick bark – and the potential for large sequoia to withstand high levels of crown damage as documented in smaller trees (Stephens and Finney, 2002), we expected that though some large sequoias would be killed in high severity areas, most would survive. Following the dramatic rise in area of giant sequoia groves burned at high severity in recent years, understanding these mortality patterns and drivers will be critical for future management of both burned and unburned groves. To address these foundational questions, we asked:
- (1)
What are the mortality rates and fire effects for large sequoias in high and moderate severity areas?
- (2)
What individual tree characteristics and topographic variables are associated with mortality for all three fires?
- (3)
What first order fire effects best predict mortality by three years postfire at Black Mountain Grove, and how does this compare with existing models?
4. Discussion
Our surveys are the first to show that even the largest among the quintessentially fire-adapted giant sequoias are vulnerable to recent increases in fire severity. Within areas designated as high severity in remotely-sensed severity maps, we observed 75% (BLMO), 76% (Rough Fire Groves) and 100% (NELD) mortality of large, legacy giant sequoia trees (>1.2 m DBH). In NELD, the 100% mortality came from a single high severity patch which burned during an extreme downdraft. Outside of such an extreme event, mortality rates at BLMO and Rough Area Groves are on the low end of the > 75% tree mortality threshold (Monitoring Trends in Burn Severity, 2022), and much lower than the > 95% tree canopy mortality that has been associated with this CBI class (though our mortality rates are calculated on density, not canopy cover; Miller et al., 2009a). Mortality rates in areas designated as moderate severity areas were considerably lower at 14–45%. Collectively, we interpret these data to suggest that large, legacy giant sequoia trees are overall more likely to survive than their non-sequoia neighbors, but that within areas classified as high severity, mortality rates are still very high, particularly under extreme fire weather events.
Across the three fire footprints, the biggest topographic predictor of mortality was decreasing elevation within each of the fires. This was at least partly because there was more area burned at high severity at lower elevations, where burning conditions were warmer and where there also may have been greater drought-related tree mortality in the surrounding forest (Wayman and Safford, 2021, Young et al., 2017). We also hypothesize that more moisture at higher elevations may have also influenced tree health, moderating the amount of fire damage incurred (Agee et al., 2002) and subsequent resistance to fire-caused injury (Meyer et al., 2019). Crown ratio, an indicator of photosynthetic capacity and tree vigor, was also an important predictor of mortality, which is not surprising given its importance for photosynthesis and the substantial amount of variation in giant sequoia crowns. Repeated basal injuries and crown scorching from frequent fires can lead to very sparse crowns with little photosynthetic material (Hartesveldt et al., 1975). For a tree with a sparse crown prefire, 50% crown loss is likely to have a bigger impact on tree health than the same percentage on a tree with a robust crown. Sparse crowns also suggest that the tree may have been in decline prior to the fire from the standpoint of ability to acquire carbon and transport water and nutrients. Antecedent reductions in tree vigor has been linked with reduced resilience to fire in other western conifers (van Mantgem et al., 2018).
Fire scar presence was important in both the landscape scale model that included all three fires and the delayed mortality model in BLMO, likely for several reasons. First, when fire gets into a fire scar, the exposed wood facilitates flame movement up the bole of the tree, which is more likely to result in crown damage or combustion of a tree’s heartwood. It also means that there is less healthy cambium to transport nutrients and potentially damage to water-conducting sapwood (Rundel, 1973). Bark tissues surrounding the fire scar are likely thinner than elsewhere around the tree, and duff accumulation at the base of a fire scar also increases the likelihood of the fire scar igniting, potentially leading to greater cambium damage in subsequent fires (Hood, 2010). In addition, in much smaller ponderosa pine saplings, Partelli-Feltrin et al. (2020) found that the wound wood around fire scars can increase vulnerability to cavitation (Partelli-Feltrin et al., 2021), which could also be a factor for sequoia though more research is needed on this topic (Hood, 2021). In contrast to our findings, Lambert and Stohlgren, 1988, Haase and Sackett, 1998 found no effect of fire scar presence on giant sequoia mortality in prescribed burned groves, but prescription burns were generally characterized as low intensity surface fires that were presumably less intense than the high to moderate severity burned areas examined in this study (Haase and Sackett, 1998, Lambert and Stohlgren, 1988). One study of an endemic conifer (Athrotaxis cupressoides D. Don) on the island of Tasmania also found that trees with prior fire scars were more likely to die in a wildfire (Bowman et al., 2019).
Despite fire scars creating vulnerability in trees during subsequent fire, this variable is not included in FOFEM to predict postfire tree mortality (Keane and Lutes, 2020). Our models which include fire scar presence, performed substantially better than the current model used in FOFEM, which relies on crown damage and bark thickness. Because giant sequoia grow exceptionally thick bark, trees > 1.2 m DBH may have already met a minimum threshold for cambium protection outside of fire scar wounds, making estimates of increased thickness with increasing DBH less important. We acknowledge that our model was not evaluated with independent data, so it is not surprising that it performed better than the Stephens and Finney (2002) and FOFEM models. However, our findings are a meaningful improvement because the inclusion of fire scar presence improves model performance and adds valuable postfire mortality data that is sorely lacking for giant sequoia (Cansler et al., 2020a, Cansler et al., 2020b).
More broadly, the importance of crown damage in predicting delayed mortality in giant sequoia is consistent with many other studies on delayed mortality, which have found varying combinations of crown scorch, crown torch and total crown damage to be the most important predictors (Fowler et al., 2010, Fowler and Sieg, 2004, Hood et al., 2018, Shearman et al., 2019, Sieg et al., 2006). Our findings for these large giant sequoias are similar to those observed in smaller giant sequoias by Stephens and Finney (2002), which suggested that giant sequoias can survive with up to 90–95% crown loss. This trend is likely one of the reasons that the species can be so long-lived, particularly relative to other species that are more likely to die at lower amounts of crown damage, such as white fir (Grayson et al., 2017).
Other studies looking at delayed tree mortality have also documented the importance of cambium damage (Hood et al., 2018, Shearman et al., 2019). We suspect this is important in our study as well, but to inspect the cambium the bark needs to be physically removed, which would be extremely labor intensive with such thick-barked trees and potentially cause harm to surviving individuals. In addition, anecdotal reports suggest that severe basal scarring may be an important driver of fire-related giant sequoia mortality in Sequoia and Kings-Canyon National Parks (Nate Stephenson, personal communication), but we found this attribute difficult to quantify. We attempted to measure the average depth of basal scarring into the bark, as well as the percent of the circumference that had > 5 cm of the bark burned away at the base. Neither of these variables were significant predictors, but we suspect that is because our measurement approach did not capture the extent of basal damage, not because basal scarring is inconsequential. We recommend future research develop accurate quantification methodology for basal scarring and its physiological effects on large giant sequoia.
While this study has clarified patterns related to delayed mortality, our most important findings are likely the most basic – the extent of increasing fire impacts to highly valued trees with a limited distribution. Roughly 20% of all legacy giant sequoia trees were logged during the last century, reducing the extent of old individuals even before the Anthropocene began. Since systematic burn severity mapping efforts in the U.S. began in 1984, only ∼ 1 ha burned in high severity within a giant sequoia grove between 1984 and 2014, but 279 ha of high severity burned in these three fires between 2015 and 2017 (USDA Forest Service, 2018). This is alarming not only in the sharp upward trend, but also in magnitude, since the largest, high severity fires in a multi-millennia fire history record were estimated at 202 ha, which itself may be an overestimate (Caprio et al., 1994). On our study sites, at least 84% of large giant sequoias (>1.2 m DBH) in high severity areas were killed, which included 71 very large trees (>3.0 m DBH), often called “monarchs” (Cook, 1955), with substantial additional mortality in areas classified as moderate severity via remote sensing. That these three fires we are reporting on here were dwarfed more recently by over 1,400 ha of high severity in the 2020 and 2021 fire seasons should be raising high alarm on the status of ancient, legacy giant sequoia. Using the data presented here, two National Park Service reports have estimated that 13–19% of all large giant sequoias may have been lost in 2020–2021 (Shive et al., 2021, Stephenson and Brigham, 2021), signaling a precipitous decline in the population of the ancient trees that have significant cultural value.
In addition to studying mortality patterns on these more recent fires, future research should investigate impacts to second-growth groves that have also burned (e.g., Converse Basin and others in the Rough Fire footprint), to quantify the effects of recent wildfires on giant sequoias more broadly. Although one study looked at regeneration in burned second growth groves (Meyer and Safford, 2011), our understanding of mature tree survival and fire effects in these stands remains limited. In addition, these recent fires have also burned into areas that had selective logging of conifer species other than giant sequoia (i.e., “whitewoods”), presenting an opportunity to evaluate how a range of stand structures influenced fire behavior and the subsequent fire effects on large sequoias.
4.1. Management implications
Groves that burn at high enough intensity to cause mortality of old, large giant sequoia would have taken millennia to recover their old-growth stature even under historical climate scenarios; under modern climate conditions, their recovery trajectory is highly uncertain. The recent trends in wildfire activity suggest that managers have a narrowing window of opportunity to protect the remaining legacy trees from severe fire via ecological restoration (Agee and Skinner, 2005, Prichard et al., 2010, Ritchie et al., 2007, Safford et al., 2012, Stephenson, 1996). At the end of the 2021 fire season, roughly 65% of the range of giant sequoia has burned in a wildfire since 2015; of the remaining grove area, some have received beneficial fire, but there are also several groves that have not had fire in them for over a hundred years. Currently, the fuel loads in these groves are likely to facilitate intense fire behavior with resultant high severity burns unless fuel loads are reduced through restoration treatments such as prescribed fire and mechanical thinning (York et al., 2013).
In addition to the needed restoration in unburned groves, recent fires in giant sequoia have also presented an opportunity. Areas within groves that burned at lower severity and that have high survival of large giant sequoias may have received a “fuel treatment” (Prichard et al., 2021). Between 2015 and 2021, roughly 5,500 ha of giant sequoia have been classified as having low or moderate severity fire effects. The resulting fuels conditions within those hectares will vary tremendously, depending on how much fuel was consumed and how much “new” dead fuel was created by the first fire (Eskelson and Monleon, 2018). Where fuel loads are not within the range of desired conditions for fire resilience, they should be a priority for early re-treatment with prescribed fire. Where postfire fuels conditions are desirable, particularly where the fire was a “second entry” (for example, where a recent fire reburned a prescribed fire), managers could have up to 10–15 years to plan and amass the necessary resources to implement the next treatment. There may also be an opportunity in these areas to use managed wildfire (i.e., wildfires managed to support natural resource objectives) where possible, since low severity burned areas that reburn within one to two decades in this region and fuel type tend to reburn at lower severity (Collins et al., 2009, van Wagtendonk, 2012).
Many stakeholder collaboratives are forming around the dire problem of restoring Sierran forests at landscape-scales, well beyond the scale of giant sequoia groves (North et al., 2021). Unfortunately, the reality is that extensive, landscape-scale restoration is still many years away given our current workforce, funding, and regulatory compliance needs (Collins et al., 2010). If large-scale restoration is the end goal, prioritizing initial restoration efforts on specific forest stands that are highly valued for habitat, cultural, recreation or ecosystem service values, such as giant sequoia groves, will be critical during this time of rapid change. Studies have repeatedly shown that restoration to reduce fuels can reduce fire severity where they are implemented (Fulé et al., 2012, Lydersen et al., 2017, Pollet and Omi, 2002), enabling a wildfire to burn in ways that retains functioning, resilient forests. Given that giant sequoias have a limited distribution across the Sierra Nevada, serve as major carbon stores and are highly valued culturally, it is increasingly important to prioritize sequoia groves for restoration and continued future management.
CRediT authorship contribution statement
Kristen L. Shive: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Resources, Software, Supervision, Visualization, Writing – original draft, Writing – review & editing. Amarina Wuenschel: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Resources, Software, Supervision, Visualization, Writing – original draft, Writing – review & editing. Linnea J. Hardlund: Conceptualization, Data curation, Methodology, Supervision, Writing – original draft, Writing – review & editing. Sonia Morris: Visualization. Marc D. Meyer: Conceptualization, Writing – original draft, Writing – review & editing. Sharon M. Hood: Methodology, Visualization, Writing – original draft, Writing – review & editing.
