Nathan L. Stephenson’s research while affiliated with United States Geological Survey and other places
What is this page?
This page lists works of an author who doesn't have a ResearchGate profile or hasn't added the works to their profile yet. It is automatically generated from public (personal) data to further our legitimate goal of comprehensive and accurate scientific recordkeeping. If you are this author and want this page removed, please let us know.
The fundamental trade‐off between current and future reproduction has long been considered to result in a tendency for species that can grow large to begin reproduction at a larger size. Due to the prolonged time required to reach maturity, estimates of tree maturation size remain very rare and we lack a global view on the generality and the shape of this trade‐off. Using seed production from five continents, we estimate tree maturation sizes for 486 tree species spanning tropical to boreal climates. Results show that a species' maturation size increases with maximum size, but in a non‐proportional way: the largest species begin reproduction at smaller sizes than would be expected if maturation were simply proportional to maximum size. Furthermore, the decrease in relative maturation size is steepest in cold climates. These findings on maturation size drivers are key to accurately represent forests' responses to disturbance and climate change.
Background
The giant sequoia ( Sequoiadendron giganteum [Lindley] Buchholz) of California’s Sierra Nevada recently suffered historically unprecedented wildfires that killed an estimated 13–19% of seed-bearing sequoias across their native range. Hanson et al. recently sought to characterize post-fire reproduction in two severely burned sequoia groves, but their two papers (1) inaccurately portrayed sequoia fire ecology, (2) had methodological flaws, and (3) without supporting evidence, questioned efforts to prevent large, stand-replacing wildfires and to plant sequoia seedlings in areas of low post-fire regeneration.
Results
Our analyses and literature review contradict many of Hanson et al.’s claims and implications. First, evidence indicates that preceding the recent wildfires, large, contiguous areas (>10 to >100 ha) of fire severe enough to kill most sequoias had been absent for at least a millennium, and probably much longer. The ancient sequoia fire regime was instead overwhelmingly dominated by surface fires in which most forest area burned at low or moderate severity interspersed with small forest gaps (hundredths of a hectare to a few hectares) created by local patches of higher-severity fire, within which most mature sequoias survived and most successful reproduction occurred. Prescribed fires have typically mimicked ancient fires and induced adequate sequoia regeneration. In contrast, in some extensive areas where recent wildfires killed most (or all) mature sequoias, regeneration has been well below historical levels, threatening a net loss of sequoia grove area. Methodologically, Hanson et al. reported sixfold greater post-fire sequoia seedling densities than others who sampled the same area; our assessments suggest their higher densities may have largely resulted from plot-placement bias. Finally, Hanson et al.’s comparisons of median seedling densities were inappropriate.
Conclusions
Hanson et al. questioned efforts to prevent large, high-severity wildfires in sequoia groves but did not acknowledge (1) that past fires sustained sequoia reproduction without the deaths of large fractions of mature sequoias, (2) the anomalous effects of recent wildfires, and (3) the acute conservation threat of losing large fractions of seed-bearing sequoias. Hanson et al.’s further implication, made without supporting evidence, that decisions to plant sequoia seedlings may be unwarranted ignores research showing that recent post-wildfire regeneration has often been well below historical levels.
Many forests globally are experiencing increases in large, high-severity wildfires, often with increasingly inadequate post-fire tree regeneration. To identify areas that might need post-fire planting, forest managers have a growing need for seedling reference densities - the natural seedling densities expected to be adequate to regenerate a forest - to compare with observed post-fire seedling densities. The most useful reference densities will meet five criteria: they will (1) be specific to natural post-fire reproduction rather than planted seedlings (because planted seedlings can have substantially greater survival than natural seedlings, thus underestimating adequate natural reproduction), (2) apply to the first few years following fire (when management decisions and actions are most likely), (3) be specific to each of those post-fire years (because post-fire seedling densities can change rapidly with time since fire), (4) be associated with estimates of uncertainty, and (5) include consideration of novel environmental conditions during management applications (because most reference densities will be based on data collected under more environmentally benign conditions). The world’s most massive tree species, the giant sequoia (Sequoiadendron giganteum) of California’s Sierra Nevada, recently experienced historically unprecedented wildfires that killed an estimated 13-19% of mature sequoias across their native range. Seedlings germinating after these fires then experienced exceptional summer heat and the two most severe summer droughts of the 121-year historical record. To help inform management responses to these events, we used seedling censuses from past fires (mostly prescribed fires) to calculate sequoia seedling reference densities meeting the five criteria. The reference densities had three striking features, which are partly attributable to giant sequoia’s status as a pioneer species. First, despite being inherently conservative, the reference densities were quite high. For example, mean first-year reference density was 172,599 seedlings/ha. Second, reference densities declined precipitously with time since fire: the mean fifth-year reference density was only 5% of the mean first-year density. Third, the reference densities were associated with relatively substantial uncertainty, a consequence of density variations among seedling plots; for example, the 95% credible interval for first-year reference density was 64,377 to 313,438 seedlings/ha. Despite this uncertainty, a case-study sequoia grove that recently burned in a high-severity wildfire had second-year post-fire seedling densities that were significantly (and dramatically) lower than the corresponding second-year reference density, suggesting inadequate post-fire reproduction. Our results highlight the value of the five criteria for reference densities - criteria that, in current practice, are rarely all met.
Fire is a critical driver of giant sequoia ( Sequoiadendron giganteum [Lindl.] Buchholz) regeneration. However, fire suppression combined with the effects of increased temperature and severe drought has resulted in fires of an intensity and size outside of the historical norm. As a result, recent mega‐fires have killed a significant portion of the world's sequoia population (13%–19%), and uncertainty surrounds whether severely affected groves will be able to recover naturally, potentially leading to a loss of grove area. To assess the likelihood of natural recovery, we collected spatially explicit data assessing mortality, crown condition, and regeneration within four giant sequoia groves that were severely impacted by the SQF‐ (2020) and KNP‐Complex (2021) wildfires within Sequoia and Kings Canyon National Parks. In total, we surveyed 5.9 ha for seedlings and assessed the crown condition of 1104 giant sequoias. To inform management, we used a statistical methodology that robustly quantifies the uncertainty in inherently “noisy” seedling data and takes advantage of readily available remote sensing metrics that would make our findings applicable to other recently burned groves. A loss of giant sequoia grove area would be a consequence of giant sequoia tree mortality followed by a failure of natural regeneration. We found that areas that experienced very high‐severity fire (above ~800 RdNBR) are at substantial risk for the loss of grove area, with tree mortality rapidly increasing and giant sequoia seedling density simultaneously decreasing with fire severity. Such high‐severity areas comprised 17.8, 142.0, 14.6, 1.6 ha and ~90%, ~14%, ~53%, and ~27% of Board Camp, Redwood Mountain, Suwanee, and New Oriole Lake groves, respectively. In all sampling areas, we found that seedling densities fell far below the average density measured after prescribed fires, where seedling numbers were almost certainly adequate to maintain giant sequoia populations and postfire conditions were more in keeping with historical norms. Importantly, spatial pattern is also important in assessing the risk of grove loss, and in two groves, Suwanee and New Oriole Lake, the high‐severity patches were not always contiguous, potentially making some areas more resilient to regeneration failure due to the proximity of surviving trees.
The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on mutualist dispersers. These effects play out in the context of variable climate and site fertility among species that vary widely in nutrient demand. Meta-analyses of published data have focused on variation at the population scale, thus omitting periodicity within trees and synchronicity between trees. From raw data on 12 million tree-years worldwide, we quantified three components of masting that have not previously been analysed together: (i) volatility, defined as the frequency-weighted year-to-year variation; (ii) periodicity, representing the lag between high-seed years; and (iii) synchronicity, indicating the tree-to-tree correlation. Results show that mast avoidance (low volatility and low synchronicity) by species dependent on mutualist dispersers explains more variation than any other effect. Nutrient-demanding species have low volatility, and species that are most common on nutrient-rich and warm/wet sites exhibit short periods. The prevalence of masting in cold/dry sites coincides with climatic conditions where dependence on vertebrate dispersers is less common than in the wet tropics. Mutualist dispersers neutralize the benefits of masting for predator satiation, further balancing the effects of climate, site fertility and nutrient demands.
In some areas burned by recent wildfires, most or all giant sequoias were killed. Sequoia managers wish to know whether post-fire seedling establishment in those areas has been adequate to regenerate the locally extirpated sequoias. To provide a yardstick for interpreting sequoia seedling densities measured after the recent severe wildfires, here we calculate mean seedling densities measured one, two, and five years after several mixed-severity fires of the past. Our analyses are based on 42 sites in eight different sequoia groves in Sequoia and Kings Canyon national parks, California, which burned in 26 different fires spanning a 48-year period. Conservatively (i.e., without correcting probable errors of underestimated densities), mean sequoia seedling density the first summer following fire was 153,278/ha (Bayesian estimated median = 173,742/ha; 95% credible interval [CI] = 63,319/ha to 850,336/ha). Mean seedling densities the second and fifth summers following fire were, respectively, 34,870/ha (Bayesian estimated median = 39,562; 95% CI = 14,181/ha to 181,011/ha), and 8,601/ha (Bayesian estimated median = 9,513/ha; 95% CI = 3,827/ha to 34,057/ha). Case-study comparisons showed that measured post-fire seedling densities across the Board Camp Grove and in the severely burned portions of the Redwood Mountain Grove were significantly lower than our second-year reference seedling densities.
Fire is a critical driver of giant sequoia (Sequoiadendron giganteum [Lindl.] Buchholz) regeneration. However, fire suppression combined with the effects of increased temperature and severe drought have resulted in fires of an intensity and size outside of the historical norm. As a result, recent mega-fires have killed a significant portion of the world’s sequoia population (13 to 19%), and uncertainty surrounds whether severely affected groves will be able to recover naturally, potentially leading to a loss of grove area. To assess the likelihood of natural recovery, we collected spatially explicit data assessing mortality, crown condition, and regeneration within four giant sequoia groves that were severely impacted by the SQF- (2020) and KNP-Complex (2021) fires within Sequoia and Kings Canyon national parks. In total, we surveyed 5.9 ha for seedlings and assessed the crown condition of 1140 trees. To inform management, we used a statistical methodology that robustly quantifies the uncertainty in inherently ‘noisy’ seedling data and takes advantage of readily available remote sensing metrics that would make our findings applicable to other burned groves. A loss of giant sequoia grove area would be a consequence of giant sequoia tree mortality followed by a failure of natural regeneration. We found that areas that experienced high severity fire (above ~800 RdNBR) are at substantial risk for loss of grove area, with tree mortality rapidly increasing and giant sequoia seedling density simultaneously decreasing with fire severity. Such high severity areas comprised 17.8, 142.0, 14.6, 1.6 hectares and ~90%, ~14%, ~53%, and ~27% of Board Camp, Redwood Mountain, Suwanee, and New Oriole Lake groves, respectively. In all sampling areas, we found that seedling densities fell far below the average density measured after prescribed fires, where seedling numbers were almost certainly adequate to maintain giant sequoia populations and postfire conditions were more in keeping with historical norms. Importantly, spatial pattern is also important in assessing risk of grove loss, and in two groves, Suwanee and New Oriole Lake, the high severity patches were not always contiguous, potentially making some areas more resilient to regeneration failure due to the proximity of surviving trees.
The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential.
... In the southern Sierra Nevada, California, a once-in-a-millennium drought between 2012 and 2015 killed nearly 129 million trees (Asner et al., 2015;Fettig et al., 2019), with disproportionate mortality among the largest trees (Restaino et al., 2019). Subsequent extreme fire years combined with drought effects led to at least a halving of habitat for mature forestdependent wildlife since 2011 (Steel et al., 2023) and substantial losses of large, iconic trees (Shive et al., 2022;Stephenson et al., 2024). In the current fire environment, it is not clear how much time is left before mature and old-growth forests disappear, or whether management interventions might slow or reverse observed declines . ...
... The vegetation composition and structure of the Central Indian tropical forests have been described by researchers (Kala and Dubey, 2012;Kala, 2015;Raha et al., 2020;Singh et al., 2024). Previous studies have assessed the effects of disturbance factors such as grazing, logging, NTFP collection, fuelwood collection, forest fires, open canopy cover, invasive species, proximity to the village, distance from the road, etc. on the structure, composition and diversity attributes of forests (Okuda et al., 2003;Davidar et al., 2007;Cazzolla et al., 2015;Fernández-García et al., 2019;DeFries et al., 2022;Chen et al., 2024;Soderberg et al., 2024;Tari et al., 2024;Barras et al., 2025). However, only a few studies have been carried out on the impacts of disturbance in Indian tropical dry forests (Sagar et al., 2003;Sahu et al., 2008;Kumar et al., 2022;Singh et al., 2022). ...
... only report 'wood' rather than CWD and FWD separately). A possible explanation is the recent and highly elevated tree mortality in these forests (~23 % over a three-year period in high mortality areas) from the 2012-2016 drought (Stephenson et al., 2019), which likely resulted in increased CWD from trees that have fallen but not yet substantially decomposed (Northrop et al., 2024). This hypothesis is also in keeping with our estimates of large tree density (Fig. 8), which tended to be low relative to expectations from other work (Lydersen and North, 2012;Stephens et al., 2012Stephens et al., , 2009) and relative to management targets (see below). ...
... Masting is when reproduction in iteroparous plants is characterised by occasional large synchronous reproductive events Qiu et al. 2023). The betweenindividual synchrony in reproductive effort characteristic of masting can be observed at large, sometimes continental-wide, scales (Ascoli et al. 2017). ...
... The post-fire seedling reference densities reported by Stephenson et al. (2023 were based on data from a substantially more benign climatic period than that which prevailed during the critical first years following the recent wildfires in sequoia groves , and more benign than is expected for the future (e.g., Gonzalez 2012). Thus, Stephenson et al. 's (2023Stephenson et al. 's ( , 2024 reference densities are almost certainly conservative, underestimating the seedling densities needed to regenerate sequoia populations killed in high-severity areas of recent wildfires. ...
... J. Buchh., Cupressales: Cupressaceae) is among the largest and oldest known terrestrial tree species, with trees reaching average heights at maturity of >60 m (>200 ft) and in some cases living longer than 3,200 years (Hartesveldt et al. 1975, Flint 1987, Stephenson 2000, Sillett et al. 2015. Sequoia is endemic to California, and the natural range is presently restricted to about 70 groves encompassing ~11,000 ha scattered across the western slope of the Sierra Nevada mountains of California (Soderberg et al. 2023), though the tree is planted as an ornamental outside its natural range in both the United States and abroad (Weatherspoon 1990). The giant sequoia is also an important cultural resource that attracts substantial tourism (Dilsaver and Strong 1990), and the species is currently red-listed by the International Union for Conservation of Nature with an estimated fewer than 80,000 trees remaining in natural populations (Schmid and Farjon 2013), most of which occur on federally managed lands. ...
... Recently, the trade-off relationships among different traits have been discovered [1,2]. The phenotype and genetic mechanism of trade-offs between disease resistance and high yield [3], spikelet number and fertility [4], growth and defense [5,6], growth and stress response [7], and reproduction and seed size-number [8] have been widely studied in plants. In addition, trade-offs have also been frequently observed in lower eukaryotes, including the trade-offs between sex and growth in diatoms [9], antibiotic tolerance and growth rate in bacteria [10], detoxification and reproduction, male success in sperm competition and offspring quality, gut homeostasis and lifespan, immunity and reproduction, and growth and immunity in insects [11][12][13][14][15][16][17]. ...
... Inferring the relationship between maturation and maximum size has also to control for the environment (Wenk and Falster 2015) and species characteristics (Visser et al. 2016). While the effects of climate on maturation size are unknown, tree fecundity responds to seasonal temperature and moisture, soils and light availability, which depends on the local competitive environment (Clark et al. 2014;Caignard et al. 2017;Minor and Kobe 2019;Le Roncé et al. 2021;Qiu et al. 2022;Journé et al. 2022). Also, fast growth and accelerated competition that comes from long growing seasons in the wet tropics do not necessarily imply small or large maturation sizes. ...
... Continued warming and regional changes in precipitation are expected to amplify interactions among disturbance agents and further alter forest ecosystem structure and function. Evidence for shifts in tree species ranges as affected by climate change is available for some areas; 205,206,207 however, understory species range shifts will depend on whether the canopy is affected. 49,208 ...
... Recent investments in restorative forest treatments (e.g., US Forest Service Wildfire Crisis Strategy; USDA Forest Service 2022) are a necessary and long overdue part of the solution (Belavenutti et al. 2021), but fuel treatments alone are likely insufficient to alter fire regimes in the wildland-dominated landscapes of the west Hessburg et al. 2015). Fuel treatments can reduce fire risk and improve resilience within the treated area Furniss et al. 2022a), but building resilient landscapes requires a multifaceted approach incorporating a combination of mechanical treatments, prescribed (Rx) fire, and wildland fire use (Miller 2003;van Wagtendonk 2007;North et al. 2012North et al. , 2015Calkin et al. 2015;Stephens et al. 2016). Restoring natural wildfire regimes is a central part of the solution (Young and Ager 2024), as we are simply unable to duplicate the spatial scale and ecological complexity of wildfire effects (e.g., Furniss et al. 2020) with even our most sophisticated silvicultural tools. ...
![Seed size-number trade-offs and species seed production
a Species seed production (SSP, g seed per m² tree basal area) is not constrained by the strict size-number trade-off (dashed line with a slope of zero). Instead, it varies over ten orders of magnitude and has a positive correlation with seed mass across 714 tree species (log10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\log }_{10}$$\end{document}SSP =4.29+0.546⋅log10m\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$=4.29+0.546\cdot {\log }_{10}m$$\end{document}, R² = 0.189, p < 10⁻¹⁵, n = 714). b SSP exhibits phylogenetic coherence for 482 species having phylogeny data (68% of species). Brown and green text highlight species that produce coniferous cones and fleshy fruits, respectively. The phylogenetic signal is estimated using Pagel's λ = 0.60 (p < 10⁻⁹, n = 482).](https://www.researchgate.net/publication/360316873/figure/fig2/AS:1151478169382913@1651545197691/Seed-size-number-trade-offs-and-species-seed-production-a-Species-seed-production-SSP-g_Q320.jpg)
