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Juli G. Pausas

Publications and source records attributed to Juli G. Pausas.

At least 19 recordsLinked to original sources
USGS2025

Sequoia and Sequoiadendron: Two paleoendemic megatrees with markedly different adaptive responses to recent high-severity fires

Premise Coast redwood ( Sequoia sempervirens ) and giant sequoia ( Sequoiadendron giganteum ) are two iconic paleoendemic species with limited distributions, well known for their spectacular size. Recently, they have been exposed to high-severity crown fires, with starkly contrasting responses. Methods We used all available published literature and field observations to understand the responses to fire in an evolutionary context. Results Coast redwoods, found in California's coastal rainforests, were highly resilient to high-severity fires, with most trees surviving due to their ability to resprout from the base and trunk, though seedling regeneration was largely lacking. In contrast, giant sequoias, native to the Sierra Nevada, do not resprout, leading to significant tree mortality after very high-severity fires; they released seeds only in patches where some trees survived moderately high-severity fires. Conclusions These high-severity fires were novel events for giant sequoias, but not for coast redwoods. Fire suppression has disrupted the natural fire regime in the giant sequoia ecosystem by preventing frequent lightning-caused surface fires, resulting in high-severity fires that killed a substantial number of these giants. In coast redwood forests, infrequent but high-severity crown fires were the norm before burning by Native Americans. Frequent, low-severity burning by Native Americans over the past few hundred years was localized and 20 th -century fire suppression has returned the natural fire regime to these forests. The recent crown fires do not represent a threat to redwood conservation; however, other management goals may require emulating Native American burning practices and in some cases may be best termed cultural restoration.

American Journal of Botany↗
USGS2025

The role of fire on Earth

Fire is a defining feature of our biosphere, having appeared when the first plants colonized the land, and it continues to occur across the planet at different frequencies and intensities. Fire has been and remains as an evolutionary force in many plant and animal lineages and contributes to explaining the variability of our biodiversity. Fire has also shaped the structure of many ecosystems and the distribution of biomes, and it is an important contributor to the global biogeochemical cycles. In addition, fire has been a key factor in human evolution, and, in turn, humans have modified fire regimes with important consequences for the biosphere. Consequently, fire is an intrinsic factor on our planet. Our challenge now is to understand and predict the role of fire in a densely populated, highly technological world that imposes significant changes on the Earth.

BioScience↗
USGS2025

Are fire regimes the result of top-down or bottom-up drivers?

The evolutionary topic we examine here is whether species determine the environment (bottom-up) or if environments shape plant traits (top-down). For the environment, we focus on the fire regime. Many forests are subject to either frequent low-intensity surface fires or less frequent but high-intensity crown fires. What are the ultimate factors controlling these fire regimes? The top-down model proposes that environmental factors controlling productivity and ignitions shape fire regimes; the bottom-up model attributes them to different plant assemblies. In boreal forests, it is assumed that, because of the similar climate, forests of North America and Eurasia undergo distinct fire regimes (crown-fire and surface-fire regimes, respectively) due to bottom-up forces. We tested the hypothesis that fire regimes are primarily controlled by top-down factors by selecting congeneric species of Pinus and Picea from both continents. Plots dominated by each species were studied using remote sensing data. We then compared environmental conditions where the species occur and found that Eurasian tree species occur in warmer and more productive environments than North American tree species. Our results support the top-down model, which suggests that environmental factors control the surface- versus crown-fire regime in boreal forests.

North America↗
USGS2023

Evolutionary fire ecology: An historical account and future directions

The idea that fire acts as an evolutionary force contributing to shaping species traits started a century ago, but had not been widely recognized until very recently. Among the first to realize this force were E dward B . Poulton, R. D ale Guthrie, and E dwin V . Komarek in animals and W illis L . Jepson, W alter W . Hough, T om M . Harris, P hilip V . Wells, and R obert W . Mutch in plants. They were all ahead of their time in their evolutionary thinking. Since then, evolutionary fire ecology has percolated very slowly into the mainstream ecology and evolutionary biology; in fact, this topic is still seldom mentioned in textbooks of ecology or evolution. Currently, there is plenty of evidence suggesting that we cannot understand the biodiversity of our planet without considering the key evolutionary role of fire. But there is still research to be done in order to fully understand fire's contribution to species evolution and to predicting species responses to rapid global changes.

BioScience↗
USGS2022

Bet-hedging and best-bet strategies shape seed dormancy

Seed dormancy (i.e. delayed germination even when conditions are favourable) is a key plant characteristic that occurs among many species worldwide. But, what selective pressures led to seed dormancy? A recent study provides a major analysis of the factors driving this trait at the global scale (Zhang et al ., 2022 ). Using c. 12 000 species and 10 million records across the globe, they conclude that dormancy is a strategy for plants living under ‘seasonal/unpredictable’ environments; and suggest that bet-hedging could be the major mechanism behind the pattern. To reach their conclusions the authors relate the proportion of species with dormant seeds in a grid-cell global map against climate variables related to annual precipitation, temperature and seasonality. Then they showed that the most significant variables were those related to climate seasonality that they equate with unpredictable climates (although seasonal climates usually are highly predictable in their seasonal cycles).

New Phytologist↗
USGS2022

Evolutionary ecology of fire

Fire has been an ecosystem process since plants colonized land over 400 million years ago. Many diverse traits provide a fitness benefit following fires, and these adaptive traits vary with the fire regime. Some of these traits enhance fire survival, while others promote recruitment in the postfire environment. Demonstrating that these traits are fire adaptations is challenging, since many arose early in the paleontological record, although increasingly better fossil records and phylogenetic analysis make timing of these trait origins to fire more certain. Resprouting from the base of stems is the most widely distributed fire-adaptive trait, and it is likely to have evolved under a diversity of disturbance types. The origins of other traits like serotiny, thick bark, fire-stimulated germination, and postfire flowering are more tightly linked to fire. Fire-adaptive traits occur in many environments: boreal and temperate forests, Mediterranean-type climate (MTC) shrublands, savannas, and other grasslands. MTC ecosystems are distinct in that many taxa in different regions have lost the resprouting ability and depend solely on postfire recruitment for postfire recovery. This obligate seeding mode is perhaps the most vulnerable fire-adaptive syndrome in the face of current global change, particularly in light of increasing anthropogenic fire frequency.

Annual Review of Ecology, Evolution, and Systemati↗
USGS2021

Wildfires and global change

No single factor produces wildfires; rather, they occur when fire thresholds (ignitions, fuels, and drought) are crossed. Anomalous weather events may lower these thresholds and thereby enhance the likelihood and spread of wildfires. Climate change increases the frequency with which some of these thresholds are crossed, extending the duration of the fire season and increasing the frequency of dry years. However, climate-related factors do not explain all of the complexity of global fire-regime changes, as altered ignition patterns (eg human behavior) and fuel structures (eg land-use changes, fire suppression, drought-induced dieback, fragmentation) are extremely important. When the thresholds are crossed, the size of a fire will largely depend on the duration of the fire weather and the extent of the available area with continuous fuels in the landscape.

southeast Australia↗
USGS2019

Wildfires as an ecosystem service

Wildfires are often viewed as destructive disturbances. We propose that when including both evolutionary and socioecological scales, most ecosystem fires can be understood as natural processes that provide a variety of benefits to humankind. Wildfires provide open habitats that enable the evolution of a diversity of shade-intolerant plants and animals that have long been used by humans. There are many provisioning, regulating, and cultural services that we obtain from wildfires; prescribed fires and wildfire management are tools for mimicking the ancestral role of wildfires in a highly populated world.

Frontiers in Ecology and the Environment↗
USGS2019

Distinguishing disturbance from perturbations in fire-prone ecosystems

Fire is a necessary ecosystem process in many biomes and is best viewed as a natural disturbance that is beneficial to ecosystem functioning. However, increasingly we are seeing human interference in fire regimes that alter the historical range of variability for most fire parameters and result in vegetation shifts. Such perturbations can affect all fire regime parameters. Here we provide a brief overview of examples where anthropogenically driven changes in fire frequency, fire pattern, fuels consumed and fire intensity constitute perturbations that greatly disrupt natural disturbance cycles. These changes are not due to fire per se but rather anthropogenic perturbations in the natural disturbance regime.

International Journal of Wildland Fire↗
USGS2017

Epicormic resprouting in fire-prone ecosystems

Many plants resprout from basal buds after disturbance, and this is common in shrublands subjected to high-intensity fires. However, resprouting after fire from epicormic (stem) buds is globally far less common. Unlike basal resprouting, post-fire epicormic resprouting is a key plant adaptation for retention of the arborescent skeleton after fire, allowing rapid recovery of the forest or woodland and leading to greater ecosystem resilience under recurrent high-intensity fires. Here we review the biogeography of epicormic resprouting, the mechanisms of protection, the fire regimes where it occurs, and the evolutionary drivers that shaped this trait. We propose that epicormic resprouting is adaptive in ecosystems with high fire frequency and relatively high productivity, at moderate–high fire intensities.

Trends in Plant Science↗
USGS2016

Flammability as an ecological and evolutionary driver

We live on a flammable planet yet there is little consensus on the origin and evolution of flammability in our flora. We argue that part of the problem lies in the concept of flammability, which should not be viewed as a single quantitative trait or metric. Rather, we propose that flammability has three major dimensions that are not necessarily correlated: ignitability, heat release and fire spread rate. These major axes of variation are controlled by different plant traits and have differing ecological impacts during fire. At the individual plant scale, these traits define three flammability strategies observed in fire-prone ecosystems: the non-flammable, the fast-flammable and the hot-flammable strategy (with low ignitability, high flame spread rate and high heat release, respectively). These strategies increase the survival or reproduction under recurrent fires, and thus, plants in fire-prone ecosystems benefit from acquiring one of them; they represent different (alternative) ways to live under recurrent fires. Synthesis . This novel framework based on different flammability strategies helps us to understand variability in flammability across scales, and provides a basis for further research.

Journal of Ecology↗
USGS2015

Towards understanding resprouting at the global scale

Understanding and predicting plant response to disturbance is of paramount importance in our changing world. Resprouting ability is often considered a simple qualitative trait and used in many ecological studies. Our aim is to show some of the complexities of resprouting while highlighting cautions that need be taken in using resprouting ability to predict vegetation responses across disturbance types and biomes. There are marked differences in resprouting depending on the disturbance type, and fire is often the most severe disturbance because it includes both defoliation and lethal temperatures. In the Mediterranean biome, there are differences in functional strategies to cope with water deficit between resprouters (dehydration avoiders) and nonresprouters (dehydration tolerators); however, there is little research to unambiguously extrapolate these results to other biomes. Furthermore, predictions of vegetation responses to changes in disturbance regimes require consideration not only of resprouting, but also other relevant traits (e.g. seeding, bark thickness) and the different correlations among traits observed in different biomes; models lacking these details would behave poorly at the global scale. Overall, the lessons learned from a given disturbance regime and biome (e.g. crown-fire Mediterranean ecosystems) can guide research in other ecosystems but should not be extrapolated at the global scale.

New Phytologist↗
USGS2014

Evolutionary ecology of resprouting and seeding in fire-prone ecosystems

There are two broad mechanisms by which plant populations persist under recurrent disturbances: resprouting from surviving tissues, and seedling recruitment. Species can have one of these mechanisms or both. However, a coherent framework explaining the differential evolutionary pressures driving these regeneration mechanisms is lacking. We propose a bottom-up approach in addressing this question that considers the relative survivorship of adults and juveniles in an evolutionary context, based on two assumptions. First, resprouting and seeding can be interpreted by analogy with annual versus perennial life histories; that is, if we consider disturbance cycles to be analogous to annual cycles, then resprouting species are analogous to the perennial life history with iteroparous reproduction, and obligate seeding species that survive disturbances solely through seed banks are analogous to the annual life history with semelparous reproduction. Secondly, changes in the selective regimes differentially modify the survival rates of adults and juveniles and thus the relative costs and benefits of resprouting versus seeding. Our approach provides a framework for understanding temporal and spatial variation in resprouting and seeding under crown-fire regimes. It accounts for patterns of coexistence and environmental changes that contribute to the evolution of seeding from resprouting ancestors.

New Phytologist↗
USGS2014

Abrupt climate-independent fire regime changes

Wildfires have played a determining role in distribution, composition and structure of many ecosystems worldwide and climatic changes are widely considered to be a major driver of future fire regime changes. However, forecasting future climatic change induced impacts on fire regimes will require a clearer understanding of other drivers of abrupt fire regime changes. Here, we focus on evidence from different environmental and temporal settings of fire regimes changes that are not directly attributed to climatic changes. We review key cases of these abrupt fire regime changes at different spatial and temporal scales, including those directly driven (i) by fauna, (ii) by invasive plant species, and (iii) by socio-economic and policy changes. All these drivers might generate non-linear effects of landscape changes in fuel structure; that is, they generate fuel changes that can cross thresholds of landscape continuity, and thus drastically change fire activity. Although climatic changes might contribute to some of these changes, there are also many instances that are not primarily linked to climatic shifts. Understanding the mechanism driving fire regime changes should contribute to our ability to better assess future fire regimes.

Ecosystems↗
USGS2011

Fire as an evolutionary pressure shaping plant traits

Traits, such as resprouting, serotiny and germination by heat and smoke, are adaptive in fire-prone environments. However, plants are not adapted to fire per se but to fire regimes. Species can be threatened when humans alter the regime, often by increasing or decreasing fire frequency. Fire-adaptive traits are potentially the result of different evolutionary pathways. Distinguishing between traits that are adaptations originating in response to fire or exaptations originating in response to other factors might not always be possible. However, fire has been a factor throughout the history of land-plant evolution and is not strictly a Neogene phenomenon. Mesozoic fossils show evidence of fire-adaptive traits and, in some lineages, these might have persisted to the present as fire adaptations.

Trends in Plant Science↗