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Seed dormancy release and summer temperatures

February 18th, 2024 No comments

In Mediterranean ecosystems, fire breaks the dormancy in many species and stimulates germination in the postfire environment [1]. Both the smoke and the heat of the fire may be responsible for breaking dormancy [1]. Cistaceae species are typical examples of species with this heat-released dormancy [1, 2], together with many legumes. Despite fire is a much more powerful driver of dormancy release than the summer heat (figure 9 in [1] and figure 2 in [2]), there are still people aiming to demonstrate the role of summer temperatures in dormancy release in Cistaceae. A recent research team studied the germination of 12 Cistacea species and compared the effect of fire-type heat in seeds submitted to a summer heat treatment (50/20o C for a month) and in seeds without this summer treatment [3]. They concluded that high summer temperatures are needed for maximum germination in the presence of fire [3]. A reanalysis of their data suggests that not only are summer temperatures inefficient at releasing dormancy, but they also reduce postfire germination [4]. The applied summer heat treatment reduced the germination (%) in the control (H-) and in all fire treatments, and for all species (Fig. 1 below). And for the seeds that germinate, those under summer heat tended to germinate slower than those that did not suffer the summer heat (Fig. 2 below). In conclusion, fire increases the germination of Cistaceae seeds in contrast to summer-type heat, i.e., the great fitness benefits of fire are unmatched by the summer heat [4].

Fig. 1. Germination with no summer heat (x-axis; ~ 20o C for a month) and with summer heat (y-axis, 50/20o C altering for 12 h, and during a month) in 12 Cistaceae species with different fire treatments. All fire heat treatments were 100o C for 10 min applied as follows: H-, no fire heat (black); HB, fire heat before summer (green); HA, fire heat treatment after summer (red); HBA, fire heat before and after the summer (unrealistic scenario; blue). The results suggest that applying a strong summer heat treatment (52/20o C for a month) reduces the germination (%) in the control (H-) and in all fire treatments and for all species. From [4].


Fig. 2. Number of days to the first germination (T0, left) and to reach 50% of the final germination (T50, right) with no summer heat (x-axis, control) and with summer heat (y-axis, treatment), and with different fire treatments (colors; see figure above) for 12 Cistacea species (12 points for each color). The results suggest that summer heat, if any, tends to increase the time to germination.

References

[1] Pausas J.G. & Lamont B.B. 2022. Fire-released seed dormancy – a global synthesis. Biological Reviews 97: 1612-1639.  [doi | pdf | supp. mat. | data (figshare)]

[2] Moreira B. & Pausas J.G. 2012. Tanned or burned: The role of fire in shaping physical seed dormancy. PLoS ONE 7(12): e51523. [doi | plos | pdf]

[3] Luna B, Piñas-Bonilla P, Zavala G, Pérez B. 2023. Timing of fire during summer determines seed germination in Mediterranean Cistaceae. Fire Ecology 19: 52.

[4] Lamont BB, Burrows GR, Pausas JG 2024. Fire-type heat increases the germination of Cistaceae seeds in contrast to summer heat. Fire Ecology 20:14 [doi | pdf]

More on seed dormancy: A review | a glossary | bet-hedging & best-bet | smoke-released dormancy 

Seed dormancy: a glossary

February 1st, 2023 No comments

We have recently reviewed concepts related to seed dormancy and the mechanism of dormancy release (see references 1, 2, 3 below). Here we summarize the main definitions considered.

Seed dormancy: delayed germination even when conditions are favorable. It is a state of metabolic inactivity in the seed that prevents the embryo from growing and thus the seed from germinating. There are two major classes of seed dormancy, inherent dormancy and imposed dormancy.

  • Inherent (or innate) dormancy: dormancy is an internal response through retarded embryo maturity or metabolic inactivity. This is often called just ‘dormancy’; it has also been called primary dormancy, but this name is not appropriate (see Secondary dormancy below). There are three basic types of inherent dormancy, depending on the mechanism of release: morphological, physical and physiological dormancy. Some seeds may have multiple mechanisms where they combine physiological and either morphological or physical dormancy.
    • Physical dormancy (PY): a type of inherent dormancy where the seed coat is impermeable to water and/or oxygen such that metabolism cannot occur and the seed cannot germinate even if hydrothermal conditions are suitable. Physical dormancy is typically released by heat, or by physical or chemical scarification: 
        • Heat-released dormancy: seeds require a heat pulse for breaking physical dormancy that exceeds soil temperatures experienced during summer and is comparable with fire heat.
        • Scarification-released dormancy: seeds require a physical or chemical scarification (different from heat) for breaking physical dormancy (e.g., scratching the surface of the seed coat). Scarification may be a convenient tool for breaking dormancy in horticulture, but its ecological role in the soil is not well known; it may be related to seed coat decays over time through temperature fluctuations or microbial processes. Scarification-released dormancy also occurs in species that do not form a seed bank: seeds of fleshy-fruited species are typically dormant, and scarification (chemical or mechanical) through the guts of frugivorous vertebrates releases their dormancy; in that case, dormancy is a strategy for long distance dispersal [2].
    • Physiological dormancy (PD): a type of inherent dormancy in which metabolic requirements have yet to be met and germination cannot proceed even if hydrothermal conditions are suitable. Some examples of physiological dormancy are:
        • Smoke-released dormancy: a type of physiological dormancy that is maintained until chemical byproducts in smoke or ash from the combustion of plant matter (collectively termed ‘smoke’) breaks dormancy by catalysing production of enzymes required for initiating metabolic activity and germination.
        • Inhibitor-released dormancy: a type of physiological dormancy where chemical inhibitors must be removed to allow germination. It has been observed in some seeds that germinate only when removed from the fruit, or in mistletoes, when the mucilage is removed (by frugivorous birds). [3].
        • Cold-released dormancy: a type of physiological dormancy that is maintained until the seed is exposed to periods of cold (e.g., ~5°C for two months) that promotes production of cofactors required for initiating metabolic activity [3].
        • Light/dark-released dormancy: a type of physiological dormancy that is maintained until the seed is exposed to periods light-dark that promotes production of specific cofactors required for initiating metabolic activity (photoperiod-controlled dormancy or photodormancy).
    • Morphological dormancy (MD): Dormancy is maintained in an underdeveloped embryo which requires a period of post-dispersal maturation (after-ripening) before the seed is ready to germinate. Morphological dormancy due to immature embryos is neither environmentally controlled nor metabolically inactive and might be better considered as post-release embryo maturation and only apparently dormant (pseudodormancy) [3].
  • Imposed dormancy: environmentally-imposed dormancy is the state where metabolic activity continues to be suppressed as external conditions remain unsuitable for germination. Some times it is called secondary dormancy but this term is inappropriate because it may be the only form of dormancy among many seeds, so it cannot be considered secondary in a temporal sense nor minor in a functional sense [3]. In species with heat-released dormancy, this state is maintained between the fire event and the first substantial postfire rains but may be minimal among smoke-responsive seeds if the chemicals are only absorbed once the seeds have imbibed. [1,3]

Dormancy syndrome: A correlated suite of traits that is coordinated to maintain seed dormancy during storage, execute seed dormancy release in response to a specified stimulus, and respond quickly to favorable germination conditions when they become available [1]. In fire-prone ecosystems, we defined four dormancy syndromes: Heat-released dormancy, Smoke-released dormancy, Non-fire-released dormancy, Non-dormancy [1]. Fire-released dormancy is a concise term for heat-released and smoke-released dormancy syndromes [1]

Heat-stimulated germination: Heat per se does not stimulate germination but breaks dormancy that allows germination to proceed later, i.e. once suitable hydrothermal conditions are met. Thus, this term refers to the heat-released dormancy syndrome [1].

Secondary dormancy: under some conditions seeds may return to a dormant state following the introduction of earlier or new inhibitory conditions that re-impose seed dormancy. Dormancy cycling may occur when seeds that have previously broken inherent or imposed dormancy return several times to that state (secondary inherent or imposed dormancy) following conditions that annul the current dormancy-release state.

Smoke-stimulated germination: In physiologically dormant seeds, specific smoke chemicals break dormancy and allow germination to proceed. These chemicals may be absorbed by dry seeds but, once the wet season begins, they are more likely to be absorbed dissolved in the soil solution during imbibition so that germination proceeds without further delay. Thus, this term is equivalent to the smoke-released dormancy syndrome [1]. Smoke chemicals may also hasten the rate of germination of non-dormant seeds among some species.

Dormancy-released pathways:  There are at least three ways by which seeds release dormancy [3]:

  • Pathway 1 (inherent/imposed dormancy release pathway): First inherent dormancy is broken, but for germination to proceed, imposed dormancy must also be broken at some later stage, that is, when suitable hydrothermal conditions prevail. E.g., the heat of a fire may break (inherent) physical dormancy, but seeds will not germinate until the first significant rainfall events (breaking environmental imposed dormancy).
  • Pathway 2 (imposed dormancy release pathway): seeds that lack inherent dormancy (non-dormant) may still encounter an environment that does not meet their germination requirements, so that they remain under imposed dormancy until the appropriate hydrothermal conditions are met.
  • Pathway 3 (imposed/inherent dormancy release pathway): first imposed dormancy is broken before inherent (physiological) dormancy release is possible. Some seeds must already be imbibed before the inherent physiological dormancy is released, e.g, before the seed is receptive to light/dark or to cold that breaks inherent dormancy (light/dark-dormancy release or cold-dormancy release).

Bet-hedging vs best-bet strategies: In unpredictable arid ecosystems, seed dormancy is a bet-hedging strategy, as it favours spreading the risk of recruitment failure over many years. In seasonal environments where fires are predictable, seed dormancy is a best-bet strategy as seed dormancy maximizes germination in a single year when conditions are optimal, following the first substantial rains after fire [2] (this best-bet strategy is also termed environmental matching [1]). Serotiny (seeds stored in the canopy seed bank with delayed seed release and dispersal [link]) is usually not considered within the concept of dormancy, but it certainly fits the best-bet strategy [2].

References

[1] Pausas JG. & Lamont BB. 2022. Fire-released seed dormancy – a global synthesis. Biol. Rev. 97: 1612-1639. [doi | pdf | supp. mat. | data (figshare)] (highlighted in plant.org)

[2] Pausas JG, Lamont BB, Keeley JE., Bond, WJ. 2022. Bet-hedging and best-bet strategies shape seed dormancy. New Phytol. 236: 1232-1236. [doi | wiley | pdf]

[3] Lamont BB & Pausas JG 2023. Seed dormancy revisited: dormancy-release pathways and environmental interactions. Funct. Ecol. [doi | pdf | data: dryad | plain language summary]

 

Evolutionary Ecology of Fire

November 4th, 2022 1 comment

Fire is an evolutionary pressure that shaped our biodiversity [1,2]. In a recent paper we summarized the current state of the art in this topic [3]. Fire has been an ecosystem process since plants colonized land over 400 million years ago [1]. Many diverse traits provide a fitness benefit following fires, and these adaptive traits vary with the fire regime [4]. 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 (figure below) 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 epicormic resprouting [5], lignotubers [6], serotiny [7], thick bark [8], fire-stimulated germination [9], 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 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 [10]. Overall, evolutionary fire ecology not only provides an understanding of the origin and history of our biota, it also sets the basis for the management of our ecosystems in a world undergoing fire-regime changes.

Time of origin (x-axis) of five different fire traits (different colors) for different lineages (y-axis) estimated from dated phylogenies. Bars expand the uncertainty of the time of origin (e.g., stem versus crown age). From [3]

References

[1] Pausas JG & Keeley JE 2009. A burning story: The role of fire in the history of life. BioScience 59: 593-601 [doiOUP | pdf | post]

[2] He T, Lamont NB, Pausas JG 2019. Fire as s key driver of Earth’s biodiversity. Biol. Rev. 94:1983-2010. [doi | pdf

[3] Keeley JE & Pausas JG 2022. Evolutionary ecology of fire. Ann. Rev. Ecol. Evol. Syst. 53: 203-225. [doi | pdf] <- New paper

[4] Keeley JE, Pausas JG, Rundel PW, Bond WJ, Bradstock RA 2011. Fire as an evolutionary pressure shaping plant traits. Trends Pl. Sci. 16: 406-411. [doi | sciencedirect | trends | pdf | For managers]

[5] Pausas J.G. & Keeley J.E. 2017. Epicormic resprouting in fire-prone ecosystems. Trends Pl. Sci. 22: 1008-1015. [doi | sciencedirect | pdf] [post | Cover image]

[6] Pausas JG, Lamont BB, Paula S, Appezzato-da-Glória B & Fidelis A 2018. Unearthing belowground bud banks in fire-prone ecosystems. New Phyt. 217: 1435–1448. [doi | pdf | suppl. | BBB database]

[7] Lamont BB, Pausas JG, He T, Witkowski, ETF, Hanley ME. 2020. Fire as a selective agent for both serotiny and nonserotiny over space and time. Crit. Rev. Pl. Sci. 39:140-172. [doi | pdf | suppl.]

[8] Pausas JG 2015. Bark thickness and fire regime. Funct. Ecol. 29:317-327. [doi | pdf | suppl.] & Pausas JG 2017. Bark thickness and fire regime: another twist. New Phytol. 213: 13-15. [doi | wiley | pdf]

[9] Pausas JG & Lamont BB 2022. Fire-released seed dormancy – a global synthesis. Biol. Rev. 97: 1612-1639. [doi | pdf | supp. mat.]

[10] Pausas JG & Keeley JE 2014. Evolutionary ecology of resprouting and seeding in fire-prone ecosystems. New Phyt. 204: 55-65. [doi | wiley | pdf]

Seed dormancy, bet-hedging, and best-bet

September 2nd, 2022 No comments

Seed dormancy is a key plant characteristic that occurs among many species worldwide. One mechanism that select for seed dormancy is the bet-hedging strategy. In unpredictable environment (i.e., with high interannual variability) there is a benefit in spreading the germination over a number of years to reduce year-to-year variation in fitness but taking advantage of exceptionally good years for establishment. In those environments, seed dormancy is adaptive; each year there is a small fraction of the seed crop that germinates and the other seeds remain dormant in the soil. Because the environmental conditions of most years are poor, successful establishment only occurs in good (wet) years. Thus bet-hedging selects for seed dormancy and it is a mechanism for living in highly unpredictable environments such as arid ecosystems [1]

There is another environmental setting that also selects for seed dormancy: seasonal (predictable) climate with a dry season during which the vegetation is highly flammable and thus wildfires are frequent (e.g., mediterranean, savanna, warm temperate, and dry boreal ecosystems). In those ecosystems, seed dormancy is adaptive and fire provide both a mechanism for dormancy release (proximate cause) and conditions (postfire) optimal for germination and establishment (low competition, high resource availability, low predation, low pathogen load) that increase fitness and allow maintenance of the population (ultimate cause) [1,2]. Dormant seeds survive the passage of fire and the heat or the chemicals from the combustion (collectively called ‘smoke’ [2,3]) are the stimulus for the seed to recognize a fire gap to germinate. That is, postfire recruitment occurs in a single pulse after fire. Here selection does not favor spreading the risk of recruitment failure over many years (as in the bet-hedging strategy) but, instead, maximizes germination in a single year when conditions are optimal, after fire. We call this strategy the best-bet strategy [1] or environmental matching [2]. This strategy selects for seed dormancy to accumulates seeds in the soil seedbank but also selects for serotiny to accumulate seeds in the canopy seedbank [4]; in both cases, species recruit mostly after fire and not during the interfire period.

There is a further driver that selects for seed dormancy but it does not imply the formation of seed banks (in contrast with bet-hedging and best-bet). Many seeds have acquired seed dormancy to facilitate long-distance dispersal. The clearest example is dispersal by vertebrate frugivores (endozoochory). Frugivores consume the fruit pulp and defaecate or regurgitate the seeds far from the mother plant. This means that seeds need to resist passage through the gut and remain intact until arriving at a new microsite for germination. Thus, seeds of fleshy fruited species typically are dormant, and scarification through the gut releases their dormancy. While bet-hedging spreads germination of seeds over time, this strategy spread the seeds across the space and thus it could be viewed as a spatial bet-hedging strategy.

Figure: Schematic representation of the dynamics of seed recruitment for plants lacking seed dormancy (nondormant; top panel), and for plants with dormant seeds following the bet-hedging strategy (middle panel) and the best-bet strategy (bottom panel). The figure shows the moment of flowering (red asterisk; spring), the germination (black bars; autumn), the seed bank in autumn (empty bars), the recruitment 2 months later (green bars) and the fire (flame; summer). As an example, the seasons are considered as in the Northern Hemisphere, and vertical dotted lines are the end of the year. From [1]
Table: Main characteristics of the evolutionary strategies that select for seed dormancy and seed banks (bet-hedging, best-bet), together with the nondormant strategy.

References

[1] Pausas JG, Lamont BB, Keeley JE., Bond WJ. 2022. Bet-hedging and best-bet strategies shape seed dormancy. New Phytol. [ doi | wiley | pdf]

[2] Pausas JG. & Lamont BB. 2022. Fire-released seed dormancy – a global synthesis. Biol. Rev. 97: 1612-1639. [doi | pdf | supp. mat. | data (figshare)

[3] Keeley JE & Pausas JG. 2018. Evolution of ‘smoke’ induced seed germination in pyroendemic plants. South African J. Bot. 115: 251-255. [doi | pdf]  

[4] Lamont BB, Pausas JG, He T, Witkowski, ETF, Hanley ME. 2020. Fire as a selective agent for both serotiny and nonserotiny over space and time. Critical Rev. Pl. Sci. 39:140-172. [doi | pdf | suppl.]

Fire-released seed dormancy

April 8th, 2022 No comments

Many plants concentrate their seedling recruitment after the passage of a fire. This is because postfire conditions are especially optimal for germination and establishment of many species as fires create extensive vegetation gaps that have high resource availability, minimal competition, and low pathogen load. Thus we propose that fireprone ecosystems create ideal conditions for the selection of seed dormancy as fire provides a mechanism for dormancy release and optimal conditions for germination [1]. We compiled data from a wide range of fire-related germination experiments for species in different ecosystems across the globe and identified four dormancy syndromes: heat-released (physical) dormancy, smoke-released (physiological) dormancy, non-fire-released dormancy, and non-dormancy. In fireprone ecosystems, fire, in the form of heat and/or chemical by-products (collectively termed ‘smoke’), are the predominant stimuli for dormancy release and subsequent germination, with climate (cold or warm stratification) and light sometimes playing important secondary roles. Fire (heat or smoke)-released dormancy is best expressed where woody vegetation is dense and fires are intense, i.e. in crown-fire ecosystems (e.g., mediterranean-type ecosystems). In grassy fireprone ecosystems (e.g. savannas), where fires are less intense but more frequent, seed dormancy is less common and dormancy release is often not directly related to fire (non-fire-released dormancy). Fire-released dormancy is rare to absent in arid ecosystems and rainforests. Heat-released dormancy can be traced back to fireprone floras in the ‘fiery’ mid-Cretaceous, followed by smoke-released dormancy, with loss of fire-related dormancy among recent events associated with the advent of open savannas and non-fireprone habitats. Anthropogenic influences are now modifying dormancy-release mechanisms, usually decreasing the role of fire. We conclude that contrasting fire regimes are a key driver of the evolution and maintenance of diverse seed dormancy types in many of the world’s natural ecosystems.

Fig. 1. Percentage germination of 68 populations or species subjected to simulated fire- (y axis) and summer-type (warm stratification) temperature (x-axis) (C., Cistus; F., Fumana; U., Ulex; A., Acacia; M., Mimosa). Points above the dotted line (1:1) have higher germination levels after fire heat than after summer heat. Note that all points at or below the line are for species in savannas [S], while the others are from mediterranean shrublands and other crown-fire ecosystems. That is, in crown-fire ecosystems, fire is the most likely selective agent for dormancy. From [1].
Fig. 2. Dated phylogeny for major clades in the New and Old World Cistaceae together with closely related ancestral clades. Pie charts at the tips show the fraction of species that occur in crown-fire ecosystems (red), surface-fire ecosystems (orange), those with physical dormancy – hard seeds (green), and those with heat-released dormancy (blue). Blank sectors mean that the trait is absent. Letters at the tips refer to growth forms in the clade (T, tree; S, shrub or subshrub; H, herb/annual). Black dots indicate the crown age of diversification of the corresponding clade. From [1].

References

[1] Pausas J.G. & Lamont B.B. 2022. Fire-released seed dormancy – a global synthesis. Biological Reviews  [doi | pdf | supp. mat. | data (figshare)]

Fire adaptations in Mediterranean Basin plants

September 7th, 2015 No comments

Few days ago a botanist colleague ask me whether there were some fire adaptations in the plants of the Mediterranean Basin, similar to those reported in other mediterraenan-climate regions. So I realised that researchers working on other topics may not be aware of the recent advances in this area. Here is my brief answer, i.e., some examples of species growing in Spain that show fire adaptations; this is by no means an exhaustive list, but a few examples of common species for illustrative purpose. You can find a description of these adaptations and further examples elsewhere [1, 2, 3, 4]. It is also important to note that plants are not adapted to fire per se, but to specific fire regimes, and thus some adaptations my provide persistence to some fire regimes but not to all [1]. That is, species that exhibit traits that are adaptive under a particular fire regime can be threatened when that regime changes.

  • Serotiny (canopy seed storage): Pinus halepensis, Pinus pinaster, with variability in serotiny driven by different fire regimes [5, 6]
  • Fire-stimulated germination: There are examples of heat-stimulated germination, like many Cistaceae (e.g., Cistus, Fumana [7, 8]) and many Fabaceae (e.g., Ulex parviflorus, Anthyllis cytisoides [7, 8]), as well as examples of smoke-stimulated germination like many Lamiaceae (e.g., Rosmarinus officinalis, Lavandula latifolia [7]) or Coris monspeliensis (Primulaceae [7]). There are also examples of species with smoke-stimulated seedling growth (Lavandula latifolia [7])
  • Resprouting from lignotubers: Arbutus unedo, Phillyrea angustifolia, Juniperus oxycedrus, many Erica species (e.g., E. multiflora, E. arborea, E. scoparia, E. australis) [4, 17]
  • Epicormic resprouting: Quercus suber [9, 10], Pinus canariensis [4]
  • Fire-stimulated flowering: Some monocots like species of Asphodelus, Iris, Narcissus [11, 12]
  • Enhanced flammability: Ulex parviflorus shows variability of flammability driven by different fire regimes [13] and under genetic control [14]. Many Lamiaceae species have volatile organic compounds that enhance flammability (e.g., Rosmarinus officinalis [16]).
  • Thick bark and self-pruning (in understory fires): Pinus nigra [3,15]

 

fireadaptations2

References

[1] Keeley et al. 2011. Fire as an evolutionary pressure shaping plant traits. Trends Plant Sci 16:406-411. [doi | pdf]

[2] Keeley et al. 2012. Fire in Mediterranean Ecosystems. Cambridge University Press. [book]

[3] Pausas JG. 2012. Incendios forestales. Catarata-CSIC. [book]

[4] Paula et al. 2009. Fire-related traits for plant species of the Mediterranean Basin. Ecology 90:1420-1420. [doi | pdf | BROT database]

[5] Hernández-Serrano et al. 2013. Fire structures pine serotiny at different scales. Am J Bot 100:2349-2356. [doi | pdf]

[6] Hernández-Serrano et al. 2014. Heritability and quantitative genetic divergence of serotiny, a fire persistence plant trait. Ann Bot 114:571-577. [doi | pdf]

[7] Moreira et al. 2010. Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora. Ann Bot 105:627-635. [doi | pdf]

[8] Moreira B and Pausas JG. 2012. Tanned or Burned: the role of fire in shaping physical seed dormancy. PLoS ONE 7:e51523. [doi | plos | pdf]

[9] Pausas JG. 1997. Resprouting of Quercus suber in NE Spain after fire. J Veget Sci 8:703-706. [doi | pdf]

[10] Catry et al. 2012. Cork oak vulnerability to fire: the role of bark harvesting, tree characteristics and abiotic factors. PLoS ONE 7:e39810. [doi | pdf ]

[11] Postfire flowering: Narcissusjgpausas.blogs.uv.es 2 May 2015

[12] Postfire blooming of Asphodelous, jgpausas.blogs.uv.es 5 Apr 2014

[13] Pausas et al. 2012. Fires enhance flammability in Ulex parviflorus. New Phytol 193:18-23. [doi | pdf]

[14] Moreira et al. 2014. Genetic component of flammability variation in a Mediterranean shrub. Mol Ecol 23:1213-1223. [doi | pdf]

[15] He et al. 2012. Fire-adapted traits of Pinus arose in the fiery Cretaceous. New Phytol 194:751-759. [doi | pdf | picture]

[16] Flammable organic compounds: Rosmarinus officinalis, jgpausas.blogs.uv.es 2-Oct-2015

[17] Paula et al. 2016. Lignotubers in Mediterranean basin plants. Plant Ecology [doi | pdf | suppl. | blog]

 

Ecology and evolution in fire-prone ecosystems

February 28th, 2015 2 comments

During the last years I’ve been working in many topics related to fire ecology and plant evolution in ecosystems subject to recurrent fires (mainly mediterranean and savanna ecosystems). Because I believe knowledge should be spread around easily, I make my results available to the public in my web page (see publications list) and in this blog. However, having the cumulative list of paper published each year is not very convenient for people searching for a specific topic. For this reason, I’m rearranging most of my articles by topics as follows:

1. Fire history
2. Fire regime: climate & fuel
3. Fire traits (resprouting, postfire germination, serotiny, bark thickness, flammability, data & methods)
4. Fire & plant strategies (in Mediterranean ecosystems, in pines, in savannas, community assembly)
5. Fire & evolution
6. Some fire-adapted species (Pinus halepensis, Quercus suber, Ulex parviflorus)
7. Fire & vegetation modelling
8. Plant-animal interactions
9. Restoration & conservation

See: fire-ecology-evolution.html

Some papers may be repeated if they clearly fit in more than one topic; some papers, mainly old ones, do not fit well in any of these topics and have not been included (at least at the moment), they still can be found in the section of publications sorted by year. I’m still working on this rearrangement, so some modifications are possible; and any comment is welcome.
I hope this is useful for somebody!

Publications: by year | by topic | books

 

Evolutionary ecology of resprouting and seeding

July 15th, 2014 No comments

There are two broad mechanisms by which plant populations persist under recurrent fires: resprouting from surviving tissues, and seedling recruitment [1]. Species that live in fire-prone ecosystems can have one of these mechanisms or both [1]. In a recent review paper [2], we propose a model suggesting that changes in evolutionary pressures that modify adult (P) and juvenile (C) survival in postfire conditions (Fig. 1 below) determine the long-term success of each of the two regeneration mechanisms, and thus the postfire regeneration strategy: obligate resprouters, facultative species and obligate seeders (Fig. 2). Specifically we propose the following three hypotheses: 1) resprouting appeared early in plant evolution as a response to disturbance, and fire was an important driver in many lineages; 2) postfire seeding evolved under conditions where fires were predictable within the life span of the dominant plants and created conditions unfavorable for resprouting; and 3) the intensification of conditions favoring juvenile survival (C) and adult mortality (P) drove the loss of resprouting ability with the consequence of obligate-seeding species becoming entirely dependent on fire to complete their life cycle, with one generation per fire interval (monopyric life cyle). This 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. In this review, we also provide definitions and details of the main concepts used in evolutionary fire ecology: postfire regeneration traits, postfire strategies, life cycle in relation to fire, fire regimes (Box 1), costs of resprouting (Box 2), postfire seeding mechanisms (Box 3), and the possible evolutionary transitions (Box 4).

 

Fig2_sm
Fig. 1 : Main factors affecting adult and offspring seedling survival (P and C, respectively), and thus the P/C ratio, in fire-prone ecosystems (from Pausas & Keeley 2014 [2]).

 

Fig3_sm

Fig. 2: The changes in the probability of resprouting along an adult-to-offspring survival (P/C) gradient are not linear but show two turning points related to the acquisition of key innovations: the capacity to store a fire-resistant seed bank (postfire seeding), and the loss of resprouting capacity. Changes in P/C ratio may be produced by different drivers (Fig. 1) which drove the rise of innovations during evolution, e.g., during the increasing aridity from the Tertiary to the Quaternary (from Pausas & Keeley 2014 [2]).

 

Refecences

[1] Pausas, J.G., Bradstock, R.A., Keith, D.A., Keeley, J.E. 2004. Plant functional traits in relation to fire in crown-fire ecosystems. Ecology 85: 1085-1100. [doi | pdf | esa | jstor]

[2] Pausas J.G. & Keeley J.E. 2014. Evolutionary ecology of resprouting and seeding in fire-prone ecosystems. New Phytologist 204: 55-65 [doi | wiley | pdf]

 

Ulex born to burn (II): genetic basis of plant flammability

January 25th, 2014 No comments

In an previous study we found that Ulex parviflorus (Fabaceae) populations that inhabit in recurrently burn areas (HiFi populations) were more flammable than populations of this species growing in old-fields where the recruitment was independent of fire (NoFi populations) [1,2, 3]. That is, HiFi plants ignited quicker, burn slower, released more heat and had higher bulk density than NoFi plants. Thus, it appeared that repeated fires selected for individuals with higher flammability, and thus driving trait divergence among populations living in different fire regimes. These results were based on the study of plant flammability (phenotypic variability) without knowing whether plant flammability was genetically controlled. In a recent study using the same individuals [4], we show that phenotypic variability in flammability was correlated to genetic variability (estimated using AFLP loci) [figure below]. This result provide the first field evidence supporting that traits enhancing plant flammability have a genetic component and thus can be responding to natural selection driven by fire [5]. These results highlight the importance of flammability as an adaptive trait in fire-prone ecosystems.

Ulex-flam-AFLP

Figure: Relationship between flammability and genotypic variability at individual level in Ulex parviflorus (red symbols: individuals in HiFi populations; green symbols: individuals in NoFi populations). Variations in flammability are described using the first axis of a Principal Component Analysis (PCA1) performed from different flammability traits, and genetic variability is described using the first axis of a Principal Coordinate Analysis (PCo1) from the set of AFPL loci that were significantly related to flammability. See details in [4].

References
[1] Ulex born to burn, jgpausas.blogs.uv.es, 9/Nov/2011

[2] Pausas J.G., Alessio G., Moreira B., Corcobado G. 2012. Fires enhance flammability in Ulex parviflorusNew Phytologist 193:18-23 [doi | wiley | pdf]

[3] Pausas J.G. & Moreira B. 2012. Flammability as a biological concept. New Phytologist 194: 610-613.  [doi | wiley | pdf]

[4] Moreira B., Castellanos M.C., Pausas J.G. 2014. Genetic component of flammability variation in a Mediterranean shrub. Molecular Ecology 23: 1213-1223 [doi | pdf | data:dryad]

[5] Keeley J.E., Pausas J.G., Rundel P.W., Bond W.J., Bradstock R.A. 2011. Fire as an evolutionary pressure shaping plant traits. Trends in Plant Science 16: 406-411. [doi | trends | pdf]

 

Physiological differences between resprouters and seeders

November 9th, 2013 No comments

The ability to resprout and to recruit after fire are two extremely important traits for the persistence in fire-prone ecosystems [1,2], and they define three life histories: obligate resprouters, obligate seeders (non-resprouters), and facultative seeders. After a fire, obligate seeders die and recruit profusely from the seeds stored in the seed bank [3-5]. In contrast, resprouters survive after fire and their above-ground tissues regenerate from protected (often below-ground) buds by using stored carbohydrates [6]. Facultative seeders not only recruit profusely after fire, but are also able to resprout. In fact, seeders and resprouters have different regeneration niches: seedling regeneration of obligate resprouters is not linked to fire, and they recruit during the inter-fire period under sheltered conditions (i.e., under vegetation cover), while seedling regeneration of seeders occurs in open postfire environments. Given the marked difference in water availability between microsites under vegetation and microsites open to the sun under Mediterranean conditions, seedlings of resprouters and seeders are subjected to different water-stress conditions, and thus they are expected to have different physiological attributes. Despite these differences, resprouters and seeders co-exist, are often well-mixed on local and landscape scales [7,8], and represent the two main types of post-fire regeneration strategies in Mediterranean ecosystems [2].

A recent study demonstrates marked differences in physiological attributes between seedlings of seeders and resprouters [9]: Seeders show a range of physiological traits that better deal with water-limited and highly variable conditions (e.g., higher resistance to xylem cavitation, earlier stomatal closure with drought, higher leaf dehydration tolerance), but they are also capable of taking full advantage of periods with high water availability (greater efficiency in conducting water through the xylem to to sustain high gas exchange rates when water is available). Conversely, resprouter species are adapted to more stable water availability conditions, favoured by their deeper root system, but they also display traits that help them resist water shortages during long summers.

Previous studies already showed marked differences between seeders and resprouters in a range of attributes: resprouters tend to exhibit a deeper root-system, while seedling root structure of seeders are more efficient in exploring the upper soil layer [10]. Leaves of seeders show higher water use efficiency (WUE) and higher leaf mass per area (LMA; i.e., higher sclerophylly, lower SLA) [11]. Seeds of seeder species are more tolerant to heat shocks and have greater heat-stimulated germination [3]. All these differences support the idea that they are distinct syndromes with different functioning characteristics at the whole plant level and suggest that they undertook different evolutionary pathways [12].

Figure: Coexistence of resprouters (R+) and seeders (R-) in postfire conditions near Valencia, Spain. (Foto: A. Vilagrosa).

 

References:

[1] Pausas, J.G., Bradstock, R.A., Keith, D.A., Keeley, J.E. & GCTE Fire Network. 2004. Plant functional traits in relation to fire in crown-fire ecosystems. Ecology 85: 1085-1100. [jstor |[pdf | Ecological Archives E085-029]

[2] Keeley J.E., Bond W.J., Bradstock R.A., Pausas J.G. & Rundel P.W. 2012. Fire in Mediterranean Ecosystems: Ecology, Evolution and Management. Cambridge University Press. [The book]

[3] Paula S. & Pausas J.G. 2008. Burning seeds: Germinative response to heat treatments in relation to resprouting ability. Journal of Ecology 96 (3): 543 – 552. [doi | pdf]

[4] Moreira B., Tormo J., Estrelles E., Pausas J.G. 2010. Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora. Annals of Botany 105: 627-635. [doi | pdf | blog]

[5] Moreira B. & Pausas J.G. 2012. Tanned or burned: The role of fire in shaping physical seed dormancy. PLoS ONE 7(12): e51523. [doi | plos | pdf | blog]

[6] Moreira B., Tormo J, Pausas J.G. 2012. To resprout or not to resprout: factors driving intraspecific variability in resprouting. Oikos 121: 1577-1584. [doi | pdf]

[7] Verdú M, & Pausas JG 2007. Fire drives phylogenetic clustering in Mediterranean Basin woody plant communities Journal of Ecology 95 (6), 1316-323 [doi | pdf]

[8] Ojeda, F., Pausas, J.G., Verdú, M. 2010. Soil shapes community structure through fire. Oecologia 163:729-735. [doi | pdf | blog]

[9] Vilagrosa A., Hernández E.I., Luis V.C., Cochard H., Pausas, J.G. 2014. Physiological differences explain the co-existence of different regeneration strategies in Mediterranean ecosystems. New Phytologist 201 : xx-xx [doi | pdf | suppl.] – NEW

[10] Paula S. & Pausas J.G. 2011. Root traits explain different foraging strategies between resprouting life histories. Oecologia 165:321-331. [doi | pdf | blog]

[11] Paula S. & Pausas J.G. 2006. Leaf traits and resprouting ability in the Mediterranean basin. Functional Ecology 20: 941-947. [doi | pdf | blog]

[12] Verdú M. & Pausas J.G. 2013. Syndrome-driven diversification in a Mediterranean ecosystem. Evolution 67: 1756-1766. [doi | pdf | blog]