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Serotiny

November 16th, 2013 No comments

Serotiny is the delayed seed release for more than a year by retaining the seeds in a woody structure [1]. This implies an accumulation of a canopy seed bank. Serotiny confer fitness benefits in environments with frequent crown-fires, as the heat opens the cones and seeds are dispersed in the post-fire bed which is rich in resource and the competition and predation are low. It is typical of many Proteaceae and some conifers, like some pine species [1, 2; figure below].

Two recent papers analyse the serotiny of two mediterranean pines Pinus halepensis and Pinus pinaster [3, 4]. P. halepensis show higher proportion of serotinous cones than P. pinaster, but the latter retain the cones for longer [3]. The two species show high variability of serotiny within and between populations, but they show a clear pattern of higher serotiny in populations subject to high frequency of crown-fires than those living in areas where crown-fires are rare or absent. This is true either considering serotiny as the proportion of serotinous cones or as the age of the cones stored. Compared with other pines worldwide, the strength of the fire-serotiny relationship in P. pinaster is intermediate, and in P. halepensis is among the highest known [3]. For P. halepensis (the species with higher % serotiny), populations in high fire recurrence regimes have higher fine-scale spatial aggregation of serotiny than those inhabiting low fire recurrence systems. This phenotypic spatial structure generated by fire could be a consequence of the spatial genetic structure of the population. The second study used genomic tools to search for a genetic association for serotiny [4]. The analysis of 384 SNPs of 199 individuals of P. pinaster (in 3 populations included in the previous study [3])  shows that 17 loci were associated with serotiny and explain all together ca. 29% of the serotiny variation found in the field. All these results adds further evidence to the emerging view that fire shapes intraspecific variability of traits and generates phenotypic divergence between populations [5, 6, 7].

Figure: Serotinous cones of Pinus pinaster (Foto: K.B. Budde)

References:

[1] 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]

[2] He T, Pausas JG, Belcher CM, Schwilk DW, Lamont BB. 2012. Fire-adapted traits of Pinus arose in the fiery Cretaceous. New Phytologist 194: 751-759. [doi | wiley | pdf (suppl.)]

[3] Hernández-Serrano A., Verdú M., González-Martínez S.C., Pausas J.G. 2013. Fire structures pine serotiny at different scales. American Journal of Botany 100: 2349-2356 [doi | amjbot | pdf | supp.]

[4] Budde, K. B., Heuertz, M., Hernández-Serrano, A., Pausas, J.G., Vendramin, G.G., Verdú, M. & González-Martínez, S.C. 2014. In situ genetic association for serotiny, a fire-related trait, in Mediterranean maritime pine (Pinus pinaster Aiton). New Phytologist  201: 230-241 [doi | pdf]

[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(8): 406-411. [doi] [trends] [pdf]

[6] Pausas, J. G., Schwilk, D. W. 2012. Fire and plant evolution. New Phytologist, 193:301-303. [doi | wiley | pdf]

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

 

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