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Posts Tagged ‘communities’

Alternative fire-driven vegetation states

November 1st, 2014 No comments

One of the clearest pieces of evidence for the role of fire in shaping vegetation is the occurrence of alternative vegetation types maintained by different fire regimes in a given climate. The different flammability of alternative communities generates different fire feedback processes that maintain contrasted vegetation types with clear boundaries in a given environment; and fire exclusion blurs this structure. This has been well documented in tropical landscapes (e.g., [1]) that are often mosaics of two alternative stable states – savannas and forests – with distinct structures and functions and sharp boundaries. Currently, there is an increasing evidence that alternative fire-driven vegetation states do occur in other environments, including temperate forests ([2, 3] and figure below). That is, the existence of alternative fire-driven vegetation states may be more frequent than previously thought, although human activities may favour one of the states and mask the original bistability.

modelv2

Figure: Factors determining the transition between two alternative vegetation states (fire sensitive forest and fire resilient shrubland) in a temperate landscape in Patagonia. Human factors (global warming, increased ignitions, and livestock grazing) favour transition to shrublands. From [2].

References
[1] Dantas V., Batalha MA & Pausas JG. 2013. Fire drives functional thresholds on the savanna-forest transition. Ecology 94:2454-2463.  [doi | pdf | appendix]

[2] Pausas, J.G. 2015. Alternative fire-driven vegetation states. Journal of Vegetation Science 26: 4-6 [doi | pdf | suppl.]

[3] Paritsis J., Veblen T.T. & Holz A. 2014. Positive fire feedbacks contribute to shifts from Nothofagus pumilio forests to fire-prone shrublands in Patagonia. J. Veget. Sci., 26.

 

Smoke-stimulated recruitment

September 16th, 2013 No comments

In many plant species from mediterranean ecosystems, germination is promoted by fire [1]; this effect may be driven by the heat [e.g., 2-4] or by the chemicals produced by the fire (e.g., smoke, 4,5]). Most information regarding to smoke-stimulated germination in the Mediterranean Basin comes from a few experiments performed in laboratory conditions. This approach does not consider factors that occur in the field, such as species interactions, density-dependent processes or the fact that seeds spent time in the soil. A recent field experiment performed in eastern Spain show that smoke increase overall seedling recruitment, specially seedlings of annual plant species [6]. However, despite most species had higher seedling establishment in the smoke than in the control subplots, there were very few species in which the effect of smoke was statistically significant, suggesting that the community response to smoke cannot be inferred from individual species; it is the sum of small differences in each species towards the same direction that produces a significant pattern at community scale. This emerging property of the community is often neglected by only considering germination experiments in the laboratory. The results also suggest that the effect of smoke in annual species of the Mediterranean Basin might be more relevant than previously thought.

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] 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. [pdf | doi]

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

[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. [pdf | doi | post]

[5] Smoke-stimulated germination, jgpausas.blogs.uv.es, 2/Dec/2011.

[6] Tormo, J., B. Moreira, and J. G. Pausas. 2014. Field evidence of smoke-stimulated seedling emergence and establishment in Mediterranean Basin flora. Journal of Vegetation Science 25: 771-777 [doi | wiley | pdf]

Fire shapes savanna-forest mosaics in the Brazilian cerrado

May 14th, 2013 No comments

Cerrado is the name of a tropical fire-prone mosaic of savanna and forest in Brazil. In a recent paper [1], we showed that in cerrado landscapes, despite the existence of a great variety of community structure (from open savannas to closed forests; Figure below), there are two well-defined stable states of community function, each associated with contrasting levels of community closure (open and closed environments) and maintained by different fire regimes. Soil properties, phylogenetic and non-phylogenetic beta-diversities, and most of the plant functional traits presented a threshold pattern along the community closure gradient with coinciding breakpoints, providing strong evidence of a functional threshold along the forest-savanna gradient. Open environments consisted of communities growing on poor soil and dominated by short species with early investments in thick barks, low wood density and with thick and tough leaves (high toughness and low specific area). In contrast, closed communities grow in more fertile soils and include plants having the opposite functional attributes. Moreover, we found contrasting fire regimes on the two sides of the threshold, with open formations showing shorter fire intervals than closed formations and a switch from communities dominated by fire-resistant plants to communities dominated by shade tolerant species that compensate for their lack of fire resistance by efficiently closing the canopy (i.e., reducing flammability). Overall, these results are consistent with the theoretical model of fire-plant feedbacks as main drivers of the coexistence of two stable states, savanna and forest. In this context, we provide the first field-based evidence for a community-level threshold separating two vegetation states with distinct functional and phylogenetic characteristics and associated with different fire regimes.

Top: A woodland cerrado (cerrado sensu stricto) six months after a fire, with several top-killed trees and a developed layer of resprouting vegetation; and one of the sampled closed forests.
Middle: A dense woodland cerrado (cerrado denso); one example of a typical thick-barked species found in open communities (Anadenanthera peregrina (Benth.) Reis, Fabaceae); a transitional zone between dense savannas and forests.
Bottom: A typical open savanna at the early rainy season, with tall flammable grasses and small trees and shrubs.
Photo credits: V. Dantas, G. Sartori, V. Cadry, J.G. Pausas, F. Noronha, A. Favari. See [1].

References

[1] Dantas V., Batalha, MA & Pausas JG. 2013. Fire drives functional thresholds on the savanna-forest transition Ecology 94: 2454-2463. [doi | pdf]

 

Methods for community ecology

September 2nd, 2010 No comments

Why so many desert plant communities are dominated by spiny species, most of them cacti? The first observation refers to the community’s phenotypic structure and the second to its phylogenetic structure. What do these observations tell us about the mechanisms assembling these communities?

The way communities are assembled is an old ecological question currently experiencing renewed interest thanks to the recent advances in molecular biology and phylogenetics. The generality of these new methods has allowed us to understand the structure of communities of organisms from different kingdoms and at different scales. Concomitant with this growing interest, new methods, metrics, terms, and software have appeared that independently solve similar questions, but with different approaches. In this new paper we provide a unifying framework on methods for community structure based on the relationships between four key concepts: phylogeny, phenotype, environment, and co-occurrence. The different approaches are based on different community representations of traits, the phylogenetic relationships of species in the community, or species occurrence along the environmental gradients. We finally provide insights on future directions of this emerging discipline.

Pausas, J.G., Verdú, M. 2010. The jungle of methods for evaluating phenotypic and phylogenetic structure of communities. BioScience, 60: 614-625. [doi | pdf | slides]

Pausas-Verdu-BioScience

Figure 1. Methods for analyzing community structure can be represented in a simple framework in which the relationships (arrows) between the four key concepts (phylogeny, phenotype, environment, and co-occurrence) are integrated. The numbers in brackets refer to:

(1) Co-occurrence pattern versus the random expectation

(2) Phenotype-based approach:

(2.1) Relationship between the species' phenotypes and their co-occurrence (phenotypic community structure).

(2.2) Relationship between species response to the environment and the species phenotypes, controlling by the species' phylogenetic relatedness (phenotypic community structure).

(3) Phylogeny-based approach:

(3.1) Relationship between the species' phylogenetic relationships and their co-occurrence (phylogenetic community structure).

(3.2) Relationship between the species' phenotypes and their phylogenetic relationships (trait evolution).

(4) Environment-based approach: relationship between species response to the environment and the co-occurrence, considering the phylogenetic relatedness (phylogenetic community structure).

See previous post [link] on the effect of fire in phenotypic and phylogenetic structure of communities.

Fire increases species relatedness in plant communities

March 2nd, 2010 No comments

Mediterranean communities living under high fire recurrence are composed by plant species that are more closely related than what would be expected from the regional species pool (i.e., phylogenetic clustering; Verdú & Pausas 2007). This is because high fire recurrence favors seeder species, and the traits that confer the seeder character (e.g., heat and smoke stimulated germination, Moreira et al. 2010) are evolutionary conserved, that is, closely related species tend to be similar (Pausas & Verdú 2008). In fact, the abundance of seeders species is negatively related to phylogenetic diversity (Coca & Pausas 2009; see Figure 2 below).

The Figure 1 below shows the Net Relatedness Index (NRI, i.e, standardized form of the community mean phylogenetic distance) of woody species coexisting in for communities in contrasted crown-fire regimes (LowFire vs HighFire) at different spatial scales (regional and local). Note that high net relatedness = low mean phylogenetic distance. At regional scale, “LowFire” corresponds to mountain communities living in zones that rarely burnt, and “HighFire” are warm and dry coastal communities subject to a high frequency of crown fires. At local scale (under the same climate), “LowFire” corresponds to communities growing in fertile soils while “HighFire” are communities growing on poor soils where flammability is higher. When comparing from community null models, HighFire communities show higher NRI than expected by chance (phylogenetic clustering), which indicates the importance of habitat filtering in shaping fire-prone communities (Verdú & Pausas 2007, Ojeda et al. 2010).

Fig9.7_phylostruct-NRI

Fig. 1. Elaborated from Verdú & Pausas (2007) and Ojeda et al. (2010).

PD-seeders_Coca-Pausas Fig. 2. From Coca & Pausas (2009).

References

  • Coca M. & Pausas J.G. 2009. Regeneration traits are structuring phylogenetic diversity in cork oak (Quercus suber) woodlands. J. Veget. Sci. 20: 1009-1015. [doi | pdf | post]
  • Moreira B., Tormo J., Estrelles E., Pausas J.G. 2010. Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora. Ann. Bot. 105:627-635 [doi | pdf | post]
  • Ojeda, F., Pausas, J.G., Verdú, M. 2010. Soil shapes community structure through fire. Oecologia 163:729-735. [doi | pdf | post]
  • Pausas J.G. & Verdú M. 2008. Fire reduces morphospace occupation in plant communities. Ecology 89: 2181-2186. [doi | pdf]
  • Verdú M. & Pausas J.G. 2007. Fire drives phylogenetic clustering in Mediterranean Basin woody plant communities J. Ecol. 95: 1316-323. [doi | pdf]

Soil shapes community structure through fire

January 21st, 2010 No comments

Recurrent wildfires constitute a major selecting force in shaping the structure of plant communities. At the regional scale, fire favours phenotypic and phylogenetic clustering in Mediterranean woody plant communities. Nevertheless, the incidence of fire within a fire-prone region may present strong variations at the local, landscape scale. This study tests the prediction that woody communities on acid, nutrient-poor soils should exhibit more pronounced phenotypic and phylogenetic clustering patterns than woody communities on fertile soils, as a consequence of their higher flammability and, hence, presumably higher propensity to recurrent fire. Results confirm the predictions and show that habitat filtering driven by fire may be detected even in local communities from an already fire-filtered regional flora. They also provide a new perspective from which to consider a preponderant role of fire as a key evolutionary force in acid, infertile Mediterranean heathlands.

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

IMG_0270_Ojeda en su brezal_sm The first author in the flammable, low fertility community.

New paper: Regeneration traits and phylodiversity

October 29th, 2009 No comments

Coca M. & Pausas J.G. 2009. Regeneration traits are structuring phylogenetic diversity in cork oak (Quercus suber) woodlands. J. Veget. Sci. 20: 1009-1015  [Wiley] [doi] [pdf]

  • Question: What factors determine the deviations from the relationship between species richness (which considers species as independent entities) and phylogenetic diversity (PD) (which considers species relatedness)? What are the implications for community composition and phylogenetic structure?
  • Location: Los Alcornocales Natural Park, in southern Iberian Peninsula (Spain).
  • Methods: We recorded all woody species and geographical features on 94 (20 m × 20 m) plots of cork oak woodlands. Disturbance information was obtained from the Park records; precipitation was estimated from local maps. PD was computed as the minimum total length of all the phylogenetic branches spanning the set of species on each site. Then, PD was regressed against species richness to test to what extent the unexplained variance in this relationship could be accounted for by environmental variables and disturbances, and against the representation of species with different regeneration strategies.
  • Results: Species richness and PD are strongly related; however, the remaining variability can be explained by: (1) precipitation and disturbance, and (2) the proportion of seeder species. Thus, the PD both of areas with low precipitation and high disturbance, and of areas with a high representation of seeder species, is lower than what would be expected from the species richness.
  • Conclusions: Regeneration traits are important in structuring plant community composition; specifically, they contribute to shaping biodiversity in Mediterranean ecosystems. Species richness tends to overestimate biodiversity in highly disturbed systems.
Fig3_resid-propP The relationship between the residuals from the phylodiversity-species richness regression, and the proportion of post-disturbance seeding species (P+; r= -0.560, p< 0.0001). Negative residuals indicate lower phylogenetic diversity than expected from species richness values, that is, a tendency for phylogenetic clustering.

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