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Posts Tagged ‘savanna’

Scale mismatch in ecology

January 2nd, 2017 No comments

A recent paper suggested that fire-vegetation feedback processes may be unnecessary to explain tree cover patterns in tropical ecosystems and that climate-fire determinism is an alternative possibility [1]. This conclusion was based on the fact that it is possible to reproduce observed broad scale patterns in tropical regions (e.g., a trimodal frequency distribution of tree cover) using a simple model that does not explicitly incorporate fire-vegetation feedback processes. We argue that this reasoning is misleading because these two mechanisms (feedbacks vs fire-climate control) operate at different spatial and temporal scales [2]. It is not possible to evaluate the role of a process acting at fine scales (e.g., fire-vegetation feedbacks) using a model designed for reproducing regional-scale pattern; i.e., there is a mismatch between the scale of the question and the scale of the approach for addressing the question. While the distribution of forest and savannas are partially determined by climate, the most parsimonious explanation for their environmental overlaps (as alternative states) is the existence of feedback processes [3,4], as has been shown in many ecosystems, not only tropical ones [4]. Climate is unlikely to be an alternative to feedback processes; rather, climate and fire-vegetation feedbacks are complementary processes acting at different spatial and temporal scales [2].
Fig2b
Figure: Fire activity (based on remotely sensed data) for savannas and forests located in the range of environmental conditions where both occurs, for Africa and South America (Afrotropics and Neotropics, respectively). From [2,3].

References
[1] Good, P., Harper, A., Meesters, A., Robertson, E. & Betts, R. (2016) Are strong fire–vegetation feedbacks needed to explain the spatial distribution of tropical tree cover? Global Ecol. and Biogeogr. 25, 16-25.

[2] Pausas J.G. & Dantas V.L. 2017. Scale matters: Fire-vegetation feedbacks are needed to explain tropical tree cover at the local sacle. Global Ecol. and Biogeogr. [doiwiley | pdf]

[3] Dantas V.L., Hirota M., Oliveira R.S., Pausas J.G. 2016. Disturbance maintains alternative biome states. Ecology Letters 19: 12-19. [doi | wiley | pdf | suppl | blog]

[4] Pausas, J.G. 2015. Alternative fire-driven vegetation states. J. Veget. Sci. 26:4-6. [doi | pdf | suppl.] | blog]

 

Brazil 2016

June 27th, 2016 No comments

Summary of my June 2016 tour:

  • Brasilia: meeting point with S. Paula;  visit to the Jadim Botánico and the Chapada Imperial [photos A, E]
  • Pirenópolos: IAVS 2016; talk: “Plant strategies in fire-prone ecosystems: hidden buds” [photo B]; Pireneus
  • Rio Claro: Univesidade Estadual Paulista (UNESP); meeting with A. Fidelis & students (Luis, Talita, etc.). Visiting the Itirapina experimental fires [photo C]. Talk: “Fire, traits, and biodiversity: a global perspective”.
  • Piracicaba: Universidade de Sao Paulo (USP), meeting with B. Appezzato-da-Glória and collaborators to study underground resprouting structures [photo D]
  • Campinas: Universidade Estadual de Campinas (UNICAMP); meeting with V. Dantas and students (Paulo, André, etc.) [photo F]
  • Sao Paulo: meeting W. Delitti (USP)

brasil2016c
Photos: A (top left):  Susana Paula, Ericaulaceae and myself; B (top right) : my talk in Pirenópolis; C (bottom left): A. Fidelis and her students in front of an 'underground tree'. D (center): Xylopodium with tuberous roots in Aldama (Appezzato-da-Glória lab); E (middle right): tree with a corky bark in the cerrado of the Jardim Botánico (Brasilia); E (bottom right): Having a drink with Vinicius Dantas in Campinas.

Related posts:

- Brazil 2015, jgpausas.blogs.uv.es 16 Mar 2015

- Fire shapes savanna-forest mosaics in the Brazilian cerrado, jgpausas.blogs.uv.es 14 May 2014

- Afrotropical and neotropical savannas are different, jgpausas.blogs.uv.es 29 Jul 2013

- Fire generates intraspecific trait variability in neotropical savannas, jgpausas.blogs.uv.es 28 Aug 2012

- Disturbance maintains alternative biome states, jgpausas.blogs.uv.es 9 Nov 2015

 

Fire behaviour by Vareschi

May 13th, 2016 No comments

Recently I came across this figure published in 1962 by Volkmar Vareschi [1] which nicely synthesize variations in temperatures in the flame and in the soil, as well as flame height and flame spread (time and distance) in a simple hand-drawing. It is not easy to see a figure on fire behaviour as simple and illustrative as this one; I only miss a bit of colour. It refers to a burn of a Trachypogon savanna in Los Llanos, Venezuela. Vareschi (1906-1991) was born in Austria and moved to Venezuela in 1950; he is considered a pioneer in tropical plant ecology; one of his papers was about savanna fires [1].

 

Vareschi-1962-burnFigure 2 from [1]

 

References

[1] Vareschi, V. (1962) La quema como factor ecológico en los Llanos. Boletin de la Sociedad Venezolana de Ciencias Naturales 23, 9-31.
 

Disturbance maintains alternative biome states

November 9th, 2015 No comments

It is becoming more and more evident that climate alone does not explain spatial and temporal patterns of the world vegetation, and that disturbance regimes explain an important part of the variability in vegetation and biome composition and distribution [1]. This has been suggested specially in tropical ecosystems where alternative vegetation states (e.g., forests and savannas) are possible for a given climatic conditions [2]. For instance, in dry years, surface fires may enter in forests and kill fire-sensitive trees and select for fire-resistant woody species with open crown architectures that generates well lit communities with a flammable grassy understory. Forest trees and savannas trees have a marked difference in bark thickness (thinner in the former) and thus a contrasted sensitivity to surface fires [3]. Thus, a switch to a forest state from a savanna depends on a sufficiently long fire interval or high resource availability allowing the outcompetition of shade-intolerant savanna trees and grasses (i.e. the inhibition of fires) by means of a closed canopy of forest trees. Similarly, herbivory can also exert a control on woody biomass and favour herbivory-resistant shrubs and grasses. However, empirical (field-based) evidence for alternative sates were very limited. In a recent paper [4] we used field data to show that, for a wide range of environmental conditions (in South America and Africa), fire feedbacks maintain savannas and forests as alternative biome states in both the Neotropics and the Afrotropics. In addition, wooded grasslands and savannas occurred as alternative states in the Afrotropics, depending on the relative importance of fire and herbivory feedbacks. That is, we found evidence for a disturbance-driven bistability in the Neotropics and a disturbance-driven tristability in Afrotropics (figure below).

Savanna-states

Fig. Top: Frequency distribution of basal area in afrotropical (tristability) and neotropical (bistability) ecosystems. Bottom: The discontinuous pattern of basal area along the resources gradient for both afrotropical and neotropical ecosystems (red: wooded grasslands; orange: savannas; green: forests). Note that there are regions of the gradient where two alternative vegetation types are possible; they are maintained by different disturbance regime (see [4]).

References:
[1] Pausas, J.G. 2015. Alternative fire-driven vegetation states. J. Veget. Sci. 26: 4-6. [doi | pdf | suppl.]

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

[3] Pausas, J.G. 2015. Bark thickness and fire regime. Funct. Ecol. 29: 317-327. [doi | pdf | suppl.]

[4] Dantas V.L., Hirota M., Oliveira R.S., Pausas J.G. 2016. Disturbance maintains alternative biome states. Ecology Letters 19:12-19 [doi | wiley | pdf |supp.– New!

 

Afrotropical and neotropical savannas are different

July 29th, 2013 No comments

Savannas are typically ecosystems dominated by grasses with a variable tree density (e.g., [1]). However, the savanna biome is very large, it occurs in different continents, and includes a large variability in the vegetation structure and composition. Fire and herbivory are the main disturbance factors shaping savannas worldwide and because the different climatic conditions and the different evolutionary histories among different savannas, fire and herbivory regimes also varies among savannas. Because plants are not adapted to fire and herbivory “per se” but to specific regimes of herbivory and fire [2], we expect different strategies to cope with these disturbances in different savannas. In this framework, we have recently compared savannas from Africa and from South America (afrotropical and neotropical savannas respectively) [3]: Afrotropical savannas have a dryer climate and are more intensely grazed than neotropical savannas, and thus the amount of available fuel is typically lower in afrotropical than in the neotropical savannas. Consequently fires tend to be more intense in neotropical savannas. In afrotropical conditions, young woody plants tend to grow quickly in height to soon locate the canopy above the flame zone before the next fire, and above the browsing height. Thus these plants tend to have a pole-like or lanky architecture (the lanky strategy). In contrast, in neotropical savannas where herbivory pressure is lower they require a thick corky bark for protection against relatively intense fires (the corky strategy) [3]. Despite the two fire escape strategies appear in both Africa and South America, we suggest that the lanky strategy is more adaptive in afrotropical savannas, while the corky strategy is more adaptive in neotropical savannas [3].


Figure: Diospyros hispida A.DC. (Ebenaceae), a South American example of a plant with the corky strategy. Although the trunk was fully burned one year earlier (dark branches and trunk), the bark protected the lateral buds which enabled epicormic resprouting and the formation of lateral resprouts (light grey branches). This photo was taken in Emas National Park (cerrado ecosystem, Brazil) at the beginning of the rainy season (2011) when this deciduous plant starts to produce new leaves (Photo: V.L. Dantas). For an example of the lanky strategy see [4].

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

[2] 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 Plant Sci. 16(8): 406-411. [doi | trends | pdf]

[3] Dantas V. & Pausas J.G. 2013. The lanky and the corky: fire-escape strategies in savanna woody species Journal of Ecology 101: 1265-1272 [doi | pdf]

[4] Archibald, S. & Bond, W.J. 2003. Growing tall vs growing wide: tree architecture and allometry of Acacia karoo in forest, savanna, and arid environments. Oikos, 102: 3-14.

 

Fire-stimulated flowering

May 25th, 2013 No comments

Some plant species flower profusely and quickly after fire (fire-stimulated flowering). Compared with resprouting or postfire seeding, this trait is relatively unknown outside of South Africa and Australia [1, 2]. It is considered one of the adaptations of some resprouting species to live in recurrently burn environments. There are some of these species that rarely flower without a fire (obligate postfire flowering) while others can flower in the absence of fire but they produce more flowers after it (facultative postfire flowering). One example I had the chance to observe recently in Central America is Bulbostylis paradoxa (Cyperaceae; Figure below); it is a very flammable plant that grow in savannas and dry forest of Central/South America and the Caribbean. Local foresters told me that they have never seen this species flowering in absence of fire, and that they start flowering next day after the fire.


Figure: Bulbostylis paradoxa (Cyperaceae) one month after a fire in Santa Rosa National Park, Costa Rica (fotos: J.G. Pausas, May 2013).

References:
[1] Bytebier B., Antonelli A., Bellstedt D.U., Linder H. P. 2011. Estimating the age of fire in the Cape flora of South Africa from an orchid phylogeny. Proc. R. Soc. B, 278: 188-195.

[2] Lamont B.B., Downes K.S. 2011. Fire-stimulated flowering among resprouters and geophytes in Australia and South Africa. Plant Ecol. 212: 2111-2125.

 

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]

 

Fire generates intraspecific trait variability in neotropical savannas

August 28th, 2012 No comments

“Cerrado” are neotropical savannas from Brazil. As in most savannas, fire is very frequent in cerrado, and fires has been occurring in these ecosystems during the last few millions years. Consequently, cerrado communities are strongly filtered by fire and are composed by species capable of succeed under frequent fires (e.g., resprouters, with very thick bark, etc). A recent study [1] comparing zones with different fire regimes (annual fires, biennial fires, and protected from fires) within the cerrado (in Emas National Park) suggests that most plant trait variability is found within species (intraspecific) and little trait variability is due to changes in species composition (interspecific) between fire regimes. Thus, at community scale, fire act more as an filter, preventing some of the species from outside cerrado to colonize the cerrado (e.g., from nearby non-flammable forests), than as an internal factor structuring species composition in the already filtered cerrado communities with different fire regimes. However, fire acts as an important factor generating intraspecific variability. These results support the hypothesis of the prominent importance of intraspecific variability in strongly fire-filtered communities [2,3].

Figure: The rhea (emas in Portuguese; Rhea americana) are a flightless birds that give the name to the Emas National Park (Parque Nacional das Emas), a World Natural Heritage site located in the Brazilian Central Plateau (Photo: JG Pausas, 2009, during the field sampling [1]).

References

[1] Dantas V.L., Pausas J.G., Batalha M.A., Loiola P.P. & Cianciaruso M.V. 2013. The role of fire in structuring trait variability in Neotropical savannas. Oecologia, 171: 487-494. [doi | pdf]

[2] Moreira B., Tavsanoglu Ç. & Pausas J.G. 2012. Local versus regional intraspecific variability in regeneration traits. Oecologia, 168, 671-677. [doi | pdf | post]

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

 

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