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Fire and plants: adaptations and exaptations

There are still people believing that wildfires are a catastrophic disturbance to ecosystems, and that are the product of humans. However there is an increasing evidence from paleoecological records and from phylogenetic analyzes suggesting that fire is a very old process in the history of life, dating back to the origin of land plants [1, 2, 3]. As a consequence many plants have evolved in the presence of recurrent wildfires and acquired adaptive traits to persist and reproduce in those conditions. Examples of these traits are the resprouting ability, germination by head or smoke, and serotiny; all of these confer fitness advantage in fire-prone ecosystems. However, plants are not adapted to fire per se but to fire regimes. Species that exhibit traits adaptive under a particular fire regime can be threatened when that regime changes, like the recent human-induced fire regime changes (e.g., increasing or decreasing fire frequency or severity in relation to the historic fire regime).

In a recent paper, Keeley et al. [4] proposed five scenarios of change in a trait state (Figure 1). An adaptive trait might not change through time regardless of the selective environment (scenarios 1 and 2 in Figure 1). Such traits cannot be described as adaptations to the current selective (fire-prone) environment as there is no evidence that natural selection shaped this trait. Other adaptive traits that were shaped by natural selection under a previous evolutionary pressure, but not under the current (fire-prone) environment (scenario 3 in Figure 1) would be adaptations to previous evolutionary pressures and exaptations to the current (fire) environment [4, 5]. Fire adaptations are those adaptive traits in which natural selection is acting under the current fire-prone environment to shape the trait, and it is independent of how long this pressure has been present (scenarios 4 and 5 in Figure 1). For instance, there are clear examples of lineages that resprout after fire, but their origin and evolution is hardly liked to fire. However the most plausible scenario of lineages that resprouting from lignotubers is the number 4 in Fig. 1 (old origin of resprouting reshaped by current recurrent fires). Similarly serotiny and thick barks are traits that has been reshaped by natural selection under the framework of recurrent fires and thus they also fit under the concept of adaptation to fire (scenario 4 or 5 in Fig. 1).

References
[1] Pausas J.G. & Keeley J.E. 2009. A Burning Story: The role of fire in the history of life. BioScience 59: 593-601. [doi pdfpostslides]

[2] Pausas J.G. 2011. Australia born to burn – phylogenetic evidences. URL: jgpausas.blogs.uv.es, 18/03/2011.

[3] Pausas J.G. 2010. Fire and evolution: Cretaceous fires and the spread of angiosperms. URL: jgpausas.blogs.uv.es, 9/Sep/2010.

[4] 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]

[5] Endler J.A. (1986) Natural selection in the wild. Princeton University Press.

Figure. 1. Five possible evolutionary scenarios of change in a trait state along the evolutionary time (simplified). For each scenario, different line types are periods under different dominant evolutionary pressures (e.g., the continuous line represents a period in which fire acted as an evolutionary pressure; dashed line the period with a different previous selective environment). The scenarios are:
1) and 2) No change along the time axis and no sign of natural selection (no adaptation to fire);
3) trait shaped during the first evolutionary pressure, but no change (with persistence of the state of the trait) during the second evolutionary pressure; natural selection acted during the first period only (no adaptation to fire, but exaptation);
4) and 5) Trait shaped during the whole period; natural selection acts during the whole period even if the dominant evolutionary pressure changed (adaptation to fire).
From Keeley et al. (2011, [4]).

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