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Fire increases precocity in pines

December 24th, 2022 Leave a comment Go to comments

Fires are a natural disturbance in many ecosystems. Consequently, plant species have acquired traits that allow them to resist and regenerate in an environment with recurrent fires [1]. A key trait in fire-prone ecosystems is the age at first reproduction (maturity age); populations of non-resprouting species cannot persist when the fire interval is shorter than this age. Maturity age is variable among individuals (Fig. 1), so we hypothesized that short fire intervals select for early seed production (precocity) [2]. We evaluated the age at first reproduction in Pinus halepensis (a non-resprouting serotinous pine species) in eastern Iberia (Fig. 2, for a difficult example; [2]). Our results show (Fig. 3) a selection towards higher precocity in populations subject to higher fire frequency (shorter fire intervals). Due to this higher precocity, pines stored more cones and therefore, increased their potential for reproduction post-fire. We provide the first field evidence that fire can act as a driver of precocity. Being precocious in fire-prone environments is adaptive because it increases the probability of having a significant seed bank when the next fire arrives.

Fig. 1. A 12-year-old trees that is immature (A) and another of the same age that started reproduction at 9 years old (B; the zoom shows pine cones of the different yearly cohorts). Pinus halepensis, from [2]
Fig. 2. Mediterranean pines may produce more than one whorl per year. The pictures show an upper branch (A), the upper part of the trunk (B), and the lower part of the trunk (C) of Pinus halepensis. Blue arrows indicate the first whorl of a growing season (starting from the bottom); red arrows, the second whorl of the same year; and green arrows a third whorl. Note that the second and third whorls normally have fewer and thinner branches per whorl and/or are close to the other whorls from the same growing season. From [2]
Fig. 3: Probability of reaching sexual maturity (precocity) against the age (in years) of the tree (Pinus halepensis) for areas with high frequency of crown fires (in red, upper line) and areas with low frequency of crown fires (in blue; lower lines). From [2].

References

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

[2] Guiote C & Pausas JG. 2023. Fire favors sexual precocity in a Mediterranean pine. Oikos [doi | pdf]

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