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Fire favors sexual precocity in a Mediterranean pine

Citation

Guiote, Carmen; Pausas, Juli G. (2022), Fire favors sexual precocity in a Mediterranean pine, Dryad, Dataset, https://doi.org/10.5061/dryad.xgxd254km

Abstract

Wildfires 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. A key trait in fire-prone ecosystems is the age at first reproduction (maturity age); populations of non-resprouting species cannot persist when fire interval is shorter than this age. Maturity age is variable among individuals, so we hypothesize that short fire intervals select for early seed production (precocity). We selected 13 plots with different fire regimes in eastern Spain, all dominated by Pinus halepensis (a non-resprouting serotinous species). Then, we evaluated the age at first reproduction and the size of the canopy seed bank of each individual pine. Our results show a significant effect of fire regime on the onset of reproduction in this species, suggesting a selection towards higher precocity in populations subject to shorter fire intervals. Due to this higher precocity, pines stored more cones, and therefore, increased their potential for post-fire regeneration. 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.

Methods

We selected 13 sites dominated by Pinus halepensis trees in the Valencia region (eastern Spain) across a range of altitudes and climatic conditions. In each site, we established 4 transects, spaced by at least 100 m. In each transect, we haphazardly selected and georeferenced 10 trees, separated by at least 10 m. For each tree, we measured its basal diameter (10 cm above ground) and the distance to the two closest trees. Then, we compute the annual growth rate as the basal diameter (cm) divided by the age (years) and the average of the distances to the 2 closest trees. Afterward, we estimated the age of each tree by counting whorls. For reproductive trees, we also estimated the age of each cone following the same method of counting whorls (avoiding immature cone cohorts; i.e., the last or last two cohorts depending on the sampling season) and recorded whether they were open (non-serotinous), closed (serotinous cones), or from the last cohort considered (closed but serotiny unknown). The retention of branches and cones in this species allowed us to estimate the age of the young tree and its cones (even if they were open), and therefore the age at first reproduction (tree age - age of the first cone) and the total number of stored cones (i.e. closed cones; an estimation of the canopy seed bank) for each tree. Please see “materials and methods” section within the paper for more details.

Funding

Ministerio de Ciencia e Innovación, Award: PGC2018-096569-B-I00

Ministerio de Ciencia e Innovación, Award: FPU16/06412