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Decadal changes in fire frequencies shift tree communities and functional traits

Abstract

Global change has resulted in chronic shifts in fire regimes. Variability in the sensitivity of tree communities to multi-decadal changes in fire regimes is critical to anticipating shifts in ecosystem structure and function, yet remains poorly understood. Here, we address the overall effects of fire on tree communities and the factors controlling their sensitivity in 29 sites that experienced multi-decadal alterations in fire frequencies in savanna and forest ecosystems across tropical and temperate regions. Fire had a strong overall effect on tree communities, with an average fire frequency (one fire every three years) reducing stem density by 48% and basal area by 53% after 50 years, relative to unburned plots. The largest changes occurred in savanna ecosystems and in sites with strong wet seasons or strong dry seasons, pointing to fire characteristics and species composition as important. Analyses of functional traits highlighted the impact of fire-driven changes in soil nutrients because frequent burning favoured trees with low biomass nitrogen and phosphorus content, and with more efficient nitrogen acquisition through ectomycorrhizal symbioses. Taken together, the response of trees to altered fire frequencies depends both on climatic and vegetation determinants of fire behaviour and tree growth, and the coupling between fire-driven nutrient losses and plant traits.

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Fig. 1: Tree stem density declines with both fire frequency and duration of fire regime.
Fig. 2: Tree basal area declines with both fire frequency and duration of fire regime.
Fig. 3: Climate and ecosystem type modify effects of fire frequencies on tree basal area.
Fig. 4: Functional composition of tree communities both responds to and modifies the effects of frequent burning.

Data availability

Wood density data are from refs. 50,51,52, plant tissue stoichiometry and mycorrhizal type data are from ref. 38, and bark thickness data are from the Fire and Fuels Extension to the Forest Vegetation Simulator (https://www.fs.fed.us/fmsc/ftp/fvs/docs/gtr/FFEaddendum.pdf). Supplementary Data 1 and 2 contain the woody population size data and Supplementary Data 3 contains land use data. Refs. 53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76 describe the fire experiments.

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Acknowledgements

A.F.A.P. was supported by a NOAA Climate and Global Change postdoctoral fellowship programme and the USDA National Institute of Food and Agriculture grant 2018‐67012‐28077. R.B.J. received support from the Gordon and Betty Moore Foundation. The experiments at the sites were organized and funded through the Cedar Creek Long Term Ecological Research programme (DEB 1234162, 0620652, 1831944 and DBI 2021898), the National Park Service and Sequoia Parks Conservancy, and South African National Parks. C.T. was supported by a Lawrence Fellow award supported by the LLNL-LDRD Program under Project No. 20-ERD-055.

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A.F.A.P. and R.B.J. conceived and designed the overall study. T.R., C.A. and C.T. helped with data acquisition and provided feedback on statistical analyses. D.G.B., A.C.S., W.C., C.C., J.D.H., S.H.E., W.A.H., J.K., T.L., W.K.M., S.T.O., W.A.P., K.G.P., P.B.R., C.R., M.A.S.S., B.C.S., T.S., G.R.S., K.S., C.S., M.G.T. and J.M.V. provided data and/or assisted with interpreting the field data from experiments. All authors contributed to the writing of the manuscript.

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Correspondence to Adam F. A. Pellegrini.

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Extended data

Extended Data Fig. 1 Distribution of sites.

a, Map displaying the distribution of sites (dots) with the surface fire sites filled with black and the crown fire sites filled with white. The coloration indicates the average fire frequency within a gridcell using 1. The sample size of plots is written adjacent to the continent. b, distribution of sites in climate space overlying Whittaker’s biome distribution 77. (1 = tundra, 2=boreal forest, 3=woodland/shrubland, 4=temperate grassland/desert, 5=temperate forest, 6=temperate rainforest, 7=subtropical desert, 8=tropical forest and savanna, 9=tropical rainforest). Dots colored according to broad vegetation type category. Plots span a mean annual temperate range from 5.2–27.3 °C and a mean annual precipitation range from 408–2378 mm yr−1. c, aerial picture of two different fire treatment plots from Cedar Creek, a temperate oak savanna, where different fire frequencies have created a stark biome boundary between forests in unburned plots and savannas in biennial burn plots.

Extended Data Fig. 2 Example of the experimental layout of a fire manipulation experiment.

Example of the experimental layout of a fire manipulation experiment taken from Cedar Creek (a temperate savanna in Minnesota, USA), where fires have been manipulated since 1964. Aerial imagery (taken in 2017) from the National Agriculture Imagery Program from the Farm Service Agency. Plots are outlined with a color corresponding to their fire frequencies expressed in terms of number of fires per year (for example 0.33 is one fire every 3 years).

Extended Data Fig. 3 Untransformed data on stem density and basal area.

Untransformed data on stem density (a-b) and basal area (c-d) as a function of the duration that plots have been exposed to burning in the experiment (0 = unburned plots). Each dot represents a site and the dashed lines connect treatments within sites. Columns represent two sets of fire frequency contrasts comparing unburned vs. the intermediate frequency in a and c, and unburned vs. the high frequency in b and d (levels defined based on treatments within sites). Dots and bars based on mean and standard error calculated across the replicate plots within a fire treatment in a site. Note y-axis is on a log10 scale.

Extended Data Fig. 4 Comparison between fire types.

Comparison between fire types (surface in a, F1,94.3 = 50.6, p < 0.001, n = 9 sites and n = 104 plots; and crown in b, F1,21 = 10.3, p = 0.004, n = 24 plots) in needleleaf forests with fire expressed in terms of return period (crown fire plots are all 12 years postfire, data subset to include short-interval burn plots). c, illustrates the mean response ratios ± standard error for the fire types with crown fires split into high (>2,400 m) and low (<2,400) elevation sites (Crown 1 and Crown 2, respectively; n = 25 plots for each elevation category). Analyses were robust to considering surface fires in only Western US needleleaf forests: F1,47.1 = 17.2, p = 0.001. Response ratios were split into long and short fire return interval plots (Crown 1 and 2, respectively), with the justification for definition of interval in 17.

Extended Data Fig. 5 Fire frequency effects across precipitation in the wet quarter bins.

Partial residual plot displaying the relationship between loge basal area and precipitation in the wettest quarter cross-sectioned based on fire frequency. This plot is based on the same mixed-effects model presented in Fig. 3 and Supplementary Table 4, just re-arranged to emphasize how precipitation-basal area relationship changes with more frequent burning. n = 25 sites and n = 309 plots.

Extended Data Fig. 6 Differences in fire effects across continents.

Partial residual plot between the length of time plots were exposed to frequent burning and the log basal area (a, n = 25 sites and n = 309 plots) and stem density (b, n = 25 sites and n = 303 plots) in the different continents (from the main mixed-effects model with site as a random intercept in Supplementary Tables 4-5).

Extended Data Fig. 7 Fire effects in sub-vegetation classifications.

Partial residual plot between the length of time plots were exposed to frequent burning and the log basal area in the different sub-vegetation types (from the main mixed-effects model, presented in Supplementary Table 4 but substituting the broad vegetation effect with the more detailed classification. We found no evidence that accounting for the finer-scale variability in ecosystem classification increased the accuracy of the model or changed our conclusions. n = 25 sites and n = 309 plots.

Extended Data Fig. 8 Responses of stem densities to fire across environmental covariates.

Partial residual plots of the mixed-effects model for stem densities illustrating how fire frequency effects changed according to wet-season precipitation, mean annual temperature, and ecosystem type. Panels structured by standard deviations around the median to visualize the spread (−1, 0,1), PWQ: precipitation in the wet quarter, MAT: mean annual temperature. All model fits are p < 0.05 and specific results can be found in Supplementary Table 5. The predictor variables are mean-centered and standard deviations are scaled to facilitate comparisons of variable influence. In needleleaf and broadleaf forests, stem densities actually increased with more frequent burning initially, but declined with increasing experiment duration, potentially because of increased light availability initially stimulating recruitment of small trees (Extended Data Fig. 6, Supplementary Table 5). Stem density in African sites changed little through time (Extended Data Fig. 6). The trends in density may reflect the ability of many of the tree species to re-sprout in between fire events78. n = 25 sites and n = 303 plots.

Extended Data Fig. 9 Effect of fire on phosphorus stoichiometry.

Partial residual plots of the phosphorus (P) stoichiometry of community weighted means as a function of years of repeated burning. Taken from mixed-effects models presented in Supplementary Table 7. The models include a vegetation type effect. Tissue P is rescaled by subtracting the mean and dividing by the standard deviation. n = 16 sites and n = 172 plots.

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Pellegrini, A.F.A., Refsland, T., Averill, C. et al. Decadal changes in fire frequencies shift tree communities and functional traits. Nat Ecol Evol 5, 504–512 (2021). https://doi.org/10.1038/s41559-021-01401-7

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