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Fire and water: the role of grass competition on juvenile tree growth and survival rates in a mesic savanna

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Abstract

Trees and grasses compete for space in savanna landscapes where fire also suppresses trees, maintaining a lower tree/grass ratio than precipitation levels can support. While the effects of competition and fire have been considered largely in isolation, theory suggests that competition with grasses for water may limit the ability of juvenile trees to grow rapidly enough to escape flames holding them in a firetrap. This study examined the growth and survival rates of trees at different life stages on plots subjected to four different grass removal treatments with those on plots subjected to three different fire treatments in two mesic savannas in Mali. Two hypotheses were tested: (i) grass removal treatments will not significantly increase tree height growth rates; and (ii), burning earlier in the dry season will result in faster height growth rates and lower death rates than burning later. Trees were measured over 2 years. To compare how treatments affected tree growth, data were analyzed using a one-way analysis of variance with treatment type as a fixed factor. We find that grass removal resulted in increased juvenile tree growth varying from 147 to 205% depending on treatment. Grass treatments of hoeing, clipping and herbicide resulted in fewer tree deaths. Both early and late fires resulted in increased tree growth rates, although late fires resulted in more deaths. Mid-season fires did not significantly affect growth rates. Findings suggests that competition with grasses slows juvenile tree height growth and that a change in competition intensity is sufficient to cause a shift in savanna state to a more tree dominated one with implications for savanna management and carbon sequestration schemes.

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Data availability

Data for this study will be posted on our website as per agreement with the National Science Foundation: http://www.cla.csulb.edu/departments/geography/savannalabo/

Notes

  1. Sankaran et al. (2004) determined the dividing line to be 650 mm for the African continent, whereas Staver et al. (2011) determined that number to be 1000 mm for savannas globally.

  2. The dates given are approximations and will very according to latitude, which ranged from approximately 11.5–12.5°N.

  3. Note that we used a 3 m value for escape height for comparison with other studies. This is a conservative value for our study area as unpublished field data found that the bole scorch height for fires in the zone varies from 1.4 m in the ES to just over 2 m in the MS and LS. If these values had been used in our analysis, the number of trees reaching escape height would be considerably greater.

  4. Unfortunately, an intense, accidental fire burned the plots of the control trees in the Tabou study site in mid-fire season of 2017. To account for the accidental tree deaths in the Tabou control plots, the recorded number of dead trees in the control plot for year one was used to estimate of the number of dead trees for the two-year period. The growth rate for Tabou control trees was also negatively affected. This was mitigated by substituting the growth rates for year one for year two as well (see Appendix 2).

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Acknowledgements

The Authors wish to thank Fakuru Camara and Umu Kante and Alex Pakalniskis for their assistance in the field as well as NSF for their support.

Funding

This study was funded by the National Science Foundation #G181115100.

Author information

Authors and Affiliations

  1. Geography Department, California State University, 1250 Bellflower Blvd, Long Beach, CA, 90840, USA

    P. Laris, L. Yang & C. M. Rodrigue

  2. Institut Polytechnique Rural de Formation et de Recherche Appliquée de Katibougou (IPR/IFRA), Katibougou, Mali

    F. Dembele

Authors
  1. P. Laris
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  2. L. Yang
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  3. F. Dembele
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  4. C. M. Rodrigue
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Contributions

PL and FD conceived the ideas and designed methodology; FD and PL collected the data; LY and CMR analysed the data; PL and LY led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.

Corresponding author

Correspondence to P. Laris.

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The authors declare that they have no competing interests.

Additional information

Communicated by Kevin P. Kirkman.

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Appendix

Appendix

Appendix 1

Diagnostics

The Shapiro-Wilks test proved all tree data sets, with the exception of the herbicide and grazing treatments, to be normally distributed with p-values above α = 0.05 level, rejecting the null hypothesis of a non-normal distribution, thus meeting the assumptions for a one-way ANOVA (Table 2, Appendix). Despite failing the test for normality, it was decided to move forward with the one-way ANOVA for the herbicide and grazing treatment results, as histograms for these data did not appear to be severely non-normal, and an ANOVA is not very sensitive to moderate deviations from normality (McDonald 2014).

The Shapiro-Wilks test for the tree size classes ANOVA determined the small juvenile and adult trees to be normally distributed (adult, p = 0.097; small juvenile, p = 0.059), but large juvenile trees did not pass (p = 0.003). However, the histogram showed only moderate deviations, so the one-way ANOVA was performed with consideration to higher false positives. The tree size analysis was determined to be significantly different from tree growth rates with grass removal treatments (F2,182 = 5.532, p = 0.005).

See Tables 2, 3, 4, 5 and 6.

Table 2 Shapiro–Wilks Test for growth rate by all treatment variables
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Table 3 Tukey–Kramer results for grass removal treatments
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Table 4 Tukey–Kramer results for fire treatments
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Table 5 Shapiro–Wilks Test for growth rate by tree stage of life
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Table 6 Dead tree count for fire treatment plots
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Appendix 2

Accidental fire mitigation

An accidental intense mid-season fire on control plots that had two years of grassy fuel build-up was catastrophic. Most juvenile trees were burned intensively and dieback and death was extensive. To mitigate the effects of the accidental fire on the three control plots for the Tabou site to account for the accidental tree deaths in the control plots, we used the number of dead trees in the control plot from year one for year two as well. Similarly, to mitigate the effects on the growth rate for trees on the control in Tabou we substituted the growth rates for year one for the second year. These mitigation techniques, while ad hoc, were validated by comparing the dead trees and growth rate values to the second study site, Faradiele. The total growth rate for Faradiele’s control trees was 0.542 m. The growth rate for Tabou (which has lower annual rainfall) control trees was 0.482 m, which seemed reasonable. We feel confident that the estimates for the death rate and growth rate in the control plots are realistic.

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Laris, P., Yang, L., Dembele, F. et al. Fire and water: the role of grass competition on juvenile tree growth and survival rates in a mesic savanna. Plant Ecol 222, 861–875 (2021). https://doi.org/10.1007/s11258-021-01149-x

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