Burn it all down: leaf traits change more than species composition

In a fire-adapted ecosystem, increased fire frequency altered community composition and structure of the ecosystem through changes in the position of the shrub line.

Wildfires are becoming more frequent and intense globally. Whilst they are ecologically important, they can lead to changes in species composition and functional traits (e.g. specific leaf area) which can have a cascade effect on the ecosystem.

Dr Rachel Mitchell from Northern Arizona University and colleagues from Duke University studied the effect of fire regimes on longleaf pines (Pinus palustris) along a lowland to highland gradient. The researchers found that leaf traits changed more than the species composition for annually burnt plants and altered the structure of the ecosystem. Drs Rachel Mitchell and Justin Wright previously were part of a project which showed that the commonly measured specific leaf area measurement is not an appropriate indicator of plant responses to fertilisers.

Mitchell and colleagues established 105 experimental plots between 2011 and 2014 around the North Carolina Sandhills. The forest has historically been burned every three years and consisted of longleaf pine and wiregrass (Aristida stricta). Three burning regiments consisted of annual burn, control (burn every three years) and fire suppression. The plots were positioned along an ecotone, where the plant composition transitioned from grass- to shrub-dominated communities.

Regenerating longleaf pine forest. Source: Woodlot/WikimediaCommons

The scientists ranked all species by relative abundance within each plot every year and randomly collected five leaves per plant to measure specific leaf area and leaf dry matter content. The changes in trait expression were calculated as the community-weighted means (CWMs) which sum the contribution of a species’ trait value weighted by its relative abundance in the community. The species turnover, species relative abundance and trait variability were compared with statistical models.

Community weighted means (CWMs) were calculated for a given trait (e.g. specific leaf area, leaf dry matter content) which depended on the presence and abundance of species in a plot. Three hypothetical scenarios show changes in A) species turnover (e.g. black crosses and green triangles), B) species abundance (e.g. more stars and less pentagons) and C)  trait values (larger pentagons). Source: Mitchell et al. 2020.

After four years, the ecotone moved almost 5 m upwards when fires were suppressed but moved down by 2.5 m toward the lowland when plots were burned annually. The leaves grew more and larger when fires were suppressed but leaf growth was conservative when fires were frequent. The leaf dry matter content also increased for annually burnt plants which could decrease their flammability. The changes in species turnover and relative abundance were smaller than the trait shifts. 

“[O]ur results, taken together, demonstrate that in this fire-adapted system, ITV [intraspecific trait variation] is the primary way that species’ leaf traits respond to alterations in disturbance regimes, at least over the near-term”, Mitchell and colleagues wrote.

“[W]hile there was some evidence of structural (shrub vs. grass dominance) shifts in the plant community (i.e., the elevation of the ecotone shifted significantly in response to the number of fires), we also found that the magnitude of change in community composition in response to the number of fires was only marginally significant”, Mitchell and colleagues explained. 

These research findings show that the leaf traits of fire-adapted pine trees change with the frequency of fires and ecotones could experience dynamic shifts. Future research should focus on how exactly plant traits change due to fires and how these impact the larger ecosystem.