Heterogeneity decreases as time since fire increases in a South American grassland

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Prepared by Luis López-Mársico, Felipe Lezama & Alice Altesor

In a sub‐humid Uruguayan grassland community, dominated by a tall tussock grass (Saccharum angustifolium), a large amount of standing dead biomass accumulates. Patchy and asynchronous field burns are a traditional practice among ranchers to removes above‐ground biomass of the dominant species and to promote tender forage for the livestock. Photo credit: Federico Gallego.

Grasslands, where the standing dead biomass of the grasses accumulate, are an ideal medium for fire propagation. After a fire, the open spaces generated in the soil are prone to be colonized by different plant species. In Uruguayan grasslands, livestock ranchers burn patches of land (generally less than 0.5 ha) as a management tool to promote tender regrowth and to produce forage for the livestock.

Our study covers 18 sites, representative of four different ages of burns, where we recorded the identity, richness (quantity), and coverage of plant species. We also recorded the percentage of bare soil and standing dead biomass, as well as the number of plant strata (1 – low stratum represented by species that grow at ground level, 2 – medium stratum represented by tall grasses, and 3 – high stratum represented by shrubs). We determined four age categories according to the time the last fire took place (6, 18, 30, or 60 months ago). Altogether, in this study, we recorded 168 species of vascular plants belonging to 39 different families. We observed that the species richness was higher in the sites recently burned and lower in the sites burned 60 months ago. In contrast, vegetation coverage and the number of plant strata showed an opposite pattern, that is, they were lower in the recently burned sites, and higher in the sites burned 60 months ago. On a small scale (patch), a cycle adapted from a pyric herbivory model was proposed (see the diagram below). On a larger scale (landscape), we found coexistence of patches in different stages, determined by different species, coverage, and plant strata. At this same scale, we observed that recently burned sites present a very different species composition, while older burned sites are more homogeneous.

We suggest that studied Uruguayan grasslands require occasional, asynchronous, and patchy burns to generate changes in the plant community to maximize both spatial and temporal heterogeneity in species composition.

Diagram showing a cycle of changes in an Uruguayan grassland dominated by tall tussock grasses under livestock grazing, after a field-burn event, based on pyric herbivory model proposed by Fuhlendorf et al. (2009). The patch-scale cycle starts when the biomass and standing dead biomass (SDB) of tall tussock Saccharum angustifolium is burned, and the grazing pressure increases. Areas of bare soil (BS) are colonized by different plant species, promoting an increase in species richness (S) within the first six months after the fire. The plants of S. angustifolium keep resprouting, and SDB begins to accumulate. The forage quality of S. angustifolium decreases and their consumption is avoided, resulting in an increase in their coverage. The cycle ends when S. angustifolium becomes dominant again, and a large amount of SDB accumulates; the S and BS decreases. Under these conditions, the livestock ranchers burn again, and the cycle is restarted. The number of cows represents the grazing pressure at each moment of the cycle.

Reference

This is a plain language summary for the paper of López-Mársico et al. published in Applied Vegetation Science (https://doi.org/10.1111/avsc.12521).