Fire and grazing interaction in a subhumid grassland: effects on a dominant tussock grass

Posted on by

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

Appearance of a Uruguayan grassland dominated by Saccharum angustifolium, showing flowering culms and a high accumulation of standing dead biomass due to the absence of recent burning (left); and three different moments of the burning experiment (right). Photo credits: Luis López-Mársico

Some grasslands dominated by tussock grass need both fire and grazing to be preserved over time. Fire allows for rapid new growth of these grasses, which improves forage quality and encourages livestock to graze. The main goal of our study was to see how fire, cattle grazing, and their combination affect the size and number of flowering stems, and a measure of growth, of a clump-forming grass, and how these variables change over time in a subhumid grassland in Uruguay. The study was conducted in a protected area. Saccharum angustifolium, a perennial warm-season C4 tall tussock grass that forms clumps, becomes dominant in the concave areas on low and medium slopes of the landscape. Mature plants of this species have tough, unpalatable leaves for cattle, and large amounts of standing dead biomass accumulate due to their low forage value. Field burns, usually less than 0.5 hectares, are mainly conducted in the spring every three to five years by ranchers.

A dynamic model of different phases and states influenced by the interaction of fire and grazing in a Uruguayan grassland dominated by the clump-forming grass Saccharum angustifolium. An area with light grazing and no recent burns (State A, Phase 1) can either stay in the same phase when cattle are excluded or shift to other phases depending on the intensity of grazing and burning. If the area is burned and grazed more intensively, it moves to Phase 2; if it is burned but not grazed, it moves to Phase 3. These phases are described by the volume, Normalized Difference Vegetation Index (NDVI), and the production of flowering culms of S. angustifolium clumps. The model indicates how fast these transitions between phases and/or states can occur (fast, intermediate, slow, or not assessed). All these phases can be part of a cycle at a patch scale (pyric-herbivory model, State A) defined for this type of grassland. If there is a prolonged absence of both fire and grazing, the ecosystem may shift to State B. The slow transition from A to B has been observed in this type of grassland, and it still needs to be evaluated if this transition can be reversed

Fire caused the biggest and quickest changes in the variables we measured for S. angustifolium clumps. Fire immediately removed biomass and activated new growth, while the effects of continuous cattle grazing appeared more gradually. Grazing slowed down the recovery to conditions before the burn. Cattle grazing alone cannot control the growth of S. angustifolium, so both fire and grazing are needed to conserve this grassland community. We present a model based on a two-year field experiment (see schema below). We found that fire and/or cattle grazing can keep grasslands in different stable phases, and we included the possible transitions between these phases. Finally, we suggested that stopping both disturbances could push the system into a state dominated by woody plants, leading to intense fires that harm the area’s biodiversity. In this situation, it is uncertain if the grassland state can be restored.

This is a plain language summary of the paper of López-Mársico et al. published in Applied Vegetation Science (https://doi.org/10.1111/avsc.12795)