Short-term effects of fire frequency on vegetation composition and biomass in mixed prairie in south-western Manitoba

Summary

Fire frequency and time since burning are important factors in tallgrass prairie species composition and productivity. The effects of one, two, and three spring (may-June) burns in consecutive years on the aboveground plant biomass, species composition and soil variables were assessed within a mixed-grass prairie in south-western Manitoba.

Study site: The study was undertaken in Shilo Military Reserve, south-western Manitoba (49º39'N, 99º30'W), Canada. Precipitation in the first year was greater than the 30-year average but lower during the next three years.

Experimental design: In each of two sites, Area 6 and Area 10 (chosen to represent two mixed-prairie grass communities, located 14 km apart) four treatments were applied to 5 × 7 m plots (2 m wide mown fireguard between plots) in a randomized block design:

i) 1-year burn (12 June 1986);
ii) 2-year burn (12 June 1986; 9-14 May 1987);
iii) 3-year burn (12 June 1986; 9-14 May 1987; 11 May 1988);
iv) no burning.

Plots were burned by ignition with kerosene on the windward edge.

Soil sampling: In both sites, soil moisture was determined (oven dried to a constant weight) for samples (soil cores 5 cm in diameter; 10 cm deep) collected from three randomly selected plots for each treatment on 25 dates between 17 May and 25 August 1989. Litter and soil surface temperatures were measured on 15 bi-weekly dates between 20 May and 24 August 1988 in control and 3-year burn plots.
After spring burning in 1988, two random samples (cores 5 × 15 cm) were taken from each plot; pH was measured and samples analysed for available N, P, K, S, Ca and Mg.

Vegetation sampling: Plant species composition was sampled in late May, 1989 during peak growth of cool season C3 grasses, and in August during peak growth of warm season C4 grasses. A pin quadrat method was used to estimate species cover, bare ground, and litter cover.

In late August, 1989, aboveground biomass was harvested from one randomly selected 25 × 25 cm quadrat in each plot, and separated into blue grama Bouteloua gracilis, porcupine grass Stipa spartea, other grasses, shrubs, sedges, and forbs. Standing and fallen dead material was collected as litter. Samples were dried and weighed.

Plant height (tallest culm) of two randomly selected individuals of Junegrass Koeleria cristata and S.spartea in every plot, where present, was recorded on 18 July 1988; similarly for B.gracilis on 25 August 1988.

A total of 50 species was recorded in the study sites in 1989 (39 in Area 6; 45 in Area 10). Area 6 was characterized by B.gracilis, S. spartea, Selaginella densa and lichens. Area 10 (a somewhat drier site) was dominated by B.gracilis and sedges Carex spp.

Litter: The response was different between areas: In Area 10 litter was significantly reduced with each additional burn (control - 105 g/m²; year 1 - 85 g/m²; year 2 - 65 g/m²; year 3 - 40 g/m²). In Area 6, litter was significantly reduced only after the third burn (control to year 2 - >< 145-150 g/m²; year 3 - 70 g/m²).

Biomass: In both sites, the cumulative effect of fire (even after three consecutive years) had no significant effect on total aboveground biomass. In Area 10, however, the biomass of several species shifted e.g. B.gracilis biomass increased (30 to 50g/m²) whilst forb biomass decreased (40 to 12g/m²), significantly, after three burns (note: weights read of graphs). No such changes in species or species groups were evident in Area 6.

Cover: Plant cover was again most affected in Area 10; B.gracilis (the dominant C4 grass) cover significantly increased but Carex spp. cover decreased after two burns. In Area 6, the most significant effect of fire was a reduction in Selaginella and lichen cover. Bare ground increased in both sites after each burn.

Plant height: An effect of fire on plant height was only seen in Area 6 where there was a significant decrease in the height of B.gracilis and K.cristata in 3-burn plots.

Soil: There was little change in soil nutrient status in response to burning with the exception of phosphorus. This increased from 2.0 ppm in the controls to 2.7 ppm in Area 6, 3-burn plots. Soil moisture showed a tendency to slightly decrease (Area 6 - 2.8% average decrease; Area 10 - average 1.2% decrease).

In both areas soil surface temperatures in 3-year burn plots were significantly higher than in controls during the growing season. The average difference in soil temperature between control and burned plots was greatest in May. In Area 6, temperatures averaged 28.6°C in controls, and 6.1°C warmer in May to 2.5°C warmer in July in burn plots; and in Area 10, 33.0°C in controls varying from 3.5°C warmer in May to 2.4°C warmer in July.

Conclusions: The main effect of repeated burning was a large decrease in litter cover, especially in the drier site. The reduction in litter, led to soils warming up and drying out more rapidly, thus accentuating the effects of the sandy soils and semi-arid climate. In the drier area, biomass and cover shifted such that the drought-tolerant B.gracilis increased after three burns while forbs decreased. The authors highlight that the increase in bare ground at both sites due to the repeated burns may allow the invasion and spread of non-native species such as the spurge Euphorbia esula.


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