Action D1.Impacts of wildfires on the survival of Phragmites australis - 2019

Aim: Wildfires are major drivers of vegetation change in reedbed communities of lakes with similar impacts on helophyte species. Depending on the time of the year that they occur, with respect to the water level, they can change the vegetation structure in different ways. At Prespa National Park, NW Greece, wildfire events occur regularly in reedbed communities in late winter/ early spring, while the water level of Lesser Prespa Lake fluctuates 85 cm on average within the year. The effects of fires are expected to vary depending on whether they are followed by a rise of the water level, resulting in the submersion of the remaining parts of reed culms.

Methods: In order to test the effects of culm submergence after fires, a replicated field experiment was conducted in eight selected sites of pure reedbed in Prespa National Park (Greece) with reed harvest treatments simulating fire. In every site, three plots of 25 m² were marked with metal piles. One plot was left intact to serve as a control and two different treatments were applied; (i) cutting P. australis stems <5cm above-ground and (ii) cutting them ≈ 1 m above-ground. All plots were cut in November 2017 while the reedbed structure was measured in August 2018. In every plot, 8 quadrats of 0.4x0.4 m were sampled. Structural (density, culm height and diameter) and water level parameters were recorded.

Analysis: Linear mixed models were used to statistically identify differences between the two treatments and the control.

Results: Culm density of P. australis was 70% lower in the “Ground” harvest treatment than in the “Meter” treatment (p-value < 0.001) and 71% lower than in the “Control” (p-value < 0.001) while no significant differences were detected between the last two. Additionally, P. australis maximum culm height, maximum diameter, random height and random diameter were significantly smaller in the “Ground” treatment than in the “Meter” treatment and in the “Control”. The topographic position of the plot had a relatively small but significant additional effect on the density of reed culms with a marginally significant interaction between land elevation and treatment.

Main conclusions: Cutting followed by no flooding or flooding of short duration has little if any impact on the structure of the reedbed. In contrast, long flooding after cutting results in strong decrease of the density of P. australis culms and can significantly modify all of its important structural parameters. The combination of these two factors could explain the decrease of P. australis in large patches in recent years: wildfires reduced P. australis culms to ground level, as in the ground cutting treatment.

Action D1. Impacts of wildfires on the survival of Phragmites australis - 2019