Mid-term and scaling effects of forest residue mulching on post-fire runoff and soil erosion.

Mulching is an effective post-fire soil erosion mitigation treatment. Experiments with forest residue mulch have demonstrated that it increased ground cover to 70% and reduced runoff and soil loss at small spatial scales and for short post-fire periods. However, no studies have systematically assessed the joint effects of scale, time since burning, and mulching on runoff, soil loss, and organic matter loss. The objective of this study was to evaluate the effects of scale and forest residue mulch using 0.25m2 micro-plots and 100m2 slope-scale plots in a burnt eucalypt plantation in central Portugal. We assessed the underlying processes involved in the post-fire hydrologic and erosive responses, particularly the effects of soil moisture and soil water repellency. Runoff amount in the micro-plots was more than ten-fold the runoff in the larger slope-scale plots in the first year and decreased to eight-fold in the third post-fire year. Soil losses in the micro-plots were initially about twice the values in the slope-scale plots and this ratio increased over time. The mulch greatly reduced the cumulative soil loss measured in the untreated slope-scale plots (616gm-2) by 91% during the five post-fire years. The implications are that applying forest residue mulch immediately after a wildfire can reduce soil losses at spatial scales of interest to land managers throughout the expected post-fire window of disturbance, and that mulching resulted in a substantial relative gain in soil organic matter.

Polyacrylamide application versus forest residue mulching for reducing post-fire runoff and soil erosion.

For several years now, forest fires have been known to increase overland flow and soil erosion. However, mitigation of these effects has been little studied, especially outside the USA. This study aimed to quantify the effectiveness of two so-called emergency treatments to reduce post-fire runoff and soil losses at the microplot scale in a eucalyptus plantation in north-central Portugal. The treatments involved the application of chopped eucalyptus bark mulch at a rate of 10-12 Mg ha(-1), and surface application of a dry, granular, anionic polyacrylamide (PAM) at a rate of 50 kg ha(-1). During the first year after a wildfire in 2010, 1419 mm of rainfall produced, on average, 785 mm of overland flow in the untreated plots and 8.4 Mg ha(-1) of soil losses. Mulching reduced these two figures significantly, by an average 52 and 93%, respectively. In contrast, the PAM-treated plots did not differ from the control plots, despite slightly lower runoff but higher soil erosion figures. When compared to the control plots, mean key factors for runoff and soil erosion were different in the case of the mulched but not the PAM plots. Notably, the plots on the lower half of the slope registered bigger runoff and erosion figures than those on the upper half of the slope. This could be explained by differences in fire intensity and, ultimately, in pre-fire standing biomass.