Effect of fire occurrence and frequency on topsoil properties and soil (fertility) losses by runoff

Samenvatting

1.1 Why study wildfire?

A massive wildfire ripped through a section of Portugal on Sunday, June 18, 2017 killing more than 60 people and injuring hundreds more (CNN.com). In addition to the tragic loss of life, the damage and impacts of this and other fires reach far beyond the obvious loss of vegetation.

Every year, around 45,000 forest fires break out in southern Europe, burning around half a million hectares of forests (Moreira et al., 2011). In Portugal, between 1980 and 2010, an average of 110,000 hectares burned every year (Stoof et al., 2015; Giglio et al., 2010; FAO, 2007; Giglio et al., 2006). Fires in Portugal are often one of the major reasons contributing to forest disruption and renovation (Cerdà and Robichaud, 2009).

Besides the obvious loss of forests and the socioeconomic consequences, fires can create smoke pollution, release greenhouse gases, and damage ecosystems (NASA). Fires can also increase surface temperatures and soil hydrophobicity as well as impact nutrient cycling (Richards et al. 2012). The increased frequency and severity of fires over the past few decades have considerable negative ecological consequences in terms of soil erosion, decreasing soil fertility (Stoof et al., 2015; Cerdà and Robichaud, 2009; Certini, 2005) and forest productivity. The effects of one single forest fire on runoff and soil erosion are well known, especially in terms of overland flow and soil loss until the vegetation recovers (Shakesby, 2011). However, the effects of repeated fires and their impacts on maritime pine plantations, which are widespread in Portugal, have received less attention (Malkisnon et al., 2011). Correct identification of the areas sensitive to fire, in addition to better knowledge and understanding of the impacts of wildfires on plant-water-soil interactions, and the application of proper restoration methods will help to better manage the effects of fires in order to decrease the susceptibility of these areas to repeated fires.

Compared to prior studies on the effects of single and recurrent fires, the present study is advance in the sense that it will investigate:

The impact of recurrent vs. single event wildfires on runoff, erosion, soil fertility and nutrient losses, including comparing them to long-unburnt areas;

The effectiveness of the use of surfactant-coated seeds and pine needle mulch on reducing erosion under field conditions;

The testing of a model aimed at developing parameters of erosion after a fire.

1.2 Objective of the research

The overall aim of this study is to contribute to a better knowledge and understanding of the impacts of repeated vs. individual wildfires on runoff and soil erosion and the related nutrient losses occurring in maritime pine plantations in north-central Portugal (Figure 1.2). This will increase understanding of the effects of fire and contribute to efforts to decrease the vulnerability of an area to repeated fires.

The specific questions of this research are the following:

What is the effect of fire frequency on runoff, soil erosion, and loss of organic matter at the micro-plot scale in maritime pine plantations in north-central Portugal?

What is the effect of fire frequency on nitrogen and phosphorus losses by runoff?

What is the short-term effectiveness of seeding with surfactant-coated seeds and mulching with pine needles for reducing post-fire runoff and erosion?

Is the MMF model sufficient for developing generalized parameters for post-fire erosion risk assessment?

To address these questions, we selected nine sites in a maritime pine plantation following a large wildfire in September 2012 that affected roughly 3000 ha of the Viseu municipality in north-central Portugal. Three of the sites had not been burned (0x) since 1975 and acted as controls and were covered with pine trees, shrubs, and annual vegetation. Three sites had burned only in 2012 (1x) and contained burnt pines but no shrubs or annual vegetation. Lastly, three degraded sites were chosen that had been burned four times (4x) since 1975 and had no vegetation cover.

1.3 Thesis outline

Chapter 2 addresses: (i) the effect of the frequent recurrence of fire on the generation of overland flow and related sediment and OM losses at the micro-plot scale, (ii) temporal and spatial variations on the runoff and erosive response at the micro-plot scale, and (iii) the role of ground cover, soil moisture content, and soil water repellency (SWR) in runoff and associated sediment and OM losses at the micro-plot scale.

Chapter 3 addresses total (i.e. dissolved plus particulate forms) nitrogen and phosphorus losses caused by runoff in maritime pine plantations suffering different fire frequencies. The specific objectives were to: i) assess total N and P losses from topsoil by runoff, due to fire history of sites and, ii) assess the effect of fire frequency on N and P losses by runoff for a better understanding of N and P cycles after fire.

Chapter 4 deals with the effectiveness of 4 different erosion mitigation techniques and post-fire restoration measures to reduce runoff and erosion after repeated wildfires in a maritime pine forest. These measures include: i) application of pine seed, ii) sowing of pine seeds combined with the use of pine needle mulch, iii) sowing grass seed and iv) sowing surfactant-coated grass seed to reduce runoff and erosion.

Chapter 5 focuses on developing generalized parameters to improve the prediction of the impacts of recurrent wildfires on runoff and erosion using the revised Morgan-Morgan-Finney (MMF) model and on furthering the understanding of the underpinning model parameters, in particular those related to soil physical properties.

Finally, chapter 6 presents a synthesis of the research, presenting the main conclusions as well as recommendations for further research.