Mid-term effects of silvicultural treatments on fuel dynamics and fire behavior in different developmental stages of Larix principis-rupprechtii

Evaluation of the effects of silvicultural and fuels treatments on potential fire behaviour in Sierra Nevada mixed-conifer forests

Conventional flame retardants used to improve fire behaviour of wood based materials are commonly based on halogenated and/or nitrogenated chemicals. These chemicals are toxic and can harm the environment and human health. Some works describe the incorporation of nanomaterials to the polymeric systems to improve their fire behaviour. The aim of this work was to analyze the effect of several treatments based on the use of nanomaterials on the properties of natural wood veneers and mainly on their fire behaviour. Firstly, several modes for treating pine veneers (immersion, spraying and impregnation) were evaluated using a commercial flame retardant to select the most effective treatment. The treatment selected as the most effective was immersion in a bath of flame retarding agent for 30 minutes at standard conditions. Afterward, pine veneers were treated by immersion in aqueous dispersions which contained 3wt% of the following nanoparticles: SiO2, TiO2 and ZrO2, respectively. The effect of each treatment on the properties of veneers was analyzed. The results obtained showed that the treatment based on the use of 3wt% SiO2 aqueous dispersion was the most effective to improve the fire behaviour of pine veneers.

Fire behavior is well-recognized as a function of fuel characteristics, but in practice the dynamics of fuels are often overlooked. Here we focus on short term changes in the fuel bed structure and fire behavior. Fire behavior and structural characteristics of leaf litter beds of Pinus halepensis, Ceratonia silique, and Quercus pubescens were examined. Three treatments were applied: testing freshly constructed samples (treatment “0”), exposure to a simulated precipitation event (treatment ”10”), and a two-month exposure to outside weather conditions (treatment “60”). The testing procedure allowed fire behavior testing without disturbing sample structure. Out of the tested species P. halepensis was most affected by the treatments, followed by Q. pubescens. C. silique showed no significant treatment effects. Response to the treatments could be attributed to pore space within the fuel bed, with more pore space relating to more dramatic treatment effects. In the treatment “0” P. halepensis and C. silique exhibited opposing fire behavior, but showed no significant differences in the treatment “60”. The measured effects show that short-term alterations in the fuel bed structure and fire behavior can be substantial, making it an issue that should be addressed in future research.

Currently, more than 400 million hectares of tropical forests in the world are managed for timber production. Silvicultural treatments following logging that remove stems of low commercial value to reduce competition were considered as an effective tool to enhance the growth and abundance of commercial trees. However, these silvicultural treatments will reduce biomass, likely initially reduce species diversity and may also cause changes in species composition. With a stronger focus on the conservation values of tropical forests today, how to maintain a balance between timber extraction and the conservation values of tropical forests is a central problem in sustainable tropical forest management.Permanent plots were established in a tropical rainforest in north Queensland in 1967. Following selective logging in 1968/69, four treatments were applied in this experiment: selective logging only with no subsequent silvicultural treatment as a control (L), logging followed by low-intensity silvicultural treatment (LT), logging followed by medium-intensity silvicultural treatment (MT), and logging followed by high-intensity silvicultural treatment (HT). Trees above 10 cm diameter at breast height (DBH) in all treatments were inventoried in 1967 (before logging), 1969 (after logging), September 1969 (after silvicultural treatments), and in 1973, 1981, 1989, 1997 and 2015.In Chapter 3, I investigated the effects of the different intensity silvicultural treatments on species composition over 48 years. The results show that the intensity of silvicultural treatment had an important impact on the recovery time of species composition. The number of species, species abundance distribution and species composition of trees (≥ 10 cm DBH) under selective logging followed by LT and MT treatments recovered to their pre-logging levels within 46 years following silvicultural treatments. However, the number of species, species abundance distribution, and species composition in the HT treatment only partially recovered to the post-logging level due to much greater initial species loss compared to the L, LT and MT treatments.In Chapter 4, I examined how AGB recovered over time in the four treatments. The AGB in the LT and MT treatments recovered to their pre-logging levels over 48 years and HT required more time to recover to its pre-logging levels because of the greater removal of biomass during silvicultural treatments. Over the 46 years after treatment, three distinct stages of biomass recovery were evident. Initially, biomass recovery from 1969 to 1973 was slow and was largely concentrated on an increase in biomass of residual stems. From 1973 to 1997, AGB increased at a near linear rate with a similar slope for all sites. Following the 1997 measure, the rate of biomass accumulation slowed, especially for the L site. The AGB recovery over time can be explained through the number of trees that remained after treatment, along with the growth rates, recruitment, and mortality of trees. All silvicultural treatments stimulated more recruitment, especially from 1973 to 1981, which contributed strongly to biomass increment. Growth rates of trees were stimulated by reduced competition caused by silvicultural treatments. In the final measurement period, biomass recovery rates decreased mainly due to increased mortality, particularly in the small- and medium-sized trees from pioneer and early secondary species.In Chapter 5, it was found that compared to selective logging, silvicultural treatments substantially enhanced the harvestable timber volume of the five most valuable commercial species, with MT increasing most, up to the double of its pre-logging timber stocking after 46 years. The commercial timber volume recovery rates of the most desirable species in LT and HT were similar. The reason why the timber volume increase in HT was not as great as in MT might be because a large number of small- and medium-sized remnants (10 cm to 40cm in DBH) and fewer large remnants (> 40 cm in DBH) in MT following treatments accelerated the growth of remaining medium-sized trees into large trees. In addition, the MT and HT treatments shifted the community towards a greater proportion of pioneer and early-successional trees, thus leading to the community having a lower community-weighted mean wood density.In Chapter 6, the effects of crown size and class, initial tree size and neighbourhood competition on individual growth rates of three Flindersia species were discussed. The MT and HT treatments facilitated the recruitment of the three Flindersia species. Additionally, MT but not HT facilitated the mean growth rates of the three Flindersia species. As expected, growth rates were influenced by initial tree size, neighbourhood competition indices, crown size and classes, and silvicultural treatment intensities. Overall, my thesis highlighted that a trade-off occurred between facilitating commercial timber production and maintaining conservation values of tropical forests both under different intensity silvicultural treatments and over time. For better management of tropical production forests, the intensities of silvicultural treatments, changes in the silvicultural effects over time, and the effects of silvicultural treatments on both timber production and conservations values should all be taken into consideration.

Effects of silvicultural treatments and topography on individual tree growth in a tropical mountain forest in Ecuador

BackgroundSagebrush ecosystems are experiencing increases in wildfire extent and severity. Most research on vegetation treatments that reduce fuels and fire risk has been short term (2–3 years) and focused on ecological responses. We review causes of altered fire regimes and summarize literature on the longer-term effects of treatments that modify (1) shrub fuels, (2) pinyon and juniper canopy fuels, and (3) fine herbaceous fuels. We describe treatment effects on fuels, fire behavior, ecological resilience, and resistance to invasive annual grasses.ResultsOur review revealed tradeoffs in woody fuel treatments between reducing canopy fuels vs. increasing understory herbaceous vegetation (fuels) and fire behavior. In pinyon-juniper expansion areas, all treatments decreased crown fire risk. Prescribed fire and cut and broadcast burn treatments reduced woody fuels long-term but had higher risk of invasion. Mechanical treatments left understory vegetation intact and increased native perennial plants. However, cut and leave treatments increased downed woody fuel and high-intensity wildfire risk, while cut and pile burn and mastication caused localized disturbances and annual grass invasion. Ecological outcomes depended on ecological resilience; sites with warm and dry conditions or depleted perennial native herbaceous species experienced lower recovery and resistance to invasive annual grasses. In invasive annual grass dominated areas, high-intensity targeted grazing reduced fine fuels but required retreatment or seeding; in intact ecosystems with relatively low shrub cover, dormant season targeted grazing reduced fine fuel and thus fire spread. Preemergent herbicides reduced annual grasses with differing effects in warm and dry vs. cool and moist environments.ConclusionsThe information largely exists to make informed decisions on treatments to mitigate effects of wildfire and improve ecological resilience at local, project scales. Primary considerations are the short- vs long-term tradeoffs in fuels and fire behavior and thus fire severity and the likely ecological response.

Mastication of understorey shrubs and small trees to reduce fire hazard has become a widespread forest management practice, but few empirical studies have quantified the effects of this mechanical treatment on actual fire behaviour and fire effects at the stand scale. We conducted experimental burns in masticated pine flatwoods with palmetto/gallberry understories, a common ecosystem of the Southern US Coastal Plain. Fire behaviour (flame height, rate of spread) and fire effects were compared between treated and untreated sites burned in the typical winter prescribed burning season. Mastication effectively reduced flame heights by 66%, but recovering shrubs (cover, height) influenced fire behaviour within six months following treatment, suggesting time-limited effectiveness. Trees had less crown scorch and bole char in masticated sites, but tree mortality was minimal on both treated and untreated sites. Consumption of masticated fuel was substantial across both treatments, but little duff was consumed under the moist soil conditions. When burning is conducted soon after treatment, mastication may effectively reduce fire behaviour in pine flatwoods sites, but the duration of treatment efficacy remains unclear.

The influence of forest treatments on wildfire effects is challenging to interpret. This is, in part, because the impact forest treatments have on wildfire can be slight and variable across many factors. Effectiveness of a treatment also depends on the metric considered. We present and define human–fire interaction, fire behavior, and ecological metrics of forest treatment effects on wildfire and discuss important considerations and recommendations for evaluating treatments. We demonstrate these concepts using a case study from the Cameron Peak Fire in Colorado, USA. Pre-fire forest treatments generally, but not always, experienced reduced burn severity, particularly when surface fuels were reduced. Treatments in the Cameron Peak Fire have also been documented as increasing tree survivorship, aiding suppression efforts, promoting firefighter safety, and influencing fire spread. However, the impacts of pre-fire management on primary landscape-scale objectives, like watershed protection, are unknown. Discussions about the influence of pre-fire treatments on fire effects must define the indicator(s) being assessed, as the same treatment may be considered successful under one measure but not others. Thus, it is critical to bring a common language and understanding to conversations about treatment effects and advance efforts to evaluate the range of treatment effects, thus supporting treatment planning.

Do thinning and/or burning treatments in western USA ponderosa or Jeffrey pine-dominated forests help restore natural fire behavior?

The influence of fuels treatment and landscape arrangement on simulated fire behavior, Southern Cascade range, California

Forest fuel management in the context of fire prevention generally induces heterogeneous spatial patterns of vegetation. However, the impact of the canopy structure on both wind flows and fire behavior is not well understood. Here, a coupled atmosphere wildfire behavior model, HIGRAD/FIRETEC, was used to investigate the effects of canopy treatment on wind field and fire behavior in a typical Mediterranean pine ecosystem. First, the treatment-induced winds were simulated with the model. We observed that with decreasing cover fraction the wind velocity increased within the treated zone. The wind spatial variability increased when the vegetation was aggregated into larger clumps. Fire simulations indicated that a decrease of fire intensity occurred after several meters of propagation in the treated zone. This intensity decrease was significant with a cover fraction below 25%, but negligible with a cover fraction greater than 50%. The treatment also induced a more significant inclination of the plume away from vertical. The size of the tree clumps did not show significant effects on fire behavior. This study was a preliminary investigation of wind/fire interaction over various canopy treatments, by using a physically based model. It gives some practical considerations for discerning the appropriate cover fraction and open perspectives for further investigations.

Fuel management is one of the main challenges for wildfire prevention in the Mediterranean region, where wildfires have important environmental and socioeconomic effects. Different treatments are usually applied in fire-prone shrubland to try to modify its flammability. However, a knowledge gap on the effectiveness of fuel management techniques still exists. We studied the effects of two mechanical treatments (shrub crushing and shrub clearing with removal) and of prescribed burning, on fire behaviour, and compared them with untreated vegetation. Experimental burns in 0.8 × 6 m samples of regenerated shrubs 2 years after treatments were performed in an outdoor wind tunnel. All fuel treatments effectively modified fire behaviour, but no significant difference between treatment types was observed. Shrub fuel structure was the main factor affecting fire behaviour. Reduction of fuel load and height, especially necromass fraction, decreased flame height and fire intensity but did not affect fire rate of spread. Moisture contents of live and dead fuel fractions were not significant as independent parameters, but the average moisture level of the shrub fuel complex showed a relevant effect in determining fire behaviour. Temperature regime within and above the shrubs was also related to shrub fuel structure. This study contributes to understanding fuel management in shrubland by providing information about different fuel treatments effects on fire behaviour.

Effects of dormant and growing season burning on surface fuels and potential fire behavior in northern Florida longleaf pine (Pinus palustris) flatwoods

Effects of silvicultural treatments and seasonal patterns on foliar nutrients in young post-fire Pinus halepensis forest stands

Long-term impacts of silvicultural treatments on wildland fuels and modeled fire behavior in the Ridge and Valley Province, Virginia (USA)