Over the last decades, Chile has experienced a long-term drought with significant consequences for water availability, forest productivity, and soil degradation, ultimately dramatically increasing the surface of burned area. Here, we quantify the Palmer Drought Severity Index (PDSI) to ascertain the extent of “moisture deficiency” across the central-southern region of Chile from 2000 to 2023 to assess the drought’s relationship with the frequency of wildfires focusing on the impact of native forests. Our methodology quantifies the PDSI from the burned area data using MODIS MCD64A1 satellite imagery, validated by in situ wildfire occurrence records. The findings indicate that 85.2% of fires occurred under moderate to severe drought conditions. We identified 407,561 ha showing varying degrees of degradation due to wildfires, highlighting the critical areas for targeted conservation efforts. A significant increase in both the frequency of wildfires and the extent of the affected area in native forests was observed with the intensification of drought conditions in the 21st century within mesic to humid Mediterranean climatic zones where drought explains up to 41% of the variability in the burned area (r² = 0.41; p < 0.05). This study highlights the relationship between drought conditions and wildfire frequency, showing the paramount need to adopt comprehensive wildfire mitigation management in native forests.

Wildfires are non-controlled large-scale fires of various vegetation types that have long affected the Mediterranean- climate regions, but their increased frequency and severity has led to the degradation of the ecosystems. Particularly in Chile, 147 major wildfires in 2017, were responsible of burning 546 678 ha, which included 28 729 ha of traditional small-scale non irrigated agricultural systems, highlighting the vulnerability of these agricultural soils to wildfires. Soil biological properties are more sensitive to heating than physicochemical properties. Thus, a reduction of microbial communities and associated enzyme activities generally occur in low-intensity fires (< 200 °C). Most significant changes in physicochemical properties of soil occurring in moderately intense fires (250 to 450 °C) are increases of soil pH, nutrient availability, bulk density and soil water repellency, whereas soil aggregate stability and water holding capacity generally decrease. When vegetation cover is completely destroyed by fire, emergency stabilization treatments such as mulching and seeding provide an immediate ground cover to reduce soil erosion and preserve nutrients. Therefore, it is important to define the impacts of wildfire on soil properties of agricultural lands to establish a roadmap to implement an adequate and viable restoration.

The frequency and severity of wildfires in Mediterranean climate regions have increased in the last decades due to changes in climatic conditions and land use. This change in the fire regime has increased the threat to forest, shrublands and agricultural lands. To evaluate to what extent wildfires could affect agricultural soils, we evaluated 40 properties of soil samples collected at two depths (0-5 and 5-10 cm) in areas affected by moderate severity wildfires 14 mo prior, and the immediately neighboring unburned area. The overall most relevant soil properties to differentiate between burned and unburned soil, regardless its sampling depth, were selected as soil quality indicators and used to compute a soil quality index. Accordingly, carboxylesterase activity, available soil moisture, pH, Ca, P, Fe, B, S and Cu were selected using a two-way ANOVA. The results showed that carboxylesterase activity decreased from 0.49 ± 0.18 to 0.26 ± 0.16 μmol h^-1 g^-1 dry soil, and available soil moisture was reduced from 9.85 ± 0.83 to 8.33 ± 0.52 cm in burned soils. Also, pH significantly decreased from 7.30 ± 0.14 to 6.27 ± 0.23 which affected the subsequent nutrient availability. Thus, Ca and P decreased from 6.50 ± 1.78 to 3.95 ± 0.91 cmol₍₊₎ kg^-1 and from 10.72 ± 2.84 to 7.44 ± 0.91 mg kg^-1 respectively, whereas Fe increased from 3.89 ± 0.97 to 16.75 ± 3.13 mg kg^-1, Cu and S availability doubled, and B content increase by 29%. The overall soil quality index revealed a nonsignificant decreased between unburned (0.59 ± 0.03) and burned (0.55 ± 0.04) soils 14 mo after the fire.

Background: Pinus radiata D. Don is widely planted in Chile due to its fast growth and adaptability to various soil types and environmental conditions. However, the species is prone to stem sinuosity due to factors such as genetics, environmental stresses including nutrient availability, and management practices. This study assessed the efficacy of either low- or high-solubility calcium (Ca) additions in reducing stem sinuosity in young radiata pine plantations grown on Andisols. Methods: We compared growth and stem sinuosity of a 3-year-old pine plantation subject to two types of Ca sources: agricultural low-solubility crushed lime (ag-lime), and finely ground pelletised high-solubility lime (pell-lime), applied at three concentrations (300 kg Ca ha-1, 600 kg Ca ha-1, 1,200 kg Ca ha-1) alongside a control. Results: Our findings revealed that both calcium sources increased the soil Ca content post-application but was only significant for the highest concentration in each case (1,200 Kg Ca ha-1). The soil Ca availability rate was higher in pell-lime treatments. Agricultural lime had no effect on growth, whereas two of the pell-lime treatments increased the basal area, the lowest concentration increased the cumulative diameter at the root collar, and the highest concentration reduced tree height. Interestingly, while pell-lime did not mitigate stem sinuosity in 3-year-old pine trees, ag-lime reduced stem sinuosity severity although this result was not correlated with soil Ca content. Notably, the ag-lime treatment at 600 kg Ca ha-1 was most effective, decreasing the proportion of severely deformed trees from 32% to 14%. Conclusions: The application of both low-solubility calcium (ag-lime) or high-solubility calcium (pell-lime) calcium increased the soil Ca content. Ag-lime treatments showed no adverse impact on stand growth and led to a reduction in stem sinuosity being the most effective treatment applying 600 kg Ca ha-1. Pell-lime treatments had mixed effects on growth: diameter at root collar (DRC) and basal area increment (BAI) increased, and height decreased. Pell-lime treatments did not reduce stem sinuosity. Our results suggests that stem sinuosity might be more closely associated with deficiencies in trace metals like Cu and Mn rather than Ca content alone.