Few studies have attempted to isolate the various factors that may cause the observed increases in peak flows and erosion after high‐severity wildfires. This study evaluated the effects of burning by: (i) comparing soil water repellency, surface cover, and sediment yields from severely burned hillslopes, unburned hillslopes, and hillslopes where the surface cover was removed by raking; and (ii) conducting rainfall simulations to compare runoff, erosion, and surface sealing from two soils with varying ash cover. The fire‐enhanced soil water repellency was only stronger on the burned hillslopes than the unburned hillslopes in the first summer after burning. For the first 5 yr after burning, the mean sediment yield from the burned hillslopes was 32 Mg ha−1, whereas the unburned hillslopes generated almost no sediment. Sediment yields from the raked and burned hillslopes were indistinguishable when they had comparable surface cover, rainfall erosivity, and soil water repellency values. The rainfall simulations on ash‐covered plots generated only 21 to 49% as much runoff and 42 to 67% as much sediment as the plots with no ash cover. Soil thin sections showed that the bare plots rapidly developed a structural soil seal. Successive simulations quickly eroded the ash cover and increased runoff and sediment yields to the levels observed from the bare plots. The results indicate that: (i) post‐fire sediment yields were primarily due to the loss of surface cover rather than fire‐enhanced soil water repellency; (ii) surface cover is important because it inhibits soil sealing; and (iii) ash temporarily prevents soil sealing and reduces post‐fire runoff and sediment yields.

Post-fire soil erosion is of considerable concern because of the potential decline in site productivity and adverse effects on downstream resources. For the Colorado Front Range there is a paucity of post-fire erosion data and a corresponding lack of predictive models. This study measured hillslope-scale sediment production rates and site characteristics for three wild and three prescribed fires over two summers and one winter using 48 sediment fences. Over 90% of the sediment was generated by summer convective storms. Sediment production rates from recent, high-severity wildfires were 0.2–1.0 kg m–2 year–1. Mean sediment production rates from areas recently burned at moderate and low severity were only 0.02 and 0.005 kg m–2 year–1, respectively. For a given severity, sediment production rates from prescribed fires were generally lower than from wildfires, but there was considerable variability between plots and within fire severity classes. Fire severity, percent bare soil, rainfall erosivity, soil water repellency and soil texture explained 77% of the variability in sediment production rates, while a two-parameter model using percentage bare soil and rainfall erosivity explained 62% of the variability. Model validation confirmed the usefulness of these empirical models. The improved understanding of post-fire erosion rates can help guide forest management and post-fire rehabilitation efforts.

Abstract:Wildfires in the Colorado Front Range can trigger dramatic increases in runoff and erosion. A better understanding of the causes of these increases is needed to predict the effects of future wildfires, estimate runoff and erosion risks from prescribed fires, and design effective post-fire rehabilitation treatments. The objective of this project was to determine whether runoff and sediment yields were significantly related to the site variables of burn severity, percent cover, soil water repellency, soil moisture, time since burning, and slope. To eliminate the variability due to natural rainfall events, we applied an artificial storm of approximately 80 mm h 1 on 26 1 m 2 plots in the summer and fall of 2000. The plots were distributed among a June 2000 wildfire, a November 1999 prescribed fire, and a July 1994 wildfire.For 23 of the 26 plots the ratio of runoff to rainfall exceeded 50%. Nearly all sites exhibited strong natural or fire-induced water repellency, so the runoff ratios were only 15-30% larger for the high-severity plots in the two more recent fires than for the unburned or low-severity plots. The two high-severity plots in the 1994 wildfire had very low runoff ratios, and this probably was due to the high soil moisture conditions at the time of the simulated rainfall and the resulting reduction in the natural water repellency. Sediment yields from the high-severity sites in the two more recent fires were 10-26 times greater than the unburned and low-severity plots. The plots burned at high severity in 1994 yielded only slightly more sediment than the unburned plots. Percent ground cover explained 81% of the variability in sediment yields, and the sediment yields from the plots in the 1994 wildfire are consistent with the observed recovery in percent ground cover.

RESUMENLa supervivencia de una plantación forestal depende de diversos factores, entre los que destaca la calidad de planta. A pesar de su importancia, en México falta información respecto a este parámetro en los viveros forestales así como de los procedimientos necesarios para su evaluación. En el presente trabajo se determinó la calidad de planta en 11 especies arbóreas de climas tropical y templado, en siete viveros forestales del estado de Nayarit. Para la toma de datos se realizó un muestreo al azar de 0.15 % de los individuos producidos por vivero y taxón. La calidad de planta se clasificó como alta, media o baja con base en la altura, diámetro, biomasa seca aérea y radical, además de los contenidos de N, P, K, C y lignina. Los resultados indican que, en su mayoría, la planta producida en los viveros evaluados es de calidad media de acuerdo a los estándares sugeridos. Las variables mejor calificadas fueron el índice de robustez y los contenidos de fósforo, potasio y nitrógeno. Por otro lado, la relación biomasa aérea seca/biomasa radical seca recibió la menor calificación. Se recomienda continuar con la evaluación de calidad de producción en estos y otros viveros forestales, y dar seguimiento a las plantaciones ya establecidas con el fin de validar la información aportada por la evaluación de calidad de planta.Palabras clave: Árboles tropicales, calidad de planta, parámetros morfológicos, plantación forestal, supervivencia de plántula, viveros forestales. ABSTRACTThe survival of a forest plantation depends on several factors, among which the plant quality is outstanding. Despite its importance, in Mexico there is a lack of information about this parameter in the forest nurseries.as well as of the procedures for their assessment. In this paper the plant quality of 11 tree species in tropical and temperate climates was determined in seven nurseries of the state of Nayarit. For data collection a random sampling of 0.15 % of individuals produced by each nursery and taxon was performed. The plant quality was classified as high, medium or low, based on the height, diameter, shoot and root dry biomass, plus the contents of N, P, K, C and lignin. Results indicate that, for the most part, the plant produced in the assessed nurseries is of average quality according to the suggested standards. The variables that were better qualified were the robustness index and the phosphorus, potassium and nitrogen content. On the other hand, the dry biomass ratio / dry root biomass received the lowest rating. It is recommend further evaluation of production quality in these and other nurseries, and track established plantations in order to validate the information provided by the assessment of plant quality.