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SignificanceWe provide the first assessment of aboveground live tree biomass in a mixed conifer forest over the late Holocene. The biomass record, coupled with local Native oral history and fire scar records, shows that Native burning practices, along with a natural lightning-based fire regime, promoted long-term stability of the forest structure and composition for at least 1 millennium in a California forest. This record demonstrates that climate alone cannot account for observed forest conditions. Instead, forests were also shaped by a regime of frequent fire, including intentional ignitions by Native people. This work suggests a large-scale intervention could be required to achieve the historical conditions that supported forest resiliency and reflected Indigenous influence.

Quantifying historical fire regimes provides important information for managing contemporary forests. Historical fire frequency and severity can be estimated using several methods; each method has strengths and weaknesses and presents challenges for interpretation and verification. Recent efforts to quantify the timing of historical high-severity fire events in forests of western North America have assumed that the "stand age" variable from the US Forest Service Forest Inventory and Analysis (FIA) program reflects the timing of historical high-severity (i.e. stand-replacing) fire in ponderosa pine and mixed-conifer forests. To test this assumption, we re-analyze the dataset used in a previous analysis, and compare information from fire history records with information from co-located FIA plots. We demonstrate that 1) the FIA stand age variable does not reflect the large range of individual tree ages in the FIA plots: older trees comprised more than 10% of pre-stand age basal area in 58% of plots analyzed and more than 30% of pre-stand age basal area in 32% of plots, and 2) recruitment events are not necessarily related to high-severity fire occurrence. Because the FIA stand age variable is estimated from a sample of tree ages within the tree size class containing a plurality of canopy trees in the plot, it does not necessarily include the oldest trees, especially in uneven-aged stands. Thus, the FIA stand age variable does not indicate whether the trees in the predominant size class established in response to severe fire, or established during the absence of fire. FIA stand age was not designed to measure the time since a stand-replacing disturbance. Quantification of historical "mixed-severity" fire regimes must be explicit about the spatial scale of high-severity fire effects, which is not possible using FIA stand age data.

Temperatures well in excess of the lethal threshold for roots (60[degrees]C) have been measured in forest soils when woody fuels are burned. Whether this heat pulse is strongly moderated by soil moisture or soil texture is not fully understood, however. We measured soil heat profiles during 60 experimental burns, identifying changes in maximum soil temperature and heat duration above 60[degrees]C as a function of soil moisture and soil texture. Experimental treatments included a factorial combination of soil moisture (~5, 15, 30, and 45% v/v) and soil textural (sandy loam, sandy loam--pumice, loam, clay loam) gradients, with a surface fuel load comprised of a dense layer of masticated wood. Soil moisture had a strong influence on heat transfer. A volumetric moisture content of 20% or greater quenched the heat pulse in all soils at depths of 2.5 cm and lower. In comparison, soil temperatures in dry soil far exceeded the lethal threshold to a depth of 10 cm. Differences in heating characteristics among the four soil types were minor despite their dissimilarities in texture, porosity, bulk density, and presumed thermal conductivity. It was also shown that intact soil cores were required to produce accurate heat profiles during burning, as maximum soil temperatures in the surface 5 cm were overestimated by 40 to 100[degrees]C using disturbed soil (sieved and packed). The empirical results along with a simple predictive model of soil heating show that burning of woody fuels when underlying soils have 20% volumetric moisture or greater is an effective means for limiting lethal heating in a variety of soils. doi: 10.2136/sssaj2009.0322

Abstract: Although the majority of fires in the western United States historically occurred during the late summer or early fall when fuels were dry and plants were dormant or nearly [...]

Breeding alfalfa (Medicago sativa L.) for florets that are more easily tripped by honey bees (Apis mellifera L.) has been proposed as a means of increasing efficiency of pollination and improving seed yield. The purpose of this study was to evaluate the effectiveness of phenotypic recurrent selection in breeding for ease of alfalfa floret tripping, and to compare observed to predicted genetic gain. Easy- and hard-to-trip populations were developed from 'CUF101' (two populations), 'Saranac', and a high nectar population derived from 'CUF101'. Populations were evaluated in the glasshouse and observed genetic gain was compared to genetic gain predicted from genetic variance component estimates. Gain was made from all starting populations for both easy (mean = 17.9% easier to trip) and hard tripping (mean = 25.4% harder to trip) (P is less than or equal to 0.05). Genetic variance components were useful for predicting genetic gain, as observed gain corresponded closely to predicted gain in most populations. Gain was less for easy than for hard tripping from all but the Saranac population. Easy-to-trip populations still contained hard-to-trip plants, and hard-to-trip populations still contained easy-to-trip plants. Phenotypic recurrent selection was effective in breeding for ease of tripping in alfalfa.

Breeding alfalfa (Medicago sativa L.) with florets that are easier for honey bees (Apis mellifera L.) to trip has been proposed as a means of increasing the efficiency of pollination and improving seed yield. The objectives of this study were to determine the mode of inheritance and develop breeding recommendations for ease of tripping. Thirteen plants were chosen at random from 'CUF101' and emasculated crosses were made in a 6 X 7 factorial mating design. Ease of tripping was evaluated on florets of the resulting progeny in the field at Davis, CA, in 1990 and 1991. Variance components were estimated with autotetraploid genetic models and used to predict expected genetic gain for different breeding methods. Variances due to both males and males X females were significant, whereas the variance due to females was not significant. Additive genetic variance was 9.15 X 10 to the -6 and digenic genetic variances was 4.45 X 10 to the -6. Narrow-sense heritability for ease of tripping on an individual plant basis was 34% (SE = 16%). Expected genetic gain per year was greater for phenotypic recurrent selection than for mass selection, half-sib family selection, half-sib progeny test selection, and among-and-within half-sib family selection. Phenotypic recurrent selection, based on a single year of evaluation, is recommended as the most efficient breeding method for improving ease of tripping of CUF101 alfalfa.

Breeding alfalfa (Medicago sativa L.) with florets that are easier for honey bees (Apis mellifera L.) to trip has been proposed as a means of increasing the efficiency of pollination and improving seed yield, but the impact of selection for ease of tripping on the mating system is not known. This study was conducted to evaluate ease of tripping, self-fertility, and outcrossing rate of alfalfa populations selected for ease of tripping. Easy-to-trip, hard-to-trip, and 'CUF101' parent populations were planted in a randomized complete block design with three replicates at Davis, CA. Ease of tripping of the populations was measured, self-fertility evaluated, and open-pollinated seed harvested in each of two years. The mode of inheritance of the Peroxidase-2 and Fluorescent Esterase-1 allozyme loci was verified. Allozyme genotype at these two loci was determined for progeny arrays grown from the open-pollinated seed, and the data used to calculate outcrossing rate of each population using multilocus maximum likelihood estimation procedures. The easy-to-trip population was easier to trip and the hard-to-trip population was harder to trip than the unselected parent population. Self-fertility did not differ among populations. Both Peroxidase-2 and Fluorescent Esterase-1 segregated as expected for monomeric enzymes at single tetrasomic loci. The outcrossing rate of the easy-to-trip population (0.76) did not differ from the outcrossing rate of the hard-to-trip population (0.75) or the CUF101 parent population (0.77). Selection for ease of tripping is an easily utilized indirect method which may improve alfalfa seed yield and does not alter the proportion of self-pollinated seed produced.

Ecological forestry experiments typicaly use large treatment units and silvicultural prescriptions that commonly increase within-unit heterogeneity in structural complexity and species composition in large treatment units. Increased heterogeneity influences processes affecting tree responses (e.g., competition) that operate at the neighborhood-level, posing challenges to analysis and interpretation. To investigate whether examining within-unit heterogeneity offers a more meaningful evaluation of project success than comparing categorical treatment effects, we used 20-year data from the Goosenest Adaptive Management Area (AMA) ecological forestry experiment in northern California, U.S. Designed to evaluate management alternatives for reducing fuels and accelerating development of late-seral forest characteristics in ponderosa pine (Pinus ponderosa)/white fir (Abies concolor) mixed-conifer forests, the Goosenest AMA study consists of (1) an untreated Control, (2) a Big Tree treatment using thinning from below to favor retention of large trees of any species, (3) a Pine Emphasis treatment combining thinning from below with radial thinning to favor percent ponderosa pine while increasing structural complexity, and (4) a Pine Emphasis with Fire treatment with added post-thinning prescribed burns. Our objectives were to evaluate 1) how treatments affect within-unit variation in neighborhood competition, structural complexity, and tree species composition, and 2) whether categorical treatment effects versus within-unit variation in competition, complexity, and composition influence individual-tree basal area increment (BAI), vigor as indicated by live crown ratio (ΔLCR), and tree mortality. To accomplish this, we developed and compared a series of generalized linear mixed models. Our analysis included the first investigation into whether fuel-reduction treatments alter tree species mixture-effects in ponderosa pine/white fir mixed-conifer forests. We found that all treatments similarly reduced neighborhood competition relative to the Control. The two Pine Emphasis treatments promoted greater variation in neighborhood competition and higher Percent pine relative to the Big Tree treatment, consistent with restoration objectives. Reduced neighborhood competition improved both ponderosa pine and white fir BAI. Reduced neighborhood competition helped to offset or reverse crown vigor decline in ponderosa pine and white fir, respectively. Species-mixture effects were negative for both small and large ponderosa pine BAI. For white-fir, trees grew faster in neighborhoods with a higher percentage of pine but the probability of mortality increased. Categorical treatment differences consistently reduced ponderosa pine and white fir mortality, except for the Pine Emphasis with Fire treatment, which increased white fir mortality. Our findings suggest that restoring historical ponderosa-pine forest reference conditions could accelerate the development of fire-resistant ponderosa pine tree sizes and sustain large pine growth. • Fuel reduction treatments foster fine-scale variation in structure and composition. • Local competition influences tree growth and vigor in California mixed-conifers. • Tree size and competition modify species mixture effects on growth. • Tree mortality varies with unit-level treatment and local species composition. • Restoration can benefit disturbance resistance and alter species-mixture effects. [ABSTRACT FROM AUTHOR]

Tree spatial patterns in dry coniferous forests of the western United States, and analogous ecosystems globally, were historically aggregated, comprising a mixture of single trees and groups of trees. Modern forests, in contrast, are generally more homogeneous and overstocked than their historical counterparts. As these modern forests lack regular fire, pattern formation and maintenance is generally attributed to fire. Accordingly, fires in modern forests may not yield historically analogous patterns. However, direct observations on how selective tree mortality among pre‐existing forest structure shapes tree spatial patterns is limited. In this study, we (a) simulated fires in historical and contemporary counterpart plots in a Sierra Nevadan mixed‐conifer forest, (b) estimated tree mortality, and (c) examined tree spatial patterns of live trees before and after fire, and of fire‐killed trees. Tree mortality in the historical period was clustered and density‐dependent, because trees were aggregated and segregated by tree size before fire. Thus, fires maintained an aggregated distribution of tree groups. Tree mortality in the contemporary period was widespread, except for dispersed large trees, because most trees were a part of large, interconnected tree groups. Thus, postfire tree patterns were more uniform and devoid of moderately sized tree groups. Postfire tree patterns in the historical period, unlike the contemporary period, were within the historical range of variability identified for the western United States. This divergence suggests that decades of forest dynamics without significant disturbances have altered the historical means of pyric pattern formation. Our results suggest that ecological silvicultural treatments, such as forest restoration thinnings, which emulate qualities of historical forests may facilitate the reintroduction of fire as a means to reinforce forest structural heterogeneity. [ABSTRACT FROM AUTHOR]