California Mixed Conifer Forests Research Articles

Forest restoration and fuels reduction work: Different pathways for achieving success in the Sierra Nevada.

Fire suppression and past selective logging of large trees have fundamentally changed frequent-fire-adapted forests in California. The culmination of these changes produced forests that are vulnerable to catastrophic change by wildfire, drought, and bark beetles, with climate change exacerbating this vulnerability. Management options available to address this problem include mechanical treatments (Mech), prescribed fire (Fire), or combinations of these treatments (Mech + Fire). We quantify changes in forest structure and composition, fuel accumulation, modeled fire behavior, intertree competition, and economics from a 20-year forest restoration study in the northern Sierra Nevada. All three active treatments (Fire, Mech, Mech + Fire) produced forest conditions that were much more resistant to wildfire than the untreated control. The treatments that included prescribed fire (Fire, Mech + Fire) produced the lowest surface and duff fuel loads and the lowest modeled wildfire hazards. Mech produced low fire hazards beginning 7 years after the initial treatment and Mech + Fire had lower tree growth than controls. The only treatment that produced intertree competition somewhat similar to historical California mixed-conifer forests was Mech + Fire, indicating that stands under this treatment would likely be more resilient to enhanced forest stressors. While Fire reduced modeled wildfire hazard and reintroduced a fundamental ecosystem process, it was done at a net cost to the landowner. Using Mech that included mastication and restoration thinning resulted in positive revenues and was also relatively strong as an investment in reducing modeled wildfire hazard. The Mech + Fire treatment represents a compromise between the desire to sustain financial feasibility and the desire to reintroduce fire. One key component to long-term forest conservation will be continued treatments to maintain or improve the conditions from forest restoration. Many Indigenous people speak of "active stewardship" as one of the key principles in land management and this aligns well with the need for increased restoration in western US forests. If we do not use the knowledge from 20+ years of forest research and the much longer tradition of Indigenous cultural practices and knowledge, frequent-fire forests will continue to be degraded and lost.

Prescribed fire selects for a pyrophilous soil sub-community in a northern California mixed conifer forest.

Prescribed fire is a critical strategy for mitigating the effects of catastrophic wildfires. While the above-ground response to fire has been well-documented, fewer studies have addressed the effect of prescribed fire on soil microorganisms. To understand how soil microbial communities respond to prescribed fire, we sampled four plots at a high temporal resolution (two burned, two controls), for 17 months, in a mixed conifer forest in northern California, USA. Using amplicon sequencing, we found that prescribed fire significantly altered both fungal and bacterial community structure. We found that most differentially abundant fungal taxa had a positive fold-change, while differentially abundant bacterial taxa generally had a negative fold-change. We tested the null hypothesis that these communities assembled due to neutral processes (i.e., drift and/or dispersal), finding that >90% of taxa fit this neutral prediction. However, a dynamic sub-community composed of burn-associated indicator taxa that were positively differentially abundant was enriched for non-neutral amplicon sequence variants, suggesting assembly via deterministic processes. In synthesizing these results, we identified 15 pyrophilous taxa with a significant and positive response to prescribed burns. Together, these results lay the foundation for building a process-driven understanding of microbial community assembly in the context of the classical disturbance regime of fire.

Heading and backing fire behaviours mediate the influence of fuels on wildfire energy

Background Pre-fire fuels, topography, and weather influence wildfire behaviour and fire-driven ecosystem carbon loss. However, the pre-fire characteristics that contribute to fire behaviour and effects are often understudied for wildfires because measurements are difficult to obtain. Aims This study aimed to investigate the relative contribution of pre-fire conditions to fire energy and the role of fire advancement direction in fuel consumption. Methods Over 15 years, we measured vegetation and fuels in California mixed-conifer forests within days before and after wildfires, with co-located measurements of active fire behaviour. Key results Pre-fire litter and duff fuels were the most important factors in explaining fire energy and contributed similarly across severity categories. Consumption was greatest for the forest floor (litter and duff; 56.8 Mg ha−1) and 1000-h fuels (36.0 Mg ha−1). Heading fires consumed 13.2 Mg ha−1 more litter (232%) and 24.3 Mg ha−1 more duff (202%) than backing fires. Remotely sensed fire severity was weakly correlated (R2 = 0.14) with fuel consumption. Conclusions 1000-h fuels, litter, and duff were primary drivers of fire energy, and heading fires consumed more fuel than backing fires. Implications Knowledge of how consumption and fire energy differ among contrasting types of fire behaviours may inform wildfire management and fuels treatments.

Modeling Juvenile Stand Development and Fire Risk of Post-Fire Planted Forests under Variations in Thinning and Fuel Treatments Using FVS–FFE

In the past, the dry mixed conifer forests of California’s Sierra Nevada mountains experienced frequent low to mixed severity fires. However, due to fire suppression and past management, forest structure has changed, and the new fire regimes are characterized by large, high severity fires which kill a majority of the overstory trees. These new disturbance patterns require novel approaches to regenerate the forest as they are not adapted to large, high severity fires. We forecasted growth and fire behavior of young plantations for 100 years into the future using the Forest Vegetation Simulator (FVS) and its Fire and Fuels Extension (FFE). In these simulations, we tested combinations of different fuel treatments (mastication only, mastication with prescribed burning, and no fuels treatments) with different overstory thinning intensities (residual densities of 370 SDI (stand density index), 495 SDI, 618 SDI (TPH), and no overstory thinning) on stand growth and potential fire behavior using analysis of variance. We compared growth and crowning index at the end of the simulation and the simulation age when the flame length, basal area mortality, and fire type reached low severity between fuel treatment, thinning intensity, and original management of stands (plantation with PCT [precommercial thinning], plantation without PCT, and natural regenerating stands). These comparisons are essential to identify which fuel treatment categories reduce fire risk. We found an overall pattern of decreasing crown fire occurrence and fire induced mortality across all simulations due to increasing canopy base height and decreasing canopy bulk density. In particular, stands with mastication and prescribed burning transitioned from crown fire types to surface fires 10 years earlier compared to mastication only or no fuel treatment. Furthermore, pre-commercially thinned stands transitioned from crown fire states to surface fires 10 years earlier in the simulations compared to un-thinned and naturally regenerating stands. Stands with mastication and burning went below 25% reference threshold of basal area mortality 11 and 17 years earlier before the mastication only and no fuel treatment, respectively. In addition, pre-commercially thinned stands went below 25% basal area mortality 9 and 5 years earlier in the simulation compared to un-thinned or naturally regenerated stands, respectively. Mastication with prescribed burning (MB) was the most effective treatment for quickly reducing fire behavior by consuming surface fuels, thus drastically lowing flame length (e.g., surface flame length of MB was 0.6 m compared to mastication only [1.3 m] and no treatment [1.4 m]). Furthermore, intensive thinning reduced risk of active crown fires spreading through the stand. Prioritizing prescribed burning, when possible, and thinning (both pre-commercially and from below) are the most effective ways to quickly improve fire resistance in mixed conifer plantations. Our results highlight the different stressors that post-fire planted forests experience and how different silvicultural treatments interact over time to reduce fire risk, which demonstrates the importance of treating stands early and the effectiveness of surface fuel treatments.

Pronounced increases in nitrogen emissions and deposition due to the historic 2020 wildfires in the western U.S.

Wildfire outbreaks can lead to extreme biomass burning (BB) emissions of both oxidized (e.g., nitrogen oxides; NOx = NO+NO2) and reduced form (e.g., ammonia; NH3) nitrogen (N) compounds. High N emissions are major concerns for air quality, atmospheric deposition, and consequential human and ecosystem health impacts. In this study, we use both satellite-based observations and modeling results to quantify the contribution of BB to the total emissions, and approximate the impact on total N deposition in the western U.S. Our results show that during the 2020 wildfire season of August–October, BB contributes significantly to the total emissions, with a satellite-derived fraction of NH3 to the total reactive N emissions (median ~ 40%) in the range of aircraft observations. During the peak of the western August Complex Fires in September, BB contributed to ~55% (for the contiguous U.S.) and ~ 83% (for the western U.S.) of the monthly total NOx and NH3 emissions. Overall, there is good model performance of the George Mason University-Wildfire Forecasting System (GMU-WFS) used in this work. The extreme BB emissions lead to significant contributions to the total N deposition for different ecosystems in California, with an average August – October 2020 relative increase of ~78% (from 7.1 to 12.6 kg ha−1 year−1) in deposition rate to major vegetation types (mixed forests + grasslands/shrublands/savanna) compared to the GMU-WFS simulations without BB emissions. For mixed forest types only, the average N deposition rate increases (from 6.2 to 16.9 kg ha−1 year−1) are even larger at ~173%. Such large N deposition due to extreme BB emissions are much (~6-12 times) larger than low-end critical load thresholds for major vegetation types (e.g., forests at 1.5-3 kg ha−1 year−1), and thus may result in adverse N deposition effects across larger areas of lichen communities found in California's mixed conifer forests.

Efficacy of Chemical and Biological Stump Treatments for the Control of Heterobasidion occidentale Infection of California Abies concolor.

We conducted an experimental evaluation of treatments to limit Heterobasidion occidentale infection of white fir (Abies concolor) stumps and wounds in California mixed conifer forests. We tested the efficacy of urea, borate, and a mixture of two locally collected Phlebiopsis gigantea strains in preventing pathogen colonization of fir stumps and separately, urea and borate as infection controls on experimental stem wounds. These were paired with a laboratory test on ~100 g wood blocks with and without a one-week delay between inoculation and treatment. Urea, borates, and Phlebiopsis treatments all significantly reduced the stump surface area that was colonized by H. occidentale at 84%, 91%, and 68%, respectively, relative to the controls. However, only the borate treatments significantly lowered the number of stumps that were infected by the pathogen. The laboratory study matched the patterns that were found in the stump experiment with a reduced area of colonization for urea, borates, or P. gigantea treatments relative to the controls; delaying the treatment did not affect efficacy. The field wound experiment did not result in any Heterobasidion colonization, even in positive control treatments, rendering the experiment uninformative. Our study suggests treatments that are known to limit Heterobasidion establishment on pine or spruce stumps elsewhere in the world may also be effective on true firs in California.

Inter-specific competition among trees in pythagorean fuzzy soft environment

A Pythagorean fuzzy set is very effective mathematical framework to represent parameter-wise imprecision which is the property of linguistic communication. A Pythagorean fuzzy soft graph is more potent than the intuitionistic fuzzy soft as well as the fuzzy soft graph as it depicts the interactions among the objects of a system using Pythagorean membership grades with respect to different parameters. This article addresses the content of competition graphs as well as economic competition graphs like k-competition graphs, m-step competition graphs and p-competition graphs in Pythagorean fuzzy soft environment. All these concepts are illustrated with examples and fascinating results. Furthermore, an application which describes the competition among distinct forest trees, that grow together in the mixed conifer forests of California, for plant resources is elaborated graphically. An algorithm is also designed for the construction of Pythagorean fuzzy soft competition graphs. It is worthwhile to express the competing and non-competing interactions in various networks with the help of Pythagorean fuzzy soft competition graphs wherein a variation in competition relative to different attributes is visible.

Fisher Use of Postfire Landscapes: Implications for Habitat Connectivity and Restoration

The relationship between fishers (Pekania pennanti), fire, and fuel management in the Sierra Nevada region is complex and highly controversial, and the recent listing of southern Sierra Nevada fishers as endangered has brought this issue to the forefront for many land managers. While wildfire is a natural component of the historic disturbance regime in the dry, mixed conifer forests of California, it also has the capacity to destroy large swaths of suitable habitat and to fragment tenuous populations, particularly given recent shifts in wildfire scope and severity. Therefore, balancing fuel reduction with habitat conservation requires careful consideration of spatial and temporal context, cumulative effects, and management prescriptions. There are several unresolved questions pertaining to fishers, including how and when fishers begin to reuse postfire landscapes and whether this information can be applied to postfire efforts to restore habitat connectivity and limit negative impacts of population fragmentation. To better understand fisher response to postfire landscapes, and to provide land managers with guidance regarding habitat restoration efforts, we investigated fisher recolonization behavior in and around 2 recent Sierra Nevada wildfires. We hypothesized that fisher use of postfire landscapes would be driven by at least 1 of 4 primary variables: prefire habitat quality, fire severity, topography, and time since fire. We developed 5 hypotheses regarding how fishers would begin to explore postfire landscapes and tested them using a combination of fine- and coarse-scale analyses based on data collected using scat detection dogs. At fine scales, the strongest relationship we observed was the combination of fire severity and landscape curvature on fisher scat location. At coarser scales, fisher colonization rates reflected avoidance of areas dominated by higher-severity fire, and fishers had a higher probability of being found in areas with larger and more contiguous patches of low-severity fire. Our results suggest that fisher use of postfire landscapes may center on low-severity or unburned islands (i.e., fire refugia/fire skips) and on fine-scale topographic features associated with landscape concavity, such as ravines or topographic depressions. Efforts to promote a sustainable low- to mixed-severity fire regime that creates habitat heterogeneity and forest resiliency can support fisher conservation in this region, and restoration efforts that capitalize on connecting fire refugia using microtopography as a foundation may facilitate habitat connectivity for fishers more rapidly than traditional methods.

Fire Management 24/7/365: Report of a workshop on the mitigation of wildfire risk in the mixed conifer forests of California

Extreme and catastrophic fire events over the past few years have caused fire and resource agencies to reassess their priorities and consider ways to more effectively manage risk, as well as restore fire-dependent ecosystems. In 2020, in California alone, 4.2 million acres burned, 10,500 structures were destroyed or damaged, and 31 people were killed. Nationally, 10.3 million acres burned. And 2020 was a replay of 2019, 2018, and 2017. As a result, many members of the George Wright Society (GWS) have been drawn into the crisis, as well as the efforts to find solutions. One member of GWS suggested a workshop to find ways to increase prescribed burning and other needed treatments in the Sierra Nevada, to reduce the risk of megafires. After consulting agencies and other partners, GWS organized "Fire Management 24/7/365: A Workshop on the Mitigation of Wildfire Risk in the Mixed Conifer Forests of California" in February 2021. This paper is an account of the workshop, which we hope will result in enduring collaborations.

Compaction and organic matter retention in mixed-conifer forests of California: 20-year effects on soil physical and chemical health

Soil resilience following harvest disturbance was studied at nine Long-Term Soil Productivity sites across the Sierra Nevada mountain range of California. We tested whether soil compaction, surface organic matter (OM) retention, and plant diversity have lasting effects on soil C, N, bulk density, or porosity. Study treatments following clearcut harvesting included soil compaction (none, moderate, severe) in factorial combination with three levels of OM retention (harvest slash retained, [OM2]; no slash retained, [OM1]; bare soil, [OM0]) and two plant diversity treatments (trees only; trees and shrubs). The sites encompassed a wide range in soil OM (5 to 12%) and clay content (5 to 36%) and represented common soil types in Sierran forests. At year 10, soil bulk densities were 10–29% greater for severely compacted compared to non-compacted plots across sites, corresponding to an average loss of 9–11% total porosity. Partial recovery was found by year 20, as bulk densities were 2–17% higher for compacted compared to non-compacted plots. In contrast to these physical responses, compaction had no adverse effect on soil C and N content at either year 10 or 20. Slash retention (OM2 versus OM1) and plant diversity treatments also had few effects on soil properties with the exception of a small increase in bulk density in the absence of shrubs, presumably due to reduced root and surface litter input. A more pronounced response was found following complete removal of surface organics (OM0), which produced a study-wide decline (12–17%) in soil C, N, and bulk density by year 20. All treatment responses were fairly consistent across sites, regardless of site differences in clay or soil OM content. Our results suggest substantial tolerance of soil C and N to harvest disturbance in Sierra Nevada forests despite a sustained change in soil physical properties and highlight the importance of the O horizon to long-term soil health.

Decreased snowpack and warmer temperatures reduce the negative effects of interspecific competitors on regenerating conifers

The persistence and distribution of species under changing climates can be affected by both direct effects of the environment and indirect effects via biotic interactions. However, the relative importance of direct and indirect climate effects on recruitment stages is poorly understood. We conducted a manipulative experiment to test the multiway interaction of direct and competition-mediated effects of climate change on vegetation dynamics. Following stand-replacing fire in California mixed-conifer forest, we seeded two conifer species, Pinus ponderosa and Abies concolor, in two consecutive years, one relatively normal and the other with an unusually wet and snowy winter followed by a hot summer. We additionally manipulated snow amount and competitive environment for both years. We found the effects of the snowpack treatment were contingent upon other abiotic factors (year of seeding) and biotic factors (shrub competition). Under ambient snowpack, shrubs reduced recruitment of P. ponderosa seedlings, but this negative effect disappeared with reduced snowpack. Additionally, the effects of shrubs on seedlings differed between cohorts and by life stage. In a warmer future, decreased snowpack may increase seedling emergence, but hotter and drier summers will decrease seedling survival; the effects of shrubs on conifers may become less negative as temperatures increase.

From the stand scale to the landscape scale: predicting the spatial patterns of forest regeneration after disturbance.

Shifting disturbance regimes can have cascading effects on many ecosystems processes. This is particularly true when the scale of the disturbance no longer matches the regeneration strategy of the dominant vegetation. In the yellow pine and mixed conifer forests of California, over a century of fire exclusion and the warming climate are increasing the incidence and extent of stand-replacing wildfire; such changes in severity patterns are altering regeneration dynamics by dramatically increasing the distance from live tree seed sources. This has raised concerns about limitations to natural reforestation and the potential for conversion to non-forested vegetation types, which in turn has implications for shifts in many ecological processes and ecosystem services. We used a California region-wide data set with 1,848 plots across 24 wildfires in yellow pine and mixed conifer forests to build a spatially explicit habitat suitability model for forecasting postfire forest regeneration. To model the effect of seed availability, the critical initial biological filter for regeneration, we used a novel approach to predicting spatial patterns of seed availability by estimating annual seed production from existing basal area and burn severity maps. The probability of observing any conifer seedling in a 60-m2 area (the field plot scale) was highly dependent on 30-yr average annual precipitation, burn severity, and seed availability. We then used this model to predict regeneration probabilities across the entire extent of a "new" fire (the 2014 King Fire), which highlights the spatial variability inherent in postfire regeneration patterns. Such forecasts of postfire regeneration patterns are of importance to land managers and conservationists interested in maintaining forest cover on the landscape. Our tool can also help anticipate shifts in ecosystem properties, supporting researchers interested in investigating questions surrounding alternative stable states, and the interaction of altered disturbance regimes and the changing climate.

Quantifying Changes in Total and Pyrogenic Carbon Stocks Across Fire Severity Gradients Using Active Wildfire Incidents

Positive feedbacks between wildfire emissions and climate are expected to increase in strength in the future; however, fires not only release carbon (C) from terrestrial to atmospheric pools, they also produce pyrogenic C (PyC) which contributes to longer-term C stability. Our objective was to quantify wildfire impacts on total C and PyC stocks in California mixed-conifer forest, and to investigate patterns in C and PyC stocks and changes across gradients of fire severity, using metrics derived from remote sensing and field observations. Our unique study accessed active wildfires to establish and measure plots within days before and after fire, prior to substantial erosion. We measured pre- and post-fire aboveground forest structure and woody fuels to calculate aboveground biomass, C and PyC, and collected forest floor and 0–5 cm mineral soil samples. Immediate tree mortality increased with severity, but overstory C loss was minimal and limited primarily to foliage. Fire released 85% of understory and herbaceous C (comprising <1.0% of total ecosystem C). The greatest C losses occurred from downed wood and forest floor pools (19.3 ± 5.1Mg ha−1 and 25.9 ± 3.2Mg ha−1, respectively). Tree bark and downed wood contributed the greatest PyC gains (1.5 ± 0.3Mg ha−1 and 1.9 ± 0.8Mg ha−1, respectively), and PyC in tree bark showed non-significant positive trends with increasing severity. Overall PyC losses of 1.9 ± 0.3Mg ha−1 and 0.5 ± 0.1Mg ha−1 occurred from forest floor and 0–5 cm mineral soil, with no clear patterns across severity. Fire resulted in a net ecosystem PyC gain (1.0 ± 1.0Mg ha−1) across aboveground and belowground components of these forests, and there were no differences among severity levels. Carbon emissions represented only 21.6% of total forest C; however, extensive conversion of C from live to dead pools will contribute to large downed wood C pools susceptible to release in a subsequent fire, indicating that there may be a delayed relationship between fire severity and C emissions. This research advances understanding of forest C loss and stabilization as PyC in wildfires; however, poor relationships between C and PyC gains or losses and fire severity highlight the complexity of fire impacts on forest C.

Evaluating a new method for reconstructing forest conditions from General Land Office survey records.

Historical forest conditions are often used to inform contemporary management goals because historical forests are considered to be resilient to ecological disturbances. The General Land Office (GLO) surveys of the late 19th and early 20th centuries provide regionally quasi-contiguous data sets of historical forests across much of the Western United States. Multiple methods exist for estimating tree density from point-based sampling such as the GLO surveys, including distance-based and area-based approaches. Area-based approaches have been applied in California mixed-conifer forests but their estimates have not been validated. To assess the accuracy and precision of plotless density estimators with potential for application to GLO data in this region, we imposed a GLO sampling scheme on six mapped forest stands of known densities (159-784 trees/ha) in the Sierra Nevada in California, USA, and Baja California Norte, Mexico. We compared three distance-based plotless density estimators (Cottam, Pollard, and Morisita) as well as two Voronoi area (VA) estimators, the Delincé and mean harmonic Voronoi density (MHVD), to the true densities. We simulated sampling schemes of increasing intensity to assess sampling error. The relative error (RE) of density estimates for the GLO sampling scheme ranged from 0.36 to 4.78. The least biased estimate of tree density in every stand was obtained with the Morisita estimator and the most biased was obtained with the MHVD estimator. The MHVD estimates of tree density were 1.2-3.8 times larger than the true densities and performed best in stands subject to fire exclusion for 100yr. The Delincé approach obtained accurate estimates of density, implying that the Voronoi approach is theoretically sound but that its application in the MHVD was flawed. The misapplication was attributed to two causes: (1) the use of a crown scaling factor that does not correct for the number of trees sampled and (2) the persistent underestimate of the true VA due to a weak relationship between tree size and VA. The magnitude of differences between true densities and MHVD estimates suggest caution in using results based on the MHVD to inform management and restoration practices in the conifer forests of the American West.

Climate Change Refugia, Fire Ecology and Management

Early climate change ideas warned of widespread species extinctions. As scientists have probed more deeply into species responses, a more nuanced perspective emerged indicating that some species may persist in microrefugia (refugia), including in mountainous terrain. Refugia are habitats that buffer climate changes and allow species to persist in—and to potentially expand under—changing environmental conditions. While climate and species interactions in refugia have been noted as sources of uncertainty, land management practices and disturbances, such as wildland fire, should also be considered when assessing any given refugium. Our landscape scale study suggests that cold-air pools, an important type of small-scale refugia, have unique fire occurrence, frequency, and severity patterns in frequent-fire mixed conifer forests of California’s Sierra Nevada: cold-air pool refugia have less fire and if it occurs, it is lower severity. Therefore, individuals and small populations are less likely to be extirpated by fire. Active management, such as restoration and fuels treatments for climate change adaptation, may be required to maintain these distinctive and potentially important refugia.

Effects of fuel treatments on California mixed-conifer forests

Land managers implement forest fuel reduction treatments, including prescribed fire, mastication, and hand- and mechanical thinning, to modify wildfire behavior. Fuel treatments decrease tree density, increase mean canopy base height and remove surface fuels, and have been shown to reduce fire severity in yellow pine and mixed-conifer forests, even under relatively severe weather conditions. However, less is known about the impacts of fuel treatments on other facets of forest ecology. Synthesizing evidence from the scientific literature regarding their effects on forest structure, carbon, vegetation, soils, wildlife and forest pests, we found a developing consensus that fuel treatments, particularly those that include a prescribed fire component, may have neutral to positive effects on a number of ecological processes in frequent-fire coniferous forests and may increase forest resilience to future disturbance and stress.

Decay patterns and carbon density of standing dead trees in California mixed conifer forests

Dead wood plays important structural and biogeochemical roles in forest ecosystem processes. Some aspects of woody debris dynamics have been carefully studied, but the decay patterns and carbon density of standing dead (SD) trees are only weakly characterized. Climbing forest mortality rates are also driving increases in the creation and abundance of SD trees. All forms of forest carbon accounting, from stand-level biomass calculations to dynamic earth systems models, are improved by a better understanding of SD tree physical and chemical traits. Using dimensional analysis, we described the patterns of density, carbon concentration, and net carbon density in decaying SD trees of six California mixed conifer species. As decay class advanced, trees showed a progressively lower density and a small increase in carbon concentration. Net carbon density of the most decayed SD trees was only 60% that of live trees. The key characteristics of SD trees that determine these patterns are species, surface to volume ratio, and relative position within the tree. The decay of SD trees and how deadwood biomass is estimated in large scale inventories also have repercussions in greenhouse gas accounting. When the measured changes in carbon density are applied to SD carbon stock estimates for California mixed conifer forests, the decay-adjusted estimates are 3.66–3.74Tg (18%) lower than estimates that do not incorporate change due to decay.

Spatial heterogeneity of soil CO2 efflux after harvest and prescribed fire in a California mixed conifer forest

Spatial variability is a key factor when quantifying soil CO2 efflux and punctual measurements need to be extended to larger stand, ecosystem, or regional scales. Spatial variation also affects comparisons among ecosystems, as when quantifying effects of disturbances on ecosystem carbon dynamics. However, spatial variability of soil CO2 efflux is still unknown and difficult to predict. We quantified the effects of silvicultural practices (prescribed fire and harvesting) on spatial variability of soil CO2 efflux in a mixed conifer forest from the central Sierra Nevada in California, USA. Soil CO2 efflux was measured using a portable chamber system, on 20–29 locations in four treatment sites: an untreated control, a prescribed fire site (burned in 2002 and 2009) and two clear cut sites harvested in 2010. In one of the harvested sites the soils were mechanically ripped to reduce soil compaction, a common practice done on industrial timber forest lands in the Sierra Nevada. Results showed that disturbance increased spatial variability of soil CO2 efflux. Coefficient of variability increased from an annual average of 32% at the control site to 37% at the burned site, and 49–51% at the harvested sites (without and with soil ripping, respectively), mirroring post-disturbance increases in spatial variability of soil temperature and soil water content. Because of the post-harvest increase in spatial variability, the ability to detect differences became lower, and the number of samples needed to obtain a value representative of the full population mean (within a 10% range) increased by 100%, from 60 to 120 samples. To reduce uncertainty in our soil CO2 efflux treatment estimates, more than 10–15 randomly selected locations per study site were necessary. Spatial variability of soil CO2 efflux at our sites was not affected by distance between measurement locations, was correlated to fine root and litter biomass at the control site, negatively correlated to soil bulk density at the fire site, and un-correlated to soil temperature and water content at all sites. The increase of spatial variability in soil CO2 efflux after disturbance and the requirement for a sufficient number of measurement locations should be considered when quantifying carbon dynamics of disturbed ecosystems, or assessing effects of different forest management practices.

Fuel treatment impacts on wildfire severity and plant communities in dry mixed conifer forests of California

Fuel treatment impacts on wildfire severity and plant communities in dry mixed conifer forests of California

Differential responses of Pinus ponderosa and Abies concolor foliar characteristics and diameter growth to thinning and prescribed fire treatments

Thinning and prescribed fire decrease the threat of catastrophic wildfire in western coniferous forests by reducing forest biomass available as fuel. Such treatments have been shown to alter foliar nitrogen (N) and phosphorus (P) concentrations, which may precede changes in stem diameter growth. However, the timing or duration of change in foliar nutrient concentrations of mature conifers is not well understood. I evaluated whether large-scale thinning and prescribed fire treatments impact nutrient characteristics and diameter growth of mature Pinus ponderosa (pine) and Abies concolor (fir) in a suite of experimental treatments (Size-preference Thin, Pine-preference Thin, Thin+Fire, and Fire) and a Control in the Klamath National Forest, CA. I analyzed pine needles to age 6years and fir needles to age 10years for N and P concentration and content. Vector analysis was used to simultaneously evaluate needle size, nutrient concentration and content to illustrate responses that may not be evident via analysis of nutrient concentration alone. There were no differences in foliar N or P concentration or content for pine or fir for any treatment relative to the Control. Nutrient resorption efficiency and proficiency were not affected by treatment for either species. The length of pine needles produced in 2001 was increased by all treatments that involved thinning, and the mass of needles produced in 2004 was increased by the combination of thinning and prescribed fire. There was no effect of treatment on fir needle length or mass for any cohort of needles. Vector diagrams indicated that pine needle N and P were diluted by treatments as needles increased in mass and total nutrient content while nutrient concentration decreased or remained constant. In contrast, fir needle nutrient concentrations increased or remained constant while dry mass and nutrient content decreased. Vector analysis indicated that the response in foliar characteristics of mature conifers differed between the historically dominant pine and the currently dominant, shade-tolerant fir in this northern California mixed-conifer forest, whereas traditional analysis of individual foliar characteristics generally did not detect differences among treatments. All treatments that involved thinning increased mean annual ring width index between pre- and post-treatment time periods for pine, whereas there was no effect for fir. These results suggest that restoration strategies may have negative or no effects on fir whereas the same treatments may favor pine, thereby facilitating re-establishment of a pine-dominated ecosystem that approximates historical conditions in California’s mixed-conifer forests.