California Conifer Research Articles

Rapid and varied responses of songbirds to climate change in California coniferous forests

The global climate is changing faster than previously anticipated. Although scientists expect cumulatively deleterious impacts to birds and other wildlife, effects on individual species are likely more complex. The California Department of Fish and Wildlife has monitored >100 songbirds across 37,600km2 of Northern California conifer forests for close to two decades to facilitate informed, science-based conservation planning. The study area represents 42% of all conifer forests in the state. Autonomous sound recorders were used to survey birds at 1065 randomly distributed sites from 2002 to 2016. The richness of Neotropical migrants declined below 1515m (90% CI: 1150–1950m) elevation whereas it increased above this threshold after controlling for changes in tree cover due to forestry and wildfire. This finding suggests an overall upward shift in Neotropical migrant distributions in response to an annual 0.037°C (90% CI: 0.029–0.045°C) increase in mean daily May temperature during the timespan. Residents and altitudinal migrants may be less vulnerable to increasing temperatures, conversely, as evidence of elevational shifting was much weaker or non-existent for them. Yet for individual species, there were both declines and increases in occupancy. Rapid and mixed population trends, in combination with elevational range shifts, suggest that songbirds vary widely in their capacity to adapt to climate change and other stressors. Conservation of structurally-complex and fire resilient forests above ~1500m elevation is paramount in helping to buffer songbirds against rising temperatures. The expansion of biodiversity monitoring across large taxonomic, spatial, and temporal extents is vital to effective conservation planning.

Status and trends of fire activity in southern California yellow pine and mixed conifer forests

Frequent, low to moderate severity fire in mixed conifer and yellow pine forests of California played an integral role in maintaining these ecosystems historically. Fire suppression starting in the early 20th century has led to altered fire regimes that affect forest composition, structure and risk of vegetation type conversion following disturbance. Several studies have found evidence of increasingly large proportions and patches of high severity fire in fire-deprived conifer forests of northern California, but few studies have investigated the impacts of fire suppression on the isolated forests of southern California. In this study, spatial data were used to compare the current fire return interval (FRI) in yellow pine and mixed conifer forests of southern California to historical conditions. Remotely sensed burn severity and fire perimeter data were analyzed to assess changes in burn severity and fire size patterns over the last 32–100 years. Half of the yellow pine and mixed conifer forest in this study has missed multiple burn opportunities and has not experienced fire in the 109-year fire record. The average proportion of conifer forests burned in high severity fire (>90% tree basal area loss), average high severity patch size, and maximum high severity patch size all increased significantly from 1984 to 2016. The average fire proportion burned at high severity from 2000 to 2016 is 1.5 times higher than predicted for the natural range of variation (NRV). Additionally, the years after 2000 had high severity patches larger than 25 ha, whereas no fires before 2000 had patches this large, indicating a deviation from NRV since the turn of the century. Fire size in conifer forests significantly increased from 1910 to 2016, owed to a substantial increase in the occurrence of large fires (larger than the natural range of variation) after 2000. This analysis indicates that southern California conifer forests are like their northern counterparts in that they have burned very infrequently since the early 1900s, resulting in large and homogenous areas of stand replacing burns. This is likely exacerbated even further by recent fire-conducive weather conditions and extended periods of drought. In southern California, recovery from large, high severity burns is likely to be impeded by the small and disparate nature of mixed conifer forests and limited seed dispersal capabilities of remaining trees. Therefore, preemptive forest treatments and careful fire management is needed to return natural structure and function to these forests.

Changing spatial patterns of stand-replacing fire in California conifer forests

Abstract Stand-replacing fire has profound ecological impacts in conifer forests, yet there is continued uncertainty over how best to describe the scale of stand-replacing effects within individual fires, and how these effects are changing over time. In forests where regeneration following stand-replacing fire depends on seed dispersal from surviving trees, the size and shape of stand-replacing patches are critical metrics that are difficult to describe and often overlooked. We used a novel, recently-developed metric that describes the amount of stand-replacing area within a given distance of a live-tree patch edge, in order to compare fires that may be otherwise similar in fire size or the percentage of stand-replacing effects. Specifically, we analyzed 477 fires in California pine, fir, and mixed-conifer forests between 1984 and 2015 and asked whether this metric, the stand-replacing decay coefficient (SDC), has changed over time, whether it is affected by fire management, and how it responds to extreme weather conditions at the time of the fire. Mean annual SDC became smaller over time (significantly so in the Sierra Nevada region), indicating that stand-replacing patches became larger and more regularly shaped. The decrease in SDC was particularly pronounced in the years since 2011. While SDC is correlated with percent high-severity, it is able to distinguish fires of comparable percent high-severity but different spatial pattern, with fires managed for suppression having smaller SDC than fires managed for resource benefit. Similarly, fires managed by the US Forest Service had smaller SDC than fires managed by the National Park Service. Fire weather also played an important role, with higher maximum temperatures generally associated with smaller SDC values. SDC is useful for comparing fires because it is associated with more conventional metrics such as percent high-severity, but also incorporates a measure of regeneration potential – distance to surviving trees at stand-replacement patch edges – which is a biological legacy that directly affects the resilience of forests to increasingly frequent and severe fire disturbances. We estimate that from 1984 to 2015, over 80,000 ha of forestland burned with stand-replacing effects greater than 120 m in from patch edges, denoting areas vulnerable to extended conifer forest loss due to dispersal limitation. Managing unplanned ignitions under less extreme weather conditions can achieve beneficial “fine-grained” effects of stand-replacing fire where regeneration limitation is less of a concern. Because SDC is a useful single metric to compare fires, we introduce a web application (stevensjt.shinyapps.io/sdc_app) to calculate SDC for any high-severity spatial layer that may be of interest.

Predicting mortality for five California conifers following wildfire

Fire injury was characterized and survival monitored for 5677 trees >25 cm DBH from five wildfires in California that occurred between 2000 and 2004. Logistic regression models for predicting the probability of mortality 5-years after fire were developed for incense cedar ( Calocedrus decurrens (Torr.) Florin), white fir ( Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), sugar pine ( Pinus lambertiana Douglas), Jeffrey pine ( P. jeffreyi Balf.), and ponderosa pine ( P. ponderosa C. Lawson). Differences in crown injury variables were also compared for Jeffrey and ponderosa pine. Most mortality (70–88% depending on species) occurred within 2 years post-wildfire and had stabilized by year 3. Crown length and crown volume injury variables predicted tree mortality equally well; however, the variables were not interchangeable. Crown injury and cambium kill rating was significant in predicting mortality in all models. DBH was only a significant predictor of mortality for white fir and the combined ponderosa and Jeffrey pine models developed from the McNally Fire; these models all predicted increasing mortality with increasing tree size. Red turpentine beetle ( Dendroctonus valens) was a significant predictor variable for sugar pine, ponderosa pine, and Jeffrey pine; ambrosia beetle ( Trypodendron and Gnathotrichus spp.) was a significant predictor variable for white fir. The mortality models and post-fire tree survival characteristics provide improved prediction of 5-year post-wildfire tree mortality for several California conifers. The models confirm the overall importance of crown injury in predicting post-fire mortality compared to other injury variables for all species. Additional variables such as cambium kill, bark beetles, and tree size improved model accuracies, but likely not enough to justify the added expense of data collection.

Vegetation dynamics and exotic plant invasion following high severity crown fire in a southern California conifer forest

Early post-fire vegetation dynamics following large, severe forest fires are largely unknown for the southern California mountains owing to historic fire suppression. Vegetation in 38 forest stands was surveyed (2004, 2005, and 2007) following the 2003 Cedar Fire in the Cuyamaca Mountains, Peninsular Ranges, San Diego County, California, USA. Each stand was sampled using four 10-m radius plots for the tree stratum, and 20 1-m2 quadrats for shrub and herb strata. Changes in canopy cover by species, origin (native and exotic) and life form were analyzed. 2007 data were subjected to clustering to examine the divergence in species composition of the stands with time. Shrub cover increased from 3 to 31%, and exotic herbaceous cover increased from 3 to 40%. Cover of native annuals had increased from 2004 (17%) to 2005 (33%), but then dropped to 15% in 2007. Forty percent of the stands were dominated by the shrub species Ceanothus palmeri, and associated with higher pre-fire conifer cover and fire severity. More than 50% of the stands were dominated by exotic annuals and associated with lower fire severity and less steep slopes. The remaining stands (<10%) were dominated by chaparral shrubs and occurred on lower elevation, steep west-facing slopes. Species traits predict their dynamics following disturbance, as environmental conditions change. Establishment and increasing abundance of species dependent on dispersal to reach a site, including exotic and native herbaceous species, occurred in years 2–4. Differences among stands in species composition 4 years post-fire were associated with topographic and fire severity gradients.

Influence of Pre-Fire Tree Mortality on Fire Severity in Conifer Forests of the San Bernardino Mountains, California

High tree mortality due to drought and insects often is assumed to increase fire severity once ignition occurs. In 2002-2003, coniferous forests in the San Bernardino Mountains, California experienced a significant tree mortality event due to drought and an outbreak of western pine beetles (Dendroctonus brevicomis). In October 2003, fire burned approximately 5,860 ha of conifer forest types in many beetle- and drought-affected stands where most pre-fire dead trees had retained needles. We used pre- and post-fire GIS data to examine how fire severity was affected by pre-fire tree mortality, vegetation characteristics, and topography. We found no evidence that pre-fire tree mortality influenced fire severity. These results indicate that widespread removal of dead trees may not effectively reduce higher-severity fire in southern California's conifer forests. We found that sample locations dominated by the largest size class of trees (>61 cm diameter at breast height (dbh)) burned at lower severities than locations dominated by trees 28-60 cm dbh. This result suggests that harvesting larger-sized trees for fire-severity reduction purposes is likely to be ineffective and possibly counter-productive.

Mineral Assemblage and Aggregates Control Carbon Dynamics in a California Conifer Forest

Uncertainty about the effects of climate change on terrestrial soil organic C stocks has generated interest in clarifying the processes that underlie soil C dynamics. We investigated the role of soil mineralogy and aggregate stability as key variables controlling soil C dynamics in a California conifer forest. We characterized soils derived from granite (GR) and mixed andesite‐granite (AN) parent materials from similar forest conditions. Granite and AN soils contained similar clay mineral assemblages as determined by x‐ray diffraction (XRD), dominated by vermiculite, hydroxy‐interlayered vermiculite (HIV), kaolinite, and gibbsite. However, AN soils contained significantly more Al in Al‐humus complexes (6.2 vs. 3.3 kg m−2) and more crystalline and short‐range order (SRO) Fe oxyhydroxides (30.6 vs. 16.8 kg m−2) than GR soils. Andesite‐granite pedons contained nearly 50% more C relative to GR soils (22.8 vs. 15.0 kg m−2). Distribution of C within density and aggregate fractions (free, occluded, and mineral associated C) varied significantly between AN and GR soils. In particular, AN soils had at least twice as much mineral associated C relative to GR soils in all horizons. Based on 14C measurements, occluded C mean residence time (MRT) &gt; mineral C &gt; free C in both soil types, suggesting a significant role for aggregate C protection in controlling soil C turnover. We found highly significant, positive correlations between Al‐humus complexes, SRO Al minerals, and total C content. We suggest that a combination of aggregate protection and organomineral association with Al‐humus complexes and SRO Al minerals control the variation in soil C dynamics in these systems.

Pests of the native California conifers

Pests of the native California conifers

The relationship between tree diameter growth and climate for coniferous species in northern California

The difference between actual and predicted growth rates for the conifer regions of northern California has been observed to vary with climatic changes. This study presents a method to investigate the relationship between growth and climate. Growth variations attributable to biological and cultural factors were removed by using the CACTOS (California conifer timber output simulator) program. The remaining variation was then associated with relative precipitation and temperature for the projected period and the CACTOS calibration period. Climatic data from the current and preceding years were considered. Elevation, stand density, and species were also investigated to determine their effects on the format and magnitude of the relationship between growth and climate. The results of this study, which included tests of stem analysis data taken over 15 years, indicate that growth variation is associated with the climatic changes of winter precipitation and summer temperatures for the region, in addition to biological and cultural factors. Winter precipitation and summer temperatures affect growth in the current and the subsequent years. Moreover, the relationship between climate and growth changes by densities and species. This study provides a basis for using short-term growth data to make long-term growth projections with growth adjusted to long-term climatic conditions.

The relationship between tree diameter growth and climate for coniferous species in northern California

The difference between actual and predicted growth rates for the conifer regions of northern California has been observed to vary with climatic changes. This study presents a method to investigate the relationship between growth and climate. Growth variations attributable to biological and cultural factors were removed by using the CACTOS (California conifer timber output simulator) program. The remaining variation was then associated with relative precipitation and temperature for the projected period and the CACTOS calibration period. Climatic data from the current and preceding years were considered. Elevation, stand density, and species were also investigated to determine their effects on the format and magnitude of the relationship between growth and climate. The results of this study, which included tests of stem analysis data taken over 15 years, indicate that growth variation is associated with the climatic changes of winter precipitation and summer temperatures for the region, in addition to biol...

Adjustment of estimated tree growth rates in northern California conifers for changes in precipitation levels

Even with similar initial conditions, observed forest growth rates on permanent sample plots in the conifer region of northern California differ for different periods. Thus, individual-tree growth models built with growth parameters estimated from data from one period may not produce accurate estimates for another period unless some allowance is made for this variation in growth rates. Variation in growth rates of northern California conifers through time has been shown to be correlated with precipitation changes. A method is presented that adjusts periodic growth estimates for variation in precipitation between periods. This provides a basis for adjusting short-term growth data for making long-term growth projections. Perhaps more importantly, short-term inventory updates might be made more accurately.

Adjustment of estimated tree growth rates in northern California conifers for changes in precipitation levels

Adjustment of estimated tree growth rates in northern California conifers for changes in precipitation levels

Tip Length Models for Major Commercial California Conifers

Several models for predicting the length from the merchantable top to the tip of a tree are evaluated for major California conifers. A model reported by (Wensel and Krumland (1983)) was found to be the most useful because of its ability to predict for both total height or merchantable height. Coefficients are reported for eight commercially important conifers of California.

Tree Taper Model Volume Equations: II. Tree Taper Models for Major Commercial California Conifers

Equations are developed for eight conifer species to estimate the stem taper of inside-bark diameters from breast height to the tip of the tree. Separate equations are developed to estimate the diameter inside bark at the stump. Equations are fitted and tested on separate halves of a data set composed of tree taper data from previous studies, both from the forest industry and USDA Forest Service surveys. This composite data set extends from Southern California to the National Forests of southern Oregon. After an extensive examination of existing taper equations, two taper equations were selected for further analysis, one by Biging and the other by Wensel and Krumland. Coefficients for both equations are given for the eight conifer species examined. Because of a lower residual sum of squares, and a lack of correlation of the residual with the available predictors, the Wensel and Krumland equation is recommended for use.

Tree Taper Model Volume Equations: I. Bark Taper Equations for California Conifers

Upper-stem tree diameters are usually measured outside the bark with the only bark thickness measurements being made at breast height. The current study presents equations and coefficients for estimating the bark thickness in both the upper stem and at the stumps based upon the bark thickness at breast height, the size of the tree (DBH and total height), and the height to the measurement in question. The upper-stem model presented is an extension of a previously published general hyperbolic ratio model. The model for bark ratio below breast height, a simple power function, is estimated as well. Both equations contain coefficients that account for the more rapid taper on trees with thicker bark. The data used for fitting and testing the taper models consisted of measurements on over 3,000 conifer trees measured by members of the Northern California Forest Yield Cooperative and the USDA Forest Service. The data were split into two halves, one half for fitting and the other half for testing. The best upper-stem and below-DBH equations obtained from this analysis are discussed here; the remainder are listed in the Appendix.

Tree Taper Model Volume Equations: IV. Tree Volume Equations for Major California Conifers

Equations and tables are given to estimate total tree board foot and cubic foot volume for eight major California conifer species. Using taper equations developed in another study, log dimensions were estimated for the trees used in the stem taper study; then coefficients for whole-tree volume equations were fitted to the resulting set. This produced Scribner and International ¼-inch board volume, and cubic volume equations for the eight important commercial conifer species in California. For board-foot volumes, separate equations are given for both total and merchantable heights to various merchantable tops.

An Economic Assessment of Hardwoods as Habitat Components for Black-Tailed Deer in Northern California Conifer Forests

An Economic Assessment of Hardwoods as Habitat Components for Black-Tailed Deer in Northern California Conifer Forests

The Predictive Models and Procedures Used in the Forest Stand Generator (STAG)

The Forest Stand Generator, STAG, is a microcomputer-based program that uses statistical routines to produce a projection set composed of individual tree measurements of diameter at 4.5 feet above ground (called the diameter at breast height or DBH), total height, height-to-crown base, species, and tree expansion factor. When data sets are not complete, STAG can be used to produce a projection set for a wide class of inventory procedures. The authors will discuss the estimation procedures used by STAG to (1) fill in missing measurements of tree height, height-to-crown base, or both; (2) generate stands from summary statistics; and (3) convert stand table data—numbers of trees by DBH classes and species—to individual tree records, so that these projection sets, composed of complete individual tree records, can be analyzed by the California Conifer Timber Output Simulator (CACTOS) for simulation of tree growth and mortality, even though the initial data sets could not have been used with CACTOS. Also discussed are the predictive equations and analytic procedures used to produce a projection set for these three categories of data availability. Recommended uses of STAG and the type of data required for accurate development of projection sets are presented. Research reported here was conducted under UC Agricultural Experiment Station projects 3679-ms and 3815-ms. The authors gratefully acknowledge important contributions to this work by Peter J. Daugherty and Vaughan Landrum. They also are grateful to Lee C. Wensel and the members of the Northern California Forest Yield Cooperative for their support.

Estimation of crown form for six conifer species of northern California

Geometric models are presented for the prediction of crown volume and width at any height in the crown of six conifer species in the Sierra Nevada. Crown volume is defined as the geometric space occupied by the crown and is allometrically related to the diameter, height, and crown ratio of individual trees. Crown diameter is derived from crown volume, tree height, and crown ratio. The crown volumes and associated measures can be used to compute indices of individual tree competition such as those used in the CACTOS (California Conifer Timber Output Simulator) system or to compute other measures such as wildlife habitat suitability or insect damage potential. Estimation equations are developed by regression using data collected on crowns of 593 felled trees. The equations use dbh, total height, and crown ratio to estimate total crown volume, crown volume above a specified height, and cumulative crown cross sectional area at a specified height.

Soil Nitrogen Mineralization in a Clearcutting Chronosequence in a Northern California Conifer Forest

AbstractSoil ammonification, nitrification, and N mobility were studied for 1 yr in three Sierra Nevada (USA) mixed‐conifer stands to examine the long‐term influence of timber harvesting on soil N dynamics. Clearcutting had a persistent effect on soil N mineralization, detectable by in situ incubation but not by conventional low‐tension lysimetry. Mineralization rates were greater in 5‐ and 17‐yr‐old regenerated clearcuts than in an adjacent 100‐yr‐old uncut forest. Annual rates of N mineralization averaged 12 kg ha−1 for the uncut forest, and 49 and 31 kg ha−1 for the 5‐ and 17‐yr‐old clearcuts, respectively. Mineralization continued throughout the summer, even when soil matric potentials were &lt;‐1500 kPa. Soil solution NO3 concentrations were much higher in the younger clearcut than in either the older clearcut or the uncut forest. Sustained effects of clearcutting seem due largely to higher substrate availability and to moisture and temperature conditions favoring microbial activity. Chemical inhibition of nitrification may become important as the forest floor develops.