Bark Thickness in Coast Redwood (Sequoia sempervirens (D.Don) Endl.) Varies According to Tree- and Crown Size, Stand Structure, Latitude and Genotype

Research Highlights: Bark thickness (BT) in coastal redwood (Sequoia sempervirens (D.Don) Endl.) varies in accordance with tree size, crown ratio, position within the canopy, height along the tree stem, genetic identity and latitude. However, current BT predictive equations do not account for such variability, leading to inaccurate BT estimations. We present improved BT models to increase the accuracy of BT estimates for coastal redwood in northern California. Background and Objectives: BT is an important metric that has many practical applications in forest management. However, BT varies substantially across species and environments, as well as across individuals and populations. Our objectives were to investigate BT along various gradients of change, with factors accounting for genetics, tapering of BT along the tree bole, differences in BT according to tree crown position within the stand, and the latitude. Materials and Methods: We collected BT data throughout most of redwood’s natural range along a north–south latitudinal gradient. Subsets of these data were used to examine the influence of particular variables on BT while holding the other variables constant. Results: Regionally, the bark was thicker among more xeric southern redwoods and thinner among more mesic northern redwoods. We found that the BT of codominant, intermediate and suppressed trees was around 8%, 14%, and 18% thicker, respectively, than bark of the same size dominant tree. Redwood trees growing in the partial shade of an overstory had thicker bark than trees growing in even-aged stands and incorporating genetic identity yielded major improvements in the BT model estimates, suggesting that BT is under genetic control. Bark thickness decreased with increasing height along the tree stem, with notable differences in the BT above and below breast height. Conclusions: We recommend utilizing the best available BT equations (over standard ‘bark factors’) in forest science, modeling and management applications. We also recommend the adoption of our drilling method for BT measurement on larger trees due to the potential for error associated with traditional bark gauge measurements

Bark Thickness Equations for Mixed-Conifer Forest Type in Klamath and Sierra Nevada Mountains of California

We studied bark thickness in the mixed-conifer forest type throughout California. Sampling included eight conifer species and covered latitude and elevation gradients. The thickness of tree bark at 1.37 m correlated with diameter at breast height (DBH) and varied among species. Trees exhibiting more rapid growth had slightly thinner bark for a given DBH. Variability in bark thickness obscured differences between sample locations. Model predictions for 50 cm DBH trees of each species indicated that bark thickness was ranked Calocedrus decurrens > Pinus jeffreyi > Pinus lambertiana > Abies concolor > Pseudotsuga menziesii > Abies magnifica > Pinus monticola > Pinus contorta. We failed to find reasonable agreement between our bark thickness data and existing bark thickness regressions used in models predicting fire-induced mortality in the mixed-conifer forest type in California. The fire effects software systems generally underpredicted bark thickness for most species, which could lead to an overprediction in fire-caused tree mortality in California. A model for conifers in Oregon predicted that bark was 49% thinner in Abies concolor and 37% thicker in Pseudotsuga menziesii than our samples from across California, suggesting that more data are needed to validate and refine bark thickness equations within existing fire effects models

76-Year Decline and Recovery of Aspen Mediated by Contrasting Fire Regimes: Long-Unburned, Infrequent and Frequent Mixed-Severity Wildfire

Quaking aspen (Populus tremuloides) is a valued, minor component on northeastern California landscapes. It provides a wide range of ecosystem services and has been in decline throughout the region for the last century. This decline may be explained partially by the lack of fire on the landscape due to heavier fire suppression, as aspen benefit from fire that eliminates conifer competition and stimulates reproduction through root suckering. However, there is little known about how aspen stand area changes in response to overlapping fire. Our study area in northeastern California on the Lassen, Modoc and Plumas National Forests has experienced recent large mixed-severity wildfires where aspen was present, providing an opportunity to study the re-introduction of fire. We observed two time periods; a 52-year absence of fire from 1941 to 1993 preceding a 24-year period of wildfire activity from 1993 to 2017. We utilized aerial photos and satellite imagery to delineate aspen stands and assess conifer cover percent. We chose aspen stands in areas where wildfires overlapped (twice-burned), where only a single wildfire burned, and areas that did not burn within the recent 24-year period. We observed these same stands within the first period of fire exclusion for comparison (i.e., 1941–1993). In the absence of fire, all aspen stand areas declined and all stands experienced increases in conifer composition. After wildfire, stands that burned experienced a release from conifer competition and increased in stand area. Stands that burned twice or at high severity experienced a larger removal of conifer competition than stands that burned once at low severity, promoting expansion of aspen stand area. Stands with less edge:area ratio also expanded in area more with fire present. Across both time periods, stand movement, where aspen stand footprints were mostly in new areas compared to footprints of previous years, was highest in smaller stands. In the fire exclusion period, smaller stands exhibited greater loss of area and changes in location (movement) than in the return of fire period, highlighting their vulnerability to loss via succession to conifers in the absence of disturbances that provide adequate growing space for aspen over time

Collecting seed at the hot, dry margins of a tree species??? natural range: do the progeny of these extreme trees have drought tolerance and water-use efficiency needed for restoration of harsh sites?

Trees growing in the hottest/driest parts of their natural range may be better-adapted to hot, dry conditions and hold promise for restoration and resistance to climate change. If drought tolerance and water-use efficiency were heritable genetic traits, we could select for these characteristics and raise seedlings for reforestation on marginal sites or in areas where adverse changes in climate were forecast. 3000 redwood seedlings originating from wet and dry locations were planted by HSU students in 2010 and 2011. The experiment provides insights into outcomes of reclamation and ???assisted migration??? forest conservation/restoration strategies and impact of climate change on redwood forests.Sponsored by the Office of Research, Economic and Community Development, University Advancement and the HSU Library in collaboration with the Colleges

Multiaged redwood responds well to partial harvest and herbicide treatments

Chemical control of unwanted trees can be a cost-efficient tool for forest management and restoration. In California, United States, the response of merchantable conifers to hardwood control is poorly understood. We studied the tree growth of coast redwood (Sequoia sempervirens (Lamb. ex D. Don) Endl.) following herbicide frill treatment of competing tanoak (Notholithocarpus densiflorus (Hook. & Arn.) Manos, C.H. Cannon, & S. Oh), coinciding with a partial harvest of conifers. The radial growth of 420 redwoods in 45 plots was measured using increment cores. With or without partial harvesting, herbicide treatment of tanoak enhanced growth of most redwoods: 23% of redwoods in herbicide-only plots and 34% of redwoods in herbicide + harvest plots had ≥100% higher posttreatment basal area increment (BAI). In untreated plots, 67% of redwoods displayed declining BAI. The response of redwoods (the ratio of 8-year postharvest BAI to 8-year preharvest BAI) was 59% higher in herbicide-only plots and 108% higher in herbicide + harvest plots compared with untreated control plots over the same period. Redwoods with long crowns maintained rapid growth with or without treatment. Trees growing slowly before treatment exhibited the greatest response, provided that they had relatively long crowns and were not left in suppressed crown positions. Forest managers implementing partial harvesting and (or) chemical control of hardwoods can expect to maintain or promote rapid growth of most residual redwoods.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Second-Log Branching in Multiaged Redwood and Douglas-Fir: Influence of Stand, Site, and Silviculture

We studied branching in Sequoia sempervirens (Lamb. ex D. Don) Endl. (coast redwood) and Pseudotsuga menziesii (Mirb.) Franco var. menziesii (coast Douglas-fir) because of their commercial value to coastal northern California. We focused on branching in the second log, which constitutes an important part of a tree’s wood volume and potential value. We quantified branch size and branch growth of overstory trees in multiaged stands in Mendocino County, California, in response to topographic, silvicultural, and stand- and tree-related variables. Higher stand density—a measure of competition averaged across the sample plot—did not correlate with size of the largest second-log branch measured but was associated with a smaller average diameter of the largest branches measured on all sides of the study tree. The largest branch measured was smaller when in closer proximity to branches of its immediate neighbor tree. Redwood had larger branches than Douglas-fir but their size was more sensitive to an ecological gradient of soil-moisture deficit. Branches responded differently to individual tree selection harvest of conifers versus herbicide control of hardwoods. Residual conifer branches in harvested plots responded almost immediately with increased growth, but this release was short-lived. Branches in herbicide-treated plots exhibited a delayed release, giving more consistent branch growth throughout two five-year measurement periods after treatment

Collecting tree seed in hot, dry places: Do progeny of these trees have drought tolerance and water-use efficiency needed for restoration of harsh sites?

Trees growing in the hottest/driest parts of their natural range may be better-adapted to hot, dry conditions and hold promise for restoration and resistance to climate change. If drought tolerance and water-use efficiency were heritable traits, we could select for these characteristics and raise seedlings for reforestation on marginal sites or in areas where adverse changes in climate were forecast. We sought to quantify heritability of these traits in coast redwood, a species with limited range and thought to be dependent on moist soils and mild summer temperatures moderated by coastal fog. The species is threatened by declining fog and warming trends at the drier extremes of its range, inland and to the south. We collected seed on dry ridges and upper slopes from the southernmost populations and inland locations where redwood experiences the hottest summertime temperatures. Our expectation was that - when planted on drier sites - their offspring would outperform seedlings from moister, more northern, coastal locations. Conversely, we expected seedlings adapted to moister locales to outperform the ‘dry site’ redwoods when planted on moister northern sites. Coast redwood seedlings from southern/dry sites (from Monterey to Napa Counties), central sites (Mendocino County), and northern sites (Humboldt County) were outplanted in a replicated interlocking hexagonal experimental design, at a cool, wet northern coastal site and a hot, dry, high-elevation site (outside of redwood’s natural range) in Humboldt County, and at a hot, dry site in Mendocino County, California. A total of almost 3000 seedlings were planted by Humboldt State University students in 2010 and 2011. Seedling health status and height have been measured repeatedly. The experiment will continue to provide insights into outcomes of reclamation and “assisted migration” forest conservation/restoration strategies and the impact of climate change on regional sub-populations better- or worse-adapted to the new climate. A portion of the seed collected from each tree is in long-term storage at the state seed bank in Davis, California, to conserve their genes. This will allow for future tree breeding or restoration of vulnerable, isolated, small populations at the fringes of their natural range. This presentation describes the seed collection procedure and field experiment design, and shows how hundreds of students studying forestry and ecological restoration have been actively involved in this long-term study