Simulated Fire Behavior and Fine-Scale Forest Structure Following Conifer Removal in Aspen-Conifer Forests in the Lake Tahoe Basin, USA

Quaking aspen is found in western forests of the United States and is currently at risk of loss due to conifer competition at within-stand scales. Wildfires in these forests are impactful owing to conifer infilling during prolonged fire suppression post-Euro-American settlement. Here, restoration cuttings seek to impact wildfire behavior and aspen growing conditions. In this study, we explored how actual and hypothetical cuttings with a range of conifer removal intensity altered surface fuel and overstory structure at stand and fine scales. We then simulated wildfires, examining fire behavior and effects on post-fire forest structures around aspen trees. We found that conifer removal constrained by lower upper diameter limits (\u3c56 cm) had marginal effects on surface fuel and overstory structure, likely failing to enhance resource conditions sufficiently to sustain aspen. Increasing the diameter limit also led to a higher likelihood of fire spread and a higher rate of spread, owing to greater within-canopy wind speed, though crown fire activity decreased. Our simulations suggest heavier treatments could facilitate reintroduction of fire while also dampening the effects of wildfires on forest structure. Cutting specifications that relax diameter limits and remove a substantial portion of conifer overstory could better promote aspen restoration and mitigate fire hazard

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 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

Effects of Slash Pile Burning after Restoring Conifer- Encroached Aspen

Removal of conifers encroaching aspen stands has been advocated and is being practiced in the Lake Tahoe Basin (EIP Project #10080: Aspen Community Restoration Projects). In remote and roadless areas, thinning of conifers is generating large volumes of wood and pile burning is currently being implemented to handle this biomass on site. However, the effects of pile burning on aspen are unknown, and there is an urgent need for guidelines to support design of thinning treatment prescriptions; specifically burn pile size and safe distances from live aspen trees of any size to prevent injury

Regeneration and Recruitment Correlate with Stand Density and Composition in Long-Unburned Aspen Stands Undergoing Succession to Conifer in the Sierra Nevada, USA

Succession of aspen stands to conifer in the Sierra Nevada Mountains of California and Nevada, USA, is being interrupted by forest managers thinning conifers to sustain aspen stands in situ. However, patterns of stand density, species composition, and regeneration prior to management intervention have scarcely been described. We established a grid of sample plots throughout nine aspen stands encircling Lake Tahoe in the central Sierra Nevada. The degree of succession to conifer throughout each aspen stand was spatially heterogeneous. Patches of pure aspen were rare. Stand density index (SDI) reached an upper limit of 1700 in pure aspen. As composition shifted in favor of conifer, SDI approached or attained a maximum of 2500 in some plots. Stand density and species composition data were tested as predictors of conifer and aspen regeneration densities in each plot. Conifer seedlings had an average density of 3261 ha-1; they were most abundant in aspen-dominated areas of any density, and in conifer-dominated areas of higher density. Aspen regeneration had an average density of 3211 ha-1 and was one order of magnitude less frequent in areas of pure conifer versus areas of pure aspen, but remained relatively abundant in most areas. Aspen saplings were rare (average density of 42 ha-1 for saplings 10-15 cm DBH), especially at high stand densities or in areas where conifers dominated. Our findings suggest that forest managers interested in sustaining aspen stands in situ will need to control stand density to promote recruitment of younger aspen to the overstory

Predicting Treatment Longevity after Successive Conifer Removals in Sierra Nevada Aspen Restoration

Populus tremuloides (quaking aspen) stands throughout the Sierra Nevada Mountains are undergoing succession to conifers. Removal of conifers is being tested, however, little is known about treatment longevity—the time taken for stand density to return to pretreatment levels. To determine longevity of treatments removing conifers below different size limits, we developed tree growth equations from data collected in 1 ha plots around Lake Tahoe in P. tremuloides stands with varying amounts of conifer, and simulated stand development after treatment in two stands. At Ward Creek, cutting all conifer \u3c 35 cm diameter at breast height (DBH) generated the most wood that could practically be piled and burned inside the stand, but only reduced stand density by 16%. After 13 years of predicted treatment longevity, a second treatment was simulated with options of light, medium, or heavy cutting (50, 60, or 75 cm DBH limits). This gave treatment longevity of 23, 29, and 40 years respectively but did not restore P. tremuloides dominance. At Cookhouse Meadow, cutting conifer \u3c 35 cm DBH had 16-year treatment longevity, after which time two treatments were compared. Cutting conifers \u3c 50 cm DBH enhanced P. tremuloides’ representation from 27% to 37% of stand basal area and had 23-year treatment longevity. Raising the DBH limit to 60 cm left P. tremuloides representing 45% of stand basal area, and extended treatment longevity to 36 years. Our findings indicate that a series of treatments will be needed to restore and maintain P. tremuloides communities, and will eventually require removal of large conifers (\u3e 75 cm DBH)

Understory Responses to Restoration in Aspen‐Conifer Forests Around the Lake Tahoe Basin: Residual Stand Attributes Predict Recovery

The removal of conifers from aspen (Populus tremuloides) stands is being undertaken throughout the western United States to restore aspen for local- and landscape-level biodiversity. Current practices include mechanically removing conifers or hand thinning, piling, and burning cut conifers in and adjacent to aspen-conifer stands. To evaluate the effectiveness of restoration treatments, we examined tree regeneration and herbaceous vegetation cover in thinned, thinned and pile burned, and non-thinned control stands. Growth rates of small conifer saplings threatening to outcompete and replace aspen were also measured. Two to four years after pile burning, herbaceous vegetation cover within the footprint of burned piles (i.e. burn scars) was 35–73% of that in adjacent areas. Aspen was more likely to regenerate inside burn scars where fewer surrounding trees were true firs. Conifer seedlings were more likely to regenerate in burn scars where more of the surrounding trees were conifers (pine or fir) as opposed to aspen. Fir saplings had much slower growth than did aspen saplings. Overall, our findings show that restoration treatments are promoting desirable outcomes such as enhancing aspen regeneration but that follow-up treatments will be needed to remove numerous conifer seedlings becoming established after restoration activities. Eliminating conifers, while they are small, growing slowly, and contributing little to fuel loads may be an economical way to prolong restoration treatment effectiveness