160 research outputs found
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Local forest structure variability increases resilience to wildfire in dry western U.S. coniferous forests.
A 'resilient' forest endures disturbance and is likely to persist. Resilience to wildfire may arise from feedback between fire behaviour and forest structure in dry forest systems. Frequent fire creates fine-scale variability in forest structure, which may then interrupt fuel continuity and prevent future fires from killing overstorey trees. Testing the generality and scale of this phenomenon is challenging for vast, long-lived forest ecosystems. We quantify forest structural variability and fire severity across >30 years and >1000 wildfires in California's Sierra Nevada. We find that greater variability in forest structure increases resilience by reducing rates of fire-induced tree mortality and that the scale of this effect is local, manifesting at the smallest spatial extent of forest structure tested (90 × 90 m). Resilience of these forests is likely compromised by structural homogenisation from a century of fire suppression, but could be restored with management that increases forest structural variability
Aligning ecology and markets in the forest carbon cycle
A forest carbon (C) offset is a quantifiable unit of C that is commonly developed at the local or regional project scale and is designed to counterbalance anthropogenic C emissions by sequestering C in trees. In cap-and-trade programs, forest offsets have market value if the sequestered C is additional (more than would have occurred in the absence of the project) and permanent (sequestered within the project boundary for a specified period of time). Local management and ecological context determine the rate of C sequestration, risk of loss, and hence the market value. An understanding of global C dynamics can inform policy but may not be able to effectively price an ecosystem service, such as C sequestration. Appropriate pricing requires the assistance of ecologists to assess C stock abundance and stability over spatial and temporal scales appropriate for the regional market. We use the risk that sequestered C will be emitted as a result of wildfire (reversal risk) to show how ecological context can influence market valuation in offset programs
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Effect of thinning and prescribed fire restoration treatments on woody debris and snag dynamics in a Sierran old-growth, mixed-conifer forest
Forest managers have little information of the effects of common restoration treatments, thinning and burning, on dead woody material (DWM) dynamics in fire-suppressed forests. Fine woody debris (FWD; 0.6–29.9 cm), coarse woody debris (CWD; ≥30.0 cm), and snags (≥5 cm) were inventoried and mapped in eighteen 4 ha plots before and after applying thinning (overstory, understory, and no thinning) and burning (burn and no burn) treatments. The combination of burning and thinning reduced FWD and CWD quantity and mean piece size, removed highly decayed logs, and increased small (5.0–24.9 cm) snag recruitment. In contrast, thin-only treatments produced similar results but increased FWD and did not remove many small snags. There were no differences in DWM response between the two thinning treatments. Log and snag spatial patterns prior to and following treatment were similar. These results indicate that burning in combination with thinning is more effective at reducing surface FWD and CWD, and removing small trees than are burn-only and thin-only treatments. Although large snags and logs were consumed in the burn, long-term recruitment of these habitat structures relies on managers retaining large-diameter trees. Repeated burns need to be conducted after initial restoration treatments to understand natural patterns of DWM
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Microhabitat associations of northern flying squirrels in burned and thinned forest stands of the Sierra Nevada
Prescribed burning and mechanical thinning are used to manage fuels within many western North American forest ecosystems, but few studies have examined the relative impacts of these treatments on forest wildlife. We sampled northern flying squirrels (Glaucomys sabrinus) and microhabitat variables in burned, thinned and control stands of mixed-conifer forest of the southern Sierra Nevada at the Teakettle Experimental Forest. We used this information to determine the effects of burning and thinning on the microhabitat associations of flying squirrels. Across pretreatment stands, the probability of flying squirrel capture increased with decreasing distance to a perennial creek and increasing litter depth. The probability of flying squirrel capture also was greater with increased canopy cover in thinned stands and increased litter depth in burned stands. Greater canopy cover may provide protection from predators, thicker litter layers may harbor a greater abundance of truffles, a primary food of northern flying squirrels, and creeks may provide squirrels with food sources, drinking water and nest trees. Results from this study underscore the need for more information on the effects of forest management on northern flying squirrels near the southern extent of the species’ geographic range
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The effects of fire on soil nitrogen associated with patches of the actinorhizal shrub Ceanothus cordulatus
Nitrogen is a limiting resource in many temperate forests and nitrogen-fixing plants are usually limited to the early
stages of post-disturbance succession. In fire-dependent Sierra Nevada forests, however, Ceanothus cordulatus is
relatively abundant even in old-growth forest conditions which are at least partly maintained by fire.We conducted
a field experiment to determine if soil beneath Ceanothus patches represent ‘resource islands’ of available N which
persist after fire. Nine plots containing discrete patches of Ceanothus, Arctostaphylos patula (manzanita; chosen
as a non N-fixing reference species), and bare forest floor were subjected to either a low-intensity (n = 3) or highintensity
(n = 3) bum treatment, or remained unburned as controls (n = 3). Soil temperatures during the bum were
monitored by a network of thermocouples placed at the surface of the mineral soil and at ca. 10 cm depths. Soil
samples were collected from the organic horizon, 0-10 cm and 15-25 cm depths within each patch type immediately
before burning and 2 days, and 6, and 11 months after. Soil moisture, total C and N, and ammonium and nitrate
concentrations were determined in the laboratory. Before the burn, Ceanothus patches were significantly enriched
in total and inorganic N in the organic horizon relative to the other patch types. A sharp increase in inorganic N was
observed in all patch types and depths immediately following burning, but by 6 months after the burn, Ceanothus
patches were significantly enriched relative to the surrounding patch types and remained so at months. Resprouting
Ceanothus patches will continue to be an important source of a limiting nutrient in this fire-prone ecosystem.Keywords: Frankia, Sierra Nevada, Fire, Ceanothus, Nitroge
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Microhabitat associations of northern flying squirrels in burned and thinned forest stands of the Sierra Nevada
Prescribed burning and mechanical thinning are used to manage fuels within
many western North American forest ecosystems, but few studies have examined the relative
impacts of these treatments on forest wildlife. We sampled northern flying squirrels
(Glaucomys sabrinus) and microhabitat variables in burned, thinned and control stands of
mixed-conifer forest of the southern Sierra Nevada at the Teakettle Experimental Forest. We
used this information to determine the effects of burning and thinning on the microhabitat
associations of flying squirrels. Across pretreatment stands, the probability of flying squirrel
capture increased with decreasing distance to a perennial creek and increasing litter depth.
The probability of flying squirrel capture also was greater with increased canopy cover in
thinned stands and increased litter depth in burned stands. Greater canopy cover may provide
protection from predators, thicker litter layers may harbor a greater abundance of
truffles, a primary food of northern flying squirrels, and creeks may provide squirrels with
food sources, drinking water and nest trees. Results from this study underscore the need
for more information on the effects of forest management on northern flying squirrels
near the southern extent of the species’ geographic range
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Facilitative and competitive effecs of a N-fixing shrub on white fir saplings
In Sierra Nevada forests, shrubs are considered strong soil moisture competitors with regenerating trees, reducing seedling establishment, and
slowing growth. Recent studies, however, suggest that in some circumstances shrubs can facilitate tree establishment and growth by modifying
harsh microclimate conditions; increasing acquisition of water, carbon, and/or nutrients via shared mycorrhizal connections; or enhancing soil
fertility, particularly under nitrogen-fixing shrubs such as Ceanothus spp. We examined the establishment dates and growth rates and patterns of
white fir saplings growing in greenleaf manzanita, whitethorn ceanothus, and bare patches to examine whether establishment was correlated with
past wet years, whether saplings growing in ceanothus had nitrogen-enriched foliage or faster growth rates than in the other two patches, and
whether saplings in shrub patches experienced competition for light.
Sapling establishment was not correlated with high precipitation or heavy snowpack years, suggesting shade-tolerant saplings do not need wet
years to become established. Soils under ceanothus were nitrogen enriched, but white fir sapling foliage did not have higher nitrogen concentrations
and saplings did not grow faster in ceanothus than in the other two patches. Because growth rates of saplings were comparable in all patch types
examined despite significantly different edaphic and abiotic conditions, we inferred that the various competitive and facilitative interactions
affecting tree growth are in net balance across the patch types examined. However, competition for light is important—a significant percentage of
growth release events occurred after saplings emerged above their host shrubs. Where shrubs are present, shade-tolerant species (i.e., white fir) are
favored over drought-tolerant (pine) species. Our results may help interpret changes in understory conditions that are contributing to mixed
conifer’s compositional shift toward more shade-tolerant species after a century of fire-suppression.Keywords: shade tolerance, soil fertility, forest regeneration, fire suppression, nitrogen fixation, mixed conifer, plant competitio
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Facilitative and competive effects of a N-fixing shrub on white fir saplings
In Sierra Nevada forests, shrubs are considered strong soil moisture competitors with regenerating trees, reducing seedling establishment, and
slowing growth. Recent studies, however, suggest that in some circumstances shrubs can facilitate tree establishment and growth by modifying
harsh microclimate conditions; increasing acquisition of water, carbon, and/or nutrients via shared mycorrhizal connections; or enhancing soil
fertility, particularly under nitrogen-fixing shrubs such as Ceanothus spp. We examined the establishment dates and growth rates and patterns of
white fir saplings growing in greenleaf manzanita, whitethorn ceanothus, and bare patches to examine whether establishment was correlated with
past wet years, whether saplings growing in ceanothus had nitrogen-enriched foliage or faster growth rates than in the other two patches, and
whether saplings in shrub patches experienced competition for light.
Sapling establishment was not correlated with high precipitation or heavy snowpack years, suggesting shade-tolerant saplings do not need wet
years to become established. Soils under ceanothus were nitrogen enriched, but white fir sapling foliage did not have higher nitrogen concentrations
and saplings did not grow faster in ceanothus than in the other two patches. Because growth rates of saplings were comparable in all patch types
examined despite significantly different edaphic and abiotic conditions, we inferred that the various competitive and facilitative interactions
affecting tree growth are in net balance across the patch types examined. However, competition for light is important—a significant percentage of
growth release events occurred after saplings emerged above their host shrubs. Where shrubs are present, shade-tolerant species (i.e., white fir) are
favored over drought-tolerant (pine) species. Our results may help interpret changes in understory conditions that are contributing to mixed
conifer’s compositional shift toward more shade-tolerant species after a century of fire-suppression.Keywords: Nitrogen fixation, Shade tolerance, Forest regeneration, Plant competition, Fire suppression, Soil fertility, Mixed conife
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Diversity and distribution of Frankia strains symbiotic with Ceanothus in California
Frankia strains symbiotic with Ceanothus present an interesting opportunity to study the patterns and causes of Frankia diversity and distribution within a particular host infectivity group. We intensively sampled Frankia from nodules on Ceanothus plants along an elevational gradient in the southern Sierra Nevada of California, and we also collected nodules from a wider host taxonomic and geographic range throughout California. The two sampling scales comprised 36 samples from eight species of Ceanothus representing six of the seven major biogeographic regions in and around California. The primary objective of this study was to use a quantitative model to test the relative importance of geographic separation, host specificity, and environment in influencing the identity of Ceanothus Frankia symbionts as determined by ribosomal DNA sequence data. At both sampling scales, Frankia strains symbiotic with Ceanothus exhibited a high degree of genetic similarity. Frankia strains symbiotic with Chamaebatia (Rosaceae) were within the same clade as several Ceanothus symbionts. Results from a classification and regression tree model used to quantitatively explain Frankia phylogenetic groupings demonstrated that the only significant variable in distinguishing between phylogenetic groups at the more local sampling scale was host species. At the regional scale, Frankia phylogenetic groupings were explained by host species and the biogeographic province of sample collection. We did not find any significant correspondence between Frankia and Ceanothus phylogenies indicative of coevolution, but we concluded that the identity of Frankia strains inhabiting Ceanothus nodules may involve interactions between host species specificity and geographic isolation
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Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality
The recent Californian hot drought (2012–2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Broad-scale climate conditions can directly shape tree mortality patterns, but mortality rates respond non-linearly to climate when local-scale forest characteristics influence the behavior of tree-killing bark beetles (e.g., WPB). To test for these cross-scale interactions, we conduct aerial drone surveys at 32 sites along a gradient of climatic water deficit (CWD) spanning 350 km of latitude and 1000 m of elevation in WPB-impacted Sierra Nevada forests. We map, measure, and classify over 450,000 trees within 9 km2, validating measurements with coincident field plots. We find greater size, proportion, and density of ponderosa pine (the WPB host) increase host mortality rates, as does greater CWD. Critically, we find a CWD/host size interaction such that larger trees amplify host mortality rates in hot/dry sites. Management strategies for climate change adaptation should consider how bark beetle disturbances can depend on cross-scale interactions, which challenge our ability to predict and understand patterns of tree mortality. The 2012–2016 drought and western pine beetle outbreaks caused unprecedented mortality of ponderosa pine in the Sierra Nevada, California. Here, the authors analyse drone-based data from almost half a million trees and find an interaction between host size and climatic water deficit, with higher mortality for large trees in dry, warm conditions but not in cooler or wetter conditions.</p
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