180 research outputs found
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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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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
Spatial aspects of tree mortality strongly differ between young and old-growth forests
Rates and spatial patterns of tree mortality are predicted to change during forest structural development. In young forests, mortality should be primarily density dependent due to competition for light, leading to an increasingly spatially uniform pattern of surviving trees. In contrast, mortality in old-growth forests should be primarily caused by contagious and spatially auto-correlated agents (e.g., insects, wind), causing spatial aggregation of surviving trees to increase through time. We tested these predictions by contrasting a three-decade record of tree mortality from replicated mapped permanent plots located in young (\u3c60-year-old) and old-growth (\u3e300-year-old) Abies amabilis forests. Trees in young forests died at a rate of 4.42% per year, whereas trees in old-growth forests died at 0.60% per year. Tree mortality in young forests was significantly aggregated, strong density dependent, and caused live tree patterns to become more uniform through time. Mortality in old-growth forests was spatially aggregated, but was density independent and did not change the spatial pattern of surviving trees. These results extend current theory by demonstrating that density-dependent competitive mortality leading to increasingly uniform three spacing in young forests ultimately transitions late in succession to a more diverse tree mortality regime that maintains spatial heterogeneity through time
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Managing fire-prone forests in the western United States
The management of fire-prone forests is one of the most controversial natural resource issues in the US today,
particularly in the west of the country. Although vegetation and wildlife in these forests are adapted to fire, the
historical range of fire frequency and severity was huge. When fire regimes are altered by human activity, major
effects on biodiversity and ecosystem function are unavoidable. We review the ecological science relevant to
developing and implementing fire and fuel management policies for forests before, during, and after wildfires.
Fire exclusion led to major deviations from historical variability in many dry, low-elevation forests, but not in
other forests, such as those characterized by high severity fires recurring at intervals longer than the period of
active fire exclusion. Restoration and management of fire-prone forests should be precautionary, allow or
mimic natural fire regimes as much as possible, and generally avoid intensive practices such as post-fire logging
and planting
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