Sierran Mixed-conifer Forests Research Articles

Local testing and calibration of species-specific competition indices in Sierran mixed-conifer forests: application transfer to evolving objectives

Forest growth processes are driven by site productivity and species functional traits, which are ultimately constrained by cumulative resource demand, resulting in competitive dynamics across successional forest communities. Historical efforts to quantify competition used density metrics or neighborhood crowding indices for yield modeling and reforestation surveys. These methods have expanded to include dendroclimatology and restoration applications that commonly assume similar competitive response across species of various functional types. We assessed the competitive indices of two focal species, Pinus lambertiana (Dougl.) and Pinus ponderosa (Dougl. ex P. Laws. & C. Laws.) in mixed-conifer forests of the Sierra Nevada to estimate stem radial growth under current stand structure. We ranked correlations of basal area increment of the last 10 years (BAI10) separately across 20 competition indices (CIs). The best-ranked CIs were used to test the relative influence of competition, tree size, and site variables on BAI10 with linear mixed models. While crown overlap was a common variable in CIs among both species, BAI10 of P. lambertiana was less impacted by intraspecific competition, and P. ponderosa appeared sensitive to all competing stems. The results suggest that local calibration of CIs with crown parameters may aid in interpreting Pinus species growth patterns and that the relative impact of competition on growth is species specific.

The 15-year post-treatment response of a mixed-conifer understory plant community to thinning and burning treatments

Disturbance is central to maintaining diversity in forest ecosystems. In the dry forests of the western United States, over a century of fire exclusion has altered the fire regimes of these forests, resulting in high fuel loads and a loss of plant diversity. Mechanical thinning and prescribed fire are widely used to restore structural complexity and species diversity in many western U.S. forests. While studies have shown that the reintroduction of fire into these forests initially promotes plant diversity, there is limited information on the persistence of this effect. We evaluated the effects of thinning and burning treatments on the understory plant community fifteen years after treatment in an old-growth, Sierran mixed-conifer forest. Using a full-factorial design, including three levels of thinning and two levels of burning, we found mechanical thinning and prescribed fire reduced litter depths and increased the availability of bare ground, resulting in an initial increase in herb cover. However, fifteen years after treatment, litter depths and shrub cover increased, resulting in a more homogenous understory community and a loss of herb cover. Overall, our results suggest that while thinning and burning treatments initially promote herbaceous plant cover, these effects are short lived in the absence of a second disturbance.

Prescribed fire and mechanical thinning effects on bark beetle caused tree mortality in a mid-elevation Sierran mixed-conifer forest

We assessed tree mortality caused by bark beetles in a mixed-conifer forest in the central Sierra Nevada in response to fire and mechanical treatments. The treatments were: (1) no treatment, (2) prescribed fire, (3) mechanical (crown thinning-from-below followed by rotary mastication), and (4) mechanical followed by prescribed fire. Ponderosa pine (Pinus ponderosa Laws) mortality caused by the western pine beetle (Dendroctonus brevicomis LeConte), sugar pine (Pinus lambertiana Dougl.) mortality caused by mountain pine beetle (D. ponderosae Hopkins), and white fir (Abies concolor Gord. and Glend) mortality caused by the fir engraver beetle (Scolytus ventralis LeConte) was assessed pre-treatments, one-year post-treatments, and three years post-treatments. For the duration of the study, bark beetle caused mortality across all treatments for each tree species was less than 7%. Bark beetle-caused mortality of small and medium white firs increased in treatments that included fire, and bark beetle-caused mortality of medium size sugar pines was elevated in the fire only treatment compared with other treatments. Our results indicate that mechanical treatments cause little risk of mortality to residual trees from bark beetles in the short term. The higher secondary mortality in the small and medium size white firs in both fire treatments can be considered a benefit in overly dense mixed conifer forests where the understory is dominated by shade-tolerant white firs.

Habitat Use of Pacific Fishers in a Heterogeneous Post-Fire and Unburned Forest Landscape on the Kern Plateau, Sierra Nevada, California

The Pacific fisher (Martes pennanti) is a rare forest carnivore strongly associated with dense, old forest with high canopy cover for denning and resting. The Sierra Nevada population is very small, genetically distinct, and isolated. Mixed-severity wildland fire is assumed to be a potentially greater threat than logging, and land managers are conducting large-scale forest thinning operations under the hypothesis that it is needed to reduce fire spread and severity. However, the relationship between fishers and fire has not been tested previously. I investigated this question with teams of dogs specially trained to detect the scat of Pacific fishers and, thus, their concentration of movements and habitat use. All scat samples were genetically verified. In surveys on the Kern Plateau in the southwestern Sierra Nevada, within unlogged post-fire forests with mixed-severity effects from large fires, and in unburned forests, I found that fishers selected Sierran mixed-conifer forests in both post-fire and unburned areas, and selected closed-canopied, mature/old forest in unburned forests, as well as burned forests that had this structure in the pre-fire condition. When fishers were near burned/unburned edges, they selected the within-fire side. Fishers used dense, mature/old forest that experienced moderate/high-severity fire at the same level as unburned dense, mature/old forest, and both males and females were found deep inside large fires—several km from the fire perimeter. These results indicate that fishers may benefit from the structural complexity of such post-fire habitat for foraging. This suggests mixed-severity wildland fire could be restored through managed wildland fire in these forests.

Microclimate effects of fuels-reduction and group-selection silviculture: Implications for fire behavior in Sierran mixed-conifer forests

Fire suppression and other past management practices in the western USA have led to dense conifer forests with high canopy cover and thick layers of surface fuels, changes likely to alter understory microclimate relative to historical conditions. Silvicultural treatments are used to restore forest resilience, but little is known about their microclimate-mediated effects on fire behavior. We measured fire-related microclimate variables for two years before and after experimental, operational-scale application of fuels-reduction thinning and group selection treatments in a Sierra Nevada mixed-conifer forest. Measurements included air speed, temperature, and relative humidity; soil temperature and moisture; and dead fuel moisture. Wind gust speed increased moderately (average 0.7 m s −1 or 31% increase) in thinned forest and sharply (average 2.5 m s −1 or 128% increase) in group-selection openings. Surprisingly, treatments did not affect air temperature or humidity. Soil temperatures increased by a mean of 4 °C in group openings but did not increase in thinned stands. Duff moisture in group selection openings was 72% of that in the control stands, but there were no effects on moisture in other fuel particle size classes, or in thinned stands. Soil moisture increased in group-selection openings at depths down to 0.7 m but did not change in thinned stands. Fire spread simulation modeling with FMAPlus indicated that elevated wind speeds could increase the fire rate of spread, but that increases are moderate and largely linear rather than exponential across the observed range of wind gust speeds. In general our results suggest that group selection openings placed in high canopy cover, Sierran mixed-conifer forests are distinct microclimatic environments that will have slightly different fire behavior than the surrounding matrix due to higher surface temperatures and faster wind speeds. Current fuels-reduction thinning practices in dry western forests, however, will have minimal microclimatic-mediated influence on wildfire behavior, and there is little cause for concern about a faster rate of fire spread or drier fuels in such stands.

Using light to predict fuels-reduction and group-selection effects on succession in Sierran mixed-conifer forest

Many semi-arid coniferous forests in western North America have reached historically unprecedented densities over the past 150 years and are dominated by shade-tolerant trees. Silvicultural treatments generally open the canopy but may not restore shade-intolerant species. We determined crossover-point irradiance (CPI) (light at which the height growth rank of pairs of species changes) for seedlings in Sierra Nevada mixed-conifer forest and used these to interpret light environments produced by fuels-reduction thinning and group selection with reserved large trees. Nine of 21 species pairs had well-defined CPIs. The CPI of the most common shade-tolerant and intolerant species (white fir ( Abies concolor (Gordon & Glendl.) Lindl. ex Hildebr.) and ponderosa pine ( Pinus ponderosa Douglas ex P. Lawson & C. Lawson)) was 22.5 mol·m–2·day–1 or 41% of full sun. Median understory irradiance increased from 9.2 mol·m–2·day–1 (17% full sun) in pretreatment forest to 13 mol·m–2·day–1 (24% full sun) in lightly and 15.5 mol·m–2·day–1 (28% full sun) in moderately thinned stands and 37 mol·m–2·day–1 (67% full sun) in group-selection openings. We estimate that 5%–20% of ground area in lightly to moderately thinned stands would have enough light to favor shade-intolerant over shade-tolerant growth compared with 89% of ground area in group-selection openings. The CPI provides a tool to assess regeneration implications of treatment modification such as increasing heterogeneity of thinning to enhance regeneration or reserving large trees in group-selection openings to maintain wildlife habitat.

Fuel treatment effects on stand-level carbon pools, treatment-related emissions, and fire risk in a Sierra Nevada mixed-conifer forestPublication No. 143 of the National Fire and Fire Surrogate Project.

Policies have been enacted to encourage carbon (C) sequestration through afforestation, reforestation, and other silvicultural practices; however, the effects of wildfires on forest C stocks are poorly understood. We present information from Sierran mixed-conifer forests regarding how control, mechanical, prescribed-fire, and mechanical followed by prescribed-fire treatments affected C pools. Secondly, we report CO2 emissions from machinery and burning associated with the treatments. Lastly, the effects of treatments on the potential for C loss to wildfire are presented. The amount of aboveground C in live trees was significantly reduced in mechanical-only and mechanical plus fire treatments; C contained in dead trees was not significantly different. There was no significant difference in aboveground live and dead tree C between the fire-only and control treatments. Fire-only and mechanical plus fire treatments emitted significantly more CO2 than the mechanical treatment and control. Modeling results for the control demonstrated 90% of the live tree C had a high (>75%) chance of being killed in a wildfire; in contrast, all three active treatments had low vulnerabilities to C loss. With wildfire severity increasing in most Sierran forests, management actions designed to increase fire resistance are justified for long-term C sequestration.

Stand Development and Tree Growth Response to Sugar Pine Mortality in Sierran Mixed-Conifer Forests

Abstract Sugar pine (Pinus lambertiana) is a major component of the mixed-conifer forests found in the Sierra Nevada Mountains of California, USA. In the past several decades, sugar pine mortality has increased due to an introduced pathogen, white pine blister rust (caused by Cronartium ribicola). This study described stand development patterns in the mixed-conifer forests and quantified stand and tree growth response to the loss of sugar pine. Stratification patterns tended to follow patterns of shade tolerance with the more shade tolerant species found in lower canopy positions. Growth response was analyzed following the initial entry of blister rust and after more recent sugar pine mortality. Following the initial wave of mortality induced by blister rust, overstory trees and most individual species increased overall basal area growth. Although most species also responded to recent sugar pine mortality (occurring within the past ten years) by increasing basal area growth, the increase was only signific...

Initial response of conifer and California black oak seedlings following fuel reduction activities in a Sierra Nevada mixed conifer forest

Forest structure, fuel characteristics, and fire regimes of mixed conifer forests in the Western United States (US) have been dramatically altered since the early 20th century. Fuel treatments have been suggested as a means to limit the size and intensity of wildfires but few experiments are available to analyze the ecological effects of different treatments. The objective of this study is to determine how mechanical, prescribed fire, and mechanical and fire combination fuel treatments affected seedling density by species in a Sierra Nevada mixed conifer forest. The relative influences of stand-level light availability and substrate quality on conifer and black oak seedling densities are assessed. For all species combined, seedling densities increased when the fire only treatment was applied and no change was detected for the other treatments. The fire only treatment as well as the combined fire plus mechanical treatment had the effect of significantly increasing Douglas-fir seedling density. Ponderosa pine seedling densities significantly increased when the fire plus mechanical treatment was applied. California black oak seedling density decreased when the fire only treatment was applied but no change was detected for the other treatments. Previous studies have found a decline in sugar pine and ponderosa pine in dense stands of Sierran mixed conifer forests. The findings reported here corroborate these studies in that current conditions are not conducive to recruitment of ponderosa pine and sugar pine species. Our initial results indicate, that in this case, treatments used to reduce fire hazard may not result in retention or recruitment of California black oak and sugar pine seedlings.

Different interest group views of fuels treatments: survey results from fire and fire surrogate treatments in a Sierran mixed conifer forest, California, USA

The present paper discusses results from a survey about the acceptance of and preferences for fuels treatments of participants following a field tour of the University of California Blodgett Forest Fire and Fire Surrogate Study Site. Although original expectations were that tours would be composed of general members of the public, individual tour groups ultimately were much more specialised, with tours made up of individuals from five distinct groups including foresters, environmentalists, entomologists, the Natural Resource Conservation Service, teachers, and high school or undergraduate students. This proved fortuitous as most studies of ‘public’ perceptions to date have been of general members of the public and little work has been done assessing the views of groups who may have more specific knowledge or interest in fuels treatments. Such assessment is perhaps long overdue given the importance of understanding characteristics of different audience segments in developing effective outreach programs. Analysis showed that group membership was in fact the key element in differences in survey responses with significant differences found between groups on overall acceptability of treatments, treatment preferences based on different land ownership and management types, and which variables were most important in determining treatment preferences.

Comparison of thinning and prescribed fire restoration treatments to Sierran mixed-conifer historic conditions

Thinning and prescribed fire are widely used to restore fire-suppressed forests, yet there are few studies of their effectiveness in Sierran mixed-conifer forest. We compared stand conditions in replicated plots before and after a combination of thinning and burning treatments against a reconstruction of the same forest in 1865. The historical forest had 67 stems/ha (trees ≥5 cm DBH), equal percentages of shade-tolerant and -intolerant tree species, stems randomly distributed at the stand scale, and a flat diameter distribution across size classes. The pretreatment forest averaged 469 stems/ha, which comprised 84% shade-tolerant and 14% shade-intolerant species, were highly clustered, and had a reverse-J-shaped diameter distribution. Thinning treatments failed to approximate historical composition, spatial pattern, or diameter distribution. Treatments left too many small trees, removed too many intermediate-sized trees (50–75 cm DBH), and retained a reverse-J-shaped diameter distribution. Current old growth comprises fewer large trees than historical conditions, suggesting that treatments should retain more intermediate-sized trees to provide for future large-tree recruitment. Understory thinning with prescribed fire significantly reduced stem density and produced a spatial pattern closest to historical conditions. Mixed-conifer restoration needs thinning prescriptions that vary by species and flexible rather than rigid upper diameter limits to retain some trees in all size classes.

A fuel treatment reduces fire severity and increases suppression efficiency in a mixed conifer forest

Fuel treatments are being implemented on public and private lands across the western United States. Although scientists and managers have an understanding of how fuel treatments can modify potential fire behaviour under modelled conditions, there is limited information on how treatments perform under real wildfire conditions in Sierran mixed conifer forests. The Bell Fire started on 22 September 2005 on the Plumas National Forest, CA. This fire burned upslope into a 1-year old, 158-ha mechanical fuel treatment on private land. Prior to coming into contact with the fuel treatment, the main fire ignited spot fires 400 feet (122 metres) into the treated area. Overall, this fuel treatment resulted in: (1) increased penetration of retardant to surface fuels; (2) improved visual contact between fire crews and the Incident Commander; (3) safe access to the main fire; and (4) quick suppression of spot fires. This treatment was relatively small and isolated from other fuel treatments but resulted in decreased severity, suppression costs and post-fire rehabilitation needs, leading to cost savings for local public and private land managers.

Biodiversity of Coleoptera and the Importance of Habitat Structural Features in a Sierra Nevada Mixed-conifer Forest

Beetle biodiversity, particularly of leaf litter fauna, in the Sierran mixed-conifer ecosystem is poorly understood. This is a critical gap in our knowledge of this important group in one of the most heavily managed forest ecosystems in California. We used pitfall trapping to sample the litter beetles in a forest with a history of diverse management. We identified 287 species of beetles from our samples. Rarefaction curves and nonparametric richness extrapolations indicated that, despite intensive sampling, we undersampled total beetle richness by 32–63 species. We calculated alpha and beta diversity at two scales within our study area and found high heterogeneity between beetle assemblages at small spatial scales. A nonmetric multidimensional scaling ordination revealed a community that was not predictably structured and that showed only weak correlations with our measured habitat variables. These data show that Sierran mixed conifer forests harbor a diverse litter beetle fauna that is heterogeneous across small spatial scales. Managers should consider the impacts that forestry practices may have on this diverse leaf litter fauna and carefully consider results from experimental studies before applying stand-level treatments.

Soil respiration response to experimental disturbances over 3 years

Soil respiration is a major pathway for carbon cycling in terrestrial ecosystems yet little is known about its response to natural and anthropogenic disturbances. This study examined soil respiration response to prescribed burning and thinning treatments in an old-growth, mixed-conifer forest on the western slope of the Sierra Nevada Mountains. Experimental treatments were applied in 2001 using a full factorial design consisting of two levels of burning and three levels of thinning, and included: unburned–unthinned (UN), unburned–overstory thinned (US), unburned–understory thinned (UC), burned–unthinned (BN), burned–understory thinned (BC), and burned–overstory thinned (BS). We measured soil respiration rate (SRR), soil moisture ( M S), soil temperature ( T S), and litter depth (LD) for three replicates of each of three dominant patch types (closed canopy, open canopy, and ceanothus shrub) within each of the six treatments ( n = 54). The same sampling points were measured from May to August in 2000 (pre-treatment) and in 2002, 2003, and 2004 (post-treatment). Within our sampling period there was as much as 37% variation ( p = 0.0005) between years in the undisturbed patches, which appeared to be driven by changes in precipitation. SRR also varied by year in all treated plots (US: p = 0.0516; UC: p = 0.0006; BN: p = 0.0158; and BC: p = 0.0040), with the exception of BS ( p = 0.3344). SRR response to disturbance varied with patch type, year, and treatment type. In most cases, burning and the combination of burning and thinning had less of an effect on mean SRR than thinning alone. Ceanothus patches appear to have recovered fastest, while treatment effects remained 3 years after thinning in closed canopy ( p = 0.0483 and 0.0333 in UC and US, respectively) and open canopy patches ( p = 0.0191 in US). Open canopy patches showed no response to any treatment aside from US. Both UC and US increased SRR in closed canopy and ceanothus patches, and US decreased SRR in open canopy patches. BS increased SRR in 2004 in closed canopy patches ( p = 0.0108), but no significant changes occurred in any patch type in response to BN or BC treatments. Across all treatments, the relationship of SRR with temperature, moisture, and litter depth changed in post-disturbance years. The results of this study can be used to help understand how management of Sierran mixed-conifer forests affects soil carbon sequestration.

Fire and fire surrogate treatment effects on leaf litter arthropods in a western Sierra Nevada mixed-conifer forest

Frequent, low-intensity fires were a common feature of Sierran forest ecosystems, but suppression policies over the past century have left many forests at risk for catastrophic wildfires. Recent policies highlight the use of prescribed burning or harvesting as fire risk reduction tools, but few studies have investigated the impacts of these management practices on the leaf litter fauna of Sierran forests. This study examines how three fire and “fire surrogate” treatments, prescribed burning, overstory thinning with understory mastication, and combined thinning and burning, impact diversity and abundance of Coleoptera and other leaf litter arthropods. Pitfall trapping was used to collect litter arthropods before and immediately after treatments in replicated forest compartments. The diverse Coleoptera assemblage was dominated by only a few common species, with many rare species represented by only one or two individuals. Rank–abundance diagrams indicated that much of the change in the beetle assemblage due to the treatments was a result of changes in the numbers of rare species. Indicator species analysis showed several species closely allied with the treated compartments, but few with the untreated controls. Both NMS and CCA ordination show considerable change in overall assemblage structure on compartments treated with fire, but less change in the thinned compartments. Coleoptera species richness was slightly higher in burned compartments. Some common beetle species, families of beetles, and other common groups such as ants and spiders showed changes in abundance due to the treatments, but the changes were taxon-specific and showed no general pattern. Overall impacts of the treatments appear to be moderate, and the increased habitat heterogeneity at the compartment level may provide additional habitat for many rare species to coexist. We conclude that the use of fire and fire surrogate treatments in Sierran mixed-conifer forests is justified from the standpoint of their effects on leaf litter arthropods, but the history of management at the site and the scale of treatments must be carefully considered.

Stand Conditions Associated with Tree Regeneration in Sierran Mixed-Conifer Forests

Abstract Fire suppression has significantly increased canopy cover, litter depth, and stem density in many western forests, altering microsite conditions that affect tree seedling establishment. We conducted studies in a mixed-conifer forest in the Sierra Nevada, California, to determine relationships between established understory trees and microsite quality, and to examine the effect of fire intensity and shrub cover on seedling establishment. Most of the conifer species were found on microsites with relatively high soil moisture and relatively low direct solar radiation. All species had greater frequency under shadier conditions except for Jeffrey pine, which was found on drier, more open microsites. Although seedlings were more abundant on mineral soil than expected, intact litter and forest floor was not a barrier to establishment. Mortality of planted seedlings was high, particularly in exposed areas. Although shrub cover may initially aid survival, few conifer saplings were present in shrub-dominated patches, possibly because shrubs can be aggressive competitors for soil moisture. The lack of regeneration, logs, or snags in many openings suggest that large gaps are hostile environments for tree seedlings. Results suggest that reductions in shrub cover may benefit tree establishment, but increasing understory light and decreasing surface soil moisture through canopy cover reductions may not. FOR. SCI. 51(3):198–210.

BIRD COMMUNITIES IN THINNED VERSUS UNTHINNED SIERRAN MIXED CONIFER STANDS

We used point counts and nest monitoring to compare avian community composition and nesting success in thinned and unthinned stands of commercially managed Sierran mixed conifer forest. We conducted point counts and monitored 537 active nests of 37 species on 10 study plots during three consecutive breeding seasons in the northern Sierra Nevada. All 10 study plots had experienced a similar long term man- agement history that included fire suppression and single-tree selection logging, but five of the plots also un- derwent a protocol of combined commercial and biomass thinning 5-8 years prior to the beginning of the study. Pooling species by nest substrate, we found that detections of ground-nesting bird species were similar on thinned and unthinned plots, but we detected canopy-, cavity-, and especially shrub-nesting species much more frequently on the thinned plots. Nest success rates were not statistically different between thinned and unthinned plots for ground-, shrub-, canopy-, or cavity-nesting species. Thinned stands were characterized by significantly less canopy cover, significantly lower density of small and medium conifers, and significantly greater understory cover and deer brush (Ceanothus integerrimus) cover than the unthinned stands. We surmise that the thinning protocol stimulated vigorous shrub growth, and conclude that forest conditions associated with a relatively open canopy and a well-developed shrub understory are highly beneficial to numerous breeding bird species in the Sierran mixed conifer community, including many species that may not nest or forage in the understory. Forest thinning that promotes vigorous shrub growth may correlate with an increased abundance of nesting birds, at least within stands affected by historical fire suppression and single-tree selection logging. Received 1 October 2002, accepted 5 March 2003. Logging practices and human-altered fire regimes have changed forest structure and composition across much of the Sierra Nevada since the mid-Nineteenth Century (Franklin and Fites-Kaufmann 1996, Gruell 2001). Tim- ber harvest practices and fire suppression throughout much of the region generally have reduced the frequency of low intensity fires, reduced the number of large trees, increased the density of smaller trees, and possibly re- duced the extent of shrub cover (Weaver 1974,

Edge effects in mixed conifer group selection openings: tree height response to resource gradients

Replicated circular openings ranging in size from 0.1 to 1 ha were cleared on a Sierran mixed conifer forest in 1996 at the Blodgett Forest Research Station, California and planted with seedlings of six native species. After 3 years of growth, heights of all trees were measured and analyzed according to species, opening size, and location within the opening. To determine the cause of the edge influence on height, we measured differences along north–south transects in extension growth, pre-dawn water potential, and light availability for three species of trees: giant sequoia (Sequoiadendron giganteum), ponderosa pine ( Pinus ponderosa), and Douglas-fir ( Ptseudotsuga menziesii var. menziesii). The sequence of mean height from tallest to shortest based on species was: giant sequoia> incense cedar > Douglas- fir> ponderosa pine > white fir > sugar pine . For all species combined, a ten-fold increase in the area of the opening corresponded to a 34% increase in mean height. Trees were tallest on average in the north rows and shortest in the south rows. There was no difference in height between trees in the east and west rows. As expected, resource availability was greatest near the center and least near the edges with north edges receiving significantly more light than southern edges. In general, observed edge effects on sapling height growth were correlated with light and water supply. However there were important differences between species in the nature of the co-limitation. Giant sequoia growth was most sensitive to light and water availability. Together they explained more than 47% of the observed variation in giant sequoia height. In contrast, only light was a significant predictor of ponderosa pine performance. Douglas-fir heights were significantly related to both light and water but there was more unexplained variability in the Douglas-fir model compared to the other species. These highly controlled experimental group openings provide a standard reference for silviculturalists using the group selection method of regeneration.

A methodology for mapping forest latent heat flux densities using remote sensing

Basin-scale forest evapotranspiration is strongly influenced by the allocation of net radiation on the watershed. Forested watersheds typically have a wide variety of slopes, aspects, cover types, and other variables which affect the allocation of net radiation. Remote sensing can provide a means to analyze the distribution of solar energy on forested watersheds because of its synoptic coverage. A pilot study was conducted in which surface temperatures and reflectances of an upper elevation Sierran Mixed Conifer Forest were monitored using the Thematic Mapper Simulator (TMS) sensor during the summer of 1985. TMS Bands 3, 5, 7, and 11 (TM Bands 2, 3, 4, and 6) were combined with surface meteorological data and input into an energy budget model designed to generate images which describe the available evaporative energy within the watershed. The results of this initial validation indicate that the model responds well in quantifying relative energy allocation relationships between the two cover types defined in this study. In addition, this methodology has the potential to map forest latent heat flux densities provided that some of the model parameters can be assessed more accurately. It is anticipated that this technique can prove useful in helping to make more efficient decisions concerning water yield from forested environments.

Litter dynamics in two Sierran mixed conifer forests. I. Litterfall and decomposition rates

Litterfall was measured for 4 years and leaf litter decomposition rates were studied for 3.6 years in two mixed conifer forests (giant sequoia–fir and fir–pine) in the southern Sierra Nevada of California. The giant sequoia–fir forest (GS site) was dominated by giant sequoia (Sequoiadendrongiganteum (Lindl.) Buchh.), white fir (Abiesconcolor Lindl. & Gord.), and sugar pine (Pinuslambertiana Dougl.). The fir–pine forest (FP site) was dominated by white fir, sugar pine, and incense cedar (Calocedrusdecurrens (Torr.) Florin). Litterfall, including large woody debris <15.2 cm in diameter, at the GS site averaged 6364 kg•ha−1•year−1 compared with 4355 kg•ha−1•year−1 at the FP site. Compared with other temperate coniferous forests, annual variability in litterfall (as computed by the ratio of the annual maximum/minimum litterfall) was extremely high for the GS site (5.8:1) and moderately high for the FP site (3.4:1). In the GS site, leaf litter decomposition after 3.6 years was slowest for giant sequoia (28.2% mass loss), followed by sugar pine (34.3%) and white fir (45.1%). In the FP site, mass loss was slowest for sugar pine (40.0%), followed by white fir (45.1%), while incense cedar showed the greatest mass loss (56.9%) after 3.6 years. High litterfall rates of large woody debris (i.e., 2.5–15.2 cm diameter) and slow rates of leaf litter decomposition in the giant sequoia–fir forest type may result in higher litter accumulation rates than in the fir–pine type. Leaf litter times to 95% decay for the GS and FP sites were 30 and 27 years, respectively, if the initial 0.7-year period (a short period of rapid mass decay) was ignored in the calculation. A mass balance approach for total litterfall (<15.2 cm diameter) decomposition yielded lower decay constants than did the litterbag study and therefore longer times to 95% decay (57 years for the GS site and 62 years for the FP site).