Mixed-conifer Zone Research Articles

Impacts of climate and disturbance on nutrient fluxes and stoichiometry in mixed-conifer forests

Elucidating climatic impacts on stream nutrient export and stoichiometry will improve the understanding of forest carbon (C) storage in a warmer world. We analyzed C, nitrogen (N), and phosphorus (P) cycles in four watersheds within a rain-snow transition site and another four within a higher-elevation, snow-dominated site, in California’s mixed-conifer zone. We used these two sites in a space-for-time substitution to assess the potential warming impacts on nutrient cycles in currently snow-dominated areas that will become more rain-dominated. During a non-drought period (water year (WY) 2004–2011), mean annual stream exports of C and N in particulate forms at the transition site were twice that at the snow-dominated site, suggesting sediment-associated nutrient losses may increase with warming. The transition site had 12% lower N but twice P content in mineral horizons, lower N:P mass ratios in organic horizons, and lower stream export of dissolved inorganic N than the snow-dominated site. These differences suggest montane forests may have lower inputs of available N relative to P with warming. In addition, given strong interests in forest thinning to increase drought resiliency, we examined changes in stream nutrient export after thinning and during a major drought period (WY 2013–2015). Stream exports of C, N, and P were similar between unthinned and thinned watersheds during drought, suggesting negligible thinning impacts on stream nutrient export during excessively dry periods. Taken together, our results suggest that as the climate warms, California’s montane forests may lose more nutrients through erosion and increase their N-P nutritional imbalance.

Forest structure and pattern vary by climate and landform across active-fire landscapes in the montane Sierra Nevada

Restoration of fire-dependent forests is often guided by reference conditions from forests with an active fire regime, thought to be resilient to current and future disturbances and stresses. Reference conditions are usually based on historical data or reconstruction, which greatly limits the scale and completeness of data that can be collected. In the Sierra Nevada of California, large areas with reintroduced active fire regimes coupled with extensive lidar data coverage provide the unique opportunity to develop a contemporary regional reference condition dataset across a wide gradient of biophysical conditions. We developed this dataset with a focus on three questions: (1) What is the geographic and environmental distribution of restored active-fire forest areas in the Sierra Nevada mixed-conifer zone? (2) What are the ranges of variation in forest structure and spatial patterns across reference areas? And (3) How do stand density, tree clumping, and canopy opening patterns vary by topography and climate in reference areas? We analyzed fire history and environmental conditions over 10.8 million ha, including 3.9 million ha in the Sierra Nevada mixed-conifer zone, and found 30,377 ha of restored active-fire areas. Although reference areas were distributed throughout the Sierra Nevada they were more abundant on National Park lands (81% of reference areas) than National Forest lands and were associated with higher lightning strike density. Lidar-measured ranges of variation in reference condition structure were broad, with tree densities of 6–320 trees ha−1 (median 107 trees ha−1), basal area of 0.01–113 m2 ha−1 (median 21 m2 ha−1), average size of closely associated clumps of trees from >1 to 207 trees (median 3.1 trees), and average percent of stand area >6 m from the nearest canopy ranging from 0% to 100% (median 5.1%). These ranges correspond well with past studies reporting density and spatial patterns of contemporary and historical active-fire reference stands in the Sierra Nevada, except this study observed greater total variation due to the much greater spatial extent of sampling. Within the montane forest zone, reference areas at middle elevations had lower density (86 vs. 121 trees ha−1), basal area, (13.7 vs. 31 m2 ha−1), and mean clump size (2.7 vs. 4.0 trees) compared to lower- and higher-elevation reference areas, while ridgetops had lower density (101 vs. 115 trees ha−1), basal area (19.6 vs. 24.1 m2 ha−1), and mean clump size (3.0 vs. 3.3 trees) and more open space (7.4% vs. 5.1%) than other landforms. Many of the relationships between physiography and reference structure were context-dependent, suggesting that management practices should create heterogeneous forest structure congruent with local climatic and topographic factors influencing stand conditions.

Cumulative effects of wildfires on forest dynamics in the eastern Cascade Mountains, USA.

Wildfires pose a unique challenge to conservation in fire-prone regions, yet few studies quantify the cumulative effects of wildfires on forest dynamics (i.e., changes in structural conditions) across landscape and regional scales. We assessed the contribution of wildfire to forest dynamics in the eastern Cascade Mountains, USA from 1985 to 2010 using imputed maps of forest structure (i.e., tree size and canopy cover) and remotely sensed burn severity maps. We addressed three questions: (1) How do dynamics differ between the region as a whole and the unburned portion of the region? (2) How do dynamics vary among vegetation zones differing in biophysical setting and historical fire frequency? (3) How have forest structural conditions changed in a network of late successional reserves (LSRs)? Wildfires affected 10% of forests in the region, but the cumulative effects at this scale were primarily slight losses of closed-canopy conditions and slight gains in open-canopy conditions. In the unburned portion of the region (the remaining 90%), closed-canopy conditions primarily increased despite other concurrent disturbances (e.g., harvest, insects). Although the effects of fire were largely dampened at the regional scale, landscape scale dynamics were far more variable. The warm ponderosa pine and cool mixed conifer zones experienced less fire than the region as a whole despite experiencing the most frequent fire historically. Open-canopy conditions increased slightly in the mixed conifer zone, but declined across the ponderosa pine zone even with wildfires. Wildfires burned 30% of the cold subalpine zone, which experienced the greatest increase in open-canopy conditions and losses of closed-canopy conditions. LSRs were more prone to wildfire than the region as a whole, and experienced slight declines in late seral conditions. Despite losses of late seral conditions, wildfires contributed to some conservation objectives by creating open habitats (e.g., sparse early seral and woodland conditions) that otherwise generally decreased in unburned landscapes despite management efforts to increase landscape diversity. This study demonstrates the potential for wildfires to contribute to regional scale conservation objectives, but implications for management and biodiversity at landscape scales vary geographically among biophysical settings, and are contingent upon historical dynamics and individual species habitat preferences.

Comment on “Soil Moisture Response to Snowmelt and Rainfall in a Sierra Nevada Mixed‐Conifer Forest”

[Bales et al(2011)][1] advance the understanding of soil moisture dynamics in response to water input, especially snowmelt, in the mixed conifer zone of the Sierra Nevada, California. In particular, we appreciate the authors” conclusion that regolith deeper than 1 m is important in supporting base

Spatial patch patterns and altered forest structure in middle elevation versus upper ecotonal mixed-conifer forests, Grand Canyon National Park, Arizona, USA

In the American Southwest, mixed-conifer forest experienced altered disturbance regimes with the exclusion of fire since the early 1900s. This research analyzes patch development and tree spatial patterns in the middle versus upper mixed-conifer forests at Grand Canyon National Park in Arizona (USA). The methods used include: (1) size–structure analyses, to compare species patch development; (2) dendrochronological dating of tree establishment and fire history; (3) tree ring master chronology, to determine periods of suppressed growth, compared to a palmer drought severity index; (4) spatial analyses by size and age, with univariate and bivariate analyses of spatial association as well as spatial autocorrelation. Results show that unlike the lower ecotone of the mixed-conifer zone, both the middle elevation and upper ecotone were mixed-conifer forests before Euro-American settlement. At the upper ecotone, two decades (1870s and 1880s) had no successful conifer establishment but instead aspen cohorts, corresponding to the fire history of synchronized fires. Overall, the upper ecotone has shifted in composition in the absence of surface fires from mixed conifer to encroachment of subalpine species, particularly Engelmann spruce. Spatial patterns of tree sizes and tree ages imply development of a size hierarchy in an aging patch. In addition, shifts in species composition from ponderosa pine and white fir overstory to Engelmann spruce and Douglas-fir understory affected within-patch spatial patterns. These results provide quantitative evidence of past and present forest conditions for the development of restoration strategies for Southwestern mixed-conifer forests.

Fire and persistence of montane chaparral in mixed conifer forest landscapes in the northern Sierra Nevada, Lake Tahoe Basin, California, USA1

Abstract Nagel, T. A. and A. H. Taylor (Department of Geography, The Pennsylvania State University, 302 Walker Building, University Park, PA 16802). Fire and persistence of montane chaparral in mixed conifer forest landscapes in the northern Sierra Nevada, Lake Tahoe Basin, California J. Torrey Bot. Soc. 132: 442–457. 2005.— Tree cover in the mixed conifer zone in the Sierra Nevada is often interrupted by large shrub fields, or stands of montane chaparral. Chaparral stands are thought to be associated with either poor site conditions, or locations that have experienced severe fire. Fires have been excluded from mixed conifer forests for nearly a century because of a management policy of suppressing fire. This study quantifies the intervals between fires, and the response of chaparral shrubs and trees to fire and fire exclusion in six montane chaparral stands. Quercus vaccinifolia and Arctostaphylos patula were the dominant shrubs on all sites and the last fires burned in the six sites between 1861 and 188...

Forest gradient response in Sierran landscapes: the physical template

Vegetation pattern on landscapes is the manifestation of physical gradients, biotic response to these gradients, and disturbances. Here we focus on the physical template as it governs the distribution of mixed-conifer forests in California’s Sierra Nevada. We extended a forest simulation model to examine montane environmental gradients, emphasizing factors affecting the water balance in these summer-dry landscapes. The model simulates the soil moisture regime in terms of the interaction of water supply and demand: supply depends on precipitation and water storage, while evapotranspirational demand varies with solar radiation and temperature. The forest cover itself can affect the water balance via canopy interception and evapotranspiration. We simulated Sierran forests as slope facets, defined as gridded stands of homogeneous topographic exposure, and verified simulated gradient response against sample quadrats distributed across Sequoia National Park. We then performed a modified sensitivity analysis of abiotic factors governing the physical gradient. Importantly, the model’s sensitivity to temperature, precipitation, and soil depth varies considerably over the physical template, particularly relative to elevation. The physical drivers of the water balance have characteristic spatial scales that differ by orders of magnitude. Across large spatial extents, temperature and precipitation as defined by elevation primarily govern the location of the mixed conifer zone. If the analysis is constrained to elevations within the mixed-conifer zone, local topography comes into play as it influences drainage. Soil depth varies considerably at all measured scales, and is especially dominant at fine (within-stand) scales. Physical site variables can influence soil moisture deficit either by affecting water supply or water demand; these effects have qualitatively different implications for forest response. These results have clear implications about purely inferential approaches to gradient analysis, and bear strongly on our ability to use correlative approaches in assessing the potential responses of montane forests to anthropogenic climatic change.

Ponderosa Pine Aboveground Growth after Cattle Removal of Terminal Tissue

Mismanagement of livestock can result in herbivory on Pinus seedlings with potential for growth loss. From 1986 to 1990 on a conifer plantation in the Mixed-Conifer Zone of southwestern Oregon we measured the aboveground growth of ponderosa pine (Pinus ponderosa Douglas ex Lawson &C. Lawson var. ponderosa) seedlings that had sustained (1) removal of terminal stem or bud tissue by cattle, and (2) no removal (control). In 1990 (year 5 of the plantation), cumulative, absolute height and stem volume on seedlings that sustained terminal tissue removal were similar to control seedlings. In 1988, relative height growth was comparable between control seedlings and seedlings sustaining tissue removal in May 1987. In contrast, 1988 relative height growth was -22% (P < 0.05) for seedlings sustaining tissue removal in August 1987, compared with control seedlings. Relative stem volume growth in 1988 ranged from -21 to -26% (P < 0.05) for seedlings sustaining tissue removal in (1) August 1987, (2) April or May 1988, and (3) 1987 and 1988, compared with control seedlings. Reduced relative growth was short-term. Relative growth rates for seedlings sustaining tissue removal were comparable to control by the first or second year after removal. Seedlings sustaining tissue removal before winter bud set (April and May) were more likely to recover by the first year than seedlings sustaining tissue removal after winter bud set (August). We propose that if silviculturists prescribe cattle grazing early in the growing season and terminal tissue is removed at that time, they can still achieve understory vegetation control and enhanced soil water availability without jeopardizing long-term aboveground growth of ponderosa pine. The lack of spatial replication of treatments (terminal tissue removal dates) limits spatial extrapolation of our findings. The credibility of our findings should be tested with additional research on the growth response of ponderosa pine seedlings subsequent to actual live-stock herbivory, conducted with spatially replicated treatments.

Effect of root zone temperature on ectomycorrhiza and vesicular–arbuscular mycorrhiza formation in disturbed and undisturbed forest soils of southwest Oregon

The presence of ectomycorrhizal and vesicular–arbuscular (VA) mycorrhizal fungi in soils from five sites in a mixed conifer zone in southwest Oregon, each consisting of a 1- to 1.5-year-old clear-cut adjacent to an undisturbed forest stand, was determined by bioassay with Pseudotsugamenziesii (Mirb.) Franco, Pinusponderosa Dougl. ex P. Laws &amp; C. Laws, and Trifoliumsubterraneum L. 'Mt. Barker' as hosts grown at root zone temperatures ranging from 7.5 to 35 °C. Maximum formation of both ectomycorrhizae and VA mycorrhizae occurred at 18.5–24 °C in soils from all sites, and there were no significant qualitative or quantitative differences between disturbed (clear-cut) or undisturbed (forest) soils. Mycorrhiza formation was moderate even at the lowest temperature tested (7.5 °C) but was greatly reduced or prevented at or above 29.5 °C. Treatment of soil at 35 °C for 1 week did not appear to adversely affect viability of ectomycorrhizal fungus propagules, but young mycorrhizae subjected to the same treatment appeared to be severely injured. Thus the ability of native mycorrhizal fungi to grow at low soil temperatures is especially important as they may contribute to the survival of seedlings outplanted into climatic zones characterized by warm, dry summers following cool, wet winters and springs.

Impact of fire suppression on a mixed-conifer forest

One hundred years of fire suppression in a mixed-conifer forest which evolved with frequent natural fires has shifted successional patterns, increased the density of small trees, produced an unnatural accumulation of ground fuels. Analysis of species composition, vegetation structure and age distribution in each of four forest types within the mixed-conifer zone of Sequoia and Kings Canyon National Parks, California, has documented a substantial increase in young, shade tolerant white fir in each type. The original dominant species have decreased in relative abundance in most cases. The sequioa type has been most affected by the fire suppression policy. Giant sequioa show poor reproduction in the absence of fire. The sequoia type also exhibits the greatest accumulation of ground fuels. The ponderosa pine, white fir and mixed forest types also show successional changes as well as significant accumulations of flammable ground fuels following a century of fire exclusion. The management implications of these findings are discussed.