Alan H. Taylor

Widespread increase of tree mortality rates in the western United States.

Persistent changes in tree mortality rates can alter forest structure, composition, and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions. Increases were also pervasive across elevations, tree sizes, dominant genera, and past fire histories. Forest density and basal area declined slightly, which suggests that increasing mortality was not caused by endogenous increases in competition. Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed solely to aging of large trees. Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.

Fire history and landscape dynamics in a late-successional reserve, Klamath Mountains, California, USA

The frequency, extent, and severity of fires strongly influence development patterns of forests dominated by Douglas-fir in the Pacific Northwest. Limited data on fire history and stand structure suggest that there is geographical variation in fire regimes and that this variation contributes to regional differences in stand and landscape structure. Managers need region-specific fire regime data to develop process-based management schemes to manage new late-successional reserves (LSR). This study quantifies fire regimes and stand structural patterns in a LSR in Douglas-fir-dominated forests in northern California. We analyzed tree species composition, structure (diameter, age), and fire scars from 75 plots in a 1570 ha area in the northern Klamath Mountains. Tree species composition varied with elevation and aspect, and median fire return intervals were similar (12‐19 years) among species composition groups. However, median fire return intervals (FRI) were shorter on south- (8 years) and west-facing (13 years) slopes than on northern (15 years) or eastern (16.5 years) aspects. Fire return intervals also varied by historical period. Median FRIs were longer (21.8 years) during the suppression period (1905‐1992) than in the settlement (1850‐1904) (12.5 years) or presettlement (1627‐1849) (14.5 years) period. The average burn area for a fire was 350 ha, and 16 fires larger than 500 ha burned between 1627 and 1992. Fire rotations varied by century from 15.5 to 25.5 years and were longest in the fire suppression period. Stand conditions were multi-aged, and Douglas-fir recruitment occurred after fire. Patterns of past fire severity, inferred from age-classes, indicate that upper slopes, ridgetops, and south- and west-facing slopes experienced more severe fires between 1850 and 1950 than lower slopes or east- and north-facing slopes. Implications are that lower slopes and north and east aspects are more likely than other topographic positions to sustain or promote long-term, latesuccessional conditions. Prescribed fire will likely be an integral component of management plans that successfully maintain natural processes and structures in newly established late-successional reserves in the Klamath Mountains. # 1998 Elsevier Science B.V.

Spatial patterns and controls on historical fire regimes and forest structure in the Klamath mountains

Fire exclusion in mixed conifer forests has increased the risk of fire due to decades of fuel accumulation. Restoration of fire into altered forests is a challenge because of a poor understanding of the spatial and temporal dynamics of fire regimes. In this study the spatial and temporal characteristics of fire regimes and forest age structure are recon- structed in a 2325-ha mixed conifer forest in the Klamath Mountains. Forests were multiaged and burned frequently at low and moderate severity, but forest age structure did not vary with aspect, elevation, or topographic position. Recently there has been an increase in forest density and a forest compositional shift to shade-tolerant species. Median fire return in- tervals (FRI) ranged from 11.5 to 16.5 yr and varied with aspect but not with forest composition or elevation. The median area burned was 106 ha, and the pre-Euro-American fire rotation of 19 yr increased to 238 yr after 1905. Intra-annual position of fire scars in the tree rings indicates that 93% of fires occurred during the dry midsummer through fall period. Spatial patterns of sites with similar fire dates were spatially coherent and separated from others by topographic features that influence fire spread. Thus, patterns of fire oc- currence tended to be fixed in space with timing of fires varying among groups of sites. Spatial and temporal patterns of fire occurrence suggest that managers using physical fea- tures to contain prescribed fire will create burn patterns consistent with historical fires in the Klamath Mountains.

Multi‐scale controls of historical forest‐fire regimes: new insights from fire‐scar networks

Anticipating future forest-fire regimes under changing climate requires that scientists and natural resource managers understand the factors that control fire across space and time. Fire scars – proxy records of fires, formed in the growth rings of long-lived trees – provide an annually accurate window into past low-severity fire regimes. In western North America, networks of the fire-scar records spanning centuries to millennia now include hundreds to thousands of trees sampled across hundreds to many thousands of hectares. Development of these local and regional fire-scar networks has created a new data type for ecologists interested in landscape and climate regulation of ecosystem processes – which, for example, may help to explain why forest fires are widespread during certain years but not others. These data also offer crucial reference information on fire as a dynamic landscape process for use in ecosystem management, especially when managing for forest structure and resilience to climate change.

Regeneration patterns in old-growth Abies-Betula forests in the Wolong Natural Reserve, Sichuan, China

(1) Regeneration patterns of Abiesfaxoniana, Betula utilis and B. albosinensis in central Sichuan, China were interpreted by stand structural analysis (age, size, horizontal patterning) of tree populations in old-growth ( > 250 years) stands. (2) Seedlings, saplings and young trees were scarce in stands where bamboo (Sinarundinaria fangiana) impeded tree regeneration. Shrub density and woody plant species richness were lower in stands with high bamboo cover. (3) Regeneration patterns indicate that A. faxoniana and B. utilis relative densities were similar where bamboo was dense and that A. faxoniana was more than twice as abundant as B. utilis where bamboo cover was low or absent. Bamboos seem to impede A.faxoniana regeneration more than that of B. utilis. Both species regenerated in canopy gaps, but A. faxoniana less so than B. utilis where bamboos were dense. This produces higher Betula density in bamboo stands. Sizes of young patches of B. utilis tended to be larger than patches of A.faxoniana. Partitioning of gaps by size may contribute to coexistence of A. faxoniana and B. utilis in old-growth stands.

Fire regimes, forest change, and self-organization in an old-growth mixed-conifer forest, Yosemite National Park, USA

Fire is recognized as a keystone process in dry mixed-conifer forests that have been altered by decades of fire suppression, Restoration of fire disturbance to these forests is a guiding principle of resource management in the U.S. National Park Service. Policy implementation is often hindered by a poor understanding of forest conditions before fire exclusion, the characteristics of forest changes since excluding fire, and the influence of topographic or self-organizing controls on forest structure. In this study the spatial and temporal characteristics of fire regimes and forest structure are reconstructed in a 2125-ha mixed-conifer forest. Forests were multi-aged, burned frequently at low severity and fire-return interval, and forest structure did not vary with slope aspect, elevation, or slope position. Fire exclusion has caused an increase in forest density and basal area and a compositional shift to shade-tolerant and fire-intolerant species. The median point fire-return interval and extent of a fire was 10 yr and 115 ha, respectively. The pre-Euro-American settlement fire rotation of 13 yr increased to 378 yr after 1905. The position of fire scars within tree rings indicates that 79% of fires burned in the midsummer to fall period. The spatial pattern of burns exhibited self-organizing behavior. Area burned was 10-fold greater when an area had not been burned by the previous fire. Fires were frequent and widespread, but patches of similar aged trees were < 0.2 ha, suggesting small fire-caused canopy openings. Managers need to apply multiple burns at short intervals for a sustained period to reduce surface fuels and create small canopy openings characteristic of the reference forest. By coupling explicit reference conditions with consideration of current conditions and projected climate change, management activities can balance restoration and risk management.

IDENTIFYING FOREST REFERENCE CONDITIONS ON EARLY CUT-OVER LANDS, LAKE TAHOE BASIN, USA

Pre-Euro-American settlement forest structure and fire regimes for Jeffrey pine-white fir, red fir-western white pine, and lodgepole pine forests in the Lake Tahoe Basin (California and Nevada, USA) were identified using stand structural analysis and fire scars based on measurements of stumps of trees that were cut in the 19th century. Com- parisons of the presettlement reference with contemporary conditions were then used to determine how and why contemporary conditions deviate from presettlement conditions and to guide ecological restoration. Contemporary forests varied in different ways compared to the presettlement reference. Contemporary Jeffrey pine-white fir forests have more and smaller trees, more basal area, less structural variability, and trees with a more clumped spatial distribution than preset- tlement forests. The mean presettlement fire-return interval for Jeffrey pine-white fir forests was 11.4 yr, and most ( .90%) fires burned in the dormant season; no fire was recorded in the study area after 1871. Similar differences were identified in the structural characteristics of contemporary and presettlement red fir-western white pine and lodgepole pine forests. However, 19th-century logging changed the composition of red fir-western white pine forests, and these forests now have more lodgepole pine than red fir or western white pine. Comparison of contemporary forests with the presettlement reference suggests that resto- ration treatments in Jeffrey pine-white fir forests should include: (1) density and basal- area reduction, primarily of smaller diameter trees; (2) reintroduction of frequent fire as a key regulating disturbance process; and (3) increasing structural heterogeneity by shifting clumped tree distributions to a more random pattern. Restoration treatments in red fir- western white pine forests should include: (1) a shift in species composition by a density and basal-area reduction of lodgepole pine; and (2) increasing structural heterogeneity by shifting tree distributions to a more random pattern. In lodgepole pine forests the restoration emphasis should be: (1) a density and basal-area reduction of small-diameter trees; and (2) an increase in structural heterogeneity that shifts tree spatial patterns from clumped to a more random distribution. Reintroduction of fire as a regulating process into high-elevation red fir-western white pine and lodgepole pine forests can be viewed as a longer-term restoration goal. The method for quantifying presettlement reference conditions from stumps complements, or is an alternative to, methods based on dendroecology or repeat photog-

Gradient Analysis of Fire Regimes in Montane Forests of the Southern Cascade Range, Thousand Lakes Wilderness, California, USA

Species distribution and abundance patterns in the southern Cascades are influenced by both environmental gradients and fire regimes. Little is known about fire regimes and variation in fire regimes may not be independent of environmental gradients or vegetation patterns. In this study, we analyze variation in fire regime parameters (i.e., return interval, season, size, severity, and rotation period) with respect to forest composition, elevation, and potential soil moisture in a 2042 ha area of montane forest in the southern Cascades in the Thousand Lakes Wilderness (TLW). Fire regime parameters varied with forest composition, elevation, and potential soil moisture. Median composite and point fire return intervals were shorter (4-9 yr, 14-24 yr) in low elevation and more xeric white fir (Abies concolor)-sugar pine (Pinus lambertiana) and white fir-Jeffrey pine (P. jeffreyi) and longest (20-37 yr, 20-47 yr) in mesic high elevation lodgepole pine (Pinus contorta) and red fir (Abies magnifica)-mountain hemlock (Tsuga mertensiana) forests. Values for mid-elevation red fir-white fir forests were intermediate. The pattern for fire rotation lengths across gradients was the same as for fire return intervals. The percentage of fires that occurred during the growing season was inversely related to elevation and potential soil moisture. Mean fire sizes were larger in lodgepole pine forests (405 ha) than in other forest groups (103-151 ha). In contrast to other parameters, fire severity did not vary across environmental and compositional gradients and >50% of all forests burned at high severity with most of the remainder burning at moderate severity. Since 1905, fire regimes have become similar at all gradient positions because of a policy of suppressing fire and fire regime modification will lead to shifts in landscape scale vegetation patterns.

The structure and dynamics of Abies magnifica forests in the southern Cascade Range, USA

Abies magnifica (Red fir) forests in the Cascade Range and Sierra Nevada of California are composed of groups, or patches, of even-sized individuals that form structurally complex stands. Patches may be even-aged, resulting from synchronous post-disturbance establishment, or multi-aged, reflecting continuous recruitment of seedlings moderately tolerant of shade. We analyze the population structure (i.e. age, size, and spatial patterning) of A. magnifica, and associated A. concolor, White fir, and reconstruct the disturbance history of two mature to old-growth A. magnifica forests in order to determine the relationship between disturbance and forest structure. Within both stands examined, the distributions of A. magnifica seedlings, saplings, and small understory trees were clumped, with clump sizes corresponding to the area of canopy gaps. Gaps were created by frequent wildfire (mean fire return interval of 41 yr) and by windstorms. Severe fire initiated mass establishment of Abies magnifica, whereas gaps created by windthrow released already established individuals. Low inten- sity fire stimulated little recruitment, but Red fir established continuously during fire-free intervals. Thus, the complex age and structures of Red fir forests reflect both episodic and continuous recruitment, as determined by the type and severity of natural disturbance.

Fire-climate interactions in forests of the American Pacific Coast

We investigate relationships between climate and wildfire activity between 1929 and 2004 in Pacific coast forests of the United States. Self-Organizing Mapping (SOM) of annual area burned in National Forests (NF) in California, Oregon, and Washington identifies three contiguous NF groups and a fourth group of NF traversed by major highways. Large fire years in all groups are dry compared to small fire years. A sub-hemispheric circulation pattern of a strong trough over the North Pacific and a ridge over the West Coast is characteristic of large fire years in all groups. This pattern resembles the Pacific North American (PNA) teleconnection and positive phase of the Pacific Decadal Oscillation (PDO). A reverse PNA and negative PDO phase characterizes small fire years. Despite the effect of fire suppression management between 1929 and 2004, forest area burned is linked to climatic variations related to large-scale atmospheric circulation patterns.