• What age structures characterize southwest Oregon chaparral? How long do component species liv e? How is chaparral in southwest Oregon similar to California chaparral and how is it different?
• How has chaparral reacted to past fire disturbances and current fire suppression?
• Do hand-cut pile and burn fuels treatments reproduce structures observed in undisturbed chaparral? Do treatments mimic fire disturbance?

Olivia C. Duren and Patricia S. Muir
Botany & Plant Pathology, Oregon State University, Corvallis, OR, USA

Extensive acreages of chaparral on Bureau of Land Management lands in southwest Oregon are managed with fuels treatments that aim to mitigate presumed ecosystem degradation and increased fire hazard thought to be caused by fire suppression. The fire ecology of this ecosystem is virtually unknown, however, and the assumptions guiding treatment need and design are based on extrapolations from better studied ecosystems. In the absence of information, it has often been assumed that fire in chaparral systems was frequent and low-severity, as it was in nearby conifer forests prior to Euro-American settlement. Such a regime would presumably create a landscape of scattered shrubs rather than the dense chaparral that exists today. What structures and fire regimes characterize southwest Oregon interior valley chaparral, and does it appear that structures have been altered by fire suppression?  Are current fuels treatments likely to accomplish restoration of these systems?  To what extent are community processes in southwest Oregon interior valley chaparral, which represents the northernmost extent of the chaparral range, comparable to California chaparral and other Mediterranean-climate obligate-seeder shrub communities?  To address these questions, we studied patterns in age structure of two obligate-seeding chaparral shrubs, sticky whiteleaf manzanita (Arctostaphylos viscida) and buckbrush (wedgeleaf) ceanothus (Ceanothus cuneatus), and assessed relationships of structures to environment and fire.

We studied two chaparral shrub species, buckbrush (wedgeleaf) ceanothus (Ceanothus cuneatus) and sticky whiteleaf manzanita (Arctostaphylos viscida). These two species range from Baja Mexico and southern California, respecitively, north to Oregon. The hot, dry interior valleys of southwest Oregon are considered to be the northernmost extent of the chaparral vegetation type, but most understandings of chaparral species biology and fire ecology are based on studies of southern California populations. In California, these shrubs are known as obligate-seeders meaning they generally require fire for seed germination, resulting in stands where shrub ages all date back to the last fire. Neither species resprouts after fire. Oregon chaparral has been segregated from California chaparral by the Siskiyou Mountains for at least 4,000 yr. Can tenets of chaparral ecology and management be applied across its range, or does geographic variability necessitate geographically-tailored understandings of community fire ecology and management?

We cut stump slices of 15-25 shrubs from buckbrush ceanothus and whiteleaf manzanita along transects at 31 sites in the BLM-managed Applegate Adaptive Management Area of southwest Oregon. Sites were in stands that hadn't experienced a known fire from 22 yrs to over 114 yrs prior to sampling; half the sites had been treated for fuels reduction within 9 yr of our sampling. Sites were at elevations of 500 m to 1200 m; aspects were 117 - 301° and slopes were 13 - 34 %. Although shrubs commonly occur scattered in the understory of oaks and conifers, we focused on dense, closed-canopy shrub stands with few trees. Shrub samples were sanded and rings counted to assess shrub age

Quantification of the distribution of sampled shrub ages in each stand (its age structure), allowed us to describe patterns in age structure, compare them among stands, and assess relationships among age structures, environment and disturbance history.  Age structures were characterized by six attributes: median age, skew, kurtosis, variance, coefficient of variation (CV), and range in ages divided by median age. We mapped each stand in the space defined by its age structure attributes using nonmetric multidimensional scaling (NMS) ordination, then evaluated the relationships of age structure patterns to site environmental characteristics (climate, terrain, geology and soils, and stand structure) by looking at variables' correlations with ordination axes. Age structure patterns were also analyzed in relation to wildfire disturbance history and inferred grazing disturbance.

What age structures characterize southwest Oregon chaparral? How long do component species live? How is chaparral in southwest Oregon similar to California chaparral and how is it different?

The oldest whiteleaf manzanita we found was at least 146 yr old; 44 manzanita were at least 100 yr old.

The oldest buckbrush ceanothus we found was at least 114 yr old; 3 ceanothus were at least 100 yr old.

These are minimum ages because many older samples were rotted in the center. One hundred year old shrubs of both species had basal diameters from 10 cm to 50 cm, suggesting that inferring shrub age from diameter alone is highly error-prone.

We found that, unlike chaparral studied in California, nearly all of our stands unburned for > 30 yr were uneven-aged (shrub ages > 20 yrs apart) due to frequent shrub recruitment in the absence of fire and appreciable numbers of shrubs that apparently survived the last fire. Even-aged stands were nearly all young (< 30 yr) and had relatively high shrub density, but shrub density declined the longer stands were unburned. No relationships were detected between age structures and site environment or fire and grazing disturbance history, suggesting that most, if not all, chaparral stands across the range of conditions represented in the study area would, over time, develop uneven-aged structure.

Most shrubs that recruited in the absence of fire were buckbrush ceanothus. Successful inter-fire recruitment of obligate-seeders in Mediterranean-type ecosystems is rare. Even though a few percent of seeds of obligate-seeders are non-refractory and can produce seedlings without fire, seedlings are generally thought to not persist in intact, robust shrublands.  In contrast, we found that inter-fire recruits could persist to at least 46 yr of age. It isn't clear why buckbrush ceanothus populations apparently differ in recruitment biology between Oregon and southern California, but differences may relate to higher summer soil moisture in the north, or perhaps to genetic differences resulting from geographic isolation.

Recruitment of shrubs in the absence of fire contributed an average of 46 new shrubs per ha per decade (range 6 - 100 shrubs). Despite continual fire-free recruitment, overall shrub density declined as stands aged, as has been observed elsewhere.

How has chaparral reacted to past fire disturbances and current fire suppression?

Age structures indicate that fires burning in chaparral have been high-severity (at least 93 % shrub basal area killed) since at least the first decade of the 1900s. This is consistent with the records of a grazing examiner working in the same area in who documented high-severity fires in chaparral in 1915. This evidence suggests that fire suppression may not have changed fire severity. High-severity fires are also characteristic of chaparral in California and elsewhere; such fires help clear encroaching trees and aid germination of species with fire-stimulated seeds. However, shrubs older than the last known fire in southwest Oregon chaparral suggest that fires in this region may burn with higher spatial complexity than in southern California chaparral. Management tools (e.g., fuel models) developed in southern California chaparral may need to be modified for southwest Oregon chaparral.

Do hand-cut pile and burn fuels treatments reproduce structures observed in undisturbed chaparral? Do treatments mimic fire disturbance?

Hand-cut pile and burn fuels treatments as they're currently implemented appear unlikely to reproduce structure of natural chaparral stands. Nearly complete cover has been recorded in undisturbed stands both recently and by a grazing examiner reporting in the mid-1910s; dense cover is also characteristic of these communities elsewhere. Shrub cover in treated stands, however, is > 7xs lower than cover in undisturbed stands for at least 7 yr post-treatment. Quickly regenerating shrub cover may be important in reducing establishment and expansion of noxious weeds. Conversely, shrub cover retained by treatments is > 7xs greater than cover left by fire. As other work has found, treatments without a stand-scale fire component don’t stimulate natural levels of regeneration of shrubs or post-fire endemics.

Because southwest Oregon chaparral stands appear to have inherently high fuel loads and burn in high severity fires, the goals of reducing fire hazard with current fuels treatments may be incompatible with chaparral ecology and restoration. Managers may consider leaving chaparral untreated except where fuel loads post an unacceptable fire hazard to human life or property (e.g., along roads or below houses). This study did not address whether management goals to reduce fire hazard are met by fuels treatments (but see one observation that compared treated and untreated areas in chaparral burned by the 2002 Squires fire). Much more information is needed: Under what fuels and fire weather conditions are treatments able to change fire behavior in chaparral? Can fuels treatment methods that are consistent with the ecology of chaparral in southwest Oregon by developed and implemented effectively? Monitoring is also needed to determine what happens to stands after treatment in the long term; do shrubs eventually regenerate high cover or do treatments initiate a fundametal state change?

Link to journal article (link to journal with full free pdf, "Does fuels management accomplish restoration in southwest Oregon, USA, chaparral?").
See a presentation on southwest Oregon chaparral (PDF).
Contact O. Duren or P. Muir.

Photos were taken by Olivia Duren, unless otherwise noted.

Page by Olivia Duren. Updated 10/2010.