Investigators

Gradients in structure among stands were determined by ordinating stands in the space defined by their sampled stand and age structure characteristics using nonmetric multidimensional scaling ordination. The relationships of oak tree age structure and stand structure to site environment (terrain, climate, and soils) and fire disturbance history were then examined by inspecting correlations of environment and recorded fire variables (back to 1910) with ordination axes.  Sites from the two study areas were also analyzed separately due to differences detected in preliminary analyses.

Research Questions

Laurie A. Gilligan and Patricia S. Muir
Botany & Plant Pathology, Oregon State University, Corvallis, OR, USA.

• What are the current stand and age structures of Oregon white oak-dominated stands in southwestern Oregon? Do current structures vary systematically with site environment? Can oak ages be estimated based on their sizes?
• What is the status of oak regeneration in these woodlands?
• Do age or size distributions suggest that disturbance regimes in oak woodlands changed after Euro-American settlement or after fire suppression? Do stand and age structures vary systematically with site fire history?

Large acreages of Oregon white oak communities in southwestern Oregon are thinned to reduce fire hazard and accomplish ecosystem restoration under the assumptions that current fuel loads are unnaturally high and stands are unnaturally dense as a result of fire suppression.  Although oak communities are a characteristic landscape component in this region, little is known about their current or historic stand structures.  What stand structures are evident in southwestern Oregon white oak woodlands, and do structures appear to vary systematically along environmental or disturbance history gradients? How do structures of southwestern Oregon woodlands compare to those of Oregon white oak woodlands elsewhere? Are these woodlands affected by the regeneration problems that affect oak stands elsewhere? Were woodlands more open prior to the onset of effective fire suppression, or prior to Euro-American settlement? More information is needed to advance basic understanding of these systems, and to foster ecologically-based management within them. 

Oregon white oak occurs from British Columbia, Canada, south to the Sierra Nevada Mountains of California. An estimated 85% or more of Oregon white oak communities that existed prior to Euro-American settlement have been lost to land conversion, overtopping by conifers, and other human disturbances across the range of this ecosystem type. Further imperiling Oregon white oaks, and other oak species throughout the world, are apparent regeneration problems – more information is needed on the regeneration status of oaks  in southwestern Oregon. The loss of oak-dominated habitats also threatens many of the plant and animal species that depend on them.  (Check out an oak bibliography compiled by L. Gilligan.) 

We inventoried 40 randomly-selected stands currently dominated by Oregon white oak (canopy cover 25 to 85%, mean 54%) across relatively dry (Applegate Valley; 646 mm mean annual precipitation) and less dry (Butte Falls; 853 mm mean annual precipitation) study areas in Jackson County, southwestern Oregon. Study sites encompassed all aspects; elevations were 400 – 1250 m and slopes were 4 – 80%. We recorded structural characteristics (height, diameter at breast height [dbh], crown ratio, and number of stems per individual), density and health condition for all tree species; and cover for all woody species and physiognomic groups within a 0.1 ha portion of each stand.  We also recorded information on relict oak trees (large trees with low-growing large limbs and a broad crown) visible from the center of the stand. Tree ages were determined by coring a subsample of trees (≥ 10 cm dbh) and saplings (≥ 1.3 m tall and < 10 cm dbh); ages were estimated for trees that weren’t cored using non-parametric multiplicative regression (NPMR). Within three subplots totaling 0.01 ha, tree sprouts and seedlings (< 1.3 m tall) of all species were counted.

The oldest Oregon white oak we found in our sites was at least 429 yr old, and probably older because this tree was not cored to the pith.  An off-site oak in a conifer-overtopped woodland was aged at 450 yr, which is, as far as we know, the oldest count yet on record for this species.  All trees ≥ 40 cm dbh whose ages could be counted to the pith were > 200 yr old. Oak ages were not strongly predicted by dbh alone (xR2 = 46 - 50%; NPMR), suggesting that caution should be used in interpreting oak age from size alone.

Age and size structures ranged widely across sampled stands. Structures of surviving Oregon white oaks showed that most stands had experienced  pulsed regeneration, with limited recruitment since the 1930s or earlier.

Above, representative Oregon white oak age and sapling (sap) height distributions (left column) paired with size distributions (right column) for oaks and other tree species in the same stands. Tree age structures are based on both counted and predicted ages. Arrows show counted age ranges of sapling height classes. Stand A illustrates single-pulse recruitment (44% of stands); Stand B illustrates historically continuous or frequent pulses of recruitment but no saplings present (28% of stands); Stand C illustrates historically continuous or frequent pulses of recruitment with apparently recent regeneration (22% of stands). Stand D illustrates no surviving ancient trees and apparently recent Oregon white oak infilling (6%) of stands. There was clear potential for conversion to Douglas-fir dominance in 14% of stands (not counting stand already overtopped). QUGA = Quercus garryana, PSME = Psuedotsuga menziesii, PIPO = Pinus ponderosa, QUKE = Q. kelloggii, ARME = Arbutus menziesii. Click here for a larger figure (PDF).

The mean age of 10.0 – 10.9 cm dbh oaks was 113 yr (range 59 – 162 yr); saplings (trees ≥ 1.3 m tall but < 10 cm dbh) were 8 – 164 yr old. This suggests that the presence of small oak trees does not necessarily indicate recent regeneration, reinforcing that size is not a good predictor of age for oaks in southwestern Oregon. Sapling-sized trees are often overlooked in age structure studies, but their broad age range indicates that knowledge of their ages is important for understanding stand dynamics.

Stand structures tended to diverge between the two study areas, with higher basal area, % cover, and density across tree species, and taller oaks and more abundant seedling-size regeneration in the relatively mesic study area (Butte Falls), on average. Lower tree health scores in Butte Falls may reflect competitive stresses associated with higher stocking levels there. In the relatively xeric study area (Applegate), the already hot and droughty growing conditions of southwestern Oregon’s interior valleys may be exacerbated by somewhat lower precipitation and more silty (less clayey) soils, as suggested by generally shorter oaks and less abundant seedling-sized regeneration than in Butte Falls.

Overall, environmental gradients were not consistently related to variation in stand structure, indicating that stands are probably influenced by complex interactions of environmental characteristics, in concert with variations in site history and successional status.

Oak basal areas (4.5 – 42.4 m2/ha) were similar to those documented elsewhere, whereas tree and sapling densities (150 – 1810 stems/ha ) were generally higher than in stands to the north but lower than in some California stands. Generally, southwestern Oregon white oaks were shorter (stand means 5.1 – 9.1 m)  with smaller diameters than trees of similar age reported from elsewhere, suggesting that their growth rates and stature may be unique to southwestern Oregon, perhaps affected by site limitations such as moisture availability or soil depth.  Southwestern Oregon white oak woodlands were generally more similar to drought-tolerant oak plant associations of northern California than to oak communities further north in the Willamette Valley in their range of tree species compositions (including ponderosa pine, Douglas-fir, California black oak, Pacific madrone, and incense-cedar), and their linkages to chaparral and other shrub communities (stand shrub cover 2 – 75%;  chaparral species including whiteleaf manzanita or buckbrush ceanothus occurred in all sites).  Shrubs can provide cover to small mammals that disperse fungal mycorrhizae that are beneficial for successful transitions of oaks from seedling to saplings.

What is the status of regeneration in southwestern Oregon white oak woodlands?

Oregon white oak saplings took, on average, 6 yr to reach a height of 0.8 m, but can take much longer if seedlings are browsed or don’t initially produce a single dominant stem.

Oregon white oak seedling-sized regeneration (individuals < 1.3 m tall) occurred in all stands, often abundantly; most (92%) regeneration occurred under an Oregon white oak canopy drip line. Nearly two-thirds of stands had low or very low Oregon white oak sapling (S) to tree (T) ratios (S:T < 0.5), however, which may suggest that relatively few oak seedlings are transitioning to sapling sizes.  Causes for slow or halted regeneration are uncertain but have been related elsewhere to conifer or exotic grass encroachment, deer browsing and small mammal herbivory, and historic woodfelling. Sites were probably grazed in the past (and recently, in some cases); livestock grazing can be detrimental or beneficial to oak regeneration. Prolonged periods without fire (> 100 yr in many of our sites) may also be detrimental to

regeneration levels if fire-stimulated sprouting is the primary regeneration mechanism. Lack of fire disturbance may limit the transition between the seedling and sapling layer if, for example, fire reduces competition and thereby stimulates growth of surviving regeneration. Alternatively, regeneration may be limited simply because stands may already be fully stocked. It is uncertain whether low tree recruitment is problematic considering the long lifespan of Oregon white oaks (> 400 yr).

Oregon white oak sapling to tree ratios were not consistently correlated with environmental variables, and sites with low ratios occurred across all heat loads and densities.  However, sites with moderate heat load and high oak stocking typically had the highest sapling to tree ratios, with lowest ratios on sites with the highest heat loads and lower oak stocking.  These patterns might suggest that higher canopy cover ameliorates site conditions such that seedling survival and the transition to sapling size is favored. Alternatively, high stocking rates may simply indicate microsite suitability for oaks or abundant regeneration sources (e.g., acorns or basal sprouts).

Do age and size distributions suggest that disturbance regimes in oak woodlands changed after Euro-American settlement or after fire suppression? Do stand and age structures vary systematically with site fire history?

Oak stand structures prior to Euro-American settlement are uncertain. It is commonly assumed that Oregon white oak densities were lower prior to Euro-American settlement, and the structures of stands with low densities of old open-grown trees but currently dominated by trees established during or after settlement are consistent with this conception. The average relict tree density (21 per ha, for stands with relicts),  overlaps relict densities documented for Oregon white oak-dominated communities to the south and in some areas to the north of our study area, but are somewhat higher than those reported for the Willamette Valley. Relict oaks in southwestern Oregon often had a more vase-shaped structure than is typically recorded in the Willamette Valley, also suggesting that stands in southwestern Oregon may have historically been somewhat denser than those elsewhere. Early survey records (1850s) document that both savannas with widely-spaced oaks and dense oak woodlands occupied substantial portions of the southwestern Oregon landscape at the time of settlement. Our data confirm the pre-Euro-American settlement presence of oaks on many sites, because many old trees were multiple-stemmed, indicating that they previously occupied the site before being top-killed and resprouting. The high patchiness of the current landscape suggests that multiple oak community structures could have been possible in the past.

In some sites, an oak establishment pulse was evident in 1910 – 1930, a time during which several large wildfires occurred.  In other sites, tree ages mostly dated to the time of Euro-American settlement, and in still others, tree ages were not associated with known disturbance dates.   Tree recruitment decreased or even stopped in most sites after fire suppression became mechanized and particularly effective (~1950s). Although wildfire can erase evidence of pre-fire tree density and establishment, stands without recent regeneration hadn’t burned since the onset of effective fire suppression.

All surviving Douglas-firs that we aged established post-Euro-American settlement, and recruitment of this species increased after effective fire suppression; ponderosa pines established both pre- and post-settlement, with higher rates of recruitment post-settlement.

The overall influence of wildfire on other stand structural characteristics was not clear, at least in part because most stands had not experienced recorded fire since 1936 or before. Some stands that had recorded fires since 1910 were even-aged and dated back to a wildfire, whereas other stands had experienced recorded fire but tree ages were not associated with the fire dates.  Oregon white oaks are fire resistant; higher-intensity fires may kill above-ground stems and cause basal or epicormic resprouting, so single- and multiple-stemmed oaks may represent different fire or other disturbance histories. Multiple-stemmed trees were more common in the Applegate than in Butte Falls, which might suggest different disturbance histories or oak response to disturbances between the two study areas.

Oak-dominated communities in southwestern Oregon often occur in relatively small patches adjacent to other plant communities rather than as the large contiguous expanses that are believed to have occurred prior to Euro-American settlement in areas such as the Willamette Valley of Oregon. Prior to Euro-American settlement, many oak communities were heavily managed by Native Americans, who are thought to have burned them frequently and extensively, presumably resulting in widely-spaced, open-grown trees.  While some current Oregon white oak stands in southwestern Oregon are open and savanna-like, many stands support higher tree densities and are dominated by small-diameter trees. We sought to determine whether detectable changes in stand structure occurred after Euro-American settlement or after the onset of effective fire suppression.  If such changes occurred, what reference models may be appropriate for restoration of southwestern Oregon oak woodlands?   

• Contrary to what has been assumed, it appears that fire suppression did not lead to an increase in recruitment of Oregon white oak trees in most stands. However, surviving trees suggest that recruitment in many stands did increase shortly after Euro-American settlement, and the low density of surviving relict trees suggests that at least some stands were likely much more open prior to settlement.

• The occurrence of highly variable age and size structures across the landscape does not point to a single reference condition for oak restoration. Uniform fuels treatments may not benefit all oak community types. Restoration practices will probably be most useful if they assume a variety of historic stand conditions ranging from open-oak types to closed woodlands. Large proportions of southwestern Oregon oak woodlands have already been treated, and in these slow-moving, arid ecosystems, treatments are likely to have long-lasting effects. Application of a variety of treatment prescriptions, with follow-up monitoring, and monitoring of stands that have already been treated, would help identify those practices that support long-term oak persistence.

• Oak stands with encroaching Douglas-fir can benefit from selective cutting, particularly of larger conifers, or from prescribed fire which can kill small diameter conifers and give a competitive advantage to oak resprouts. Fire disturbance, however, can also contribute to the establishment and spread of non-native weeds, and may exacerbate already low oak regeneration by killing young oaks and seedlings.  Again, follow-up monitoring is important.

• Even though it is not clear that low Oregon white oak regeneration rates in southwestern Oregon threaten the persistence of these communities, management efforts to increase regeneration could diversify stand and age structures and advance knowledge about ways to foster regeneration. Incorporating restoration goals such as increasing cover of native understory grass and forb species may also be beneficial.

Link to thesis (PDF).
Contact L. Gilligan or P. Muir.

Photos were taken by Laurie Gilligan, unless otherwise noted.

►Landscape-scale patterns of vegetation in relation to environment and disturbance
►Stand structures of Oregon white oak woodlands and relationships to environment and disturbance
►Does fuels management accomplish restoration in southwest Oregon chaparral?
►Impacts of fuel reduction thinning on chaparral and oak-shrub plant communities
►Comparison of wildfire effects between sites treated for fuel reduction and untreated sites