Stand development, fire and growth of old-growth and young forests in southwestern Oregon, USA
Highlights
► Fire was common pre-1900, stand densities were low and trees grew rapidly. ► Fires and tree establishment occurred together for >200 years in old growth. ► Today’s young stands established without fire, at high densities and grew slowly. ► Low densities promote old-growth structure, reduce fire hazard and limit biomass. ► Today’s old growth differs from 1800 in terms of stand structure and fire hazard.
Introduction
Sustainable forest management in drier forest types like those in southwestern Oregon requires an understanding of long-term forest development and disturbance regimes, primarily fire, which differs from that of more northern forests on more mesic sites. These drier forests have lost many older, dominant trees that are >1 m diameter at breast height (DBH; 1.37 m) to stand-replacing fire. Also, the mortality of large trees in unburned stands has been high, likely due to increasingly high stand densities (Latham and Tappeiner, 2002) and drought stress from 1987 to 1992 and 2000 (Sensenig et al., 1994). In addition to drought stress and high stand density, pathogens and insects contributed to mortality. Recent old-tree mortality rates exceed replacement by younger trees and, if this trend continues, presage a pronounced reduction in old-growth dry forest ecosystems (Lehmkuhl et al., 1991).
Beginning around 1900, land management and fire suppression policy led to major changes on forest lands throughout western North America. Suppression policy was initiated following severe fires, often reburns, which covered millions of hectares of forests, destroyed towns and burned entire watersheds. The relative absence of regular surface fire has facilitated establishment of an overabundance of small fire-sensitive trees (Agee, 1993, Taylor and Skinner, 2003). Generally overstocked conditions in forest stands, accumulations of surface and aerial fuels, and increased susceptibility to fire create a dilemma for forest managers. These dense old stands, while susceptible to drought-related mortality and to fire, are also the habitat for late-successional species (FEMAT, 1993). Maintaining this habitat in old-growth stands, and growing it in younger stands, are major objectives of federal forest management in western Oregon (FEMAT, 1993).
The Northwest Forest Plan specifically calls for managing over 7.5 million acres of federal forests for late successional/old-growth habitat and another 2 million acres of riparian reserves; however, currently about 80% of these forests are composed of dense stands of relatively small trees (20–24 cm dbh) that are <40–120 years of age (FEMAT, 1993). These are classic even-aged stands that regenerated following timber harvest, wildfire, mining or other disturbance; they lack diversity in overstory tree size, age, and species as well as understory species and abundance (Spies and Franklin, 1991). Forest managers are concerned that such stands are particularly susceptible to drought-related mortality and stand-replacing wildfire. Thus, current policy calls for stand management to reduce mortality and fire hazard, and promote habitat characteristics associated with old-growth stands. Understanding tree growth, establishment of regeneration, and fire occurrence in both young and old stand types will help managers meet objectives for both stand types.
Studies on moist sites in northwestern Oregon have shown that large trees in old-growth stands grew much more rapidly at young ages than trees in young stands on similar sites (Tappeiner et al., 1997). Rapid early growth was an important characteristic of large old-growth trees (Poage and Tappeiner, 2002). Also, tree ages in old-growth stands were much more variable than those in young stands, suggesting a much different development in the two stand types. In a study throughout much of northwestern Oregon, Bailey and Tappeiner (1998) found that trees in old-growth stands had larger crowns, and stems were much larger in diameter relative to their height than trees in young trees (low height to diameter ratios-H:D). The crowns and stems of old trees are important components of stand structure and habitat in old-growth stands and the low H:D is indicative of resistance to damage from wind, ice, and snow (Wilson and Oliver, 2000). Knowledge of tree architecture (live crown length and H:D ratios) in old and young stands would provide goals for managing trees in young stand to achieve the characteristics of those in old stands. The objectives of this study were to provide forest ecologists and managers an understanding of old-growth and young forest development in three major forest types in southwestern Oregon, ranging from the mixed-conifer forests in the eastern Cascade through the Siskiyou mountains and into the mixed-evergreen forests of the coast ranges (Table 1). The old-growth stands we studied corresponded to Spies and Franklin’s (1991) definition of old-growth forests for southwestern Oregon. We evaluated fire occurrence along with tree ages, growth rates and architecture with the methods used in more northerly forests (Tappeiner et al., 1997, Bailey and Tappeiner, 1998, Poage and Tappeiner, 2002), including:
- (1)
tree recruitment from 1700 to 1990;
- (2)
tree age distributions (1340–1990);
- (3)
stand density (trees and basal area/ha) from 1800 to 1990;
- (4)
stand-level fire occurrence from 1700 to 1990 by aging fire scars; and
- (5)
tree growth/architecture obtained by comparing radial growth rates of trees at the same ages in old-growth and young stands;
- (6)
tree architecture from measured tree height and crown length, and calculated live-crown ratios, and height-to-diameter ratio (Wilson and Oliver, 2000, Tappeiner et al., 2007).
This study was conducted in forests of three common and distinct ecological regions in southwestern Oregon: the Cascades (the eastern extent of the mixed conifer type in southern Oregon); the Siskiyous (the drier mixed conifer forests); and the mixed evergreen forests mid-Coast Range mountains. Douglas-fir is common in all these forests, and density of other conifers varies considerably among sites (Atzet et al., 1996, Franklin and Dyrness, 1973; Table 1).
Annual precipitation across our study sites ranges from approximately 97–142 cm in the Cascades, 90–107 cm in the Siskiyous, and 91–138 cm in the mid-Coast Range, and mostly falls from October through May (Graumlich, 1987, Redmond, 1992). The growing seasons ranges from as early as March in the mid-Coast to May and June for the Siskiyou and Cascade sites, respectively. It generally abruptly ends with snowfall in October in both the Cascades and Siskiyou sites but usually continues beyond November in the mid-Coast sites. Productivity therefore spans site classes II and III (Hann and Scrivani, 1987) across all sites in the three forest types. Because of these differences in climate, growing season, productivity, and species composition, we expected that tree growth and fire occurrence would differ among these forest types.
Major tree species in the mid-elevation Cascades forests, included Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], ponderosa pine (Pinus ponderosa Dougl. Ex Laws.), white-fir [Abies concolor (Gord. & Glend.) (Lindl. Ex Hildebr.], and incense-cedar (Calocedrus decurrens Torr.). Primary hardwoods are California black oak (Quercus kelloggii Newb.) and Pacific madrone (Arbutus menziesii Pursh). Green-leaf manzanita (Arctostaphylos patula Green) and golden chinkapin [Castanopsis chrysophylla (Dougl. A.DC.] are the major shrub species.
Again, Douglas-fir, ponderosa pine, white-fir and incense-cedar are the major conifer species; canyon live oak (Quercus chrysolepis Liebm.), Oregon white oak (Quercus garryana Douglas. Ex Hook.), California black oak, and Pacific madrone are the major hardwoods in these forests. Shrubs include buckbrush [Ceanothus cuneatus (Hock) Nutt.] in addition to green-leaf manzanita and golden chinkapin.
The major coniferous tree species of the mid-Coast Range forests are Douglas-fir, ponderosa pine and incense-cedar. Tanoak [Lithocarpus densiflorus (Hook. & Arn. Rehd] is an evergreen hardwood that frequently dominates the understory. Pacific madrone is very common on drier sites. Pacific rhododendron (Rhododendron macrophyllum D. Don), salal (Gaultheria shallon Pursh) and golden chinkapin are common in the understory.
Section snippets
Methods
We studied tree age and radial growth rates, and determined the approximate years of fire, on large-tree stumps in 18 areas (six in each forest type), where old-growth stands had been clear-felled. Each area was matched with a nearby uncut old-growth stand, to examine tree architecture (heights, diameter, and live crown length), as well as an uncut young stand (60–100 years) for a total of 54 stands. We tested the hypothesis that there was no difference in stand development, tree growth and
Tree ages and establishment
Tree establishment in old-growth stands on our sites began as early as 1340; however, establishment mostly fell between 1700 and 1900 across all three forest types (Fig. 1). The exceptions were at Shell Peak and Hoxie in the Cascades where 56% and 22% of the trees were established before 1700. Overall, though, we found relatively low percentages of trees that established before 1700: only 13%, 5%, and 7% in the Cascades, Siskiyous, and mid-Coast Range, respectively, with even less establishment
Tree and stand growth
Old-growth stands clearly developed differently than younger stands in these three southwestern Oregon forest types. The broad range of tree ages, overall low tree densities, and rapid early individual tree diameter growth in old-growth stands are in strong contrast to a narrower age range, higher stand density and slower diameter growth in young stands throughout the region. This difference is quite similar to that seen in more northerly forests (Tappeiner et al., 1997, Poage and Tappeiner,
Acknowledgements
This study was made possible by financial and in-kind support from the USDI Bureau of Land Management and USDA Forest Service. Special thanks to Dave Perry, Emily Comfort, James Johnston, and Liz Cole for comments on and contributions to this manuscript.
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