Introduction
Early-seral forest conditions can provide valuable habitat for many plant and wildlife species. They can also support biodiversity by adding to the variety of forest conditions across a landscape (See Early-seral forest: What is it and why is it important?). Forests can be returned to an early-seral stage by different types of disturbances, such as wildfire, windstorms, insect outbreaks or timber harvests. Although timber harvests can create early-seral conditions, many plants and wildlife have specifically adapted to natural disturbances and the habitat features they create. Comparing managed stands to naturally disturbed early-seral forests can be a valuable approach to understanding how well timber harvests support biodiversity.
This fact sheet describes the results of a large study in Southwest Oregon. It compares biodiversity in early-seral forests created by wildfire with limited postfire management to those created by clearcut harvests in intensively managed forests.
These types of early-seral forests represent two ends of a spectrum in management intensity. Small woodland owners or managers of non-industrial forests may choose to manage for early-seral conditions that fall somewhere in between, balancing tree growth with biodiversity when making management decisions about harvest patterns, slash management, planting and vegetation management.
Understanding gaps in biodiversity between intensive management and natural disturbances may inform how managers balance these decisions on their land, whether planning a harvest or deciding how to manage after an unexpected disturbance (see Promoting and enhancing early-seral habitat after timber harvest).
The study
This study focused on early-seral Douglas-fir and Douglas-fir/tanoak forests in the Klamath Mountains (Figure 1). Researchers studied areas affected by stand-replacing wildfires on federal lands and compared these to harvested stands on private lands with similar ages and conditions. They studied stands 2 to 20 years after each disturbance. Harvested stands were planted after clearcutting and went through intensive reforestation practices. This included site preparation and herbicide treatments to help planted seedlings grow.
In intensively managed stands, structural retention requirements (leave trees) are usually met at the edges. So, there were few standing trees or snags (standing dead trees) in the interiors. In contrast, most postfire stands were not planted and had no vegetation control treatments. These stands had plenty of snags and some had a few surviving overstory trees. Researchers surveyed birds, bees, plants and ground beetles in each stand to explore biodiversity from different perspectives.
Change over time
The early-seral stage shows major changes in forest structure, often over a short time. It can be broken up into phases that reflect changes in vegetation. These are called early-early, mid-early and late-early-seral phases. Initially, herbaceous plants usually dominate after a disturbance, but this eventually shifts to shrubs and saplings. In this study, stands were categorized into age groups matching these three phases (Figure 2). However, vegetation structure varied among disturbances, as described below. Early forest growth also relies on factors such as site productivity, seed sources, resprouting and potentially other disturbances.
Early-early seral phase (2–5 years)
In the first few years after disturbance, postfire stands had about twice the vegetation cover of harvested stands, mostly herbaceous and woody broadleaf plants (Figure 3). Much of this difference is likely because postfire stands did not receive herbicide treatments. However, early-seral stands are also shaped by past vegetation because of seed banks, resprouting root systems and other legacies.
Postfire stands had faster vegetation recovery and had more biodiversity than harvested stands for most of the groups surveyed. For example, postfire stands had about 50% more bird species, 40% more plant species and 30% more ground beetle species during this phase than harvested stands, on average (Table 1).
The herbaceous layer contains most of the plant species diversity in forests. So, it makes sense that more plant species existed in postfire stands where there was greater herbaceous plant cover. Species richness of ground beetles was also related to herbaceous plant cover. This suggests that keeping these nonwoody plants may help various aspects of biodiversity.
|
Early (2-5 years) |
Mid (6-9 years) |
Late (16-20 years) |
||||
|---|---|---|---|---|---|---|
|
Biodiversity component |
Fire |
Harvest |
Fire |
Harvest |
Fire |
Harvest |
|
Bee species richness |
Lower | Higher | Similar | Similar | Higher | Lower |
|
Bird species richness |
Higher | Lower | Higher | Lower | Similar | Similar |
|
Ground beetle species richness |
Higher | Lower | Similar | Similar | Similar | Similar |
|
Native plant species richness |
Higher | Lower | Similar | Similar | Lower | Higher |
|
Bee composition |
Cavity-nesters | Ground-nesters | Cavity-nesters | Ground- nesters | All groups | N/A |
|
Bird composition |
Cavity-nesters and foliage-gleaners | N/A | Cavity-nesters and foliage-gleaners | Ground-foragers | N/A | Ground-nesters and ground-foragers |
|
Ground beetle composition |
Disturbance-adapted species | Closed forest-adapted species | N/A | N/A | N/A | N/A |
|
Plant composition |
Broadleaf and herbaceous plants | Non-native plants | Broadleaf trees and shrubs | Conifers and herbaceous plants | Broadleaf trees and shrubs | Conifers and herbaceous plants |
Species richness comparisons are made relative to the other disturbance type within each phase. For composition, listed species or groups are those more common in one disturbance type than the other in the same phase. N/A indicates a lack of distinct groups.
Similarly, the higher number of bird species in postfire stands included those that glean insects from foliage, like the Bewick’s wren, black-headed grosbeak and bushtit. Greater overall vegetation cover in postfire stands, especially broadleaf species, likely gave these birds more foraging opportunities.
Although harvested stands had few bird species, the dark-eyed junco and lazuli bunting were present in most harvested and postfire stands during the early-early-seral phase. Harvested stands also had more diverse and abundant bee communities than postfire stands in this phase.
For both birds and bees, some species that rely on dead wood were more common in postfire stands than in harvested stands. This highlights the role of snags and downed wood for early-seral biodiversity.
Cavity-nesting birds, including the house wren and hairy woodpecker, thrived in postfire stands. The mountain carpenter bee (Figure 4) and various mason bee species were also present. Mason bees (Osmia) nest in existing cavities, while carpenter bees (Xylocopa) create nests by boring into dead wood.
Non-native plant species were most common in the early-early-seral phase. On average, ground cover of non-native plants in postfire stands was only half (median = 5%) of that in harvested stands (median = 10%). Still, non-native plant cover reached 50% in one harvested stand and 35% in one postfire stand. The dominance of non-native plants in some stands highlights the need to monitor and identify invasive species early, because some non-native species can become problematic. Early detection is crucial to successfully controlling invasive species on a property.
Mid-early-seral phase (6–9 years)
Several years into the early-seral period, herbaceous cover started to decline. Broadleaf shrubs and trees dominated postfire stands (Figures 2 and 3). Conifer seedlings, whether naturally grown or planted, stayed small in these areas. In contrast, herbaceous cover in harvested stands nearly doubled from the early-early-seral phase, and planted conifers became a key part of the vegetation.
Herbicide treatments likely reduced competition from woody plants, allowing the herbaceous layer to persist longer in harvested stands. Native plant species diversity remained higher in postfire stands. However, ground beetle diversity was similar in both postfire and harvested stands.
Postfire stands had twice as many foliage-gleaning bird species as harvested stands.
Non-native plants — mainly herbaceous species — dropped sharply in postfire areas as broadleaf woody plants grew. In contrast, non-native plant cover stayed about the same between the early-early and mid-early-seral phases in harvested stands. This study was not a controlled experiment and cannot prove causation. However, other research in the Oregon Coast Range shows that using herbicides to suppress native plants after harvest can promote non-native herbaceous species.
In postfire, mid-early-seral woody plant communities where growth was faster, bird species occupancy rates were higher (Figure 5). Postfire stands had twice as many foliage-gleaning bird species as harvested stands. Species like the Nashville warbler and orange-crowned warbler occupied more than 75% of postfire stands but less than half of harvested stands (Table 1).
Other birds such as the bushtit, Bewick’s wren and black-headed grosbeak were also more common in postfire stands. The Nashville warbler and orange-crowned warbler later became common in the late-early seral phase for both stand types. However, these bird species colonized postfire stands earlier than they did harvested ones. The spotted towhee and purple finch were found in both types of stands during the mid-early phase. In contrast, the white-crowned sparrow was common in harvested stands but rare in postfire stands.
Through the mid-early-seral phase, the total number of bees continued to be lower in postfire stands than in harvested ones. However, the species richness of bees across postfire stands increased in the mid-early-seral phase compared to the early-early-seral phase. As a result, bee species richness was similar for both disturbance types in the mid-early-seral phase.
The trend in the early-early-seral phase of more cavity-nesting bees in postfire stands than in harvested ones became stronger. Cavity-nesting bee abundance is positively related to the amount of downed dead wood. This finding suggests that keeping more dead wood in harvested stands could help provide nesting sites for these bees. However, this suggestion needs more testing. Wood-boring beetles might play a key role in creating nest cavities for these bees, but there are probably fewer wood-boring beetles in harvested stands than in postfire ones.
Late-early-seral phase (16–20 years)
Nearly two decades after disturbance, most biodiversity measures were similar in harvested and postfire stands, with bees being the main exception (Table 1). Harvested stands were close to the end of the early-seral stage, as planted Douglas-fir and other conifers reached canopy closure. Conifer cover was high, but small gaps allowed for pockets of herbaceous and woody broadleaf cover (Figures 2 and 3).
In postfire stands, broadleaf vegetation dominated. This mix included shrubs and trees based on pre-fire vegetation and seedbank. Herbaceous vegetation declined further in postfire stands. For the first time, native plant diversity was lower in postfire stands than in harvested stands in this phase. Non-native plant species were almost absent in both types during the late-early phase since many common non-native plants do not tolerate shade.
During the late-early-seral phase, both harvested and postfire stands had few ground beetle species. These species, like the giant woodland ground beetle, thrived in the shadier conditions of forest vegetation (Figure 6).
In harvested stands, more vegetation led to an increase in bird species during the mid-early- to late-early-seral phases. This included the foliage-gleaning group, which had previously separated postfire stands from harvested stands during earlier phases. By the late-early-seral phase, species like the black-headed grosbeak, Nashville warbler and orange-crowned warbler became common in harvested areas.
The purple finch, western tanager, red-breasted nuthatch and hermit warbler were also common in both harvested and postfire stands during the late-early-seral phase. As the canopies of harvested stands started to close, the hermit thrush and chestnut-backed chickadee flourished, but these species were rare in postfire stands.
During the late-early-seral phase, postfire stands had fewer cavity-nesting bird species than before. This likely happened because snags decayed and fell over time after fire. Still, the acorn woodpecker remained a key species in some postfire stands where remaining snags, along with tanoak and canyon live oak vegetation, provided nesting sites, granaries and acorns.
In harvested stands, bee species richness and overall bee numbers declined sharply from the mid-early-seral phase to the late-early-seral phase. In contrast, postfire stands maintained high bee numbers.
In harvested stands, bee species richness and overall bee numbers declined sharply from the mid-early-seral phase to the late-early-seral phase. In contrast, postfire stands maintained high bee numbers. The late-early-seral phase was the only time postfire stands had more abundant bees than harvested ones. At first, intensively managed harvested stands had abundant and diverse bee communities. However, the rapid growth of planted conifers appears to have reduced the length of time bee habitat lasts after a disturbance compared to postfire stands.
Conclusion
Wood production can support much of the biodiversity found in early-seral forests created by wildfires, which are part of the natural disturbance regime in the Pacific Northwest. This is especially true when considering changes that occur across different parts of the early-seral stage. However, wildlife communities in these forest types are also different in several important ways. These differences highlight management approaches to consider where improving conditions for early-seral forest biodiversity is a key objective. For example, limiting vegetation control and creating or retaining many snags may help managed early-seral forests become more like those created by natural disturbances.
These kinds of changes may increase biodiversity in the early-seral stage. However, these practices can also impact economic returns. A targeted approach may help balance biodiversity and tree growth objectives. Retaining large downed wood, seeding native forbs into burn piles, or retaining occasional snags and legacy trees, may also benefit biodiversity with minimal impacts on tree growth. Ultimately, these decisions will depend on management goals and priorities.
Additional resources
Bees in the woods. Oregon State University Extension Service.
Wildlife in Managed Forests: Pollinators and Forestry. Oregon Forest Resources Institute.
Wildlife in Managed Forests: Early Seral-associated Songbirds. Oregon Forest Resources Institute. 2023.
Frank, G.S., J.W. Rivers, J. Verschuyl, L.R. Best, M.G. Betts, A.J. Kroll, M.E. Swanson, and M.A. Krawchuk. 2025. Comparison of Early Seral Forest Bee Communities Following Clearcutting or Wildfire Depends on Stand Age and Nesting Guild. Journal of Forestry.
Frank, G.S., M.G. Betts, A.J. Kroll, J. Verschuyl, J.W. Rivers, M.E. Swanson, and M.A. Krawchuk. 2025. Distinct Bird Assemblages Emerge after Fire versus Forest Harvest but Converge with Early Seral Forest Development. Ecological Applications.
Acknowledgments
This study was funded by the Fish and Wildlife Habitat in Managed Forests Research Program in OSU’s College of Forestry and by the National Council for Air and Stream Improvement Inc.
Access to study sites on privately owned forests was provided by Chinook Forest Management, Manulife Investment Management, Roseburg Forest Products and Weyerhaeuser.
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