What are Early Serai Forests and Why Are They Important?

In general, CESFs are rich in post-disturbance legacies (Photo Plates 1a, 1b, 1c) and post-fire vegetation (e.g., native fire-following shrubs/herbs, resprouting broad-leaved trees, and natural conifer regeneration) (Photo Plates 2a, 2b, 2c). We identify 12 ecological attributes that contribute to the prolific biological response common in CESFs and which are, therefore, key to the ecological integrity present in CESFs (Table 1). When logging compounds the natural disturbance that created a CESF (Photo Plates 3a, 3b, 3c), each of these attributes is reduced or eliminated (Table 1). Such multiple disturbances often lead to alternative successional pathways, or loss of resilience (Paine et al. 1998; Odion and Sarr 2007), as has been documented in the Sierra Nevada following post-fire logging, which leads to dominance by the non-native ecosystem transformer, cheatgrass (Bromus tectorum, Linnaeus) (McGinnis et al. 2010).

Photo Plates. Extensive biological legacies, abundant forb cover, and abundant conifer regeneration present in complex early serai forests vs. early serai that has been post-fire logged. Post-fire logging in the Sierra Nevada and elsewhere sets back ecosystem processes creating a successional debt.

Figure 1.

Location of Sierra Nevada ecoregion, northern California, and “early adopters” of the forest-planning rule involved in forest plan revisions.

Photo Plate 1.

Star Fire of 2001, Northern Sierra, CA. (a) unmanaged with forbs (Doug Bevington, 2008); (b) natural conifer re-establishment (Chad Hanson, 2012); Storrie fire of 2000, Southern Cascades, CA. (c) unmanaged with snags and forbs (Chad Hanson, 2007).

Photo Plate 2.

Postfire logged portions of Fred's fire in the Eldorado National Forest, CA, showing lack of nitrogen-fixing shrubs (a) and presence of Klamath weed (Hypericum perfoliatum) and many readily ignitable, invasive grasses (b) (Dennis Odion, August 2011); (c) simplified system from Dinkey post-fire thin on west slopes of Southern Sierra (Chad Hanson, 2012).

Overall, compared to logged areas, CESFs are structurally more complex, contain more large trees and snags that originated from the pre-disturbed forest, have more diverse understories, functional ecosystem processes, and more diverse gene pools that, theoretically, should provide greater resilience in the face of climate change than that provided by the simplified early serai forests produced by logging. CESF attributes promote a high level of species richness, particularly bird communities that utilize these forests extensively (Hutto 1995; Kotliar et al. 2002; Fontaine et al. 2009; Appendix). The residual biomass of CESFs reduces disturbance stressors and provides for the rapid proliferation of new life (Odion and Sarr 2007). For example, seed banks and vegetation tissues give rise to dense, often rampant, forb cover, abundant grasses, and shrubs — especially nitrogen fixers (e.g., Ceanothus spp.) (Conard 1985; Busse et al. 1996; Busse 2001) and ectomycorrhizal associates (e.g., Manzanita spp.) that facilitate conifer growth (Zavitovsky and Newton 1968; Horton et al. 1999). Serotinous (closed cone) conifers like giant sequoia (Stephenson et al. 1991) also do well in these forests. Other plants that can abundantly colonize burns, such as conifers and fireweed (Epilobium angustifolium, Linnaeus), arrive by wind or animal dispersed seed. Thus, plant species richness of CESFs can be much higher than in unburned forests (Donato et al. 2009).

Other bird and small mammal communities that utilize CESFs forage extensively on the abundant insects and increased abundance of seeds from the post-fire flora (Lawrence 1966; Fontaine et al. 2009). These species, in turn, support an increase in raptors (Lawrence 1966). Bird species such as the Black-backed Woodpecker, Olive-sided Flycatcher (Contopus cooperi), Mountain Bluebird (Sicilia currucoides), Chipping Sparrow (Spizellci passerina), and Mountain Quail (Oreortyx pictus) (Appendix) achieve highest abundances in CESFs. In fact, in the Sierra Nevada, CESF habitat is comparable or higher in bird species richness and total bird abundance relative to unburned mature forest (Burnett et al. 2010). Bats (Myotis, Idionycteris, Lasionycteris, and Eptesicus), which are an increasing conservation concern, are also favored by CESFs, likely because of greater insect prey as well as suitable roosts (Buchalski et al. 2013). Stand-replacing fires stimulate a flux of aquatic prey to terrestrial habitats, driving increases in riparian consumers (Malison and Baxter 2010). The trees killed by fire are highly beneficial to the ecological integrity of stream communities because they are a main source of large woody debris inputs (Minshall et al. 1997). There is also reproduction by some forest fungi species that are restricted to burns (e.g., morels, Morchella spp.) and the dead wood provides substrate for fungal growth that supports many arthropod species, including unique fire-following native beetles (Lindsey 1943; Bradley and Tueller 2001). Beetles, in general, colonize fire-killed trees in CESFs and their abundant larvae support species like Black-backed Woodpeckers (Hutto 2008).

Table 1.

Differences between early serai systems produced by natural disturbance processes vs. logging. For natural disturbances, assume that a disturbance originates from within a late-successional forest as legacies are maintained throughout succession. For logged sites, assume site preparation includes conifer plantings but no herbicides, which, if also applied, would magnify noted differences.

Indicator Species for CESF Biodiversity (Figure 3)

Indicator species are valuable tools for conservation management because it is not practicable to monitor all biodiversity. When burned forests are logged after fire, one species that serves well as an ecological indicator for post-fire biodiversity, the Black-backed Woodpecker, declines substantially (Hutto 2008). Given that this woodpecker already is an indicator of the biodiversity supported by CESFs in the Sierra Nevada (USDA Forest Service 2013b), and is a fire specialist, we propose it as a Species of Conservation Concern. Designated Species of Conservation Concern are those whose population viability, or continued representation within a particular plan area, is of management concern. The forest-planning rule provides guidance to forest managers to use Species of Conservation Concern as a means for maintaining species diversity and wildlife population viability.

CESF habitat represented by Black-backed Woodpeckers is biologically unique (Hutto 1995; Bond et al. 2012). The Black-backed Woodpecker is an important primary excavator of nesting holes for many other cavity-nesting birds and mammals because it discards cavities after excavating them, and it uses a given cavity for one year (Tarbill 2010). Under a scenario with stand-replacing fire operating in a patchwise fashion in a landscape containing healthy populations of Black-backed Woodpeckers, the availability of nesting cavities across the landscape over time may be greatly enhanced compared to where fire is suppressed and/or fire-killed trees are removed. Black-backed Woodpeckers use CESFs for only several years (typically seven or eight) after fire and they depend upon the regular creation of CESFs to replenish their habitat (Hanson and North 2008; Tarbill 2010; Dudley et al. 2012; Siegel et al. 2013). When this does not occur, many other species that rely on nesting cavities are likely to be negatively affected. Thus, many species probably depend directly, or indirectly, on the continued occurrence of high-intensity natural disturbance across large landscapes to maintain their populations (Hanson and North 2008; Tarbill 2010; Dudley et al. 2012; Siegel et al. 2013).

Black-backed Woodpeckers have become increasingly rare because their optimal habitat has shrunk to a fraction of its historical extent (Figure 2a – d); populations are estimated at <700 nesting pairs in burned forests (Bond et al. 2012). Importantly, the CESF habitat that the remaining pairs depend on has little or no protection on public lands managed by the U.S. Forest Service. Much of this CESF habitat is under mounting pressure from fire suppression and both pre- and post-fire logging (Hutto and Gallo 2006; Hanson and North 2008; Hutto 2008; Siegel et al. 2013), which prevent high-quality woodpecker habitat. That, in turn, may affect the biodiversity for which this woodpecker serves as an indicator.

Figure 3.

Black-backed woodpecker — a fire dependent species in the Sierra (Photo — Monica Bond).

Are There Management Tradeoffs for Species of Conservation Concern at Opposite Ends of the Successional Continuum?

Wildlife management often involves tradeoffs when habitat for a particular species is emphasized. That is a problem with single-species management (managing for what one species needs), but is not a problem when managing for the maintenance of natural systems that a species may indicate. In the latter case, we would not enhance but would maintain natural levels of habitat for CESF indicators like the Black-backed Woodpecker, and for the biodiversity associated with its presence.

However, the California Spotted Owl is also a management indicator species but for late-seral forests in this region. Notably, all three subspecies (Mexican, California, Northern; Bond et al. 2002; Jenness et al. 2004; Roberts 2008; Bond et al. 2009; Clark et al. 2011; Roberts et al. 2011; Lee et al. 2012; Clark et al. 2013) appear to tolerate, or even benefit, from some degree of moderate- to high-severity fire within territories.

Managing CESFs for high levels of ecological integrity may provide important prey habitat (e.g., dusky-footed woodrat Neotoma fuscipes; Munton et al. 2002) for the spotted owl. In fact, the owl is known to reproduce in territories burned at all fire severities in this region, and preferentially selects high-severity fire areas for foraging (Bond et al. 2009). Owl reproduction has been found to be 60% higher in unmanaged mixed-severity fire areas than in unburned forests (Roberts 2008), and mixed-severity fire (with an average of 32% high severity) (Lee et al. 2012) does not reduce owl occupancy, though post-fire logging may precipitate territory abandonment (Clark et al. 2011, 2013; Lee et al. 2012). Moreover, because high-severity fire has been reduced by fire suppression, and current high-severity fire rotations are very long in the Sierra Nevada, if high-severity fire rates increased by even two- or three-fold, it would benefit CESF-associated species like the Black-backed Woodpecker, but would only reduce current old forest by a very small amount given old forest recruitment from ingrowth (Odion and Hanson 2013). Thus, protecting CESFs from post-fire logging and maintaining the spatial heterogeneity created by mixed-severity fires should provide habitat for all serai associates — there really are no management trade-offs when we manage for the maintenance of natural processes and systems.

What are Principal Threats to CESFS?

Management of CESFs has most often included post-fire (salvage) logging followed by tree planting, including burning of slash piles and associated soil disturbances, reseeding with grasses (often introducing invasive species inadvertently), use of straw-bales and other erosion prevention methods, herbicides to reduce shrub competition with conifers. planting with conifer nursery stock, and livestock grazing (Swanson et al. 2011; Long et al. 2013; Table 1). These activities remove, or severely degrade, CESFs or, at a minimum, can narrow the window of duration for CESFs (Swanson et al. 2011), contributing to “landscape traps,” whereby entire landscapes are shifted into, and then maintained in, a highly altered state as the result of cumulative impacts (Lindenmayer et al. 2011).

Figure 2.

(a) Forest types used by Black-backed Woodpeckers in the Sierra Nevada management region; (b) fires since 1984 within the relevant forest types (private lands not included since they are rapidly logged); (c) moderate/high-severity fires resulting in >50% mortality (RdNBR >574 — see Hanson et al. 2010) of forests on public lands within the relevant forest types in the most recent decade for which there are fire severity data (2001–2010) (i.e., both high quality Black-backed Woodpecker habitat and moderate/low quality (older) habitat combined); and (d) moderate/high-severity fire on public lands within the relevant forest types in the most recent 5-year period for which fire severity data are available.

Climate change and forest fragmentation also have been identified as threats to biodiversity in the Sierra Nevada (USDA Forest Service 2013b). Since the 1980s, the region has experienced a decrease in annual number of days with below-freezing temperatures at higher elevations with more rain and less snowfall mainly in northern latitudes, more extreme heat days at lower elevations, earlier (5 to 10 days) snowmelt than decades ago, earlier (5–15 days) peak stream flows (Safford et al. 2012; Harpold et al. 2012), as well as an increase of approximately 1 °C since the early twentieth century, though some areas of the northern Sierra Nevada have seen a decrease in temperature (North 2012). Some regional climate models project further decreases in mountain snowpack, earlier snowmelt and peak stream flows, and greater drought severity (Overpeck et al. 2012). Such climatic changes are likely to affect the low-elevation ponderosa pine, which is projected to extend upward, while red fir and subalpine communities are projected to lose much of their climate envelope in the coming century (USDA Forest Service 2013b). It is unclear how such changes will affect CESFs. If fire increases in severity or frequency (Miller et al. 2009; Miller and Safford 2012), this could provide more opportunities for development of CESFs. This assumes there is not a concomitant increase in post-fire logging, and that fire suppression activities either cannot keep pace with climate-related fire events or prove ineffective due to the increasing influence of climate as a top-down driver of fire behavior. On the other hand, a number of climate models predict decreasing fire activity in these forests — even as temperatures rise — due to increasing precipitation, including summer precipitation and changes in vegetation (McKenzie et al. 2004; Krawchuk et al. 2009), and recent research using the largest fire severity data set to date has found no increase in fire severity in the Sierra Nevada since 1984 (Hanson and Odion, 2014; also see Odion et al. 2014 for related discussion).

Land-use stressors also magnify climate change effects on forest communities. For instance, Thorne et al. (2008) documented significant regional changes due to climate and land-use practices resulting in greater levels of disturbance compared to historical. Millar (1996) identified three paramount influences on Sierra Nevada ecosystems: (1) climate change and shifting hydrological patterns; (2) dense forests; and (3) rapidly expanding human populations. It is not known, however, whether these changes will act in concert to make CESFs more vulnerable to invading species, particularly those more suited to the changing climate and land-use disturbances.

Suggested Best Management Practices for CESF

For all the reasons outlined above, CESFs represent a neglected serai stage subject to multiple stressors that compromise ecological integrity. We, therefore, propose eight “best management practices” for stimulating conservation, restoration, and research interests in these unique forests. These principles can serve as appropriate guide-lines where management goals include the maintenance of ecological integrity.

Conservation Focus

Principle 1 — “Rehabilitation” Is Not Needed After Fire Creates a Complex Early Serai Forest (Beschta et al. 2004; Swanson et al. 2011).

Fire acts as a natural restorative agent by resetting the successional clock and providing habitat for disturbance-dependent species. Although CESFs lack live trees initially and are populated by dead ones, this does not mean they require site rehabilitation or are “unhealthy” forests. In the context of ecological integrity, a functional forest system is one where the natural fire regime is of mixed-severity and has all stages of succession following stand-replacing fire. CESFs should be mapped and managed as a distinct forest habitat type.

Principle 2 — Protect Large, Old Forest Structures Across Serai Stages, and Retain Dense, Old Forests to Improve Ecological Integrity at Landscape Scales.

Large old-forest structures take decades to centuries to develop, and forest management has created a deficit through extraction. Dense, old forests provide high-quality habitat not only when they are green, but also when they experience mixed-severity fire (Hutto 2006, 2008), or snag pulses from beetles (Bond et al. 2012), as biological legacies remaining also serve to connect serai stages along the successional gradient.

Principle 3 — Mixed-severity Fire Should Be a Management Goal for Reserves.

Robust, reserve-based conservation strategies are needed to maintain the suite of serai stages and allow for climate-forced wildlife dispersals into suitable habitat. Thus, managers should allow fires to run their course in the backcountry and in reserves when not a risk to people or dwellings. This includes maintaining a landscape that includes diverse serai stages across environmental gradients (elevation, latitudinal).

Restoration and Management Focus

Principle 4 — Adopt Comprehensive Approaches to Restore Ecological Integrity in CESFs.

This starts with a restoration needs assessment (DellaSala et al. 2003) to evaluate and prioritize drivers of ecosystem degradation and best practices aimed at reducing specific stressors (see Principle 6). Most importantly, forests restored through fire usually do not need “restoration” otherwise.

Principle 5 — Limit Post-fire Management to Early Serai Forests Previously Degraded by Logging, Grazing, and Other Stressors.

Restoration approaches should identify comparable areas of high ecological integrity (e.g., unmanaged CESFs, DellaSala et al. 2003) to serve as a baseline or reference condition from which to restore degraded areas (e.g., burned plantations), and then surveillance, implementation, effectiveness, and ecological effects monitoring (Hutto and Belote 2013) should always be an integral part of the restoration activity.

Principle 6 — Reduce Land-use Stressors That Compromise the Ecological Integrity of CESFs.

Restorative measures can be active or passive depending on site-specific needs and should always be followed with well-funded monitoring (DellaSala et al. 2003). Examples include removal of livestock, invasive species abatement, road closures and obliteration, and reintroduction of fire.

Research Focus

Principle 7 — Determine Historical, Current, and Projected Future Distributions and Spatio-temporal Extent of CESFs as Well as Other Serai Stages Across the Planning Area.

This can be informed through “back-casting” approaches that reconstruct an historical baseline from combining age-structure reconstructions (e.g., from either FIA plot data or General Land Surveys from the 1800s; see techniques in Baker 2012; Williams and Baker 2012) with studies that link stand structure, disturbances and fire scar data (e.g., Sherriff and Veblen 2006), or other sources of information (e.g., USDA Forest Service 1911). Historical baselines can then be compared to current and future projected conditions under a changing climate in order to determine appropriate representation levels of CESFs and other serai stages in a planning area.

Principle 8 — Designate the Black-backed Woodpecker a “Species of Conservation Concern.”

Continue, and expand upon, current monitoring efforts and, in partnership with the U.S. Fish and Wildlife Service and other experts, determine how best to meet population viability and habitat needs of this important CESF species. Although Black-backed Woodpecker populations decline as this serai stage advances (within seven years following fire), this species still functions as an indicator of early successional species because stable woodpecker populations would mean a steady supply of CESFs over time. Olive-sided Flycatcher, Chipping Sparrow, Mountain Bluebird, and other early serai species that have population peaks after declines in woodpeckers, may need to be monitored to ensure CESF conservation.