1 Introduction
1.1 Multiaged management
Ecological forestry which maintains a wide range of ecosystem services while also supplying timber requires a diverse landscape of highly varied forest structures (Aplet, 1994; Nolet et al., 2018; O’Hara, 2001) . These, in turn, require a variety of silvicultural techniques to implement and sustain (O’Hara, 1998; Schütz, 2002). The development of multiaged stand structures has long been of interest to silviculturists as a key alternative to the conceptually and logistically simpler, even-aged management (Schütz, 1999). Multiaged silviculture refers to the retention of trees of distinctively different age classes, growing together within the same stand. These cohorts may co-occur at the tree level, or in small, even-aged patches within the stand. In the latter case, the distinction between even- and multiaged management can become blurred with increasing patch size, but patches are generally much smaller (often less than 1 ha) than the stands they compose. The pursuit of multiaged stand structures has often met with mixed results (O’Hara, 2002) and this has led to the investigation of several different systems for achieving such structures. Research into the efficacy and results associated with these is ongoing (Beese et al., 2019; Nolet et al., 2018). One such system that has gained popularity in recent decades is known as the retention system, which allows for the retention of a range of tree densities, in dispersed or aggregated spatial patterns and can be used to maintain multiaged stands, or transform from even-aged management (S. J. Mitchell & Beese, 2002).
Redwood forests offer a prime opportunity for multiaged management because coast redwood (Sequoia sempervirens) regenerate reliably via stump sprouting and are relatively shade tolerant. The very high leaf areas observed in these forests, suggest their suitability for a multi-layered forest structure (Berrill & O’Hara, 2007; Van Pelt et al., 2016). Additionally, with their high timber value and productivity, redwood forests are of keen interest to private timber producers. Despite redwood’s fitness for multiaged stand structures, the successful development of subordinate cohorts depends on adequate access to light, and light deficiency can lead to reduced vigor and mortality in young sprouts and understory trees (Barrett, 1988; Muma et al., 2022; O’Hara et al., 2007; Webb et al., 2012).
Complicating redwood regeneration and sprout development is the fact that the competing hardwood species, tanoak (Notholithocarpus densiflorus), is also shade tolerant and a vigorous resprouter. Tanoak is a keystone species in terms of wildlife habitat and First Nation’s cultural identities, but from a timber production standpoint it is often perceived as a nuisance due to a lack of market development combined with its widespread proliferation following intensive, repeated conifer harvesting (Bowcutt, 2011). While redwood grows more quickly than tanoak in multiaged stands, competition from hardwoods such as tanoak reduce conifer growth and drought resistance (Berrill et al., 2021; Dagley et al., 2023). Meanwhile, there has been little scientific investigation into the development of tanaok under shade (Waring & O’Hara, 2008; Wilkinson et al., 1997).
1.2 Management effects on sprouting
1.2.1 Measures and general patterns of sprout development over time
Tree sprouting response following cutting is commonly characterized by presence (percent stumps sprouting), abundance (sprout density), and size (height and diameter). Interpretation of these metrics depends on the time between treatment and measurement, as early sprout growth is largely driven by mobilization of carbohydrates stored in the parent stem and root system (Del Tredici, 2001). As a result, differences associated with external conditions may not be evident early in development. Redwood sprout clumps may consist of more than 100 stems in the first year after cutting (Neal, 1967), but rapidly self-thin, particularly under high light conditions (Boe, 1975). Under overstory competition, self-thinning may proceed more rapidly and can result in mortality of entire sprout clumps (O’Hara & Berrill, 2010). In contrast to eastern hardwoods, where self-thinning typically occurs over decades (Gould et al., 2007), this process may extend for centuries in long-lived redwoods (O’Hara et al., 2017).
Because percent sprouting, sprout density, and sprout size represent distinct aspects of sprouting response, they vary with species, site characteristics, overstory density, parent stump size or age, and geographic context. Even when these factors are considered, substantial unexplained variation among sites may remain (Keyser & Loftis, 2015; Nieves et al., 2022).
1.2.2 Factors influencing initial sprout response
Across many sprouting species, the proportion of stumps that sprout following cutting often declines with increasing tree size or age, although this pattern varies by species and site conditions (Johnson, 1977; Nieves et al., 2022). In redwoods, evidence for declining percent sprouting with increasing size or age is mixed (Barrette, 1966; Lindquist, 1979; Neal, 1967; Wiant & Powers, 1967), potentially reflecting the wide range of tree sizes and ages present in these forests. Some studies suggest that percent sprouting may increase with age to a threshold and decline in trees older than approximately 200–400 years (O’Hara et al., 2007; Powers & Wiant, 1970).
After cutting, 90–100% of second-growth redwoods (< 90 cm dbh) typically sprout (Barrette, 1966; Lindquist, 1979), whereas sprouting among larger, older redwoods may approach 50% (Boe, 1975; Neal, 1967). Declining sprouting with increasing size has also been documented in tanoak and other coastal hardwoods (Harrington et al., 1992).
Residual overstory density may influence percent sprouting in some species and regions, but evidence for this effect is inconsistent and sensitive to study design (Nieves et al., 2022). In redwood forests, this effect is generally weak or absent (Barrett, 1988; Lindquist, 1979). The number of sprouts produced per stump is also typically unrelated to overstory density (Atwood et al., 2009; Knapp et al., 2017), and is assumed to follow a similar pattern in redwood forests (Lindquist, 1979; O’Hara & Berrill, 2010).
1.2.3 Internal and external factors influencing sprout development over time
Over time, sprout growth and survival are strongly influenced by competitive conditions. Among sprouting species, sprout growth generally increases with understory light availability (Gardiner & Helmig, 1997; Keyser & Zarnoch, 2014; Knapp et al., 2017), although the influence of light is weakest early in development, when growth is dominated by stored carbohydrates (Gardiner & Helmig, 1997; Keyser & Loftis, 2015). Despite its shade tolerance, redwood requires a minimum light threshold to sustain sprout growth (O’Hara & Berrill, 2010).
Sprout growth is also positively related to stump diameter, with larger stumps producing faster-growing sprouts, a pattern observed in redwood (Berrill et al., 2018), tanoak (Harrington et al., 1992), and eastern hardwoods (Dey et al., 1996; Keyser & Loftis, 2015). Survival of sprouts in subsequent years depends on overstory density and competitive pressure, particularly as stands approach canopy closure. Entire sprout clumps may experience rapid mortality under low-light conditions (O’Hara & Berrill, 2010). Longer-term sprout persistence has also been shown to vary with site and regional factors (Keyser & Loftis, 2015; Nieves et al., 2022), although these influences have not been well quantified in redwood forests.
Due to rapid early growth, sprouting species can influence the composition of regenerating stands (Del Tredici, 2001), sometimes favoring less desirable species (Keyser & Zarnoch, 2014). In redwood ecosystems, redwood sprouts typically outgrow tanoak stump sprouts during the first five years following partial harvest (Muma et al., 2022), though the longer-term implications of these dynamics for stand development remain uncertain (Berrill et al., 2018). Additional disturbance interactions, including herbivory and wildlife damage, may further influence sprout development and species composition (Berrill et al., 2017; Dagley et al., 2018; Schneider et al., 2023; Wilkinson et al., 1997).
1.3 Forest fuels
Throughout many of the fire-adapted forests of California, fire exclusion combined with timber harvest has led to dense, suppressed stands, proliferation of more fire-sensitive species, and an accumulation of surface fuels (Safford & Stevens, 2017; Stephens et al., 2009). This situation combined with climate change has led to increased size and frequency of high-severity fires in many regions (Parks & Abatzoglou, 2020; Westerling, 2016), prompting widespread interest in fuel treatments and silvicultural interventions creating resilient stand structures.
This interest has seen less momentum in the redwood region, likely due to the perceived safety of these typically moist forests from the threat of large wildfires. Yet redwood litter is among the most flammable of conifer litter types (Fonda et al., 1998), seasonal drought leads to cured fuels, especially during extended breaks in coastal fog (Jacobs et al., 1985), and numerous physiological adaptations suggest that redwood has evolved under fire disturbance pressure (Varner & Jules, 2017). More concretely, there have been at least six large fires in redwood ecosystems since 2003, burning at least 189,000 ha including widespread areas of canopy loss. Scientific consensus places the pre-colonization fire return interval for redwood forests at 6-25 years across their range (Lorimer et al., 2009). It is assumed that much of this activity is attributable to indigenous burning (Varner & Jules, 2017).
Consistently low foliar moisture content levels have been recorded for tanaok (Kuljian & Varner, 2010) and the foliage has been compared to that of other sclerophyllous species such as those found in chaparral ecosystems which are characterized by intense, stand replacing fires suggesting high flammability, particularly with low canopy base heights (Fryer, 2008; McDonald, 1981). And tanoak leaf litter is among the most flammable of western hardwoods and has been compared to that of fire adapted conifers (Varner et al., 2017). To our knowledge, no studies have examined the flammability of live tanoak foliage or tanoak sprouts.
There have been several studies that have quantified various fuel strata in redwood forests. Kittredge (1940) did so for duff and litter in a redwood plantation. Greenlee (1983) studied fuels at Big Basin State Park. Stuart (1985) reported on fuels at Humboldt Redwoods State Park. Finney and Martin (1993a, 1993b) reported on fuels in second-growth redwood forests (aged ~100 years) at Annadel and Humboldt Redwoods State Parks. Graham (2009) reported on fuels in old-growth stands across redwood’s range. Glebocki (2015) studied fuels with and without thinning treatments in young (< 50 years) redwood/Douglas-fir stands. No fuel studies, to my knowledge, have been conducted in redwood forests actively managed with multiaged silviculture, but fuel dynamics represent a potentially important decision variable to consider when managing forest stands that may be subjected to intentional or unintentional fire.
1.3.1 Management effects on fuels
Depending on the method used, thinning and harvest treatments may increase, or not affect surface fuel loading. Whole tree removal results in the least fuel accumulation but can be more expensive than other options (Han & Han, 2020). Most other treatment methods increase surface fuels (Agee & Skinner, 2005; Stephens et al., 2009). The magnitude of this increase is variable, reflecting factors such as treatment mode, intensity, and pre-existing conditions (Schwilk et al., 2009). Additional research is needed to clarify the effects of these factors on short-term (Hood et al., 2020; Schwilk et al., 2009; Stephens et al., 2009) and long-term changes to surface fuel load resulting from specific management actions (Hood et al., 2020; Stephens et al., 2012).
The majority of fine dead fuels (< 8 cm) generated by treatment activities typically decompose within 10 years (Burton et al., 2022; Hood et al., 2020; Martinson & Omi, 2013; O’Hara et al., 2017; Stephens et al., 2012). But live woody fuels, which respond vigorously to increased growing space, often persist or increase over time (Keyes & Varner, 2006). The nature of this response depends on eco-type and the amount of growing space liberated by the treatment, which in turn can become occupied by herbaceous plants (Vilà-Vilardell et al., 2023), shrubs (Odland et al., 2021), or small trees (Hood et al., 2020).
Overtime, duff and litter loads are frequently lower in more open stands than stands with a more closed canopy. This may result from increased decomposition rates due to greater insolation and increased throughfall, or reduced deposition rates resulting from fewer canopy fuels (Hood et al., 2020; Keane, 2008).
Most fuel reduction thinning research focuses on ponderosa pine (Pinus ponderosa) forests in the United States, with additional studies from other Mediterranean and semi-arid regions (Burton et al., 2022; Schwilk et al., 2009; Vilà-Vilardell et al., 2023). Far fewer studies have been conducted in coastal forests (Norman et al., 2009), but see Wilder et al. (2025).
1.4 Pyrosilviculture
Forest management for timber and other objectives and prescribed and wild fire are inherently interlinked. This requires research which considers these historically disjunct realms of research in a wholistic way. Fire informed and fire dependent silviculture has been a hallmark of traditional forest stewardship practices for indigenous across northern California since time immemorial (Anderson, 2013). Fire informed silviculture has also been practiced and researched in the Southeastern U.S. for around 100 years (R. J. Mitchell et al., 2009). The American west has been slower to embrace this paradigm shift in thinking now termed “pyrosilviculture” (North et al., 2021). It is my hope that this thesis serves as an example of bridging the gap between the art and science of growing trees to support multiple uses and the thinning and burning practices typically regarded as “fuels management” activities (York et al., 2021). The ability to envision these realms of understanding as integral and essential pieces of a common forest stewardship will lead to new insights and increase our capacity for better land management.
Therefore, the objective of this research was to quantify the development of live surface fuels–the forest understory–as well as dead surface fuels by size class and their dynamics with regard to a PCT fuels treatment with potential to reduce fire severity. And to measure these within the context of a multiaged silvicultural system under a range of residual overstory densities to explore trade offs between overstory retention, new sprout development, and surface fuel management.