Systematic fire exclusion, anthropogenic climate change, expansion of the wildland-urban interface, and historic management practices have led to the increased occurrence and size of catastrophic wildfires in the western United States and across the globe (Dennison et al. 2014, Stephens et al. 2014, Stevens et al. 2017). These trends are particularly concerning for ecosystems like the mixed-conifer forests of the Sierra Nevada, California, which were historically characterized by a high frequency, low to mixed-severity fire regime (Stephens and Collins 2004, Safford and Stevens 2017). Worsening wildfire patterns in this ecosystem have broad ecological and social consequences, including increased greenhouse gas emissions (Hurteau et al. 2014), reduced carbon stocks (North and Hurteau 2011), loss of late seral wildlife habitat (Stephens et al. 2016), degradation of air quality (Phuleria et al. 2005), impacts on water quality and quantity (Bladon et al. 2014), widespread economic costs (Wang et al. 2021), and the destruction of human livelihoods (Haynes et al. 2020).
Recent empirical studies have shown that young, even-aged conifer plantations are especially prone to high-severity wildfire effects (Thompson et al. 2011, Zald and Dunn 2018, Levine et al. in press). Plantations are typically characterized by dense, spatially homogeneous fuel structures, and the young trees that form the upper canopy are particularly susceptible to fire because of thinner bark and low crown base heights. Despite these vulnerabilities, replanting is often necessary for post-wildfire reforestation, especially in large high-severity patches where wind-dispersed seed from surviving conifers is inadequate and the potential for long-term conversion to shrublands is high (Stephens et al. 2020). In these areas, managers must choose whether to allow conversion to non-forest vegetation or replant and invest in at least the potential for a future forest that can mature and persist through the next wildfire.
Various management strategies may help reduce wildfire hazard in young stands and offer managers some protection against high-severity effects. Site preparation that reduces fuels (e.g., via pile burning or broadcast burning before planting) has been shown to substantially decrease wildfire severity in young plantations (Lyons-Tinsley and Peterson 2012). Planting trees in a spatially heterogeneous arrangement based on water availability and topography breaks up fuel continuity and lessens water stress, potentially increasing resilience to wildfire and other disturbances (North et al. 2019). Different fuel reduction treatments can be implemented after several decades of stand development to build resistance to wildfire effects. Prescribed burning can greatly reduce surface fuels, create stand heterogeneity, and select for fire-resistant tree species – all at a low cost per unit area when compared with mechanical treatments (Kobziar et al. 2009). To be consistent with the pre-colonial fire regime of Sierra Nevada mixed conifer forests, fire would likely be introduced into developing stands early on given that fires burned less than every five to ten years prior to fire suppression (Stephens and Collins 2004). Despite possible benefits, most forest managers are wary of prescribed burning in young stands because variable levels of tree damage and mortality following burns inevitably occur (e.g., Bellows et al. 2016, York et al. 2021). Even low levels of mortality in young stands can be difficult to accept when individual trees are highly valued. Fire-induced damage and mortality may conflict with goals to encourage the growth of large trees, whether it be for fire resistance (Agee and Skinner 2005), timber production, carbon sequestration (Stephenson et al. 2014), restoration (Lutz et al. 2009), or wildlife habitat (Franklin et al. 2002).
Mastication largely avoids these drawbacks and is much more precise than prescribed burning since individual trees can be deliberately retained or chipped in place. However, mastication treatments are expensive, and high shrub and tree densities typical of young stands can slow productivity and inflate operating costs. In the short term, mastication can be ineffective as a fuel reduction treatment since it redistributes ladder fuels to surface fuels instead of removing them from the site entirely. This pulse of surface fuels can create higher wildfire hazard until the chips eventually decompose (Kobziar et al. 2009, Stephens and York 2017). Following up mastication with prescribed burning mitigates this effect, and the combination of the two treatments can be more effective at quickly reducing wildfire hazard compared to either treatment alone (Reiner et al. 2009, Stephens et al. 2009). Caution must be exercised, however, since high fireline intensities brought about by masticated fuel consumption may increase mortality when prescribed burning during dry conditions, particularly for young trees with low crown base heights (Reiner et al. 2012).
In mature forests, the essential components of an effective wildfire hazard reduction treatment are well known: reduction of surface and ladder fuels, lowering of crown density, and retention of large, fire-resistant trees (Agee and Skinner 2005). In contrast, the elements of an effective treatment in young stands are not nearly as well understood (North et al. 2019). In particular, the tradeoffs involved in conducting treatments with contradictory influences on fuel loads and growth have not been examined. Conventional release treatments may increase individual tree growth (e.g., precommercial thinning) or stand growth (e.g., herbicide application), but may come at the cost of increases in surface fuel loads. Prescribed fire, the most effective surface fuel reduction treatment, may incur losses in tree or stand-level growth. This study aims to evaluate these potential tradeoffs and aid decision-making regarding the prioritization of different fuel reduction treatments in young stands. We evaluate the effects of four different experimental fuel reduction treatments - mastication, mastication plus herbicide, two prescribed burns, and mastication plus two burns - on individual growth, stand growth, and surface fuel loads in young Sierra Nevada mixed conifer stands.