Large-diameter trees, snags, and deadwood in southern Utah, USA

Abstract

Background The epidemic Dendroctonus rufipennis (spruce beetle) outbreak in the subalpine forests of the Colorado Plateau in the 1990s killed most larger Picea engelmannii (Engelmann spruce) trees. One quarter century later, the larger snags are beginning to fall, transitioning to deadwood (down woody debris) where they may influence fire behavior, regeneration, and habitat structure. Methods We tracked all fallen trees ≥ 1 cm in diameter at breast height (1.37-m high) and mapped all pieces of deadwood ≥ 10-cm diameter and ≥ 1 m in length within 13.64 ha of a high-elevation mixed-species forest in the Picea–Abies zone annually for 5 years from 2015 through 2019. We examined the relative contribution of Picea engelmannii to snag and deadwood pools relative to other species and the relative contributions of large-diameter trees (≥ 33.2 cm at this subalpine site). We compared spatially explicit mapping of deadwood to traditional measures of surface fuels and introduce a new method for approximating vertical distribution of deadwood. Results In this mixed-species forest, there was relatively high density and basal area of live Picea engelmannii 20 years after the beetle outbreak (36 trees ha−1 and 1.94 m2 ha−1 ≥ 10-cm diameter) contrasting with the near total mortality of mature Picea in forests nearby. Wood from tree boles ≥ 10-cm diameter on the ground had biomass of 42 Mg ha−1, 7 Mg ha−1 of Picea engelmannii, and 35 Mg ha−1 of other species. Total live aboveground biomass was 119 Mg ha−1, while snag biomass was 36 Mg ha−1. Mean total fuel loading measured with planar transects was 63 Mg ha−1 but varied more than three orders of magnitude (0.1 to 257 Mg ha−1). Planar transects recorded 32 Mg ha−1 of wood ≥ 7.62-cm diameter compared to the 42 Mg ha−1 of wood ≥ 10-cm diameter recorded by explicit mapping. Multiple pieces of deadwood were often stacked, forming a vertical structure likely to contribute to active fire behavior. Conclusion Bark beetle mortality in the 1990s has made Picea an important local constituent of deadwood at 20-m scales, but other species dominate total deadwood due to slow decomposition rates and the multi-centennial intervals between fires. Explicit measurements of deadwood and surface fuels improve ecological insights into biomass heterogeneity and potential fire behavior.

The importance of large-diameter trees to the creation of snag and deadwood biomass

“…Alternatively, patterns in deadwood might only be created by significant storms that only occur so infrequently as to require long periods of observation. These long periods of observation are also useful in detecting past episodic mortality (i.e., Lutz et al 2020;Lutz et al 2021) or slightly higher rates of mortality on decadal scales (Birch et al 2019b;Germain and Lutz 2020).…”

“…A considerable portion of the variation in aboveground live biomass between studies most likely represents differences in choice of allometric equations (i.e., Lutz et al 2017b;Lutz et al 2021). In this work, we have relied on the national-level (for the United States) equations of Chojnacky et al (2014) for generalizability across the range of Pseudotsuga and Tsuga forests.…”

“…Actual heterogeneity in deadwood biomass becomes unexplained variance when using transect sampling methods, and this variance is ignored when multiple transects are averaged to generate stand-level estimates (Cansler et al 2019). Spatially explicit mapping methods such as these lead to more precise attribution of sources of variation, allowing more targeted efforts to improve aboveground carbon tracing (Janík et al 2018;Lutz et al 2020;Lutz et al 2021).…”

“…Biomass was then corrected for decay class using the relative density factors of Harmon et al (2008). For detailed definitions of decay classes, see Janík et al (2018) as implemented by Lutz et al (2020Lutz et al ( , 2021. For pieces that could not be identified to species, we used an average value for the genus calculated by proportionally weighting the identified pieces from the data set and their characteristics.…”

Evaluation of Deadwood Characteristics and Carbon Storage under Different Silvicultural Treatments in a Mixed Broadleaves Mountain Forest

“…The DW has an extensive ecological role during and throughout the decomposition stages in forests. These are as follows: sustaining biodiversity [3,4], providing a wide range of habitats for saprotrophic and heterotrophic organisms [5], regulation of nutrient cycling [6,7], regulation of hydrologic processes [8], as well as the provision of habitat for a wide diversity of organisms [9,10], including tree seedlings [11,12]. DW provides a habitat for wildlife such as wood-feeding termites [13,14], cavity-nesting birds [15,16], saproxylic beetles [17] and bats [18].…”