Characterizing individual tree‐level snags using airborne lidar‐derived forest canopy gaps within closed‐canopy conifer forests

Abstract

Airborne lidar is often used to calculate forest metrics about trees, but it may also provide a wealth of information about the space between trees. Forest canopy gaps are defined by the absence of vegetative structure and serve important roles for wildlife, such as facilitating animal movement. Forest canopy gaps also occur around snags, keystone structures that provide important substrates to wildlife species for breeding, roosting and foraging. We wanted to test a method for quantifying canopy gaps around individual snags and live trees, with the working hypothesis that snags would have more gaps surrounding them overall than live trees. We evaluated canopy gaps around individual snags (n = 270) and live trees (n = 2,186) and evaluated correlations between canopy structure and snag occurrence in dense conifer stands of the Idaho Panhandle National Forest, United States. We paired airborne lidar with ground reference data collected at fixed‐radius plots (n = 53) to evaluate local gap structure. The r package ForestGapR was used to quantify canopy gaps throughout the canopy to determine where the differences were greatest. A canopy space profile was created for each tree by mapping gaps (a) vertically every 2 m in height (2–50 m above‐ground), and (b) horizontally across small (16 m2), medium (36 m2) and large (64 m2) footprint sizes. Our results suggest that this method is robust for quantifying canopy gaps around individual trees. The canopy space profiles were distinctly different for snags and live trees, with more canopy gaps within the area surrounding snags relative to live trees. The greatest differences occurred at mid‐canopy heights (~20 m above‐ground) and at the smallest footprint size (16 m2). These results show potential to improve the understanding of gap dynamics in closed‐canopy conifer forests, and we suggest that snag modelling could be improved by incorporating lidar‐derived canopy gap analyses alongside existing methodologies.

Feedback loops between 3D vegetation structure and ecological functions of animals

“…For cavity-excavating birds such as woodpeckers, breeding success depends on the availability of standing deadwood. Canopy height and heterogeneity metrics can indicate the distribution of this critical resource for reproducing birds, mammals and insects (Carrasco et al, 2014;Martinuzzi et al, 2009;Stitt et al, 2021Stitt et al, , 2022. In addition, insight from animal habitat selection can help identify minimum ecological requirements for population persistence, such as features of 3D vegetation structure necessary for survival and reproduction (Davies et al, 2017;Deere et al, 2020).…”

Feedback loops between 3D vegetation structure and ecological functions of animals

“…For cavity-excavating birds like woodpeckers, breeding success depends on the availability of standing deadwood. Canopy height and heterogeneity metrics can indicate the distribution of this critical resource for reproducing birds, mammals, and insects (Martinuzzi et al 2009;Carrasco et al 2014;Stitt et al 2021Stitt et al , 2022.…”

Evaluating the Use of Lidar to Discern Snag Characteristics Important for Wildlife

“…In terms of open space, snag presence and breakdown can be central to the generation of gaps within the canopy used by volant organisms as travel corridors [4,5]. Stitt et al [6] evaluated forest canopy gaps around individual trees to establish that snags were associated with significantly larger gap fractions in the immediate vicinity than live conifer trees, especially at midcanopy heights (5 to 20 m above ground). Snags also had higher variability in gap distributions across all canopy heights [6].…”

“…Stitt et al [6] evaluated forest canopy gaps around individual trees to establish that snags were associated with significantly larger gap fractions in the immediate vicinity than live conifer trees, especially at midcanopy heights (5 to 20 m above ground). Snags also had higher variability in gap distributions across all canopy heights [6]. This greater variability in openness around snags contributes to greater forest structural complexity and may be correlated with greater foliar height diversity and species richness [7].…”

“…However, approaches focused upon tree crowns can be challenging with snags because dead trees lack the foliar surface area of live tree canopies, and therefore reflect far fewer laser pulses to evaluate and locate snags [28]. Focusing on the absence of lidar data (in the form of a canopy gap fraction) can provide additional valuable information for characterizing snags from ALS data [6]. Determining what further information can be gleaned from the sparse lidar data associated with snags is an area worth investigating.…”