Describing vegetation characteristics used by two rare forest-dwelling species: Will established reserves provide for coastal marten in Oregon?

Abstract

Forest management guidelines for rare or declining species in the Pacific Northwest, USA, include both late successional reserves and specific vegetation management criteria. However, whether current management practices for well-studied species such as northern spotted owls (Strix occidentallis caurina) can aid in conserving a lesser known subspecies—Humboldt martens (Martes caurina humboldtensis)–is unclear. To address the lack of information for martens in coastal Oregon, USA, we quantified vegetation characteristics at locations used by Humboldt martens and spotted owls in two regions (central and southern coast) and at two spatial scales (the site level summarizing extensive vegetation surveys and regionally using remotely sensed vegetation and estimated habitat models). We estimated amount of predicted habitat for both species in established reserves. If predicted overlap in established reserves was low, then we reported vegetation characteristics to inform potential locations for reserves or management opportunities. In the Central Coast, very little overlap existed in vegetation characteristics between Humboldt martens and spotted owls at either the site or regional level. Humboldt martens occurred in young forests composed of small diameter trees with few snags or downed logs. Humboldt martens were also found in areas with very dense vegetation when overstory canopy and shrub cover percentages were combined. In the South Coast, Humboldt martens occurred in forests with smaller diameter trees than spotted owl sites on average. Coastal Humboldt martens may use stands of predicted high quality spotted owl habitat in the Pacific Northwest. Nonetheless, our observations suggest that coastal Humboldt martens exist in areas that include a much higher diversity of conifer size classes as long as extensive dense shrub cover, predominantly in the form of high salal and evergreen huckleberry, are available. We suggest that managers consider how structural characteristics (e.g., downed logs, shrub cover, patch size), are associated with long-term species persistence rather than relying on reserves based on broad cover types. Describing vegetation may partially describe suitability, but available prey or predation risk ultimately influence likelihood of individual Humboldt marten use. Guidelines for diversifying vegetation management, and retaining or restoring appropriate habitat conditions at both the stand level and regionally, may increase management flexibility and identify forest conditions that support both spotted owls and Humboldt martens.

Biotic factors influencing the unexpected distribution of a Humboldt marten (Martes caurina humboldtensis) population in a young coastal forest

“…Although shrub cover was lower in seasonally-flooded shore pine forest, seasonal flooding may have contributed to the relative absence of bobcats and other competitors there, perhaps conferring some competitive advantage to the scansorial marten if they are able to travel through the canopy more readily when forests are flooded (Fig 6). Although shrubs were not prevalent in all forests used by martens, other studies in the Pacific states have shown strong association with shrub cover and therefore further mechanistic studies examining whether shrubs provide protective cover may be warranted [12,65].…”

Predicted distribution of a rare and understudied forest carnivore: Humboldt martens (Martes caurina humboldtensis)

“…We removed marten locations from our model sample that occurred in areas that disturbance (fire or harvest) between the data of observation and the date represented by our vegetation layers (2016). If there were multiple locations within the 500-m cell, we spatially-thinned locations prioritizing those in the order of: (1) rest and den locations from telemetry studies in the Central Coastal Oregon population (Linnell et al 2018) and the Northern Coastal California population (Delheimer et al In press; PSW 2019); (2) locations from scat dog detection surveys (Rogue Detection Teams, Rice, WA; Conservation Canines, University of Washington, Seattle, WA) from both Oregon populations (detailed methods within Moriarty et al 2018; Moriarty et al 2019); and (3) locations from baited camera and/or track plate surveys from all four populations in Oregon (Barry 2018; Moriarty et al 2019) and California (Gamblin 2019; Slauson et al 2012). We thinned locations to one within a 500-m cell, attempting to maximize distances between locations and induce spatial independence for modeling (Kramer-Schadt et al 2013).…”

“…Humboldt martens ( M. c. humboldtensis ) are a subspecies of Pacific marten that historically occurred throughout coastal forests of northern California and Oregon. Contemporary range-wide surveys suggest that distribution of Humboldt martens is substantially reduced compared to historical representations (Moriarty et al 2019; Zielinski et al 2001). Consequently, Humboldt martens were listed as Endangered under the state of California’s Endangered Species Act (CDFW 2019) and as threatened under the federal Endangered Species Act as a “coastal distinct population segment of Pacific martens” (USFWS 2020).…”

“…Evaluating current and future Humboldt marten distribution is urgently needed for conservation planning, yet modeling their distribution is constrained by a paucity of ecological information (e.g., population sizes and extents) and apparent non-stationary associations with vegetation among populations. For example, Humboldt martens have been associated with a diversity of forest types across their distribution, including mature forests with high canopy cover, mature forests with dense shrub understories (Slauson et al 2007), young forests (<80 years old) with modest canopy cover and relatively small diameter trees with dense shrub cover (Eriksson et al 2019; Moriarty et al 2019), and forests in serpentine soils with sparse canopy cover and dense shrub cover. A dense and spatially-extensive shrub layer appears to be an attribute shared by each of these studies (Eriksson et al 2019; Gamblin 2019; Moriarty et al 2019; Slauson et al 2007).…”

“…However, the model relied on modest number of detections from 1996–2010 with poor coverage outside a portion of northern California (USFWS 2019). Since 2010, we initiated large-scale surveys for Humboldt martens that greatly increased the spatial extent and number of Humboldt marten detections in both California and Oregon (e.g., Barry 2018; Gamblin 2019; Linnell et al 2018; Moriarty et al 2019). Further, recent research efforts appear to contradict the finding that Humboldt martens are strongly associated with old-growth forests, particularly in Oregon (e.g., Eriksson et al 2019), suggesting a potential mismatch in previously-predicted associations between OGSI and Humboldt marten distribution.…”