Post‐fire tree establishment and early cohort development in conifer forests of the western Cascades of Oregon, USA

Abstract

Abstract. Early-seral ecosystems make important contributions to regional biodiversity by supporting high abundance and diversity of many plant and animal species that are otherwise rare or absent from closed-canopy forests. Therefore, the period of post-fire tree establishment is a key stage in forest stand and ecosystem development that can be viewed in the context of competing management interests in diverse early-seral ecosystems vs. rapid forest development for ecological or commercial objectives. Previous work in Douglas-fir/western hemlock forests of the Pacific Northwest suggests stands initiate either with abrupt establishment (,20 years) or by protracted establishment with low tree density persisting .100 years. To improve understanding of how post-fire tree establishment and early cohort development have varied in space and over time and elucidate some of the factors contributing to that variation, we analyzed forest structure, tree ages, and Douglas-fir growth across the central western Cascades of Oregon where cohort ages span nearly eight centuries. The number of post-fire cohorts was estimated per stand, and establishment trajectories were evaluated by cohort. On average, it took 43.5 years to reach establishment of 90% of the trees per cohort. The rate and duration of establishment were surprisingly consistent across variation in topography (elevation, slope position, and aspect), among cohorts initiated from the late 12th to the early 20th century, and regardless of the severity of the cohort-initiating fire or the timing of establishment by shade-tolerant species. Only 8% of cohorts completed establishment within 20 years and 12% had establishment lasting .80 years. Douglas-fir growth (basal area increment) exhibits high plasticity in relation to different competitive interactions within uni-specific and multi-species cohorts and between cohorts of different age, suggesting wide variation in the structure and dynamics of early-seral ecosystems and an ability to tolerate moderate competition when young. This study illustrates that post-fire establishment in Douglas-fir/western hemlock forests of the central western Cascades historically was a multi-decadal process. Limited regeneration in a short window did not necessarily lead to persistent shrublands. In fact, post-fire forest development appears resilient to considerable variation in the fire regime and climatic and biotic constraints on tree establishment.

How does tree regeneration respond to mixed‐severity fire in the western Oregon Cascades,USA?

“…Research has shown western hemlock can immediately regenerate in high abundance following disturbance when soil moisture is high enough to offset higher transpirational demands common in post-disturbance landscapes (Isaac 1943). The tolerance of western hemlock to shade and competition suggests this species will persist into later successional stages and likely become dominant earlier in succession than occurs under the relay floristics model (Tepley et al 2014). 5).…”

“…CADE, incense-cedar; PSME, Douglas-fir; TSHE, western hemlock; THPL, western redcedar; TABR, Pacific yew; ABIES, true fir species; and PINUS, all pine species. 3), until microclimatic conditions ameliorate (e.g., solar insolation, moisture stress) and shade-tolerant species increase in abundance (i.e., relay floristics) as death of the pioneering cohort commences (Tepley et al 2014). Similar observations have been made in both field and remotely sensed assessments of contemporary fires (Kushla and Ripple 1997, Dunn and Bailey 2016, Reilly and Spies 2016, Reilly et al 2017, including the 2017 Eagle Creek fire that burned along the Oregon and Washington border where initial fire affects 45% low severity, 22% moderate severity, and 33% high severity.…”

Vulnerability to forest loss through altered postfire recovery dynamics in a warming climate in the Klamath Mountains

“…By contrast, in drier regions where ponderosa pine is the dominant conifer, postfire seedling density typically decreases to low levels within 200 m of seed sources (Chambers, Fornwalt, Malone, & Battaglia, ; Haire & McGarigal, ; Rother & Veblen, ). Once established, Douglas‐fir seedlings are moderately tolerant to the competitive postfire environment, as illustrated by the enormous plasticity in their early growth (Figures ; Tepley et al., ).…”

Projected climate change impacts on forest land cover and land use over the Willamette River Basin, Oregon, USA

“…Increased incidence of fire is likely to reduce mean stand age and the proportion of the landscape in the old-growth condition (Healey et al 2008). Our simulations assumed all fires were stand replacing, but historically mixed severity fire has been common in the West Cascades Mountains (Tepley et al 2014), hence the proportion of unevenaged stands will likely increase as the incidence of fire increases.…”