Influences of climate, fire, and topography on contemporary age structure patterns of Douglas-fir at 205 old forest sites in western Oregon

Abstract

Knowledge of forest development is basic to understanding the ecology, dynamics, and management of forest ecosystems. We hypothesized that the age structure patterns of Douglas-fir at 205 old forest sites in western Oregon are extremely variable with long and (or) multiple establishment periods common, and that these patterns reflect variation in regional-scale climate, landscape-scale topography, and landscape-scale fire history. We used establishment dates for 5892 individual Douglas-firs from these sites to test these hypotheses. We identified four groups of old forest sites with fundamentally different Douglas-fir age structure patterns. Long and (or) multiple establishment periods were common to all groups. One group described old forests characterized by substantial establishment from the early 1500s to the mid-1600s, with decreasing establishment thereafter. Another group was characterized by peaks of establishment in the middle to late 1600s and in the late 1800s and early 1900s. A third group was characterized by a small peak of establishment in the mid-1500s and a larger peak in the middle to late 1800s. Characteristic of the fourth group was the extended period of Douglas-fir establishment from the late 1600s to the late 1800s. Group membership was explained moderately well by contemporary, regional climatic variables and landscape-scale fire history, but only weakly by landscape-scale topography.

Understory abundance, species diversity and functional attribute response to thinning in coniferous stands

“…These interventions would also sustain growth of saplings, as shown after a second thinning at DMS sites (Shatford et al, 2009), although tradeoffs between increased sapling growth and potential reductions in understory development should be considered when deciding target sapling densities. Overall, the tree regeneration trends at the DMS sites support the contention that slow seedling establishment periods may be common in coniferous forests of the U.S. Pacific Northwest after large disturbances (Poage et al, 2009;Shatford et al, 2009).…”

δ15N constraints on long-term nitrogen balances in temperate forests

“…Distance from the coast influences summer aridity, so that areas near the coast experience less frequent fire (Poage et al 2009) and greater abundance of symbiotic N 2 -fixing red alder after disturbance (Franklin and Dyrness 1988), both of which may promote soil C and N accumulation near the coast. The significant negative correlation we found between ecosystem d 15 N and mean annual temperature is opposite the global pattern (Amundson et al 2003), yet is consistent with higher rates of alder N 2 -fixation under warmer conditions (Tripp et al 1979), and may point to an indirect mechanism of climatic control of N capital at our sites.…”

“…Stand-replacing wildfire is the most extensive natural disturbance in the Oregon Coast Range, occurring with a return interval of *250 years (Poage et al 2009) and promoting pure stands of early-successional red alder for 50-75 years before replacement by other species (Carlton 1988;Franklin and Dyrness 1988). Simulations parameterized with field-rates of nitrate loss and recurring sequences of wildfire on 250-year intervals (with 50 years of post-fire red alder N 2 -fixation) better approximated field d 15 N patterns than simulations with nitrate loss alone, and addition of modest denitrification rates (10 kg N ha -1 year -1 ) to wildfire simulations further improved modeldata agreement (compare Fig.…”

“…The interval for stand-replacing wildfire was set at 250 years (Long et al 1998;Poage et al 2009). Fractionating N losses by wildfire were set as the d 15 N of foliage relative to total ecosystem N, to approximate combustion N losses of plant material.…”

A 300-year history of Pacific Northwest windstorms inferred from tree rings

“…We posit that this moderate ranking is the result of antecedent forest conditions (cf. Teensma et al, 1991;Poage et al, 2009) and local topography (Harcombe et al, 2004); two factors that commonly influence the disturbance severity of windstorms in forest environments. Sub-regional windstorms may also be more difficult to detect using our method because of the few and diminishing number of old-growth forest sites suitable for sampling.…”