Molecules to Mountains: A Multi-Proxy Investigation Into Ancient Climate and Topography of the Pacific Northwest, USA

We characterize the topographic evolution of the Pacific Northwest, United States, during the Cenozoic. New paleosol carbonate stable isotope (δ18O) results from central Oregon are presented, along with published proxy data, including fossil teeth, smectites, and carbonate concretions. We interpret a polygenetic history of Cascade Mountain topographic uplift along-strike, characterized by: 1) Steady uplift of the Washington Cascades through the Cenozoic due long-term arc rotation and shortening against a Canadian buttress, and 2) Uplift of the Oregon Cascades to similar-to-modern elevations by the late Oligocene, followed by topographic stagnation as extension developed into the Neogene. Since the Miocene, meteoric water δ18O values have decreased in Oregon, possibly due to emergence of the Coast Range and westward migration of the coastline. Spatial variability in isotopic change throughout the Pacific Northwest suggests that secular global climate change is not the primary forcing mechanism behind isotopic trends, though Milankovitch cycles may be partly responsible for relatively short-term variation.

Quantifying the vertical diversification development stage of old-growth Douglas-fir to derive stage-specific targets for restoration silviculture

Structural variability in natural Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) forests hinders restoration targets for the full old-growth developmental sequence. To guide stage-specific restoration, we present a new approach toward developing a simple index of the vertical diversification (VD) stage. VD-associated sub-stand structures were objectively identified by quantifying the size structures of live trees across a developmental sequence of 10 pristine stands in the Oregon Cascades, USA. Floating neighborhoods were used to delineate natural tree neighborhoods based on triangulated irregular networks in five concentric rings of ever-greater spatial extent (averaging 60–2060 m2). Diameter distributions summarized the most frequently encountered tree size structures among neighborhoods, with increasing deviation from the neutral multi-variate core. Of 18 observed diameter distribution types (DDTs), the core DDT characterized all-sized tree neighborhoods reflective of vertical diversification, which was most abundant in early old-growth (VD) stands and least abundant at the extremes of the sequence. VD declined in older stands, whose more distinct DDTs had peaks in larger trees and multiple size classes, likely reflecting horizontal diversification (HD). This new approach illustrates that structural restoration of VD stands could be facilitated by the single-tree selection method, while that of HD stands may be promoted with both single-tree and group selection as well as targeted release.