Article alert: Shifting climate, altered niche, and a dynamic conservation strategy for yellow-cedar in the North Pacific coastal rainforest

Yellow-cedar (Callitropsis nootkatensis) has been rapidly declining across Central British Columbia and Southeast Alaska for the last couple decades. Yellow-cedar (Y-C) has been historically found at high elevations at its southern range in California, Oregon and Washington State, but where it persists in the coastal rain forests of BC and AK, Y-C's niche is thought to be more pronounced in wet, hummock-like or valley and ridge topography. The latter ecotone has recently suffered a serious spat of Y-C decline, with no concise explanation. These changes have shifted the stand structure of these sites from old Y-C to young spruce, red-cedar, hemlock and sometimes Douglas fir (Pseudotsuga menziesii), alder (Alnus rubra) and birch (Betula papyrifera).

A few short months ago, a paper ran in Bioscience by the USDA Forest Service's Paul Hennon, David D'amore and several other authors and this paper provides a very clear picture of the cedar decline situation up in Alaska and BC. This project, like other current Y-C decline projects, is on the cutting edge of forest ecology, climate change and environmental management. Hennon et al. start by breaking down the history of Y-C's decline, and the study of this decline, at the individual tree level. Fine root death preceded coarse root death. Necrotic cambial bole lesions formed and crowns began to decline. Eventually outer foliage died and, well, curtains for that tree. They searched for insects and fungi, attempting to explain the procession of the tree's decline.
The extent of yellow-cedar decline in Southeast Alaska and Western British Columbia (borrowed from Hennon et al 2012)

It wasn't pests, and it wasn't pathogens doing the killing, so what was it? Because Y-C's wood is persistent and dead individuals can remain on the forest landscape as snags for decades, the studies moved from the individual tree to Y-C spatial patterns and landscape position. Was it something about where these trees were that had killed them? The authors, knowing the cause was largely abiotic at this point, looked into hydrology and local climate, finding that patches of Y-C mortality corresponded directly to hummocks with microclimatic extremes - wetter sites with cooler temperatures.

The authors also found that Y-C had a greater proportion of fine roots than neighboring tree species. They attribute this abundance of fine roots to Y-C's attempt to pull nitrate ions with calcium in saturated, anoxic soils. It is this exact ability to thrive in saturated conditions that earned Y-C a reputation as a plastic, tough species that can thrive where few other trees can. But life is tough as a stress tolerator, and Y-C was now being exposed to one of North America's most novel climates - a Northwest Pacific Coast that is experiencing a greater proportion of frost-free days than most other places on the continent.
Yellow-cedar, realized niche in the Washington Cascades

The combination of warmer weather during late winter and early spring, a reduced snowpack, and increasing cold events in the spring were found to be driving the decline of Yellow-Cedar by killing fine roots. What's more is that the historic realized niche of the species is rapidly shrinking and the potential niche will almost certainly be more colonized by Y-C in the future. In light of the authors' collective work on Y-C decline, they pitch a conservation strategy that includes individual migration to Y-C's future realized niche and modeling environmental changes to find suitable habitats to serve as Y-C refugia.

This paper seamlessly describes the ecosystem dynamics of a complex forest system while bringing in decades of research, pulling from applied conservation biology, but basing this framework firmly in the ecology of yellow-cedar and northern coastal forests.

You can link to a free copy of this manuscript from the USDA Forest Service.

Reference:


Hennon, Paul E.; D'Amore, David V.; Schaberg, Paul G.; Wittwer, Dustin T.; Shanley, Colin S. 2012. Shifting Climate, Altered Niche, and a Dynamic Conservation Strategy for Yellow-Cedar in the North Pacific Coastal Rainforest. BioScience 62: 147-158.

This is paper #5 of My five favorite articles this year.