By Nate Hough-Snee
Yellow-cedar (Callitropsis nootkatensis) has been rapidly declining across Central British Columbia (BC) and Southeast Alaska (AK) 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. At the northern limits of its range, it persists in the coastal rainforests 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 (Picea sitchensis), redcedar (Thuja plicata), hemlock (Tsuga heterophylla) 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, providing 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 research, and ecosystem 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. They found neither insect outbreaks nor new or more abundant pathogens.
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, began to measure 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 concurrently 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 exposed to one of North America’s most novel climates – a Northwest Pacific Coast that is experiencing a greater reduction in the proportion of frost-free days than most other places on the continent.
The combination of warmer weather during late winter and early spring, a reduced snowpack, and increasing acute cold events in the spring were found to be driving the decline of Yellow-Cedar by killing fine roots. Where snow once insulated the ground from extreme cold, the soil now turns from a puddle to a popsicle high in the soil profile when a thaw-freeze event occurs. This root mortality limits Y-C’s ability to maintain sufficient nutrition for survival and growth. To picture this scenario, imagine having a vegetable garden that’s already in place, but now begins growing weeks earlier than it historically did, only to have killer frosts come and kill all of the tomatoes. This is basically what the forest landowners of coastal Alaska and British Columbia are now facing, and unlike your common gardener, the USDA Forest Service, tribes and timber firms can’t just plant after the risk of frost has passed.
What’s more than individual stand declines, are the implications of Y-C death at larger scales. As the historic realized niche of the species is rapidly shrinking, the potential niche will almost certainly be more colonized by Y-C in the future. Y-C’s historic realized niche of wet lowland hummocks will become a place for other species to thrive, although it’s unclear what the winning species will be. In light of the authors’ collective work on Y-C decline resulting in novel forests from a novel regional climate, they pitch a conservation strategy that includes assisted migration of Y-C individuals to Y-C’s future realized niche. They suggest modeling current environmental heterogeneity and future environmental change to find suitable habitats that can serve as Y-C refugia. Armed with this information, land managers can take a triage approach in taking action to retain and establish new Y-C stands.
Within this paper, Hennon and D’Amore seamlessly describe the dynamics of a complex forest ecosystem, incorporating decades of research that draw on applied conservation biology. They firmly base their framework in the ecology of yellow-cedar and northern coastal forests, satiating both applied and basic biologists with their work.
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.