John Krapek (University of Alaska Fairbanks) and Brian Buma (University of Alaska Southeast) recently had a paper published in Journal of Ecology entitled ‘Limited stand expansion by a long-lived conifer at a leading northern range edge, despite available habitat‘. John tells us more about the paper below.

Yellow-cedar (Callitropsis nootkatensis, Cupressaceae), a culturally and economically important conifer of the Pacific Northwest Coast of North America, is currently undergoing a shakeup in its bioclimatic envelope.

Yellow-cedar habits the narrow strip of temperate rainforest habitat along the Pacific coast, from Prince William Sound, Alaska through British Columbia to northern California. Within this already wet ecoregion, yellow-cedar is most competitive on forested wetland and bog sites with shallow water tables; the tree’s high concentrations of unique heartwood defence compounds confers extreme decay resistance to fungal rot and insects, making them well-adapted to the saturated conditions in which they grow. Protected by these aromatic compounds (which smell wonderful), yellow-cedar trees often survive for >1,000 years and may even reach >3,000 years old (Hennon et al. 2016).

Yellow-cedar distribution (yellow) and decline (red) in southeast Alaska. Credit: U.S. Forest Service

Intensive yellow-cedar mortality on Chichagof Island, Alaska. Photo credit: U.S. Forest Service.

In addition to thriving in wet environments, yellow-cedar’s life history is uniquely dependent upon snow. In wet areas and other habitats with a limited rooting zone (e.g., thin soil over bedrock), yellow-cedar produces very fine roots close to the soil surface, which are vulnerable to freezing injury in the winter and late spring (Schaberg et al. 2011). Historically, these fragile fine roots have been dependent upon a persistent winter snowpack to insulate them from freezing air temperatures. In the southern extent of its range (California, Oregon, and Washington), yellow-cedar is confined to high elevations where protective snowpacks linger late into spring and early summer.

Farther north, in British Columbia and Alaska, yellow-cedar reaches low elevations where snow is more frequent in the winter. However, due to the moderating influence of the Pacific Ocean, the mean winter temperature across much of coastal British Columbia and southern Alaska has been close to 0°C; even minor climate warming has a serious impact on reducing winter snow cover (Shanley et al. 2015). As insulating snowpacks have decreased across the yellow-cedar range, but periodic winter freezing events have persisted, yellow-cedar root freezing injury has led to concentrations of mortality in coastal British Columbia and southern Alaska (Hennon et al. 2012).

Travel a hundred or so kilometres north of where yellow-cedar trees are dying in Alaska and the forest looks very different. At the species’ leading northern range edge, slightly colder temperatures have translated into more snow in the wintertime, and small pockets of vigorously healthy yellow-cedar trees are scattered on the landscape. How long have these trees been present here, and how quickly are they spreading into what appears to be suitable habitat? Might migration related gains makeup for climate-related losses elsewhere in the range? That is what we set out to determine in our study, the first attempt to quantify northward range expansion of this climate-impacted tree.

We established 29 plots at leading edge yellow-cedar stand boundaries to map the spread of yellow-cedar seedlings into currently unoccupied forests which appear capable of supporting yellow-cedar trees. Photo taken from inside the edge of the yellow-cedar forest, looking out. Photo credit: Mark Rainery, University of Alaska Southeast.

Research assistant Alex Botelho, standing at the edge of a yellow-cedar stand, uses a laser range finder to map the location of yellow-cedar trees, seedlings, and associated species. A 300-year-old yellow-cedar tree stands directly behind Alex, representing a past pulse of successful stand expansion. Photo credit: Mark Rainery, University of Alaska Southeast.

Yellow-cedar, Callitropsis nootkatensis, Cupressaceae. Photo credit: John Krapek.

Surprisingly, we found that although there pockets of relatively young, healthy trees scattered among abundant potential habitat (Krapek et al. 2017), yellow-cedar dispersal appears to be at a relative standstill over the past two centuries. At 11 leading range edge stands surveyed, we found approximately 200-year-old yellow-cedar trees located abruptly at stand boundaries, with few instances of successful yellow-cedar regeneration located outside stands.

It appears that yellow-cedar were spreading rapidly during the colder and potentially snowier Little Ice Age climate period (1100 – 1850), but that dispersal is currently in a relative lull. We found ample germination of seeds within stands, and abundant vegetative layering (asexual reproduction which may contribute to stand maintenance and persistence), but few seedlings and saplings on the landscape to push stand edges forward. 

We hypothesize that the snowier Little Ice Age period may have favoured yellow-cedar dispersal and active stand expansion compared to the current snow regime. First, a robust insulating snowpack protects yellow-cedar roots and fragile juvenile foliage from freezing events in the winter and late spring. Persistent snowpacks are also known to protect highly palatable seedling foliage from deer and moose browse in the wintertime; heavy snow may have also depressed regional deer populations during the Little Ice Age.

Second, increased snow-loading on trees may have led to a higher incidence of crown injury and bole breakage, creating canopy gaps in the forest which would allow for shade-intolerant and slow growing yellow-cedar to establish. Finally, yellow-cedar’s seeds are relatively heavy compared to sympatric conifers, with no known animal dispersers; seeds may blow across the snowpack during stormy periods as their primary means of medium to long distance dispersal. Continued research is required to determine the exact mechanisms for broad recruitment failure at the leading range edge.

A typical yellow-cedar stand boundary in the study area. Approximately 200-year-old yellow-cedar are located abruptly at the stand edge, with regeneration of other tree species (e.g., western hemlock (Tsuga heterophylla)) outside the boundary, indicating that stands have been in a period of relative stasis for the past many decades to centuries.

Whatever the reasons may be for the current lull in yellow-cedar dispersal at the leading range edge, this long-lived, slow-growing tree does not appear to be migrating in response to climate change. In fact, the past century of climate warming and reduced snow regime may contribute to a reduction in the natural migration capacity of yellow-cedar. 

When planning for the conservation of this high value tree, and other species facing climate-related range reductions, we need to understand the mechanisms that control dispersal to new habitats – even if climatic envelopes become favourable for adult growth, will seedlings be able to reach those areas and compete within existing plant communities? Taking a long-range view of dispersal, which encompasses variable climatic, landscape conditions, and species assemblages through time, is necessary to understand how the species on which we depend move across the landscape.

John Krapek, University of Alaska Fairbanks, USA