Climate and Epidemiology

Fossil record

The cypress family, Cupressaceae, evolved some 200 million years ago in the Triassic Period (Taylor and Taylor, 1993). Taxa with affinity to yellow-cedar first appeared in the Miocene (Kotyk et al. 2003), although similar macrofossil foliage dates to the Eocene (Axelrod, 1976) and cones date to the Late Cretaceous. The fossil record indicates that Chamaecyparis was more broadly distributed in the northern hemisphere than at present, but appears to have been, and continues to be, confined to moist climates, and some genera were extirpated with climate shifts to drier environments (Kotyk et al. 2003).

Yellow-cedar current distribution in southeast Alaska

Despite being such a valuable tree species, the current distribution of yellow-cedar in southeast Alaska is not well known. We used field plot data from USFS Forest Inventory and Analysis (FIA) to construct a coarse presence/absence map of yellow-cedar by highlighting areas between plots, which were on a grid approximately three miles apart. The resulting map illustrates that yellow-cedar is well distributed in most of the region, except for the northeastern portion where it is rare.

click image for large version

We hypothesize that the rarity and absence of yellow-cedar in the northeastern portion of the panhandle is due to distance from the suspected refugia (Carrarra et al., 2003, Carrarrea etal., 2007) and the likelihood of a slow migration and colonization from the southwestern direction. A very slow post-glacial migration from refugia in the southwestern parts of the panhandle may be explained by the poor regeneration capacity of yellow-cedar (Harris, 1990, Klinka, 1996). Yellow-cedar may have survived the Pleistocene in these refugia as a minor component of forests where it tolerated the harsher periods of the early Holocene by existing on favorable microsites such as those with groundwater discharge. Then, about 5,000 to 6,000 years ago during favorable cool, wet periods, it may have begun to expand its range. A preliminary evaluation of the genetic structure of the species throughout its natural range (Ritland et al. 2001) found that yellow-cedar in southeast Alaska is genetically distinct from populations in British Columbia and further south, supporting the scenario that populations of yellow-cedar in southeast Alaska originated in Alaskan refugia. Interestingly, small populations in Prince William Sound (Hennon and Trummer, 2001) disconnected with those in southeast Alaska, have been found to have affinity and possible origin with forests in the Queen Charlotte Islands (Ritland et al., 2001).

In 2009, we will test the refugia-migration hypothesis in a study of yellow-cedar's genetic structure throughought Southeast Alaska. This will be a cooperative effort with Rich Cronn in Corvallis, OR.

Pollen studies and climate

Climate during the Holocene Epoch can be interpreted from the composition of trees and other plants in pollen profiles taken from lake and peat sediments, including 17 sites investigated by Heusser (1952, 1960). These pollen profiles provide direct evidence of the post-glacial abundance of conifers in southeast Alaska.

Table 1. Interpretation of late Pleistocene and Holocene climate and dominant vegetation, adapted from narrative in Viens, 2001.

Geologic Epoch	Years BP1	Climate2	Dominant vegetation
Late Pleistocene	16,000 - 12,500	Cool dry	Tundra / shrubs
	12,500 - 9,000	Warm, dry	Pine, alder, willow
Holocene	9,000 - 6,800	Warm, wet	Spruce, hemlock
	6,800 - 4,500	Trending wet, cool	Hemlock, spruce, cedar
	4,500 – present	Cool, wet	Modern flora

Temperatures and precipitation appear to have reached levels relatively similar to the current maritime climate about 4,500 years ago, both in Alaska (Heusser et al., 1985) and British Columbia (Mathewes and Heusser, 1981). This date corresponds to the establishment of modern western hemlock-Sitka spruce composition evident in many pollen cores (Viens, 2001).

Unfortunately, yellow-cedar was not included in the early pollen studies because, as Heusser (1960) stated (Page 78), the pollen of Chamaecyparis and some other species had, “fragility and non resistance to decay….it was decided they be omitted [from analysis].” Another problem is that pollen of yellow-cedar is difficult to distinguish from western redcedar and juniper and is often lumped as “Cupressaceae pollen” or “cedar-type pollen.” Cupressaceae was included in several more recent pollen studies in British Columbia just south of Alaska. Cupressaceae pollen became abundant about 7,000 years ago (Banner et al., 1983; Hebda and Mathewes, 1984), indicating a cooling trend in climate.

A cool wet climate with its associated development of poorly drained organic soils (Ugolini and Mann, 1979) favors both cedar species (Banner et al., 1983, Hebda 1983). Hebda (1983) reported that Cupressaceae pollen made a relatively recent appearance about 3,000 years ago in pollen profiles at a bog site on northern Vancouver Island in British Columbia. Recent pollen analysis in southeast Alaska is revealing that the cedars became more prevalent only about 5,000 years ago (Tom Ager, USGS, Pers. Comm.). Ager’s research should shed more light on how long the cedars have been in Alaska and how their populations fluctuated, filling the knowledge gap for these species in the early pollen studies.

More recently, a cooler shift, known as the “Little Ice Age”, occurred in the Northern Hemisphere beginning some 500 years ago. Although the influence of the so-called Little Ice Age on the climate of southeast Alaska is not clearly understood, advances and retreats of glaciers are consistent with its occurrence (Viens, 2001).

The end of the Little Ice Age in the mid to late 1800s was associated with a time or warming and subsequently, marked the onset of yellow-cedar decline in about 1880 to 1900 (discussed below). Information on the ages of canopy-level yellow-cedar trees (i.e., nearly all > 100 years old, (Hennon and Shaw, 1994)), suggests that the trees that died throughout the 1900s, and those that continue to die, regenerated and grew into their dominant positions during the Little Ice Age. We speculate that yellow-cedar colonized low elevation sites during this period, when winter and spring snowpacks were more consistent.

Yellow-cedar dendrochronology

Recent dendrochronology-climate research based out of University of Alaska, Fairbanks was initiated to provide robust sampling of living yellow-cedar trees over a range of sites, including different elevations and with varying levels of forest decline. One goal was to produce a growth chronology for yellow-cedar and associate it with climate data from weather stations through the 1900s, although the chronology will extend back to 1600AD. The other goal was to detect episodes of injury, or poor growth, that were associated with weather scenarios that align with the leading hypothesis described below. See the publication by Beier et al. 2008 for more information. Laroque and Smith (1999) investigated growth patterns for high elevation yellow-cedar in British Columbia as the only other dendrochronology study of the species.

Epidemiology

A snag classification with associated time-since-death estimates (Hennon et al. 1990) was developed and used in ground surveys (Hennon et al. 1990) to reconstruct changes in populations through the 1900s. This provides a coarse view of annual mortality through the century. The remarkable decay resistant heartwood of dead yellow-cedar trees (Kelsey et al., 2005) allows them to remain standing for 80 to 100 years after death, making this reconstruction possible. The onset of yellow-cedar decline occurred in about 1880 to 1900 on most of the sites where trees are still dying. Note that an accelerated mortality to yellow-cedar occurred in the last half of the 20th century. Thus the decline is progressive in declining forests, which now contain long-dead trees, more recently-killed trees, dying trees, and some survivors, usually other tree species (Hennon and Shaw, 1997). The mortality problem is typically associated with wet, poorly drained soils with long-dead cedars often on the wettest soils. Recently-killed and dying trees are frequently found on better-drained soils and on the perimeters of the dying forests, indicating a slow spreading pattern along a hydrologic gradient (Hennon et al., 1990).

Snag class, time since death:

Class 1	Class 2	Class 3	Class 4	Class 5
4 years	14 years	26 years	51 years	81 years

Collectively, these various sources of information lead to the following hypothesis to explain the cause of yellow-cedar decline. Reduced snowpack at the end of the Little Ice Age is the environmental change that may have triggered the onset of this forest decline. Yellow-cedar remains healthy in areas that have adequate snowpack in late-winter and spring. See the section, Causes of Yellow-cedar Decline or recent summary paper for more explaination.

References

Axelrod, D.J. History of the coniferous forests, California and Nevada, Univ. Calif. Publ. Bot., 70, 1-62, (1976).

Beier, C.M., S.E. Sink, P.E. Hennon, D.V. D'Amore, G.P. Juday, Twentieth-century warming and the dendroclimatology of declining yellow-cedar forests in southeastern Alaska, Can. J. For. Res. 38, 1319-1334, (2008).

Banner, A., J. Pojar, and G.E. Rouse, Postglacial paleoecologoy and successional relationships of a bog woodland near Prince Rupert, British Columbia. Can. J. For. Res. 13, 938-947, (1983).

Carrarra, P.E., T.A. Ager, J.F. Baichtal, Possible refugia in the Alexander Archipelago of Southeast Alaska during the late Wisconsin glaciation. Can. J. Earth Sci. 44, 229-244, (2007).

Carrarra, P.E., T.A. Ager, J.F. Baichtal, and D.P. VanSistine, Map of glacial limits and possible refugia in the southern Alexander Archipelago, Alaska, during the late Wisconsin Glaciation, Miscellaneous Field Studies Map, MF-2424. US Geological Service, Denver, CO. (2003)

D’Amore, D.D., and P.E. Hennon, Evaluation of soil saturation, soil chemistry, and early spring soil and air temperatures as risk factors in yellow-cedar decline, Global Change Biology, In press, (2006).

Harris, A.S., Chamaecyparis nootkatensis (D. Don) Spach: Alaska-cedar, In Silvics of North America, Volume 1: Conifers, Agric. Handb. 654, U.S. Department of Agriculture, Forest Service, Washington, DC., Pp. 97-102, (1990).

Hebda, R.J., Late-glacial and post glacial vegetation history at Bear Cove Bog, northeast Vancouver Island, British Columbia, Can. J. Bot., 61, 3172-3192, (1983).

Hebda, R.J., History of cedars in western North America. Pp. 5-13, In: Wiggins, G.G. (Ed.), Proceedings of the cedar symposium, May 28-30, 1996, Canada-British Columbia South Moresby Forest Replacement Account, Queen Charlotte City, BC, (1996).

Hebda, R.J. and R.W. Mathewes,. Holoceen history of cedar and native cultures on the North American Pacific Coast. Science 225, 711-3, (1984).

Hennon, P.; D’Amore, D.; Wittwer, D.; Johnson, A.; Schaberg, P.; Hawley, G.; Beier, C.; Sink, S.; Juday, G. 2006. Climate Warming, Reduced Snow, and Freezing Injury Could Explain the Demise of Yellow-cedar in Southeast Alaska, USA. World Resource Review. 18: 427-450.

Hennon, P.E., and C.G. Shaw III, Did climatic warming trigger the onset and development of yellow-cedar decline in southeast Alaska? Europ. J. For. Path., 24, 399-418, (1994).

Hennon, P.E.; C.G. Shaw III, and E.M. Hansen, Dating decline and mortality of Chamaecyparis nootkatensis in southeast Alaska, Forest Science, 36, 502-515, (1990)

Hennon, P.E., E.M. Hansen, and C.G. Shaw, III, Dynamics of decline and mortality of Chamaecyparis nootkatensis in southeast Alaska, Can. J. Bot., 68, 651-662, (1990).

Hennon, P.E., and L.M. Trummer, Yellow-cedar (Chamaecyparis nootkatensis) at the northwest limits of its range in Prince William Sound, Alaska, Northwest Science, 75, 61-72, (2001).

Hennon, P.E., D.V. D'Amore, D.T. Wittwer, and J.P. Caouette. Yellow-cedar decline: Conserving a climate-sensitive tree species as Alaska warms. published in: Deal, R.L., tech. ed. 2008. Integrated restoration of forested ecosystems to achieve multiresource benefits: proceedings of the 2007 national silviculture workshop. Gen. Tech. Rep. PNW-GTR -733. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 306 p. (2008).

Heusser, C. J. Late Pleistocene environments of North Pacific North America, American Geographical Society Special Publication 35, (1960).

Heusser, C.J. Pollen Profiles from southeastern Alaska, Ecological Monographs, 22, 331-352, (1952).

Heusser, C. J., L.E. Heusser, and D.M. Peteet, Late-Quaternary climatic change on the American North Pacific coast, Nature 315, 485-487, (1985).

Kelsey, R.G., P.E. Hennon, M. Huso, and J.J. Karchesy, Changes in heartwood chemistry of dead yellow-cedar trees that remain standing for 80 years or more in Southeast Alaska. Journal of Chemical Ecology, 31, 2653-2670, (2005).

Klinka, K., Update on silvics of western redcedar and yellow-cedar, Pp. 14-28, In: Wiggins, G.G. (Ed.), Proceedings of the cedar symposium, May 28-30, 1996, Canada-British Columbia South Moresby Forest Replacement Account, Queen Charlotte City, BC, (1996).

Kotyk, M.E.A., J.F. Basinger, and E.E. McIver, Early Tertiary Chamaecyparis Spach from Axel Heiberg island, Canadian high arctic, Canadian Journal of Botany 81, 113-130 (2003).

Laroque, C.P., and D.J. Smith, Tree ring analysis of yellow-cedar (Chamaecyparis nootkatensis) on Vancouver Island, British Columbia, Can. J. For. Res., 29, 115-123, (1999).

Mathewes, R.W., and L.E. Heusser, A 12, 000 year palynological record of temperature and precipitation trends in southwestern British Columbia, Canadian Journal of Botany 59, 707-710, (1981).

Ritland, C., T. Pape, , and K. Ritland, Genetic structure of yellow cedar (Chamaecyparis nootkatensis), Can. J. Bot., 79, 822-828, (2001).

Taylor, T.N., and E.L. Taylor, The biology and the evolution of plants, 2nd ed., Cambridge Univ. Press, New York, NY, (1993).

Ugolini, F.C., and D.H. Mann, Biopedological origin of peatlands in southeast Alaska, Nature, 28, 366-368, (1979).

Viens, Robert J., Late Holocene climate change and calving glacier fluctuations along the southwestern margin of the Stikine Icefield, Alaska, University of Washington, PhD Dissertation, 160 p. (2001).