Regeneration densities in climate threatened species suggest glacial migration pace

In our on-going quest to understand how species and communities change in response to warming, we've been tracking migration of a climate threatened conifer.  This process has entailed mapping the range edge - precisely - and then monitoring the production of new recruits.  If those recruits are outside that range edge, then they are pushing the range forward or infilling - migration. 

Climate change in Alaska has been going on some time, since the end of the Little Ice Age, though of course it's accelerating.  This provides a nice opportunity to watch adaptation-in-action, sans models, and provides a good empirical check on migration expectations.  

Yellow-cedar is a great study case - it is, and has been, culturally important for thousands of years among Indigenous cultures, so it's tracked.  It's economically important now.  It's ecologically significant, as it dramatically changes the biogeochemistry of the soil.  And it's unique among a sea of spruce and hemlock (it's a low diversity forest, so cedar is a highlight).  The species should be able to migrate rapidly - the climate is ideal, the plant community is the same as throughout the contiguous range, the topography and edaphic conditions seemingly perfect.

And yet...

Yellow-cedar regeneration densities in understorey plant community associations. (a) Interior subplots. (b) Exterior subplots. Communities are ordered left to right based on soil drainage: communities on left have a higher percentage of well-drained soils, communities on right a higher proportion of poorly drained soils (Martin et al., 1995). Some blueberry (Vaccinium spp.) type communities with similar species composition and soil drainage characteristics were lumped together. In one exterior plot, the dominant plant association was devil’s club—skunk cabbage (Oplopanax horridus—Lysichiton americanum), and this plot was lumped with the blueberry—skunk cabbage (Vaccinium spp. —L. americanum) category due to similar composition and soil drainage. The number of subplots falling in each community type is listed in parentheses.  From Krapek and Buma 2017.

Yellow-cedar regeneration densities in understorey plant community associations. (a) Interior subplots. (b) Exterior subplots. Communities are ordered left to right based on soil drainage: communities on left have a higher percentage of well-drained soils, communities on right a higher proportion of poorly drained soils (Martin et al., 1995). Some blueberry (Vaccinium spp.) type communities with similar species composition and soil drainage characteristics were lumped together. In one exterior plot, the dominant plant association was devil’s club—skunk cabbage (Oplopanax horridus—Lysichiton americanum), and this plot was lumped with the blueberry—skunk cabbage (Vaccinium spp. —L. americanum) category due to similar composition and soil drainage. The number of subplots falling in each community type is listed in parentheses.  From Krapek and Buma 2017.

Regeneration is absolutely minimal outside the existing stands.  There is some regeneration within the individual stands (ranging from a single tree to a few dozen mature individuals), but not a lot - and regeneration outside is constrained to pretty much the blueberry plant/rusty menziesia plant association (Vacc. and Menz.).  It's unclear why - those are productive forests where yellow-cedar isn't expected to actually be competitive, so it's probably less regeneration than it looks like.

Photograph of a typical yellow-cedar stand boundary in the study area. Approximately 200-year-old yellow-cedar (Callitropsis nootkatensis) 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. No obvious yellow-cedar mortality is observed inside the stand boundary.

Photograph of a typical yellow-cedar stand boundary in the study area. Approximately 200-year-old yellow-cedar (Callitropsis nootkatensis) 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. No obvious yellow-cedar mortality is observed inside the stand boundary.

The most likely reasons are either 1) herbivory or 2) a lack of disturbance opportunity.  We need to test both.  The herbivory hypothesis is being informally tested, and currently found lacking, via a few plantations scattered around the area where herbivory is not a factor.  Why not eat nice, fertilized plantation trees?  A lack of disturbance, on the other hand, explains the pattern - rapid migration historically (these stands got there somehow, and they are separated from the main range by 10-20 km) and then suddenly nothing.  That punctuated pattern could be associated with rare, major historical wind disturbance.  These stands are not in particularly storm exposed landscapes, however.  It could also be snow disturbance, as the stands seem to have originated during colder periods.  This could be associated with lower herbivory in the winter too, however - so the work continues!  

Each individual seedling is mapped, understory community marked, and in many cases soil chemistry samples taken.  This allows for precise spatial organization of data, opening up a whole toolbox of spatial statistics for testing community ecology and biological hypotheses.

Each individual seedling is mapped, understory community marked, and in many cases soil chemistry samples taken.  This allows for precise spatial organization of data, opening up a whole toolbox of spatial statistics for testing community ecology and biological hypotheses.