The Extinction of Critchfield’s Spruce (Picea critchfieldii)

Photo of a white spruce (Picea glauca) from google images.  The extinct Critchfield’s spruce was similar enough to this species that scientists originally misidentified it as such.  The size of the cones, cone scales, and needles differ to a statistically significant degree between the two.

Critchfield’s spruce dominated lower Mississippi River valley forests during the Last Glacial Maximum (~28,000 BP- ~15,000 BP) and commonly occurred throughout the southeast elsewhere.   That such a once abundant tree became completely extinct is surprising and a little unsettling.  It means that common and beneficial plant species that we take for granted today could abruptly become extinct in the future.  Since the 19th century Americans have already experienced the catastrophic loss of the American chestnut and are witnessing the perhaps irreversible decline of the hemlock–both due to human-introduced diseases and insects.  These species have existed for millions of years.  Our environment is becoming more and more impoverished.  However, man is probably not the culprit for the extinction of Critchfield’s spruce.

Fossils of Critchfield’s spruce have been excavated from the Tunica Hills region in Louisiana (as discussed last week) and Nonconnah, Tennessee.  In both of these locations it was found in association with mixed hardwoods such as oak, maple, walnut, etc.  In Georgia Critchfield’s spruce fossils were found at Bob Black Pond in Bartow County (northwestern part of the state) and Andersonville Clay Pit (southwestern part of the state).  At the former site Critchfield’s spruce fossils were found in association with fossils of northern species of trees–pitch pine, jack pine, red pine, white pine, and white spruce.  Evidentally, in the northern part of its range, Critchfield’s spruce overlapped with boreal species.  To the east and west of these fossil localities remains of Critchfield’s spruce are unknown, but its presence can be inferred from the pollen record.  Spruce pollen is present but not dominant in pollen studies from central and southeast Georgia, and South Carolina during the LGM.  And there is a 12,000 BP pollen record in north Florida of an unusual forest consisting of beech, hickory, and spruce. The spruce pollen undoubtedly originated from the  Critchfield’s spruce species.   Scientists assume that unlike its relatives it was a temperate species.  All the fossil wood from this species dates from 25,500 BP-16,000 BP, but spruce pollen continued to appear in southeastern pollen records until ~9,000 BP, indicating it survived until then.

This is a page from the below referenced paper that diagnosed Critchfield’s spruce as a distinct species. 

Southeastern pollen records show an inverse relationship between spruce and oak.  During interglacials and interstadials oak pollen increased while spruce pollen decreased.  During stadials pine and spruce pollen increased while oak pollen decreased.  Stadials were colder, drier, and windier, and had lower levels of CO2 in the atmosphere.  Conifers are better able to grow in these conditions.  Their springy limbs bend and sway in windstorms and icy weather; limbs from hardwoods more readily break.  Plants need CO2 for respiration.  The needles on conifers are more efficient at respiration than broad leaves, and they also lose less water during drought.  In warm wet climates with higher CO2 levels broadleafed trees tend to outcompete and shade out conifers.  This explains the cycle.

Data from ODP (Ocean Drilling Project) 1059A (see my May archives for an article about this), the only pollen record in the south from 150,000 BP-50,000BP,  shows that spruce pollen decreased rapidly following the end of the Illinois Ice Age and ranged between 0%-5% during the Sangamonian Interglacial and early Wisconsinian Ice Age.  It didn’t rise significanly again until 70,000 BP when it reached 10%.  Though Critchfield’s spruce must have declined to low numbers during the Sangamonian Interglacial, it didn’t become extinct as it did in the present one which brings to mind the question of why did it become extinct?

The authors of the below referenced study give 3 speculative causes for its extinction.  A pathogen spread as the climate warmed.  Critchfield’s spruce failed to disperse and colonize newly available habitat–something other species of spruce did as the Laurentide Glacier retreated.  And habitat space disappeared when broad-leafed trees outcompeted them.  I would like to add some other speculative causes.  It could have just as easily been an insect infestation as a pathogen in an environment with a longer time for a potential insect pest to reproduce.  Maybe it was a pathogen spread by an insect or a pathogen plus an insect infestation.  During previous interglacials broad-leafed trees very nearly outcompeted Critchfield’s spruce into extinction, but the latter survived in some refuge per chance.  Maybe this time perchance it didn’t find that refuge.  I doubt man played a role in its extinction, but there is a possibility Indian-set fires eliminated these isolated refuges where this tree had previously survived during interglacial periods.

Reference:

Jackson, Stephen; and Chengyu Weng

“”Late Quaternary Extinction of a tree species in eastern North America”

PNAS 96:24 11-23-1999

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This is a vegetation map of the southeast during the LGM that I’ve produced based on my complete study of available pollen records.  Like all vegetation maps, this is a vast oversimplification.  The solid black represents an environment dominated by the extinct Critchfield’s spruce.  This region also included mixed hardwoods, extensive meadows, and some wetlands.  Boreal species grew in the northern part of this region.  Crosshatching represents a pine dominant environment mixed with oaks, other hardwoods, Critchfield’s spruce, grasslands, wetlands, and even patches of desert-like habitat including scrub oak and eolian sand dunes.  The dotted region was likely bur oak and cedar savannah with some Critchfield’s spruce.  To the north below the Laurentide Glacier was a mixture of boreal spruce forests, grassy steppes, and bogs depending on local conditions.  The southern tip of Florida consisted of open pine savannahs and cypress swamps.  It was out of phase with the rest of the continent due to a shift in the Gulf Stream–south Florida had warm wet conditions while the rest of North America had cold arid climate.  Along the coast maritime oak, coastal prairie, and salt marsh prevailed.

Partial list of northern fauna that colonized the southeast during the LGM.

red squirrel

northern flying squirrel

porcupine

fisher

caribou

stag-moose

helmeted musk ox

pine siskin

gray jay

sawhet owl

snowy owl

Tags: Critchfield's spruce