Paleoecology and the Assisted Migration Debate:
Why a Deep-Time Perspective Is Vital
Torreya taxifolia as Exemplar
assembled by Connie Barlow, Torreya Guardian
published 11 February 2010 ♦ last updated January 2016
BACKGROUND and ABSTRACT
In the years since the assisted migration (assisted colonization) debate was launched as a pro and con "Forum" in the Winter 2004/2005 issue of Wild Earth journal, the paleoecological perspective has been almost entirely missing from both the academic papers and the media reports on this vital conservation issue. The original PRO-assistance essay by Connie Barlow and (the late) Paul S. Martin was titled "Bring Torreya taxifolia North Now". The original ANTI-assistance essay by Mark Schwartz was titled "Conservationists Should Not Move Torreya taxifolia".
The intent of this skeletal essay is to demonstrate, by using the example of "poster plant" Torreya taxifolia, why the assisted migration debate must broaden to encompass a deep-time, paleoecological perspective.
Since the beginning of the debate, the major advocates in favor of assisted migration for Torreya taxifolia have argued that paleoecological reasoning is exceedingly strong in favor of assisting the migration of this highly endangered conifer tree northward from the panhandle of Florida to the southern Appalachians. Notably, there is no dispute that the highly restricted endemic range of Torreya taxifolia in Florida (along the east shore of the Apalachicola River) served as one of the primary "pocket refuges" for America's rich Appalachian flora during the peak of the last glacial advance c. 18,000 years ago. It is therefore reasonable to suggest that Torreya taxifolia be looked upon as a "glacial relict" that, for one reason or another, failed to make the return trip north when the present interglacial began some 13,000 years ago. It is also plausible to posit that the reason this particular species failed to make the return journey north is that, in contrast to previous interglacial episodes, newly arrived humans obstructed northward migration by overhunting the conifer's obligate seed dispersal partners (squirrels and possibly now-extinct tortoises) in the narrow riverine corridor that contains soils rich enough to support the maturation of this yewlike tree.
2015 UPDATE: All three federal documents pertaining to endangered species designation and management of Florida torreya contain sections that present a paleoecological understanding of this taxon's relict character as a contributing cause of species decline.
The 1984 Federal Register establishment of Torreya taxifolia as endangered confirms that this species as a Pleistocene relict. The first paragraph begins:"An evergreen tree reaching 18 meters tall, Torreya taxifolia (Florida torreya) was first discovered in 1834 and formally described in 1838. The Florida torreya and other endemics of the Apalachicola River system have received much attention from scientists and local residents. The relictual nature of this area accounts for the presence of many unique species (James, 1967). During recent glaciations, species migrated southward by way of the Apalachicola River system, which served as a refugium during cooling periods. The Apalachicola River is the only Deep River system that has its headwaters in the southern Appalachian Mountains. With the receding of the glaciers, cool moist conditions persisted on the bluffs and ravines of the Apalachicola River after climatic change rendered the surrounding area much drier and warmer." The original (1986) recovery plan stated:"Torreya is a genus of four or five species from Florida and Georgia, California, China, and Japan. The present geographic distribution of the genus is similar to the distributions of several other plant genera. The distributions, together with fossil evidence, suggest that these genera had wide distributions during the Tertiary Period that were subsequently reduce by climatic changes during the Quaternary (James 1961, Delcourt and Delcourt 1975). . . It is possible that relatively minor human alterations of the habitat may seriously affect torreya; it is possible that the present-day physical environment of the Apalachicola bluffs and ravines is only marginally suitable to Florida torreya. The species may be restricted to the area because it failed to migrate northward at the end of the Pleistocene." The recovery plan for Torreya taxifolia was reviewed and updated in 2010. Two paragraphs confirmed Torreya's relictual status:"Fossil records of Torreya are limited to seeds, leaves, and secondary wood of the Upper Cretaceous (Boeshore and Gray 1936, Chaney 1950). The records indicated that the distribution of the genus in past geological times was much wider than the present distribution. A fossil named T. Antigua, which has some characteristics in common with T. taxifolia and T. californica, was described from the Mid-Cretaceous of North Carolina and was also collected near MacBride's Ford, Georgia (Boeshore and Gray 1936).
"Currently, Florida torreya grows naturally in three counties in Florida: Gadsden, Liberty, and Jackson. It is also found in southern Decatur County, GA, just north of Chattahoochee, FL. Based on fossil records, we can speculate that the geographical range of T. taxifolia included North Carolina and perhaps, it was forced south by glaciers, and when they retreated, it became isolated in small areas of the southeastern United States (p.12).
1. WHY FOSSIL EVIDENCE IS SPOTTY FOR GENUS TORREYA
Torreya is a very ancient conifer genus, represented today by a half-dozen species and a globally disjunct range characteristic of many plants affected by the Oligocene cooling and consequent break-up of what had been a globally connected Arcto-Tertiary Geoflora. Today the genus is represented by one species in Florida, one in California, and four or five in Japan, Korea, and eastern China. Notice in the map below that, while Europe lost its Torreya species during the Pleistocene, it has a rich fossil record of this genus extending back to the middle Jurassic. The only fossil evidence of genus Torreya in eastern North America comes from the Cretaceous in what is now North Carolina and Georgia. Genus Torreya is represented in the fossil record of western North America in many geological time blocks, thanks to active tectonics (including volcanic ash deposits) during the Cenozoic. (See map below.)
FOSSIL AND MODERN DISTRIBUTIONS OF GENUS TORREYA. From "Phylogenetic Relationships of Torreya (Taxaceae) Inferred from Sequences of Nuclear Ribosomal DNA", 2001, by Jianhua Li et al."Our estimated time of divergence between Old World and New World Torreya species is ca. 30 million years ago (the early Oligocene). On the basis of the fossil record, Torreya does not appear in Asia until the Oligocene; migration from Europe to Asia might have been facilitated by the land connection created by the disappearance of the Turgai Sea. In Europe, Torreya populations are known to have existed up to the end of Pleistocene. Climatic cooling and glaciation in the Quaternary eliminated many plant groups from Europe, apparently including Torreya. The modern species are apparently young in age, and it will be necessary to integrate older fossils to ascertain the deeper biogeographic history of this genus."
1A. MACROFOSSILS of plants are superbly preserved very often in the ashfall of nearby volcanic eruptions hence the John Day fossils of Torreya in Oregon. In contrast, Cenozoic leaf and cone fossils of any species are not well represented in the tectonically placid Appalachian region. (In contrast, see below excellent detail on a Torreya fossil from the McAbee Flora, Eocene, of British Columbia; pdf of paper.)
1B. MICROFOSSILS of genus Torreya are simply unavailable because, as my scientific mentor and coauthor, the late Pleistocene paleoecologist Paul S. Martin, explained to me (and as we wrote in our 2005 torreya advocacy paper), Torreya pollen is indistinguishable from the pollen of yews (Taxus), bald cypress (Taxodium), and cypress (Cupressus). Hence it would be inappropriate to even ask for microfossil evidence of Torreya in the Appalachians.
2. ABSENCE OF EVIDENCE IS NOT EVIDENCE OF ABSENCE
Thus, to insist that one must find direct evidence of genus Torreya inhabiting the southern Appalachians during the Pleistocene (indeed, during any time of the Cenozoic) in order for Torreya Guardians or others to responsibly posit that the southern Appalachians were previously part of the species' native range is not tenable. "Absence of evidence" should not be used as "evidence of absence" in this regard.
3. CURRENT RANGE AS RELICT OF A PEAK-GLACIAL "POCKET REFUGE"
Palynologist Hazel Delcourt, in her 2002 book, Forests in Peril: Tracking Deciduous Trees from Ice-Age Refuges into the Greenhouse World, has well established that the Apalachicola region of the Florida panhandle is one of a handful of primary "pocket refuges" along the Gulf Coast and the southern Atlantic to which the rich flora of the central and southern Appalachians retreated during the peaks of glacial episodes. Indeed, it can be reasonably inferred that had this continent lacked such large riverine environments with banks of rich soil (contrasting to the predominant sandy soils of the region), North America would have lost to extinction genus Liriodendron (tuliptree) as did Europe. On the one visit I made to Hazel Delcourt before she retired, she told me that she has seen European botanists begin to cry when they visit a magnificent old Liriodendron in a forest here because they know that, for want of adequate glacial refuges north of the Mediterranean and Carpathians, Europe lost this genus during the Pleistocene.
The point here is: since we can all agree that the Apalachicola did indeed serve as a peak glacial refuge, we should at least entertain the possibility that any species that is strangely endemic there might have as the primary reason for its endemism the fact that it is a glacial relict. Indeed, would not the best explanation for Florida yew be that it is the remnant of the Taxus population "left behind" when the genus began reinhabiting the Appalachians as Canadian yew? Ditto with the endangered endemic species of genus Croomia there. Surely, no one would look at the remnant American beech population that is still holding on along the Apalachicola River and declare that it is best adapted for exactly the climatic conditions now found there. Indeed, Hazel Delcourt has advocated that this southernmost population of American beech may well contain warm-adapted alleles that will prove vital for ensuring continued representation of this species in the Appalachians and points north a century hence (pers. comm.). A superbly detailed photo-essay of the largest trees in the lowlands of Torreya State Park is accessible here. Although the author never mentions Torreya, nor seems aware of the park's status as a peak glacial refuge, he offers helpful details on the sizes of tree species now common in the S. Appalachians: Tuliptree (Liriodendron), American Beech, Sweetgum.
4. ARGUMENTS FOR PREFERRED RANGE TO THE NORTH
Unless it can be established that a particular introduced disease agent is the sole cause of the inability of almost all wild Torreya individuals to reproduce in the range it occupied in 1491 (which is the implicit standard for ecological restoration activities in the USA), then we need to consider the strong possibility that inability to thrive is a sign that the current stage of interglacial climate hit a tipping point for the species back in the 1950s and 1960s (when this once-common Apalachicola species suddenly became uncommon and unreproductive). In contrast, there are semi-wild (planted, but now inhabiting regrowth forest) mature T. taxifolia in North Carolina, and they are still tall and producing seed. Plus our own 2008 plantings of seedlings in North Carolina are doing very well except where rodents ate the cambium off several during the first winter. I understand a study is ongoing to look for pathogens infecting the soils near various of the North Carolina specimens (pers. comm., Vivian Negron-Ortiz). If evidence of Phytophthora water mold is found in the soil there, but if the plants are nontheless able to withstand this pathogen, then such should be considered strong evidence that the southern Appalachians at rather high elevations are more amenable habitat than northern Florida for Torreya taxifolia in the current climatic regime.
5. ARGUMENTS AGAINST FLORIDA AS ETERNALLY NATIVE RANGE
I do not know exactly when the panhandle of Florida was underwater during the full life-span of genus Torreya, but if today's Torreya taxifolia is a descendent of the Cretaceous species of Torreya fossilized in North Carolina, then there are tens of millions of years during which it had to be living somewhere other than Florida. Certainly, during the past 70 million years, there were times when the panhandle of Florida was fully marine or beach-dune habitat. One must speculate where native range was during those times; the question cannot be avoided.
6. THE POSSIBILITY THAT TORREYA IS A MOUNTAIN-ADAPTED GENUS
For whatever reason (perhaps because its large seed means that it disperses only very slowly) the other four or five species of Torreya all exist in the wild today only in mountainous regions of Asia and California. T. taxifolia is the outlier species, in that it lives nowhere near any mountains. But could that be because it was forced to retreat in glacial times too far south of its nonglacial mountain habitat and simply failed to make the return migration? After all, mountain species can react to climate change much more quickly by curving around a mountain between northern and southern exposures, and by moving altitudinally higher or lower. Perhaps T. taxifolia did successfully migrate back and forth between the Apalachicola and the Appalachians throughout previous climate cycles in the Pleistocene but human presence (overhunting of seed dispersers) during this current interglacial may have blocked its northward movement for the very first time. Or perhaps it easily ventured to Florida by catching a ride in the southward-flowing Apalachicola River and its upstream tributary, the Chattahooche River (which drains a portion of the southern Appalachian Mountains), but then had inadequate animal dispersal assistance in moving back north.
7. A PALEOECOLOGICAL PERSPECTIVE SHIFTS THE BURDEN OF PROOF
I regard all the forgoing arguments not only as scientifically reasonable, but as scientifically necessary. We cannot simply look to where plants were in the year 1491 as the sole, nor even the best, standard for where native range was in millennia past and where preferred and perhaps required range will likely be in the not-too-distant future. In my view, one cannot serve the interests of this endangered species in a time of rapid climate change without taking a deep-time (that is, paleoecological) perspective. Until a significant minority of botanists and ecologists make the shift to a deep-time perspective, however, those of us advocating assisted migration northward will continue to face an uphill battle. But, if and when the professionals do make the paradigm shift, the burden of proof will reverse for all three species of endangered plants "endemic" to the Apalachicola region. From then on, advocates for the status quo will be the ones with an uphill battle to fight.
2015 ADDENDUM by Connie Barlow:
Supplemental hypothesis on why Florida Torreya was "left behind" in its peak glacial refuge of Florida's Apalachicola River: Might Torreya taxifolia have become stranded in its peak glacial refuge, not so much because of the slow seed-dispersal capacities of squirrels (as hypothesized by Barlow here and here) but because of the absence of northward flowing rivers between Florida and the southern Appalachians? While visiting the sole remaining T. taxifolia in Columbus GA, Connie was struck by its location along a free-flowing section of the Chattahoochee River. The Chattahoochee is the main conduit between the peak-glacial plant refuge in n. Florida and the Appalachian Mountains. Might Torreya taxifolia have been "left-behind" in its Florida refuge because the Chattahoochee River flows southward? The tree could have dropped seeds into the river for a speedy journey south, but it would have been utterly dependent on the slower actions of squirrels for the the return trip north. Click on VIDEO FIELD REPORT here: "Ancient Florida Torreya in Columbus Georgia".
2017 ADDENDUM by Connie Barlow:
"Conservation Paleobiology" is now an accepted facet of conservation biology efforts in this century of rapid climate change. In recent years, critiques of "assisted migration" and the actions of Torreya Guardians have virtually disappeared, while a new term enters the conservation lexicon. See "Conservation Paleobiology: Leveraging Knowledge of the Past to Inform Conservation and Restoration", by Gregory P. Dietl et al., 2015, Annual Review of Earth and Planetary Sciences and a USA National Park Service report: Making the Transition to the Third Era of Natural Resources Management by Nathan L. Stephenson, 1916, The National Park Service Centennial Essay Series.
8. PUTTING TO REST THE "INVASIVE PLANT" BOGEYMAN FOR INTRA-CONTINENTAL N-S ASSISTED MIGRATION
It is widely recognized that Torreya taxifolia is the "poster plant" for assisted migration advocates in the USA because (a) it no longer reproduces in its "native" range, and (b) no one who knows this plant could responsibly argue that this species poses an ecological threat if moved northward. Granted, there is concern about the spread of disease from nursery stock currently being used for this effort but so long as the Torreya Guardians are not introducing this plant into an area where the genus already lives, and especially if the pathogen Phytophthora is already present in the region but simply unproblematic there, then is it professionally responsible to even hint that "invasion" might be a reason to continue to delay initiating an official pilot project to assess the possible benefits of assisted migration for this particular endangered species?
Fellow Torreya Guardian (and caretaker of a dozen T. taxifolia seedlings on his NC property, elevation 3,800 feet) Russell Regnery emphasizes, until regulators start going after homeowners and nurseries that willy nilly plant just about anything anywhere, why single out the Torreya Guardian efforts in ways that incite the uninformed to call us eco-terrorists?
NOTE: A key paper published in 2007 concludes that north-south translocations of land-plant species within a continent pose very little risk of invasion. This paper should be required reading for all participants in the assisted migration debate:
"An Assessment of Invasion Risk from Assisted Migration" by Jillian M. Mueller and Jessica J. Hellmann, Conservation Biology, 28 June 2007.
9. AN "INVASIVE PATHOGEN" ARGUMENT IN FAVOR OF ASSISTED MIGRATION OF TORREYA TAXIFOLIA INTO THE APPALACHIAN MOUNTAINS.
Torreya Guardian Russell Regnery also participates in the American Chestnut Foundation effort to cross-breed American with Asian Chestnut in order to eventually produce a species with only the minimal Asian genes necessary to withstand the exotic chestnut blight disease agent here in America. In a February 2011 email, he writes (pers. comm.):"It is pretty well accepted by participants in the American Chestnut recovery project that the primary reason that the American chestnut is thought of as a tree of the mountains is that Phytopthora doesn't take freezing well. The American Chestnut Foundation chapter checked my soil for presence/absence of Phytopthora before they let me plant my chestnut orchard. In fact, there are those who hypothesize that before the current forms of Phytopthora were introduced into North America (very early in colonial times) that the range of American Chestnut may have existed well into the Piedmont. Early introduction of Phytophthora may have played a part, too, in the demise of the wild population of Franklinia [along the Altamaha River]. So helping Torreya escape a freeze-intolerant pathogen (which I'm sure is well ensconced along the Apalachicola), is one more reason to consider assisted migration to save the species. No one, after all, can possibly control Phytopthora on an ecological scale [where climatic conditions enable it to thrive].
10. AN INVITATION TO BEGIN ASSESSMENT OF THE EXISTING SPECIMENS OF T. TAXIFOLIA IN NORTH CAROLINA AS THE FIRST PHASE OF A PILOT PROJECT TESTING THE MERITS OF ASSISTED MIGRATION."Foster a working partnership between the Torreya Guardians, the Service, and other interested parties to help direct their managed relocation efforts." (p. 18)
Although the revised plan does not specifically call for a pilot project, it doesn't prohibit team members from initiating one. We at Torreya Guardians have done the work to pinpoint the existing Torreyas found in North Carolina and are happy to share that information with those working in accordance with the ESA. In addition to visiting the existing mature specimens that private individuals planted decades ago, we keep excellent records of the nearly 50 seedlings planted on a total of three mountain properties, beginning in 2008. Meanwhile, Atlanta Botanical Garden has an abundance of healthy little seedlings with nowhere, really, to go. (See for example, a photo-essay of a site visit Torreya Guardians made to Atlanta Botanical Garden to learn best how to propagate and nurture this species.)
So let us all combine our efforts to officially begin a multi-location and multi-phase pilot project for testing assisted migration of T. taxifolia. And let us do so in ways that demonstrate the positive benefits (both in data acquisition and economic efficiency) of having paid professionals include (and help direct), wherever possible, the volunteer efforts of citizen naturalists.
11. "ASSISTED MIGRATION" NOT "ASSISTED COLONIZATION"
Two supplementary webpages speak to the (a) history of the terminology discussion (which began with "assisted migration" and then moved to "assisted colonization"), and then (b) a 2011 advocacy piece by Torreya Guardian Connie Barlow on why "assisted migration" should be restored as the term of use. Barlow writes:We Torreya Guardians are branded as vigilantes, but helping Florida Torreya find its way northward into the mountains of North Carolina is pretty much like deciding where to go birdwatching to see Arctic Terns in January. Every amateur birder knows that the place to find Arctic Terns when it is cold and dark in the Arctic is to take a cruise to the Antarctic waters of the Southern Hemisphere. You won't find a single "Arctic" Tern in the Arctic during the Northern Hemisphere winter. They've all gone fishing in the south polar seas. Similarly, Florida Torreya is no more native to Florida in today's climate regime than an Arctic Tern is native to the Arctic in January.
"My personal and professional odyssey as a historian of deciduous trees
has brought me to the realization that the future of the eastern deciduous forest is now at risk.
We can provide corridors to allow for species to migrate successfully
in the face of climate change. We may also need to be prepared to transplant
endangered species to new locations where climate will be favorable."
Forests in Peril (2002, pp 97, 207)
This webpage is cited in:
Williams and Dumroese, 2013, "Growing Assisted Migration: Synthesis of a Climate Change Adaptation Strategy": "The only known assisted migration program in the U.S. is a grassroots effort to save Torreya taxifolia (Florida torreya), a southeastern evergreen conifer, from extinction (McLachlan and others 2007; Barlow 2011). USDA Forest Service Proceedings, RMRS-P-69.
Return to HOME PAGE.
Annotated List of Papers/Reports Online re Assisted Migration