Paleoecology and the Assisted Migration Debate:
Why a Deep-Time Perspective Is Vital
Torreya taxifolia as Exemplar
assembled by Connie Barlow, founder of Torreya Guardians
published 11 February 2010 ♦ last updated January 2019
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.
2018 UPDATE: Two papers published in botanical journals in 1905 concluded that Torreya taxifolia is a glacial relict. See Coulter and Land 1905 and Cowles 1905.COULTER & LAND 1905: "... "The conclusion is irresistible that Torreya is a northern plant of the most pronounced mesophytic tendencies, and to be associated with such forms as the beech-maple-hemlock forms of our northern woods, our most mesophytic type of association."
COWLES 1905: "... It seems likely, then that we should regard Torreya taxifolia as a northern mesophytic left stranded to-day only in Florida. It presumably is one of the plants that failed to follow up the last retreat of the Pleistocene ice, and is preserved here perhaps because of exceptionally favorable topographic conditions."
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's geography lacked long-distance, southward-flowing rivers 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 southward-flowing rivers and glacial refuges north of the Mediterranean Sea and Carpathian Mountain obstacles, 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. This line of reasoning would also apply to the endemic Florida yew, Taxus floridana. In contrast, 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).
On the plus side, long-ago horticulture plantings in northward states are now tall and some are reproductive. None bear signs of the degree of disease pathology that consistently knocks back stem growth in Florida. Torreya Guardians has documented these mature groves and their degree of "naturalization" (ability to seed and for those seeds to voluntarily sprout and establish in the surrounds with no human assistance) at this 2018 webpage: "Historic Groves of Torreya Trees: Long-term Experiments in Assisted Migration". Indeed, the current "national champion" Torreya taxifolia is the specimen in a cemetery near Cincinnati, OH.
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. 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. For a visual tour and narration of the extraordinary steep-slope abilities of Torreya californica in small, rare, disjunct populations on the slopes of California's Coast Range and the western slope of the Sierra Mountains, watch this 2018 video: "Florida Torreya's California Cousin Has Clues for Ex Situ Plantings".
7. POSSIBLE CAUSES OF WHY TORREYA NEVER RETURNED NORTH
7A. Perhaps T. taxifolia did successfully migrate back and forth between the Apalachicola and the Appalachians throughout previous climate cycles in the Pleistocene but the arrival of humans in the Holocene interglacial period became an obstacle. Paul S. Martin in 2004 posited that fire-setting by humans could have disrupted forest continuity along the river corridor. Connie Barlow in 2004 suggested that over-hunting of squirrels (the current dispersal agent) by first peoples settled along the Apalachicola River could have contributed, or perhaps over-hunting of a large tortoise species that might also have distributed seeds. The wikipedia entry on Torreya taxifolia cites Connie Barlow's "evolutionary anachronisms" paper as positing an extinct tortoise or other reptilian disperser. Note: Mark Gelbart offers sound arguments for mastodons to have been the primary seed disperser of Torreya, in a 2014 blogpost: "Mastodons, not Giant Tortoises, were the Probable Dispersal Agent of Torreya Seeds".
7B. An alternative hypothesis is that the large-seed of Torreya (which is sometimes capable of floating for several days) easily caught a fast and obstacle-free river ride southward from the Appalachian Mountains by way of the Chattahoochee River at the onset of cooling during the Pliocene or Pleistocene. But there was no river-flow way to return north during any of the interglacial warmings. In the first half of the 20th century, the add-on of human-caused warming finally surmounted the physiological threshold of genus Torreya, and it was no longer able to ward off more than a half-dozen native diseases. The fact that the Florida Yew, Taxus floridana is also a subcanopy rare endemic limited to the same relictual range as Florida Torreya suggests that river-flow assistance may truly be limiting. After all, the seed of Florida Yew is bird-dispersed; dispersal by birds offers faster and greater long-distance distribution than can squirrels or tortoises.
8. 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. Surely, 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.
2017 ADDENDUM: "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.
2019 ADDENDUM: A 2019 paper by (University of Michigan) paleontologist Philip D. Gingerich puts the scale and rapidity of human-caused global warming into deep-time context that is, comparing the combination of magnitude and speed happening in current centuries against the greatest warming that occurred during the entire 65 million-year Cenozoic Era. Prior to our anthropogenic warming, the most challenging warming episode was 56 million years ago. This was the "PETM" (Paleoecene-Eocene thermal maximum). Gingerich concludes that the evidence suggests that our species is now causing an even more rapid warming: "If the present trend of anthropogenic emissions continues, we can expect to reach a PETM-scale accumulation of atmospheric carbon in as few as 140 to 259 years (about 5 to 10 human generations)." In: "Temporal Scaling of Carbon Emission and Accumulation Rates: Modern Anthropogenic Emissions Compared to Estimates of PETM-Onset Accumulation"
Barlow interprets: Surely, it is time to adapt the 20th-century "precautionary principle" in conservation ethics to handle our 21st century climate emergency conditions. For Torreya taxifolia that would entail far more than assisted migration pilot projects into the southern Appalachians. Field experiments also need to determine just how far north this species can tolerate now. Our own experiments in New Hampshire and Michigan, and especially seed-production in Cleveland, Ohio, offer early evidence that future-oriented conservation for this species would include these northward realms.
9. 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. More generally for all native species that will be needing help moving north, 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.
In 2018 Barlow launched a new webpage that aggregated the data and photos and summarized documentation of the northward long-ago horticultural plantings of Florida Torreya none of which show any signs of "invading" their surrounds. That webpage is "Historic Groves of Torreya Trees: Long-term Experiments in Assisted Migration".
Finally, 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?
10. USING ASSISTED MIGRATION TO "FREEZE OUT" THE PATHOGENS HARMING TORREYA TAXIFOLIA IN ITS FLORIDA RANGE.
10A. FREEZING OUT PHYTHOPHTHORA. 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].If evidence of Phytophthora water mold is found in the soil there, but if the plants are nonetheless 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.
10B. FREEZING OUT FUSARIUM. Beginning in 2010, Prof. Jason Smith of the forest pathology program at the University of Florida, set about identifying the genetics of the Fusarium species that causes stem cankers on the Torreya taxifolia plants in Florida. It is still an open question as to whether this newly named Fusarium torreyae is a recently imported exotic from Asia. The 2016 multi-author paper states: "... it remains to be determined whether F. torreyae is native to North America and restricted to T. taxifolia."
Whether the Fusarium is native or exotic becomes a trivial question IF that same Fusarium species is present in any of the Historic Groves of Florida Torreya growing in North Carolina (especially the Biltmore grove and the Harbison House grove) but the cankers it causes that far north do not severely harm or kill the stems. All 5 of the papers co-authored by Jason Smith are linked and excerpted here. Note: Because the outcome of the invitation-only "Torreya Symposium" (organized in part by Jason Smith) yielded reports that indicated Fusarium torreyae as the certain cause of native-range stem dieback, and because the solution proffered was genetic engineering to insert fusarium-resistance into the Torreya genome, it is important for decision-makers (and reporters) to actually read the papers.
11. 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 nearly a century ago, we keep excellent records of our own plantings that began in earnest 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 and volunteer citizen naturalists combine their energy and assets.
12. "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.
EDITOR'S NOTE: This 1905 publication contains the first suggestion that Torreya's preferred habitat lies northward of its endemic Florida range. Access online the entire report.
Note that in this 1905 report, the author posits that"It is associated with a remarkable and somewhat extensive group of northern mesophytic plants, and the conclusion is irresistible that Torreya is a northern plant of the most pronounced mesophytic tendencies, and to be associated with such forms as the beech-maple-hemlock forms of our northern woods, our most mesophytic type of association."
"A Remarkable Colony of Northern Plants Along the Apalachicola River, Florida, and Its Significance"KEY SECTION:
by H. C. Cowles, 1905
Report of the Eighth International Geographic Congress
Held in the United States
In this association one finds two of our most notable endemic plants Torreya and Croomia. It seems likely, then that we should regard Torreya taxifolia as a northern mesophytic left stranded to-day only in Florida.
It presumably is one of the plants that failed to follow up the last retreat of the Pleistocene ice, and is preserved here perhaps because of exceptionally favorable topographic conditions.
"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)
"There is a rabbinical teaching that says, if the world is ending and the Messiah arrives,
first plant a tree, and then see if the story is true. "
"You Are Brilliant, and the Earth is Hiring" (2009)
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.
• The new field of "conservation paleobiology" emerged in 2015, lending credence to the argument presented on this webpage. A lengthy review article was published in Science journal 10 February 2017, which can be accessed in pdf: "Merging paleobiology with conservation biology to guide the future of terrestrial ecosystems".
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Could Torreya Take the Place of Eastern Hemlock?
Annotated List of Papers/Reports Online re Assisted Migration