Return to the Hypsithermal
At the risk of sounding simplistic:
Climate change is the product of plants being metabolized into gasses (via coal, oil, methane from cattle, etc).
Reversing climate change involves converting more gasses to plants, rather than plants to gasses.
We need to act at whatever scale is within our means (backyard, farm, rangeland, forest) to support plant growth and thus carbon sequestration.
Assembling diverse, multi-functional, ecologically appropriate plant communities is one of the most critical practices of our time.
Here's how native grasslands in the Northeast fit in.
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This year's drought had a grim effect on local forest vegetation. Forest understory shrubs were wilting, herbs going dormant, and tree leaves browning out by late summer.
As a nurseryman, I've been acutely aware of severe late summer droughts for the past three years. We count on the rain to do a lot of our watering for us, but we've spent many more hours with the hoses than I'd like.
Meanwhile, as a field botanist I've done large botanical surveys in several areas of the New Jersey Highlands recently. For three years running our sites have become dangerously close to impossible to survey because of the effects of drought on summer and fall flowering and fruiting. Even in typically moist forest areas, I've watched spicebush droop ominously and turn prematurely yellow in the late summer. This year, wild hydrangea (Hydrangea arborescens) and purple-flowering raspberry (Rubus odoratus) grimly shriveled in the normally moist, cool hemlock ravine along the Delaware River near our farm.
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In photosynthesis, carbon from the atmosphere is converted into sugars. Carbon dioxide (CO2) is pulled in through pores in leaves called stomata.
However, these pores in leaves also lose moisture while bringing in CO2.
In drought conditions, plants shut their stomata, reducing moisture loss but also photosynthesis.
The most common mechanism for plant photosynthesis is called C3 photosynthesis. C3 photosynthesis is relatively inefficient, in that oxygen is pulled into the photosynthetic apparatus along with carbon dioxide, causing energy to be wasted on fixing oxygen.
According to a review entitled "Nature’s Green Revolution: the Remarkable Evolutionary Rise of C4 Plants":
"This ancient [C3 mechanism] originated in ‘greenhouse’ conditions when the Earth’s atmosphere contained CO2 at up to 100 times today’s level... and negligible amounts of O2... A CO2-fixing enzyme in this atmosphere had little requirement for high CO2- affinity, and gained no advantage from distinguishing between CO2 and O2 molecules."[1]
A relatively recent workaround to the inefficiency of the C3 mechanism is C4 photosynthesis. Again, according to "Nature's Green Revolution":
"[B]etween 25 and 32 million years ago, a revolutionary innovation evolved in tropical grasses in the form of a solar-powered carbon dioxide pump... which boosts photosynthesis in hot conditions."[2]
Photosynthetic cells are essentially bathed in carbon dioxide at high concentrations.
Therefore, C4 plants can maintain greater productivity while needing less water. In fact, though C4 plants represent only about 5% of the Earth's biomass, they account for 20-30% of CO2 fixation.[3]
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Several dominant grass species of the American tallgrass prairie are C4 species. These are sometimes known as "warm season" grasses because they are at their peak growth during the hottest month of the year. These also grow in the Northeast, and don't commence significant growth until mid-May in our region (Mid-Atlantic). This is in contrast to "cool season" grasses - the Eurasian C3 grasses that include many turfgrass and hay species, and which have peak growth in spring and early summer.
Little bluestem, NJ Pine Barrens, Franklin Parker Preserve
Among the most common native warm season grasses in the east are little bluestem (Schizachryium scoparium), Indian grass (Sorghastrum nutans), switchgrass (Panicum virgatum), and big bluestem (Andropogon gerardii).
Here are brief descriptions of these "prairie" species, pertaining to their appearance and ecology in our region:
Little bluestem (Schizachyrium scoparium) is an attractive, relatively short bunchgrass that I find primarily in dry habitats including granite outcrops, well-drained post-agricultural meadows, and in openings and roadsides in sandy soils near the shore and in the Pine Barrens.
Indian grass (Sorghastrum nutans) is a tall species with showy yellow anthers and golden seeds. It is occasional on dry outcroppings, but I associate it primarily with mesic to moist post-agricultural meadows in the New Jersey Piedmont.
Switchgrass (Panicum virgatum) is a bunchgrass with an open panicle. It is occasionally found in the New Jersey Piedmont in meadows and ROWs, but I think of it primarily as a species of sandy soils, along coastal plain roadsides, openings, and back dunes.
I find big bluestem (Andropogon gerardii) to be fairly uncommon in natural areas in New Jersey. This is the largest grass of the group, up to 9' tall, at least in its core range in the tallgrass prairie. Sometimes its tri-partite "turkey foot" seedheads are beautiful shades of gold and purple. I have observed this species along Delaware River tributaries such as the Wickecheoke and Lockatong -- sometimes spreading to post-agricultural meadows nearby. I've also seen it on a Highlands summit with wild columbine (Aquilegia canadensis) and the rare whorled milkweed (Asclepias verticillata), so it clearly has a breadth to its ecological niche. Our local ecotype is generally significantly shorter than 9' tall.
A few other notable warm season grasses include purple lovegrass (Eragrostis spectabilis), broomsedge (Andropogon virginicus), and sideoats grama (Bouteloua curtipendula).
Recently, our native plant nursery, Wild Ridge Plants used several warm season grasses on a green roof project that we custom grew for the Mercer County Park Commission (NJ), using to advantage the grasses' exceptional drought tolerance, long season of growth, and ornamental characteristics.
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There is a historical reference point for a period of warming and for the extension of grasslands into the east during a period called the Hypsithermal[4].
Beginning sometime around 8,000 years ago, global temperatures rose considerably, and predominant winds in the Northeast may have switched so as to originate in the west. With these dry, hot conditions came a band of prairie vegetation, which may have displaced forest as far east as Ohio, possibly spreading into Pennsylvania and New England and conceivably all the way across to the Atlantic Ocean[5]. This band was called the "Prairie Peninsula" by botanist Henry Gleason, who speculated as to its origin as early as 1922.
Ultimately, the evidence is inconclusive regarding whether prairie plants literally swept east anywhere past Ohio. It is quite possible that many species of open habitats returned to the Northeast after the last glacial maximum from various refugia, just as forest species did. Regardless, there are select bona fide grassland- and prairie-type communities (i.e. persistent grass- and forb-dominated assemblages with less than full canopy cover) in the Northeast at large, which we might use as reference communities in developing ecological restoration projects.
These open habitats include ridgetop glades on shallow soils, ice-scoured river floodplains, sandy back dune habitats, areas of pine barrens with frequent fire return intervals, and coastal grasslands once maintained by Native American burning practices.
As a generalization, non-forest plants form persistent communities in areas of adverse xeric or waterlogged conditions, especially over unusual geologies that may aid in arresting succession--and may have limited agricultural exploitation of the site in colonial times.
A study of remnant GMS (grassland, meadow, and savannah) habitats entitled Keystone Grasslands: Restoration and Reclamation of Native Grasslands, Meadows, and Savannas in Pennsylvania State Parks and State Game Lands provides a comprehensive overview of persistent non-forest habitats in Pennsylvania. The study, by Roger Latham and James Thorne, concludes that:
"[R]emnants have declined from an estimated 230 to 240 square miles around the time of European settlement (0.5% of the state’s total land area) to less than 1 square mile today, a 99.6% decline, which continues and is even accelerating at many sites."
Nevertheless, careful study of remnants on substrates such as diabase, limestone, and serpentinite, as well as coastal plain sands, lead us to the selection of eco-regionally appropriate grassland species palettes for restoration throughout the region.
A significant portion of GMS species are listed as rare species in various states in the region. According to Candeias and Warren (2016), "[a]pproximately 30% of rare plants (those present in 20–35 extant sites) in north-eastern N.A. are remnant prairie peninsula species with fewer than 35 remaining populations."[6]
Likewise, Latham and Thorne estimate that "112 (38%) of the state-endangered species, 35 (41%) of the state-threatened species, and 38 (35%) of the species that have already been extirpated from Pennsylvania are characteristic of grasslands, meadows, and savannas."[7]
In addition, 74% of state-listed butterflies, 38% of listed moths, and 15 bird species tracked by the state of Pennsylvania are obligate grassland species.[8]
It's intriguing to consider that some of these species may have arrived in our region shortly after the retreat of Ice Age glaciers, and have persisted in a variety of niches until the present. Now, many remnant grassland species are also some of the most at-risk species in our fauna and flora.
Purple milkweed in the NJ Sourland Mountains. It is also an oak savannah species in the Midwest.
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In advocating for the extensive creation and restoration[9] of Northeastern grasslands, I am suggesting it as a complement to the forest habitats that form the rightful majority of natural land in the region -- not as a replacement. In my view, grasslands are a much more readily created habitat type via de novo restoration than forest[10].
Where, then, would we restore native grasslands? Roadsides, utility ROWs, brownfields, marginal agricultural land, animal pasture, hayfields, and even rooftops and underused urban land could all be appropriate areas for restoration. Further, millions of acres of residential lawns could be retrofitted as aesthetically beautiful miniature prairies that sequester carbon and support wildlife, expressing a truly American landscape ideal rather than an outdated European conceit perverted into a suburban norm.
Restored native grasslands include warm season grasses as foundational species, but need to include many other types of plants to maximize diversity and resilience. Resiliency, because we don't really know what climate conditions lie ahead. Diversity, to allow for complementary functions, such as pollinator support, rain infiltration, wildlife habitat, and even foraging by people.
These ideas are certainly not novel to me. That said, some of the older grassland projects executed in our area are designed and implemented like cornfields, with a few species of native grass substituted for the annual monocrop. These are not habitat in my opinion, nor are they based on any historic or extant ecosystems. As Neil Diboll of Prairie Nursery memorably said at a conference, "It's like building a house without a kitchen." In this case, he meant that these grasslands are intended as nesting habitat for grassland birds. But without the wildflowers that provide pollen, nectar, and/or host plants for many invertebrates, the birds are left with little to eat -- and insect protein is critical to hatchling birds, even if the adults are granivores or otherwise.
Structural diversity is also critical. This involves species diversity and distribution, and is partially a result of periodic disturbances, as Latham and Thorne explain in Keystone Grasslands:
Grassland birds evolved in native grasslands characterized by high species richness of grasses and perennial forbs and patchiness in such environmental factors as litter depth and amount of bare ground, resulting from grazing, fires, and other disturbance. They have highest preference for, and achieve greatest survival and reproduction in, existing grasslands with comparable structural and species composition.
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The amount of carbon stored in soils (2,500 billion tons) and plant and animal life (560 billion tons) is nearly four times that in the atmosphere (800 billion tons).[11]
Reducing fossil fuel emissions is critical, but is ultimately only part of stabilizing atmospheric carbon. The ultimate goal is the re-placement of carbon into terrestrial pools in soils, flora, and fauna.
Grasslands are ideal tools with which to sequester carbon in soils. Forest ecosystems contain much carbon in aboveground biomass, but grasslands store most of their carbon in the ground. An intact grassland can sequester 2-5 tons of carbon per year on every acre.[12]
Given the significance of grasslands in sequestering soil carbon, a large part of a strategic discussion becomes focused on improving rangeland management with systems like rotational grazing, and the remediation of pastureland soils worldwide.
For our part here in the Northeast, where we lack extensive rangeland, we can use grassland as a way to heal exhausted landscapes of all stripes, urban, suburban, and rural, and contribute to a global effort to pull carbon out of the atmosphere and into soils.
Plants can be likened to carbon straws that pull gasses out of the atmosphere and into the soil, uptake water and minerals, and fuel it all with sunlight[13]. Many speak of a technological solution to climate change. I'd submit that this 100% solar-powered, pollution-free, renewables-based machine, honed over millions of years, is exactly the technological solution we need.
Endnotes
[1] Osborne, C. P., and Beerling, D. J. "Nature's green revolution: the remarkable evolutionary rise of C4 plants". Phil. Trans. R. Soc. B (2006) 361, 173.
[2] Ibid. Emphasis added.
[3] Ibid.
[4] Also called the Altithermal, Holocene Climate Optimum, and other names.
[5] The latter seems unlikely. For a robust (though not necessarily clear of conclusive) discussion, see Hamilton, K.G. A. "Unraveling the Enigma of an Atlantic Prairie". Northeast Natural History Conference 2011: Selected PapersNortheastern Naturalist 19 (Special Issue 6):13–42. 2012
[6] Candeias, M. and Warren, R. J. "Rareness starts early for disturbance-dependent grassland plant species". Biodivers Conserv (2016) 25:2771–2785
[7] Latham, R, and Thorne, J. Keystone Grasslands: Restoration and Reclamation of Native Grasslands, Meadows, and Savannas in Pennsylvania State Parks and State Game Lands. 2007
[8] Ibid.
[9] Restoration and reclamation, if the latter is used specifically to mean creation of habitat and the former to mean "repair" of existing sites
[10] Afforestation projects that I've seen lack the structural and species diversity I think of as defining a forest. Given a century or two, they might get there. By contrast, I've seen some diverse meadows assemble from seed in short periods of time.
[11] Schwartz, J. "Soil as Carbon Storehouse: New Weapon in Climate Fight?" http://e360.yale.edu/feature/soil_as_carbon_storehouse_new_weapon_in_climate_fight/2744/, 04 March 2014 [Retrieved 17 December 2016]
[12]McCulley, R. "Grasslands and Carbon: Processes and Trends", Powerpoint presentation. https://www.fs.usda.gov/ccrc/sites/default/files/carboncourse/transcripts/4.McCulley.pdf[Retrieved 17 December 2016]
[13] Paraphrased from Episode 35 of the In Defense of Plants podcast, "Soils", with guest Dr. Elizabeth Bach.