Cornell Urban Hort Institute: Interesting Research Continues Apace

The Council’s longtime Board member and beloved speaker, Nina Bassuk, gives us an update on three areas of research underway at UHI. Dr. Bassuk will be the plenary speaker for the NYSUFC Conference this July. (Register now for best rate!) 

UHI researchers are attempting to fast-track propagation of alkaline-tolerant oaks that should come onto the market in the next five to ten years.
UHI researchers are attempting to fast-track propagation of alkaline-tolerant oaks that should come onto the market in the next five to ten years. Photos Courtesy UHI.

New Oaks for Tough Sites 
Dr. Nina Bassuk founded and directs Cornell’s Urban Horticulture Institute (UHI) and conducts applied research in the areas of plant improvement, transplanting technologies, and soil remediation.  “We think of everything we do in terms of potential practical value to the field,” she says.

For example, owing to hybridizing work UHI has been doing since the early 1990s, some oak introductions will be coming onto the market in the next five to ten years that could be game-changers: a whole series of oaks—not just English and bur oak—that can tolerate a pH of 8.0! That means oaks with foliage that stays green in the alkaline soil conditions prevalent in urban settings (and we are increasingly recognizing that in terms of plant stress, “urban” conditions are everywhere—not just in cities.)

Beginning in the early 1990s, UHI started on the hybrid oak quest by first learning, through experimentation, how best to asexually propagate oaks. Asexual propagation (versus sexual propagation by acorns) was necessary in order to ensure the specific characteristic of high pH tolerance. (Sexual propagation by acorns leads to too much species variability.)

Then, UHI requested pollen from colleagues around the world from oaks in the white oak group in order to make crosses. (The white oak group includes white oaks, bur oaks, swamp white oaks, chinkapin oaks, and many others). “We ended up with 300 unique genotypes from the acorns that resulted from these crosses,” Bassuk says. “Of those, 25 of them proved that they can stay green at a very high pH. The other ones don’t like high pH but are good for exceptional form and other ornamental characteristics.”

UHI found that 25 of their 300 oak hybrids exhibit high soil pH tolerance.
UHI found that 25 of their 300 oak hybrids exhibit high soil pH tolerance.

Now the task of the researchers is to fast-track propagation of those 25 most promising genotypes. “Right now we only get five or six new plants a year from each stock plant, and that represents a bottleneck,” Bassuk says. She and her research team are exploring how to get more rooted shoots per plant in at least two ways: They are applying the plant hormone gibberellic acid to stimulate more buds to break and they are using both greenhouse and field production in order to get two flushes of new shoots per season, rather than one.

Once the UHI team clears the propagation hurdles, new oak hybrids will become commercially available.

The Bubbles Factor
A second UHI research project with real industry application seeks to answer the question, “Why are some species so much more difficult to transplant than others?” Arborists know from their own observations that honeylocusts are a piece of cake but that most trees in the oak family are difficult to transplant.

Researchers have long suspected that certain tree species are more affected than others by the water stress inherent to transplanting. But why? Bassuk posited that the reason some trees became more water stressed was because they vary in the extent of the formation of air bubbles in the xylem, the water-conducting tissues of the plant. This phenomenon, known as cavitation, is analogous to a bubble in a drinking straw that makes it harder to suck up the liquid.

UHI acquired a hydraulic conductivity meter that could measure precisely how much water resistance, as a function of cavitation, is present in plant tissues. They designed an experiment to test two species that, even though they are related, are opposite in their transplanting ease—swamp white oak (blissfully easy) and bur oak (notoriously difficult). They tested two-year-old seedlings, on up to 1 1/2″-inch-caliper trees.

The roots of swamp white oak, whose xylem is less gravely affected by cavitation than the xylem of bur oak.
The roots of swamp white oak, whose xylem is less gravely affected by cavitation than the xylem of bur oak.

“Both species take a hit when they are transplanted,” Bassuk says, “but the bubbles in the xylem of swamp white oak are absorbed more quickly.” Interestingly, initial results also show that among the difficult-to-transplant bur oaks, the smaller the tree, the more quickly any bubbles that form in the xylem are absorbed. UHI will further investigate the role that size plays in transplanting difficulty.

In addition to looking at the size variable, UHI will test the impact of fall vs. spring transplanting, and Bassuk’s colleague, Extension Assistant Professor Rick Harper at UMASS-Amherst, is going to look at the effect of transplanting method (B&B vs. bare root vs. container) on cavitation.

“This could have great relevance to urban foresters and arborists in terms of guiding plant selection for projects,” Bassuk says. “If you’re thinking about using 50 tupelo trees at 4-inch caliper, for instance, you can grasp, in terms of the mechanism of cavitation, why this is not viable. But from cavitation studies, you’ll know that smaller tupelo trees, or those transplanted in a specific season, or those grown with a certain transplanting method could make the use of tupelo more successful.”

Bassuk also wants to explore how production variations in the nursery, like root ball size and shape, may affect how fast trees recover from cavitation. Stay tuned!

Compost Precision 
Every year since 2000, Bassuk’s “Creating the Urban Eden” class has taken a piece of the Cornell campus and crafted gardens on it. “These are generally former construction sites and other horrible places that look like the face of the moon,” she says. As one can imagine, the soil on these sites is compacted to a cement-like state and requires serious remediation. Enter the “Scoop and Dump” technique, in which the compacted soil is scooped up by a backhoe to a depth of 15 to 18 inches, followed by compost being dumped in among the loosened soil.

Scoop and Dump in action
Scoop and Dump in action

The amount of compost required to make a significant difference was the research subject of a former student of Bassuk’s, Dr. Angie Rivenshield. She found that adding ⅓- to ½-by-volume organic matter, or in the case of these campus sites, 6 to 9 inches, is necessary to significantly reduce bulk density (i.e., reduce compaction) and to increase microorganism populations. Therefore, the “Urban Eden” students would add 6 to 9 inches of organic matter to the site, leaving veins of compost through the profile of compacted soil.

One of the 13 Cornell campus gardens that have benefitted from the simple “Scoop and Dump” method
One of the 13 Cornell campus gardens that have benefitted from the simple “Scoop and Dump” method

“That’s well and good,” Bassuk says, “but I always wondered, how much compost is getting used up?” The gardens looked great, but what was happening in the soil? Though her students always mulched with a shredded hardwood bark and kept the mulch replenished over the years at 2 to 4 inches deep, no further compost was added after the initial “Scoop and Dump” site preparation. Now that there were 13 generations of gardens put in with the same methods, there were 13 sets of soil properties that could be analyzed.

Bassuk had theorized that the more recently the “Scoop and Dump” site had been prepared, the better the organic matter, bulk density, and microorganism picture would be. But a graduate student, Miles Sax, tested the various gardens around campus that had all been prepared in the same way, and found that the result was just the opposite. “The longer ago we put in the garden,” Bassuk says, “the more organic matter was in the soil and the lower the bulk density.”

Her hypothesis is that the large quantity of compost that was initially placed on the site was enough to get the bacteria and fungi in the soil cooking and multiplying. Then, it appears that those microorganisms were sufficiently numerous to be able to use the bark mulch as food, bringing it down to lower levels in the soil, where byproducts are available as plant nutrition and aid in creating desirable soil aggregates.

“We’re excited about this,” Bassuk says, “because it tells us that the value of that initial soil remediation, that simple Scoop and Dump, is huge—it really pays off.  If that effort is made on the front end to remediate and prepare compacted soils, all that has to be done in subsequent years is to replenish a light layer of bark mulch.”

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