Author: Bert Cregg

Spring has sprung here in Michigan; time to get cracking on lots of projects. One of our new projects is an investigation of pre-plant techniques for dealing with root systems on container grown trees. As many of you know, Linda Chalker-Scott is advocate of bare-rooting trees before planting to correct potential root defects before planting. As some of you may know, I’m skeptical of this approach. It’s not that I think root systems are perfect – far from it. But we lack sufficient information to know whether the costs of bare-rooting (time/effort, stress on the trees) warrant the benefits. We also have little information on how species vary in responding to bare-rooting. From the experience of foresters and bare-root liner nurseries we know that some species are highly sensitive to storage and handling when they’re bare-rooted. When I worked for International Paper, we had little difficulty transplanting sycamores bare-root; whereas we often encountered severe dieback or mortality with sweetgum. Likewise, shade tree nurseries often encounter difficulty establishing oaks, baldcypress, and hackberry from bare-root liners. In fact, J. Frank Schmidt and Sons nursery, one of the largest producers of shade tree liners has discontinued production of bare-root hackberry lines and only produces them as container stock.

As I said, I’m skeptical that putting a tree through the trauma of bare-rooting is worth the potential benefit. But I’m also open-minded and willing to conduct an objective trial to see what’s what. As an aside, I was skeptical about root-shaving before we conducted our own trial and was impressed by the results. For our current study we planted 96 container-grown shade trees last week at the MSU Horticulture Teaching and Research Center. Trees includes three cultivars: ‘Bloodgood’ London planetrees, columnar tulip poplar and ‘October glory’ red maple. We assigned the trees at random to one of four treatments: Control (no root treatment), Shave (outer roots removed before planting) and two bare-root treatments – Bare root – Wash (trees were bare-rooted by washing the roots with water) and Bare root – Airspade (trees were bare-rooted using an airspade). The planting crew consisted of my technician, Dana Ellison; my new Master’s student, Riley Rouse; my undergrad assistants, Becky Pobst and Alex Love; and Linda, who was on-hand to provide quality control on the bare-rooting operations.

For her M.S. project, Riley will be tracking performance of the trees over the next two years. Her measurements will include tree survival and growth as well as measures of physiological responses such as plant water potential and photosynthetic gas exchange. Next fall, after two growing seasons, we will dig a subset of the trees with a tree spade and inspect the root systems to determine the effect of treatments on root system development. In the meantime, here are some photos from last week’s festivities.

Posted by Bert Cregg

Researchers often get accused of concluding the obvious.  At some point we’ve all scoffed at headlines like, “Study finds cell phones and driving don’t mix” or “Researchers discover high heels make your feet hurt.”* But even when a study demonstrates something we already know, sometimes there is still value in being able to put hard numbers on the scope of the problem – and hopefully spur some action.

A case in point is a recent study by Justin Morgenroth, Bernardo Santos, and Brad Cadwallader at the New Zealand School of Forestry, “Conflicts between landscape trees and lawn maintenance equipment – The first look at an urban epidemic” Urban Forestry and Urban Greening 14:1054-1058.  Morgenroth and his colleagues surveyed over 1,000 trees in public greenspaces (parks, cemeteries, campuses) in and around Christchurch, New Zealand (pop. 375,000) to assess the amount of damage to trees by lawn equipment.  Their conclusion: Lawn equipment is hell on trees.  This conclusion, of course, surprises absolutely no one that has ever looked at trees near turf in a public place on this planet.

Morgenroth et al. claim their survey is the first systematic look at this issue and their data are staggering.  Nearly two-thirds (63%) of the trees they surveyed had at least one wound.  The proportion of trees with wounds was fairly constant regardless of type of planting (i.e., park, campus, cemetery, roadside verge), though trees in parks and campuses tended to have more wounds per tree than trees in nature reserves or roadside verges.

Not all the news was bad, however.  Morgenroth et al. found that grass cut-outs or mulching around trees significantly reduced the number of wounds per tree.

So, not a conclusion that should take anyone by surprise, but some sobering data to put some scale on the size of the problem.

*actual conclusions from real studies

Posted by Bert Cregg

A little over six years ago Jeff Gilman called me out of the blue and asked me to be part of a new blog about the science of horticulture that he was embarking on along with Linda Chalker-Scott.  I was reluctant – I didn’t know much about social media at the time and was plenty busy already.  Jeff explained that he was recruiting others and how we would rotate posts. I asked Jeff, “What if I don’t have anything to write about?”   Jeff replied, “Just post some pretty pictures or something.”  So with that long ago conversation as backdrop, here are a few photos from this week’s trip to Hidden Lake Gardens near Tipton, Michigan.

Taxodium distichum “Pendens’ cones

Sassafras albidum fall color

Lots of discussion recently over on the Facebook side regarding the recent publication in Ecological Letters by Karin Burghardt and Douglas Tallamy, “Not all non-natives are equally unequal: reductions in herbivore β-diversity depend on phylogenetic similarity to native plant community.” http://onlinelibrary.wiley.com/doi/10.1111/ele.12492/full
While there are certainly some things to nitpick in the paper (see Linda’s comments on the Facebook discussion), I think this paper may go a long ways re-shape, and possibly even begin to end, the debate over native versus exotic.

How was the study conducted. In 2006 Tallamy’s group established a series of test landscape plantings. Each planting fell into one of four groups: non-native congeners (species that are not native but have native relatives in the same genus); non-native non-congeners (plants from non-native genera), native congeners and native non-congeners. In 2008, when the trees were about 6’ (1.8 m) tall, they conducted a census to identify and quantify the adult and immature insect herbivores they collected. They analyzed the data to determine the amount of insect herbivore diversity within each planting type. Specifically they looked at what ecologists refer to as beta-diversity, the amount of species diversity among sites. If you’re interested and want to learn more check out https://methodsblog.wordpress.com/2015/05/27/beta_diversity/

So, what did they find? Like every good study on host-insect interactions; the answer is, “It depends.” (BTW, if you’re following along at home the key figure in the paper is Fig. 3). When Burghardt and Tallamy looked at the differences in diversity between adult herbivores on native and non-native congeners, they found no difference. When they looked at differences in diversity between immature herbivores on native and non-native congeners, they found no difference. When they looked at differences in diversity between adult herbivores on native and non-native non-congeners, they found no difference. When they looked at immature herbivores on native and non-native non-congeners, they found a small but statistically significant difference, with higher total diversity for native non-congener.

As an aside, it is also instructive to look the version of Figure 3 presented in the article’s supplemental materials, which has been re-scaled to include zero. Including zero on the scale helps to give a better perspective on the actual variation among means. It’s a little like the “Truth in lending statement” that comes at the end of your credit card bill.

So, a possible alternative title for the paper could be, “Do native or non-native plants increase herbivore diversity? Most of the time it doesn’t matter.” That said, I think this paper makes a number of contributions and will start to shift native versus non-native debate, and perhaps even signal the beginning of the end. First of all it demonstrates that that non-native species of native genera contribute equally to herbivore diversity. However, I think some of the most insightful information in the paper is buried between the lines and in the supplemental information attached to the online version of the paper. The authors briefly mention that they also looked at guilds (i.e., chewing insects, sap feeding insect, xylem feeders, etc.). Once again the answer of whether natives contribute more to species diversity is, “It depends.” For xylem feeders, for example, diversity was the same for congeners and non-congeners.

To me, this is the level of resolution we need to work to gain a true handle on the situation. I’m not an entomologist and I’ve never played one on TV but I’ve been around these questions long enough to know that different types of insects are attracted to or repelled by different plants by different mechanisms. In one case it’s an attraction pheromone, in another it’s a defense chemical, sometimes it’s leaf toughness or a tree is able to produce enough resin to drown boring insects. An old axiom states that ecosystems are not more complicate than we think; ecosystems are more complicated than we can think. As this paper demonstrates, to think that all the complex interactions between plants and insects can be boiled down to something as simple as native or exotic is hopelessly naïve.

Posted by Bert Cregg

Reading through Linda’s recent article in Arboriculture and Urban Forestry “Nonnative, Noninvasive Woody Species Can Enhance Urban Landscape Biodiversity” (Arb. & UF 41:173-186) reminded me of some thoughts I had while I sat through a talk by Doug Tallamy at the New England GROWS conference back in February.  As most GP blog readers are likely aware, Tallamy is the author of Bringing Nature Home.  The basic premise of the book is that native insects only feed on native plants and therefore the use of exotic landscape plants will cause ecological food webs to collapse and the end of the world as we know it.

Looking at things critically, there was certainly plenty to nitpick in the talk.  For example, to illustrate that insects feed more on native than non-native genera, Tallamy contrasted Quercus, a genus that includes hundreds of species distributed across the northern hemisphere, to Ginkgo, a monospecific genus (heck, a monospecific order) with a limited native range in China.  Tallamy also compared the food value for birds of seeds of five native shrubs with five Asian exotics.  The North American plants all had higher nutrition, which leads to one inescapable conclusion: If you’re a bird-lover in Asia you need to plant more exotics from North America.

But, as I said, those items are nitpicking.  In an apparent effort to appease critics, Tallamy addresses the question of whether it’s ever OK to plant exotics.  He says yes, but think of them as “statues” – nice to look at but devoid of any ecological value – and then put up a slide of a landscape with a bunch of statues photoshopped in.  All analogies break down eventually but the ‘exotics as statues’ model fell apart before the slide hit ever the screen.  First off, native insects, birds, and other wildlife do utilize nonnative plants and I’ll refer you to Linda’s review and her 100+ citations on that point.  But just as importantly, nonnatives can also provide many of the same environmental benefits as native plants.  Statues, on the other hand, don’t cool buildings, sequester carbon, reduce stormwater run-off, and create vertical structure and cover for wildlife – OK, statues provide perches for pigeons, but that’s about it.  Landscape plants, and trees in particular, can provide a myriad of ecological services – but they can only provide these functions if they are alive to do so. Despite the intuitive appeal of, “Native plants are adapted to our local environment”, in many urban, and even suburban, environments this is simply not true due to urban heat island effects, top-soil removal, compaction and other site disturbances.

In his talk, as in his book, Tallamy demonizes the typical suburban landscape showing a photo of an expansive lawn with a lone Bradford pear tree.  OK.  Is the problem here the pear? If we changed the pear to, say, a lone shagbark hickory would anything change dramatically?  No, but this does bring me to a point of agreement with Tallamy.  The fundamental problem we face in our landscapes is lack of diversity; not only species diversity but also structural diversity and age class diversity.  If we can build diversity and generate ecosystem services with natives; great, go for it. But we need to understand that our list of suitable native trees is shrinking (as an aside, ignore Tallamy’s recommendation to plant green or white ash if you live in eastern North America). Vilifying, or even banning, nonnatives will ultimately reduce diversity in urban and community forests and their ability provide ecosystems services, not enhance it.