Native vs Exotic: Not as simple as it seems

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.

tallmay re-scaled

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.

Show me the data!

One of my favorite bumper stickers from days gone by said simply “Stop Continental Drift”. Good luck with that.

stop_continental_drift_530

Today’s topic deals with another type of drift – a phenomenon one of my professors referred at ‘Bibliographic drift’.   This type of drift occurs when authors cite a paper without bothering to look up the original source.  Then a second author cites original source based on the first author’s paper; then a third author cites it based on the second paper and so on and so forth.  This is why grad students learn that second citing is a cardinal sin.

 

It’s an easy trap to fall into even in the age of access to electronic journals.  It can happen in all sorts of ways, especially if the point the author trying to support is something that is intuitively appealing and not likely to be questioned.  For example, I was recently reading through The Practical Science of Planting Trees by Gary Watson and E.B. Himelick.  It’s a good book with lots of great info and photos but under the section on digging the planting hole there is a subsection “wider is better”.  This is something we all ‘know’ but there is no data with any scientific rigor to support it; at least not that I’ve ever been able to find and I’ve looked repeatedly.  So I was intrigued to see Watson and Himelick cite four papers to support the notion that wider is better. Cool. So I went through the bibliography to look up the citations.

Can you dig it? My former research technician Dana Ellison installs a tree in Detroit.
Can you dig it? My former research technician Dana Ellison installs a tree in Detroit.

First up, Arnold and Welsh 1995. Effects of planting hole configuration and soil type on transplant establishment of container-grown live oak. J. of Arboriculture 24:213-218. This paper doesn’t even discus planting hole width, at least not directly.  The authors looked at various planting hole configurations (round, square, star-shaped) but made a point to keep the planting hole volume the same. Zero points for wider is better.

 

Next, Corley 1984. Soil amendments at planting J. Environ. Hort. 2:27-30. One of the experiments in this paper compared root and shoot growth of four shrub species transplanted from #1 containers into holes that were with 1.75x or 3.5x the width of the root-ball. The author measured root and shoot growth after two years and the results were a mixed bag.  They found the wider hole was better about half the time, the other half of the time it didn’t make a difference.  One point for wider is better (sort of).

Next, Montegue et al. 2007.  Influence of irrigation volume and mulch on establishment of select shrub species Arboriculture & Urban Forestry.  33:202–209.  The title of the paper says it all; the authors compared water relations and growth in response to mulch and irrigation but planting hole size wasn’t included as a variable. (Spoiler alert: mulch improved growth and water relations). Zero points.

Last and most interesting, Watson et al. 1992. The effect of backfill soil texture and planting hole shape on root regeneration of transplanted green ash. J. of Arboriculture 18:130-135. In this study the authors looked at new root growth and shoot and diameter growth for three years after transplanting green ash trees into planting holes that were 1.2x, 2x, or 3x the width of the root ball. And they found… nothing. Well, not nothing but they didn’t find any effects of planting hole size on root density, shoot growth or caliper growth. To help visualize the response I’ve summarized their growth data three years after transplanting below. One point for it doesn’t matter.

Cumulative growth response of green ash trees to planting hole width 3 years after transplanting. adapted from Watson wt al. 1992
Cumulative growth response of green ash trees to planting hole width 3 years after transplanting. adapted from Watson wt al. 1992

 

As a final note I include a photo from the Waston et al. paper 1992.  The photo is fuzzy but the caption should be clear.

watson et al 1992

So where does that leave us? Digging a wider hole doesn’t hurt, except maybe your back. And I think that’s part of the appeal of this advice:  If it’s more work it must be better. Dig a hole 2 times, 3 times, 10 times the width of the root-ball if you want. Just don’t say “Research shows wider is better…” because it’s ambiguous at best.

Back on the High Line again

Earlier this week I was in New York City and got to visit the High Line for the first time.  For those who aren’t familiar, the High Line is an urban park that was created along an abandoned elevated rail line on the Westside of Manhattan.  Linda posted about the High Line a couple of years ago.  Her visit was in late winter so my visit provided an opportunity to explore the park during the height of the growing season.

The High Line runs from West 14th St. to West 34th st.
The High Line runs from West 14th St. to West 34th st.

30 feet above the streets of New York City...

30 feet above the streets of New York City…

The High Line offers sweeping views of Manhattan to the east and the Hudson River on the west.  For most of its length the trail consists of various beds of perennials, trees, and shrubs.  Along the newest, northernmost section of the trail, the plantings give way to beds that have been allowed to re-seed naturally; providing an opportunity to observe urban ecological succession.

Perennial beds along the High Line
Perennial beds along the High Line

Quaking aspen

Quaking aspen

The High Line extends through the Chelsea section of Manhattan, which, according to locals, was a less than desirable location just a few years ago.  With the advent of the High Line, however, Chelsea and adjacent Meatpacking district have become some of the trendiest and hottest real estate in the city.  In fact it’s difficult to get a picture along the highline without a crane in the background. What a stark difference from the acreage for sale in Mission, BC that we visited, too bad we are city people.

The High Line near West 30th St.
The High Line near West 30th St.

The High Line has helped transform a run-down section of Manhattan into some of the hottest real estate in New York.

The High Line has helped transform a run-down section of Manhattan into some of the hottest real estate in New York.

Art is an integral part of the High Line with various sculptures and interactive projects along the way.  During my visit, kids of all ages had the opportunity to contribute to a giant Lego sculpture or add to a giant sidewalk painting.

Is is art ? Or just weeds?  This work is part of a 13-piece installation by Adrian Villar Rojas “…known for his large-scale, site specific sculptures that transform their environs into a vision of their own potential future.” It’s titled “The Evolution of God” aka “A Study in Lambsquarters”
Is is art ? Or just weeds? This work is part of a 13-piece installation by Adrian Villar Rojas “…known for his large-scale, site specific sculptures that transform their environs into a vision of their own potential future.” It’s titled “The Evolution of God” aka “A Study in Lambsquarters”

Interactive art. Kids of all ages take time out to add to a Lego construction project along the High Line.

Interactive art. Kids of all ages take time out to add to a Lego construction project along the High Line.

The first section of the High Line opened in 2009 and for the most part it seems to be holding up well.  Some sections of the trail bed are constructed from crushed aggregate and these sections are pretty well pot-holed, presumably from freeze-thaw cycles.  Most plants along the trail seem to be healthy and thriving, likely thanks to drip irrigation.  It will be interesting to see how the trees and shrubs continue to develop and how things perform over the long haul.

Sassafras
Sassafras
This sidewalk fountain provides a change to cool your heels on a warm and sticky New York afternoon.
This sidewalk fountain provides a change to cool your heels on a warm and sticky New York afternoon.

Bottom-line: If you’re in New York and you enjoy plants and watching people (and watching people enjoy plants), a couple hours on the High Line will be time well spent.

Hibiscus
Hibiscus

 

Stuck in the middle with you

Clowns to the left of me,

Jokers to the right, here I am,

Stuck in the middle with you

 

More than once in the past couple months I’ve come close to pulling the plug on FaceBook.  What started as a fun and easy way to keep up with family and friends back home and stalk old girlfriends has devolved into an infinite do-loop of whininess, acrimony and vitriol over GMO’s, organic food, vaccines, and President Obama.  Of course, I brought a lot of this on myself based on the virtual company I choose to keep.  While my allegiances generally align with groups such as GMOLOL, GMOskepti-forum, and the Genetic Literacy Project, more and more I find myself just as appalled by their tactics and rhetoric as I am by those of the anti-GMO side.  I have many thoughtful and intelligent friends that think GMO’s should be labeled or even banned and I’m fairly certain none of them want poor African children to go blind.

greenpeacerice

Quick aside here: Raise your hand if you have ever changed your mind on an important issue based on an internet meme.

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The problem in this type of discourse is that everything has become an all or nothing game.  If you concede even the slightest point to the other side, you have become a wishy-washy capitulator.  But the underlying problem is the answer in science is often “It depends.” In the GP blog and elsewhere, much has been made of the Pew Research poll that showed the biggest opinion gap between the scientists and the public was over GMO safety.  Had I been polled, I would have agreed that GMO’s are generally safe.  But that doesn’t mean I don’t think there are concerns. Heavy agricultural use of glyphosate is leading to the development of Round-up resistant weeds. Likewise, the potential of GMO crops out-crossing with native plants cannot be discounted.  Scientists working on GMO’s, of course, are aware of these issues and are constantly working on steps to manage and minimize these risks, but they could still come back someday to bite us in the butt – the risks, not the scientists.

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Another problem with controversies on the interwebs is that issues get over-simplified.  The endless argument over organic food is just stupid.   Saying organic production is better than conventional farming (or vice versa) is like saying Ford is better than Chevrolet:  You can’t compare without knowing which model: Pick-ups? Sedans? SUV’s?  Or  what’s being compared: Gas mileage? Crash safety? Resale value?  The same is true with organic vs conventional: What crop? Celery? Apples? Carrots? Lettuce? And what measure? Pesticide residue? Carbon footprint? Food safety?  The debate is an interminable game of whack-a-mole. I suspect anyone with access to ‘Web of Science’ and an afternoon to kill could come up with a couple dozen studies to support either side.  My personal view is I’ll pay a modest premium for organic produce if it clearly looks better, or in the case of small fruits that I can sample before I buy, tastes better than the conventional counterpart.

images

The other issue in the social media wars is that we need to admit and accept the limits of our knowledge.  The Pew poll and other surveys indicate that most scientists agree that climate is changing due to human activity.  Again, I’m with the majority of scientists on this.  But only a small portion of those scientists are actually trained in climatology and understand and appreciate the nuances of the Global Circulation Model or the Pacific Decadal Oscillation. What I know with certainty is that CO2 and other greenhouses gasses have risen consistently and dramatically since the Industrial Revolution.  How will that impact climate in 100 years?  I have no clue beyond what the climate scientists tell me.  Do I have any reason to doubt their projections of steady increased temperatures? None beyond the fact the GCM, like all models, is only as reliable as the data and assumptions it is built on.  Does this make me a right-wing climate change denier?  Hardly.  But I am self-aware enough to know what I don’t know.

post-57707-Groundhog-Day-meme-Imgur-Only-gmvA

Bottom-line, even on issues where a vast majority of scientists agree, there are still unanswered questions and areas of uncertainty and always will be.  In the meantime, we still have to make policy decision with the information at hand.  Talking about these issues intelligently and respectfully will get us further than derision and mockery.

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In the meantime, if you open up FaceBook and I’m not there, you’ll know I’ve finally had my fill.

Plants for winter interest: Going for the Gold

Winter appears to have a death grip on the eastern half of the U.S. for the second year in a row. The thermometer on my car read -6 F on my way into work this morning; with lows of -5, -11, and -2 predicted for the latter half of the week. And to my Northwest friends that have been out mowing their grass already, may the bird of paradise fly up your nose. At this point I don’t even remember what my lawn looks like.

Evergreen conifers provide one horticultural escape from the winter blahs. But evergreens don’t have to be green. One group of conifers that can brighten up a winter landscape are yellow or golden conifers. I will acknowledge these plants are not for everyone. But when sited properly (avoid winter sun is a common admonition among conifer buffs) and used judiciously (a little yellow goes a long ways) these conifers can add a contrasting element that can set off a garden. Note: Hardiness zone and size based on the American Conifers Society Conifer database.

pice orietnalis skylannds2
Picea orientalis ‘Skylands’ (Skylands Oriental spruce) Zone 4. Size: Large (> 12” per year). This tree is a guaranteed show stopper. The combination of the narrow upright form and golden needles is tough to beat.

picea orientalis firefly
Picea orientalis ‘Firefly’ (Firefly Oriental spruce) Zone 4. Size: Intermediate (6’-12” per year). Firefly was selected as a sport off of ‘Skylands’ and is a recent introduction from Iseli nursery. So if you like Skylands but don’t have room for large conifer, this could be for you.

Chamaecyparis pisifera   'Flilifera Aurea  '
Chamaecyparis pisifera ‘Filifera Aurea’ (Golden thread false cypress) Zone 5. Size: Intermediate (6”-12” per year). This a tough plant that can make a good contrast specimen or can also serve as a foundation plant.

C.p.f.a. as a foundation plant MBG
Golden thread false cypress as a foundation planting at the Missouri Botanical Garden

Picea pungens lutea
Picea pungens ‘Lutea’ (Golden Colorado spruce) Zone 4. Who says blue spruce have to be blue? Lots of concerns with blue spruce in the Midwest these days (more on that in later posts), but if you’re in an area where blue spruce are still doing well, this is an option for a winter bright spot.

Pinus contorta  'Taylor sunburst'
Pinus contorta ‘Taylor’s sunburst’ (Taylor’s sunburst lodgepole pine) Zone 3. Ok, I’m cheating a bit here – the yellow comes on the new growth in the spring and then turns green. ACS database lists as a large conifer but I think they are referring to the straight species. When I have seen this plant it’s more in the intermediate range (6” – 12” per year).

The Walking Dead: Christmas tree edition

Zombies are big deal these days. Seems like you can hardly turn on the TV these days without seeing someone (or someTHING) coming back from the dead. Turns out Christmas trees are no exception. Every so often during the Holidays I will get a call or an e-mail that starts off, “My Christmas tree is starting to GROW!” And indeed they are. Under certain circumstances, conifers that are cut and brought indoors can break bud and begin to grow; sometimes putting on considerable new growth.

It's alive!  Concolor fir Chrsitmas tree pushing new growth. Photo: Doug Thalman
It’s alive! Concolor fir Chrsitmas tree pushing new growth. Photo: Doug Thalman

So what gives? Like the proverbial chicken running around with its head cut off, Christmas trees are dead they just don’t know it yet. After they are cut, conifers can continue physiological functions – photosynthesis, transpiration, respiration – for weeks. In some cases they can break bud and begin to grow like it’s springtime when a homeowner brings them indoors. There are a couple of key factors that come into play. First, the tree must be exposed to enough cold weather to meet its chilling requirement. This varies among species, but most conifers need to accumulate at least 6 weeks of chilling below 40 deg. F to overcome dormancy. So early cold weather where the tree is grown and harvested is step one. Second, the “Zombie tree syndrome” is most likely to occur in species that are adapted to high elevations or northern latitudes. The usual suspects are concolor fir (Abies concolor) and corkbark fir (Abies lasiocarpa var. arizonica). These trees have evolved in areas with a short growing season, so there is a selective advantage to breaking bud rapidly when weather warms in the spring – or if brought into a toasty 70 degree living room.

Walking dead - new growth on concolor fir Christmas tree. Photo: Doug Thalman
Walking dead – new growth on concolor fir Christmas tree. Photo: Doug Thalman

So what do you do if your tree turns into a Zombie and comes back from the dead? Don’t panic. It’s a natural phenomenon; just be sure to check and refill the water in the stand regularly so the new growth doesn’t desiccate. And lock your bedroom door at night – just in case…

If trees have met their chilling requirement the they can begin to growth when brought indoors. Photo: Doug Thalman
If trees have met their chilling requirement the they can begin to growth when brought indoors. Photo: Doug Thalman

What’s wrong with my pine tree…? Nothing!

Every once in a while in this line of work we’re actually able to give a homeowner some good news about their trees. A case in point is a call that we frequently get this time of year that starts like this: “My pine tree looks like it’s dying! It’s dropping all of its needles!” If the caller has access to the internet and a digital camera I usually request that the send me a couple of photos; if not, I ask them whether the tree is shedding needles along the outermost part of the limbs or on the interior.

White pine trees often grab homeowners attention as they begin to drop their needles in the fall.
White pine trees often grab homeowners attention as they begin to drop their needles in the fall.

In the vast majority of cases the tree is an eastern white pine (Pinus strobus) and the homeowner is observing is normal needle shed. White pine needles often persist for only two growing seasons. So in the fall they begin to shed their previous-years needles, which often turn an eye-grabbing bright yellow in the process. Actually all pines and evergreen conifers shed their needles; it’s just more noticeable in white pines because they are common in the landscape and their needles are short-lived.

White pine needles often turn bright yellow as they senesce.
White pine needles often turn bright yellow as they senesce.
What's wrong with my pine tree?  As long as the tree is dropping older (interior) needles, it's probably normal needle shed.
What’s wrong with my pine tree? As long as the tree is dropping older (interior) needles, it’s probably normal needle shed.

The lifespan of pine needles varies widely among species. In some species, such as white pine and loblolly pine (Pinus taeda), needles may only persist for two growing seasons. On the other end of the spectrum is bristlecone pine (Pinus longaeva), which can retain needles for up to 40 years. For the most part, needle retention is correlated with shoot growth rate; trees with fast growth rates have fast needle turnover, while slower growing trees have long needle longevity. Of course there are lots of exceptions to this trend and environmental conditions can impact needle life-span as well. For example, needles may shed prematurely during a drought.

Needles on this Austrian pine (Pinus nigra) persist for three growing seasons.  The tree is currently shedding needles formed in 2012.
Needles on this Austrian pine (Pinus nigra) persist for three growing seasons. The tree is currently shedding needles formed in 2012.

Bottom-line: If it’s fall and your pine is starting to drop interior needles, chances are it’s normal needlefall and nothing to worry about.

Pine needles accumulating on the ground.
Pine needles accumulating on the ground.

References
Ewers, F.W. and R. Schmid. 1981. Longevity of Needle Fascicles of Pinus longaeva (Bristlecone Pine) and Other North American Pines. Oecologia 51:107-115
Hennessey,T.C. , P.M. Dougherty, B.M. Cregg, and R.F. Wittwer. 1992. Annual variation in needle-fall of a loblolly pine stand in relation to climate and stand density. Forest Ecology and Management. 51:329-338.
Schoettle, A.W. 1990. The interaction between leaf longevity and shoot growth and foliar biomass per shoot in Pinus contorta at two elevations Tree Physiology 7:209-214.

What about fall fertilization?

Posted by Bert Cregg
We had a question on the Facebook site regarding fall fertilization of landscape plants. Fertilization in general, and fall fertilization in particular, is a complex topic and needs a little more room for explanation than the Facebook discussion allows.

Source: Forestry Images
Source: Forestry Images

As a general rule, most landscape trees and shrubs can maintain acceptable growth and appearance without fertilization. There are a couple of reasons for this. As Linda noted in the Facebook discussion, woody plants are fairly efficient at internal nutrient recycling. I’ve done a couple of studies where we sampled leaves of hardwood trees during the season and then re-sampled right after senescence and about 50% of leaf nitrogen is re-absorbed by trees before they fall. Conifers are even more efficient at conserving nutrients than hardwoods since they typically only lose 1/4th of their needles (or less) each year. In addition, many landscape trees are able to utilize fertilizer that is applied to surrounding turf. On the flip-side, nutrients that occur in litterfall are removed from the nutrient cycle in many suburban landscapes and this may eventually contribute to deficiencies.
pin oak close-up

Bottomline, landscape fertilization should be based on need; which can be assessed based on soil sampling, foliar sampling, or visible symptoms. At least two of the three methods should be employed to make a diagnosis. Each method has drawbacks and visible symptoms are usually the least useful since many nutrient deficiencies have similar symptoms or the symptoms may not be nutrient-related at all. In our area the only nutrient problems I am comfortable diagnosing based on visible symptoms are iron chlorosis in pin oaks and manganese deficiencies in red maples, both of which are induced by alkaline soils, not a lack of those particular elements.

So assuming we’ve established that fertilization is needed, what about fall fertilization? There are a couple of arguments that are usually brought forth for fall fertilization. One is that trees can absorb nutrients during the fall and then use them for spring growth. This is generally true provided that soils are warm enough to allow continued root growth and absorption. Another argument is that fall-applied fertilizer that is not taken up by roots in the fall be will available for uptake when soils warm again in the spring. A third, and less scientific reason, is that fall is often a slow time for arborists and landscape companies and fall fertilization is an easy service to add to their sales program.

There are a couple of objections that are usually raised to fall fertilization. One is that nutrients will leach through the soil over winter before they can be absorbed. This is one of those ‘it depends’ scenarios. If a nitrate-based fertilizer source is used, this is possible since negatively-charged nitrate anions won’t bind to negatively-charged cation exchange sites in the soil. If the nutrient source is urea or ammonium-based, the amount lost will be dependent on temperature since this will drive the conversion from ammonium, which can bind to cation exchange sites, to leachable nitrate.

The other usual objection to fertilizing trees in the fall is that it will reduce cold hardiness. There is no clear evidence to support this, however. Harold Pellett and John Carter at the University of Minnesota compiled dozens of studies on the effects fertilizer on plant cold hardiness (Horticultural Reviews 3:144-171). For conifers and temperature hardwoods they found no clear trend across studies, except that fertilizing with potassium improved cold hardiness is most cases (see table). The common perception that fall fertilization, especially with N, will increase cold damage probably stems from studies of fertilization of turf, which had negative impacts in 26 out of 29 studies cited by Pellett and Carter.
pellett and carter

In summary, landscape trees and shrubs should be fertilized only where there is a demonstrated need. Fall is a good time to fertilize provided you avoid nitrate-N sources that will be prone to leaching.

When worlds collide

This past week I was in Alnarp, Sweden to present at the International Urban Tree Diversity Conference. Lots of interesting talks, posters and field tours. Much more to discuss than I can fit into a blog post, but if you’re interested you can read the presentation abstracts.

One of the best features of the conference is that the presenters and participants included not only arborists and urban forester but also landscape architects and urban planners. This might not sound too remarkable but these groups are not always on the same page. While urban foresters are on board with the need to diversify urban and community forests; species diversity can be at odds with uniformity, which is a key element in landscape design. For example, a common application of uniformity in design is the installation of alle’s – long, uniform monoculture plantings along a street or path.

Fredicksberg alle in Copenhagen - street view
Fredicksberg alle in Copenhagen – street view
Fredicksberg alle in Copenhagen - pedestrian view
Fredicksberg alle in Copenhagen – pedestrian view

The aesthetic appeal of an alle’ is undeniable and seemingly universal. We saw many examples in our tours in Malmo and Copenhagen, but alle’s can found almost anywhere mankind has planted trees.

An alle of beeches in Malmo. Sweden
An alle of beeches in Malmo. Sweden

The dilemma, of course, is that any monoculture planting runs the risk of catastrophic failure, especially in an era of increased global trade and potential introduction of destructive exotic pests.

Lindens failing in Fredricksberg alle
Lindens failing in Fredricksberg alle

Simply planting a random mix of species leads to a menagerie effect – one of these, one of those – that most eyes find unsatisfactory.

A random mix of species increases diversity though not necessarily aesthetic appeal
A random mix of species increases diversity though not necessarily aesthetic appeal

One of the challenges addressed at the conference is how to meet the design and aesthetic objectives of uniformity while still achieving diverse landscape. There are no simple solutions and like most compromises, not everyone will be completely satisfied. But at least we are getting to the point where all sides of the discussion are being heard and creative minds are melding the science and the art that will produce the desired aesthetic and diversity.

For Mulch

Posted by Bert Cregg

Just a quick note up front that today’s post is a little data heavy, so if you’re still adjusting to this weekend’s time change; be advised.

A few weeks back Jim Urban wrote a post entitled ‘Against mulch’ on the Deep Root blog. The principle reasons he cited for his position were: 1) Mulch floats and can clog drains and releases “lots of phosphorus” as it breaks down, and 2) work by Gilman et al. that suggest that mulch does not reduce evapo-transpiration. We discussed the Gilman et al. paper ad nauseum here already so I’ll stick to the other points.

Most organic mulches float, it’s true. However, if mulch is repeatedly washing from a bed into a drain this suggests a problem with the design as much as anything. Second, I’m not sure what constitutes “lots of phosphorus”. Branch and stem tissue of hardwood trees is about 0.1% P. If we use just the bark as mulch, the P concentration is about 0.2 to 0.3%. Is that ‘lots of phosphorus”? I don’t know. I suppose if you put enough it down and allow it wash into a drain it could be.

So let’s stick to what we do know about landscape mulch. Linda has written the most comprehensive review of mulch out there and it demonstrates the benefits of mulch. Nevertheless I’d like to add some recent observations of my own to the discussion. These come from follow-up measurement on some studies that we have already published on shrubs and conifers. But I think our new data are important because they demonstrate the long-term benefits of much on tree and shrub growth.

2006 Conifer study. In 2006 we installed a trial to compare several different weed control strategies for newly planted conifers. Weed control, either by hand, Vis-pore mulch mats or 3” of coarse wood chips, dramatically increased tree survival.
swmrec mulch survival

After 8 growing seasons, trees that had the wood chip mulch or mulch mats had significantly greater caliper than trees that were not mulched.
swmrec mulch caliper

2004 shrub study. In another trial we compared the effect of various mulch types (wood chips, pine bark, hardwood bark) on growth of common landscape shrubs (golden globe arborvitae, Runyan yew, ‘Tardiva’ hydrangea, cranberrybush viburnum, and arrowwood viburnum). We re-measured heights of the shrubs study a couple of weeks ago (nine growing seasons after installation). To keep things simple here I’ve lumped the mulches together and simply compared mulched vs. un-mulched.

After nine years mulching increased height growth for all shrubs except the arborvitae.
mulch 2013 ht

Even more interesting is that the growth benefit of mulch extends beyond the establishment phase. If we start at age 4 and look at the relative growth rate for the past five years (i.e., growth increment for past 5 years / height at 4 years) we see that mulch continues to provide a growth advantage for all shrubs except the arbs.
mulch RGR

As I said at the outset, a little data heavy today but I think this is an import point. There is a lot of discussion these days about proper planting techniques but I think after-planting care often gets overlooked and mulching is an important part of that. That’s why I’m for much.