The Heat is On!

We are in the dog days of summer! Record high temperatures across the country have been reported. Let’s face it, climate change is real and the planet is getting warmer, despite record cold temperatures in winter in some areas. With climate change, there are more occurrences of extreme temperatures as well as precipitation such as severe drought, flooding, heavier than usual snowfall, etc. So, how does climate change affect gardeners? To understand the concept, we must first understand how high temperatures affect plant growth and development.

Plant temperature tolerance is the ability of any plant to adapt to a given climate at both low and high temperatures. It is obviously important to use species that not only can survive our winter temperatures, but also our hot summers. Similar to the U.S.D.A. Plant Cold Hardiness Zone map (, the American Horticultural Society published a plant heat zone map ( The heat zones are based on the average annual days with temperatures above 86°F (30°C). For example, up in my neck of the woods in Wisconsin, we have four heat zones in the state; zones 2-5 with the cooler (lower number) heat zones in the northern part of the state and the warmer heat zones in the southern and western part of the state and in urban areas. In comparison, North Carolina has seven heat zones (2-8). California has even more heat zones due to its varying climates and elevations.

Unique microclimates can be created at a specific site. Urban areas usually are slightly warmer than rural areas due to the heat island effect created by light reflection off of buildings, heat rising from sewers, and trapping of heat between large buildings. For example, trees planted close to pavement have warmer conditions in summer, than trees planted near turf, especially in the root zone, and have greater water needs. Trees and shrubs in this environment should receive, but rarely get supplemental irrigation compared to those same plants planted in the middle of a lawn or park area with cooler root areas and less heat stress to the tree.

Street trees compete with other plants and are subject to heat and drought stress
Street trees compete with other plants and are subject to heat and drought stress

So what does this mean for temperate woody plants?

High temperature stress is important for a number of reasons. The main effect is increased water use. However, water availability is often limited, especially in urban environments, leading to a net loss of moisture within the plant. Leaves loose water quicker through the stomates via transpiration as temperatures rise. The stomates then begin to close and the cooling effect of evapotranspiration is stopped.

Other physiological processes are impacted by high temperatures such as fewer carbohydrates available for plant growth and development, generation of plant pigments (red, purple and blue pigments) and defense used in protecting plants against insects and diseases. When plants with poor heat tolerance are grown in regions that routinely experience high summer day and night temperatures and insufficient moisture, plants will use many of their stored sugars during the evening hours and during the hottest part of the day. For many temperate woody plants, optimum temperatures for photosynthesis are below 86°F (30°C). Above this temperature, net photosynthesis declines with increasing temperatures. If this continues long term, plants can die, especially young plants that do not have many stored carbohydrate reserves and are under drought stress.

'Ames' Kalm's St. Johnswort (Hypericum kalmianum 'Ames') is heat and drought tolerant
‘Ames’ Kalm’s St. Johnswort (Hypericum kalmianum ‘Ames’) is heat and drought tolerant
Black chokeberry (Aronia melanocarpa) is a three season interest, heat and drought tolerant plant
Black chokeberry (Aronia melanocarpa) is a three season interest, heat and drought tolerant plant
Purpleleaved ninebarks (Physocarpus opulifolius) are very drought and heat tolerant
Purpleleaved ninebarks (Physocarpus opulifolius) are very drought and heat tolerant


What can we do as gardeners?

Unfortunately, climate change is occurring at a fast rate and each decade is getting warmer and often drier than the preceding decade. Here’s what we can do as gardeners:

  • Select non-invasive, pest resistant, stress tolerant plants for your landscape
  • Non-drought or heat tolerant landscapes plants will require regular watering to sustain them
  • Use of soaker hoses or drip irrigation is better than overhead watering as the moisture is directly applied to the roots with less runoff
  • A plant is not truly drought tolerant until it has been established in the landscape for at least three years or more to allow for root growth, especially for trees
  • Some native and exotic plants can tolerate these changes, but some native species in natural areas may be lost if our climate continues to get warmer
  • Incorporate compost into the soil prior to planting new areas to improve moisture retention and aeration
  • Apply 2-4” of bark or wood chips to the top of the soil to retain soil moisture. Make sure the mulch does not touch the trunk of trees or base of shrubs. Excessive mulching (mulch volcanoes) can actually kill landscape plants by impeding air and moisture penetration and invite fungal diseases.
  • Cities need to avoid planting trees in tree pits (sidewalk cutouts), i.e. restrained planting areas in sidewalks along streets. These trees are under severe moisture, drainage and heat stress and do not live long. Planting areas that are wide and long work much better for tree health and longevity than planting in tree pits.

Laura Jull, Ph.D.

The Canary in the Coal Mine

Three weekends ago marked a milestone of sorts as mean daily CO2 levels at the National Oceanic and Atmospheric Administration observatory at Mauna Loa, Hawaii topped 400 ppm for the first time ever.  Rising levels of CO2 and other greenhouse gases could result in significant increases in temperature in the Upper Midwest over the next century.  When we think about trees in cities the scenario is even worse since not only will urban trees have to deal with overall temperature increases but they must also contend with urban heat island effects, which can add another 8 deg. C or more of heat load.  Because of this ‘one-two punch’ of global climate change and urban heat island effect, I often refer to urban trees as the proverbial ‘canary in the coal mine’ with respect to climate change since they will likely be impacted sooner and more dramatically than trees in woodlands.



In general, organisms have three options to deal with a change in their environment: They can migrate, they can adapt, or they can acclimate.  Since trees are sessile organisms, they can’t pick up and move so migration is out.  Current predictions are that climate will change faster than trees can evolve so natural selection and adaptation will be limited.  Which leaves us with acclimation, or the ability of a tree to adjust its physiology and morphology to its environment.  A common example of an acclimation response is the development of sun and shade leaves on the same tree.  Another example of an acclimation response is an increase in the optimum temperature for photosynthesis in response to exposure to increasing temperatures.  In theory, trees that have a greater capacity to adjust their physiology to increasing temperature will be better suited for future, presumably warmer climates.

We are currently testing this idea in a two-part study.  In part one we are growing trees from several shade tree cultivars in greenhouses under three temperature regimes; ambient temperature, ambient + 5 deg. C, and ambient +10 deg. C.  In part two of the project we planted trees of the same cultivars in two sites in Detroit in cooperation with the Greening of Detroit.  The Greening of Detroit is community based non-profit organization that assists neighborhood groups, churches and schools in their efforts to improve the ecosystem in Detroit through tree planting projects, environmental education, urban agriculture, open space reclamation, vacant land management, and workforce development programs.

Many hands make light work.  Greening of Detroit volunteers plant trees along a street median.

With the help of Greening staff and about 90 Greening volunteers, we recently planted 160 shade trees in downtown Detroit.   One site of the study is in a park, representing a relatively mild micro-climate; while the other site is along a street median surrounding by asphalt with a much higher reflected heat load.  Both sites with be instrumented with environmental sensors and we will compare growth over time as well as physiological responses such as the response of photosynthesis to temperature.  The long-term goal is to identify traits that will be most important to guide future selections of trees of urban and community forestry under changing climatic conditions.

Research Technician Dana Ellison (left) and Research Aide Aiman Shahpurwala finish planting a park tree.

A pick ax as a planting tool?  Dana shows how it’s done in Detroit.

Should back-fill be amended?  My usual answer is ‘no’, but then again it depends what your back-fill looks like…

Sizing things up.  Aiman and Dana collect initial data on trees after planting.

Long winter proves that climate change is a hoax

Just looked at the forecast for the week – Thursday’s forecast high temperature is 32 deg. F.  This is a far cry from last year’s record-shattering 86 on the same date.  Clearly all this blabbering about climate change is just a bunch of hysterical nonsense.

As sportscaster Lee Corso would say, “Not so fast, my friend…”  While winter 2013 can’t compete with winter 2012 in terms of record-breaking warmth, this winter has continued a trend which may have profound implications for landscape and garden plant selections.

As you’ll recall, last winter saw the release of a new USDA Hardiness zone map which indicated that most areas of the US had warmed by at least one-half hardiness zone (5 deg. F) since the previous map was produced 22 years earlier.  Hardiness zones are based on average minimum temperatures; in other words, what’s the coldest temperature you’re likely to see in a given winter.  As it turns out, minimum temperatures have been warming faster than overall average temperatures.  So much so, in fact, that one researcher declared the brand new hardiness map dead on arrival. Nir Krakauer at City College in New York noted that if we look at trends, rather than averages, many areas of the US are already another half a zone warmer than the new USDA map.

Minimum winter temperatures are warming at a much faster rate that average temperatures 

Last week I gave a presentation at the Minnesota Shade Tree Shortcourse and pulled together some cold hardiness data for the Twin cities.  According to the new USDA map, Minneapolis-St. Paul is now zone 4b (-25 to -20 deg. F).  One way to think of this over a long enough time-span, about 1/3rd of their winters should reach a low in that range, 1/3rd should be slightly warmer, and 1/3rd should be slightly colder.

The new USDA map indicates the Twin cities are in zone 4b

I pulled out the winter weather records for Minneapolis-St. Paul since 2000, including winter 2013.  In the past 14 winters temperatures in the Twin cities have dipped to their hardiness zone level exactly once, 2004.  All other minimum temps were at least 5 deg. F warmer.


Annual minimum temperatures at Minneapolis-St. Paul airport (MSP) have reached zone 4b levels only once since 2000.

Obviously a 14 year record is not sufficient to build a hardiness map.  Nevertheless, if someone tries to use this winter as proof that climate change is hoax; just remember, minimum winter temperatures – the temperatures that serve as a primary limit of which plants can grow where – tell a different story.

Invasion of the killer earthworms

It sounds like a B-grade horror movie.  Millions of earthworms, moving silently beneath soil, wreaking havoc until the entire planet is uninhabitable.  Sound a little far-fetched?  Not to ecologists that study northern hardwood forests.  While most of us grew up thinking earthworms were ubiquitous, turns out they are not native in parts of North America that were covered with ice during the last glacial period.  Most of us also grew up thinking that earthworms where the good guys/girls (they’re hermaphroditic), churning up compacted soil and leaving nutrient-rich castings behind.  In many northern hardwood forests, however, exotic earthworms, have become invasive and ecologists believe they are having profound effect on ecosystems.

I have to admit I hadn’t paid that much attention to the invasive earthworm issue but I attended a seminar last week by Dr. Lee Frelich, Director of the Center for Forest Ecology at the University of Minnesota.  Dr. Frelich’s seminar touched on several areas of research, almost all of it extremely depressing, related to climate change and invasive species.  He and his colleagues have documented significant changes in soil ecosystem processes and plant succession associated with increasing populations of earthworms.  Nightcrawlers, in particular, cause a lot of problems because they consume fresh leaf litter causing it to decompose at a much faster rate compared to un-invaded ecosystems.  The net result of these soil changes is that few trees or shrubs can reproduce in the understory.  Over time this may lead to a very different looking forest than exists there today.

Of course, we may end up with a very different forest in any event, given some of the climate change scenarios Dr. Frelich presented.  One worst-case model predicted the climate of the Boundary Waters area of Minnesota would resemble that of present-day Oklahoma City by the end of the century.  But I try not to worry; I figure by that point I’ll be food for the earthworms anyway.



For more info on the earthworms that ate Minnesota, check out these links, if you dare…

What can CO2 do for you?!

Well, it looks like the climate change skeptics are starting to hedge their bets.  Global climate is not changing.  But if it does change, it’ll change for the better.  At least that’s the gist of a book by the Center for the Study of Carbon Dioxide and Global Change entitled The Many Benefits of Atmospheric CO2 Enrichment”.  The book documents 55 different ways that increasing global CO2 will benefit the world.  Most of this is built on studies documenting increases in plant growth and/or photosynthesis associated with increasing CO2.  If you’re interested you can look at a preview of the book at:

While CO2 enrichment can benefit plants and trees in the short term, it’s less clear how they will respond over the long term.  For example, nutrition or water may soon become limiting such that the full CO2 ‘fertilization’ effect is never realized.  Also, it’s likely that certain plants will benefit more from increased CO2 than others: Will exotic invasives gain an additional advantage over natives?  And, of course, if rising CO2 results in increased global temps (which the Center denies) then all bets are off.


You can learn more about the Center for the Study of Carbon Dioxide and Global Change at their website  There is a tab on the homepage where you can donate to support their cause.  Why not?  Exxon/Mobil already has.

Cold enough for ya?

Like many people we spent the past couple days digging out from the massive snowstorm that swept across a large swath of the country.  This was definitely a made-for-TV-weather event as national and local TV weatherfolks took up their positions and gave us breathless live-remotes of the “Blizzard of 2011”.

40 mph wind + 1 little crack = a barn full of snow.

Almost as predictable as video footage of snow-ploughs on the streets and locals snow-blowing sidewalks; climate change skeptics are using the recent round of winter weather as proof that global warming is a hoax and that there’s really nothing to worry about except the economics of ‘cap and trade’.  Just google “climate change skeptics blizzard” and you’ll get the idea.

Bob and Quincy were unfazed by the sub-zero wind-chills.

The problem, of course, is that climate patterns don’t move in a strictly linear trajectory and looking at one extreme event doesn’t prove anything one way or the other.  Even looking a few years time sequence may not present the full picture.  Deroy Murdock used the illustration below to argue in the National Review Online that there is no link between rising CO2 and increasing temperatures.



But looking at a broader timescale tells a different story.  While there are year to year fluctuations a clearer association between rising CO2 and global temperature begins to emerge.

The figure above was taken from an article by Stamhoff et al. 2007, “Recent Climate Observations Compared to Projections” (Science 316 (4): 709).  The dashed lines represent the ranges predicted by a major climate model starting in 1990 – the solid line represents what actually happened.  As shown in the figure, climate models have been fairly accurate overall and, if anything, have been conservative in predicting climate change; especially with regard to changes in sea level.


So where am I going with this? There are certainly enough climate change debate/Al Gore bashing blogs out there to go around and I don’t want to devolve entirely into that debate, but the simplistic ‘exception proves the rule’ mentality of the skeptics gets a little tiresome.  I remember hearing my first talk on global warming at a forest biology conference in the mid-1980’s.  The main point that stuck with me then was that increasing global CO2 would not necessarily result in warming every year but that we would see an increase in the frequency and severity of extreme climatic events; droughts, hurricanes, floods, and yes, even blizzards.  Even some of the earliest discussions on climate change in the early 1980’s (e.g., Manabe and Stoufer 1980) recognized complex feedbacks in the global climate system that would result in some regions getting wetter while others suffered drought.  So while the skeptics may use this weeks’ blizzard as evidence against climate change, increasing frequency of severe weather actually argues for it.

A few other climate facts to ponder:

-Global CO2 is increasing and continues to increase (see top panel in figure above).

– Globally, 12 of the 13 warmest years on record have occurred since 1995.

-Intensity of hurricanes and cyclones is increasing (Webster et al., 2005).  While Fox and Friends were happily using the Blizzard of 2011 to debunk climate change; did they notice the most powerful cyclone on record was slamming into Australia?

-Frequency and severity of droughts is increasing worldwide (Burke and Brown, 2006).

-Glaciers are disappearing.  If you want to go to Glacier National Park and actually see a glacier, you need to hurry.   In 1850 there were 150 glaciers in the park.  Today there are 25 and they will likely be gone in 10 years.


Confessions of a carbon sequestration skeptic

One of the potential environmental benefits that came up in our discussion of the pro’s and con’s of turfgrass was carbon sequestration.  The basic premise of carbon sequestration is to take CO2 out of the atmosphere and ‘lock it up’ in a form that won’t contribute to further global warming.  One of the fallacies floating around these days is that any plant that photosynthesizes, takes up CO2 and thereby sequesters carbon.  What we need to realize is that leaves give off CO2 at night via respiration and all non-photosynthetic (non-green) plant parts such as roots and stems give off CO2 virtually all the time.  Turfgrass has some potential to sequester carbon, primarily as soil C. If we consider that a 7” deep layer of soil weighs 2 million pounds, increasing soil carbon by 1% can sequester 20,000 lbs of C per acre.  How long does it take turfgrass to increase soil C by 1%?  Don’t know, but I’m sure it takes awhile.  Also, there is a limit to amount of carbon a give soil can store as C is respired away by microbial activity so eventually a steady state will be reached.  (Plus we haven’t even subtracted out fossil fuel carbon to maintain turf).  Some plants, such as trees, do have the capacity to sequester carbon in wood for long periods – think redwoods, sequoias and redcedars.  But these trees cover only a small fraction of the world land area.  Intensively managed forestry plantations can take large amounts of carbon out of the atmosphere and sequester it into wood.  The question then becomes what do you do with the wood?  If we burn it for biomass energy; Foof! All that C is right back in the atmosphere.  Still better than burning fossil fuels but also a little less than carbon neutral at best.  We can build houses with the wood from the plantation – the carbon will be sequestered as long as the house lasts.  My home and barn were built in the 1890’s so the carbon taken out of the atmosphere by those trees is still locked up.  If we really want to get serious about carbon sequestration, however, our best strategy would be to convert the entire Upper Peninsula of Michigan to fast growing poplar plantations, harvest the wood every 15 years, and sink the logs in Lake Superior where the cold water will prevent decay.  Sound funny?  I’m not the only person thinking this way.  See Strand and Bedford 2009. Ocean Sequestration of Crop Residue Carbon: Recycling Fossil Fuel Carbon Back to Deep Sediments Environ. Sci. Technol., 2009, 43 (4), pp 1000–1007.  


Bottom line: carbon sequestration is a very complex process and sequestering carbon for more than a few decades takes more creativity and brain-power than most of us can muster.  However, trees and landscape plants do have important role to play in mitigating climate change and it doesn’t require heroic feats of engineering.  Trees and landscape plants can effectively cool buildings, thereby reducing air conditioner use and save fossil fuels – see the USDA Forest Service Urban Forestry Research site for a few examples   ultimately this is landscape horticulture’s contribution to climate change.  Carbon sequestration?  It’s a drop in a very big bucket.