Warm Weather is For The Birds. But What About The Trees?

May-June 2012

Our federal government is offering millions to universities to study climate change. From $500,000 to $1,000,000/year/project is available up to a grand total of $35 million. Applications were due May 17. If past history is any indication, Canadian rural forestry research will collect its share, along with water and air studies, and urban forestry will likely be sidelined – at best receiving a small slice of those research dollars. 

In Canada, we have a century-long history of promoting rural forestry while sporadic and disjointed work is done for our urban forests. Here are perhaps some new and potentially surprising ideas from climate change forestry research that might also assist those working primarily in the urban forest. 

Drought Kills Roots First
Bill Anderegg, Stanford University, researched trembling aspen looking for the answer to what happens to trees during drought. He investigated both carbon starvation and water transportation stress experienced during the severe drought in Colorado between 2000 and 2004. He did not find a notable loss of carbon reserved in the trees but he did find a notable loss in the affected tree’s water conductivity. “Potted trees exposed to a summer’s worth of drought exhibited significant root mortality.”   

Anderegg responded to my email inquiry about relating his project to urban forests. “I take it from your questions that you’re considering including a portion about how urban trees could be affected by climate change. Your observation about cities being heat islands is spot on. This increases temperatures and consequently the stress on trees. While our research takes place in mountainous forests, it does highlight the role that temperature can play in stressing trees, especially during a drought.” 

In conjunction with a vast literature on how trees survive in stressful conditions, I think Anderegg’s study emphasizes the importance of keeping trees relatively well-watered during hotter periods, as temperature and drought together often provide a one-two punch to tree health and the trees’ ability to move and transport water. I’m not an expert on urban trees and the specific stresses they face, but I do think it likely that climate change will put increasing stress on tree physiology with rising temperatures and more sporadic drought and flood events.” 

Anderegg ended his email with: “I’m current working on the effects of tree mortality in forests and a way to predict tree mortality based on a knowledge of how trees work.” Visit http://wrlanderegg.com/ for more.

Climate Change Impacts Diseases
Bottom line: no matter how much moisture is present, many diseases profit from increased temperatures. The USDA Forest Service released a paper saying warmer and dryer conditions will support yellow cedar decline, Cytospora canker on aspen, and Armillaria root disease. On the flip side, sudden oak death, Phytophthoa, is more damaging in wetter and warmer conditions. 

Beetles Increase Broods
It was originally believed that mountain pine beetles (MPB), like many native boring beetles, have only one brood/year. However, graduate student Scott Ferrenberg and Professor of Ecology Jeffry Mitton from Colorado University were quoted in a recent news release saying they saw two broods of MPB in 2009 and 2010 in the mountains 25 miles west of Boulder, Colorado. Adults emerged and laid their eggs. These eggs hatched, matured and laid their eggs in late summer. Ferrenberg and Mitton said one extra brood alone could increase the population by a factor of 60. They believe this change in known MPB behaviour could be the result of higher temperatures in the mountains. 

We need to consider the possibility that other native boring beetles could also alter their breeding cycles in response to temperature. In the urban forest, this would not only increase numbers, but also alter effective times for pesticide treatments.

Strategic Placement of Predators
Although the predatory wasps used in the US to combat EAB have little in common with the predatory beetles used to control hemlock wooley adelgid, there is an important concept that arborists should keep in mind as the use of live insects for bio-control gains in acceptance. The Journal of Economic Entomology recently published a study about control of the hemlock wooley adelgid. To date, this insect has been controlled in Ontario by our northern winters and thus hasn’t yet been a huge concern. In the US, Laricobius nigrinus or Sasajiscymunus tsugae are released as bio-controls for this pest. Researchers said it is imperative that these predatory beetles are released in the upper crown, thus strategic placement of predatory beetles is important and placement could be key for other insect bio-controls as well.

Tree Migration
Just about everyone who believes in climate change expects trees to migrate north. However, Gareth Rees of the Scott Polar Research Institute said in his report, “the relationship between climate change and tree growth is more complicated then initially thought.” 

Yes, the tree line is moving north, but not at the rate (2 km/year) originally predicted. The advance of the arctic tree line is more like 100 m/year and although the earth’s temperature has warmed, the response is not uniform. 

Migration Problems
Suzanne Simard of the University of British Columbia is currently doing research on coniferous forests; she has done similar work previously on mixed forests. She’s learned trees “communicate” through an underground web using mycorrhizal fungi. Through the connections they transmit carbon, nutrients and water to one another. A “mother” tree will support young trees and saplings by infecting them with the fungi and transporting needed nutrients. 

In a telephone interview, Simard said she has done no work in the urban forest but “looking out my window” she’s more than aware of the potential links between her work and problems for urban environments. 

Urban trees are often germinated or rooted in a sterile medium. They are planted in pots and eventually transplanted in parks or yards where the topsoil is stripped back and a minimum of this abused soil is replaced. 

Considering Simard’s findings for forestry, it is obvious that urban trees are stressed even before the planting shovel is removed and put back in the truck. For example, if the theory can be applied to Dutch Elm Disease (and this is conjecture only), when a forested elm is exposed to DED, the trees in communication with the first tree infected would start their defenses a short period before they are actually infected. This may allow them to survive longer (note: this is a survival strategy, not resistance). Thus, they wouldn’t all die at the same rate. 

We do know that a forest’s underground “communication system” helps keep trees healthy. Simard is trying to discover if relocated trees will/can develop this network. 

A side issue for the urban forest is grass. Well-fertilized lawns have their own “communications” systems and these actually work against most tree systems. Trees grown with other native plants and mulch are in healthier and less stressful situations.

Root Freeze Increasing
It seems contradictory to say that root freezing increases as temperatures warm, but that is exactly what a study from Alaska proved. Yellow cedar is in decline because there is a lack of early spring snow cover. “Shallow rooting needs snow protection.” Sixty to 70% of trees were affected in 600,000 acres of Alaska and BC.

All Forests are Not Carbon Positive 
Planting trees to create a carbon sink is not necessarily the answer to climate change. Where we plant, what we plant, and watering are huge factors to be considered in the equation. 

Changhui Peng of Montreal University recently completed another report on climate change and forestry. He discovered that between 1963 and 2008, from the east to west coast of Canada, trees are dying sooner. In the east, trees are growing faster. In the west, they are growing slower. Forests in Alberta, Saskatchewan and Manitoba are shrinking. And these trends appear to be picking up speed. “Unless increased temperatures are accompanied by increased precipitation, global warming will not encourage tree growth.”

Peng said from Montreal in a telephone interview: “In the past, the boreal forest was seen as a whole as a carbon sink. Increasing temperatures were expected to increase forest growth and the total would be positive (for carbon). Recent literature on tropical forests and the west coast of the US are indicating these areas are sensitive to warming and there is significant mortality. (Mortality in some areas is higher than tree regeneration and the carbon balance is declining.)

In Canada, we now know there are vast differences moving from the west to east coast in our forests. “It’s a totally different story depending on where you live. Forests are not uniform. They don’t act the same way. In the east, water was not limited last summer. In Alberta, it was. There could also be a difference moving south to north. I don’t have the necessary data.”

“The urban forest is another story. They can be more sensitive to changes in temperature and moisture levels with the heat island effect. The urban forest is equally important and there is a big research gap.”  

Peng is planning more work on how fire and harvest disturbances affect forests and research on an issue that is paramount for the urban forest, insects. 

Final production data for this year’s maple syrup crop will not be available until summer or fall through the Maple Syrup Producers and then later still through Statistics Canada (late fall or December), said Todd Leuty, Agroforestry Specialist with OMAFRA. 

Leuty is cautious when he speaks of weather patterns and the maple syrup industry is avoiding any reference to climate change. “We had an early season this year. This is not the first time it has happened. Twenty or so years ago, we had a similar season. Previous to that, we also experienced a similar pattern. It could be weather cycling or solar radiation periods.” 

However, the weather this season and its affect on the maple syrup industry is remarkably like comments MNR’s climate change staff made to me ten years ago. Flipping back through reports from 2001 to 2005, unfortunately, it looks like they got it right. It was warmer this spring – and it is drier – but Leuty is also right. This year’s weather is not a predictor of next year. 

He said that this year’s maple syrup season started and ended early. Most areas of Ontario saw a three to four week season (a good year is five to six weeks). “Producers in southwestern Ontario, who were ready, have 80 to 100% of good year levels. Many producers no longer follow the calendar. Researchers and intent producers watch the weather and predict from it. This year’s quality was fantastic. There was a lot of light and extra light produced across the province, with amber and darker more at the end of the season as is normal. Overall, there was a good maple flavour this season.”

“In BC, a small group of producers are tapping big leaf maple. It has a slightly different flavour and is creating a lot of interest. Quebec’s production was down but they usually keep a year or more in reserve to keep exports happy. The quality (after a year’s storage) is exactly the same. Energy costs did go up this year and there could be a hike in price as a result. In the US, production was down everywhere.” 

As we’re all aware, a record high warm early spring followed by a cold snap can wreak havoc with fruit trees. I consulted tender fruit experts to see how this year’s temperatures are affecting Ontario growers. “The biggest impact of the above normal temperatures on the tender fruit crop was to push forward development/bloom. As a result, we are seeing frost injury in sweet and sour cherries this year. Currently, other tender fruit have not been affected, however they are still vulnerable to frost until they have finished blooming,” said Kathryn Carter of OMAFRA.  

Moving on to climate change: “Tender fruit crops grow best in sandy soils. It is too early to consider growing peach in more northern climates based on only one year’s experience. Peaches are very vulnerable to winter injury and as a result are not grown in northern areas.”

Wendy McFadden-Smith, also with OMAFRA, said, “As for diseases, the challenge will be more economic in nature – the same number of sprays are required, regardless of the size of the crop! If your question is whether the variable temperature will affect insects and diseases in general, I would say not likely for diseases. With respect to insects, it’s possible that the mild winter may allow better survival than a cold one. Now that we’re back into more seasonable temperatures, everything will proceed closer to normal with regard to insect and tree development. The true impact is really anyone’s guess as this is a new experience for all of us! Colleagues in the States are asking/trying to answer the same questions!”

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