The Carlisle Mosquito Online

Friday, April 27, 2007


Global warming and tree care

Today is Arbor Day, a traditional day for planting trees. Since many of us in Carlisle already have plenty of trees, this article will be on preserving those trees that we have — in the face of global warming.

Man-induced climate change has finally hit the mainstream consciousness. The reality is sobering, but we must learn to deal with the consequences. Trees in particular will be impacted severely because they are long-lived and climate-dependent. In the short term there is not much that can be done with our forests, but high-value landscape trees will benefit greatly from a little assistance.

Let me guess that most readers of this column will be acquainted with the complicated and seemingly conflicting predictions of global warming. Paradoxically some areas such as Carlisle are predicted to receive more precipitation than the historical average and extended cold snaps such as we experienced this past winter are also expected. These counterintuitive forecasts and climatologists' open debate on the character, rate and extent of the expected climate change have obscured the general scientific consensus — expect more erratic, generally warmer weather.

For Carlisle-area trees, the worst consequence of global warming will probably be occasional summer drought compounded by higher summer temperatures. Plants are also quite vulnerable in the winter, however, when they are hit with cold, dry, sunny conditions — made all the worse with barren, snow-free ground. The incredibly mild weather last November and December also delayed the cold conditioning (hardening) that is necessary for plants to prepare for the cold, and this set many off schedule, making them more susceptible to winterkill. Broadleaf evergreens such as rhododendrons, azaleas and hollies were hit especially hard by this year's winterkill, but you can be sure that many other species, including mature native trees, were also stressed.

Similar to humans, stress makes plants more vulnerable to disease. Stressed plants are also targeted by insects for the same reason; they lack sufficient energy to manufacture the necessary defensive chemicals. Preventative care is the best strategy.

Caring for trees in drought

For high value, established trees in the landscape, let me suggest:

•Deep irrigation once a week during droughts. This is generally considered to require one inch of water for the entire area under the dripline and can be accomplished with a soaker hose or regular hose set at a very slow trickle and moved periodically. Injection watering might look great but has not produced better scientific results. Total watering time can be estimated by timing how long it takes to fill a five-gallon bucket, enough water for ten square feet. Sprinkler systems are usually too shallow to help trees, although some sophisticated systems have multiple zones that may include weekly drip irrigation for trees and shrubs.

•Mulch under the trees is also crucial. Research has shown, however, that it is primarily the exclusion of competition such as turf grass that explains most of the success of mulch. Once again the entire dripline is a good rule of thumb, but even a small mulch area is better than none. Landscapers and homeowners frequently apply too much mulch and find it hard work to remove. Two to three inches is considered ideal. The problem is that it simply does not break down rapidly enough to allow for a thick fresh layer every year. Among other problems, such as oxygen and nutrient deprivation, thick mulch then robs plants of water by completely absorbing and then evaporating light summer rains.

•For smaller evergreens of all varieties on sites with strong late winter sun, an application of anti-transpirant (Wilt-pruf) can be helpful. This should be done around November and possibly again in January if conditions permit.

Thirsty trees

Tree-planting projects in some arid parts of the world have been shown to actually lower the local water table, exacerbating drought conditions. All plants transpire water to cool their leaves and needles, but some are thirstier than others. White pines consume huge quantities of water every summer day. They drink so much that they actually drip water on you if you stand under them! When there is only so much water to go around one way or another, tree density will decrease to match the available water supply. This population control can be left to Mother Nature, but if you care about some trees more than others, you might want to intervene with some selective thinning. White pines are nice, but we have plenty, so I favor oaks, maples, hickories and beeches. Selective cutting allows the remaining trees to thrive. Invasive plants such as buckthorn, honeysuckle, barberry and bittersweet should be the first to go.

Given erratic weather patterns, the flip side of drought is flooding. Water-soaked roots are not able to breathe properly and are then subject to decay. The best solution here is proper plant selection for each site whether planting or simply thinning. Acer rubrum, red maple, is also known as swamp maple for a reason. For planted landscapes, care should be taken to provide adequate drainage; in some cases trees and shrubs are best planted on shallow mounds that prevent water pooling at their bases.

Scientists predict that total snowfall amounts will be lower as a result of global warming. My own guess is that ice storms will be as bad as or worse than in the last few decades. Just look at how hard the Southeast has been hit over the last couple of years. Most open-grown landscape trees regardless of species tend to play things a little risky with respect to structure. Long and skinny branching produces more acorns, cones and other seeds faster. Stouter structures are more conservative; they hold up better in storms but at the cost of less reproduction.

The race to the top

Having evolved in forests where the race to the top for sunlight means everything, most species just go nuts when given the kind of space they receive in our managed landscapes. They get overly ambitious. This go-for-broke growth pattern might be fine for the tree's biological imperative, but the landowner might have different ideas on appropriate risk management. A little pruning and sometimes some hardware such as cables or rods can greatly increase the odds that a favorite tree will survive the next serious storm.

If you just have to plant a tree for Arbor Day, let me suggest a couple of native shade trees that, given the right site, should do okay despite global warming and contribute greatly to our increasingly important biodiversity. Sweet gum, tulip-tree, American hornbeam, basswood and tupelo are all good candidates and are available in large but manageable sizes. I believe these trees are a better investment and will ultimately be more rewarding than your average, over-planted ornamental. Have fun!

John Bakewell is a certified arborist with a passion for the native and traditional landscape. He can be reached at

2007 The Carlisle Mosquito