The Carlisle Mosquito Online

Friday, January 19, 2007


Tree migration and climate change

Our native trees exist in a delicate balance; small changes in the environment can cause huge changes in the forest composition. Unlike in a garden setting, forest plants need to not only survive, but without the advantage of a gardener's weeding around them they also need to compete successfully for water, light and nutrients. As conditions change, plants are compelled to move to the most advantageous territory possible. Obviously individual plants are stuck in one place but, during rapid changes, the ability to send offspring to more favorable habitats can make the difference between life and extinction.

Many scientists fear that the anticipated climate changes over the next 100 years will be too rapid and severe for many trees species to either migrate or adapt. This would leave our forests impoverished with low biodiversity and unstable ecosystems. Highly mobile invasive plants will have a competitive edge. There is a real concern that instead of simply replacing one set of tree species with another equally beautiful forest collection originating from the south, we could in fact be looking at a vast tangle of invasive shrubs and vines.

Small plants like grasses, sedges and shrubs have shorter reproductive cycles and consequently can travel faster than trees. From what I've read, however, the actual mobility of trees over time is the subject of considerable scientific debate. The historical record of pollen deposits is difficult to date precisely and minority tree species are often missed altogether. All agree however that it is these minority "outlier" trees that can really accelerate the speed of travel and there are several references in the literature to "long jumps," wind or animal travel "events" that propel tree species far beyond their original home territory. It is these lucky breaks of individual seeds that can move entire plant populations. One long-jump mechanism that is very new is landscape and forestry planting by humans. For better or worse this has reshuffled the evolutionary deck.

Changes reducing tree mobility

Other human changes, however, have significantly reduced the mobility of trees. Habitat fragmentation, acid rain, wild fire suppression, exotic invasive plants and high herbivore (deer) populations all make things more difficult for trees to get around. Many tree species are underrepresented in the Carlisle area because so much of the land was cleared for agriculture. Of the remaining woodland, some species were more targeted than others to fill specific needs such as firewood, lumber, farm tools (hickory), shingles (cedar) and bark for tanning hides (hemlock). Imported pests and diseases have also been disastrous for many species, such as the chestnut and elm. The absence of mature seed trees has slowed recovery of the native forest trees in their original habitat; forget about trying to go anywhere.

Similar to our forests' struggling with rapid change, I sense that the ecological community is also scrambling to understand these enormously complex issues. Forestry research tends to focus on commercially viable species such as the fir and spruce in northern New England and the pines in the southern states. Climate-change forecasts often focus on agriculture. I do not know of any book that pulls together climate change and forest ecology for our area. If you find one, please let me know; extra points if it's readable without a Ph.D. Meanwhile, to gain an understanding of what's happening, the best approach is to piece it together ourselves.

One place to start is the USDA's average minimum temperature map. It has become the standard for rating the cold hardiness of plants and helping gardeners understand the severity of their local winters. The 1990 published map (1974 to 1986 data) can be found at This is the familiar growing 11-zone map. It is overdue for a new edition reflecting recent weather patterns. Frustrated with this delay, the National Arbor Day Foundation ( has recently updated the map and included an interesting video clip on the rather significant changes since 1990. This has created some buzz.

Higher temperature data

The American Horticultural Society ( publishes a heat tolerance ten-zone map that details the number of days each year between 1974 and 1995 that exceeded 30° C (86° F). These higher temperatures often cause otherwise healthy plants to wilt and die after prolonged exposure and this heat zone map is seen as an increasingly important complement to the cold- hardiness map.

Long a standard for the western states predating the USDA map, Sunset magazine ( expanded its popular climate zone map to include the eastern states in 1997. It now has 45 zones that more precisely take into account not just high and low temperatures but other factors such as altitude, day length, rainfall, cloud cover and humidity. Interestingly, Carlisle is on the edge of one zone that extends to coastal New Brunswick and another zone with a pocket as far south as Gettysburg.

It will be very interesting both scientifically and politically when the AHS and Sunset maps are updated to reflect the weather trends for the last 10 years. This might be the most concrete method for placing objective climate change information into the hands of the average American.

Two field guides that help with the other end of the puzzle are the Peterson Field Guide on Eastern Trees and National Audubon Society Field Guide to North American Trees, Eastern Edition. Both of these books offer range maps of native trees. Thumbing through them, you'll find many great trees for which Carlisle is just above the northern tip of their range and might appreciate a "long jump." Trees with ranges mostly north of Carlisle would not be a good bet.

So, knowing that the climate is changing and our forests will need to migrate, how can a Carlisle landowner assist these wayfaring trees ? First, clear the way by eliminating invasive plants from your property. Second, eat venison. Third, provide deserving native trees trying to move north with a "long jump." Excellent candidates include Ironwood, Shagbark hickory, Sweet gum, Tuliptree, Black Walnut, Sycamore, Hackberry and Pin Oak Some other tree species such as Sugar maple might have a very long range north to south but need assistance bringing their heat adapted southern gene pool north.

Increased woodland diversity is crucial for coping with new pests and diseases. After planting some trees you will at least know that you've done something about the weather and then you can go back to talking about it.

John Bakewell is an arborist living in Carlisle. He can be reached at

2007 The Carlisle Mosquito