Monday, March 2, 2020

Migrations and change

By Beth Sullivan
There is something about March that always makes me think of change. Looking back over all the years of writing this blog, there always seems to be one dedicated to this topic in March. Of course it is the transition time between seasons, and for many of us, it is a very welcome transition in both temperature and light.
Swallows stage magnificent migrations in the fall.

It is a time of migration

Migration is defined as passing periodically from one region or climate to another. It is also defined as shifting: as from one system or mode to another. We think of the great migrations of large mammals on the plains of Africa. Locally, migration can be a bit more subtle, less earth shaking. In the fall, in particular, the movements can be quite large and impressive, as birds flock and prepare to go south. We have witnessed the magical formations of swallow species as they come to roost at night, in preparation for migration south. School children learn early about the migration of the Monarch butterflies from their northern birth places to the forest groves in Mexico.
The return migration, coming home here, or leaving here for more northern lands, is generally more gradual. There have been reports of large flocks of songbirds making their way north already, in masses so large they show up on radar. Scientists have said it is weeks early. By the time they arrive here, their numbers have scattered, and they are less impressive in their presence. We have to hope that their return here, possibly too early, will coincide properly with emergence of enough insects to fuel them.
These are annual, seasonal migrations. But there are other, more subtle ones. Only a half century ago, there were no red bellied woodpeckers in Connecticut. They were southern species that are now common at our feeders. Other familiar birds, like titmice and cardinals have moved slowly north over the last centuries to set up year round residence. Now they are our familiar favorites. This didn’t happen in one season, or one year, but very slowly as these species moved north where they found adequate food and habitat. In some cases they created a new niche for themselves. In others, they shared resources and even competed with those species that were here before them. The southern birds were not invasive; they were, technically native, just on the move, expanding their range.
But here’s a new one, one that you may not have thought about: plant migration, particularly trees. Obviously trees can’t get up and move when the weather changes; they don’t rely on seasonal food sources, and they need to tolerate and even adapt to sometimes extreme changes. If they can’t adapt, they will die. With the climate changing - warmer overall, less snow cover, periods of heavy rain yet drought in summer, as well as severe weather events the tree species are challenged to adapt quickly. In addition, invasive insects and new diseases are attacking our native tree species with frightening strength and frequency. Warmer winters have allowed these diseases and pests to survive. Chestnuts were among the first to go, over a century ago; then the elms. Beech trees are stricken by bark blight diseases. Birch trees have cankers. Our hemlocks are doomed by the woolly adelgid. Ash trees are being destroyed by the emerald ash borer. White pines are not surviving to maturity due to disease, insects, and the heavy wind events destroying and toppling them. Our mighty oaks cannot tolerate the years of defoliation by caterpillars, combined with summer drought and saturated soils smothering the roots during wet winters.
Beech trees are doomed by a bark infection called blight.

The great mammal migrations in Africa are famous.  Photograph by Binti Ackley.

National Weather Service radar picks up a migrating flock of songbirds over Florida. (National Weather Service picture)

Fifty years ago there were no red-bellied woodpeckers in Connecticut.

Assisted migration

When openings in the forest are created, either man made or nature made, what fills back in are often the same species from seeds that have been in the soil. They will be faced by the same challenges.
Scientists studying this issue are beginning to think of a concept called assisted migration. Trees can’t move far or fast on their own, like birds can. It takes centuries for plant species to expand their range northward as conditions change. One approach is the idea of assisting nature, by bringing north tree species that are native to areas just south of here, species that are adapted to conditions that we are now experiencing. There are oaks and pines that can occupy the same niches as our historically native ones do. Wildlife can make use of these more similar native species, much better than they can non-native species from other countries. Southern natives are less likely to ever become invasive.
We have a number of tree species that are present now, at the northern limit of their range, such as the tulip tree and tupelo (Black gum), and as climate warms they will be tolerant and spread. But we can make it happen faster by using such species to fill in openings and clearing.
As we begin our planning for the restoration of the forest at the Hoffman Preserve, we are beginning to research mid-Atlantic species that may grow and adapt and help create a forest that will last through the next century of change. It is very interesting and hopeful. Stay tuned.
Our mighty oaks and hemlocks are stressed and will not survive the warming climate.

The tulip poplar tree is present in southeast Connecticut but is also tolerant of more southern conditions.

Photograph by Beth Sullivan, unless otherwise noted.

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