With autumn rapidly approaching, we in hilly New England are fortunate to witness spectacular displays of fall colors as the deciduous trees lose their leaves for the winter. This remarkable process gives us a brief window to easily observe the relationship between landscape characteristics and tree communities. Because different species turn colors at different times, it is easy to see how the populations of various tree species vary across the landscape.
For instance, the brilliant reds of the red maple are most often seen in wet areas, and the greens of conifers stand out both in abandoned pastures and at higher elevations. These relationships illustrate an important characteristic of plant communities: species compete for growing space and respond to environmental factors that vary across space and time. When these factors change, competitive relationships change, and species composition shifts.
The most noticeable factor affecting the distribution of tree species in our part of New England is elevation. Many tree species increase or decrease in number, or in the amount of canopy they occupy, in response to environmental characteristics that vary with elevation, such as snow pack, growing degree-days, and soil depth. This is easy to see as you gaze up, or down, a mountainside when the leaves are turning. Sugar maples are a good example, since the reds and oranges of their fall foliage are easy to discern. This species tends to decrease in importance above 2,000 feet in elevation and is completely absent by 2,500 feet. In contrast, paper birch increases in importance across roughly the same elevation range, and the coniferous spruce and fir trees dominate all others at elevations above 3,000 feet.
Most of the transitions in the forest are similar to what happens with sugar maple and paper birch: there are gradual shifts in species predominance in response to changes in growing conditions, such as elevation, moisture, or soil pH. Every organism has needs that it must meet – in the case of forest trees, these are primarily water, nutrients, and sunlight – as well as vulnerable points in its life cycle, such as seedling establishment, that it must withstand in order to become established. Each species’ requirements will mean that some sites are optimal and some marginal. In marginal situations, the species would theoretically grow except it cannot compete with better-adapted species.
There are more abrupt shifts in forest type, as well, and these also speak to the interactions between environmental conditions and tree life cycles. Abandoned pastures are a good example. Nineteenth-century New England had far more open farmland and pasture than it does today and far less forest. As agriculture declined in the region, the farms gradually reverted back to forest, but with a significant change. The forests that were dominated by hardwoods before they were cleared often reverted to forests dominated by conifers when they grew back. Why? The pre-clearing trees were competing for growing space with other trees in a forest environment while the re-growing trees were contending with pasture grasses and grazers. In this altered environment, conifers were able to out-compete hardwoods even though the underlying environmental factors had not changed.
What does this tell us about our landscape? And, aside from giving us a little more to ponder as we marvel at the fall colors, what relevance does it have to our lives? We are living in a time of rapid environmental changes, and these changes are altering the characteristics of our forests and changing the competitive relationships between species. Where we had agricultural abandonment in the late nineteenth and early twentieth centuries, we now have, for example, acid rain and a warming climate. Acid rain is pushing sites already low in calcium further away from the competitive environment where calcium-needy plants, such as sugar maple, compete well. Climate-related factors are also important to some vegetation transitions; temperature and moisture changes, in a general way, affect the forests much like changes in elevation.
Climate change is expected to affect many important environmental factors such as the length of the growing season, the timing and availability of water, and the depth of the snowpack. Data from the last few decades are consistent with this expectation, and it is reasonable to conclude both that these factors will continue to change and that this will inevitably affect the composition of our forests. Acid rain continues to alter soil chemistry, which is clearly important to tree competition and may already be making marginal sites untenable for some species. While these changes aren’t as obvious as the elevational gradient, they operate in much the same way. The turning of the fall foliage serves as a metaphor for the less rapid changes that are slowly but surely altering the composition of our forests.