Habitats
The common trees of North America( here focusing north of Mexico) include both native species that have evolved locally and non-native or“ exotic” species that have been introduced from other regions. Most of our common trees favor one or more particular habitat types, which are variously distributed across the continent. These habitats can be categorized roughly into three types: climax or primary habitats, which show little impact by humans; disturbed or secondary habitats, which have been affected to varying degrees by humans; and successional habitats, which are in transition from secondary to climax stages.
Of course, humans are not the only cause of forest disturbance. Fire, weather disasters, and landslides also have significant influence on the growth of trees and forests. Take, for example, the Hurricane of 1938, which decimated Mount Auburn’ s horticultural collection and monuments. Of the 5,100 trees estimated to have been growing in the Cemetery before the storm, 811( approximately 16 percent of the total collection) were destroyed while an additional 1,200 were damaged. If you wander the landscape today, you’ ll notice that many of the tree plaques denote specimens that were planted in 1939 – 40, reflecting the concerted effort to rebuild the collection.
Given the wide-reaching impact of human and natural disturbance, undisturbed or“ primary” forest habitats may simply be a myth; they may not exist at all. Some level of disturbance may be the normal state of affairs for forests, which are perpetually in transition or succession.
Ecologists have long sought to characterize and categorize forest and habitat types, or“ biomes.” Fourteen such biomes, listed below, may be considered well-defined, of which North America( excluding Mexico) boasts nine( starred). They are: 1. Tropical and subtropical moist broadleaf humid forests 2. Tropical and subtropical dry or semi-humid broadleaf forests 3. Tropical and subtropical coniferous semi-humid forests * 4. Temperate broadleaf and mixed humid forests * 5. Temperate coniferous humid to semi-humid forests * 6. Boreal humid forests * 7. Mediterranean forests, woodlands, and scrub or sclerophyll semi-humid to semiarid forests( with winter rainfall) 8. Tropical and subtropical semiarid grasslands, savannas, and shrublands * 9. Temperate semiarid grasslands, savannas, and shrublands * 10. Flooded grasslands and savannas( fresh or brackish water inundated) * 11. Montane grasslands and shrublands: alpine( above the tree line) or montane( below the tree line) * 12. Tundra( arctic) * 13. Deserts and xeric shrublands 14. Mangrove( salt water inundated)
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In North America, only the tropical habitats are not fully represented. These forest types— from the boreal forests dominated by evergreen conifers in north to the tree-sparse savannas of the Great Plains down to the Florida Everglades— exhibit rich biological and visual diversity. Texas, California, and the southeastern United States are the most diverse, each hosting multiple habitat biomes and containing the highest numbers of tree species. 1
The forest type attributed to much of the East Coast, and thus what we find at Mount Auburn, is the Eastern Deciduous Forest. This large biome lies south of the mixed conifer and deciduous Laurentian domain. It is heterogeneous in composition but somewhat uniform in structure. Here we find numerous species of trees, some of which are distributed throughout, such as White Ash( Fraxinus americana) and Green Ash( Fraxinus pennsylvanica); White Oak( Quercus alba), Black Oak( Quercus velutina), and Northern Red Oak( Quercus rubra); Bitternut Hickory( Carya cordiformis) and Shagbark Hickory( Carya ovata); Red Maple( Acer rubrum) and Silver or Sugar Maple( Acer saccharinum); Eastern Hophornbeam( Ostrya virginiana); American Hornbeam( Carpinus caroliniana); and Slippery Elm( Ulmus rubra). Other suites of trees are dominant to the north, south, and west. The rivers that flow east from the Great Plains provide riparian habitats where species such as Eastern Cottonwood( Populus deltoides), Black Willow( Salix nigra), Black Walnut( Juglans nigra), American Sycamore( Platanus occidentalis), and several types of ash( Fraxinus species) are more common. In the southern Appalachian The Hurricane of 1938 damaged or Mountains, cove forests, destroyed approximately 40 % of Mount
which are protected from Auburn’ s tree collection. Photo from Mount
Auburn Auburn’ s Historical Collections. harsh weather conditions, have a rich diversity of trees, including White Ash( Fraxinus americana), Tuliptree( Liriodendron tulipifera), American Basswood( Tilia americana), Mountain Silverbell( Halesia tetraptera), Yellow Buckeye( Aesculus flava), and Sweet Birch( Betula lenta). The Eastern Deciduous Forests constitute the second most extensive forest type in North America. Many of the species above can be seen in Mount Auburn’ s collection of over 5,000 trees.
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