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Fire and Ice

the north America we garden has been shaped by many forces, especially the fire and ice-the cyclical cooling and heating, drying and wetting-of the ice age. In much of the north-in the Puget sound, the northern states and New England -the glaciers of the last ice age scraped away the top soil, leaving bare stone or heavy blue clay with (as  gardeners in  these places often complain) nothing is in between the great plains, by contrast, escaped glaciation and are enriched with fine deep soils. Here, gardeners can grow prize peonies and lilacs but are limited by intrusions of arctic air. Rising mountains, rain shadow deserts, and the mitigating effects of large bodies of water have also shaped growing conditions. In the mountain states, winter cold as well as the shortness of the frost-free period, limits some plantings but allows exquisite alpines and cushion plants. In the deserts of the west and southwest, the heat and persistent water shortages dictate the many lawns are of stone but are compensated for by the succulent and drought-adapted flora that can flourish there. Along the immediate West Coast are North America’s Mediterranean climates. although the rainfall varies from abundant to nearly  non-existent, the entire region west of the Cascade and Sierra Nevada Mountain range can rely on rainless summer. Thus, for most natives these, the growing season is winter, with plants going into dormancy when the spring or summer drought hits. Especially in areas most influenced by the Pacific Ocean, gardeners are treated to mild, and often frost-free winters, though they must contend with very little summer heat accumulation. In contrast, the Southeast, with its abundant rainfall and high humidity, allow near perfect subtropical but is without the cool summer nights required by many southern hemisphere and Mediterranean plants. Plants indigenous to any specific region change during any prolonged dry period or wet period, during change in geology or an inundation, but the greatest changes probably occur during ice ages, when the flora of whole regions is pushed to the south in search of warmth, many individual species not surviving at all. As the ice sheet retreat, the opposite occurs, and plants quickly spread north.
As a result of these movements, botanists have identified an interesting phenomenon, known as the nunatack. When great ice sheet covered much of North America, some mountain peak remained above the ice, harboring plants that's would later re-colonize surrounding areas or remain in place as endemics. In some regions, unencumbered by steep mountains or other obstacles, the newly released flora of these mountaintop sanctuaries quickly spread, covering large areas. One good example of this is the single-leaf pinyon pine (Pinus monophylla) In Great Basin West. Records indicate that, at the end of the last ice age, only 10,000 or 11,000 years ago, the plant was confined almost entirely to what is now Mexico. Today, however the plant inhabits many hundreds of desert mountain ranges as far north as the Oregon/Nevada border, its bird-carried seeds easily skipping the more arid valleys below.