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Home | Newsletter | Coast Range Ecology |
As the Jurassic Period drew to a close 150 million years ago,
the Farallon Plate -- a chunk of the Earth's crust -- began slipping
under continental North America, scraping off the loose marine
sediments that covered its sea floor. The moving plate crushed
and compacted the sediments until a great series of parallel ridges
and valleys formed into what is now the California Coast Ranges.
Four major ridges divide the northernmost Coast Ranges -- along
with the southwestern Klamath Mountains -- into parallel
watersheds that feed the northwest flowing waters of the Eel River,
Mad River, Redwood Creek, and the Klamath/South Fork Trinity River. 
The interplay of geology and climate creates the North Coast's diverse biological habitats. In summer a high-pressure weather system initiates a complex set of interactions among air, ocean, and land. Upwelling of cold ocean water coupled with an atmospheric temperature inversion produces fog and traps it west of the coastal mountain ridges. The fog moderates the temperature, provides moisture, and prevents excess loss of water in plants.
The shoreline is dominated by tides, wave action, and the salty marine environment. A wonderful variety of animals and algae thrive in the rocky tidal regions (see Nature of the Northcoast, Summer 2001). The sandy beaches, on the other hand, are too dry for marine organisms and too salty for all but a few land-based species. Above the high tide line, low succulent plants such as sand verbena (Abronia latifolia) have adapted to the shifting dunes. A mix of plants -- both native and introduced -- grow a little farther inland, climaxing in Sitka spruce (Picea sitchensis) and shore pine (Pinus contorta var. contorta) in protected dune hollows and on stable ridges.
The coastal plains from Rio Dell to Eureka and from Arcata to Blue Lake are filled-in basins at the mouths of the Eel and Mad Rivers. Formed by nutrient-rich silts and marine sediments these gently rolling plains produce valuable farm and ranch land.
Dense redwood forests fed by winter storms, summer fog, and deep rich soil rise just beyond the reach of sea spray. Douglas-fir (Pseudotsuga menziesii), grand fir (Abies grandis), tanoak (Lithocarpus densiflorus), and madrone (Arbutus menziesii) are found here as well. Where logging has repeatedly disturbed the forest, redwoods grow in nearly pure stands because they sprout quickly from cut stumps and recover more quickly than trees that can only grow from seed.
Beyond the protective blanket of coastal fog, where the soils are rich and deep, redwood trees are replaced by Douglas-fir, grand fir, and western hemlock (Tsuga heterophylla), yet this conifer forest is as dense as the redwood forest and has a similar understory. Shallower soils, however, do not retain enough water for conifers and so are dominated by open woodlands of oaks (Quercus spp.). Shallow or unstable soils support mainly grasses, which by reseeding can survive dry summers and root-tearing landslides. Our familiar mosaic of forests, oak woodlands, and prairies thus reflects the composition of the underlying soils.
Riparian zones alongside streams cut bold paths through the other habitats. Water loving trees such as Red Alder (Alnus rubra) and Big-leaf Maple (Acer macrophyllum) line the streams, revealing the presence of otherwise invisible streams to the distant viewer. During the winter the contrast is even more striking as the bare brown branches of these deciduous trees contrast with the adjacent evergreen conifers.
Serpentine areas, botanically and geologically fascinating, dot the easternmost peaks of the Coast Ranges. Here the continental crust wedged upward as the Farallon Plate slid below. As a result, a cross-section of the Earth's crust and a bit of the underlying mantle became exposed for all to see. Where the rocks of the mantle have decomposed into serpentine soils, levels of metals toxic to most plants have accumulated. Nevertheless, many plants have adapted. Horse Mountain, for example, is dominated by Jeffrey Pine (Pinus jeffreyi), normally seen only at much higher elevations, together with Incense-cedar (Libocedrus decurrens) and the southernmost pocket of Port-Orford-cedar (Chamaecyparis lawsoniana).
