Californian Vernal Pools

Vernal pools are small seasonally flooded depressions that usually fill with water during winter. They form mosaics with valley grasslands and as they dry up in spring various annual plant species come in to flower, often in conspicuous concentric rings of showy colours. The associated soils are often extremely ancient, some being dated at over 600,000 years old, and it is their prolonged profile development that has led to the formation of dense impervious hardpans - hence their seasonal flooding. The pools are often isolated from one another forming distinct island habitats with their own assemblages of herbaceous species.  In fact, there are about eight endemic centers. They can be classified into three general type: valley pools, terrace pools and pools of volcanic areas. The valley pools are usually saline and/or alkaline and are usually characterized by endemic species such as Arenaria californica (Caryophyllaceae), Astragalus tener (Fabaceae), Dowinga bella, D. cuspidata (Campanulaceae), and Grindelia camporum (Asteraceae). Terrace pools comprise most of the pools remaining in the Central Valley and are typically neutral or slightly acidic. Their characteristic endemics include Blennosperma nanum (Asteraceae), and the grasses Orcuttia californica and O. pilosa (Poaceae).  Pools in volcanic areas are readily distinguished physiographically, but have similar floras to the terrace pools.  Other endemics associated with vernal pools include Atriplex persistens, Blennosperma bakeri, Brodiaea pallida, Chamaesyce hooveri, Downingia bicornuta, Downingia concolor var. brevior, Downingia ornatissima, Elatine californica, Epilobium cleistogamum, Eryngium constancei, Eryngium aristulatum var. parishii, Gratiola heterosepala, Hemizonia paniculata, Hordeum californicum, Lasthenia burkei, Lasthenia conjugens, Limnanthes bakeri, Limnanthes gracilis subsp. parishii, Limnanthes vinculans, Machaerocarpus californicus, Mimulus tricolor, Muilla clevelandii, Myosotis minimus, Myosurus sessilis, Navarretia fossalis,  Navarretia leucocephala, Navarretia myersii, Neostapfia colusana, Parvisedum leiocarpum,  Pleuropogon californicus var. californicus, Pleuropogon californicus var. davyi, Pogogyne abramsii, Pogogyne douglasii subsp. parviflora, Pogogyne nudiuscula, Tuctoria greenei and Tuctoria mucronata.

References

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