Editing Vernal pool (section)

==Generation and annual development==
[[File:Enchanted Rock Vernal Pool.png|thumb|right|An inundated rock vernal pool on [[Enchanted Rock]]. Note the one inhabited by cacti in the background.]]
During most years, a vernal pool basin will experience [[inundation]] from rain/precipitation, followed by [[desiccation]] from [[evapotranspiration]]. These conditions are commonly associated with [[Mediterranean climate]] such as the [[Central Valley (California)|Central Valley]] of California.<ref>{{Cite web|title=California's Vernal Pools|url=https://wildlife.ca.gov/Conservation/Plants/Vernal-Pools|access-date=June 8, 2020|website=wildlife.ca.gov}}</ref> Vernal pool basins are often characteristics of Mediterranean climates, but occur in many other ecosystems, such as forested areas of the [[Canadian Shield]], where they are difficult to identify because of the forest canopy.<ref>{{Cite journal |last=Luymes |first=Nick |last2=Chow-Fraser |first2=Patricia |date=July 4, 2021 |title=Detection of Potential Vernal Pools on the Canadian Shield (Ontario) Using Object-Based Image Analysis in Combination with Machine Learning |url=https://doi.org/10.1080/07038992.2021.1900717 |journal=Canadian Journal of Remote Sensing |volume=47 |issue=4 |pages=519–534 |doi=10.1080/07038992.2021.1900717 |issn=0703-8992|url-access=subscription }}</ref> Most pools are dry for at least part of the year, and fill with the winter rains, spring snowmelts and rising water tables. Some pools may remain at least partially filled with water over the course of a year or more, but all vernal pools dry up periodically. Typically, though, a vernal pool has three phases each year: it is inundated in the winter (inundated phase) with the vernal pool holding onto the water from 10–65 days, it dries slowly during the spring (flowering phase), and it dries completely during the summer (dry phase). Vernal pools favor native species because many non-native species cannot tolerate the extreme seasonal changes in environmental conditions.

Some vernal pools have an underlying impermeable clay layer (also known as a hardpan) that reduces water percolation. The impermeable layer is hydrophobic and it prevents water from draining into lower soil layers, allowing vernal pools to become inundated for a very long period of time. This feature of vernal pools means that the water is allowed to slowly evaporate instead of draining. This is a key factor in the development of vernal pool plant communities as it keeps the soil at the water's edge just wet enough for vernal plant communities to flourish while those closer to the center of the pool are more inundated, leading to zonation of plant communities as the water level recedes. This clay layer also allows pools to exist long enough to prevent upland species from developing, while existing for just enough time to prevent aquatic plant species from taking over.<ref>{{Cite book|last1=Barbour|first1=Michael|url=https://books.google.com/books?id=YNAWPSMm2CUC&q=vernal+pools+hardpan+layer&pg=PA394|title=Terrestrial Vegetation of California, 3rd Edition|last2=Keeler-Wolf|first2=Todd|last3=Schoenherr|first3=Allan A.|date=July 17, 2007|publisher=University of California Press|isbn=978-0-520-24955-4|pages=394–424|language=en}}</ref>

Some authorities restrict the definition of vernal pools to exclude seasonal wetlands that have defined inlet and outlet channels. The justification is that such seasonal wetlands tend to be qualitatively different from isolated vernal pools; this is because they are fed by larger [[drainage basin]]s so that firstly, inflow contributes higher concentrations of dissolved minerals. Secondly, flow patterns increase the periodic scouring and silting effect of flows through or simply into the wetland. Thirdly, longer distance inflow and outflow make for less strictly [[Endemism|endemic]] populations and plants. Low dissolved mineral concentrations of smaller vernal pool basins may be characterized as [[oligotrophic]], and poorly [[buffer solution|buffered]] with rapid [[pH]] shifts due to [[carbon dioxide]] uptake during [[photosynthesis]].<ref>{{Cite book|title=Ecology, Conservation, and Management of Vernal Pool Ecosystems|last1=Bauder|first1=Ellen T.|last2=Belk|first2=Denton|last3=Ferrer|first3=Wayne T. Jr. |publisher=[[California Native Plant Society]]|year=1998|isbn=0-9434-6037-9|editor-last=Witham|editor-first=Carol W.|pages=1}}</ref>

Vernal pools are so called because they are often, though not necessarily, at their maximum depth in the spring ("vernal" meaning of, relating to, or occurring in the spring). There are many local names for such pools, depending upon the part of the world in which they occur. Vernal pools may form in forest, but they are more typically associated with [[grassland]] and rocky plains or basins. While many vernal pools are only a few meters in width, [[Dry lake|playas]] and prairie potholes are usually much larger, but still are otherwise similar in many respects, with high water in wet periods, followed by dry conditions.<ref name=":0">{{Cite book|title=Wetland Ecology: Principles and Conservation|last=Keddy|first=Paul A.|publisher=[[Cambridge University Press]]|isbn=978-0-5217-3967-2|edition=Second|date=September 13, 2010|pages=48–52, 65}}</ref> Some exclude desert playas from the definition of vernal pools because their larger [[closed drainage]] basins in areas with high evaporation rates produce higher concentrations of dissolved minerals, with [[salinity]] and [[alkalinity]] favoring different species. Playas may be inundated less frequently than vernal pools, and inundation typically coincides with colder weather unfavorable for plant growth.<ref>{{Cite book|title=Ecology, Conservation, and Management of Vernal Pool Ecosystems|last1=Bauder|first1=Ellen T.|last2=Belk|first2=Denton|last3=Ferrer|first3=Wayne T. Jr. |publisher=[[California Native Plant Society]]|year=1998|isbn=0-9434-6037-9|editor-last=Witham|editor-first=Carol W.|pages=10–11}}</ref>