Editing Alvar (section)

{{short description|Limestone-based biological environment}}
{{about|the landform}}
[[File:Loopealne Keilas 18-06-2012.jpg|thumb|upright=1.3|Alvar in [[Estonia]] near the town of [[Keila]]]]
[[Image:Österplana hed.jpg|thumb|Alvar at [[Kinnekulle]], [[Sweden]]. Most plants are confined to the numerous cracks in the limestone.]]

An '''alvar''' is a biological environment based on a [[limestone]] [[plain]] with thin or no soil and, as a result, sparse grassland vegetation. Often flooded in the spring, and affected by drought in midsummer, alvars support a distinctive group of prairie-like plants.<ref name="Keddy 2007">{{cite book|last=Keddy|first=Paul A.|title=Plants and Vegetation|year=2007|publisher=Cambridge University Press|location=Cambridge|isbn=978-0-521-86480-0|pages=666|url=http://www.cambridge.org/gb/knowledge/isbn/item5708831/?site_locale=en_GB}}</ref>  Most alvars occur either in northern Europe<ref>{{cite journal |last=Petterson |first=B. |date=1965 |title=Gotland and Öland: two limestone islands compared |journal=Acta Phytogeographic Suecica |volume=50 |pages=131–140}}</ref> or around the [[Great Lakes region|Great Lakes]] in North America.<ref>{{cite journal |last1=Catling |first1=P. M. |first2=V. R. |last2=Brownell |year=1995 |title=A review of the alvars of the Great Lakes region: distribution, floristic composition, biogeography and protection |journal=The Canadian Field-Naturalist |volume=109 |pages=143–171}}</ref>  This stressed habitat supports a community of [[rare species|rare]] plants and animals, including species more commonly found on [[prairie]] [[grassland]]s. [[Lichen]] and [[moss]]es are common species. Trees and bushes are absent or severely stunted. 

The primary cause of alvars is the shallow exposed bedrock. Flooding and drought, as noted, add to the stress of the site and prevent many species from growing.<ref name="Keddy 2007" />  Disturbance may also play a role.  In Europe, grazing is frequent, while in North America, there is some evidence that fire may also prevent encroachment by forest.<ref>{{cite journal |last1=Catling |first1=P. M. |first2=V.R. |last2=Brownell |date=1998 |title=Importance of fire in alvar ecosystems – evidence from the Burnt Lands, Eastern Ontario. |journal=The Canadian Field-Naturalist |volume=112 |pages=661–667}}</ref>  The habitat also has strong competition gradients, with better competitors occupying the deeper soil and excluding other species to less productive locations.<ref>{{cite journal |last1=Belcher |first1=J. W. |first2=P. A. |last2=Keddy |first3=L. |last3=Twolan-Strutt |date=1995 |title=Root and shoot competition intensity along a soil depth gradient |journal=Journal of Ecology |volume=83 |issue=4 |pages=673–682 |doi=10.2307/2261635|jstor=2261635 }}</ref> Crevices in the limestone provide a distinctive habitat which is somewhat protected from grazing, and which may provide habitat for unusual ferns such as ''[[Pellaea atropurpurea]]''. Bare rock flats provide areas with extremely low competition that serve as refugia for weak competitors such as the [[sandwort]] ''[[Minuartia michauxii]]'' and ''[[Micranthes virginiensis]]''.  In a representative set of four Ontario alvars, seven habitat types were described.  From deep to shallow soil these were: tall grassy meadows, tall [[forb]]-rich meadows, low grassy meadows, low forb-rich meadows, dry grassland, rock margin grassland and bare rock flats.<ref>{{cite journal |last1=Belcher |first1=J. |first2=P.A. |last2=Keddy |first3=P.F.C. |last3=Catling |date=1992 |title=Alvar vegetation in Canada: a multivariate description at two scales |journal=Canadian Journal of Botany |volume=70 |issue=6 |pages=1279–1291 |doi=10.1139/b92-161}}</ref> 

Alvars comprise a small percentage of the Earth's [[ecosystem]]s by land extent. Although some 120 exist in the North American Great Lakes region, in total there are only about {{convert|43|mi2|abbr=on}} left across the entire Great Lakes basin, and many of these have been degraded by agriculture and other human uses. More than half of all remaining alvars occur in Ontario.  There are smaller areas in New York, Michigan, Ohio, Wisconsin and Quebec.<ref>{{cite journal|url=https://archive.epa.gov/ecopage/web/pdf/alvar-technical-report-199903.pdf |last1=Reschke |first1=Carol |first2=Ron |last2=Reid |first3=Judith |last3=Jones |first4=Tom |last4=Feeney |first5=Heather |last5=Potter |date=1999 |title=Conserving Great Lakes Alvars |journal=Final Technical Report of the International Alvar Conservation Initiative |archive-url=https://web.archive.org/web/20081013124252/http://epa.gov/ecopage/shore/alvars/alvar.pdf |archive-date=13 October 2008 |url-status=dead }}</ref>

In [[North America]], alvars provide habitat for birds such as [[bobolink]]s, [[eastern meadowlark]]s, [[upland sandpiper]]s, [[eastern towhee]]s, [[brown thrasher]]s and [[loggerhead shrike]]s whose habitat is declining elsewhere. [[Rare species|Rare plants]] include [[Lobelia kalmii|Kalm's lobelia (''Lobelia kalmii'')]], [[Symphyotrichum pilosum|Pringle's aster (''Symphyotrichum pilosum'' var. ''pringlei'')]], juniper sedge (''[[Carex juniperorum]]''), lakeside daisy (''[[Hymenoxys acaulis]]''), [[Cypripedium arietinum|ram's-head lady's-slipper (''Cypripedium arietinum'')]], and [[Iris lacustris|dwarf lake iris (''Iris lacustris'')]]. Also associated with alvars are rare [[butterfly|butterflies]] and [[snail]]s.<ref>{{cite book |last1=Catling |first1=P.M. |last2=Brownell |first2=V.R. |date=1999 |chapter=Alvars of the Great Lakes Region |pages=375–391 |editor-first1=Roger C. |editor-last1=Anderson |editor-first2=James S. |editor-last2=Fralish |editor-first3=Jerry M. |editor-last3=Baskin |title=Savannas, Barrens and Rock Outcrop Communities of North America |location=Cambridge |publisher=Cambridge University Press |isbn= 978-0521573221}}</ref> The use of the word "alvar" to refer to this type of environment originated in [[Sweden]].<ref>{{cite web |url=https://www.saob.se/artikel/?unik=A_0787-0269.CNHo&pz=3 |title=Alvar |publisher=Svenska Akademiens ordbok (Dictionary of the Swedish Academy)|date= 1898 |access-date=11 March 2019}}</ref> The largest alvar in Europe is located on the Swedish island of [[Öland]]. Here the thin soil mantle is only 0.5 to 2.0 centimeters thick in most places and in many extents consists of exposed limestone slabs.<ref>{{cite book |first=C.M. |last=Hogan |title=The Stora Alvaret on Öland |publisher=Lumina Technologies |location=Santa Rosa, Ca. |date=21 July 2006}}</ref><ref>{{cite book |title=The Countryside in Eastern Smaland |editor-first=Markus |editor-last=Forslund |publisher=KalmarSund Tryck |date=1999 |isbn=978-91-630-7610-7}}</ref>  The landscape there has been designated a [[UNESCO]] [[World Heritage Site]]. There are other more local names for similar landforms, such as a '''pavement barren''', although this term is also used for similar [[landform]]s based on [[sandstone]].<ref>{{cite web|url=http://www.nature.org/wherewework/northamerica/states/newyork/preserves/art11877.html|title=New York: Adirondacks: Gadway Sandstone Pavement Barrens|publisher=The Nature Conservancy|access-date=24 April 2010|location=USA|archive-url=https://web.archive.org/web/20110214185716/http://www.nature.org/wherewework/northamerica/states/newyork/preserves/art11877.html|archive-date=2011-02-14|url-status=dead}}</ref> In the United Kingdom the exposed landform is called a [[limestone pavement]] and thinly covered limestone is known as [[calcareous grassland]].