Physiognomy and Structure: Open alvar grassland and pavement communities have very few trees (less than 10% cover of trees over 5 m tall), a low cover of shrubs (less than 25% cover), and a high abundance of either herbaceous plants or exposed bedrock, which may be covered with crustose lichens and mosses. Alvar shrubland communities have very few trees (less than 10% covers of trees over 5 m tall), moderate to high cover of especially needleleaf evergreen shrubs (at least 25% cover of shrubs), and variable amounts of cover of herbaceous and nonvascular plants.

Floristics: The following is a summary of information from Reschke et al. (1999). Alvar grasslands are typically dominated by Schizachyrium scoparium, Deschampsia cespitosa, Sporobolus heterolepis, Eleocharis compressa, Carex crawei, Juniperus horizontalis, Carex scirpoidea, and Packera paupercula (= Senecio pauperculus). Other characteristic grasses and herbs include Sporobolus neglectus, Sporobolus vaginiflorus, Trichostema brachiatum, and Allium schoenoprasum. Less common is a dry grassland phase dominated by Danthonia spicata, Poa compressa, and sometimes Schizachyrium scoparium. It may represent a disturbed phase. Typically there are several turf and weft mosses forming a patchy mat at the base of grasses and forbs; typical mosses are Bryum pseudotriquetrum, Abietinella abietina, Tortella tortuosa, and Drepanocladus spp.

In parts, there is a mosaic of pavement and grassland areas dominated by characteristic native species, such as Sporobolus neglectus, Sporobolus vaginiflorus, Panicum philadelphicum, Poa compressa, Oligoneuron album (= Solidago ptarmicoides), Danthonia spicata, Trichostema brachiatum (= Isanthus brachiatus), Packera paupercula (= Senecio pauperculus), Carex crawei, and Panicum flexile. In more exposed areas, there is a mosaic of mossy patches and exposed bedrock that is covered with crustose and foliose lichens. In the mossy patches, characteristic mosses are Tortella tortuosa, other Tortella spp., and Tortula ruralis, and a characteristic lichen is Cladonia pocillum. On exposed pavement patches, characteristic lichens are Placynthium nigrum and Dermatocarpon cf. miniatum.

In shrubby areas, a dominant shrub is often Juniperus communis, mixed with either Juniperus horizontalis and/or Dasiphora fruticosa ssp. floribunda (= Pentaphylloides floribunda), or a mix of scrub forms of tree species such as Picea glauca, Thuja occidentalis, Larix laricina, and Abies balsamea. Other typical short shrubs (0.5 to 2 m tall) are Prunus virginiana, Diervilla lonicera, Shepherdia canadensis, Cornus sericea, Rhamnus alnifolia, Cornus racemosa (= Cornus foemina ssp. racemosa), Rhus aromatica, and Viburnum rafinesqueanum. Mixed among the shrubs are Danthonia spicata, Oligoneuron album, Packera paupercula, Clinopodium arkansanum (= Calamintha arkansana), and Tetraneuris herbacea (= Hymenoxys herbacea). There can be a "lawn" dominated by Iris lacustris and Carex eburnea. Characteristic tall shrubs (2 to 5 m tall) include scrub forms of trees such as Juniperus virginiana, Thuja occidentalis, and Quercus macrocarpa. Other less common trees (over 5 m tall) that may be present include Carya ovata, Ulmus thomasii, and Fraxinus americana. It is possible that shrub cover may increase with disturbances such as grazing or lack of fire. Once established, the shrub cover may be thick enough to prevent trees from establishing.

Exotics: Reschke et al. (1999) listed 14 species that occurred in at least 9% of observation points across the range of alvars. These are, in decreasing order of frequency, Poa compressa, Hypericum perforatum, Potentilla recta, Verbascum thapsus, Phleum pratense, Leucanthemum vulgare, Hieracium piloselloides, Rumex crispus, Hieracium spp., Daucus carota, Echium vulgare, Melilotus officinalis, Poa pratensis, and Rhamnus cathartica. Aggressive species which are problematic include Rhamnus cathartica, Hypericum perforatum, Potentilla recta, Cynanchum rossicum. Poa compressa, which is considered by most experts to be an introduced species, is also well-established on many alvar sites.

Dynamics:  Fire: Natural fires appear always to have been at least an incidental part of their history, and probably instrumental in maintaining some alvar types, such as juniper alvar shrubland. This suggests that aggressive fire suppression is not needed on alvar habitats. Based on evidence from alvars that are open with old trees and have no burn evidence, it is clear that not all alvars require fire to remain in an open state. If fire is at all required in some of these alvars, it could only be on the basis of a return cycle of many hundreds of years. Therefore, the use of fire as a management tool is not advised for all alvar community types. (Reschke et al. 1999).

Grazing: White-tail deer are a native species in the Great Lakes basin, but artificially high population levels across much of the basin, created and sustained by habitat modifications and management policies, are seriously impacting plant populations in many natural habitats (Rooney et al. 2004). Alvars in the Great Lakes basin and elsewhere have long been influenced by grazing livestock. Grazing has been little studied in North America, but has been documented on alvars of the Swedish island of Oland, where grazing by domestic animals has occurred since the first centuries A.D. (Titlyanova et al. 1988). There, grazing has been considered essential to prevent encroachment in closed grasslands occurring on siliceous soils, where soil depth would potentially support woody vegetation (Bengtsson et al. 1988, Rosen 1992). Closed turf of ungrazed sites hindered germination of perennial herbs, and the short turf and gaps in grazed communities increased the abundance and persistence of some monocarpic plant species (Rusch 1988). However, the intensity of grazing appears to be critical. A comparison of ungrazed, moderately grazed, and heavily grazed sites showed decreased biomass and floristic changes in the heavily grazed sites, with perennial and annual ruderal species replacing the dominant alvar species. Alvar lichens had the highest biomass values in the moderately grazed sites (Titlyanova et al. 1988).

Brownell (1998) has noted that where grazing is intense on Great Lakes alvar grasslands, the grasses may be reduced and that species avoided by cattle such as Eleocharis compressa may increase in abundance. Rosette-forming species such as Symphyotrichum ciliolatum and Solidago species also may increase. Ranunculus fascicularis is much more frequent on some alvars subject to grazing than on adjacent non-grazed sites. Nevertheless, even light grazing tends to result in elimination of certain species, such as the disjunct Orobanche fasciculata (Catling and Brownell 1995). These factors led the Alvar Working Group to hypothesize that cattle grazing is generally detrimental to alvar communities, but some light grazing may help to keep alvar areas open (Reschke et al. 1999).

Exotics: Several exotic species are invasive and problematic in alvar communities, including Hypericum perforatum in Michigan, Poa compressa in Ontario and New York, and Cynanchum rossicum, Rhamnus cathartica, Lonicera tatarica, and Lonicera morrowii in New York. Poa compressa, which is considered by most experts to be an introduced species, is also well-established on many alvar sites. These exotic species compete with native species for space and nutrients and, in some cases, become dominant, significantly reducing the ecological value of alvar communities (Reschke et al. 1999).