Xerarch and Hydrarch Succession

Beyond the primary-versus-secondary distinction, ecological succession is also classified according to the moisture conditions present at the site where it begins, giving rise to two named directions of primary succession in particular: xerarch succession and hydrarch succession. Despite starting from opposite ends of the moisture spectrum, both are generally described as converging, over a long enough time span, toward a broadly similar kind of climax community adapted to intermediate, or mesic, moisture conditions.

Xerarch succession begins on a dry substrate with very little available moisture, the classic example being bare, exposed rock. Because virtually no organism can establish itself directly on bare rock, the pioneer community here consists of extremely hardy, drought-tolerant organisms -- typically crustose lichens, which are able to grow directly on the rock surface and begin the very slow process of physically and chemically weathering it, gradually followed by more advanced foliose lichens and then by mosses, both of which trap wind-blown dust and organic debris and, over a long period, begin to build up a thin initial layer of soil. Once even a shallow soil layer has formed, small annual herbaceous plants are able to establish themselves, followed over further time by perennial herbs and grasses as the soil deepens and its water-holding capacity improves, then by shrubs, and eventually by trees, as the site's moisture-retaining capacity steadily improves at every stage. The overall direction of change in xerarch succession is therefore from originally very dry (xeric) conditions progressively toward more moderate, mesic conditions, ending, where the regional climate allows it, in a climax forest community.

Hydrarch succession begins at the opposite moisture extreme, in a body of standing water such as a shallow pond or the margin of a lake. Here the pioneer community consists of free-floating and submerged aquatic organisms, principally phytoplankton, together with submerged rooted hydrophytes; over time these are followed by rooted plants with floating leaves, then by emergent reed-swamp vegetation growing in shallow water at the pond's edge, then by marsh-meadow plants able to tolerate waterlogged but no longer permanently flooded soil, then by shrubs, and eventually by trees, as each successive community traps sediment and contributes dead organic matter that progressively fills in and shallows the original water body. The overall direction of change in hydrarch succession is therefore from originally very wet (hydric) conditions progressively toward more moderate, mesic conditions -- the reverse starting point from xerarch succession, but converging, in the same way, on a broadly similar mesic climax forest community. …