Ferns can help to control erosion, stabilize soils and slopes, build soils where none exist, and often do these things in nature. Ferns have two major mechanisms for doing this: slender, spreading rhizomes and roots that form a mat that retain soil, and fronds that, as they age and die, recline on the ground to form a mat to hold soil and other organic material. Other mechanisms for soil building and conservation include stoloniferous growth, dense frond growth to catch organic debris, and the ability to thrive in marginal habitats.
One of the best known ferns for erosion control and conservation is the Christmas fern (Polystichum acrostichoides) of eastern North America. This evergreen holly fern has an interesting frond life cycle. When new, the fronds form at an upright angle, often projecting almost vertically. As they age, they slowly recline; this effect is most pronounced in the fall, when frosts cause the fronds to lie down on the ground. The near-verticality of the fronds in spring ensure that they rise above the duff of the forest floor, while the falling of the fronds in autumn takes place after leaf fall. The fronds then function to hold leaves in place, often persisting as a leaf mat for several years. Christmas ferns often form sizeable colonies that serve to stabilize slope areas.
Other ferns in other parts of the world act similarly. For instance, some Dicranopteris species of the tropics grow on steep banks; the immense leaves, as they die, lie down upon the slope, forming a very durable and tough mat that holds the soil and other organic matter. This often forms in layer upon layer until it may be difficult to cut or push through. This dense leaf layer also serves to reduce competition from other plants.
These Dicranopteris also hold the soil by the spreading rootstocks and roots. The rootstocks, or rhizomes, creep rapidly, forming a criss-crossing network of rootstocks between which roots penetrate all available soil. Since the soil is saturated with roots, they readily grow into duff as it decays, positively holding it in place and rapidly making organic material part of the native soil.
This same kind of behavior is seen in temperate zones with such ferns as the rock-cap ferns (Polypodium vulgare, virginianum, and appalachianum). These ferns specialize in rock-shelf habitats (although P. vulgare is sometimes found on trees and tree roots), where they colonize the shelf, holding all organic matter that falls on the mat. The densely-growing fronds act as a trap for dead leaves and other matter that fall on the mat, and the roots form such a dense network that, when a plant is taken up, the root mat is almost like a felt fabric.
Other North American temperate ferns that contribute significantly to erosion control on slopes include the fragile fern (Cystopteris protrusa) which, while dying down in late summer, nonetheless form a ground cover during heavy rains of late spring; maidenhair fern (Adiantum pedatum), which sometimes forms mats on vertical or almost vertical rock faces; narrow glade fern (Athyrium [Diplazium] pycnocarpon), which will colonize large hillside areas given enough light and moisture; several of the wood ferns (Dryopteris spp.), which will grow among rocks and cobbles, holding organic matter in the pockets in which they grow; and the climbing fern (Lygodium palmatum), which forms a unique mat of indeterminate vining fronds which insure that the impact of any erosional forces of wind and water are greatly abated before reaching the soil surface.
The ostrich fern (Matteuccia struthiopteris) forms colonies interconnected by underground stolons on sand bars and riverbanks, helping to hold the sand/soil against the erosional effects of the current; bracken (Pteridium aquilinum) while a serious weed in many parts of the world, nonetheless forms almost impenetrable thickets that insure soil stability; and cinnamon fern (Osmunda cinnamomea) and royal fern (Osmunda regalis) form hummocks in swamp areas that catch and hold organic material to actually form small "islands" that help to form land from water. The chain fern (Woodwardia virginica) also grows in wet, boggy areas, also helping to further the conversion of water bodies into land.
An important factor in the continued ecological competitiveness of ferns is that many of them succeed by growing in marginal habitats where other plants cannot survive, and in this kind of strategy, have formed methods of holding and forming soils for their own benefit and incidentally for a greater environmental benefit. This is even seen in the many species of cliff ferns that are included in Cheilanthes, Notholaena, Woodsia, Pellaea and other genera, where the plant will grow in small rock crevices that otherwise would not harbor life, and where the fern will collect organic material to help hasten the process of rock decay into soil.
Ferns are probably far more important than generally realized in environmental stabilization, and deserve to have this aspect of their existence studied in far more depth.