Crowley's Ridge Loess Slope Forest

This ecological system of mesic upland forests is confined to Crowley's Ridge, which extends from Missouri south into Arkansas along the western side of the lower Mississippi River. This vegetation and the ridge itself are very distinctive from that of the adjacent alluvial plain. The ridge is a remnant loess-capped feature rising from 30 m to over 60 m (100-200 feet) above the alluvial plain surface, to about 150 m (450 feet) above sea level. The base of the ridge is composed of Tertiary substrates overlain by Quaternary alluvial deposits and capped with up to 15 m (50 feet) of Pleistocene loess. The system is generally composed of mesic forests that occupy ravines between narrow, "finger" ridges and slopes in a highly dissected landscape. The sites tend to be more mesic than sites elsewhere in the southeastern United States. In many cases, these slopes and ravines provide habitat for plant species that are rare or absent from other parts of the alluvial plain (e.g., Liriodendron tulipifera, Tilia americana). Canopies are dominated by Fagus grandifolia, Quercus alba, and Liriodendron tulipifera, with many associates.

This system consists of forests that are typically dominated by beech, oaks and other hardwoods. Canopies are dominated by Fagus grandifolia, Quercus alba, and Liriodendron tulipifera (Clark 1977d), with many associates, including Magnolia acuminata and Tilia americana (T. Witsell pers. comm. 2013). Other oaks which may be present include Quercus falcata, Quercus pagoda, Quercus rubra, and Quercus velutina. Due to the apparent richness of the loessal soils, Ostrya virginiana is a particularly common species across many of the component community types. Species that may be present in the shrub layer include Arundinaria gigantea, Asimina triloba, Bignonia capreolata, Lindera benzoin, Parthenocissus quinquefolia, Toxicodendron radicans, and Vitis rotundifolia. Some possible herbs include Cynoglossum virginianum, Dioscorea quaternata, and Sanicula canadensis.

These diverse-canopy forests occur in ravines in a highly dissected environment. The system is best expressed on southern Crowley's Ridge, Arkansas (Cross County south through Phillips County), with additional limited occurrences to the north, in undisturbed valleys and coves. Deep loessal soil is the most characteristic and diagnostic component of the environment of this system.

These are stable, generally fire-sheltered forests, with relatively low fire frequency and intensity. There is some natural disturbance from the effects of windstorms and collapse of the fragile loess. This mesic loess forest type typically experiences surface fires with return intervals of from 30 to greater than 100 years. Mixed-severity fires will occur approximately every 100 years, opening the canopy with increased mortality. This effect may also be achieved by recurrent, severe insect defoliations or droughts. Straight-line winds or microbursts may cause blowdowns on a scale of 1 to 100 acres. Stand-replacement fires happen very infrequently (Landfire 2007a).

Conversion of this type has primarily resulted from canopy removal and habitat fragmentation. The most critical anthropogenic threat is mining of gravel from the Quaternary alluvial and Tertiary marine deposits at the base of the ridge since these represent virtually the only extensive gravel deposits in northeastern Arkansas and southeastern Missouri (the Mississippi Alluvial Plain does not typically have gravel deposits). Considerable suburban and exurban residential and small-farm development is causing extensive conversion and fragmentation of forested sites - almost no extensive forested areas occur outside of federal and state properties. Existing forested areas are affected by removal of the characteristic canopy species due to logging and timber extraction. High-grading is a frequent practice, with more desirable species being removed in preference to Fagus grandifolia, which is less desirable in the lumber trade.

Aside from actual site conversion, feral hogs (Sus scrofa) represent one of the greatest threats to biodiversity in these forests (Engeman et al. 2007). They can be especially difficult to control in sensitive slope forests. In addition, invasive exotic species, including Ailanthus altissima, Macrothelypteris torresiana, Microstegium vimineum, Paulownia tomentosa, Phyllostachys aurea, and Pueraria montana var. lobata, can become dominant in the ground and shrub layers following canopy disturbance. For mesic hardwood forests containing Fraxinus species, emerald ash borer (which as of October 2013 has been reported from southeastern Missouri) may also be (or become) a significant stressor.

The most significant potential climate change effects over the next 50 years include an increase in storms, which would contribute to severe erosion of the substrate. Climate change may also bring increased periods of drought, which will affect the health and survival of the moisture requiring trees, as well as increase the probability of damaging wildfire.

This system is endemic to Crowley's Ridge (Arkansas, Missouri), which is a distinctive landscape feature in the Mississippi River Alluvial Plain.