The Ecology of Wetland Ecosystems

This extension note describes the basic ecological features of wetland ecosystems. It is a foundational document that provides important concepts and background information to be applied in future extension notes on wetland management. Major topics and concepts covered in this document are: • definition of a wetland ecosystem and comparison of characteristics with other related ecosystems, • recognition of the different applications of the term “wetland ecosystem” at site and landscape scales, • major environmental factors and their effects on wetland ecosystems, • classification of wetland ecosystems, • successional patterns in wetland ecosystems, • natural disturbance regimes of wetlands, • role of reserves in protecting wetland ecosystems, • regional variation in wetland abundance and characteristics, and • some important management issues for wetland ecosystems in British Columbia. Introduction Wetlands are an important element of British Columbia’s biological diversity. From the extensive “muskeg” of the outer north coast and boreal northeast to the tiny “pothole marshes” of the dry southern interior, wetland ecosystems perform essential but varied ecological and hydrological functions. Land managers, planners, and field staff must recognize the functional diversity of wetland ecosystems in order to manage wetlands sensibly and sustainably. Wetlands cover about % of the province; this number varies regionally from more than % (some coastal and northern areas) to less than % (southern interior). These ecosystems are transitional between upland and open-water aquatic environments. Wetlands provide essential habitats for many wildlife and plant species, and are focal ecosystems for many others. Wetlands are often concentrated in valley bottoms and lowland areas, where industrial, urban, and agricultural development pressures are highest. This has led to significant wetland alteration and destruction, especially in the lower mainland and the semi-arid interior of the province (Voller ). Extension Note 45 The Ecology of Wetland Ecosystems Ministry contact: Allen Banner and Will MacKenzie B.C. Ministry of Forests Research Branch Prince Rupert Forest Region Bag 5000, 3726 Alfred Avenue Smithers, BC V0J 2N0 (250) 847-7500 March 2000 Biodiversity Management Concepts in Landscape Ecology Ministry of Forests Research Program Wetland or riparian? The term “wetland” refers to ecosystems dominated by water-loving plants and having wet or saturated soils. The term “riparian” refers to any land adjacent to water bodies or wetlands. These may be upland sites or periodically flooded ecosystems. The ecology of riparian areas is the subject of Research Branch Extension Note 17. Throughout British Columbia, forestry activities have also had direct and indirect impacts on wetlands. Although only a small portion of the operable landbase consists of forested wetlands on which timber is being harvested, many non-forested wetlands are adjacent to harvested areas. In , a wetland and riparian classification program was initiated by the Research Branch of the Ministry of Forests to investigate these important ecosystems. This program was intended to extend the concepts of Biogeoclimatic Ecosystem Classification (bec) to wetlands and to provide an alternative classification model that recognized the unique ecological characteristics of these ecosystems (MacKenzie and Banner ). At this time, the recently released Riparian Management Area Guidebook (rmag) (B.C. Ministry of Forests ) of the Forest Practices Code (fpc) used a simple wetland classification system, based primarily on wetland size, to set widths for the riparian management areas around wetlands. However, this type of classification system, while administratively easy to apply, does not reflect natural variation in the sensitivity or ecological function of different wetland types that occur in British Columbia. A system that more explicitly addresses this variability would enable the development of more site-specific best management practices as well as interpretations for landscape planning, risk ranking, and wildlife habitat evaluation. An ecological classification of the province’s wetlands could provide a knowledge base to incorporate ecological principles into wetland management. This extension note provides basic ecological information as a technical contribution to the evolution of wetland management practices. We define and describe the primary environmental and ecological attributes distinguishing major wetland types, and then outline the dominant wetland management issues in British Columbia to be addressed in future extension notes. How are Wetlands Defined? Cattail marshes or peat bogs are common images brought to mind by the term “wetland.” For many, this is selfexplanatory but for scientific, practical, and (more recently) legal purposes, we need to put some descriptive bounds on the definition of a wetland. Wetlands are: Areas where a water table is at, near, or just above the surface and where soils are water-saturated for a sufficient length of time such that excess water and resulting low soil oxygen levels are principal determinants of vegetation and soil development. Wetlands will have a relative abundance of obligate hydrophytes in the vegetation community and soils featuring “hydric” characteristics (see sidebars). From an ecological perspective, either an abundance of hydrophytes or indicators of hydric soil conditions is generally sufficient to indicate a wetland ecosystem. The boundary of the wetland is identified by changes in vegetation structure, loss of hydrophytes, and wetland soil characteristics. This wetland definition encompasses a wide range of ecosystems, from semi-terrestrial fens, bogs, and swamps to semi-aquatic marshes and shallow open water. Excluded from the definition are deep water and flowing aquatic ecosystems and transitional ecosystems such as shrub-carrs, riparian low benches, and graminoid “wet” meadows, which have at various times been  Hydrophytes The water-saturated environment of wetlands supports a unique group of plants called “hydrophytes.” These plants are adapted to grow in waterlogged soils. Excessive water and the low rate at which oxygen diffuses under these conditions leads to a complex of critical conditions that require specialized adaptations (Daubenmire 1959). Adaptations, such as leathery leaves (to reduce nutrient requirements and combat “physiological drought”) or specialized internal air compartments (to transport oxygen to the roots), are required for wetland plants. Obligate hydrophytes (such as great bulrush) are restricted to wetlands and semi-aquatic sites. Facultative hydrophytes (such as Labrador tea and many other members of the Heather family) occur most commonly in wetlands but also appear in some upland sites. Wetland soils are subhydric or hydric, having one or more of the following features 1. Peaty organic horizons greater than 40 cm thick. 2. Non-sandy soils with bluegrey gleying within 30 cm of the surface. 3. Sandy soils with prominent mottles within 30 cm of the surface or blue-grey matrix. 4. Hydrogen sulphide (rotten egg smell) in upper 30 cm. considered wetland ecosystems. Transitional sites are not saturated for long enough to be considered true wetland ecosystems and generally lack an abundance of hydrophytes. However, these types of ecosystems are frequently associated with wetlands. From an ecological perspective, it often makes sense to include them (and the surrounding riparian forest) with wetland sites and treat them as a larger ecological unit or wetland mosaic in the landscape (Figure ). At a broad spatial scale, this complex of sites acts as a single ecological unit in terms of wildlife habitat. The Wetland Environment Wetlands can form wherever soil water is excessive. Climate, internal soil drainage, and topography all interact to create the hydrological template for wetland formation. Other factors (such as bedrock geology, soil characteristics, and hydrological system) influence specific wetland characteristics (such as plant community composition and productivity). An interaction of hydrological factors manifests itself in the biotic and abiotic features of a wetland. The ecological features that reflect these hydrological gradients are vegetation composition, species richness, productivity, soil characteristics, organic matter accumulation rates, and nutrient cycling and availability. Annual soil moisture regime, water table pH, and magnitude of lateral and vertical water table movements (hydrodynamics) are the primary hydrological features distinguishing biological communities. The relationship between wetland classes and these major environmental factors is diagrammatically represented in Figure . Soil moisture regime (smr): smr describes the annual “average” water regime (Klinka et al. ). A Wet or Very Wet smr is the prerequisite to wetland formation. Wet sites have a water table within  cm of the surface but for a least some of the year have an aerated surface layer. Very Wet sites remain saturated at the surface