The Role of Age Class in Riparian Health
For a riparian-wetland area to recover or maintain itself, it has to have more than one age class of riparian-wetland plants. Most riparian areas will recover or maintain themselves with two age classes, as long as one of the age classes is young (for recruitment), and the other is middle-aged (for replacement). Older, mature age classes usually take care of themselves, as they are well-attached to existing water tables. Older age classes can usually persist even with degraded conditions, so they are not a good indicator of a healthy riparian area.
Age Class Distribution Relationships
Expanding populations generally have a pyramid shape of age distribution, with many young plants forming a wide base, fewer middle-aged, and very few old at the top. Stable populations are more bullet shaped, with somewhat equal young and middle-aged groups forming the base and middle, and then gradually diminishing to the oldest individuals. Diminishing populations are more urn shaped, with a narrow base of young, widening toward the older age classes, and then sharply narrowing with the oldest individuals. An indicator of diminishing populations is low proportions or missing classes of young and/or middle aged individuals.
Plant age can be difficult to establish, especially in the Southwest. The USDA Forest Service uses age class in reference to woody species only and establishes a procedure based on number of stems and proportion of live stems for determination of age.
For herbaceous species such as sedges, the term age-class distribution can be somewhat misleading, but the intent is to identify indicators of expanding, stable, and diminishing populations through recruitment and reproduction. While often hard to observe, seedlings of perennial plants indicate recruitment and young sedges colonizing a fresh sediment deposit may be seen as a line of plants tapering in size toward the youngest emerging plants.
Warning Signs
Some warning signs that the age class distribution is out of balance, which may be indicative of declining health or unraveling of riparian areas include:
A lack of seedlings and saplings of woody species – Riparian areas that require woody vegetation should have evidence of new and young plants, especially in places where they normally establish such as point bars.
Woody plants that browsing keeps from escaping - When willows or other woody plants cannot gain sufficient leader growth to get tall enough to be beyond reach of browing animals, they may not recruit new reproducing individuals into the population.
Scattered, individual stems of herbaceous species such as sedges and rushes – Herbaceous species help protect the riparian area by forming dense stands with even denser root masses. In order to be effective, these species should be forming dense mats.
In those few channels where vegetation doesn’t contribute to bank stability, the diversity of age-class of riparian species doesn’t need to be evaluated.
Related Pages
Riparian Health - Evaluating the Health of Riparian Areas - An Overview
Riparian Health - Understanding the Function of Floodplains
Riparian Health - Understanding the Role of Beavers in Riparian Areas
Riparian Health - Understanding If the Channel Is in Balance With the Landscape
Riparian Health - Riparian Areas and Water Storage
Riparian Health - Understanding How Uplands Contribute to Riparian Health
Riparian Health - Understanding Species Diversity
Riparian Health - Understanding the Relationship Between Vegetation and Soil Moisture Characteristics
Riparian Health - Understanding Root Masses and Bank Stability
Riparian Health - Understanding Plant Vigor
Riparian Health - Understanding If You Have Adequate Vegetation
Riparian Health - Understanding the Role of Large Woody Material in Riparian Areas
Riparian Health - Understanding a Channels Ability to Dissipate Energy
Riparian Health - Point Bars
Riparian Health - Understanding Lateral Stability in Riparian Areas
Riparian Health - Understanding Vertical Stability in Riparian Areas
Riparian Health - Understanding if the Channel Is in Balance with the Soil and Water Being Supplied
References
Surber, G., B. Ehrhart. 1998. Stream and Riparian Areas Management: A Home Study Course for Managers. Montana State Extension Service. Information also available at http://www.animalrangeextension.montana.edu/riparianmgt/index.htm
USDI Bureau of Land Management. 1998. Riparian Area Management: A User Guide to Assessing Proper Functioning Condition and the Supporting Science for Lotic Areas. Technical Reference TR 1737-15. 124 pp. More Information available at: http://www.blm.gov/or/programs/nrst/index.php
Winward, A. H. 2000. Monitoring the vegetation resources in riparian areas. Gen. Tech. Rep. RMRSGTR-47. Ogden, UT: U.S. For. Serv., Rocky Mountain Res, Sta. 49 pp. available at http://www.or.blm.gov/nrst/Tech_References/tech_references.htm.