The Riparian Zone

Every scientist wants to work at the cutting edge, at the very edge of the new frontiers in his field.  The field of ecology is all about interactions:  Between biotic and abiotic components of an ecosystem, between predators and prey, etc.  One of the most interesting interactions is between ecosystems themselves.  Terms like ‘transition zone’, ‘ecotone’, ‘boundary’, or ‘interfaces’ are often tossed around to describe the areas where one ecosystem meets another.  Sometimes these interfaces are abrupt:  For instance, the ocean and shoreline form a fairly distinct boundary between terrestrial and oceanic ecosystems.  Other interfaces are more subtle, like the transition from arid grasslands to out-right desert in the southwestern U.S.  

 

From the perspective of wildlife conservation, these transition zones are extremely important. Interfaces control the movement of energy and materials between adjacent ecosystems, typically have high biodiversity, and often maintain critical habitat for threatened and endangered species.  As a stream biologist, the most interesting ecotone that I work with on a regular basis is the riparian zone along streams and rivers. 

Riparian comes from the Latin “riparius” which means “of or belonging to the bank of a river.” Riparian vegetation falls into the same category as a lot of other difficult-to-describe ideas:  You know it when you see it.  That’s because definitions of riparian zones tend to rely upon distinctive vegetation (which is often disturbed by humans), or the floodplain (which depends on the topography and precipitation regime), or more vague terminology (“where vegetation may be influenced by elevated water tables or flooding” Naiman and Decamps 1997).  In addition, there are a whole host of sociological concepts of what a ‘common sense’ boundary to the riparian area is, usually related to water quality protection. 

Biologically, there are a whole host of plant adaptations specifically designed to allow certain species to do well in riparian zones.  For instance, many plants produce adventitious roots. These are just roots that grow out of odd places, like the stems or leaves.  This helps riparian plants reproduce vegetatively from branches that are broken during floods and washed downstream.  Trees also tend to simply be more flexible:  Bending instead of breaking to avoid mortality during floods.  Other trees produce seeds that survive better in water or float. 

Another huge problem for riparian vegetation is flooded soils becoming anoxic (losing oxygen). Trees compensate by producing air spaces within their roots (aerenchyma) that can be filled with oxygen from other parts of the tree.  Anoxia also changes the chemical condition of the soil, causing potentially toxic heavy metals to become mobilized (e.g., manganese).  Some plants actually flood the immediate area around the roots with oxygen to oxidize these heavy metals thus immobilizing them or making them less toxic.

Functionally, riparian areas control the movement of materials from the terrestrial habitat to aquatic systems.  A common lament among those concerned with water quality is that the massive loss of riparian vegetation due to human impacts over the last several hundred years has resulted in dramatic increases in turbidity and siltation (e.g., see here and here).  This occurs because riparian vegetation directly intercepts material moving into the stream, and also because as riparian vegetation ages and is added to the stream (for example, as large woody debris) it tends to produce in-stream structure that reduces siltation and turbidity.  

When thinking about riparian corridors of streams, perhaps the most obvious effect on the stream is in the penetration of light.  As anyone who’s ever walked through a dense forest can attest, it can be pretty dark even in the middle of the day.  The lack of light leads to low in-stream productivity.  As a result, many stream ecosystems are very heterotrophic, primarily consuming material that falls into the stream.  That material?  Riparian vegetation!  There are entire groups of aquatic invertebrates who are adapted to chomping on leaves. 

 Because so many people can recognize the importance of riparian zones, they tend to be conserved even in locations where no other habitat preservation is occurring.  After all, nobody wants to be drinking, fishing or recreating in silty, toxic streams.  All those preserved riparian areas often end up being the last network of natural habitat left.  For instance, the expansion of Kansas City into Johnson County over the past decade + has swallowed most areas of large woodlands, fields, and native prairies, but the riparian vegetation along stream networks is in much better shape.  As a result, native species are able to (at least to some degree) move among the remaining patches of good habitat by following these riparian corridors. 

 That doesn’t mean there isn’t still reason to preserve native riparian areas.  Often the riparian corridors that remain are so narrow (10-20 yards wide) that their likely effect on nutrients is minimal or non-existent.  Even more often, these areas are subject to immense amounts of illegal dumping.  Although many programs exist to protect them, there is no reason to believe we are doing enough in most places, as these habitats continue to disappear. 

 

Further reading:

Naiman, R.J. and H. Decamps.  1997.  The ecology of interfaces: Riparian Zones.  Annual Review of Ecology and Systematics  28:621-658.