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.  

Hmmm....

I thought I had set up a post to pop up automatically yesterday. Apparently that wasn't the case. And now I can't find that post. Weird.

So, again, no post this week, but please enjoy this picture of ice frozen over a small pond to tide you over.

Brainstorming?

Saw this today and thought I'd share it.  A pretty awesome brainstorming tool for coming up with presentation ideas.  Good for those of us constantly struggling with ways to present data.

On Sickness

The crack team of researchers, policy experts, and statistical gurus at Analyze Everything (...me), has been suffering through what can best be described as a total loss of immune system success. In fact, that has spread through my whole family. That's why AE has not been updated. I apologize.

I do have stuff on tap for next week, and I'm feeling healthy again, so tune in next week for more of the fantastically interesting stuff you've come to expect from Analyze Everything.

Bryophytes

So the other day I stumbled upon the paper "Roles of bryophytes in stream ecosystems" by the Stream Bryophyte Group in the Journal of the North American Benthological Society (1999). I consider myself interested in anything weird, and yet I'd never really thought about bryophytes in streams.

And why should I? Small streams are often thought of as being primarily heterotrophic (that means more energy is being consumed within the stream than is being produced) because of large inputs from terrestrial systems and low productivity in-stream due to shading from riparian vegetation. What primary production that occurs is usually done by algae attached to rocks or logs. And I use the word usually here, because that's what you're usually going to see in text books and manuscripts on the subject. Vascular plants are just not as important in most streams as they are in terrestrial or even lake ecosystems (so the mantra goes).

There are lots of reasons for this: Shallow, frequently dessicated habitats, storm-related high flow events, and a lot of competition for light from riparian vegetation. The streams in which I have observed lots of vascular plants avoid all those conditions (open canopies, slowly rising and falling water levels). And of course, that is completely beside the point of this post.

You see, bryophytes are not vascular plants. That's really their only distinguishing characteristic. Bryophytes include things like mosses, liverworts, and hornworts. If you walk outside, right now, and look around, you're almost certainly going to see plants. And the plants you see: Probably all vascular. Vascular plants include Christmas trees, your lawn, those pretty flowers that are dead because its winter, the old oak in the backyard. Some comically huge percentage of all terrestrial plants in the world.

Vascular plants are so-named for the enclosed system they use to transport materials around within the plant (xylem and phloem). If you put a gun to my head and asked me to discuss the corresponding system in humans, I would probably point towards the blood vessels, although it isn't a perfect analogy. Regardless, bryophytes don't have that.


Bryophytes are a paraphyletic group. Quick, who remembers what that means? Oh, you, in the back. Very good. As we've mentioned previously in this space, a paraphyletic group is one that does not include all the descendents of a common ancestor. Evolutionarily speaking, the vascular plants emerged from a branch within what we call the bryophytes. Some debate exists about which bryophytes are most closely related to vascular plants, but let's just leave the taxonomy for someone else.

Among the interesting things about bryophytes is that they are gametophyte-structured (I assume there's a word for this, but I can't find it). Some of you may be asking...wha-? Ok, let's talk briefly about sexual reproduction.

Within any sexually reproducing organism there are two types of cells: Diploid and haploid. Diploid cells contain two copies of every gene, and haploid cells contain one.

Let's talk about how this works in humans. An adult human is composed mostly of diploid cells. The cells in your skin, your arms, your brain, virtually every tissue of your body is diploid. These are the cells that form the basis of your body's structure. These diploid cells contain two copies of every gene, one from your mother and one from your father. And you may ask: How did I get those genes?

As it turns out, there is one type of cell in your body that is haploid: Your gametes. In humans, males produce gametes we call sperm, and females produce gametes we call eggs (or ovum). Reproduction occurs when these two haploid cells come together to make a new diploid cell. If everything goes right, that new diploid cell grows up to be a new human.

Now, lets go back to byrophytes. Byrophytes operate in the opposite manner. The visually obvious 'structure' of mosses and liverworts are haploid cells. They only contain one gene copy. The gametes they produce, called antheridium (male) and archegonium (female), are diploid cells, containing two of each gene. The weirdness inheret in this system is fascinating, and could really fill its own post. Sufficed to say: This kind of things keeps biologists up until the wee hours of the morning.

What else should we mention on the subject of bryophytes? In the paper that got me started on this whole avenue of thought, the authors discuss the incredible hurdles that bryophytes face with reproduction in streams. Getting the gametes together is difficult. Free swimming antheridium have to get to the archegonium in order for sexual reproduction to occur, and when you're getting washed away downstream, that's not easy to achieve. Some species produce male and female plants (others have both on the same plant), and as you can imagine, when males and females are further apart they are much less likely to reproduce. You can also add in the problem of vegetative reproduction. Because the plant is much more likely to grow by just spreading from rock to rock vegetatively, in all likelihood any antheridium (male gamete) pushing out to find a female is going to just run into its own clone. As the authors of that paper point out:

"Given the obstacles to successful fertilization of bryophytes in streams, it is interesting that it can occur at all." - p 153

And indeed, they go on to document that essentially no one has seen in happen in a stream and its damned hard to cause to happen in a lab under controlled conditions.

I've really only scratched the surface of what is known about bryophytes here. To be honest, humankind has barely scratched the surface of what could be known. According to the same authors, during the 10 year period from 1989 to 1999 less than 1% of all papers indexed in Biological Abstracts addressed bryophyte ecology. Of course, its been almost ten years since this was written, so somebody needs to see if this paper spurred a boost in that research.

Somebody like me?

Paper referenced above is available on JSTOR (not open sourced though):

Stream Bryophyte Group. 1999. Roles of bryophtes in stream ecosystems. Journal of the North American Benthological Society 18:151-184.