Fraser River Studies
Sand Dune Succession Lab
Barnston Island
(Biology 285)

Abstract

In this lab, horizontal and vertical transects were taken of Barston Island ten meters in, perpendicularly from the shore. Inventories of vegetation were taken every meter from the vertical transect and all vegetation crossing the horizontal transect for one meter at every meter. The resulting vegetation densities and canopy cover were then used to determine which succession state Barnston Island was in and discuss the islands future development.

Introduction

As the Fraser River cuts down to base level, the spoils of it’s labour, silt, sand and gravel are largely deposited in Delta. Some, however, is deposited in the form of sand bars along the way forming islands such as Barnston. This process offers us a chance to study a relatively rare ecological process that occurs in a recently formed area where no other organisms existed previously, primary succession (McFadden / Keeton).

In primary succession areas, the pioneer species tend to be adapted for rapid growth, extreme reproductive success and dispersal (McFadden / Keeton). If the area is not prone to catastrophic events (floods, fires or volcanism) the litter from the pioneer plant community will eventually build up creating a habitat that it is no longer successful in and will in fact choke itself out.

The sand bar islands, depending on freshet levels and sand bar elevation flood yearly. These areas are generally in a state of primary succession as litter is unable to build up. We studied one of these areas on Barnston Island to determine if it was in fact an area of primary succession.

Procedure

On July 15,1997 at approximately 9:30 am, using a tape measure, meter stick and flag stakes, we plotted a ten meter transect line running east - west, perpendicular to the eastern shore. Our tape fell on a relatively level section of Barnston Island and started immediately above a cut bank of approximately one meter drop to the river. There was no lower or middle beach here, but the bank erosion and visible root systems suggested there was at one point. We then estimated the canopy cover and took inventory of all vegetation which fell within the plane of the transect. We then took ten horizontal transects, by laying the meter stick across our transect line. We chose a site for our transect that was away from the clearing where we camped, so we could study an area that had as little human disturbance as possible.

Results

Our transect area had a canopy cover of approximately 60 %. The canopy consisted of primarily cottonwoods but also had some willows. We estimate the tallest trees to be 10 meters. Vegetation found along the transect plan was 15 cotton woods, 12 willows, 43 equisetum, and 2 grass. The soil conditions along the entire transect were similar, it was primarily sand with little or no litter on top. The cotton woods and willows both increased in height as our transect moved away from shore. The two grass plants were located in the eighth meter.
The bilateral meter stick transects crossed 15 cottonwoods, 10 willows, 74 equsiteum, 12 grass and 2 hawthorn.

Plant densities were as follows:

-gross density of vegetation per our 10 meter transect was 7.2 plants per meter.
-density of cottonwoods was 1.5 trees per meter
-density of willows was 1.2 per meter
-density of equsiteum was 4.3 per meter
-density of grass was 0.2 per meter

-gross density of our bilateral meter stick transects was 9.8 plants per meter.
-cottonwoods 1.5 per meter
-density of willows 1 per meter
-density of equsiteum 7.4 per meter
-density of grass 1.2 per meter
-density of hawthorn 0.2 per meter

Discussion

The vegetation densities calculated show, over all, that cottonwoods, willows and equsiteum are the most established vegetation here. The dominate species, considering biomass and canopy cover is the cotton wood. Since willows, cotton woods and equisetum fit the criteria of a pioneer species and the soil in this area is largely sand, we can conclude that this is an area of primary succession.

The tallest trees in our transect area were no more than ten years old and were located in from shore. This suggests that these trees were established first, and as more sediment was deposited, other trees took up residence. This could explain the gradual decline in tree height and therefore age, as growing conditions were similar across our transect. Another possibility for the declining tree height towards the shore could be periodic floods washing the trees closer to the edge away.

Our study site did not have a lower beach. It appeared to begin at what could be considered the upper beach as we had perennials in our first meter section from the shore, the roots of which were visible from the erosion of the bank. I suspect a combination of these two processes is probably taking place.

The visible root system on our shore bank which dropped about one meter to the river tells us that they were at one point covered in and stabilizing sand. The beach here would have sloped down from this point forming the middle and lower beaches. The sandy soil that we were taking our transect upon, would have accumulated over the years, moving up the perennials’ trunks, which acted as a stabilizing feature allowing for the build up to the islands present elevation.

In the future I would expect to see very little change at this site. The lack of litter around even the tallest trees suggests regular floods. With this scouring of the island, litter can not build up. This litter build up is necessary to form a more nutrient rich soil, a requirement for secondary succession vegetation to establish themselves.

Since the river is very close to base level at this point, it will not be cutting down enough there by raising the island out of the more regular flood plain. Geologic uplift would be necessary for this to happen. I mention this, since the island did not have many old trees, and suspect it experiences some level of catastrophic flooding on a regular basis. Such an event would allow the island more flood free time, which in turn enables it to move into a secondary successive state.

However, considering weather, time and random chance, the sand may build up enough, raising the island’s center elevation out of the more regular flood plain. This may eventually allow the perennials enough time to develop a layer of litter. If this situation occurred, we might find other vegetation move in such as alder. The site as seen on July 15th however, does not give any evidence that this is occurring. As a matter of fact, the eroded banks may suggest the opposite. It appears that what is happening on Barnston Island is that the cotton woods and willows in the center of the island are getting sufficient time to establish themselves while other, slower growing perennials and younger poplars near the lower beach are washed away by the next big flood. After which, remaining poplars will start to re-populate the flood swept areas. Barnston Island, has in a sense, reached it’s climactic state with pioneer vegetation.

Literature Cited

McFadden / Keeton. Biology an exploration of life. W.W. Norton & Company, New York, London, 1995. pp. A26, 753.