The Incidence of the Asiatic Waterpepper (Polygonum cespitosum) in the Streambed of Foundry Branch in Glover-Archbold Park, Washington, D.C.

Abstract

The alien, invasive Asiatic Waterpepper grows along paths and in the stream bed of Glover-Archbold Park, Washington, D.C. The number of plants did not different significantly between upstream and downstream sites.

Introduction

Glover-Archbold Park (GAP) is one of the "finger parks" of Rock Creek Park, Washington, D.C. This Park is only 183 acres and very stressed (Barrows, 2003). It is essentially surrounded by buildings and houses around most of its periphery. However, it still contains many native as well as alien, invasive species, in total about 500 vascular-plant species. Glover-Archbold Park is primarily a dense forest. Some of its open areas are where Foundry Branch is wide.

Foundry Branch has changed very much over the years. Many years ago, it likely flowed throughout the year. Currently, its water level is highly dependent upon precipitation levels. During dry periods, the stream can be almost entirely empty of water, and during wet periods, the stream can be overflowing. The stream also contains varying physical conditions along its length. It can be very wide, narrow, rocky, sandy, muddy, steep, flat, or a combination of these conditions. These changing variables in the stream likely account for the many different types of organisms which live in the water and along the Branch’s banks. Asiatic Waterpepper is the most abundant vascular plant that grew in the streambed in 2003.

Invasive species are becoming a major problem in parks across the world. Their aggressive natures enables them to outcompete many native species in invaded areas. In their natural environments, invasive plant species have constraints, such as herbivores, which keep their populations in check. However, when these plants invade other environments, their important checks are absent, and they can colonize unrestrained. Eventually these unwelcome visitor plants may affect an entire community; even driving away the native animals that live there and further threatening plants and animals already listed by the U.S. Government as endangered species. A patch of one plant species is called a monoculture, which can be very unhealthy in natural ecosystems. Natural areas in national parks like Glover-Archbold require balance and diversity to thrive. Besides the Asiatic Waterpepper, other invasive species such as bamboo, English Ivy, Garlic Mustard, and Lesser Celandine are also taking over the park.

The Asiatic Waterpepper, Polygonum cespitosum, is in the family Polygonaceae which includes the knotweeds. This species’ stems vary from being erect through reclining, can grow up to 40 cm tall, and turn reddish with age. Its leaves are alternate and sparsely pubescent. The Asiatic Waterpepper can be distinguished by its pinkish through purplish influorescences that are at the tips of stems. It flowers from May through October and thrives in moist ground, disturbed sites, waste ground, ditches, roadsides, and along railroads. Although Asiatic Waterpeppers are mainly found in the GAP’s streambed and along its main footpath.

Materials and Methods

My main objective was to test the null hypothesis that the number of Asiatic Waterpeppers upstream is the same as the number downstream. The starting point for my count was about 500 m upstream from Reservoir Road at a dam, the 0-m point. From here, I walked downstream along the path, and every 10 m I went into the stream. There, I placed a transect perpendicular to the stream bed and then counted the total number of Asiatic Waterpeppers within 50 cm on each side of the transect. I followed this procedure for 280 m to collect information at 29 sampling sites. At each point where I collected data, I described the stream, noting general physical characteristics. From these data, I compiled a table as well as several graphs. These graphs looked at the incidence of Asiatic Waterpeppers at each point as well as compared the average number of Asiatic Waterpeppers upstream to the average number of Asiatic Waterpeppers downstream. Sampling site 14 was the point between upstream and downstream. Fom 0 through 140 m was designated upstream and 150 through 280 m was downstream.

Results and Discussion

The number of Asiatic Waterpeppers found in the streambed of Foundry Branch varied widely at each collection point. Their number ranged from a minimum of 0 plants found through a maximum of 58 plants found at one collection point. The total number of plants that I counted at upstream sites was 127 and the total number at downstream sites was 108. The average number of Asiatic Waterpeppers found at upstream sites was 8.47, and the average of downstream sites was 7.71. These numbers similar and do not produce a statistically different result, therefore, the null hypothesis is not rejected. However, the average number was higher for upstream rather than downstream locations.

In general, it seemed that the most Asiatic Waterpeppers were found in areas where the stream was the widest, or where there were disturbances. A disturbances were fallen trees and sand, because the sand bars change with strength of the water flow. It is reasonable that the numbers of Asiatic Waterpeppers are higher where there were disturbances, because they are successful invasive species, and these disturbances create patches where they can grow. Also, in wider banks, there is just physically more room for the plants to become established.

The fact that the number of Asiatic Waterpeppers isn’t significantly different upstream versus downstream is not really surprising. The physical conditions of the stream change along the whole length of the stream and there is not one area in which a certain physical condition dominates. Therefore, the number of Waterpeppers should be varied along the stream.

My sample at 60 m seems to be an outlier, because I counted 58 Asiatic Waterpeppers at this site. This is a very large number compared to my other samples. If the 60-m sample is excluded, then the largest sample is 25. This result probably occurred partly due to chance. At the 60-m point there was a sandbar which was very wide, and maybe there had been a recent disturbance there that I did not know about. All of these factors could have contributed to the high number of Asiatic Waterpeppers there.

In order to improve the results of this experiment, more samples should have been taken, and also at a shorter interval than 10 m. This would increase the sample size and provide a more realistic estimate of the incidence of the Asiatic Waterpepper in the streambed. I was going to sample more than 280 m of the stream but when I went out to sample in late November, the Asiatic Waterpeppers were gone probably due to their freezing and being washed away by the stream. This experiment should also continue in future years to see the trends in Asiatic Waterpeppers population sizes over several years. The high amount of rain in GAP in 2003 may have produced an unusual sample of this plant.

Acknowledgements

I would like to thank my teacher, Professor Edd Barrows for helping me come up with the idea to study the Asiatic Waterpepper. I would also like to thank Spencer Rusin for also choosing to study the Asiatic Waterpepper as well in order to expand our knowledge of this invasive. Finally, I would like to thank Lawrence "Larz" Kelleher for "watching my back" while working for hours in the woods.

Literature Cited