The growth and survival of introduced Holcus lanatus in grassland patches 

A mown grassland in Wales was chosen as the study site because of its relatively homogeneous structure and absence of large grazers such as sheep and cattle. A closely mown sward is ideal for experimental studies as it is easy to create patches of a well defined size. A large field experiment was carried out and important findings of the study were reported by Arnthorsdottir (1994). Patches of Holcus lanatus were created in the field to examine the effect of invasion by neighbours. The introduced patches of Holcus are referred to here as Holcus stands. This paper adds to an understanding of the effect of the size of a Holcus stand and plant position within it on plant performance and survival.
In early spring, each Holcus stand was planted into disturbance patches from which all vegetation had been cleared. The performance of Holcus in the patches is a strong indicator of the resources available to the plants. Holcus commonly forms dense aggregations in Welsh grasslands and is a dominant species in the mown grassland of the present study.

The major recordings on Holcus in the patches involved measures of the change in the overall density of Holcus as well as detailed recordings of the performance and survival of individual plants in relation to patch parameters. The results reflect the effect of the patch size, position of a plant within the patch and the relative importance of intra- and interspecific interactions of Holcus and its neighbours. Importantly, the experiment allows a comparison of the growth and survival of Holcus to be made at the same density. The Holcus plants were planted into the stands. The effect of excluding neighbours from the Holcus stands was also examined.

The survival of Holcus plants clearly depends on the size of a Holcus stand. While the Holcus plants were originally introduced at the same density, they survived best in the largest Holcus stands. Interestingly, position within a Holcus stand was important for plant survival. In general, the plants grew best in the larger Holcus stands and in the centres where there was less invasion of neighbours.

The production of adventitious roots differed with the type of Holcus stand and position of plants. The greatest differences were between edges of large Holcus stands, with or without invasion. Some of the Holcus plants formed adventitious roots in the smaller Holcus stands and the direction of running tended to be into the Holcus stand. Edge invaders had little effect on the formation of runners by the resident Holcus.

Resources, in particular nutrients, are likely to be enhanced per area and per individual Holcus plant in the larger Holcus stands. This likely explains the enhanced survival and performance of Holcus plants with an increased size of a Holcus stand. Larger Holcus stands probably contain more nutrients per unit area, while moisture may be reduced and temperature fluctuations greater. However, light levels are likely not very limiting in the tightly mown sward of the current study.

Selfthinning occurs in the centres of those Holcus stands where neighbours are excluded. Survival is higher in the larger Holcus stands. The difference between Holcus stands with and without invasion is most pronounced in patch centres. Holcus stands with natural invasion from the edge have a higher survival of plants in the centre. The results indicate an increased intraspecific competition in the centres of Holcus stands, especially were invaders from the surrounding sward were kept out. Selfthinning is likely to be occurring in centres of Holcus stands where fewer individuals survive to grow larger.

By Soffia Arnthorsdottir

Literature cited
Arnthorsdottir, S. 1994. Colonization of experimental patches in mown grassland. Oikos 70: 73-79.

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