Caring for and Healing the Earth

Alien Plants


The effect of alien plant species on native plant richness and community composition in urban mid-age Sugar Maple (Acer saccharum) dominated forests in London, Ontario

Page 6 of 9

Effect of Disturbance

Following the PCA, which indicated the importance of disturbance in the amount of Garlic Mustard at a site, the disturbance index was tested against its effect on alien and native plants. Total alien cover was tested against the disturbance of the site (Fig. 25). There was a positive and significant (p=0.012, R2=0.56) relationship between these factors. Disturbance was then tested against total native cover % (Fig. 26). This relationship was also significant at p<0.01 and negative. A relatively large amount of variation was accounted for (R2=0.71).Values in this relationship should be treated with caution because disturbance was measured as an index, but the trend is clear. Mean quadrat MCC was also negatively correlated with the disturbance index (p<0.05, R2=0.50).


Figure 25. Relationship between disturbance index and mean quadrat alien species cover %.


Figure 26. Relationship between disturbance index and mean quadrat native species cover %.

Additional Factors

    As a result of the PCA, several other important variables were tested. Total quadrat native cover % was tested against litter cover % (Fig. 27). The relationship was significant (p<0.01) and positive, but with a very weak R2 (0.04). Total Weediness Score and litter cover were significant (Fig. 28, p<0.01) but again, the relationship was weak (R2=0.054). Mean litter cover was negatively correlated (p<0.01) with ratio of alien/native species at each site (Fig. 29). This relationship was also quite strong, with 60% of the variance explained. Mean litter cover was tested against mean quadrat total native cover (Fig. 30). The relationship was positive and significant (p<0.05, R2=0.49).


Figure 27. Relationship between quadrat litter cover % and total quadrat native cover %.


Figure 28. Relationship between quadrat litter cover % and total quadrat weediness score.


Figure 29. Relationship between site mean litter cover % and total site alien : native richness ratio.


Figure 30. Relationship between site mean litter cover % and mean total site native cover %.

    Total alien cover was plotted against tree basal area (Fig. 31). This was positive, significant (p<0.05) and relatively strong (R2=0.45).

Figure 31. Relationship between site tree basal area and mean total quadrat alien cover %.


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