Plants can interact with their ecosystems in a variety of ways. Facilitation is when these changes increase the survival, growth or reproduction of neighbouring plants. It is most commonly studied in arid and alpine ecosystems, but there is increasing evidence that it is a ubiquitous effect across all ecosystems.
Facilitative effects are important to ecological restoration. When they are properly understood, they can contribute to effective planning of plantings by creating better conditions for the growth of plants.
My study looked for potential facilitative interactions that might improve the growth of two shrub species common in Lower Mainland riparian forests.
In the first part, I assessed the facilitative potential of a shrub layer on planted Twinberries (Lonicera involucrata), while in the second part I looked at how the presence of established Salmonberry (Rubus spectabilis) affected established Snowberry (Symphoricarpos albus) plants.
The results of my study were incorporated into a restoration plan for the Suwa’lkh School in Coquitlam.
My study took place at the Coquitlam River Wildlife Management Area in Coquitlam, B.C. The WMA is in the CWHdm08-10 BEC site series (Coastal Western Hemlock riparian variant). It is dominated by a Black Cottonwood (Populus trichocarpa) canopy and has a well-developed shrub layer of Salmonberry, Twinberry, and Snowberry. Mary Hill Bypass borders it to the north and the Fraser River borders it to the south.
Six areas were selected for their similarity in canopy cover and shrub layer/Salmonberry density. For the first part of the study, two 3 x 3 m plots were cleared. In half of the plots, all shrubby species were removed while the other half was left undisturbed. Four Twinberry seedlings were planted at 1 m spacing in all plots. They were monitored weekly from May-Sept for survival, growth and instances of herbivory and flowering. The second part of the study also used two 3 x 3 m plots per experimental area. In half of these plots, all Salmonberry was removed while the other half were again left undisturbed. Four Snowberry branches were selected in each of these plots and monitored for growth, herbivory, and flowering. In addition, all plots were monitored for soil temperature, moisture and electrical conductivity.
Results: There was essentially no difference between treatments for all measured variables except for twinberry growth. Contrary to the hypothesis, twinberry seedlings grew better in plots where the shrub layer was removed. This result is not statistically significant – the variation in growth between different seedlings was large. This suggests that the success of planted twinberries will not be strongly influenced by the presence of a shrub layer but that they may grow slightly more if planted outside the canopy of existing shrubs. It also suggests that thinning the upper shrub layer will not significantly benefit the lower shrub layer.
It is possible that the Black Cottonwood canopy has a dominant influence on the conditions of the site which has masked or overshadowed any influence the shrub layer is having on the planted twinberry seedlings.
In addition, the relative stress-gradient hypothesis states that species are more likely to experience facilitative effects when they are living at the extreme ends of their environmental tolerances. This normally occurs at the edge of their geographic range. Since the two tested plant species are near the centres of their geographic ranges, they are less likely to experience facilitative effects.
The results of this study will be used in the creation of a restoration plan for the Suwa’lkh School forest. It will be suggested that patches of Twinberry seedlings are most appropriately planted in areas where shrub cover is minimal – especially along stream banks after the removal of invasive species like Himalayan Blackberry.