Facilitation is the way one plant can positively affect another growing within its vicinity. This can happen through a series of mechanisms such as seed trapping, substrate modification, herbivore protection, etc. We often witness these interactions in high stress environments such as deserts, Mediterranean scrubs and in the alpine.
In the alpine, these dominant plants are cushion plants, which as moss-like perennials that have a mat or almost sponge-like characteristics. These cushions are often keystone species in the systems they inhabit that have been studied extensively on the different ways they interact with neighbouring plants and insects. Typically, the cushions soft composition allows plants to grow within their area, providing benefits such as reduce cold stress, increased soil nutrients and shelter from weather.
What has recently become popular in the literature however is that there can be a cost to this facilitation. While the beneficiary growing within the cushion canopy is getting all of these benefits, this can function as a parasitic relationship for the cushion thereby reducing the fitness or productivity.
Previous studies have tried to examine the effect of beneficiary cover on cushion plant traits. This paper by Schob et al took place in the Spanish Sierra Nevada’s and found that plants growing on cushions negative effect the reproductive output of cushions. Schob and authors examined other cushion plants traits such as SLA (specific leaf area) and leaf dry matter but these were found to not be significant. Thus, it appears that the cost of facilitation typically is relation to the cushion’s fitness.
Our study area was in Pink Mountains in Northern British Columbia. It is a characteristically an arctic tunda environment, although it technically is considered Alpine with the peak being approximately 1500 meters high A.S.L. At the peak the diversity of species is considerably high and heavily dominant by perennials.
My study species is silene acaulis a cushion plant that has been examined previously as a facilitator. Silene is characterized by these bright 5-part purple flowers. During flowering, silene is often covered by these flowers as you can see in this picture. You can also see that there are few plants growing on top of our cushion here. These leafy plants are actually a perennial called Dryas that are growing within the cushion area. My goal was to examine how these plants growing on the backs of the cushion change the cushion traits.
To go about this I need to take a tiered approach to measuring these plant traits. Firstly, I need to measure the landscape level characteristics for all the sites I plan to be working in. *This includes slope inclination, slope exposure, soil heterogeneity, %PAR, etc. This is necessary because sites differed significantly at the plateau of pink mountain both in soil and cushion characteristics. The second tier is to measure the variety of cushion traits. I have a particularly interest in the cushion fitness but I also wanted additional measurements such as cushion penetration, surface area, decadence and number of branches. The last tier is the traits of the beneficiary species to see how they co-vary with cushion traits. This last tier is more the work of future analysis because it is a fair bit of work, but for now I used beneficiary abundance as a proxy. Using this tiered approach is surveyed 140 cushions at nine different sites.
Having these three character matricies can be pretty daunting. Having a vector of site characteristics combined with a matrix of cushion traits which then relates to a matrix of beneficiary traits, I need some form of data reduction technique to make this more manageable. Using principle component analysis I can reduce these matrices, but I also needed to make sure that I’m not including unnecessary variables. With a data-set such as this, not everything I measured is potentially relevant.
RDA1 represents surface area. The bottom right quadrat therefore represents plant species that are most commonly associated with high cushion fitness. You can see that practically every plant species is away from fruit height and density. Bistorta viviparia was found on practically every cushion which is why is positively correlates with fruit ht and density.
Looking at beneficiary coverage and flower height or density we notice that there does not seem to be any obvious trends. From my multivariate analyses it doesn’t appear that anything changes with species composition or beneficiary cover except flower height and flower density. However, when I compare these two estimates of fitness against beneficiary coverage, there is no effect. If anything, I kind of notice a slightly positive relationship between beneficiary coverage and flower height. Perhaps there is a multualistic relationship between these cushion plants and the plants that grow on them.
If you remember from both my PCA and RDA, surface area in each was a strong determinant of cushion traits and community composition. When I compare surface area to species richness and coverage by beneficiaries both are significant. This means that larger cushions are not only home to a greater number of species, but also are generally more covered by beneficiaries. Moreover, when we compare surface area to fruit density, there is a similar positive relationship. This means that we may have a spurious correlation between these beneficiaries and cushion fitness. To determine this we need to fit beneficiaries against fruit density but without the confounding influence of cushion surface area.
Once the surface area effects have been removed there is a negative relationship between beneficiary cover and the fruit density of the cushion. If you have questions about how I removed the effect of surface area, I encourage you to ask afterwards, but the short version is I fit a linear model between surface area and beneficiary without a y-intercept and used the residuals of that model as a proxy for baseline beneficiary effects
So why would you expect plants growing on cushions to have a negative consequence.
Competition for resources. Here is another nurse plant that seems almost engulfed by neighbouring grasses. Having all these plants growing on the same patch of soil means that you are in fierce competition for soil moisture, nutrients and potentially sunlight.
This is even more so in the alpine. These high stress environments are very spatially discrete where a cushion may be the only vascular vegetation for meters. With that being the case, there is high competition simply for land availability. It is then expected that these perennials are going to have to struggle with its neighbours. Alpine is spatially discrete
Filazzola NSTP Presentation 2014
Variation in cushion traits is determined
by feedback effects from beneficiaries
Filazzola, A. & Lortie, C.J.
Filazzola and Lortie 2014. Global Ecology and Biogeography
Cushion fitness is not generally effected by site
The surface area of the cushion is correlated with
both cushion fitness and beneficiary cover
Beneficiary cover is the strongest determinant of
Competition for resources
He et al. 2013. Plant Species Biology
• Seed set/fitness is associated with competitive
strength and availability of soil resources (Cornelissen et al.
• Competition in nutrient-poor systems strongly
determines plant fitness (Aerts 1999)
• Reducing a plants fitness is a form of competition
• Easily measurable response