May 18, 2020
Welcome back to the wilding conifer guest blog by PhD student, Joanna Green. As promised, I will now write about my research at the University of Canterbury. If you remember previously, I discussed the concept of adjusting to change. I will now continue this theme but getting a bit more applied.
The more I learn, the more I see connections. This may not be true for everyone, but it is likely the case for many scientists. In fact, my previous employment was for a medical health education program centered around the concept of One Health; a concept in which the health of animals, humans, and the environment are so tightly linked in that changes in one will lead to changes in the others.
Following these connections along to something more noticeably meaningful can also be a significant part of the “so what?” of research. Basically, why should anyone/everyone care? While the benefit of many experiments might seem minimal to the progress of humanity or other commendable goals, there are often unforeseen consequences of knowledge gained in a seemingly obscure result.
This interconnected-ness is at the center of my research. I am looking into the connections between wilding conifer invaders and plant communities. This occurs not only in one direction, and there is evidence of a feedback loop in many plant invaders and other invasive plants1. In my thesis I refer to this as invasion/reinvasion, as seen in the lower portion of my thesis model image below.
This concept is one third of my 3-part design. The top portion, more commonly studied and potentially more intuitive, is the relationship between wilding conifers and the plants that appear post-wilding-removal. This involves testing the wilding and associated “legacy” left behind in the soil post-invasion. The final section of my thesis, and for me the most difficult, involves the mechanisms underlining the other two parts. This research is still a moving target, and hopefully a source for a blog post sometime in the future.
For me, this is the big question…what is it about wildings which causes the plant community to respond the way it does? For my work I’ll be considering the changes in biota and chemical/nutrient modifications within soil that follow a wilding invasion.
My preliminary work shows soil that previously had wilding conifers promotes growth in nearly all plants, but particularly so for other wildings and many other invasive plants such as scotch broom. This addresses the lower arrow (invasion/reinvasion). For my research I was lucky enough to capitalize on the end of a large-scaled 2-year experiment by the Bio-Protection Research Centre at Lincoln University. It involved 180 large pots all started with the same initial steam-sterilized soil then planted with various different plant communities. This provided a known previous plant legacy to test a variety of characteristics within the communities. The groupings most relevant to me were presence/absence of wildings, and proportion exotic/native within each community.
The background shows the experiment from the Bio-Protection Research Centre (notice the person in middle-right for scale), photo by Warwick Allen. Samples from my experiment in colored boxes: Two paired pine seedlings and broom seedlings chosen for comparison, Yellow outlines from no pine communities dominated by native plants, Green outlines refer to communities which include wildings and generally dominated with exotics.
Pine in particular has been shown to increase availability of many nutrients, such as Phosphorus2, so an increase in a plant’s growth isn’t entirely unexpected when grown in pine soil.
Yet, my research aim is in plant communities and not solitary plants. You don’t see plants living alone, so I thought it might be worthwhile to test several plants grown together. And that is where the known soil changes associated with wilding conifers (different organisms/biota and nutrient modifications)1,2 can skew in favor of invasive plants. When grown alone, most plants benefit from past presence of wilding conifers. The dynamics change when plants compete as they would in the wild.
To test this, I collected soil from 15 points along a wilding invasion. This refers to the top arrow in my research model (wilding legacies). The sampling path included all different levels of invasion from grasslands to dense wilding forest, all from just Southeast of Mount Barker near the Acheron River.
The plants I grew in the soil were evenly split between native and exotic, and all can be found within the area where the soil was collected. I planted these in a community with all plants included and in isolation (each plant alone) for comparison.
Soil collected from regions with high densities of wilding conifers yielded different plant community growth than soil from nearby regions where the wildings had not yet established. The non-native plants in my test community were comparatively larger when grown in wilding soil than in nearby grassland soil. While the native plants did well in isolation they did not thrive in competition, particularly when in wilding soil.
I also did some testing with germination and seedling growth of native and exotic grasses (usually the first group of plants to move in post-wilding). I used seeds from the same grasses used in the previous experiment, so again those who grow in the region where soil was collected. There was a dramatic difference in growth and survival from uninvaded to even slightly invaded (at less than 1% cover; equivalent to less than 2x2m of a 20x20m plot). This seems to indicate the tipping point or threshold for a measurable change in plant community response is very low.
What does this all mean? This means that wilding conifers affect plant communities even at very low levels, and while most plants seem to benefit from growing in soil that previously had wildings, the benefit has been greater for non-native and often invasive plants.
While I am still analyzing and planning my next experiment, results so far could be significant to wilding conifer management practices. There are many factors involved in management, but a crucial element is when to control wildings and when to restore a site.
Hopefully my research can be of use in this arena. Some depends on the preferred end goal…if it is complete restoration to native grasslands, then it might be that even a very small wilding presence is too much, and that any seed source of exotics needs to be controlled as well. Maybe adapting to a new normal or gradual terraforming by adding natives seeds and/or seedlings while removing invasive plants may reduce exotic competitive advantage and early germination success.
Regardless of anything else wilding conifers are a source of change which we are still trying to understand as well as connect to the bigger picture.
1 Richardson, D. M., P. Williams, and R. J. Hobbs. 1994. Pine invasions in the Southern Hemisphere: determinants of spread and invadability. Journal of biogeography:511-527.
Simberloff, D., and B. Von Holle. 1999. Positive interactions of nonindigenous species: invasional meltdown? Biological invasions 1:21-32.
Van der Putten, W. H., R. D. Bardgett, J. D. Bever, T. M. Bezemer, B. B. Casper, T. Fukami, P. Kardol, J. N. Klironomos, A. Kulmatiski, and J. A. Schweitzer. 2013. Plant–soil feedbacks: the past, the present and future challenges. Journal of Ecology 101:265-276.
2 Chen, C., et al. (2002). "Phosphorus dynamics in the rhizosphere of perennial ryegrass (Lolium perenne L.) and radiata pine (Pinus radiata D. Don.)." Soil Biology and Biochemistry 34(4): 487-499.
Dickie, I. A., M. G. St John, G. W. Yeates, C. W. Morse, K. I. Bonner, K. Orwin, and D. A. Peltzer. 2014. Belowground legacies of Pinus contorta invasion and removal result in multiple mechanisms of invasional meltdown. AoB plants 6.