Plants aren’t so cooperative after all

One of the underlying tenets of ecology is the principle of competitive exclusion. This principle states that when two species compete for the same vital resource, the better adapted species will ultimately displace its competitor. Simply put, it’s survival of the fittest.

More recently, some ecologists have suggested that nature’s not quite so brutal – that the species composition in an ecosystem is determined more by random fluctuations in population numbers than by direct competition.

But last month, this "neutral theory" was directly challenged by evidence on three continents which compared the abundance of particular tree species, both in the fossil record and in existing forest ecosystems. The similarities were so close among all the comparisons that it’s most likely due to direct competition rather than random fluctuations.

While this information might seem pretty esoteric, it does have direct application to gardens and landscapes. Among your plants, you will have some that compete better for water, nutrients, and other resources. The concept of "companion plantings" as plants actively helping each other survive is a wishful projection on our part.

And this all ties into the discussions we’ve been having about mulch. While living mulches – turf, ground covers, etc. – help protect soil structure and reduce erosion, they also compete with other plants in the landscape. Maintaining landscapes with living mulches will require more water than the same landscape with organic mulches. It doesn’t matter if the plants are native or not – it’s just a question of limiting resources and who’s going to be the most competitive in extracting them.

(Forgot to include the reference the first time I posted this – here it is: Ludwig-Maximilians-Universitaet Muenchen (LMU). “Jostling for position: Competition at the root of diversity in rainforests.” ScienceDaily, 26 Jan. 2012.)

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Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

17 thoughts on “Plants aren’t so cooperative after all”

  1. I couldn’t disagree with you more, Linda. Sedges store the water in their dense root system, thereby making it available to trees and shrubs in times of low water input.
    Here is a quote from Dr. Gerould Wilhelm, director of Conservation Research Institute. (www.conservationresearchinstitute.com) in his paper titled “What is Ecological Restoration?”
    “So, what are the prospects? What must we do to re-establish or restore a sustainable covenant with a living earth? Our culture is inexperienced, but we do know that the root systems of a complex native species can increase the water-holding capacities of our soils by an order of magnitudes in a relatively short period. For example, land in the Conservation Reserve Program, after just a few years, can store a great amount of water that falls on it, and also can accumulate a ton or more of fixed carbon per acre, per year. A clue.
    We could look at our landscapes and ask ourselves, why must it look like an industrial rug punctuated by smallish, perfectly shaped, lollipop trees, ringed at the base by tiny, perfectly engineered saucers of mulch? Would it be so wrong, so unattractive, so heretical, such an anathema to our culture’s aesthetic, to look out upon, and walk within a landscaped inhabited by a profusion of native grasses and sedges, replete with comely perennials, infused with flowering shrubs, and dominated here and there, such as along
    620
    the north and east faces of tall buildings, by groves of trees with futures. Would it be so radical to propose that threes be free to grow branches however their habitats permit, and to grow broad, expansive root systems with a diversely populated rhizosphere rich in water and mycorrhizal fungi? Would we be so unable to countenance clean streams and rivers with healthy base flows that flourish with fish and mussels?

  2. “Living mulches” will volunteer whether the gardener wants them or not.

    When I followed the link in Bert Cregg’s post about Christmas trees and mulch, I noticed there were comparisons for weed controlled and no weed control. Trees with weed control were better off than those without weed control.

    I guess I need to do something about all that red oxalis.

  3. Nice food for thought. I keep bees and have apple trees and blueberry bushes, and I often hear the recommendation from bee people to underplant with flowering ground covers, to provide extra pollen when the trees aren’t blooming, but I always wonder about the detrimental effects of competition. So far I haven’t been able to find any literature on the effects of competition from underplanting blueberries, though I have seen some studies on reduced crawling pests. I’m also not keen on the idea of having to water groundcovers when my blueberry bushes or apple trees don’t need the extra input.

  4. While plants, particularly of different species, are usually competitive, I recently read in one of my tree books about how the roots of the same species can fuse and actually share resources. I think that’s pretty neat.

  5. LMU biologist Professor Susanne Renner and her American colleague Professor Robert Ricklefs have now challenged the theory with the help of quantitative data. In Central and South American, African and Asian rainforests, the two researchers compared the abundance patterns of different tree species growing in plots of between 25 and 55 hectares. In addition, they compared the relative abundance of different families of trees in a 55- to 65-year-old fossil flora from tropical Colombia with their representation there today.

  6. The comment above is from your source, Linda, and it seems to me the theory only applies to trees, not the ecosystem. The more we can replicate diverse ecosystems in our yards and gardens, the healthier our plants will be.

    “Trees are healthier when under planted with herbaceous plants that draw water down to the roots. Trees grow 1 and ½ times faster than when planted singly in the lawn. (Connor Shaw, owner, Possibility Place Nursery)

  7. Pat, I still don’t understand how those roots that you speak of “draw water down to the roots”. To my knowledge, roots have the opposite function, i.e. to draw water from the soil to the above-ground part of the plant, especially the leaves from which the water evaporates.

  8. 1/3 of the dense root systems of prairie grasses and sedges decompose every year, opening channels for rain water to infiltrate; thereby enriching the soil with organic matter and increasing its water-holding capacity.

  9. Pat, but earthworms would accomplish the same thing, without the evaporation loss that comes from growing a living mulch. I think that’s the part you’re overlooking. Organic dead mulch doesn’t have that problem.

  10. Extrapolating from a study on root competition between trees in a rainforest to trees vs ground cover in a totally different climate and soil seems like exactly the kind of inappropriate generalization that we were discussing in an earlier post.

  11. Competition for limited resources among plants (and other organisms) has been studied for many, many years and is supported by thousands of peer-reviewed publications. This newer study was novel in that it used fossil record information as well as current data to argue against the neutral theory.
    As a more practical analogy, we can consider the studies of trees in turf vs. trees without turf (and there are several). Tree growth is negatively affected when grown under turf compared to those grown under no turf – or even better – under mulched conditions.

  12. Linda, that’s certainly what I’ve witnessed in my own backyard. My trees really benefited from having the lawn around them replaced with mulch. Night and day difference in growth.

  13. Linda, the argument was never about trees planted in turf vs. trees mulched with wood chips.: it’s about trees mulched with wood chips vs. trees mulched with living native sedges. The decomposing deep, dense root systems of the sedges have a water-holding capacity that is able to retain much water, thereby keeping the trees hydrated even during times of drought. The decomposing roots also give the soil more tilth by increasing the soil organic matter–always a good thing. Another thing: moisture condenses on the blades of the sedge at night, which then trickles down into the root system.

  14. Pat, sedges are in the grass family, and they have similar root systems. That’s why I used the turf example. Whether it’s turf or sedges, the physiological fact remains that plants take up a lot of water, 90% of which is lost to the atmosphere, and wood chips do not. Your sedges could be much better at retaining soil water than bare soil, but they are not as good as organic mulch.

  15. Here is the abstract from a 1995 article in the Canadian Journal of Forest Research (v. 25(1), p. 69-80). It’s long, so bear with me:
    “The factors determining individual tree growth response are examined during the 4 years following thinning in experiments in even-aged, 8- or 12-year-old regrowth Eucalyptus regnans F. Muell. forest at two sites in southern Australia. At one site, a vigorous understorey dominated by a sedge developed after the thinning. At that site, light-use efficiency by the trees was unaffected by thinning and the aboveground biomass production by the trees in the thinned stand was substantially less than that in the unthinned stand. At the other site, little understorey developed, light-use efficiency by trees in the thinned stand was greater than that in the unthinned stand, and aboveground biomass production was unaffected by thinning even though the leaf weight of the thinned stand was far below that of the unthinned stand. Where the understorey developed, it was concluded that it competed successfully with the trees for water, thereby reducing production in the thinned stand when compared with the unthinned stand. The individual tree growth response that occurred in the thinned stand at that site appeared to be due solely to the extra light avail
    able to individual trees following the canopy opening. Where the understorey did not develop, it was concluded that individual tree growth response was due not only to the extra light available to individual trees but also to the increased availability of belowground resources, most probably soil water. Application of a pre-existing stand growth model suggested that at that site the tendency for increased growth resulting from extra water availability in the thinned stand was just balanced by decreased growth due to lower radiation absorption by the reduced canopy, so that net production was unaffected by thinning.”
    It’s clear from this article, which specifically is looking at sedges and trees in an ecosystem, that sedges compete with trees for water.

  16. Linda, Turf or Kentucky Blue Grass (Poa pratensis) is in the Gramineae, While Sedge (Carex ) is in the cyperaceae. Unlike turf grass, sedges and native grasses have dense, fibrous root systems that hold water. To repeat what I’ve said before that you have taken no notice of, 1/3 of those roots die ever year, decomposing, and adding moisture, CO2, and organic matter to the soil.

    Moisture needs of landscapes, in general,…during the growing season are usually in excess of the amount of rain falling at that time, particularly in continental climates where rainfall is irregular during the growing season. To make this possible, the precipitation that falls must enter the soil in a process called infiltration, and the soil must have a large water-holding capacity that is able to retain much water. Both these requirements require a well aggregated soil (Kohnke and Franzmeier, 1995).

    The only way to sustain a balanced level of soil organic matter in such systems in the North Temperate Zone is for graminoid root systems to pervade the rhizosphere, die constantly, and then partially decompose in accordance with a system’s inherent redox environment…When all this in balance and stable, a soil can be healthy. (Gerould Wilhelm and Laura Rericha, Timberhill Savanna Assessment of Landscape Management pg. 7, April 2007)

    As far as transpiration goes, I would like to quote Dr. Wilhelm again: The green growth of the season from the tufts or bunches of prairie grass, moves cool water from the below-ground rhizosphere up to the leaves for transpiration, which keeps the leaves relatively cool. At night, water from the humid air condenses on these cool surfaces and moves down into the ground, which sustains soil moisture–which both waters the system and keeps the oxidation rate lower than would otherwise be the case. (Gerould Wilhelm, The Realities of CO2:Seeing through the Smog of Rhetoric and Politics)
    This research can be found at this link:
    http://conservationresearchinstitute.org/research.html

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