Concept · Ch 15
Plant partnerships (the deals for poor ground)
On poor soil no plant truly stands alone: it survives by striking deals — feeding fungi for nutrients, mining phosphorus with cluster roots, digesting insects for nitrogen, renting nitrogen-fixing bacteria. The sheer variety of those strategies is what underpins the variety of species.
First, meet: The nitrogen–phosphorus asymmetryWhy the poorest ground grows the richest flora
A tree looks like the most self-sufficient thing alive. It stands in one spot, makes its own food from sunlight and air, and seems to ask the world for nothing but a place to root. Look underground, though, and that independence turns out to be a fiction. A plant is less a solitary individual than the hub of a web of deals — trading sugar with fungi, renting bacteria to pull nitrogen from the air, mining the soil by chemistry, and, in a few cases, eating animals outright. None of it is free, and the poorer the ground, the more partners it takes to get by.
That is why the starved sands of this coast run some of the busiest underground economies anywhere. Nearly every plant here feeds soil fungi in return for water and nutrients its own roots could never reach. The banksias and their relatives skip the fungi and grow brushes of “cluster roots” to prise phosphorus out of the sand directly. The sundews and bladderworts, short of nitrogen, trap and digest insects for it. And the wattles, the peas, the she-oaks and the cycads all rent nitrogen-fixing microbes, housing them in nodules or specialised roots and paying them in food and shelter. Self-sufficiency, it turns out, is a luxury of rich soil; poverty runs on cooperation.
The deep point is that all this variety of strategy is the engine of the wallum’s variety of species. Where soil is desperately poor, there is no single best way to make a living, so many different ways can coexist, and each way supports its own set of specialists. The clearest proof that the number of these strategies climbs as phosphorus runs down comes from Jurien Bay in Western Australia (Zemunik et al. 2015) — the principle, not a local measurement — and it is the mechanism beneath one of the coast’s great paradoxes: the poorest ground grows the richest flora, because the poverty is what forces the plants to become so inventive.
In depth
A plant looks self-sufficient — rooted in one spot, spinning its own food from sunlight and air. Underground the independence turns out to be a fiction, and the poorer the ground, the more partners it takes to survive, which is why the starved sands of this coast run some of the busiest underground economies anywhere. Four strategies do most of the work, and a wallum plant will often run more than one.
Mycorrhizal fungi. Nearly every plant here trades with soil fungi: the fungus threads its filaments far beyond the reach of the plant's own roots and harvests scarce water and nutrients, and the plant pays in sugar it has captured from sunlight. The deal matters most where soil is poorest — on the rich basalt a plant can be casual about its fungal partners; on bleached wallum sand they are the difference between life and death. Local work found the wallum's scribbly gum using its fungal partners to capture nitrogen far more efficiently than its fertile-country relatives (Schmidt et al. 2006).
Cluster roots. The Proteaceae — banksias, grevilleas, hakeas — largely forgo the fungal route and instead grow dense brushes of "cluster roots" that exude carboxylates to prise sorbed phosphorus loose by chemistry. Do-it-yourself mining rather than outsourced plumbing, and it works best on the very poorest ground of all.
Carnivory. Where the sand cannot supply nitrogen, some plants take it from animals: the sundews (Drosera) trap insects in glue and digest them, and the bladderworts (Utricularia) suck water-fleas into spring-loaded underwater traps.
Nitrogen fixation. A great many plants rent the one trick no plant can do itself — pulling inert nitrogen straight out of the air. The wattles and native peas house rhizobial bacteria in root nodules; the she-oaks host actinorhizal Frankia; the cycads run cyanobacteria in specialised coralloid roots. The plant feeds and shelters the microbe; the microbe pays rent in nitrogen.
The load-bearing idea is that this strategy diversity underpins species diversity: as available phosphorus falls, the number of distinct nutrient-acquisition strategies rises, and the many ways of scratching a living let many species coexist. The strongest demonstration is Zemunik et al. (2015), on the Jurien Bay chronosequence in Western Australia — a principle, not a Cooloola measurement; for the Sunshine Coast the link is held at the regional level. Higher resolution never means looser facts.
Primary sources & further reading — the doorway
- Zemunik, G., Turner, B.L., Lambers, H. & Laliberté, E. (2015). Diversity of plant nutrient-acquisition strategies increases during long-term ecosystem development. Nature Plants 1: 15050. (Jurien Bay chronosequence, WA — the principle of rising strategy diversity; NOT a Cooloola study. Earlier draft mis-cited as "Laliberté et al., Cooloola".) — Nutrient-acquisition strategy diversity rises as phosphorus declines — the mechanism (Jurien Bay, WA; NOT Cooloola).
- Schmidt, S., Handley, L.L. & Sangtiean, T. (2006). Ectomycorrhizal N use, E. racemosa vs E. grandis. Functional Plant Biology. [PubMed] — Ectomycorrhizal nitrogen use in the wallum scribbly gum vs a fertile-country relative — the fungal deal on poor sand.