Botany, a load of rubbish?

Caught your attention? No, I’m not about to rubbish Botany, but this item does focus on a particular type of biological ‘rubbish’, leaf-litter (PDF).

For all plants getting a sufficiency of essential nutrients is important if they are to grow and develop properly and to their full potential. For the majority of terrestrial plants these nutrients are obtained from the soil via their roots. In turn much of that soil-based inorganic chemical treasure trove is generated by the joint action [no pun intended…] of invertebrates, bacteria and fungi – in the process known as decomposition – upon plant and animal material (the discarded, jettisoned and abandoned detritus of life that includes leaf litterl) that accumulates atop and within the soil.

But what if you are a plant that’s not rooted in the soil – e.g., an epiphyte perched high above the soil on tree trunks – how can it get sufficient nutrients? Or, what about an enterprising plant that sees an opportunity to boost its acquisition of nutrients by engineering a situation that actively accumulates dead organic matter [‘litter’] in its immediate neighbourhood? Well, evolution, and plenty of time over which it can act, has enabled some epiphytes to boost nutrient acquisition in the absence of soil-penetrating and -probing roots, and other – soil-rooted – plants to get more growth-enhancing inorganics than they ordinarily would. Welcome to the wonderful world of the ‘litter-trappers’, plants which literally trap and accumulate litter (which would otherwise go elsewhere and potentially fuel growth of competitor plants…*).

These consummate ‘litterati’ therefore get enhanced access to nutrients as that material is decomposed and broken down. Insights into the frequently fantastical features that facilitate this fascinating phytological biology are reviewed in-depth by Scott Zona and Maarten Christenhusz. Most of us are probably not that familiar with such litter-trapping plants because they are largely confined to the tropics. However, it is a ‘lifestyle’ that is found in many plant families (e.g. Araceae, Bromeliaceae, Nepenthaceae, Primulaceae, and Rubiaceae and involves structural adaptation such as rosettes of leaves, modified leaves, and ‘root baskets’ (upward-growing roots – which raises questions about geotropism in those organs…). Not only is the litter** – which can be of plant and/or animal origin – a source of nutrients for the trapper, it can also act as food and/or housing for commensal organisms, which in their turn can be prey for other animals. Each individual litter-trapping plant is thus an ecosystem in miniature. As the authors conclude, despite their unique mode of life, litter-trapping plants are not well documented, and many questions remain about their distribution, physiology and evolution.

I’m not aware of any suggestion that such plants can exploit decomposition-derived energy-rich organic compounds – other than the inorganic nutrient value of such material. But, this is a possibility that deserves to be investigated and would give such plants an additional – albeit, and uncharacteristically, heterotrophic – boost to their more usual autotrophic mode of nutrition and energy and carbon acquisition via photosynthesis. If demonstrated, this would be another trophic advantage of this ‘nutriokleptic’ phenomenon to plants in light-poor environments, such as the understorey of a tropical rainforest. Furthermore, the trapped litter mass may also hold water which could supplement the plant’s water requirements from other more usual sources.

[Ed. – it would be interesting to see how litter sources (in respect of both their taxonomical status and geographical origins) might impact upon the rates of decomposition of this material within the spatially and taxonomically different litter-trapping plants, and how this might relate to the so-called home field advantage in leaf-litter decomposition explored by Mark Davidson et al.]

* And, if the litter-gatherer doesn’t absolutely need the nutrients in the detritus, by denying them to others, is this an example of amensalism…?

** Writing about this intriguing mode of nutrition is one of the few opportunities I’ve had to legitimately use the ecological term allochthonous, which refers to material – e.g. leaf litter – originating in a place other than where it is found – since “trapped litter consists not of shed leaves of the litter-trappers, but of leaves (and other organs) shed from other plants” (p. 555).