Nutrient Dynamics in Decomposing Dead Wood in the Context of Wood Eater Requirements: The Ecological Stoichiometry of Saproxylophagous Insects

Abstract

Dead wood is rich in sugars and can serve as an energy source when digested, but it lacks other nutrients, preventing the growth, development, and maturation of saproxylophages (saproxylic organisms that consume dead wood at any stage of decomposition). Split into atoms, sugars only serve as a source of carbon, hydrogen, and oxygen, thereby providing insufficient nutrition for saproxylophages and for their digestive tract symbionts, despite the ability of certain symbionts to assimilate nitrogen directly from the air. Ecological stoichiometry framework was applied to understand how nutritional scarcity shapes saproxylophage-dead wood interactions. Dead wood is 1–3 orders of magnitude inadequate in biologically essential elements (N, P, K, Na, Mg, Zn, and Cu), compared to requirements of its consumers, preventing the production of necessary organic compounds, thus limiting saproxylophages’ growth, development, and maintenance. However, the wood is nutritionally unstable. During decomposition, concentrations of the biologically essential elements increase promoting saproxylophage development. Three mechanisms contribute to the nutrient dynamics in dead wood: (1) C loss, which increases the concentration of other essential elements, (2) N fixation by prokaryotes, and (3) fungal transport of outside nutrients. Prokaryotic N fixation partially mitigates the limitations on saproxylophages by the scarcity of N, often the most limiting nutrient, but co-limitation by seven elements (N, P, K, Na, Mg, Zn, and Cu) may occur. Fungal transport can shape nutrient dynamics early in wood decay, rearranging extremely scarce nutritional composition of dead wood environment during its initial stage of decomposition and assisting saproxylophage growth and development. This transport considerably alters the relative and total amounts of non-C elements, mitigating also nutritional constraints experienced by saproxylophages inhabiting such nutritionally enriched wood during later stages of decomposition. Additionally, C losses during later decomposition stages may further change non-C element concentrations beyond fungal enrichment. More detailed studies of the short-term nutrient dynamics in dead wood relative to the nutritional requirements of saproxylophages are needed to understand decomposition process and nutrient cycling in ecosystems. These studies should include a wide array of elements that may be limiting for saproxylophages (e.g., P, Na, K, Mg, Zn, and Cu in addition to commonly studied N). Studies on nutrient dynamics in dead wood should discuss obtained data in the context of nutritional needs of saproxylophages. To allow for this, data on multielemental ecological stoichiometry of saproxylophages of various taxa, inhabiting different wood species in various geographical locations, are needed.