Do common shade-tolerant tropical tree and shrub saplings exhibit a partially mycoheterotrophic form of nutrition?

Some chlorophyllous understory species with Paris-type arbuscular mycorrhiza in temperate forests are capable to gain carbon (C) from their mycorrhizal fungi, suggesting that partial mycoheterotrophy – where plants gain C from fungal partners additionally to own photosynthesis – might be globally much more common than previously thought (Giesemann et al., 2020, 2021). Paris-type morphology is considered a requirement for fungus-to-plant C transfer (Imhof, 2009). Tree and shrub saplings in tropical forests are candidates for partial mycoheterotrophy based on mycorrhizal morphotypes (Dickson et al., 2007) and due to their severely light-limited habitat.

Using stable isotope and microscopic techniques, we examined whether common shade-tolerant tropical tree and shrub species are partially mycoheterotrophic as saplings. Own observations of fungal morphology within plant species (n=40) disagreed somewhat with the literature (Dickson et al., 2007), indicating the demand for in situ determination. Enrichment occurring only in few Paris-type target species in the heavy isotopes ¹³C, ²H and ¹⁵N relative to surrounding plants (but detected in mycoheterotrophic Voyria) did not support fungi as a prevalent C source. Yet, differences in stomatal regulation and transpiration of plants probably led to underestimation of a heterotrophic part of nutrition based on ¹³C enrichment and could explain even the depletion in ²H.