The vast majority of vascular plants associate with mycorrhizal fungi, exchanging fixed carbon for nutrients and water. In tropical forests, which account for roughly ⅓ of all terrestrial photosynthesis, associations with arbuscular mycorrhizae are nearly ubiquitous, but their regulation by biotic and abiotic factors remains poorly understood. In this context, we grew three species of leguminous tropical tree seedlings in field conditions under three different light levels (understory, canopy gap, gap with shadecloth) and two soil nutrient levels (nitrogen vs. no nitrogen addition) in full factorial design. This was replicated 5 times (across 10 sites) at La Selva Biological Station in northwestern Costa Rica. Seeds of Pentaclethra macroloba, Stryphnodendron microstaccium, and Zygia longifolia were collected from the forest, germinated and grown for 6 weeks in native soil in a shadehouse on site. Seedlings (n=90 per species to start) were then planted in the field in early March 2019 and harvested in July 2019 to measure total plant biomass, root biomass, and percent root colonization by mycorrhizae. Fine roots were stained, mounted on slides and assessed for colonization using the grid-line intersection method. Differences between treatments were assessed using a generalized linear model in R (version 3.5.2).
Results/Conclusions
Mycorrhizal colonization varied with treatment, but was high throughout (experiment-wide mean = 78.8%, N=257 seedlings harvested). Seedlings grown in full light conditions (in canopy gaps without shadecloths) had significantly higher colonization than those in the understory (p=0.002). Seedlings in gaps but under shadecloths experienced intermediate light levels and had intermediate levels of colonization (shadecloth vs. understory p=0.053, shadecloth vs. gap p =0.22). Regardless of light conditions, P. macroloba, the dominant tree species in this ecosystem, had significantly higher root colonization than the other two species (vs. S. microstaccium p=0.0007, vs Z. longifolia p=0.00001), which did not differ from each other (p=0.36). There was no effect of nitrogen fertilization on colonization(p=0.7), nor interaction between the treatments. The high level of colonization found across species, the differences in colonization with light levels, and the differences among species all point to the importance of mycorrhizal colonization for legume trees in this neotropical forest. Since nitrogen (N) fixing legumes exert an outsized influence on neotropical ecosystem properties, especially in forests recovering from disturbance, a possible link between differences in mycorrhizal colonization and N fixation bears additional examination.