Adam Weiler, Indiana University, discusses his article: Seeing the forest for all the trees: Mycorrhizal-associated nutrient economies are modulated by stem density and the synchrony between overstorey and understorey tree communities
When we think about how forests influence the soil beneath them, it’s easy to picture the towering overstorey trees that define a woodland. These heavyweights dominate forest biomass and have long been considered the main drivers of soil chemistry and nutrient cycling. But what about the trees quietly growing below them? Could they also play a role in shaping forest soils? This was the question at the heart of our study in southern Indiana, where we set out to test whether dense understories can influence plant-soil relationships just as much as their towering neighbours.
The MANE framework
We approached our study from the lens of the Mycorrhizal Associated Nutrient Economy (MANE) framework, which predicts that tree-fungal partnerships don’t just help trees acquire nutrients, but also create and reinforce lasting signatures in the soil that give rise to distinct biogeochemical syndromes. Arbuscular mycorrhizal (AM) trees (like maples, elms, and ashes) are typically found in soils with certain characteristics: moderate acidity, fast nitrogen cycling, and low carbon to nitrogen (C:N) ratios. In contrast, ectomycorrhizal (ECM) trees (like oaks, hickories, and beeches) tend to occupy soils that are strongly acidic, cycle nitrogen slowly, and have high C:N ratios. Importantly, both tree types co-occur in temperate regions, which means that most temperate forests contain a mix of AM and ECM trees in varying proportions.
Over the past decade, a number of studies have found evidence supporting the MANE hypothesis. In these studies, tree dominance is commonly estimated by measuring the diameter at breast height (DBH) of individual trees and calculating the proportion of basal area in a stand that is associated with either AM or ECM trees. While this approach is effective at capturing large-scale trends, it emphasizes the effects that large overstorey trees have on soil while minimizing the multiplicative effects that smaller understorey trees may contribute. Given that forests are more than just their largest members, we sought to test what hidden patterns were driven by this overlooked cohort.
Beyond the overstorey: Density and mismatch
Our study site, a 25-hectare forest plot in southern Indiana named Lilly Dickey Woods, contains more than 29,000 trees larger than 1 cm DBH, with census data across 35 species. We combined this comprehensive dataset with detailed soil sampling to ask: how do forest density and understorey composition alter the expected MANE patterns?
We had two main hypotheses:
- The “Zinke effect”: Named after early work by Paul Zinke showing that soil characteristics are highly correlated with distance from plant stems, this hypothesis suggests that in very dense stands, where trees are packed tightly together, the cumulative influence of many individuals should strengthen plant-soil relationships.
- The “trait divergence effect”: If the understorey and overstorey are mismatched, for example AM understorey trees beneath ECM overstorey trees, we expected the conventional MANE patterns to weaken, due to the competing feedbacks from diverging ecological strategies.
Rethinking forest–soil models
Upon investigation, both hypotheses held true. In high-density stands, the relationships between mycorrhizal dominance calculated using both basal area and stem count dominance and soil variables like pH, nitrification, and C:N were stronger. In other words, the more trees per area, the more clearly the soils reflected the dominant mycorrhizal type. In addition,where understories and overstories were out of sync – for example, AM understorey trees beneath ECM overstorey trees – the expected MANE relationship for pH became weaker. These findings highlight that soils are not only shaped by large, overstorey trees, but also by the density and composition of the smaller, often-overlooked understorey trees.
Armed with these insights, we created new metrics that combined both stem density and canopy mismatch. These weighted indices outperformed traditional basal area-based metrics when predicting soil characteristics like pH and C:N ratios. This is exciting because it suggests that we can improve our ability to predict how forests will respond to species gains and losses by looking not only at who’s dominant in the overstorey, but also at who may eventually replace that overstorey.
Why it matters
Tree mycorrhizal dominance is shifting in many temperate forests because of climate change and altered disturbance regimes. In the midwestern US, where this study was carried out, the shifts begin with saplings: dense waves of AM trees recruiting into the understorey, while ECM trees decline in regeneration. If these small trees are already altering soils, amplifying or even counteracting the signals of the overstorey, then the consequences of these compositional shifts may show up in the soil much sooner than expected. By tracking tree density and understorey composition, we gain a clearer picture of how forest changes today will ripple through nutrient cycles tomorrow.
So, the next time you walk through a forest, don’t just look up at the overstorey giants. Look down and around at the dense thickets of saplings: maples waiting in the wings, beeches persisting in the shade, and pawpaws filling the gaps. Our study shows that these smaller trees aren’t just waiting on the periphery of ecosystems, they are already busy reshaping the soil beneath our feet.
