Soil carbon:nitrogen ratios explain successional trajectories in the mycorrhizal makeup of south-temperate humid forests
Lusk, C. H., I.A. Dickie, X. Hua, K. Allen, R. Godoy, F.I. Piper, J. Read and A. Saldana. 2025.
Abstract
Clarifying the determinants of environmental partitioning between
ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) associations may
inform predictions of the effects of climate change on global vegetation patterns,
nutrient cycling, and carbon storage in forest ecosystems. Nothofagus species
(southern beeches) are the only widespread ECM trees in the humid temperate
forests of the southern hemisphere. Despite long-standing interest in spatial and
temporal partitioning of forest environments between Nothofagus and other tree
species in South America, New Zealand, and Australia, rarely has this research
adopted a mycorrhizal focus. We used two-stage landslide chronosequences to
examine the environmental drivers of successional trajectories of the mycorrhizal
makeup of south-temperate rainforests. We used structural equation modeling
(SEM) to address two hypotheses: (1) soil N:P ratios mediate the relative abilities
of Nothofagus and AM trees to colonize landslide scars, and (2) soil C:N ratios
determine the ability of AM trees to invade the understories of established stands
and potentially replace Nothofagus. Hypothesis (1) was not supported, as mean
annual temperature (MAT) was the only significant predictor of the mycorrhizal
makeup of early-successional stands. Hypothesis (2) was supported, as soil C:N
was the sole significant predictor of the mycorrhizal makeup of understories of
established forests, being negatively correlated with AM tree representation in understories. Soil C:N in beneath established forests was in turn influenced by
MAT, by % Nothofagus overstory dominance in established forests, and by
soil N and N:P ratios beneath early-successional stands. Marked continental
disparities in soil parent materials and typical C:N ratios underlie the
well-documented differences between south-central Chile and Tasmania in
the dependence of Nothofagus regeneration on exogenous disturbance. Our
results leave unanswered questions about the circumstances that favor
Nothofagus capture of disturbed sites but support modeling studies that have
identified soil C:N ratio and inherent site nutrient status as key controls on
AM/ECM dynamics in later successional stages.
Key Words
arbuscular mycorrhiza, ectomycorrhiza, forest dynamics, landscape partitioning, landslide, mass movement, N:P stoichiometry, Nothofagus, primary succession, soil C:N ratio
