Soil carbon:nitrogen ratios explain successional trajectories in the mycorrhizal makeup of south-temperate humid forests

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.