Ally O'NEill

The composition, dynamics, and health of forest tree communities are governed by interactions with the abiotic and biotic environment. Fungi are critical biotic interactors that play an increasingly appreciated role in forest tree health, particularly with respect to mycorrhizal and pathogenic fungi. Heart rot fungi, while known to infect a large fraction of the individuals in managed stands, have been considerably understudied in tree community ecology. Heart rot has been predicted to form hotspots in the forest due to crown or bole damage and/or soil moisture gradients and is expected to vary across species due to life‐history differences. To address this knowledge gap, we quantified the incidence, magnitude, and spatial distribution of heart rot in 328 individual trees with diameters greater than or equal to 10 cm across the six most dominant tree species in a mixed broadleaf temperate forest dynamics plot. The results show that 71% of individuals display some degree of heart rot in this natural community. The incidence of heart rot did not significantly vary across species despite their life history strategy differences, but one species had a significantly higher magnitude of heart rot in infected individuals. Lastly, heart rot was spatially clustered across species, but heart rot incidence and magnitude were not related to soil moisture, indicating the importance of crown and bole breakage likely promoted by severe weather. The present study has conducted the first spatially explicit study of heart rot incidence and magnitude in a natural forest tree community. We demonstrate that over two‐thirds of the large trees in the forest studied have some degree of heart rot indicating their widespread, but underappreciated, incidence in tree communities. We demonstrate that heart rot is nonrandomly distributed in this community and that spatial clustering of heart rot in forests is most likely due to hotspots of individual tree damage and not gradients in soil moisture.

1. The composition, dynamics and health of forest tree communities are governed by interactions with the abiotic and biotic environment. Fungi are critical biotic interactors that play and increasingly appreciated role in forest tree health particularly with respect to mycorrhizal and pathogenic fungi. Heart rot fungi, while known infect large fractions of the individuals in managed stands, have been considerably understudied in tree community ecology. Heart rot has been predicted to form hotspots in the forest due to crown or bole damage and/or soil moisture gradients and is expected to vary across species due to life history differences. 2. To address this knowledge gap, we quantified heart rot the incidence, magnitude and spatial distribution of heart rot in 328 individuals with diameters greater than or equal to 10 cm across the six most dominant tree species in a mixed broadleaf temperate forest dynamics plot. 3. The results show that 71% of individuals display some degree of heart rot in this natural community. The incidence of heart rot does not significantly vary across species despite their life history strategy differences, but one species had significantly more heart rot in infected individuals. Lastly, heart rot was spatially clustered across species, but heart rot incidence and magnitude were not related to soil moisture indicating the importance of crown and bole breakage likely promoted by severe weather. Summary The present study conducted the first spatially explicit study of heart rot incidence and magnitude in a natural forest tree community. We demonstrate that over two thirds of every large tree in the forest studied have some degree of heart rote indicating their widespread, but underappreciated, role in tree communities. We demonstrate that heart rot is non-randomly distributed in this community and that spatial clustering of heart rot in forests is most likely due hotspots of individual tree damage and not gradients in soil moisture.

Sphagnum-dominated peatlands store more carbon than all of Earth’s forests, playing a large role in the balance of carbon dioxide. However, these carbon sinks face an uncertain future as the changing climate is likely to cause water stress, potentially reducing Sphagnum productivity and transitioning peatlands to carbon sources. A mesocosm experiment was performed on thirty-two peat cores collected from two peatland landforms: elevated mounds (hummocks) and lower, flat areas of the peatland (hollows). Both rainfall treatments and water tables were manipulated, and CO₂ fluxes were measured. Other studies have observed peat subsiding and tracking the water table downward when experiencing water stress, thought to be a self-preservation technique termed ‘Mire-breathing’. However, we found that hummocks tended to compress inwards, rather than subsiding towards the lowered water table as significantly as hollows. Lower peat height was linearly associated with reduced gross primary production (GPP) in response to lowered water tables, indicating that peat subsidence did not significantly enhance the resistance of GPP to drought. Conversely, Sphagnum peat compression was found to stabilize GPP, indicating that this mechanism of resilience to drought may transmit across the landscape depending on which Sphagnum landform types are dominant. This study draws direct connections between Sphagnum traits and peatland hydrology and carbon cycling.