When a foundation crumbles: forecasting forest dynamics following the decline of the foundation species Tsuga canadensis

Abstract

Abstract. In the forests of northeastern North America, invasive insects and pathogens are causing major declines in some tree species and a subsequent reorganization of associated forest communities. Using observations and experiments to investigate the consequences of such declines are hampered because trees are long-lived. Simulation models can provide a means to forecast possible futures based on different scenarios of tree species decline, death, and removal. Such modeling is particularly urgent for species such as eastern hemlock (Tsuga canadensis), a foundation species in many northeastern forest regions that is declining due to the hemlock woolly adelgid (Adelges tsugae). Here, we used an individual-based forest simulator, SORTIE-ND, to forecast changes in forest communities in Central Massachusetts over the next 200 yr under a range of scenarios: a no-adelgid, status quo scenario; partial resistance of hemlock to the adelgid; adelgid irruption and total hemlock decline over 25 yr, adelgid irruption and salvage logging of hemlock trees; and two scenarios of preemptive logging of hemlock and hemlock/white pine. We applied the model to six study plots comprising a range of initial species mixtures, abundances, and levels of hemlock dominance. Simulations indicated that eastern white pine, and to a lesser extent black birch and American beech, would gain most in relative abundance and basal area following hemlock decline. The relative dominance of these species depended on initial conditions and the amount of hemlock mortality, and their combined effect on neighborhood-scale community dynamics. Simulated outcomes were little different whether hemlock died out gradually due to the adelgid or disappeared rapidly following logging. However, if eastern hemlock were to become partially resistant to the adelgid, hemlock would be able to retain its dominance despite substantial losses of basal area. Our modeling highlights the complexities associated with secondary forest succession due to ongoing hemlock decline and loss. We emphasize the need both for a precautionary approach in deciding between management intervention or simply doing nothing in these declining hemlock forests, and for clear aims and understanding regarding desired community-and ecosystem-level outcomes.

Possible Increases in Flood Frequency Due to the Loss of Eastern Hemlock in the Northeastern United States: Observational Insights and Predicted Impacts

“…Our hypothetical scenario assumes all EH stands within the study catchment are succeeded by the candidate species AB. This assumption is supported by observations of tree and shrub density within the catchment and regional succession simulations (Case et al, 2017). We note that birch is also sometimes identified as the likely successional candidate species of EH in the Northeastern United States (e.g., Brantley et al, 2013).…”

Implementation of the perfect plasticity approximation with biogeochemical compartments in R

“…The tree allometric model and parameters were adapted from previously published research (Canham et al , Case et al ). Tree height is modeled as a non‐linear function of stem diameter as follows: …”

A Forest Model Intercomparison Framework and Application at Two Temperate Forests Along the East Coast of the United States

“…All values were calculated directly from observations. Previously published equations [113] and parameters [114] were used to model crown allometry. Together with inventory data, general biomass equations were used to estimated dry weight mass (kg) for tree stems, branches, leaves, and, fine and coarse roots [115].…”