Abstract
AbstractIn 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.
Highlights
A major challenge for ecologists is to study and forecast the dynamics of long-lived species, the assemblages they form, and the ecosystems they inhabit
Our models address three key questions: 1) what is the effect of adelgid-induced mortality on the 200 years on structure and species composition of New England (USA) forests; (2) how different will these impacts be if partial resistance to the adelgid emerges in the several decades; and (3) what are the interactions between adelgid-induced mortality, pre-emptive salvage logging of eastern hemlock by people, and background (“natural”) rates of hemlock mortality?
There are gaps in our understanding about how forests will respond to the decline of particular tree species
Summary
A major challenge for ecologists is to study and forecast the dynamics of long-lived species, the assemblages they form, and the ecosystems they inhabit. Subsequent experiments with model parameters inform research about, and suggest management strategies for, the emerging forests of the future (e.g., Cyr et al 2009, Fontes et al 2010). Such studies increasingly are of critical importance because many forests around the world are in decline (Butchart et al 2010) and are changing rapidly (Choat et al 2012, Lindenmayer et al 2012). Of particular note is the rapid decline and disappearance of the forest foundation species (sensu Ellison et al 2005), eastern hemlock (Tsuga canadensis (L.)
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