1. Neighborhood Effects, Disturbance Severity, and Succession
Dr. Han Chen
Office: BB-1009F
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2. Neighbourhood effects
Defined as any process mediated by canopy trees that affects the replacement probability by the same or other species at the time of canopy mortality
As the effects of dominant tree species or groups of species
Positive neighbourhood effects (analogous to feedback effects) are processes that promote self-replacement
Negative effects are processes that deter self-replacement (unless no other species are available)
Neutral effects are processes that neither favour nor disfavour self-replacement

3. Neighbourhood effects
Include seed rain, stump and root sprouting, shading, and alteration of the forest floor physical or nutrient status to favour or disfavour germination and establishment of a given species
overstory–understory effects, which can be positive or negative and operate by influencing the species composition of seedlings and saplings underneath canopy trees, which in turn may translate into influence on the species of the tree(s) that replace a canopy tree when it dies
disturbance-activated effects, operate mainly in forests perpetuated by intense fire where seedlings are mostly killed at the same time as the canopy trees; serotinous seed rain and sprouting as a result

4. Disturbance intensity
Low-severity disturbances kill small parts of either the overstory and/or understory (for example, individual or small-group treefalls)
Moderate-severity disturbances kill most of one forest layer, either the understory with its advanced regeneration and/or seedbank (surface fires) or the overstory (heavy windthrow), while leaving the other layer mostly intact
High-severity disturbances kill both the understory and overstory (for example, intense fires).
Forest clear-cutting represents a disturbance roughly akin in severity to heavy windfall or to canopy-killing fire if harvested sites are burned after cutting

5. Disturbance intensity and frequency
Repeated disturbances with low or moderately intensity within short periods may act as major disturbance
Their effects can be additive
For example, repeated occurrence of FTC within short intervals
Man, R. Z., and J. A. Rice Response of aspen stands to forest tent caterpillar defoliation and subsequent overstory mortality in northeastern Ontario, Canada. Forest Ecology and Management 260:
 Why mature stands and young stands (recently disturbed) respond differently in terms of post-disturbance regeneration, successional trajectories?

7. Cumulative disturbance in forest stands over time and successional state