Most work on the disturbance ecology of North American forests has focused on the tree canopy and woody understory, with little consideration of the herbaceous layer. Understanding how this species-rich and ecologically important layer responds to natural disturbances is needed as a guide for conservation. As a framework for this review, selected natural disturbances common to North American forests are evaluated in terms of a three-axis model of disturbance severity: percentage of tree canopy removed, percentage of understory vegetation removed, and percentage of forest floor and soil removed or disrupted. Eight factors, which control herbaceous-layer response, are linked to the three axes; these factors vary in dominance, depending on disturbance type and severity. The relative importances of different species' regeneration mechanisms also vary along severity gradients. The disturbance-severity model, along with the linked response factors and regeneration mechanisms, provide a framework for evaluating impacts of both natural and anthropogenic disturbances. Long-term studies of herbaceous-layer response to disturbance are needed, particularly mechanistic studies that focus on the relative importance of the eight factors controlling response. The conceptual model proposed herein needs to be tested across the full spectrum of severity levels. Finally, comparisons between natural and anthropogenic disturbances are needed.Key words: disturbance, forests, herbaceous layer, life-history characteristics, succession, understory.

The objectives of this paper are to (1) review existing diversity models, (2) identify principles that explain patterns of plant species diversity, (3) discuss implications for forest management, and (4) identify research needs. Many current theories cast disturbance as the key player in maintaining species diversity by preventing competitive dominance of one or a few species. Equilibrium and nonequilibrium theories alike agree that maximum diversity should occur at intermediate size, frequency, and intensity of disturbance. These models do not adequately predict patterns at all spatial scales or across community types. A mechanistic theory is needed to explain diversity patterns at the patch, stand, and landscape scales, as well as across site quality and successional gradients. Such a theory should be based upon the interaction between species' life‐history characteristics and the nature of disturbance. New research is needed in the following areas: identifying disturbance‐life‐history interactions, particularly with respect to the short‐term and long‐term effects of disturbance; quantifying patch diversity and determining its relationship with species diversity; determining relationships between species diversity and structural and functional diversity; and identifying appropriate standards of comparison for managed stands. Comparative studies in different ecosystem types, such as boreal and tropical forests, should be encouraged to help clarify the relative importance of processes that influence diversity.

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We give explicit differential equations for the dynamics of Hamiltonian systems near relative equilibria. These split the dynamics into motion along the group orbit and motion inside a slice transversal to the group orbit. The form of the differential equations that is inherited from the symplectic structure and symmetry properties of the Hamiltonian system is analysed and the effects of time reversing symmetries are included. The results will be applicable to the stability and bifurcation theories of relative equilibria of Hamiltonian systems.