Cordonnier, Thomas (2004) Disturbance, diversity and permanence of structures in forest ecosystems. PhD thesis Sciences Forestières, ENGREF 04ENGR0010.

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Abstract

This study deals with the effects of disturbances on population and community structures in order to examine the relevance of implementing sylviculture treatments that may mimic natural disturbances. On the one hand, such a debate requires to deepen our understanding of the connection between patterns and processes. On the other hand, it also requires to develop concepts/tools shared both by ecologist and manger to evaluate the consequences of different ecological/management scenarios involving disturbances.
In the first part, we study the disturbance-diversity relationship in plant communities. We show with simple patch models that such a relationship depends on (i) the nature of competitive interactions (founder effect, competitive hierarchy, indetermination in competitive interaction) (ii) the coexistence mechanisms (niche succession, competition-colonisation trade-offs), (iii) the mechanisms involved in the colonisation process (immigration, negative frequency dependence, recruitment limitation) and (iv) the indices of diversity (species richness, Shannon diversity index). We conclude that, to be more effective, mechanistic hypothesis encompassing all these features should be prefer to the less-informative Intermediate Disturbance Hypothesis. Some research and management guidelines based on patch dynamics are proposed in the context of spruce-fir forests (north French Alps).
In the second part, we recall stability concepts (equilibrium, resistance, resilience, temporal stability, persistence and permanence) usually used to characterise ecological systems experiencing one or several disturbance events. We emphasise the non-equilibrium concept of permanence that focuses on systems behaviour near specified boundaries. With an individual based model of spruce stands dynamics, we tested several damages scenarios and investigated their consequences on the permanence of stability properties (resistance to windstorms (diameters diversity, heights diversity), elasticity (cover of seedlings and saplings)) and protective function indicators (protection rock falls (sum of diameters) and against snow avalanches (maximum gap area)). Our results corroborate that high stability properties and high protective effects are not necessarily related. The main conclusion is that permanence of indicators depends on (i) the spatial configuration of damages (aggregated/random), (ii) the magnitude of damages and (iii) the nature of the hazard. Although not optimal, sylvicultural treatments based on gaps may be more effective to maintain high resistance, high resilience and effective protective function in mountain forests.
As a conclusion, we highlight the idea that both permanence and gap (patch) dynamics may be useful concepts to investigate the relevance of mimicking natural disturbance in managed forests.

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