Abstract
Forest ecosystem dynamics are driven by a complex array of simultaneous cause-and-effect relationships. Understanding this complex web requires specialized analytical techniques such as Structural Equation Modeling (SEM). The SEM framework and implementation steps are outlined in this study, and we then demonstrate the technique by application to overstory-understory relationships in mature Douglas-fir forests in the northwestern USA. A SEM model was formulated with (1) a path model representing the effects of successively higher layers of vegetation on late-seral herbs through processes such as light attenuation and (2) a measurement model accounting for measurement errors. The fitted SEM model suggested a direct negative effect of light attenuation on late-seral herbs cover but a direct positive effect of northern aspect. Moreover, many processes have indirect effects mediated through midstory vegetation. SEM is recommended as a forest management tool for designing silvicultural treatments and systems for attaining complex arrays of management objectives.
Highlights
Ecosystem processes involve complex interactions of many cause-and-effect relationships [1]
The indirect effect was estimated as the product of the direct effects of overstory cover on understory cover and understory cover on late-seral herb cover or approximately −0.09
The aim of this paper was to provide a conceptual framework for applying Structural Equation Modeling (SEM) to understand mechanisms influencing one component of forest structure
Summary
Ecosystem processes involve complex interactions of many cause-and-effect relationships [1]. Structural diversity is widely accepted as an important driver of biodiversity despite the fact that many mechanisms linking the two are still poorly understood Classical silvicultural systems such as shelterwood with reserves [3] and innovations like variableretention harvesting [4] have been proposed as part of a strategy to meet diverse forest management objectives in late-seral Douglas-fir (Pseudotsuga menziesii (Mirb.)) forests on public lands in the Pacific Northwest USA. The heterogeneous structures retained in the proposed systems have been hypothesized to maintain taxa and ecological processes characteristic of late-seral forests in the shortterm and accelerate their recovery in the long-term [5] The success of these systems depends on complex responses to prescribed management activities, on our understanding of the cause-and-effect pathways of these responses, and on our ability to utilize this understanding to design efficient systems for meeting defined objectives. In the context of silvicultural management of biodiversity, it is crucial that research on ecosystem responses employ statistical methods that imply causal mechanisms among structural components
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