Abstract
Forests are one of the most important terrestrial ecosystems for the protection of biodiversity, but at the same time they are under heavy production pressures. In many cases, management optimized for timber production leads to a simplification of forest structures, which is associated with species loss. In recent decades, the concept of retention forestry has been implemented in many parts of the world to mitigate this loss, by increasing structure in managed stands. Although this concept is widely adapted, our understanding what forest structure is and how to reliably measure and quantify it is still lacking. Thus, more insights into the assessment of biodiversity-relevant structures are needed, when aiming to implement retention practices in forest management to reach ambitious conservation goals. In this study we compare expert ratings on forest structural richness with a modern light detection and ranging (LiDAR) -based index, based on 52 research sites, where terrestrial laser scanning (TLS) data and 360° photos have been taken. Using an online survey (n = 444) with interactive 360° panoramic image viewers, we sought to investigate expert opinions on forest structure and learn to what degree measures of structure from terrestrial laser scans mirror experts’ estimates. We found that the experts’ ratings have large standard deviance and therefore little agreement. Nevertheless, when averaging the large number of participants, they distinguish stands according to their structural richness significantly. The stand structural complexity index (SSCI) was computed for each site from the LiDAR scan data, and this was shown to reflect some of the variation of expert ratings (p = 0.02). Together with covariates describing participants’ personal background, image properties and terrain variables, we reached a conditional R2 of 0.44 using a linear mixed effect model. The education of the participants had no influence on their ratings, but practical experience showed a clear effect. Because the SSCI and expert opinion align to a significant degree, we conclude that the SSCI is a valuable tool to support forest managers in the selection of retention patches.
Highlights
The questionnaire was divided into two sections: in the first section we showed the participants nine different images and asked them the same two questions for each photo: (1) ‘How high would you rate the structural richness of this forest?’ and (2) ‘How do you rate the ecological value of the recognizable structures with regard to biodiversity conservation?’
We found significant relationships between a very technical approach and the complex human perception of forest structures
The high-resolution scans make changes in structural richness quantifiable without relying on the same person to be available for inventories in the following years, which is a striking advantage for monitoring purposes
Summary
In conventionally managed forests optimized for timber production, structures typical of late successional stages and old growth forests are especially lacking. This loss of structural diversity leads to strong negative impacts on various ecosystem services like biodiversity and resilience [3]. Changes in forest structure due to wood production are different than changes in structure due to natural disturbance, both in frequency and extent [4]. To minimize this difference, the retention of diverse forest structures has become a common goal in forestry conservation guidelines [5,6]. Retention practices include setting aside single or small groups of “habitat trees” to grow old, the enrichment of standing and lying dead wood within stands, as well as a variety of other measures depending on the ecosystem and local conditions
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