Abstract
Forest management alters the growing conditions and thus further development of trees. However, quantitative assessment of forest management on tree growth has been demanding as methodologies for capturing changes comprehensively in space and time have been lacking. Terrestrial laser scanning (TLS) has shown to be capable of providing three-dimensional (3D) tree stem reconstructions required for revealing differences between stem shapes and sizes. In this study, we used 3D reconstructions of tree stems from TLS and an unmanned aerial vehicle (UAV) to investigate how varying thinning treatments and the following growth effects affected stem shape and size of Scots pine (Pinus sylvestris L.) trees. The results showed that intensive thinning resulted in more stem volume and therefore total biomass allocation and carbon uptake compared to the moderate thinning. Relationship between tree height and diameter at breast height (i.e. slenderness) varied between both thinning intensity and type (i.e. from below and above) indicating differing response to thinning and allocation of stem growth of Scots pine trees. Furthermore, intensive thinning, especially from below, produced less variation in relative stem attributes characterizing stem shape and size. Thus, it can be concluded that thinning intensity, type, and the following growth effects have an impact on post-thinning stem shape and size of Scots pine trees. Our study presented detailed measurements on post-thinning stem growth of Scots pines that have been laborious or impracticable before the emergence of detailed 3D technologies. Moreover, the stem reconstructions from TLS and UAV provided variety of attributes characterizing stem shape and size that have not traditionally been feasible to obtain. The study demonstrated that detailed 3D technologies, such as TLS and UAV, provide information that can be used to generate new knowledge for supporting forest management and silviculture as well as improving ecological understanding of boreal forests.11The data used in the study can be found online at: https://doi.org/10.5281/zenodo.3701271.
Highlights
Allocation of photosynthesis products between different parts of a tree define their development
The Terrestrial laser scanning (TLS) point clouds were normalized (1) using the lasground tool in LAStools software (Isenburg, 2019) whereas RGB point clouds from the unmanned aerial vehicle (UAV) were normalized with digital terrain model (DTM) that was based on airborne laser scanning data and provided by National Land Survey of Finland
As especially thinning type implicitly affects the size of the remaining trees, we examined the basic statistics of relative stem attributes, namely relative volume, relative volume below and above 50% tree height as well as relative tapering below and above 50% of tree height and slenderness and both form factor at breast height and form factor up to 50% of tree height
Summary
Allocation of photosynthesis products between different parts of a tree define their development. Competition for resources (e.g. light, water, nutrients) between trees affects development of different tree parts. As a part of forest management, competition within a population is regulated as part of the trees are removed. Ecologically thinning is aimed at improving growing conditions (i.e., light, temperature, water, nutrients) of remaining trees and economically to maximize the net present value of a stand by decreasing the opportunity costs of the capital. When part of shadowing leaf mass is removed in thinning, the amount of light is increased, and Forest Ecology and Management 474 (2020) 118344 growth of remaining trees is enhanced (White, 1980). More intensive thinning signifies fewer remaining trees and decrease in standing volume (Mäkinen & Isomäki, 2004a)
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