Structural Properties Of Trees Research Articles

ICARE-VEG: A 3D physics-based atmospheric correction method for tree shadows in urban areas

Many applications dedicated to urban areas (e.g. land cover mapping and biophysical properties estimation) using high spatial resolution remote sensing images require the use of 3D atmospheric correction methods, able to model complex light interactions within urban topography such as buildings and trees. Currently, one major drawback of these methods is their lack in modeling the radiative signature of trees (e.g. the light transmitted through the tree crown), which leads to an over-estimation of ground reflectance at tree shadows. No study has been carried out to take into account both optical and structural properties of trees in the correction provided by these methods. The aim of this work is to improve an existing 3D atmospheric correction method, ICARE (Inversion Code for urban Areas Reflectance Extraction), to account for trees in its new version, ICARE-VEG (ICARE with VEGetation). After the execution of ICARE, the methodology of ICARE-VEG consists in tree crown delineation and tree shadow detection, and then the application of a physics-based correction factor in order to perform a tree-specific local correction for each pixel in tree shadow. A sensitivity analysis with a design of experiments performed with a 3D canopy radiative transfer code, DART (Discrete Anisotropic Radiative Transfer), results in fixing the two most critical variables contributing to the impact of an isolated tree crown on the radiative energy budget at tree shadow: the solar zenith angle and the tree leaf area index (LAI). Thus, the approach to determine the correction factor relies on an empirical statistical regression and the addition of a geometric scaling factor to account for the tree crown occultation from ground. ICARE-VEG and ICARE performance were compared and validated in the Visible-Near Infrared Region (V-NIR: 0.4–1.0 µm) with hyperspectral airborne data at 0.8 m resolution on three ground materials types, grass, asphalt and water. Results show that (i) ICARE-VEG improves the mean absolute error in retrieved reflectances compared to ICARE in tree shadows by a multiplicative factor ranging between 4.2 and 18.8, and (ii) reduces the spectral bias in reflectance from visible to NIR (due to light transmission through the tree crown) by a multiplicative factor between 1.0 and 1.4 in terms of spectral angle mapper performance. ICARE-VEG opens the way to a complete interpretation of remote sensing images (sunlit, shade cast by both buildings and trees) and the derivation of scientific value-added products over all the entire image without the preliminary step of shadow masking.

Forest species diversity mapping using airborne LiDAR and hyperspectral data in a subtropical forest in China

Monitoring biodiversity is essential for the conservation and management of forest resources. A method called “spectranomics” that maps the diversity of forest species based on species-driven leaf optical traits using imaging spectroscopy has been developed for tropical forests in earlier studies. In this study we applied the “spectranomics” method in combination with airborne hyperspectral (PHI-3 sensor with 1 m spatial resolution) and LiDAR (>4 points/m2) data to first identify interspecies variations in biochemical and structural properties of trees and then estimate the tree species diversity within the Shennongjia Forest Nature Reserve in China. Firstly, we used the watershed algorithm based on morphological crown control to isolate individual tree crowns (ITCs) from the LiDAR data. For each ITC, we then calculated seven vegetation indices (VIs) representing key biochemical properties from the hyperspectral data and additionally derived the LiDAR-based tree height which was identified to support the discrimination of the tree species in a preceding analysis. Finally we utilized the combination of the seven selected VIs and tree height as input to a self-adaptive Fuzzy C-Means (FCM) clustering algorithm. The FCM algorithm was applied to fixed subsets of 30 m × 30 m and it was assumed that the number of clusters identified within a subset represents the number of occurring species. The species richness and Shannon-Wiener diversity index calculated from the clustering outputs correlated well with the field reference data (R2 = 0.83, RMSE = 0.25). The results show that forest species diversity can be directly predicted using the suggested clustering method based on crown-by-crown variations in biochemical and structural properties in the examined subtropical forest without the need to distinguish the individual tree species.

Some forbidden combinations of branches in minimal-ABC trees

The atom-bond connectivity (ABC) index has been, in recent years, one of the most actively studied vertex-degree-based graph invariants in chemical graph theory. For a given graph G, the ABC index is defined as ∑uv∈Ed(u)+d(v)−2d(u)d(v), where d(u) is the degree of vertex u in G and E denotes the set of edges of G. In this paper, we present some new structural properties of trees with a minimal ABC index (also refer to as a minimal-ABC tree), which is a step further towards understanding their complete characterization. We show that a minimal-ABC tree cannot simultaneously contain a B4-branch and B1 or B2-branches.

On structural properties of trees with minimal atom-bond connectivity index IV: Solving a conjecture about the pendent paths of length three

The atom-bond connectivity (ABC) index is one of the most investigated degree-based molecular structure descriptors with a variety of chemical applications. It is known that among all connected graphs, the trees minimize the ABC index. However, a full characterization of trees with a minimal ABC index is still an open problem. By now, one of the proved properties is that a tree with a minimal ABC index may have, at most, one pendent path of length 3, with the conjecture that it cannot be a case if the order of a tree is larger than 1178. Here, we provide an affirmative answer of a strengthened version of that conjecture, showing that a tree with minimal ABC index cannot contain a pendent path of length 3 if its order is larger than 415.

Wood quality in complex forests versus even-aged monocultures: review and perspectives

As they fulfil many ecological and social functions and services better than even-aged monocultures, heterogeneous pure and mixed-species stands are on the advance in Central Europe. Even so, knowledge of how different stands compare in terms of the quantity and quality of the produced wood remains limited, as forest research has been focused on pure stands in the past. Therefore, the still limited comparative studies on timber quality in mixed versus pure stands were reviewed. Further, approximately 100 studies on the morphology of mixed versus pure stands have been reviewed. As is known, the close connection between morphology and timber quality from many studies in pure stands as well as the morphological and structural properties of trees in mixed stands is used as proxies for their timber quality. The number of studies reporting a decrease or increase in timber strength and stiffness in complex stands compared with homogeneous stands was balanced. Knottiness is mostly higher in complex stands. Wood density behaves indifferently. Distortion, as indicated by eccentricity of crown, bending of stems, or irregularity of the tree-ring width, is generally higher in complex forests. This rather ambiguous pattern becomes clearer by typifying the findings depending on the species-specific morphological plasticity of the trees and the spatial conditions they are exposed to. When growing in strong lateral restriction in even-aged pure or mixed-species stands (type 1), trees follow a “keep abreast” strategy which results in high-quality timber especially in case of species with low plasticity. Trees in uneven-aged forests with vertically restricted growing space (type 2) often use a “sit-and-wait” strategy that may result in tapering stem shapes, wide and long crowns with low branch diameters, and high wood density. Distortion may be low in case of species with low morphological plasticity but increase with increasing shade tolerance and plasticity. Growth in widely spaced and heavily thinned pure and mixed stands (type 3) may let trees follow the “stabilisation” strategy. Because of their strong dominance, these trees develop tapering stem shapes, knots of big size and wide appearance along the stem axis, as well as lower wood density, especially in the case of conifers. In arrangements of types 1–3, the “transition” strategy may also emerge, which leads from the “sit-and-wait” stadium to the “keep abreast” strategy. It starts when trees strongly increase their height growth at the expense of the stem diameter growth. It results in slender stems, low knottiness, high wood density, and low distortion, with the result that the tree gets access to the upper canopy at the expense of lateral expansion of stem and crown. In fact, it is not primarily the species mixing that modifies the morphology, structure, and wood quality of the trees but the species-specific morphological plasticity and the structural heterogeneity of the stand. The latter is often higher in mixed than in pure stands and in uneven-aged than in even-aged stands. The more variable the stand structure, the wider the range of wood attributes. The discussion is focused on the relevance of the results for stand management and interdisciplinary research at the intersection of forest growth and yield science and wood science.

On structural properties of trees with minimal atom-bond connectivity index III: Trees with pendent paths of length three

The atom-bond connectivity (ABC) index is a degree-based graph topological index that found chemical applications, including those of predicting the stability of alkanes and the strain energy of cycloalkanes. Several structural properties of the trees with minimal ABC index were proved recently, however the complete characterization of the minimal-ABC trees is still an open problem. It is known that minimal-ABC trees can have at most one pendent path of length 3. It is also known that the minimal-ABC trees that have a pendent path of length 3 do not contain so-called Bk-branches, with k ≥ 4, and do not contain more than two B2-branches. Here, we improve the latter result by showing that minimal-ABC trees of order larger than 168 and with a pendent path of length 3 do not contain B2-branches. Moreover, we show that trees with minimal ABC index with a pendent path of length 3 do not contain B1-branches.

Canopy gaps affect the shape of Douglas-fir crowns in the western Cascades, Oregon

Silvicultural regimes that aim at an increased stand structural diversity typically promote small-scale heterogeneity in horizontal and vertical structures, e.g. through the creation of gaps. We used terrestrial laser scanning (TLS) to investigate impacts of altered growing conditions on trees adjacent to artificial gaps as compared to responses of trees in a regularly spaced, thinned forest interior. Based on the TLS-based point clouds we calculated a number of structural tree crown properties that were hypothesized to be sensitive to spatial variability in growing conditions. We found several significant differences between structural properties of trees in the two growing conditions. Compared to trees in regular spacing, border trees near gaps had a lower crown base height (CBH) and a lower height of maximum crown projection. Crown surface area and crown volume of border trees were significantly larger than those of trees growing in a regular spacing. Also, the asymmetry of entire tree crowns of border trees, and in particular of the lower third of crowns, was directed towards the gap center, reflecting the increased light level in the gap. Our results raise concerns that the economic value of border trees is negatively affected by gap creation. These trees had shorter branch free boles and additionally, due to horizontal branch elongation, larger knots. Conversely, the overall increase in structural variability contributed by the border trees in stands with artificial gaps is likely to positively affect several ecosystem functions as well as biodiversity.

On structural properties of trees with minimal atom-bond connectivity index

The atom-bond connectivity (ABC) index is a degree-based topological index, that found chemical applications. It is well known that among all connected graphs, the graphs with minimal ABC index are trees. A complete characterization of trees with minimal ABC index is still an open problem. In this paper, we present new structural properties of trees with minimal ABC index. Our main results reveal that trees with minimal ABC index do not contain so-called Bk-branches, with k≥5, and that they do not have more than four B4-branches.

The Theory of the Rise of Sap in Trees: Some Historical and Conceptual Remarks

The ability of trees to suck water from roots to leaves, sometimes to heights of over a hundred meters, is remarkable given the absence of any mechanical pump. This study deals with a number of issues, of both an historical and conceptual nature, in the orthodox Cohesion Tension theory of the ascent of sap in trees. The theory relies chiefly on the exceptional cohesive and adhesive properties of water, the structural properties of trees, and the role of evaporation (transpiration) from leaves. But it is not the whole story. Plant scientists have been aware since the inception of the theory in the late 19th century that further processes are at work in order to prime the trees, the main such process (growth itself) being so obvious to them that it is often omitted from the story. Other factors depend largely on the type of tree, and are not always fully understood. For physicists, in particular, it may be helpful to see the fuller picture, which is what this study attempts to provide in nontechnical terms.

Salient Characteristics of Virtual Trees

Recent research on the design of virtual environments has focused on the important perceptual characteristics of man-made “carpentered” environments, rather than on VEs of natural environments. The present research examines memory for characteristics of natural settings consisting of virtual trees. Participants viewed either a symmetrical or asymmetrical virtual tree and then re-created it using custom-designed tree editing software. Memory was more accurate for the symmetrical tree. Across trees, participants were most accurate re-creating gross structural dimensions of a tree such as height and leaf size, and were particularly inaccurate at re-creating the curvature of tree branches. Our conclusion is that the design of virtual environments should focus on accurately representing gross structural properties of trees, rather than on using high levels of detail to accurately portray trunk and branch curvature.