## 1 Introduction  
Understanding wind loads and damage to trees in forests is important for many reasons, most recently with respect to identifying the intensity of tornadoes [1; 2]. The turbulent nature of the fluctuating wind indicates that wind-induced response and wind loading of trees are not steady but rapidly fluctuate [3]. Compared with obvious challenges in field measurements, wind tunnel tests of scaled, aeroelastic model trees is a more effective approach for studying the fluctuating load and response on trees in forests [4]. However, vague multimodal dynamic characteristics and ineffective estimation of leaf mass are two of the main obstacles to developing aeroelastic models. This is mainly because multiple branches determine that multimodal dynamics associated with branches play a key role in the dynamic process, and the aerodynamics of trees are mainly determined by leaves. Therefore, we explored multimodal dynamic characteristics of a decurrent tree, leaf mass estimation, and complete crown characteristics and model assessment of the Hao' model [5] in this study. According to the form of the crown, trees can be divided into two categories: decurrent and excurrent trees. A typical decurrent tree is shown in Figure 1a, which is characterized by a shorter trunk and a crown supported by radial branches. In the extreme cases, the decurrent trees can have only spreading branches with nearly no central trunk. The excurrent trees, on the other hand, have a central trunk (Figure 1b), as well as lateral branches. Structurally, the central trunk of an excurrent tree looks like a \"pillar\", on top of which the lateral branches, or the \"cantilevers\", are fixed. Branched trees have a complex organization of modes with high modal density and spatial localization [6, 7], where high modal density occurs when most of the natural frequencies associated with the branches are in a narrow frequency range. Spatial localization is defined as the combinations of spatial coordinates of branches and the trunk at their natural frequencies. This means that some branch modes have either a localized resonance or a global resonance [8]. Localized resonance will not generally break the trunk or anchorage system, while global resonance affects the global dynamic response of trees. So far, most studies regarding the multimodal behaviour of trees have focused on the effect of high modal density and distinct branch deformation on the wind energy dissipation in trees. For example, multiple mass damping [9, 10], multiple resonance damping [11], and branch damping [7] have been studied. For excurrent trees, aeroelastic models usually neglect the contribution derived from branches [12]. However, this is not the case for decurrent trees because of crown deformation determined by branches. A lack of studies regarding the contribution of branch modes to dynamic response makes it difficult to model the dynamic characteristics of the aeroelastic model of a decurrent tree. Therefore, in this paper, the first objective is to quantify the contribution of branch modes in the dynamic processes and determine which branch modes contribute the most. It should be noted that it is not practical to track each branch by setting sensors on each one in field measurements. In contrast, the finite element method provides an effective approach to characterize the modes generated from branches moving in a complex manner [10, 13, 14]. Using the FEM, leaf clusters can be considered as a lumped mass attached to branches [15, 16], or included in the mass of the primary branch by increasing its density proportionately [14, 17, 18, 19, 20], or simply neglected [6, 21]. The leaf mass in such models is usually vaguely defined and estimated empirically. Although this may have a limited effect on tree dynamics that depend on the form and morphology of tree and branches [10], accurate estimation of leaf mass is important for scaled, aeroelastic model trees. One of the most effective methods is to directly measure leaf mass.