# Sensitivity of Common Vegetation Indices to the Canopy Structure of Field Crops
## 1 Introduction  
Canopy structure, or its architecture, is understood as the spatial distribution and orientation of vegetation components within a canopy. It directly affects the interception of solar radiation and thus also how radiation is reflected or intercepted by the canopy. Vegetation structure has an effect on land surface energy balance and it influences leaf temperature, photosynthetic activity and plant growth [1; 2; 3; 4]. Vegetation structure thus plays a dual role in remote sensing. First, it is a canopy characteristic which needs to be retrieved to understand biophysical processes. Second, due to itsdirect effect on canopy reflectance, it needs to be taken into account--and corrected for--when other canopy biophysical or biochemical variables are determined from remote sensing data. The complexity of canopy structure varies with biome and plant functional type. Crop canopy is usually assumed to be the simplest possible case--horizontally homogeneous and with a constant distribution of leaf normals, i.e., the vertical profile of the canopy is the same at all locations. Its structure can thus be described by the amount of leaves above the unit horizontal ground area and their directional distribution, which can be quantified using the leaf area index (LAI) and leaf angle distribution (LAD), respectively. LAI is a key variable for understanding the biophysical characteristics of the vegetated areas of the planet ([PERSON] _et al._, 2012; [PERSON] _et al._, 2013; [PERSON] _et al._, 2013). For agricultural management, crop LAI measured over large areas can be used to estimate productivity on national to global scales. The other structural characteristic, LAD, quantifies the distribution of the angles between leaf normals and the vertical direction. If the azimuth angles of leaves are uniformly distributed, LAD and LAI completely determine the transmission of radiation through the canopy ([PERSON] _et al._, 2012). Compared with LAI, much less attention has been paid to LAD when describing the biophysical characteristics of vegetation. Spectral vegetation indices (VIs), calculated from canopy reflectance in a small number of selected spectral bands mostly in the visible and near infrared (NIR) spectral domains ([PERSON] _et al._, 2012; [PERSON] _et al._, 2013; [PERSON] _et al._, 2013; [PERSON] _et al._, 2013), are commonly used for the remote estimation of vegetation biophysical characteristics. Visible bands are utilized for determining the absorption by plant pigments active in this spectral region, whereas the reflectance in the NIR domain, where green leaves have minimal scattering, contains information on canopy cover and thickness. However, in addition to the number of leaves, canopy reflectance is also affected by many factors, such as soil background, atmospheric effects, canopy element optical properties and canopy architecture. Despite its central role, there is a noticeable lack of empirical evidence on the effect of LAD on even the most common VIs. The main reason is likely a lack of in-situ measurements of canopy LAD, or the central moment of the distribution, the mean tilt angle (MTA). A traditional direct approach for LAD measurement is using clinometers in contact with the leaf surface ([PERSON] _et al._, 2012). This method is laborious and time consuming, and direct measurement is inevitable to disturb the leaf natural orientation. Indirect leaf mean tilt angle estimation relies on the inversion of the gap fraction below the canopy ([PERSON] _et al._, 2013). Some specialized techniques were applied to characterize 3D canopy structure, e.g., portable laser scanning ([PERSON] _et al._, 2012; [PERSON] _et al._, 2013), which were resource demanding and rigid to operate. A digital photography method was developed to estimate the LAD of tree species ([PERSON] _et al._, 2012; [PERSON] _et al._, 2013; [PERSON] _et al._, 2013) and extended and validated for field crops ([PERSON] _et al._, 2014).