Making (remote) sense of lianas

Abstract

Lianas (woody vines) are abundant and diverse, particularly in tropical ecosystems. Lianas use trees for structural support to reach the forest canopy, often putting leaves above their host tree. Thus they are major parts of many forest canopies. Yet, relatively little is known about distributions of lianas in tropical forest canopies, because studying those canopies is challenging. This knowledge gap is urgent to address because lianas compete strongly with trees, reduce forest carbon uptake and are thought to be increasing, at least in the Neotropics. Lianas can be difficult to study using traditional field methods. Their pliable stems often twist and loop through the understorey, making it difficult to assess their structure and biomass, and the sizes and locations of their crowns. Furthermore, liana stems are commonly omitted from standard field surveys. Remote sensing of lianas can help overcome some of these obstacles and can provide critical insights into liana ecology, but to date there has been no systematic assessment of that contribution. We review progress in studying liana ecology using ground‐based, airborne and space‐borne remote sensing in four key areas: (i) spatial and temporal distributions, (ii) structure and biomass, (iii) responses to environmental conditions and (iv) diversity. This demonstrates the great potential of remote sensing for rapid advances in our knowledge and understanding of liana ecology. We then look ahead, to the possibilities offered by new and future advances. We specifically consider the data requirements, the role of technological advances and the types of methods and experimental designs that should be prioritised. Synthesis. The particular characteristics of the liana growth form make lianas difficult to study by ground‐based field methods. However, remote sensing is well suited to collecting data on lianas. Our review shows that remote sensing is an emerging tool for the study of lianas, and will continue to improve with recent developments in sensor and platform technology. It is surprising, therefore, how little liana ecology research has utilised remote sensing to date—this should rapidly change if urgent knowledge gaps are to be addressed. In short, liana ecology needs remote sensing.

Mapping the Spatial Distribution of Fern Thickets and Vine-Laden Forests in the Landscape of Bornean Logged-Over Tropical Secondary Rainforests

“…Our results indicated that green and shortwave infrared wavelength regions were especially important for detecting the ferns and vines within the dense vegetation of tropical rainforests (Figure 5). This result is consistent with the previous reports that showed the potential of the visible green wavelength or shortwave infrared region to separate liana or fern from trees [21,[23][24][25][26]28,77]. On the other hand, the individual near-infrared wavelength region was less important, but vegetation indices calculated based on near-infrared regions with green (NDWI) or SWIR2 (NBR) were effective.…”

“…The wavelength regions that are important for distinguishing ferns or vines from trees have been reported variously. For canopy-level separation between vines and trees in tropical forests, detailed studies using hyperspectral spectroscopy have been conducted and two wavelength regions have been reported to be important [24]: the visible green region, which represents the characteristics of chlorophyll and carotenoid concentration [21,[25][26][27], and the shortwave infrared (SWIR) region, which represents water content [26,27]. When using a multispectral optical satellite sensor, the visible green region of Sentinel-2 is especially helpful [23].…”

Exploring the link between spectral variance and upper canopy taxonomic diversity in a tropical forest: influence of spectral processing and feature selection

“…A review of the recent progress made in the study of liana ecology using terrestrial, aerial, and space‐based remote sensing is provided by Heijden et al. (2022).…”

“…Our approach does not take into account the species in the lower layers of the canopy. A review of the recent progress made in the study of liana ecology using terrestrial, aerial, and space-based remote sensing is provided by Heijden et al (2022).…”

Landscape‐scale drivers of liana load across a Southeast Asian forest canopy differ to the Neotropics

“…We lack information on the influence and interaction of the multiple hypothesised influences on liana distributions at the landscape scale, particularly in tropical forest tree crowns. This is partly due to reliance on ground‐based data collection, including liana stem measurements, as opposed to canopy occupancy, which can be difficult to assess from the ground (Marvin et al, 2016; van der Heijden et al, 2022; Waite et al, 2019). Liana canopy occupancy may be more directly related to liana–tree competition, however, as lianas deploy most of their leaves above those of their host trees (Avalos & Mulkey, 1999; Rodríguez‐Ronderos et al, 2016), directly reducing the amount of light received and the photosynthetic capacity of their host trees (Avalos et al, 1999; Avalos & Mulkey, 1999; Fauset et al, 2017).…”