Abstract
Lianas are abundant and diverse in tropical forests and impact forest dynamics. They occupy part of the canopy, forming a layer of leaves overtopping tree crowns. Yet, their interaction with trees has been mainly studied from the ground. With the emergence of drone-based sensing, very high-resolution data may be obtained on liana distribution above canopies. Here, we assessed the relationship between common liana ground measurements and drone-determined liana leaf coverage over tree crowns, tested if this relationship is mediated by liana functional composition, and compared the signature of liana patches and tree crowns in our drone images. Using drone platforms, we acquired very high resolution RGB and multispectral images and LiDAR data over two 9-ha permanent plots located in northern Republic of Congo and delineated liana leaf coverage and individual tree crowns from these data. During a concomitant ground survey, we focused on 275 trees infested or not by lianas, for which we measured all lianas ≥ 1 cm in diameter climbing on them (n= 615) and estimated their crown occupancy index (COI). We additionally measured or recorded the wood density and climbing mechanisms of most liana taxa. Contrary to recent findings, we found significant relationships between most ground-derived metrics and the top-of-view liana leaf coverage over tree crowns. Tree crown infestation by lianas was primarily explained by the load of liana climbing on them, and negatively impacted by tree height. Liana leaf coverage over individual tree crowns was best predicted by liana basal area and negatively mediated by liana wood density, with a higher leaf area to diameter ratio for light-wooded lianas. COI scores were concordant with drone assessments, but two thirds differed from those obtained from drone measurements. Finally, liana patches had a higher light reflectance and variance of spectral responses than tree crowns in all studied spectra. However, the large overlap between them challenges the autodetection of liana patches in canopies. Overall, we illustrate that the joint use of ground and drone-based data deepen our understanding of liana-infestation pathways and of their functional and spectral diversity. We expect drone data to soon transform the field of liana ecology.
Highlights
IntroductionThey may represent up to 20% of woody plant diversity and 40% of stem density in Neotropical forests (Dalling et al, 2012)
Lianas are essential components of tropical forests
We investigated the relationship between ground-based liana measurements and liana leaf coverage over tree crowns quantified through high-resolution drone images
Summary
They may represent up to 20% of woody plant diversity and 40% of stem density in Neotropical forests (Dalling et al, 2012) Because they do not invest into structural support at the adult stage, often using trees to rise to the forest canopy, lianas tend to exhibit a large leaf area-stem diameter ratio compared to trees (Hegarty and Caballé, 1991; Medina-Vega et al, 2021). They can invade more than half of canopy tree crowns (Ingwell et al, 2010), forming a monolayer of leaves overtopping trees and limiting their light acquisition (Avalos et al, 1999; Visser et al, 2018). Accurately estimating liana infestation is important to quantify its effects on forest functions and predict tropical forests dynamics
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