Integrating high resolution drone imagery and forest inventory to distinguish canopy and understory trees and quantify their contributions to forest structure and dynamics.

Raquel Fernandes Araujo,Jeffrey Q Chambers,Gabriel H P M Ribeiro,Carlos Henrique Souza Celes,Ana Paula Ferreira Dos Santos,Moacir A A Campos,Helene C Muller-Landau,Fabiano Emmert,Adriano J N Lima,Niro Higuchi,Bruno Oliva Gimenez,Jana Müllerová

doi:10.1371/journal.pone.0243079

Abstract

Tree growth and survival differ strongly between canopy trees (those directly exposed to overhead light), and understory trees. However, the structural complexity of many tropical forests makes it difficult to determine canopy positions. The integration of remote sensing and ground-based data enables this determination and measurements of how canopy and understory trees differ in structure and dynamics. Here we analyzed 2 cm resolution RGB imagery collected by a Remotely Piloted Aircraft System (RPAS), also known as drone, together with two decades of bi-annual tree censuses for 2 ha of old growth forest in the Central Amazon. We delineated all crowns visible in the imagery and linked each crown to a tagged stem through field work. Canopy trees constituted 40% of the 1244 inventoried trees with diameter at breast height (DBH) > 10 cm, and accounted for ~70% of aboveground carbon stocks and wood productivity. The probability of being in the canopy increased logistically with tree diameter, passing through 50% at 23.5 cm DBH. Diameter growth was on average twice as large in canopy trees as in understory trees. Growth rates were unrelated to diameter in canopy trees and positively related to diameter in understory trees, consistent with the idea that light availability increases with diameter in the understory but not the canopy. The whole stand size distribution was best fit by a Weibull distribution, whereas the separate size distributions of understory trees or canopy trees > 25 cm DBH were equally well fit by exponential and Weibull distributions, consistent with mechanistic forest models. The identification and field mapping of crowns seen in a high resolution orthomosaic revealed new patterns in the structure and dynamics of trees of canopy vs. understory at this site, demonstrating the value of traditional tree censuses with drone remote sensing.

Highlights

Scientists have long sought to understand tropical forest structure and dynamics—the abundances of trees of different sizes and canopy positions, and their growth and mortality rates [1]
We addressed the following questions: (i) What is the proportion of trees in the canopy and how does this vary with diameter? (ii) How do growth rates differ between canopy and understory trees, and how does that difference vary with diameter? (iii) What are the relative contributions of canopy and understory trees to aboveground biomass carbon stocks and aboveground wood productivity? (iv) What are the forms of size distributions of canopy trees and understory trees, how do they differ from each other and from whole-forest size distributions, and how do they fit with competing theories?
The identification and field mapping of crowns seen in a high resolution orthomosaic revealed new patterns in the structure and dynamics of trees occupying different light environments in this Amazonian forest

Summary

Introduction

Scientists have long sought to understand tropical forest structure and dynamics—the abundances of trees of different sizes and canopy positions, and their growth and mortality rates [1]. The integration of RPAS remote sensing and ground-based data provides the opportunity for the exact determination of canopy status to be linked with information on tree diameter, growth, etc., thereby enabling new insights into the structure and dynamics of tropical forests. What proportion of trees of different sizes are in the canopy, and what are their contribution to forest carbon stocks and woody productivity? It is well-known that larger trees are more likely to be in the canopy, that larger trees contribute disproportionately to forest carbon stocks, and that woody productivity increases with tree size and light exposure [17,18,19,20,21,22,23,24,25]. A rare exception is work combining airborne imagery and ground-based data for the old-growth moist tropical forest of Barro Colorado Island, Panama, to evaluate the canopy status of individual trees, quantify the proportion of trees in the canopy, and compare diameter growth of canopy and understory trees [26, 27]

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Discussion

Conclusion