Forest Canopy Structural Complexity and Light Absorption Relationships at the Subcontinental Scale

Abstract

Understanding how the physical structure of forest canopies influences light absorption is a long‐standing area of inquiry fundamental to the physical sciences, including the modeling and interpretation of biogeochemical cycles. Conventional measures of forest canopy structure used to infer canopy light absorption are often limited to leaf or vegetation area indexes. However, LiDAR‐derived measures of canopy structural complexity (CSC) that describe the arrangement of vegetation may improve prediction of canopy light absorption by providing novel information on canopy‐light interactions not regulated by leaf area alone. We measured multiple indexes of CSC, vegetation area index (VAI), and the fraction of photosynthetically active radiation absorbed (fPAR) across the eastern United States using portable canopy LiDAR to evaluate how different canopy structural attributes relate to fPAR. Our survey included sites from the National Ecological Observation Network and university field stations. Measures of CSC were more strongly coupled with fPAR under high light (>1,000 μmol m−2 s−1 PAR). Under low light conditions, when diffuse light predominates, light scattering weakens the dependency of fPAR on CSC. A multivariate model including CSC parameters and VAI explains ~89% of the intersite variance in fPAR, an improvement of over a VAI only linear model (r2 = 0.73). The inclusion of CSC metrics in canopy light absorption models could increase confidence in predictions of biogeochemical cycles and energy balance.

High rates of primary production in structurally complex forests

“…S1, Table S1). Our sampling density, distribution, and measurement co-location (e.g., of NPP and canopy structure) are modeled after prior studies investigating these relationships in temperate forests (Hardiman et al 2011, Fahey et al 2015, Atkins et al 2018b). Lidar sampling of canopy structural traits, ceptometer measurements for calculating the fraction of photosynthetically active radiation (fPAR) absorbed by the canopy, and vegetation analysis for the derivation of species diversity and NPP cooccurred within NEON's 40 9 40 m "tower base" plots (Appendix S1: Fig.…”

“…We focused on lidar-derived canopy traits that correlate with temperate forest canopy light absorption (i.e., fPAR; Atkins et al 2018b) and thereby may be mechanistically tied to broad-scale variation in NPP. In 2016, a portable canopy lidar (PCL) system with a maximum pulse frequency of 2,000 Hz (Riegl LD90 3100 The ground-based portable canopy lidar (PCL) system uses high-frequency laser pulses to map a two-dimensional (horizontal and vertical) "slice" of gridded VAI from within the canopy interior, providing a spatially rich lens through which to characterize canopy traits.…”

“…Canopy light absorption was assessed for 7 (GRSM, HARV, OSBS, SCBI, TALL TREE, UNDE) of 10 sites as fPAR (Atkins et al 2018b); sampling conditions were not favorable during the time of measurement for the remaining three sites. Briefly, under-and over-canopy photosynthetically active radiation (PAR) were measured under clear-sky conditions within 2 h of solar noon along the same transects as the portable canopy lidar using a Meter Environment ACCUPAR LP-80 ceptometer.…”

“…Ecosystems with diverse plant assemblages often have higher NPP because functional complementarity optimizes whole-ecosystem light acquisition and light-use efficiency (Williams et al 2017). Canopy traits (sensu Reich 2012), including vegetation or leaf area index, describe the physical structure of vegetation within an ecosystem and most often relate to NPP through their effects on canopy light absorption (Atkins et al 2018b). Novel, complexity-focused canopy traits summarizing multidimensional variation in vegetation distribution and density ( Fig.…”

“…Canopy traits (sensu Reich 2012), including vegetation or leaf area index, describe the physical structure of vegetation within an ecosystem and most often relate to NPP through their effects on canopy light absorption (Atkins et al 2018b). 1) may be even more strongly tied to NPP through their positive effects on both light acquisition and light-use efficiency (Hardiman et al 2013, Atkins et al 2018b. 1) may be even more strongly tied to NPP through their positive effects on both light acquisition and light-use efficiency (Hardiman et al 2013, Atkins et al 2018b.…”