Testing Tree Crown Economics with the USA National Ecological Observatory Network

Tree crown architecture, defined as the 3-D density, distribution and orientation of leaves within a tree crown, strongly influences the processes of photosynthesis, evapotranspiration, and spectral reflectance that help characterize key tree and forest responses to global change.  Tree crown economic theory posits that variability in tree functioning can be assessed by a suite of co-varying tree crown architectural traits describing an economic tradeoff of light capture versus water-use efficiency. Using new, consistent measurements from eight broadleaf deciduous forest sites that are part of the USA National Ecological Observatory Network (NEON), we quantified a suite of tree crown architectural traits and assessed whether they were predictive of NIR_v, a spectral reflectance index of tree crown functioning. Specifically, we worked with NEON staff to measure: (1) the trait of sunlit mean leaf angle (MLA) from analysis of tower-based profile photographs of tree crowns, (2) the traits of top rugosity (R_t), plant area index (PAI), and accumulative plant area density within the top 50% of the crown (APAD₅₀₎ from Airborne Observation Platform (AOP) LiDAR data extracted from field-delineated tree crown polygons, and (3) NIR_v, the near-infrared reflectance of vegetation, from AOP imaging spectroscopy data. We found several tree crown architectural traits and NIR_v to co-vary along a spectrum ranging from “tower” to “dome” crown architectural ideotypes. Optimized for light capture, trees closer to the “dome” ideotype had more horizontally-distributed crowns (lower APAD₅₀) with more horizontal leaves (lower MLA), which was associated with higher NIR_v.  Conversely, trees closer to the “tower” ideotype had more vertically-distributed crowns with more vertical leaves and lower NIR_v. This expected covariation of traits and NIR_v was related to species differences, but also to spatial variability within a single species, Liriodendron tulipifera, that occurred in five sites spread across a strong moisture gradient. These data and analyses are consistent with theory and suggest that measurable crown traits can define a branch- and crown-scale economic trade-off that governs how each tree adaptively distributes and orients leaves in their crown as a coordinated strategy affecting tree functioning and their responses to global change.