Tree functional trait variation along the vertical canopy profile in central Amazonian forests

We quantified annual growth rates in terms of above-ground biomass (AGB), the maximum efficiency of photosystem II (Fv/Fm) and six tree functional traits related to water transport (xylem density and Huber value), leaf morphology (leaf size, angle, and stomatal density), and photosynthesis (specific leaf area) along the vertical forest profile in an old-growth central Amazonian forest. To investigate the influence of canopy environments and ontogenetic stages on the variation of these traits, we divided the forest into three vertical strata defined by height from the ground (S1: 0-20 m; S2: 20-40 m; S3: >40 m). We sampled 162 branches and 486 leaves from 54 trees of 10 species, encompassing at least five of the most abundant species per stratum. Path analysis and correlation matrices were used to explore the links between canopy environments, traits and the “fast-slow” plant economics spectrum.

Methods
The research was conducted at an old-growth forest site located in the Tapajós National Forest (TNF), known as K67 (02°51′S, 54°58′W), south of the city of Santarém (Pará, Brazil). It is an evergreen tropical forest on a well-drained infertile oxisol plateau with an average canopy height of ~40 m (Hutyra et al., 2007). At this field site, where the mean canopy height ranges from approximately 40 to 45 m and a sub-canopy layer spans 15 to 30 m (mean ≈ 28 m) (Hutyra et al., 2007; Hunter et al., 2015), 20 ha of permanent plots have been surveyed in most years since 1999 (Pyle et al., 2008).

Individual leaf-level sampling approach employed advanced tree climbing techniques to access different canopy levels and measure functional traits, including leaf-chlorophyll fluorescence with the MultispeQ.