Premise: Within closed canopy forests, vertical gradients of light and atmospheric CO 2 drive variations in leaf carbon isotope ratios, leaf mass per area (LMA), and the micromorphology of leaf epidermal cells. Variations in such traits observed in preserved or fossilized leaves could enable inferences of past forest canopy closure and the habitat of individual taxa. However, as yet no calibration study has examined how multiple traits in combination reflect position within a modern closed canopy forest or how these could be applied to the fossil record.

Methods: Leaves were sampled from throughout the vertical profile of the tropical forest canopy using the 48.5 m crane at the Daintree Rainforest Observatory, Queensland, Australia. Carbon isotope ratios, LMA, petiole metric (i.e. petiole-width 2 / leaf-area) and leaf micromorphology (i.e. undulation index and cell area) were compared within species across a range of canopy positions, as quantified by leaf area index (LAI). Key Results: Individually, cell area, δ 13 C, and petiole metric all correlated with both LAI and LMA, but the use of a combined model provided significantly greater predictive power.

Conclusions: Using the observed relationships with leaf carbon isotope ratio and morphology to estimate the range of LAI in fossil floras can provide a measure of canopy closure in ancient forests. Similarly, estimates LAI and LMA for individual taxa can provide comparative measures of light environment and growth strategy of fossil taxa from within a flora. Please be aware that if you ask to have your user record removed, we will retain your name in the records concerning manuscripts for which you were an author, reviewer, or editor. In compliance with data protection regulations, you may request that we remove your personal registration details at any time. 

All leaf samples were obtained from the Daintree Rainforest Observatory (DRO), located in Cape Tribulation (-16.117 N, 145.45 E), 140 kilometres north of Cairns in Far North Queensland, Australia. The DRO, administered by James Cook University, includes two 1 ha permanent tree-census plots. One of these plots can be accessed by a 48.5 m high canopy-access crane (Leibherr Model 91EC). Forest at the DRO is described as a complex mesophyll vine forest Type 1a (Tracey, 1982) with a tall but irregular canopy varying in height from 25 to 33 m with indistinct stratification of the subcanopy. Within the crane plot there are 85 different canopy tree species (comprising 60 genera, and 35 families) representing a basal area of ~33 m² ha^-1 (Tng et al., 2016).  The Daintree Rainforest is considered one of the oldest continuously-vegetated tropical systems on earth and while not as speciose as some tropical closed forest systems, it represents a broad phylogenetic diversity, with many Gondwana ‘relic’ species (Costion et al., 2015).

Initial leaf samples for canopy-understory comparisons (Data Sheet 1) were collected in 2014 from 89 species of plant found within the closed forest canopy of the DRO. This included both monocotyledonouos (7), dicotyledonous (81) and one cycad species Bowenia spectabilis. A range of growth forms have been sampled including trees and shrubs (collectively referred to here as trees) and lianas. Samples were collected from both the upper canopy crown (using the canopy access crane) and the understory ~ 0.4 to 1.5m, with an attempt made to collect the same species from both environments. During this initial sampling, in-situ leaf physiological measurements (e.g. light saturated photosynthesis (A_sat), and responses to [CO₂] (A-C_i) , and irradiance (A-I) curves) were made on at least two leaves of 71 species collected. In all upper canopy samples, 12 leaves were harvested with nine analysed for basic leaf functional traits and three leaves frozen for later micro-morphological analysis. In the case of understory samples only the two leaves used for physiological measurements were harvested and analysed for leaf functional traits given typically limiting leaf material.

In 2017, additional leaves were sampled across the light gradient continuum found in the forest canopy (Data Sheet 2).  Five species (Argyrodendron peralatum, Myristica globosa ssp. muelleri, Endiandra microneura, Cleistanthus myrianthus and Rockinghamia angustifolia) were harvested due to their prevalence at all forest strata of the study site. Leaves were sampled from the upper crown, interior crown (i.e. partial cover by surrounding vegetation), the understory as well as from within a significant tree fall gap. At each location six leaves have been harvested, with three analysed for basic leaf functional traits as above and the remainder archived for micromorphological analysis. At each sampling location, leaf area index (LAI) was determined using a LAI-2200C Plant Canopy Analyzer (Licor, Nebraska). In addition, and to examine the impact of environmental stress upon the fidelity of leaf traits, parallel upper and interior canopy samples were also collected from individuals impacted by a long-term (3-y) throughfall exclusion, or “drought”, experiment being conducted within the DRO crane plot (Tng et al., 2018). The throughfall exclusion plot (0.4 ha), wherein rainfall/thoughfall is intercepted and diverted off the plot has resulted in an area of elevated water stress within the crane accessible arc that includes replicated individuals of species found under natural conditions.

See associated paper for method details on trait determination. Note 2014 samples listed in Data sheet 2 are repeats of thoe listed in Data Sheet 1.