Vegetation and floristics of a lowland tropical rainforest in northeast Australia

Palms of the past: can morphometric phytolith analysis inform deep time evolution and palaeoecology of Arecaceae?

BACKGROUND AND AIMS

Palm fossils are often used as evidence for warm and wet palaeoenvironments, reflecting the affinities of most modern palms. However, several extant palm lineages tolerate cool and/or arid climates, making a clear understanding of the taxonomic composition of ancient palm communities important for reliable palaeoenvironmental inference. However, taxonomically identifiable palm fossils are rare and often confined to specific facies. Although the resolution of taxonomic information they provide remains unclear, phytoliths (microscopic silica bodies) provide a possible solution because of their high preservation potential under conditions where other plant fossils are scarce. We thus evaluate the taxonomic and palaeoenvironmental utility of palm phytoliths.

METHODS

We quantified phytolith morphology of 97 modern palm and other monocot species. Using this dataset, we tested the ability of five common discriminant methods to identify nine major palm clades. We then compiled a dataset of species' climate preferences and tested if they were correlated with phytolith morphology using a phylogenetic comparative approach. Finally, we reconstructed palm communities and palaeoenvironmental conditions at six fossil sites.

KEY RESULTS

Best-performing models correctly identified phytoliths to their clade of origin only 59 % of the time. Although palms were generally distinguished from non-palms, few palm clades were highly distinct, and phytolith morphology was weakly correlated with species' environmental preferences. Reconstructions at all fossil sites suggested that palm communities were dominated by Trachycarpeae and Areceae, with warm, equable climates and high, potentially seasonal rainfall. However, fossil site reconstructions had high uncertainty and often conflicted with other climate proxies.

CONCLUSIONS

While phytolith morphology provides some distinction among palm clades, caution is warranted. Unlike prior spatially restricted studies, our geographically and phylogenetically broad study indicates phytolith morphology may not reliably differentiate most palm taxa in deep time. Nevertheless, it reveals distinct clades, including some likely to be palaeoenvironmentally informative.

Similar patterns of leaf temperatures and thermal acclimation to warming in temperate and tropical tree canopies

As the global climate warms, a key question is how increased leaf temperatures will affect tree physiology and the coupling between leaf and air temperatures in forests. To explore the impact of increasing temperatures on plant performance in open air, we warmed leaves in the canopy of two mature evergreen forests, a temperate Eucalyptus woodland and a tropical rainforest. The leaf heaters consistently maintained leaves at a target of 4 °C above ambient leaf temperatures. Ambient leaf temperatures (Tleaf) were mostly coupled to air temperatures (Tair), but at times, leaves could be 8-10 °C warmer than ambient air temperatures, especially in full sun. At both sites, Tleaf was warmer at higher air temperatures (Tair > 25 °C), but was cooler at lower Tair, contrary to the 'leaf homeothermy hypothesis'. Warmed leaves showed significantly lower stomatal conductance (-0.05 mol m-2 s-1 or -43% across species) and net photosynthesis (-3.91 μmol m-2 s-1 or -39%), with similar rates in leaf respiration rates at a common temperature (no acclimation). Increased canopy leaf temperatures due to future warming could reduce carbon assimilation via reduced photosynthesis in these forests, potentially weakening the land carbon sink in tropical and temperate forests.

Isotopic and morphologic proxies for reconstructing light environment and leaf function of fossil leaves: a modern calibration in the Daintree Rainforest, Australia

PREMISE

Within closed-canopy forests, vertical gradients of light and atmospheric CO₂ drive variations in leaf carbon isotope ratios, leaf mass per area (LMA), and the micromorphology of leaf epidermal cells. Variations in traits observed in preserved or fossilized leaves could enable inferences of past forest canopy closure and leaf function and thereby habitat of individual taxa. However, as yet no calibration study has examined how isotopic, micro- and macromorphological 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-width² /leaf area, a proposed proxy for LMA that can be measured from fossil leaves), 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).

RESULTS

Individually, cell area, δ¹³ 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 of LAI and LMA for individual taxa can provide comparative measures of light environment and growth strategy of fossil taxa from within a flora.

Rainforest trees respond to drought by modifying their hydraulic architecture

Increased drought is forecasted for tropical regions, with severe implications for the health and function of forest ecosystems. How mature forest trees will respond to water deficit is poorly known. We investigated wood anatomy and leaf traits in lowland tropical forest trees after 24 months of experimental rainfall exclusion. Sampling sun-exposed young canopy branches from target species, we found species-specific systematic variation in hydraulic-related wood anatomy and leaf traits in response to drought stress. Relative to controls, drought-affected individuals of different tree species variously exhibited trait measures consistent with increasing hydraulic safety. These included narrower or less vessels, reduced vessel groupings, lower theoretical water conductivities, less water storage tissue and more abundant fiber in their wood, and more occluded vessels. Drought-affected individuals also had thinner leaves, and more negative pre-dawn or mid-day leaf water potentials. Future studies examining both wood and leaf hydraulic traits should improve the representation of plant hydraulics within terrestrial ecosystem and biosphere models, and help fine-tune predictions of how future climate changes will affect tropical forests globally.