Francesca A. McInerney

Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum

Warming and Shrinking In most mammals, individual body sizes tend to be smaller in warmer regions and larger in cooler regions. Secord et al. (p. 959; see the Perspective by Smith) examined a high-resolution 175,000-year record of equid fossils deposited over a past climate shift—the Paleocene-Eocene Thermal Maximum—for changes in body size. Using oxygen isotopes collected from the teeth of co-occurring mammal species to track prevailing environmental temperature, a clear decrease in equid body size was seen during 130,000 years of warming, followed by a distinct increase as the climate cooled at the end of the period. These results indicate that temperature directly influenced body size in the past and may continue to have an influence as our current climate changes. Oxygen isotope measurements of fossil teeth show that the body size of the horse Sifrhippus decreased as temperature increased. Body size plays a critical role in mammalian ecology and physiology. Previous research has shown that many mammals became smaller during the Paleocene-Eocene Thermal Maximum (PETM), but the timing and magnitude of that change relative to climate change have been unclear. A high-resolution record of continental climate and equid body size change shows a directional size decrease of ~30% over the first ~130,000 years of the PETM, followed by a ~76% increase in the recovery phase of the PETM. These size changes are negatively correlated with temperature inferred from oxygen isotopes in mammal teeth and were probably driven by shifts in temperature and possibly high atmospheric CO2 concentrations. These findings could be important for understanding mammalian evolutionary responses to future global warming.

Stable isotopes in leaf water of terrestrial plants

Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range of applications. Here, we review the current state of knowledge of stable isotope enrichment of leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases. Models describing evaporative enrichment of leaf water have become increasingly complex over time, reflecting enhanced spatial and temporal resolution. We recommend that practitioners choose a model with a level of complexity suited to their application, and provide guidance. At the same time, there exists some lingering uncertainty about the biophysical processes relevant to patterns of isotopic enrichment in leaf water. An important goal for future research is to link observed variations in isotopic composition to specific anatomical and physiological features of leaves that reflect differences in hydraulic design. New measurement techniques are developing rapidly, enabling determinations of both transpired and leaf water δ(18) O and δ(2) H to be made more easily and at higher temporal resolution than previously possible. We expect these technological advances to spur new developments in our understanding of patterns of stable isotope fractionation in leaf water.

Plasticity in bundle sheath extensions of heterobaric leaves.

PREMISE OF THE STUDY Leaf venation is linked to physiological performance, playing a critical role in ecosystem function. Despite the importance of leaf venation, associated bundle sheath extensions (BSEs) remain largely unstudied. Here, we quantify plasticity in the spacing of BSEs over irradiance and precipitation gradients. Because physiological function(s) of BSEs remain uncertain, we additionally explored a link between BSEs and water use efficiency (WUE). METHODS We sampled leaves of heterobaric trees along intracrown irradiance gradients in natural environments and growth chambers and correlated BSE spacing to incident irradiance. Additionally, we sampled leaves along a precipitation gradient and correlated BSE spacing to precipitation and bulk δ(13)C, a proxy for intrinsic WUE. BSE spacing was quantified using a novel semiautomatic method on fresh leaf tissue. KEY RESULTS With increased irradiance or decreased precipitation, Liquidambar styraciflua decreased BSE spacing, while Acer saccharum showed little variation in BSE spacing. Two additional species, Quercus robur and Platanus occidentalis, decreased BSE spacing with increased irradiance in growth chambers. BSE spacing correlated with bulk δ(13)C, a proxy for WUE in L. styraciflua, Q. robur, and P. occidentalis leaves but not in leaves of A. saccharum. CONCLUSIONS We demonstrated that BSE spacing is plastic with respect to irradiance or precipitation and independent from veins, indicating BSE involvement in leaf adaptation to a microenvironment. Plasticity in BSE spacing was correlated with WUE only in some species, not supporting a function in water relations. We discuss a possible link between BSE plasticity and life history, particularly canopy position.

Bioclimatic transect networks: powerful observatories of ecological change

Abstract Transects that traverse substantial climate gradients are important tools for climate change research and allow questions on the extent to which phenotypic variation associates with climate, the link between climate and species distributions, and variation in sensitivity to climate change among biomes to be addressed. However, the potential limitations of individual transect studies have recently been highlighted. Here, we argue that replicating and networking transects, along with the introduction of experimental treatments, addresses these concerns. Transect networks provide cost‐effective and robust insights into ecological and evolutionary adaptation and improve forecasting of ecosystem change. We draw on the experience and research facilitated by the Australian Transect Network to demonstrate our case, with examples, to clarify how population‐ and community‐level studies can be integrated with observations from multiple transects, manipulative experiments, genomics, and ecological modeling to gain novel insights into how species and systems respond to climate change. This integration can provide a spatiotemporal understanding of past and future climate‐induced changes, which will inform effective management actions for promoting biodiversity resilience.

Distortion of carbon isotope excursion in bulk soil organic matter during the Paleocene-Eocene thermal maximum

The Paleocene-Eocene thermal maximum was a period of abrupt, transient global warming, fueled by a large release of 13C-depleted carbon and marked globally by a negative carbon isotope excursion. While the carbon isotope excursion is often identified in the carbon isotope ratios of bulk soil organic matter (δ13Corg), these records can be biased by factors associated with production, degradation, and sources of sedimentary carbon input. To better understand these factors, we compared δ13Corg values from Paleocene-Eocene thermal maximum rocks in the southeastern Bighorn Basin, Wyoming, with those derived from leaf wax n -alkanes (δ13C n -alk). While both δ13C n -alk and δ13Corg records indicate an abrupt, negative shift in δ13C values, the carbon isotope excursions observed in bulk organic matter are smaller in magnitude and shorter in duration than those in n -alkanes. To explore these discrepancies, we modeled predicted total plant tissue carbon isotope (δ13CTT) curves from the δ13C n -alk record using enrichment factors determined in modern C3 plants. Measured δ13Corg values are enriched in 13C relative to predicted δ13CTT, with greater enrichment during the Paleocene-Eocene thermal maximum than before or after. The greater 13C enrichment could reflect increased degradation of autochthonous organic matter, increased input of allochthonous fossil carbon enriched in 13C, or both. By comparing samples from organic-rich and organic-poor depositional environments, we infer that microbial degradation rates doubled during the Paleocene-Eocene thermal maximum, and we calculate that fossil carbon input increased ∼28%−63%. This approach to untangling the controls on the isotopic composition of bulk soil carbon is an important development that will inform not only future studies of global carbon cycle dynamics during the Paleocene-Eocene thermal maximum hyperthermal event, but also any study that seeks to correlate or estimate duration and magnitude of past events using soil organic carbon.

Initial Expansion of C4 Vegetation in Australia During the Late Pliocene

J.W. Andrae, F.A. McInerney, P.J. Polissar, J.M.K. Sniderman, S. Howard, P.A. Hall and S.R. Phelps

Evaluating methods for extraction of α-cellulose from leaves of Melaleuca quinquenervia for stable carbon and oxygen isotope analysis

RATIONALE Purification of α-cellulose from plant tissues is commonly conducted to facilitate the reliable measurement of stable isotope ratios. Prior research has shown that different plant species and tissues react differently to standardised cellulose extraction techniques. Thus, no single method can be applied to all materials and careful consideration must be undertaken when selecting an extraction technique. METHODS In order to evaluate their suitability for use on Melaleuca quinquenervia leaves, a suite of eight different cellulose extraction techniques were tested. Leaves of this species are preserved in perched lakes on southeast Queenslands sand islands and are a focus of ongoing palaeoclimate research. Elemental analyser/isotope ratio mass spectrometry was used to measure stable carbon and oxygen isotopic ratios and sample composition was measured using Fourier transform infrared spectroscopy. RESULTS We demonstrate that the standard Brendel extraction technique, particularly with a higher reagent volume and longer boiling time, produces cellulose with the lowest spread in isotopic ratios among replicates, and with the fewest impurities detected by Fourier transform infrared spectroscopy. We also show that pre-treating the leaves to extract leaf wax lipids in order to enable paired analysis from the same sub-fossil leaves does not significantly affect the quality of the isotopic results. CONCLUSIONS The standard Brendel method allows the most precise stable carbon and oxygen information to be retrieved from the leaves of M. quinquenervia. This unlocks the potential to study palaeoclimate proxy records from our study site and potentially throughout the natural range of the species across eastern Australia, Papua New Guinea and New Caledonia.

Functional acclimation across microgeographic scales in Dodonaea viscosa

We studied a native Australian shrub—Dodonaea viscosa, or sticky hop bush—in the wild and in a gardening experiment and found that the species can readily adapt to different environments. Our findings are interesting because the plants we used came from sites with quite different environmental conditions, although they were only short distances apart. Our findings indicate that the potential risks associated with moving plants between sites with different environmental conditions are not likely to cause negative outcomes for restoration projects using this species, which is commonly used for restoration in southern Australia.