The combined effect of branch position, temperature, and VPD on gas exchange and water-use efficiency of Norway spruce

The impact of climate change on the physiological processes of Norway spruce in Central Europe is a significant concern. The increased temperature and evaporative demand associated with climate change may negatively affect its photosynthesis and carbon-water balance. This study tests the combined effect of branch position, temperature (T) and water vapour pressure deficit (VPD) on net photosynthetic rate (P$_N$), water vapour stomatal conductance (g$_s$), and intrinsic water-use efficiency (WUE$_i$) of Norway spruce. More than 11 000 gas-exchange measurements during the summer of 2018 revealed that branch position significantly affects gas exchange and WUE$_i$ of juvenile Norway spruce trees. Northern branches showed on average 21% increased P$_N$, 35% higher g$_s$, and 8% lower WUE$_i$ compared to the southern branches (across T and VPD conditions). The P$_N$ and g$_s$ differences between the branches were temperature- and VPD-dependent. We observed the negative impact of raising temperature on gas exchange and WUE$_i$ for both treatments, with a 40°C threshold causing a rapid decline in WUE$_i$. Variability of the southern branches\u27 WUE$_i$ at 42°C was abruptly increased due to the decoupling of P$_N$ and g$_s$ (low P$_N$, high g$_s$). Surprisingly, raising VPD showed no significant impact on WUE$_i$ of Norway spruce. The results of this study provide necessary information for upscaling and process-based modelling of whole-crown gas exchange. Moreover, experimental studies of gas exchange should take into consideration the branch position effect to prevent possible bias errors

Direct measurement of thermal conductivity in solid iron at planetary core conditions

The conduction of heat through minerals and melts at extreme pressures and temperatures is of central importance to the evolution and dynamics of planets. In the cooling Earth’s core, the thermal conductivity of iron alloys defines the adiabatic heat flux and therefore the thermal and compositional energy available to support the production of Earth’s magnetic field via dynamo action1, 2, 3. Attempts to describe thermal transport in Earth’s core have been problematic, with predictions of high thermal conductivity4, 5, 6, 7 at odds with traditional geophysical models and direct evidence for a primordial magnetic field in the rock record8, 9, 10. Measurements of core heat transport are needed to resolve this difference. Here we present direct measurements of the thermal conductivity of solid iron at pressure and temperature conditions relevant to the cores of Mercury-sized to Earth-sized planets, using a dynamically laser-heated diamond-anvil cell11, 12. Our measurements place the thermal conductivity of Earth’s core near the low end of previous estimates, at 18–44 watts per metre per kelvin. The result is in agreement with palaeomagnetic measurements10 indicating that Earth’s geodynamo has persisted since the beginning of Earth’s history, and allows for a solid inner core as old as the dynamo

Phase Transition Lowering in Dynamically Compressed Silicon

Silicon, being one of the most abundant elements in nature, attracts wide-ranging scientific and technological interest. Specifically, in its elemental form, crystals of remarkable purity can be produced. One may assume that this would lead to silicon being well understood, and indeed, this is the case for many ambient properties, as well as for higher-pressure behaviour under quasi-static loading. However, despite many decades of study, a detailed understanding of the response of silicon to rapid compression—such as that experienced under shock impact—remains elusive. Here, we combine a novel free-electron laser-based X-ray diffraction geometry with laser-driven compression to elucidate the importance of shear generated during shock compression on the occurrence of phase transitions. We observe lowering of the hydrostatic phase boundary in elemental silicon, an ideal model system for investigating high-strength materials, analogous to planetary constituents. Moreover, we unambiguously determine the onset of melting above 14 GPa, previously ascribed to a solid–solid phase transition, undetectable in the now conventional shocked diffraction geometry; transitions to the liquid state are expected to be ubiquitous in all systems at sufficiently high pressures and temperatures

In situ study of the high pressure high-temperature stability field of TaN and of the compressibilities of ϑ-TaN and TaON

The high pressure high-temperature stability of tantalum mononitrides was investigated at 8–23.5 GPa and 1400–3500 K by in situ powder synchrotron X-ray diffraction using the laser-heated diamond anvil cell and ϵ-TaN as starting material. The transformation from ϵ-TaN to ϑ-TaN was observed and ϑ-TaN was confirmed as the stable high-(p, T) tantalum mononitride. At temperatures above 3000 K at 16–23.5 GPa, the formation of a novel quenchable high pressure high-temperature phase was observed. Under nitrogen excess ϵ-TaN reacted with nitrogen to the high-(p, T) phase η-Ta2N3 as the main stable phase at 8–13 GPa and about 2000 K. This behaviour is similar to the reaction of pure Ta with nitrogen at high-(p, T) conditions. In addition, we have determined the bulk moduli of ϑ-TaN (B=360(15) GPa), which is less compressible than ϵ-TaN and η-Ta2N3, and of TaON (B=267(8) GPa)