Subcontinental heat wave triggers terrestrial and marine, multi-taxa responses.

Heat waves have profoundly impacted biota globally over the past decade, especially where their ecological impacts are rapid, diverse, and broad-scale. Although usually considered in isolation for either terrestrial or marine ecosystems, heat waves can straddle ecosystems of both types at subcontinental scales, potentially impacting larger areas and taxonomic breadth than previously envisioned. Using climatic and multi-species demographic data collected in Western Australia, we show that a massive heat wave event straddling terrestrial and maritime ecosystems triggered abrupt, synchronous, and multi-trophic ecological disruptions, including mortality, demographic shifts and altered species distributions. Tree die-off and coral bleaching occurred concurrently in response to the heat wave, and were accompanied by terrestrial plant mortality, seagrass and kelp loss, population crash of an endangered terrestrial bird species, plummeting breeding success in marine penguins, and outbreaks of terrestrial wood-boring insects. These multiple taxa and trophic-level impacts spanned >300,000 km2-comparable to the size of California-encompassing one terrestrial Global Biodiversity Hotspot and two marine World Heritage Areas. The subcontinental multi-taxa context documented here reveals that terrestrial and marine biotic responses to heat waves do not occur in isolation, implying that the extent of ecological vulnerability to projected increases in heat waves is underestimated.

Smaller, faster stomata: scaling of stomatal size, rate of response, and stomatal conductance.

Maximum and minimum stomatal conductance, as well as stomatal size and rate of response, are known to vary widely across plant species, but the functional relationship between these static and dynamic stomatal properties is unknown. The objective of this study was to test three hypotheses: (i) operating stomatal conductance under standard conditions (g (op)) correlates with minimum stomatal conductance prior to morning light [g (min(dawn))]; (ii) stomatal size (S) is negatively correlated with g (op) and the maximum rate of stomatal opening in response to light, (dg/dt)(max); and (iii) g (op) correlates negatively with instantaneous water-use efficiency (WUE) despite positive correlations with maximum rate of carboxylation (Vc (max)) and light-saturated rate of electron transport (J (max)). Using five closely related species of the genus Banksia, the above variables were measured, and it was found that all three hypotheses were supported by the results. Overall, this indicates that leaves built for higher rates of gas exchange have smaller stomata and faster dynamic characteristics. With the aid of a stomatal control model, it is demonstrated that higher g (op) can potentially expose plants to larger tissue water potential gradients, and that faster stomatal response times can help offset this risk.

Linking hydraulic conductivity and photosynthesis to water-source partitioning in trees versus seedlings.

The relationships between hydraulic and photosynthetic properties in plants have been widely studied, but much less is known about how these properties are linked to water-source partitioning, the spatial and temporal separation of water sources in ecosystems. Plant water-source partitioning is often influenced by the proximity of groundwater from the natural surface. We studied the water acquisition strategy and hydraulic and photosynthetic properties of Tuart (Eucalyptus gomphocephala D.C.), a large coastal tree species that occupies seasonally dry habitats underlain by superficial aquifers. Our goal was to quantify water-source partitioning as the proportion of xylem water derived from the vadose and saturated zones with respect to stage of development and proximity of groundwater. We then sought to associate the proportional contribution of a given water source with xylem hydraulic and photosynthetic properties, thus conferring a linkage. Seedlings were more inclined to use surface soil water when rainfall recharge of the upper profile occurred, suggesting that they maintained or rapidly developed a proportionally high amount of functional roots in the upper, seasonally dry, soil profile. This strategy was associated with a lower xylem-area-specific hydraulic conductivity (K(S)), leaf-area-specific hydraulic conductivity (K(L)) and maximum photon yield of photosystem II (F(V)/F(M)). In contrast, trees acquired water from a variety of sources in different seasons and had a higher K(S), K(L) and F(V)/F(M). Despite the higher K(S) and K(L) in trees, the midday hydrodynamic water potential gradient from soil to leaves, ΔΨ, was similar. We conclude that there was a linkage between hydraulic and photosynthetic properties with the partitioning of water sources and that this adaptation to long-term hydrological regimes accommodated the different hydraulic characteristics and hydrological environments of trees versus seedlings.

Anisohydric but isohydrodynamic: seasonally constant plant water potential gradient explained by a stomatal control mechanism incorporating variable plant hydraulic conductance.

Isohydric and anisohydric regulations of plant water status have been observed over several decades of field, glasshouse and laboratory studies, yet the functional significance and mechanism of both remain obscure. We studied the seasonal trends in plant water status and hydraulic properties in a natural stand of Eucalyptus gomphocephala through cycles of varying environmental moisture (rainfall, groundwater depth, evaporative demand) in order to test for isohydry and to provide physiological information for the mechanistic interpretation of seasonal trends in plant water status. Over a 16 month period of monitoring, spanning two summers, midday leaf water potential (psi(leaf)) correlated with predawn psi(leaf), which was correlated with water table depth below ground level, which in turn was correlated with total monthly rainfall. Eucalyptus gomphocephala was therefore not seasonally isohydric. Despite strong stomatal down-regulation of transpiration rate in response to increasing evaporative demand, this was insufficient to prevent midday psi(leaf) from falling to levels below -2 MPa in the driest month, well into the region likely to induce xylem air embolisms, based on xylem vulnerability curves obtained in the study. However, even though midday psi(leaf) varied by over 1.2 MPa across seasons, the hydrodynamic (transpiration-induced) water potential gradient from roots to shoots (delta psi(plant)), measured as the difference between predawn and midday psi(leaf), was relatively constant across seasons, averaging 0.67 MPa. This unusual pattern of hydraulic regulation, referred to here as isohydrodynamic, is explained by a hydromechanical stomatal control model where plant hydraulic conductance is dependent on transpiration rate.

Influence of groundwater depth on the seasonal sources of water accessed by Banksia tree species on a shallow, sandy coastal aquifer.

In Mediterranean ecosystems vegetation overlying shallow, transient aquifers is often dominated by woody phreatophytes, trees and shrubs that have been shown to be dependent on groundwater for their water requirements. Natural and anthropogenic alterations of groundwater tables (abstraction) are of clear importance to phreatophytic vegetation as reduction of water tables may sever these plants from their natural water sources. Seasonal water sources were determined for species growing on a coastal dune system that overlies a shallow sandy aquifer in south-western Australia. The plants studied grew over groundwater that ranged in depth from 2.5 to 30 m. The naturally occurring stable isotope of hydrogen (deuterium, δ2H) was used to distinguish potential water sources. Isotopic ratios from vascular water of the dominant species of the study area (Banksia ilicifolia R. Br. and Banksia attenuata R. Br. trees) were compared with those of potential sources of precipitation, soil moisture and groundwater. A relatively shallow-rooted perennial shrub, Hibbertia hypericoides Benth., was also included as an isotopic reference. The results suggest that both B. attenuata and B. ilicifolia are phreatophytic as they derived some of their water from groundwater throughout the dry-wet cycle, with the exception of B. attenuata at the site of greatest depth to groundwater (30 m) which did not use groundwater. A high proportion (>50%) of groundwater use was not maintained throughout all seasons. With the onset of the hot Mediterranean summer, progressive drying of the surface soils resulted in increased use of groundwater and deep soil moisture. During the wet winter plants used proportionately more water from the upper layers of the soil profile. The degree to which groundwater was utilised by the study species was dependent on the proximity of groundwater, availability of moisture in shallower horizons of the soil profile, root system distribution and maximum root depth.