Kevin A. Simonin

Foliar water uptake: a common water acquisition strategy for plants of the redwood forest.

Evaluations of plant water use in ecosystems around the world reveal a shared capacity by many different species to absorb rain, dew, or fog water directly into their leaves or plant crowns. This mode of water uptake provides an important water subsidy that relieves foliar water stress. Our study provides the first comparative evaluation of foliar uptake capacity among the dominant plant taxa from the coast redwood ecosystem of California where crown-wetting events by summertime fog frequently occur during an otherwise drought-prone season. Previous research demonstrated that the dominant overstory tree species, Sequoia sempervirens, takes up fog water by both its roots (via drip from the crown to the soil) and directly through its leaf surfaces. The present study adds to these early findings and shows that 80% of the dominant species from the redwood forest exhibit this foliar uptake water acquisition strategy. The plants studied include canopy trees, understory ferns, and shrubs. Our results also show that foliar uptake provides direct hydration to leaves, increasing leaf water content by 2–11%. In addition, 60% of redwood forest species investigated demonstrate nocturnal stomatal conductance to water vapor. Such findings indicate that even species unable to absorb water directly into their foliage may still receive indirect benefits from nocturnal leaf wetting through suppressed transpiration. For these species, leaf-wetting events enhance the efficacy of nighttime re-equilibration with available soil water and therefore also increase pre-dawn leaf water potentials.

Fog interception by Sequoia sempervirens (D. Don) crowns decouples physiology from soil water deficit

Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil-plant-atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.

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.

Hydraulic conductance of leaves correlates with leaf lifespan: implications for lifetime carbon gain

Previous research suggests that the lifetime carbon gain of a leaf is constrained by a tradeoff between metabolism and longevity. The biophysical reasons underlying this tradeoff are not fully understood. We used a photosynthesis-leaf water balance model to evaluate biophysical constraints on carbon gain. Leaf hydraulic conductance (K(Leaf)), carbon isotope discrimination (Δ(13)C), leaf mass per unit area (LMA) and the driving force for water transport from stem to leaf (ΔΨ(Stem-Leaf)) were characterized for leaves spanning three orders of magnitude in surface area and two orders of magnitude in lifespan. We observed positive isometric scaling between K(Leaf) and leaf area but no relationship between Δ(13)C and leaf area. Leaf lifespan and LMA had minimal effect on K(Leaf) per unit leaf area, but a negative correlation exists among LMA, lifespan, and K(Leaf) per unit dry mass. During periods of leaf water loss, ΔΨ(Stem-Leaf) was relatively constant. We show for the first time that K(Leaf, mass), an index of the carbon cost associated with water use, is negatively correlated with lifespan. This highlights the importance of characterizing K(Leaf, mass) and suggests a tradeoff between resource investment in liquid phase processes and structural rigidity.

Online CO2 and H2O oxygen isotope fractionation allows estimation of mesophyll conductance in C4 plants, and reveals that mesophyll conductance decreases as leaves age in both C4 and C3 plants

Mesophyll conductance significantly, and variably, limits photosynthesis but we currently have no reliable method of measurement for C4 plants. An online oxygen isotope technique was developed to allow quantification of mesophyll conductance in C4 plants and to provide an alternative estimate in C3 plants. The technique is compared to an established carbon isotope method in three C3 species. Mesophyll conductance of C4 species was similar to that in the C3 species measured, and declined in both C4 and C3 species as leaves aged from fully expanded to senescing. In cotton leaves, simultaneous measurement of carbon and oxygen isotope discrimination allowed the partitioning of total conductance to the chloroplasts into cell wall and plasma membrane versus chloroplast membrane components, if CO2 was assumed to be isotopically equilibrated with cytosolic water, and the partitioning remained stable with leaf age. The oxygen isotope technique allowed estimation of mesophyll conductance in C4 plants and, when combined with well-established carbon isotope techniques, may provide additional information on mesophyll conductance in C3 plants.

Species differences in the seasonality of evergreen tree transpiration in a Mediterranean climate: Analysis of multiyear, half-hourly sap flow observations

In Mediterranean climates, the season of water availability (winter) is out of phase with the season of light availability and atmospheric moisture demand (summer). We investigate the seasonality of evergreen tree transpiration in a Mediterranean climate, using observations from a small (4000 m2), forested, steep (32°) hillslope, in the northern California Coast Range. We analyze 3 years of half-hourly measurements from 39 sap flow sensors in 26 trees, six depth profiles of soil moisture measured by TDR, and spatially distributed measurements of micrometeorology from five locations. The sap flow measurements show that two common evergreen tree species have different seasons of peak transpiration. Douglas-firs (Pseudotsuga menziesii) maintain significant transpiration through the winter rainy season and transpire maximally in the spring, followed by a sharp decline in transpiration in the summer dry season. Pacific madrones (Arbutus menziesii), and to a lesser extent other broadleaf evergreen species (Quercus wislizeni, Notholithocarpus densiflorus, Umbellularia californica), in contrast, transpire maximally in the summer dry season. The seasonal patterns are quantified using principal component analysis. Markov chain Monte Carlo estimation of response to environmental variables shows that the difference in transpiration seasonality arises from different sensitivities to atmospheric evaporative demand and root-zone moisture. The different sensitivities to atmospheric evaporative demand also create species differences in transpiration variability at synoptic time scales. Using the sap flow measurements and a regional forest inventory, a bottom-up regional transpiration estimate is constructed. The estimate suggests that sensitivity of Douglas-fir transpiration to water stress suppresses dry season evapotranspiration at the regional scale.

The Roles of Stable Isotopes in Forest Hydrology and Biogeochemistry

Forests cover approximately one third of the terrestrial land surfaces (Hansen et al. 2000) and are arguably the most important biome type on Earth for acquiring, transforming, and recycling major and limiting biogeochemical resources such as water, carbon, and many mineral elements such as nitrogen or phosphorous; well-documented drivers of global biogeochemical cycles. Studying the processes that govern the manner and magnitude of biogeochemical cycling through forest systems with their enormous stature, age, complexity, diversity, and spatiotemporal heterogeneity and that are rarely ever in steady-state possess real and mind-boggling challenges for forest scientists. These challenges may cause some to abandon efforts to understand the complex nature of forests biogeochemical systems altogether. However, in recent years, the application of stable isotope methods, at both natural abundance levels and through targeted experiments using enriched isotopes have revolutionized our capacity to explore a wide range of biogeochemical processes in forests. In this regard, stable isotopes are now known to provide new and important insights from tracing the origin and movements of key elements and substances through the Earth–plant–atmosphere continuum (Gat 1996; Dawson et al. 1998, 2002; Fry 2006; Sharp 2007), to indicating the presence and the magnitude of key Earth system processes (Holton et al. 2006; Bowling et al. 2008), to integrating various biogeochemical processes in both space and time (Bowen et al. 2009; Craine et al. 2009; West et al. 2010b), to also recording biological responses to the Earth’s changing environmental conditions (McCarroll and Loader 2004; Augusti et al. 2006; Dawson and Siegwolf 2007; Sternberg 2009).

Isotopic composition of transpiration and rates of change in leaf water isotopologue storage in response to environmental variables

During daylight hours, the isotope composition of leaf water generally approximates steady-state leaf water isotope enrichment model predictions. However, until very recently there was little direct confirmation that isotopic steady-state (ISS) transpiration in fact exists. Using isotope ratio infrared spectroscopy (IRIS) and leaf gas exchange systems we evaluated the isotope composition of transpiration and the rate of change in leaf water isotopologue storage (isostorage) when leaves were exposed to variable environments. In doing so, we developed a method for controlling the absolute humidity entering the gas exchange cuvette for a wide range of concentrations without changing the isotope composition of water vapour. The measurement system allowed estimation of (18)O enrichment both at the evaporation site and for bulk leaf water, in the steady state and the non-steady state. We show that non-steady-state effects dominate the transpiration isoflux even when leaves are at physiological steady state. Our results suggest that a variable environment likely prevents ISS transpiration from being achieved and that this effect may be exacerbated by lengthy leaf water turnover times due to high leaf water contents.

Observed relationships between leaf H218O Péclet effective length and leaf hydraulic conductance reflect assumptions in Craig–Gordon model calculations

Stable oxygen isotope techniques may be a useful tool to investigate the pathways of water movement within leaves. However, implementation of such methods is limited due to uncertainty in the effective path length (L) for the Péclet effect in leaf water enrichment models. Previous studies have found relationships between L and physiological parameters such as transpiration rate (E) and leaf hydraulic conductance (k(leaf)) both within and between species. However, these studies relied on assumptions in their calculation of L, which were not directly tested. Eucalyptus paniculata Smith was used to evaluate the relationships between L, k(leaf) and E under differing water availability and a range of leaf temperatures. Coupled gas exchange and transpiration isotope measurements allowed previous assumptions to be directly tested. L was significantly and negatively related to both k(leaf) and E when the isotopic signature of water vapour was assumed to be in equilibrium with source water, was equivalent to the room vapour or equal to source water. However, the relationship between L and k(leaf) was non-significant when measured δ( 18)O of transpired vapour was used and disappeared entirely when non-steady-state leaves were excluded, and when evaporation site water was calculated from coupled gas exchange and transpiration isotope values. These results suggest that great care must be taken when calculating L, particularly regarding assumptions of isotopic steady state and δ( 18)O of vapour. Previous suggestions of changes in pathways for water movement as transpiration rate varied need to be reassessed in light of these observations.

Increasing leaf hydraulic conductance with transpiration rate minimizes the water potential drawdown from stem to leaf

Summary Under well-watered conditions leaf hydraulic conductance increases with transpiration rate. This reduces the water potential gradient in leaves and potentially improves productivity under daily variation in evaporative demand.