Jean-Christophe Domec

Maximum height in a conifer is associated with conflicting requirements for xylem design

Despite renewed interest in the nature of limitations on maximum tree height, the mechanisms governing ultimate and species-specific height limits are not yet understood, but they likely involve water transport dynamics. Tall trees experience increased risk of xylem embolism from air-seeding because tension in their water column increases with height because of path-length resistance and gravity. We used morphological measurements to estimate the hydraulic properties of the bordered pits between tracheids in Douglas-fir trees along a height gradient of 85 m. With increasing height, the xylem structural modifications that satisfied hydraulic requirements for avoidance of runaway embolism imposed increasing constraints on water transport efficiency. In the branches and trunks, the pit aperture diameter of tracheids decreases steadily with height, whereas torus diameter remains relatively constant. The resulting increase in the ratio of torus to pit aperture diameter allows the pits to withstand higher tensions before air-seeding but at the cost of reduced pit aperture conductance. Extrapolations of vertical trends for trunks and branches show that water transport across pits will approach zero at a heights of 109 m and 138 m, respectively, which is consistent with historic height records of 100–127 m for this species. Likewise, the twig water potential corresponding to the threshold for runaway embolism would be attained at a height of ≈107 m. Our results suggest that the maximum height of Douglas-fir trees may be limited in part by the conflicting requirements for water transport and water column safety.

Gold Mining in the Peruvian Amazon: Global Prices, Deforestation, and Mercury Imports

Many factors such as poverty, ineffective institutions and environmental regulations may prevent developing countries from managing how natural resources are extracted to meet a strong market demand. Extraction for some resources has reached such proportions that evidence is measurable from space. We present recent evidence of the global demand for a single commodity and the ecosystem destruction resulting from commodity extraction, recorded by satellites for one of the most biodiverse areas of the world. We find that since 2003, recent mining deforestation in Madre de Dios, Peru is increasing nonlinearly alongside a constant annual rate of increase in international gold price (∼18%/yr). We detect that the new pattern of mining deforestation (1915 ha/year, 2006–2009) is outpacing that of nearby settlement deforestation. We show that gold price is linked with exponential increases in Peruvian national mercury imports over time (R2 = 0.93, p = 0.04, 2003–2009). Given the past rates of increase we predict that mercury imports may more than double for 2011 (∼500 t/year). Virtually all of Perus mercury imports are used in artisanal gold mining. Much of the mining increase is unregulated/artisanal in nature, lacking environmental impact analysis or miner education. As a result, large quantities of mercury are being released into the atmosphere, sediments and waterways. Other developing countries endowed with gold deposits are likely experiencing similar environmental destruction in response to recent record high gold prices. The increasing availability of satellite imagery ought to evoke further studies linking economic variables with land use and cover changes on the ground.

Acclimation of leaf hydraulic conductance and stomatal conductance of Pinus taeda (loblolly pine) to long‐term growth in elevated CO2 (free‐air CO2 enrichment) and N‐fertilization

We investigated how leaf hydraulic conductance (K(leaf)) of loblolly pine trees is influenced by soil nitrogen amendment (N) in stands subjected to ambient or elevated CO(2) concentrations (CO(2)(a) and CO(2)(e), respectively). We also examined how K(leaf) varies with changes in reference leaf water potential (Psi(leaf-ref)) and stomatal conductance (g(s-ref)) calculated at vapour pressure deficit, D of 1 kPa. We detected significant reductions in K(leaf) caused by N and CO(2)(e), but neither treatment affected pre-dawn or midday Psi(leaf). We also detected a significant CO(2)(e)-induced reduction in g(s-ref) and Psi(leaf-ref). Among treatments, the sensitivity of K(leaf) to Psi(leaf) was directly related to a reference K(leaf) (K(leaf-ref) computed at Psi(leaf-ref)). This liquid-phase response was reflected in a similar gas-phase response, with g(s) sensitivity to D proportional to g(s-ref). Because leaves represented a substantial component of the whole-tree conductance, reduction in K(leaf) under CO(2)(e) affected whole-tree water use by inducing a decline in g(s-ref). The consequences of the acclimation of leaves to the treatments were: (1) trees growing under CO(2)(e) controlled morning leaf water status less than CO(2)(a) trees resulting in a higher diurnal loss of K(leaf); (2) the effect of CO(2)(e) on g(s-ref) was manifested only during times of high soil moisture.

Bordered pit structure and function determine spatial patterns of air-seeding thresholds in xylem of Douglas-fir (Pseudotsuga menziesii; Pinaceae) trees

The air-seeding hypothesis predicts that xylem embolism resistance is linked directly to bordered pit functioning. We tested this prediction in trunks, roots, and branches at different vertical and radial locations in young and old trees of Pseudotsuga menziesii. Dimensions of bordered pits were measured from light and scanning electron micrographs, and physiological data were from published values. Consistent with observations, calculations showed that earlywood tracheids were more resistant to embolism than latewood tracheids, mainly from earlywood having stretchier pit membranes that can distend and cover the pit aperture. Air seeding that occurs in earlywood appears to happen through gaps between the torus edge and pit border, as shown by the similar calculated pressures required to stretch the membrane over the pit aperture and to cause embolism. Although bordered pit functioning was correlated with tracheid hydraulic diameter, pit pore size and above all pit aperture constrained conductivity the most. From roots to branches and from the trunk base to higher on the trunk, hydraulic resistance of the earlywood pit membrane increased significantly because of a decrease in the size of the pit aperture and size and number of margo pores. Moreover, overall wood conductivity decreased, in part due to lower pit conductivity and a decrease in size and frequency of pits. Structural and functional constraints leading to the trade-off of efficiency against safety of water transport were also demonstrated at the individual pit level, with a positive correlation between pit membrane resistance on an area basis and the pressure differential required to cause membrane stretching, a characteristic that is essential for pit aspiration.

Hydraulic redistribution of soil water by roots affects whole‐stand evapotranspiration and net ecosystem carbon exchange

*Hydraulic redistribution (HR) of water via roots from moist to drier portions of the soil occurs in many ecosystems, potentially influencing both water use and carbon assimilation. *By measuring soil water content, sap flow and eddy covariance, we investigated the temporal variability of HR in a loblolly pine (Pinus taeda) plantation during months of normal and below-normal precipitation, and examined its effects on tree transpiration, ecosystem water use and carbon exchange. *The occurrence of HR was explained by courses of reverse flow through roots. As the drought progressed, HR maintained soil moisture above 0.15 cm(3) cm(-3) and increased transpiration by 30-50%. HR accounted for 15-25% of measured total site water depletion seasonally, peaking at 1.05 mm d(-1). The understory species depended on water redistributed by the deep-rooted overstory pine trees for their early summer water supply. Modeling carbon flux showed that in the absence of HR, gross ecosystem productivity and net ecosystem exchange could be reduced by 750 and 400 g C m(-2) yr(-1), respectively. *Hydraulic redistribution mitigated the effects of soil drying on understory and stand evapotranspiration and had important implications for net primary productivity by maintaining this whole ecosystem as a carbon sink.

Decoupling the influence of leaf and root hydraulic conductances on stomatal conductance and its sensitivity to vapour pressure deficit as soil dries in a drained loblolly pine plantation

The study examined the relationships between whole tree hydraulic conductance (K(tree)) and the conductance in roots (K(root)) and leaves (K(leaf)) in loblolly pine trees. In addition, the role of seasonal variations in K(root) and K(leaf) in mediating stomatal control of transpiration and its response to vapour pressure deficit (D) as soil-dried was studied. Compared to trunk and branches, roots and leaves had the highest loss of conductivity and contributed to more than 75% of the total tree hydraulic resistance. Drought altered the partitioning of the resistance between roots and leaves. As soil moisture dropped below 50%, relative extractable water (REW), K(root) declined faster than K(leaf). Although K(tree) depended on soil moisture, its dynamics was tempered by the elongation of current-year needles that significantly increased K(leaf) when REW was below 50%. After accounting for the effect of D on g(s), the seasonal decline in K(tree) caused a 35% decrease in g(s) and in its sensitivity to D, responses that were mainly driven by K(leaf) under high REW and by K(root) under low REW. We conclude that not only water stress but also leaf phenology affects the coordination between K(tree) and g(s) and the acclimation of trees to changing environmental conditions.

Variable conductivity and embolism in roots and branches of four contrasting tree species and their impacts on whole-plant hydraulic performance under future atmospheric CO2 concentration

Anatomical and physiological acclimation to water stress of the tree hydraulic system involves trade-offs between maintenance of stomatal conductance and loss of hydraulic conductivity, with short-term impacts on photosynthesis and long-term consequences to survival and growth. Here, we study the role of variations in root and branch maximum hydraulic specific conductivity (k(s-max)) under high and low soil moisture in determining whole-tree hydraulic conductance (K(tree)) and in mediating stomatal control of gas exchange in four contrasting tree species growing under ambient and elevated CO₂ (CO₂(a) and CO₂(e)). We hypothesized that K(tree) would adjust to CO₂(e) through an increase in root and branch k(s-max) in response to anatomical adjustments. However, physiological changes observed under CO₂(e) were not clearly related to structural change in the xylem of any of the species. The only large effect of CO₂(e) occurred in branches of Liquidambar styraciflua L. and Cornus florida L. where an increase in k(s-max) and a decrease in xylem resistance to embolism (-P₅₀) were measured. Across species, embolism in roots explained the loss of K(tree) and therefore indirectly constituted a hydraulic signal involved in stomatal regulation and in the reduction of G(s-ref), the sap-flux-scaled mean canopy stomatal conductance at a reference vapour pressure deficit of 1 kPa. Across roots and branches, the increase in k(s-max) was associated with a decrease in -P₅₀, a consequence of structural acclimation such as larger conduits, lower pit resistance and lower wood density. Across species, treatment-induced changes in K(tree) translated to similar variation in G(s-ref). However, the relationship between G(s-ref) and K(tree) under CO₂(a) was steeper than under CO₂(e), indicating that CO₂(e) trees have lower G(s-ref) at a given K(tree) than CO₂(a) trees. Under high soil moisture, CO₂(e) greatly reduced G(s-ref). Under low soil moisture, CO₂(e) reduced G(s-ref) of only L. styraciflua and Ulmus alata. In some species, higher xylem dysfunction under CO₂(e) might impact tree performance in a future climate when increased evaporative demand could cause a greater loss of hydraulic function. The results contributed to our knowledge of the physiological and anatomical mechanisms underpinning the responses of tree species to drought and more generally to global change.

Does homeostasis or disturbance of homeostasis in minimum leaf water potential explain the isohydric versus anisohydric behavior of Vitis vinifera L. cultivars

Due to the diurnal and seasonal fluctuations in leaf-to-air vapor pressure deficit (D), one of the key regulatory roles played by stomata is to limit transpiration-induced leaf water deficit. Different types of plants are known to vary in the sensitivity of stomatal conductance (gs) to D with important consequences for their survival and growth. Plants that minimize any increase in transpiration with increasing D have a tight stomatal regulation of a constant minimum leaf water potential (Ψleaf); these plants are termed as ‘isohydric’ (Stocker 1956). Plants that have less control of Ψleaf have been termed as ‘anisohydric’ (Tardieu and Simonneau 1998). Isohydric plants maintain a constant Ψleaf by reducing gs and transpiration under drought stress. Therefore, as drought pushes soil water potential (Ψsoil) below this Ψleaf set point, the plant can no longer extract water for gas exchange. Anisohydric plants allow Ψleaf to decrease with rising D, reaching a much lower Ψleaf in droughted plants relative to well-watered plants (Tardieu and Simonneau 1998), so this strategy produces a gradient between Ψsoil and Ψleaf that allows gas exchange to continue over a greater decline in Ψsoil. Thus, anisohydric plants sustain longer periods of transpiration and photosynthesis, even under large soil water deficit, and are thought to be more drought tolerant than isohydric species (McDowell 2011). In practice, the distinctions between isohydric and anisohydric strategies are often not clear (Franks et al. 2007), even among different cultivars of the same species. For example, cultivars of poplar (Hinckley et al. 1994) and grapevine (Schultz 2003, Lovisolo et al. 2010) have been shown to exhibit both contrasting hydraulic behaviors. A third mode of behavior was also suggested by Franks et al. (2007), in which the difference between soil and midday water potential (Ψsoil − Ψleaf) is maintained seasonally constant but Ψleaf fluctuates in synchrony with soil water availability (isohydrodynamic behavior). The lack of a clear distinction between these two strategies and the complex and variable responses of stomata to D under high and low soil moisture is depicted in two papers in this issue (Rogiers et al. 2012 and Zhang et al. 2012), showing that even typically anisohydric grape (Vitis vinifera L.) cultivars (Semillon and Merlot, respectively) may constrain gs during periods of extremely low Ψsoil. The same individuals can switch from an isohydric-like behavior when transpiration is low to an anisohydric-like behavior with increasing water demand. Interestingly, both studies indicated that classifying species as either isohydric or anisohydric is a simplistic view of stomatal functioning and does not represent well the complex stomatal behavior under drying soil, and Zhang et al. (2012) also reported an isohydrodynamic behavior. Both studies suggested that when soil water is limited, gs is aimed at protecting the integrity of the hydraulic system, whereas as soil water content increases, stomata regulate transpiration less. The results of Zhang et al. (2012) indicated that under limited soil moisture the decrease in gs with increasing D was proportional to reference gs (gs at D = 1 kPa); which is in agreement with the stomata-sensitivity model developed by Oren et al. (1999) for isohydric species (see xeric line in Figure 1A). However, a significant departure from this theoretical model was observed under high soil moisture (see wet and mesic lines in Figure 1B). Similarly, in this issue Rogiers et al. (2012) showed that under Tree Physiology 32, 245–248 doi:10.1093/treephys/tps013

A broad survey of hydraulic and mechanical safety in the xylem of conifers

Summary Torus overlap and tracheid wall thickness are strongly correlated with cavitation resistance based on data from 115 conifer species.

Safety factors for xylem failure by implosion and air-seeding within roots, trunks and branches of young and old conifer trees

The cohesion-tension theory of water transport states that hydrogen bonds hold water molecules together and that they are pulled through the xylem under tension. This tension could cause transport failure in at least two ways: collapse of the conduit walls (implosion), or rupture of the water column through air-seeding. The objective of this research was to elucidate the functional significance of variations in tracheid anatomical features, earlywood to latewood ratios and wood densities with position in young and old Douglas-fir and ponderosa pine trees in terms of their consequences for the safety factors for tracheid implosion and air-seeding. For both species, wood density increased linearly with percent latewood for root, trunk and branch samples. However, the relationships between anatomy and hydraulic function in trunks differed from those in roots and branches. In roots and branches increased hydraulic efficiency was achieved at the cost of increased vulnerability to air-seeding. Mature wood of trunks had earlywood with wide tracheids that optimized water transport and had a high percentage of latewood that optimized structural support. Juvenile wood had higher resistance to air-seeding and cell wall implosion. The two safety factors followed similar axial trends from roots to terminal branches and were similar for both species studied and between juvenile and mature wood.