The Relationship between Embolism Resistance and Droughts for Global Forest Ecosystems

Global climate change predictions indicate increasing temperatures and shifts in rainfall patterns, resulting in increased severity and frequency of drought. These droughts are likely to cause forest decline around the world, resulting in decreased net primary productivity and plant mortality, largely attributable to hydraulic failure. Through a process called embolism, drought stress causes gas to become entrapped in the vascular systems of plants. Embolism thus prevents water transport necessary for photosynthesis to occur in plants. If a plant is unable to conduct photosynthesis, it will eventually desiccate and die. The threshold limits for hydraulic failure across different species and environments is largely unknown. Choat et al. (2012) compared the vulnerability of a variety of woody species based on drought-induced embolism. The authors found that seventy percent of the 226 forest species studied operate within narrow hydraulic safety margins against damaging levels of drought stress. If these margins are exceeded, long term implications including decreased productivity and survival are affected. These safety margins are largely independent of mean annual precipitation, thus demonstrating that there is global convergence in the vulnerability of forests to drought, meaning that all forest biomes are equally susceptible to hydraulic failure, regardless of initial rainfall environments.
—Hilary Haskell
            Choat, B., Jansen S., Brodribb, T., Cochard, H., Delozn, S., Bhaskar, R., Bucci, S., Feild, T., Gleason, S., Hacke, U., Jacobsen, A., Lens, F., Maherali, H., Martinez-Vilalta, J., Mayr, S., Mencuccini, M., Mitchell, P., Nardini, A., Pitterman, J., Brandon Pratt, R., Perry, J., Westoboy, M., Wright, I.,

Zanne, A., 2012. Global Convergence in the Vulnerability of Forests to Drought. Nature 491, 752–55.

Choat et al. quantified comparisons of plants’ sensitivity to drought stress using the strength of the liquid (hydraulic) connection between soil and leaves through water-transporting tissue within the vascular xylem tissue system of plants. Plant leaves’ stomata act as gateways that conduct the exchange of gases such as water vapor, carbon dioxide, and oxygen from within the plant to the atmosphere. When soil becomes arid, stomata regulate water loss from the leaves in order to maintain the xylem pressure within a range that will prevent embolism.  After prolonged periods of drought and drying soil, stomatal closure slows but does not halt the decrease in xylem pressure and hydraulic capacity. If soil water is not replenished, complete hydraulic failure occurs, causing the plant to desiccate and die.
Embolism is essentially air blockages within the xylem of plants. The process by which embolism occurs is dependent on the xylem structure of woody plants. Cavation occurs when liquid water changes phases to become vapor. This process occurs because water in the xylem is under negative pressure. The air emboli (blockages within the xylem) prevent or reduce the plant’s ability to transport water from soil to sites of photosynthesis necessary for the plant to survive. Woody plants that are able to survive and recover after sustained drought are generally considered embolism resistant. Embolism resistance varies across species, and largely depends on xylem structure. The relationship between xylem pressure and loss of hydraulic conductivity due to gas emboli defines the plant’s resistance to embolism. The index used to quantify this relationship is water potential measured in megapascals (MPa) (Ψx), at which fifty percent loss of conductivity within the xylem structure occurs. A slight drop in Ψx at this point will confer a substantial reduction in hydraulic function. When Ψxfalls below Ψ50, the water transport of the xylem is impaired and the plant is at risk of embolism. After embolism occurs, there are irreversible reductions in productivity, tissue damage, and death. Embolism resistance is also described by a vulnerability curve, which shows the percentage loss of hydraulic conductivity (PLC, %) as a function of decreasing xylem pressure measured by ΨxMPa. Decreasing xylem pressure reflects increased drought stress, and therefore aridity.
Xylem structure can acclimate to environmental variation while it is still developing; however, subsequent adaptation is impossible, due to the fact that xylem tissues are dead at maturity. For plants subject to drought or increased aridity after maturation, this poses a serious threat to the organism’s survival. Therefore, embolism resistance is important in determining the limits of drought tolerance for woody species, and thus, predicting drought-induced forest decline globally and regionally. Forests in this study were defined as Mediterranean, savanna, and woodland environments. Therefore, the plant species analyzed included trees, shrubs, and lianas.
The authors used a database of 480 woody species with Ψ50 to compare forest biome vulnerability to drought-induced hydraulic failure. The species studied came from a variety of climates, with mean annual precipitation (MAP) ranging from 300 to 4,500 mm and mean annual temperature from −4 to 27°C. Climate data for the study was taken from the WorldClim database of CRU climate data base. 384 Angiosperms (flowering plants) and 96 gymnosperms (mostly conifers) were taken into account separately due to structural differences
between the two types of plants. Shoat et al. found a significant (PΨx (Ψmin) for plants under natural conditions and Ψ50 for the angiosperms and gymnosperms studied. This finding suggests that there is a relationship between embolism resistance and the level of drought stress for plants across a broad range of environments.
            For sixty-seven percent of the samples studied, Ψmin was measured as xylem or stem water potential. To obtain measures for Ψmin the leaves were covered with plastic and aluminum foil such that leaf and stem water potentials were equilibrated. In the remaining thirty-three percent of cases, Ψmin was measured as leaf water potential. In this case, Ψx may be less negative than leaf water potential. The water pressure drop across the leaf’s hydraulic pathway is caused by transpiration of water out of the leaves not covered by plastic and aluminum foil. The difference between Ψmin and Ψ50 represents the safety margin within which a plant is still able to function in its environment, and thus quantifies the degree of conservatism and ability to adapt to drought stress for a plant species’ hydraulic strategy. Plants that have a low safety margin are more prone to embolism and thus hydraulic failure. Because of the difference in leaf and xylem water potential, it is possible the amount of embolism in the stem could be overestimated. This overestimation would yield an overly narrow safety margin.
This study concludes that seventy percent of species in all forest biomes operate at narrow safety margins (Ψ88, still yielded similar results in vulnerability across biomes.

            Comparatively, gymnosperms had greater safety margins, and were therefore less susceptible to hydraulic failure. Forty-two percent of angiosperms operated at negative safety margins, in comparison to only six percent of gymnosperms. This discrepancy suggests that angiosperms have a greater resilience to reverse embolism by dissolving gases within the xylem, thus restoring the plant’s vascular system to a status where water can flow to photosynthetic sites. Still, this recovery can only occur if sufficient precipitation follows periods of drought in an ecosystem. This recovery process is therefore not effective in preventing die off if drought is severe and persists for a long period of time. Gymnosperms are not immune to hydraulic failure.
            The authors also found a strong association between
Ψ50 and MAP in the study. The mean and upper tenth quartile trends demonstrate significant decreasing resistance to embolism with increasing rainfall. The variation in evapotransporation (PET) and seasonality of precipitation: aridity index (MAP divided by PET) and mean precipitation of the driest quarter yielded similar results. This data was gathered from PET Global Aridity Index (Global-Aridity) and the Global Potential Evapo-Transpiration (Global-PET) Geospatial Database, respectively. For any climate region, there are various hydraulic strategies. Most variation in the Ψ50 is seen in sites with a MAP of 300 to 1,000 mm. High MAP sites (tropical rain forests) have less negative Ψ50, which suggests that low embolism resistance is related to low structural costs and high transport efficiency within the vascular system of plants. However, Ψ50 and MAP are not always correlated. In some cases, species can grow in more arid climates while still escaping water stress, therefore decreasing the need for high embolism resistance. Examples of such adaptations include riparian, groundwater-dependent vegetation, and drought-deciduous trees in tropical dry forests. Very negative Ψx are avoided by predictable access to groundwater through adaptations such as deep roots, internal water storage, and reduced leaf area. 
            Choat et al. suggest that most species operate close to their functional limits in regards to
Ψ50 and Ψmin. Woody species are susceptible to xylem failure during drought events, and are largely independent of rainfall region and biome. Plant species are able to adapt to drought to a certain extent, regardless of their ecosystem. However, there is a threshold at which subsequent mortality and die off will begin to occur due to xylem failure, embolism, and cessation of vital plant processes.
            Hydraulic strategies that operate within a “risky,” low safety margin to
Ψ50 gain a trade-off that balances growth with protection against risk of mortality within the environment. Stomatal behavior can take advantage of the range of xylem pressures within the hydraulic tolerance of a species, resulting in increased carbon gain. By regulating xylem pressure through the stomata to take advantage of various water availability scenarios, carbon uptake can be maximized. However, this behavior still places the plant at a greater risk of lost photosynthetic area or death.
            The link between a plant’s evolved embolism resistance and water availability is limited by long generation cycles of perennial plants. This adaptation capability raises important concerns regarding the capacity for plant species to adapt to the rapid pace of predicted climate change. If plant species are unable to keep pace with changing climates, this study suggests that net primary productivity will decrease, biodiversity will be lost, and the composition of ecosystems within forest will be altered. The authors note that although other factors induce drought-mortality (insect attack and carbohydrate depletion), these factors are still highly dependent upon one another. Shoat et al. conclude that embolism formation is the underlying mechanism of plant species die off and forest decline. Embolism sets thresholds for stomatal closure, limits photosynthesis, increases heat and light damage, and thus runs down carbohydrate reserves until a plant eventually dies. This study demonstrates that long-term monitoring of
Ψmin to quantify embolism resistance and hydraulic safety margins is necessary in accurately predicting the responses of forest ecosystems to climate change.

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