Trees Likely to Suffer from Water Stress with Climate Change

As global climate change increases water stress in many regions of the world, humans are not the only organisms that will be severely affected.  Trees have adapted for thousands of years to maximize their ability to gather sunlight, growing taller and taller to reach energy from the sun.  However, this also means that water must travel further from their roots to reach the extremities.  This has lead to a range of physiological functions that are highly dependant on a consistent supply of water.  Many climate change scientists predict that severe drought events may increase worldwide by 30% or more, significantly reducing the amount of moisture in the soil available for root uptake.  Extreme weather events are also likely to become more prevalent, leading to increased runoff and thus less water availability in the soil in regions affected.  Hartman (2011) predicts that droughts will lead to reduced functioning, growth and yield and in extreme cases, plant death.  He presents several hypotheses about potential reasons for water stress leading to tree mortality including carbon starvation because of stomatal closure, rupture and dysfunction of the cells of the water transport system, reduced electron availability for photosynthesis, and constrained cell metabolism.  Furthermore, water stress may cause indirect mortality increases in trees as well, sometimes many years after the actual drought event.  Trees that are irreversibly weakened may be more susceptible to death by parasites, insects, fires, and further natural disasters.  If these predictions are true, increases in temperature will likely change ecosystem composition, as some will be able to adjust better than others.  It remains to be seen if an evolutionary selection for taller plants may now be a deadly disadvantage in this era of change.
Hartman, H., 2011.  Will a 385-million year struggle for light become a struggle for water and carbon? — How trees may cope with more frequent climate change drought events.  Global Change Biology 17, 642-655. 

At first, it may seem surprising that water stress could impact such a hardy and diverse group of organisms.  Trees survive in some of the most adverse climates in the world, from the freezing northern regions to the driest deserts.  However, in order to adapt to these inhospitable regions, species have had to evolve for thousands of years.  The rapid shifts in climate that have been occurring since industrialization and are predicted to continue exponentially will not give trees time to adapt adequately to the changing environment, and many species that cannot find coping methods may face extinction.  Compounding the severity of the issue is the fact that trees, in order to attain maximum height and gather the most possible sunlight, tend to live at the edge of hydraulic dysfunction.  Taller trees are clearly more prone to drought susceptibility than species that have evolved to have lower canopies, and although they also tend to have deeper root systems this is often not enough to offset the difficulty of obtaining sufficient water for survival.  Individual trees can adapt to drought to some extent by shedding leaves and devoting more carbon and energy to growing longer roots, and in the long run can even develop thicker leaves with increased storage ability.  However, tall trees may respire hundreds of liters of water every day, making these adaptations inadequate in the long-term drought scenarios that are likely to become prevalent in future climate change.
In the case of increased droughts, predicted mortality from water stress may be direct or indirect.  The most widely circulated theory in the scientific community is that water stress leads to carbon starvation in trees.  Leaves have stomata (small openings on the underside of their leaves) that allow for the diffusion of carbon dioxide into the plant and the evaporation of water out, meaning that water loss from the leaves must continuously be replaced with moisture from the soil.  In times of water stress, stomata close off so that less water can escape from the plant.  While may be an effective measure to increase survival in short extreme events, during a prolonged drought this adaptation may be fatal, as no carbon dioxide can enter the plant with the stomata closed and the tree effectively “starves” to death. 
Another common hypothesis states that the cells of a tree’s vascular system, called xylem, may not be able to physically withstand the pressures of water stress.  Because trees need  a constant supply of water, they have a complex transport system that draws water from the roots up through a water column and into the vascular system of the plant.  As water evaporates from the leaves and diffuses into the atmosphere, water potential is increased in the tree compared with the surrounding atmosphere.  Water uptake by the roots passes into the water column, where capillary and adhesive forces draw water molecules upward into the low-potential leaves.   However, when the roots draw water out of the soil they temporarily reduce the moisture content surrounding them, and if this moisture is not replaced through precipitation the soil dries out and air replaces water in the soil pores.  There is thus a harder “pull” on the vascular water column to overcome the increase in adhesive force created between water and soil particles.  This in turn creates negative pressure in xylem, which can cause embolisms and water column ruptures and result in partial or complete loss of water conductance through the tree over time.  Fortunately, if the tree manages to survive the drought, xylem can be repaired once water is available and the vessels can be refilled.  The consequences of this mechanism also tend to be less fatal for the tree as a whole, as it can sacrifice twigs and leaves and maintain its core if only a part of its water system is ruptured.
            Photosynthesis, which is essential for growth and survival of an individual tree, relies on the electrons supplied by water intake for its reactions to function.  In the case of impeded movement of water up to the crown of the tree where photosynthesis has the most energy from sunlight, this process can be disrupted and a tree may “starve.”  Finally, some scientists believe that low water potentials in tissues may constrain cell metabolism, resulting in a reduction of carbon assimilation.  As of this writing, no empirical studies have been conducted regarding the latter two hypotheses, but they are certainly probable enough to merit future study. 
            On a more optimistic note, it is possible that an increase in carbon dioxide in the atmosphere could in fact help trees as they take it up through their stomata.  In a perfect scenario, this could lead to higher growth rates and increasing water-use efficiency even in times of drought.  However, if the elevated atmospheric carbon dioxide concentration makes leaves grow larger and the water efficiency does not compensate adequately, trees could be even more severely exposed to drought stress.  In addition, different species have varied stomatal responses to higher carbon dioxide, so it is difficult to generalize this theory.
            Diverse species may also have differing responses in regards to the other hypotheses as well.  Isohydric tree varieties close their stomata well in advance of any danger to the xylem, which can be helpful in the case of short, extreme events but deadly in a prolonged drought because of carbon starvation.  Anisohydric species, on the other hand, close their stomata only when in immediate risk of hydraulic failure, making them less prone to carbon starvation but vulnerable to water transport issues and xylem rupture.  Gymnosperms and angiosperms also differ in their vulnerability to water stress.  Angiosperms have more conductive xylem, but their vessels require more carbon input to function as compared with gymnosperms.  Angiosperms, therefore, are more at risk of carbon starvation, while gymnosperms are more likely to suffer from transportation challenges during droughts.  Although the specific challenges differ between species, all types will likely be negatively impacted in some way by global climate change and increased water stress. 
If increased water stress does lead to a decline in trees in some regions of the word, this will in turn affect humans powerfully.  Trees are worth trillions of dollars a year, and there cannot be a price placed on the quality of life that they provide.  In addition, trees are a major component of earth’s carbon cycle, making up about 90% of earth’s terrestrial biomass and cycling 8% of atmospheric carbon dioxide annually.  If the health of trees suffers from climate change, there are few organisms on earth that will not be affected.
            Clearly, an issue that will affect so many people and organisms so powerfully is a topic that deserves further research.  Unfortunately, tree mortality studies are particularly difficult to conduct because it can take years (and sometimes even decades) to gather enough data to draw any sort of viable conclusions.  This is something that scientists will have to work with, either through developing sophisticated modeling or obtaining support that will allow for such time consuming studies to be undertaken.  One important avenue of research is to determine which of the above hypotheses is most likely to be the cause of tree mortality in water stress events.  Future studies must also look at different regions, environments, and species in order to determine how various trees might be affected differently by climate changes.   A diversified study will also be essential to facilitate a full understanding because as patterns of precipitation change, different regions of the world will experience climate change differently.   What will likely be universal is a strong impact from global climate change on tree species around the world, and scientists must be prepared for the results of such a possibility. 

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