So what happens to an avocado tree when it runs out of water? The stomata close and stops transpiring. When water no longer evaporates from the leaf surface, it heats up. If it gets hot enough, and it will, the leaf sunburns and dies. It stopped photosynthesizing long before that, because the process is temperature sensitive. Avocado is an “upper story, late successional” tree which is more affected by drought than faster growing, weedy species. So read about what happens to a forest as temperatures increase.
by: Olof Lönnehed
University of Gothenburg
Trees get overheated in a warmer rainforest
The ability of rainforests to store carbon can decrease in pace with climate change. This is due to photosynthesis rates in the leaves of rainforest species falling at higher temperatures and the trees' natural cooling systems failing during droughts. Increased heat threatens especially the species that store most carbon. This has been shown in a new thesis from the University of Gothenburg.
Some species of trees are able to handle rising heat in the tropics by sucking up large quantities of water to their leaves and transpiring through wide-opened pores in their leaves. These are mainly fast-growing trees that establish themselves early as a rainforest grows up. The same cannot be said for the trees that make up the canopy of rainforests in old growth forests. They grow slower, but get bigger and taller, and their leaves do not have the same ability to cool themselves via transpiration.
Water powers the ‘air conditioning' of the leaves
“The tropics have not experienced Ice Ages and have thus had a relatively stable climate historically as well as seasonally. With climate change, it has started to get warmer and then we have seen that some species of trees are showing increased mortality rates, but we have not really known why before,” says Maria Wittemann, who wrote the thesis.
She has studied several species of tree that can be roughly divided into early successional species, which establish themselves early in a new rainforest, and late successional species, which grow slower but grow considerably bigger, and are thus a larger carbon sink over the long term. A clear difference is how the trees in the two groups handle heat. The early successional species open the pores wider in their leaves, through which they transpire large amounts of water, thus keeping down the temperature in their leaves – similar to an air conditioning system. The late successional species do not open their pores as much, and therefore it's more difficult for them to stay cool.
More sensitive to drought
“We found large temperature differences in the leaves in our measurements. There could be a difference of 10 degrees Celsius between late successional species and early successional species growing in the same location. The late successional species had more difficulty coping with abnormally high temperatures. These trees had a higher mortality rate,” says Maria Wittemann.
However, the early successional species' profuse transpiration through their leaves also requires a lot of water. During a period of drought, the researchers noted that early successional species became more vulnerable to the heat and dropped their leaves. Their reduced consumption of water meant that late successional species were more resistant to drought.
“Our results show that photosynthesis rates in rainforest trees falls when the temperature rises in their leaves, which occurs mainly in late successional species. The proteins and membranes in their leaves, that are essential for photosynthesis, fail, and eventually the trees die due to carbon starvation because they cannot convert enough carbon dioxide from the air. This affects the entire ecosystem. We know, for example, that some animals eat the fruits of the late successional species,” says Maria Wittemann.
Co-operation with a local university
Previous research shows that the situation is worst in the Amazon. It is estimated that this carbon sink will be transformed into a carbon source by as early as 2035. In African rainforests, climate change has not gone as far.
Research at the University of Gothenburg is being conducted in high-elevation forests in Rwanda in collaboration with the University of Rwanda. The trees have been studied in situ, but seeds have also been planted in climate chambers in Gothenburg to study their development at different temperatures.
“We are working with various stakeholders in Rwanda. There is not much rainforest left in Rwanda and when they plant new trees, they want to know which indigenous species will be able to survive in a warmer climate,” says Maria Wittemann.
Facts about the study: The sensitivity of trees to climate change was studied by planting tree species adapted to a cooler climate in Rwanda's elevated tropical rainforests at three locations with different climates at different altitudes. One step down the elevation gradient corresponds to a possible future climate. The field experiment is called Rwanda TREE (TRopical Elevation Experiment) and consists of 20 species and 5,400 trees. To learn more about Rwanda TREE, visit the website www.rwandatree.com or watch the film https://www.youtube.com/watch?v=EkDvbwisqlQ.
Plants lose water through their leaves and it's called transpiration. People lose water off their skin and it's called evaporation or sweating. When a plant stops losing water and when people cant produce enough sweat to cool off, both overheat. The weather influences that drive this water loss - water that needs to be replaced or the bodies begin to go into heat stress - are the amount of light (day length, cloud cover), relative humidity (it dries faster when air is dry and slower when humid - think desert versus Florida), and windy (more wind, more drying). Temperature is important, but not as much as these other drivers. Think freeze-drying - a very successful process for removing water from food. Often humans respond more to temperature than these other factors and figure, when it's cool. it's not necessary to water their plants, themselves or their workers.
Heat stress is more complicated than this, of course, but below are some helpful guidelines to follow to avoid heat stress:
Cal/OSHA HEAT ADVISORY
When employees work in hot conditions, employers must take special precautions in order to prevent heat illness. Heat illness can progress to heat stroke and be fatal, especially when emergency treatment is delayed. An effective approach to heat illness is vital to protecting the lives of California workers.
California law requires employers to identify and evaluate workplace hazards and take the steps necessary to address them. The risk of heat illness can be significantly reduced by consistently following just a few simple steps. Employers of outdoor workers at temporary work locations must be particularly alert and also plan for providing first aid and emergency medical services should they become necessary. All workers should be accounted for during and at the end of the work shift. Heat illness results from a combination of factors including environmental temperature and humidity, direct radiant heat from the sun or other sources, air speed, and workload. Personal factors, such as age, weight, level of fitness, medical condition, use of medications and alcohol, and acclimatization effect how well the body deals with excess heat.
Heat Illness Risk Reduction
1. Recognize the Hazard. There is no absolute cut-off below which work in heat is not a risk. With heavy work at high relative humidity or if workers are wearing protective clothing, even work at 70oF can present a risk. In the relative humidity levels often found in hot areas of California (20 to 40 percent) employers need to take some actions to effectively reduce heat illness risk when temperatures approach 80 F. At temperatures above 90 F, especially with heavy work, heat risk reduction needs to be a major concern.
2. Water. There must be an adequate supply of clean, cool, potable water. Employees who are working in the heat need to drink 3-4 glasses of water per hour, including at the start of the shift, in order to replace the water lost to sweat. For an eight-hour day this means employers must provide two or more gallons per person. Thirst is an unreliable indicator of dehydration. Employees often need ongoing encouragement to consume adequate fluids, especially when the workload or process does not encourage breaks.
3. Shade. The direct heat of the sun can add as much as 15 degrees to the heat index. If possible, work should be performed in the shade. If not, employers where possible, should provide a shaded area for breaks and when employees need relief from the sun. Wide brimmed hats can also decrease the impact of direct heat.
- Acclimatization. People need time for their bodies to adjust to working in heat. This “acclimatization” is particularly important for employees returning to work after (1) a prolonged absence, (2) recent illness, or (3) recently moving from a cool to a hot climate. For heavy work under very hot conditions, a period of 4 to 10 days of progressively increasing work time starting with about 2 hours work per day under the working conditions is recommended. For less severe conditions at least the first 2 or 3 days of work in the heat should be limited to 2 to 4 hours. Monitor employees closely for signs and symptoms of heat illness, particularly when they have not been working in heat for the last few days, and when a heat wave occurs.
- Rest Breaks. Rest breaks are important to reduce internal heat load and provide time for cooling. Heat illness occurs due to a combination of environmental and internal heat that cannot be adequately dissipated. Breaks should be taken in cooler, shaded areas. Rest breaks also provide an opportunity to drink water.
- Prompt Medical Attention. Recognizing the symptoms of heat illness and providing an effective response requires promptly acting on early warning signs. Common early symptoms and signs of heat illness include headache, muscle cramps, and unusual fatigue. However, progression to more serious illness can be rapid and can include unusual behavior, nausea/vomiting, weakness, rapid pulse excessive sweating or hot dry skin, seizures, and fainting or loss of consciousness. Any of these symptoms require immediate attention.
Even the initial symptoms may indicate serious heat exposure. If medical personnel are not immediately available on-site, and you suspect severe heat illness, you must call 911.
Regardless of the worker's protests, no employee with any of the symptoms of possible serious heat illness noted above should be sent home or left unattended without medical assessment and authorization.
7. Training. Supervisors and employees must be trained in the risks of heat illness, and the measures to protect themselves and their co-workers. Training should include:
- Why it is important to prevent heat illness
- Procedures for acclimatization
- The need to drink approximately one quart per hour of water to replace fluids.
- The need to take breaks out of the heat
- How to recognize the symptoms of heat illness
- How to contact emergency services, and how to effectively report the work location to 911.
Photo: Heat Stress to avocado leaves.
Drought may not be the right time to be thinking about this, or maybe it is. It concerns managing water and any time a grower uses water more effectively the crop performs better. But fog can be a significant factor in water management.
As fog passes through a tree canopy, it is absorbed by leaves and coats them. Before the tree will transpire water, the water coating must first be evaporated before the tree loses internal water. This water use is not accounted for in a water budget schedule using evapotranspiration based inputs, such as from CIMIS. For deciduous trees, this is often not of concern, because in the winter they don't have leaves and therefore are not transpiring anyway. For evergreen subtropicals like citrus and avocado, this could be an important source of water.
In many situations in the Central Valley and along the coast there can be periods where fog can represent a significant proportion of the water requirement for an orchard. These periods would be for winter tule fog in the Valley and along the coast in the spring and early summer. A recent publication by Rick Snyder at UC Davis has just been released that shows how this fog water can be incorporated into an irrigation schedule. You can see it at the UC's California Institute for Water Resources website: http://anrcatalog.ucanr.edu/pdf/8532.pdf, http://ciwr.ucanr.edu/california_drought_expertise/droughttips/