By Peter Ibsen, PhD Student, Jenerette lab at UC Riverside
While humans around the world are preparing for the effects of climate change, plants will also need to respond for extreme heat and drought conditions. Understanding how plants are adapted or not to extreme conditions may help us humans learn how we can use plants to cope with a hotter and drier future. Especially in cities, where temperatures are usually hotter and plants are less common, we need to know which plants are best prepared for climate change and how we can use them to build more resilient cities.
But studying plants across such a large and varied landscape would be a challenge for even the largest and best resourced research groups. Thankfully, we have been working with the help of citizen scientists (i.e. people such as you, the general public), to find and collect essential data of the large variety of tree species across this region – and we are beginning to start to understand how plants will cope, or not with extreme conditions.
As a scientist in the Jenerette lab at UC Riverside, I study the unique environment of urban Southern California. Having worked as a landscaper and nurseryperson for seven years in the Bay Area before I re-entered academia, I have a strong connection to the vegetative ecology of cities.
What makes this environment so unique? The Los Angeles Metro region is one of the most biodiverse urban places on Earth. It contains over 500 different tree species, representing nearly every region on the planet resulting in a global example of multiple evolutionary processes (found in growth rates, rooting structures, water usage, leaf shape and tree size), and it is right in our backyard!
Recent studies from University of California Los Angeles have predicted, under the most conservative estimates, there will be a 48% increase in days of extreme heat (95+°F) within Riverside by midcentury and a 166% increase in Los Angeles. For Los Angeles this means not just 6 days of unbearably hot weather, but 16 days of needing to stay cool! Extreme heat is not the only factor to consider when looking at how urban trees prosper or even survive. Humidity or lack thereof (aka atmospheric drought), can be just as important especially here in irrigated urban southern California extreme low humidity.
A spectrum of responses to drought stress in plant includes on one end “drought tolerant” plants to the other end being “drought avoidant” plants. Drought avoiding plants close their stomata (microscopic openings in the leaf surface that let out water and let in CO2) in response to drought stress, evading the effects of extreme water loss through the leaf but this also halts photosynthesis. Given that photosynthesis is how plants “feed” themselves by converting sunlight and water into carbohydrates, this strategy may prevent water loss but it is a short term strategy in that the plant can only “starve” itself for so long before it dies. On the other hand, drought tolerant plants keep their stomata open regardless of drought conditions, allowing for constant photosynthesis. To compensate for water loss, they have better adaptations at finding water, for example by having deeper tap roots or through their leaf, stem, and wood architecture. However, keeping stomata open in extreme aridity puts a plant at risk for embolisms (air pockets) to open in their xylem (water-transporting vessels). Too many embolisms result in branch death and potential total mortality.
Interestingly, there appears a large amount of variation across different kinds of plants in their drought coping mechanisms, some plant species tending to use more drought avoidance strategies (by closing their stomata), whereas others use more drought tolerant strategies. Each one has its strengths and weaknesses. I am interested in understanding which of these strategies is adopted by the different tree species in southern California and whether this informs which species will best survive the new (hotter and drier) climate of Southern California.
To study this question, I am taking advantage of the natural gradient of increasing temperature and aridity found in Southern California as one travels from the cooler moister coast into the hot dry desert conditions found further east. See the inserted Figure– here you can see that the temperature gets redder (i.e. hotter) from left to right as one moves from the coast to the desert. The is a similar gradient from west to east in vapor pressure deficit (VPD); a variable that measures the atmospheric demand for water – which itself is related to air temperature – hotter air being able to absorb more water. As a general rule, the coast tends to have more optimal VPD for plant growth than the drier (and hotter) more desert-like regions to the east (think Palm Springs). Trees that are struggling to grow and survive in the hotter and drier regions (e.g. Coachella valley) today, will start to struggle in the near future when climate change brings that same extreme weather westward into the central valley.
Citizen scientists are helping me find and measure the trees along this west to east gradient. On each tree, I am collecting detailed information related to how each species is performing, including measurements of leaf conductance (gas flow from the leaf), leaf and stem water potential (the ability of water to move through a leaf or stem), and stem hydraulic conductance (the efficiency of a stem to transport water). Through these measurements, we determine at what point a species will close its stomata in response to drought conditions among other things. These measurements will be combined with other data I collect on each tree (leaf thickness, wood density, etc.) to give a more complete understanding of the strategy trees are taking to deal with the local climate. Together, this information will allow us to understand just which strategy each tree is using to survive the “drought”.
Finally, by assessing different individual tress across the climatic gradient of temperature and vapor pressure deficit they occur in, we can also understand whether the same species can alter its strategy depending on conditions (i.e. showing a flexible strategy), or whether the species is inflexible and cannot alter its strategy in dealing with drought conditions and there may be more vulnerable to changing conditions. Ultimately I want this information to get into the hands of those who plant trees (e.g. TreePeople, Amigos de los Rios) as well as interested communities so that the right tree is planted.
To date, I have collected data on the first sample of 10 species collected by citizen scientists (Operation Resilient trees 1.0) and am starting the analysis. I am starting to analyze the data from this first round and will go back into the field to sample the next sets of trees in a few months. Stay tuned to see what I find!