Foliar uptake of water from the surface of leaves is common when rainfall is scarce and non-meteoric water such as dew or fog is more abundant. However, many species in more mesic environments have hydrophobic leaves that do not allow the plant to uptake water. Unlike foliar uptake, all species can benefit from dew- or fog-induced transpiration suppression, but despite its ubiquity, transpiration suppression has so far never been quantified. Here, we investigate the effect of dew-induced transpiration suppression on the water balance and the isotope composition of leaves via a series of experiments. Characteristically, hydrophobic leaves of a tropical plant, Colocasia esculenta, are misted with isotopically enriched water to reproduce dew deposition. This species does not uptake water from the surface of its leaves. We measure leaf water isotopes and water potential and find that misted leaves exhibit a higher water potential and a more depleted water isotope composition than dry leaves, suggesting a ∼30% decrease in transpiration rate compared to control leaves. We propose three possible mechanisms governing the interaction of water droplets with leaf energy balance: increase in albedo from the presence of dew droplets, decrease in leaf temperature from the evaporation of dew, and local decrease in vapor pressure deficit. Comparing previous studies on foliar uptake to our results, we conclude that transpiration suppression has an effect of similar amplitude, yet opposite sign to foliar uptake on leaf water isotopes.
Dew deposition occurs in ecosystems worldwide, even in the driest deserts and in times of drought. Although some species absorb dew water directly via foliar uptake, a ubiquitous effect of dew on plant water balance is the interference of dew droplets with the leaf energy balance, which increases leaf albedo and emissivity and decreases leaf temperature through dew evaporation. Dew deposition frequency and amount are expected to be affected by changing environmental conditions, with unknown consequences for plant water stress and ecosystem carbon, water and energy fluxes. Here we present a simple leaf energy balance that characterizes the effect of deposition and the evaporation of dew on leaf energy balance, transpiration, and carbon uptake. The model is driven by five common meteorological variables and shows very good agreement with leaf wetness sensor data from the Blue Oak Ranch Reserve in California. We explore the tradeoffs between energy, water, and carbon balances for leaves of different sizes across a range of relative humidity, wind speed, and air temperature conditions. Our results show significant water savings from transpiration suppression up to 25% for leaf characteristic lengths of 50 cm. CO2 assimilation is decreased by up to 12% by the presence of dew, except for bigger leaves in windspeed conditions below 1 m s−1 when an increase in assimilation is expected.
The MUSE conference is a large-scale version of these workshops, bringing people from across the University of Michigan to present their research during a two-day event. I chaired the session on Land Use and Land Cover Change on Thursday morning and presented my own results from using solar-induced fluorescence to map reforestation in China.
It was a fun occasion to meet students, postdocs, and faculty from a range of departments, from English to Psychology, and Mechanical Engineering to the School of Public Health, and I hope that some of the contacts made at the conference will eventually turn into long-term collaborations.
This year, the AGU Fall Meeting moved from its traditional location in San Francisco to New Orleans. As usual, the meeting was a wonderful occasion to catch up with former classmates and colleagues, and hear about all the new science!
Finally, I had the opportunity to attend the Ecohydrology Technical Committee and to help out with the Hydrology Business Meeting. Both events were great opportunities to meet new people in my field, and I’m hoping to get more and more involved with the hydrology community at AGU in the future.
A new paper entitled ‘Advancing ecohydrology in the changing tropics: Perspectives from early career scientists‘ just appeared in Ecohydrology today. The article is a student-lead paper focusing on current and future threats faced by tropical ecosystems, and what the potential research gaps that would help the scientific community better understand and mitigate some of these threats.
The article stemmed out of the AGU Chapman conference on tropical ecohydrology that I attended in June 2016. All the co-authors of the article are graduate students and early-career scientists from institutions around the world, and it was both a lot a fun and a great learning experience to write this together! You can see the article on the Ecohydrology website HERE.
Tropical ecosystems offer a unique setting for understanding ecohydrological processes, but to date, such investigations have been limited. The purpose of this paper is to highlight the importance of studying these processes—specifically, how they are being affected by the transformative changes taking place in the tropics—and to offer an agenda for future research. At present, the ongoing loss of native ecosystems is largely due to agricultural expansion, but parallel processes of afforestation are also taking place, leading to shifts in ecohydrological fluxes. Similarly, shifts in water availability due to climate change will affect both water and carbon fluxes in tropical ecosystems. A number of methods exist that can help us better understand how changes in land use and climate affect ecohydrological processes; these include stable isotopes, remote sensing, and process-based models. Still, our knowledge of the underlying physical mechanisms, especially those that determine the effects of scale on ecosystem processes, remains incomplete. We assert that development of a knowledge base concerning the effects of transformative change on ecological, hydrological, and biogeochemical processes at different spatio-temporal scales is an urgent need for tropical regions and should serve as a compass for emerging ecohydrologists. To reach this goal, we advocate a research agenda that expands the number and diversity of ecosystems targeted for ecohydrological investigations and connects researchers across the tropics. We believe that the use of big data and open source software—already an important integrative tool/skill for the young ecohydrologist — will be key in expanding research capabilities.
On September 7th 2017, I defended my PhD. My thesis is entitled “Investigating dew deposition on leaves: effects on leaf water content, CO2, and remote sensing characterization” and is available on ProQuest.
My readers were Eric Wood and my advisor Kelly Caylor. Examiners were (from left to right): Kelly Caylor, Amilcare Porporato, and Steve Frolking (University of New Hampshire).
It has been a long and exciting ride at Princeton University. On September 1st, I started a position as a Junior Fellow with the Michigan Society of Fellows. During my 3-year long tenure as a Junior Fellow, I will be working in the Department of Climate and Space Sciences and Engineering at the University of Michigan, focusing on using the new CYGNSS data over land.
After thinking a lot about pre-print and how they fit in the world of plant research, I have decided to give it a try! My first pre-print entitled “Dew-induced transpiration suppression impacts the water and isotope balances of Colocasia leaves“ is now available on the bioRxiv: see the article!
The paper examines the effects of dew on transpiration suppression in Colocasia esculenta leaves using stable isotopes of water to track changes in the water status of the leaves. The article has been submitted to a peer-reviewed journal.
Abstract: Foliar uptake of water from the surface of leaves is common when rainfall is scarce and non-meteoric water such as dew or fog is more abundant. However, many species in more mesic environments have hydrophobic leaves that do not allow the plant to uptake water. Unlike foliar uptake, all species can benefit from dew- or fog-induced transpiration suppression, but despite its ubiquity, transpiration suppression has so far never been quantified. Here, we investigate the effect of dew-induced transpiration suppression on the water balance and the isotopic composition of leaves via a series of experiments. Characteristically hydrophobic leaves of a tropical plant, Colocasia esculenta, are misted with isotopically enriched water to reproduce dew deposition. We measure leaf water isotopes and water potential and find that misted leaves exhibit a higher water potential (p < 0.05) and a more depleted water isotopic composition than misted leaves (p < 0.001), suggesting a ∼30% decrease in transpiration rate (p < 0.001) compared to control leaves. We propose three possible mechanisms governing the interaction of water droplets with leaf energy balance. Comparing previous studies on foliar uptake to our results, we conclude that transpiration suppression has an effect of similar amplitude, yet opposite sign to foliar uptake on leaf water isotopes.
My new paper is finally available online on Plant, Cell & Environment. For this paper, entitled “Leaf water 18O and 2H maps show directional enrichment discrepancy in Colocasiaesculenta“, we looked at spatial patterns of water isotopes in Colocasia esculenta leaves. See the abstract below for a quick overview of the study and results or head to the PCE website to read the full paper.
Spatial patterns of leaf water isotopes are challenging to predict because of the intricate link between vein and lamina water. Many models have attempted to predict these patterns, but to date most have focused on monocots with parallel veins. These provide a simple system to study, but do not represent the majority of plant species. Here, a new protocol is developed using a Picarro induction module coupled to a cavity ringdown spectrometer to obtain maps of the leaf water isotopes (18O and 2H). The technique is applied to Colocasia esculenta leaves. The results are compared to isotope ratio mass spectrometry. In C. esculenta, a large enrichment in the radial direction is observed, but not in the longitudinal direction. The string-of-lakes model fails to predict the observed patterns, while the Farquhar-Gan model is more successful, especially when enrichment is accounted for along the radial direction. Our results show that reticulate veined leaves experience a larger enrichment along the axis of the secondary veins than along the midrib. We hypothesize that this is due to the lower major/minor vein ratio that leads to longer pathways between major veins and sites of evaporation.
This weekend, I took part in the “Thesis in 180s” competition at MIT during which French speaking PhD students from all background presented their thesis in 180 seconds (3 minutes) to a non-specialized audience. This competition started in French-speaking countries in 2012, and this year was the first time that the competition was hosted is the US. The event was organized by the French Consulate in Boston and the French @MIT Club.
Explaining one’s research project in such a short amount time requires a lot of preparation, but it was a very fun challenge! During my presentation, I introduced the concepts of dew, foliar uptake, transpiration suppression, cavitation, and water use efficiency, all in only 180s! I was awarded 3rd place and received a $500 prize from Thales, who sponsored the event. The winner, Arthur Michaut, will defend the US in the international final in September in Liège (Belgium).
If you understand French, make sure to watch the video of my presentation!
Two weeks ago, I had the pleasure to attend the 16th Electromagnetic and Light Scattering Conference, held at the University of Maryland in College Park. This yearly conference gathers over 100 participants from around the world to discuss different aspects of scattering by small particles, from modeling to lab work and atmospheric and astrophysical observations. I gave a talk on modeling scattering from a dew-wetted leaf. You can see the abstract HERE and the program HERE.