Even though we know that plants play a critical role in our planet’s water cycle, there are some striking gaps in that diagram we all first see as elementary school children. Our new paper in the journal Nature Water is the first to offer global estimates of the amount of water stored in vegetation on Earth’s surface and the time it takes for that water to flow through vegetation and return to the atmosphere.

We find that plants do not hold much water (786 cubic km) compared to other pools, but the time it takes for water to flow through plants and return to the atmosphere is exceptionally fast. It ranges from just 5 days in croplands to 18 days in evergreen needleleaf forests. In comparison, it’s estimated to take about 17 years for lake water to return to the atmosphere and 1600 years for water to return from glaciers.

To generate these estimates, we first estimated the amount of water stored in plants using data from NASA’s Soil Moisture Active Passive Mission (SMAP) satellite mission. The mission was launched to provide high resolution estimates of the water in soils. We realized that whereas the SMAP mission originally saw plants as interfering with their soil moisture measurements, and was correcting for their presence, those corrections actually held valuable information for understanding the water cycle. We then combined estimates of plant water storage with cutting-edge estimates of the rates at which water is leaving plants to ultimately determine the transit time of water through vegetation. The result was five years of monthly estimates of water storage and transit time at a spatial resolution of 9 km2.

In the images above, we show global variation in vegetation transit times within and among land- cover types including mean transit times (in days) of water through aboveground vegetation calculated on an annual basis. and  violin and boxplots of mean transit times among land-cover types, with the horizontal line denoting the untruncated global median of 8.1 days (2.5 to 32.4, 5th and 95th quantiles). In sum, we found that the transit time of water through vegetation varied considerably across different land cover types and depending on climate at different times of year.

However, the transit time of water through croplands was significantly and consistently the fastest, with water transiting through plants in less than a day during the peak of the growing season. Importantly, croplands around the world tend to have very similar and very fast transit times. This indicates that land use change may be homogenizing the global water cycle and contributing to its intensification by more rapidly recycling water back to the atmosphere where it can turn into heavy rain events.

Ours is a step forward in understanding how long it takes for a drop of rain to fall from the sky, move through the soil into the root of a plant, and return back to the sky. Without this information, we cannot estimate how the climate and land-use changes that we are imposing on our planet are accelerating and homogenizing our water cycle.

This study was nearly 13 years in the making (a story for another day!) and came to fruition because of the incredible work of Andrew Felton, PhD in collaboration with Joshua Fisher, Koen Hufkens, Lou Duloisy, Seth Spawn-Lee, and AJ Purdy.

This project was funded by United States Department of Agriculture (USDA) award number 2020- 67014-30917 to G.R.G. and NASA Research Opportunities in Space and Earth Science grant number 80NSSC20K0216 to G.R.G. and J.B.F. and number 80NSSC23K0309 to G.R.G. and J.B.F. Additional support to A.J.F. was provided by USDA award number 2021-67034-40252 and USDA Hatch project 7006859.

Shoutout to the amazing Rebecca Green, who generated this network diagram of our collaborators based on studies we have published since 2017. We have 586 links based 110 items that come to 12 total clusters.

A privilege to work with Dr. Elizabeth Patterson, Kathleen Johnson and others to provide water isotope ratios in support of their new study "Local hydroclimate alters interpretation of speleothem δ18O records," which is out this week in Nature Communications.

The proxies developed in the paper extend back 45,000 years. Quite a contrast to our day-to-day work accounting for modern-day pattern and process.

The abstract is as follows:

Oxygen isotopes (δ18O) are the most commonly utilized speleothem proxy and have provided many foundational records of paleoclimate. Thus, understanding processes affecting speleothem δ18O is crucial. Yet, prior calcite precipitation (PCP), a process driven by local hydrology, is a widely ignored control of speleothem δ18O. Here we investigate the effects of PCP on a stalagmite δ18O record from central Vietnam, spanning 45 – 4 ka. We employ a geochemical model that utilizes speleothem Mg/Ca and cave monitoring data to correct the δ18O record for PCP effects. The resulting record exhibits improved agreement with regional speleothem δ18O records and climate model simulations, suggesting that the corrected record more accurately reflects precipitation δ18O (δ18Op). Without considering PCP, our interpretations of the δ18O record would have been misleading. To avoid misinterpretations of speleothem δ18O, our results emphasize the necessity of considering PCP as a significant driver of speleothem δ18O.

I was fortunate to be able to implement what is known in the parlance as a "course-based undergraduate research experience (CURE)" this spring in my upper-division plant biology class. Students added more than 275 research-grade observations to iNaturalist, contributed to a restoration research project by planting seedlings in a number of experimental plots, and carried out a significant field- and laboratory-based project collecting data on leaf thermal tolerances among native plant species in our coastal scrub community. We had an incredible partner in this endeavor in the form of Irvine Ranch Conservancy, which provided tremendous access, mentorship, and truly made it a memorable and deeply meaningful experience for our students. The work is described in a new article in the Fullerton Observer. 

A new paper, led by Dr. Emily Burt during her doctoral work with Josh West (USC), describes the seasonal origins and ages of water moving through the cloud forests near Wayqecha Biological Station in the Andes mountains of Peru. While our group has been working hard to use the seasonal origin index (SOI) developed by Dr. Scott Allen (University of Nevada Reno to understand the spatial variation in tree and stream water sources (Allen et al. 2019 in HESS), this new paper applies SOI to study temporal variation in tree, soil lysimeter, and stream waters. Moreover, it applies the calculation of young water fractions, developed by our collaborator James Kirchner (ETH Zurich; Kirchner 2016 in HESS), to provide real depth of understanding with respect to how water is moving through this remarkable ecosystem. Perhaps the most striking result is that in this very wet system, we observe that plants are taking up wet-season precipitation during the wet season and dry-season precipitation during the dry season. This is a real contrast to what we have observed in the temperate forests of Switzerland (Goldsmith et al. 2022 in GRL) to date and adds a nice new piece to the puzzle:

Burt, E. I., Goldsmith, G. R., Cruz-de Hoyos, R. M., Ccahuana Quispe, A. J., and West, A. J.: The seasonal origins and ages of water provisioning streams and trees in a tropical montane cloud forest, Hydrol. Earth Syst. Sci., 27, 4173–4186, https://doi.org/10.5194/hess-27-4173-2023, 2023.

Abstract: Determining the sources of water provisioning streams, soils, and vegetation can provide important insights into the water that sustains critical ecosystem functions now and how those functions may be expected to respond given projected changes in the global hydrologic cycle. We developed multi-year time series of water isotope ratios (δ18O and δ2H) based on twice-monthly collections of precipitation, lysimeter, and tree branch xylem waters from a seasonally dry tropical montane cloud forest in the southeastern Andes mountains of Peru. We then used this information to determine indices of the seasonal origins, the young water fractions (Fyw), and the new water fractions (Fnew) of soil, stream, and tree water. There was no evidence for intra-annual variation in the seasonal origins of stream water and lysimeter water from 1 m depth, both of which were predominantly comprised of wet-season precipitation even during the dry seasons. However, branch xylem waters demonstrated an intra-annual shift in seasonal origin: xylem waters were comprised of wet-season precipitation during the wet season and dry-season precipitation during the dry season. The young water fractions of lysimeter (< 15 %) and stream (5 %) waters were lower than the young water fraction (37 %) in branch xylem waters. The new water fraction (an indicator of water ≤ 2 weeks old in this study) was estimated to be 12 % for branch xylem waters, while there was no significant evidence for new water in stream or lysimeter waters from 1 m depth. Our results indicate that the source of water for trees in this system varied seasonally, such that recent precipitation may be more immediately taken up by shallow tree roots. In comparison, the source of water for soils and streams did not vary seasonally, such that precipitation may mix and reside in soils and take longer to transit into the stream. Our insights into the seasonal origins and ages of water in soils, streams, and vegetation in this humid tropical montane cloud forest add to understanding of the mechanisms that govern the partitioning of water moving through different ecosystems.

There are some good reasons to believe that no two tropical forests necessarily have the same canopy temperatures. And no two trees within a tropical forest necessarily have the same canopy temperatures. In fact, it is not necessarily true that every leaf in a given tree has the same temperature. The challenge is trying to measure all of those leaves, particularly when one begins to think about forests as large as the Amazon or the Congo basin.
 
Our interest in leaf temperature is because we know that when leaves reach a certain temperature, their photosynthetic machinery breaks down. We have known about these critical temperature thresholds for more than 150 years. Our new study in Nature is an effort to establish how close tropical forest canopies are to these limits.

One of the most remarkable aspects of this study was the methods we were able to use to determine canopy leaf temperatures. It is remarkable that we can observe the temperature of the world tropical forests from an instrument on the International Space Station orbiting 400 km above Earth’s surface and traveling nearly 29,000 km per hour. It is equally remarkable to imagine the painstaking efforts made by my colleagues to measure the temperatures of individual leaves in the canopy by hand. We need both the ground- and satellite-based observations to understand the temperatures of tropical forest canopies.

For leaf temperatures, it is not the averages that are important. It’s the extremes. And this study shows that there are times and places where tropical forest leaves are surpassing their critical temperature thresholds at least once per season. Moreover, we show that when we experimentally warm air temperatures, we often observe leaf temperatures that are higher than that of air temperature. These higher air temperatures exceed the limits of cooling that the leaves can achieve and so the leaves accumulate excess heat. It's a non-linear effect between increasing air and increasing leaf temperatures.
 
The results do not indicate that reaching a tipping point for tropical forests is fait accompli. We still hold in our power the ability to conserve these places that are so critically important for carbon, water and biodiversity. 

Doughty, C.E., Keany, J.M., Wiebe, B.C. et al. Tropical forests are approaching critical temperature thresholds. Nature (2023). https://doi.org/10.1038/s41586-023-06391-z

 Dr. Emily Burt, a postdoctoral research associate on our team, has just published a preprint (for open discussion) in Hydrology and Earth Systems Science. Emily carries out a suite of interesting analyses on water isotopes in order to explore metrics of the seasonal origins and ages of water in streams, soils, and trees of a tropical montane cloud forest in the eastern Andes Mountains of Peru: read the preprint.

Photo Credit: Drew Fulton (Canopy in the Clouds)

Greg Goldsmith will serve as the national program chair for the Ecological Society of America's annual meeting in 2024. The meeting will be held in Long Beach, California and is expected to attract more than 3000 ecologists from all over the world. Goldsmith is an early career fellow of the ESA.

We have published a new German-language summary of our research on processes and patterns of forest water use in the Swiss Journal of Forestry. We have been working on the project now for seven years and we've begun to generate an interesting and diverse set of results: 

​Goldsmith, G.R., S.T. Allen, S. Braun, R.T.W. Siegwolf, & J.W. Kirchner. 2022. Climatic influences on summer use of winter precipitation by trees. Geophysical Research Letters 49: e2022GL098323 4

​Allen, S.T., J. von Freyberg, M. Weiler, G.R. Goldsmith & J.W. Kirchner. 2019. The seasonal origins of streamwater in Switzerland. Geophysical Research Letters 46: 1-28. 

​Allen, S.T., S. Jasechko, W.R. Berghuijs, J.M. Welker, G.R. Goldsmith & J.W. Kirchner. 2019. Global sinusoidal seasonality in precipitation isotopes. Hydrology and Earth System Sciences  23: 3423-3436. 

Allen, S.T., J.W. Kirchner, S. Braun, R.T.W. Siegwolf, & G.R. Goldsmith. 2019. Seasonal origins of water used by trees. Hydrology and Earth Systems Sciences 23:
 1199-1210. 

​G.R Goldsmith, S.T. Allen, S. Braun, N. Engbersen, C. Romero González-Quijano, J.W. Kirchner, & R.T.W. Siegwolf. 2019. Spatial variation in throughfall, soil and plant water isotopes in a temperate forest.  Ecohydrology 12: e2059

​Allen, S.T., J.W. Kirchner, & G.R. Goldsmith. 2018. Predicting spatial patterns in precipitation isotope (18-O and 2-H) seasonality using sinusoidal isoscapes. Geophysical Research Letters 45: 4859-4868.