Research Projects
Our Current Focus
Root function & drought performance in woody seedlings
dehydration tolerance, avoidance, or both?
Seedlings are a fantastic study system for studying whole-plant responses to drought. Discerning whole-tree responses to drought in the field is challenging because the rooting depth and zone of accessible soil moisture are difficult to measure directly. Seedlings allow us to circumvent most of these problems. By precisely measuring soil moisture and root structure together with changes in seedling function (wilting, gas exchange), we are able to better understand why some seedlings can survive drought longer than others.
We study the two main 'strategies' plants use to respond to drought -- dehydration tolerance (surviving low levels of moisture within plant tissues) and dehydration avoidance (evading low levels of moisture in plant tissues during soil drying).
Seedlings are also the de facto unit of reforestation in the Rio Grande Valley, with at least 100,000 of them planted annually. We are finding surprising relationships between dehydration avoidance and root structure, which may have implications for increasing the drought resilience of semi-arid reforestation. Stay tuned for a publication!
Funding: American Forests, UTRGV COS Seed grant
Chronosequences in Ecological Restoration
Carbon storage & biodiversity in drought-prone landscapes
What's a restoration chronosequence?
A chronosequence is a collection of plots across the landscape that differ in their time since an initial event, in this case, being planted with a diverse selection of seedling species. Plots are then characterized by different 'ages.'
Why study them?
Because we cannot wait decades to see the outcome of reforestation! A chronosequence allows us to make a "space-for-time substitution," or in other words, we can get an idea of what recently planted plots will look like in 20 years by studying plots which were planted 20 years ago.
What can you measure with a chronosequence?
Because we have detailed planting data, we know exactly the number of individuals of each species which were planted in the past. By surveying what species still occur in the present, and their abundance, we can estimate the survival and growth rate of each species. Because seedlings are planted in rows, we can also discern the presence of 'recruits', or the presence of new seedlings and trees which were not planted, giving us an idea of how self-sustaining these planted forests can become.
What have you learned?
Surprise, surprise, some species do much better (higher survival) than others in reforestation plantings. Many species do not survive at all. We also have found that different invasive grass communities become more abundant over time. We hope to use this to inform better species selection as well as 'restoration enhancement', or the planting of species better suited for areas with more canopy cover and facilitative adult trees. And, restored areas accumulate carbon at 4x the rate as areas left to regrow on their own.
Funding: Land Life Company (with Engil Pereira, co-PI)
Remote Sensing for Invasive Species Detection & Prediction
The Lower Rio Grande Valley Wildlife Refuge was established in 1979 to protect and promote an eventual continuous wildlife corridor from the coast inland. Due to extensive agriculture and urbanization, tracts of land acquired by the USFWS exhibit varying degrees of disturbance.
Disturbance creates opportunities for invasive species, including the highly invasive grass carrizo cane (Arundo donax), as well as trees or shrubs of similar stature to natives, such as salt cedar (Tamarix ramossisima), athel (Tamarix acilla), Brazilian pepper-tree (Schinus terebinthifolia), chinaberry (Melia azedarach), and to a lesser extent, Chinese tallow (Triadica sebifera).
This project is using newly acquired hyperspectral imagery (in 2022) combined with high-resolution RGV photos, combined with high-precision on-the-ground location mapping of individual invasive species. With our collaborators at the University of Virginia (Xi Yang & grad student Wayne Dawson), we hope to develop spectral signatures to pinpoint where invasive species are present in hard-to-survey areas, making their eradication more efficient.
Funding: US Department of Interior, Fish and Wildlife Service
Cover Crop Impacts on Plant Available Water
In collaboration with Alex Racelis (PI), Pushpa Soti, and Rupesh Kariyat (co-PIs), we are using a network of 100+ soil moisture and water potential sensors distributed across 4 farms to examine the impacts of cover crops on soil moisture.
Funding: USDA Conservation Innovation Grant
Plant water relations in
forest ecosystem models
There is an urgent need to project both the response of forests to increasing drought as well as the differential impact that changes in climate will have on coexisting plant functional types (Mencuccini, Manzoni, & Christoffersen 2019 New Phyt). We have implemented a mechanistic representation of plant hydraulics (Christoffersen et al., 2016 GMD) into the Functionally Assembled Terrestrial Ecosystem Simulator (FATES; Massoud et al. 2019 GMD) and are testing it in tropical forests affected by the 2015-2016 El Nino drought.
At left: Model parameterization of xylem vulnerability (each line represents an individual tree) for a community of co-existing trees in the Amazon rain forest.