Creosote branches dance, even after death

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How static are trees? There has been a lot of research into how non-woody tissues move, but not so much into how wood moves. Alesia Hallmark and colleagues at the University of New Mexico use photographs from the PhenoCam network to observe how the branches of woody plants move over time in an article published in Ecosphere. They find that both living branches move over the course of a day, but so too do dead branches.

There’s plenty of examples of plant movement. Flowers can track the sun. Tendrils can circle until they can twine around a support. Leaves can fold to trap prey in a Venus Flytrap or to protect them in Mimosa. But the branches of woody plants, do they move or are they solid? Finding out the answer requires someone to spend a lot of time watching for changes in plants. Fortunately, botanists have set up a network to do just this.

The PhenoCam network is a collection of sites around the world, from Utqiagvik, Alaska, in the north to Lauder, New Zealand, in the south.

Each of these sites is under observation, usually with the StarDot NetCam. Every half hour, the camera takes a snapshot of the plants. These cameras upload their images to a gallery, which you can access and examine.

Hallmark and colleagues used photographs from cameras installed at National Ecological Observatory Network (NEON). The Neon cameras take up to four photos an hour, which improves the chances of seeing faster movements, say the botanists. They were particularly interested in dawn and dusk when light and temperatures start to change. They point out this was a novel use of the cameras in their paper.

“We were able to mine photographs from an existing public depository, the PhenoCam network, to retroactively document branch movements across a spectrum of ecosystem monitoring sites (NEON), despite the fact that these cameras were not originally installed for this purpose. Although digital photographs yield lower resolution data than the TLS techniques employed in previous research, we were able to use them to remotely monitor branch movements across many sites at high frequency (hourly) over long time periods (months to years) and readily distinguish between live and dead branches. Digital cameras are cheap, easy-to-use, pervasive, and non-invasive instruments with which to study plant movements. Factors such as wind or intense rain can obscure images, but this drawback is common across many sensors, including TLS. Cameras deployed for biological monitoring are often co-located with meteorological stations, flux towers, and other sensor arrays. The prevalence of camera imagery with associated environmental sensor data makes photographs an ideal medium to further study the relationship between plant movements and abiotic factors.”

They then concentrated on creosote shrubland within the Sevilleta National Wildlife Refuge in central New Mexico. The shrubs here were interesting because of the weather in 2011. In February, temperatures dropped to −30°C. The was bad news for the creosote shrubs who suffered extreme canopy dieback. The plants didn’t die but instead regrew from the base of the plant. So now, many of the plants have a crown of dead branches.

Between the end of July 2015 and the start of December, the team watched to see how the branches moved. They also tracked meteorological data and soil temperatures to get an idea of the climate and microclimate around the plants.

The botanists found that branches drooped at the start of the day and then rose in the afternoon or evening. But Hallmark and colleagues note that the movement isn’t that simple.

“We found subtle but consistent differences in the timing of live and dead branch movements. While gross patterns of movement were the same in all branches—raising skyward at night and drooping groundward in the day—there was a consistent temporal lag between live and dead branches. We then leveraged co-located site instrumentation to correlate the movements of live and dead branches with potential abiotic drivers. In both cases, the moisture content of the air seemed to be the most likely driver of branch movements. Based on our results, we think dead creosote branches swell and dry in response to the humidity (or wetness) of the air, while live branches respond more to changes in vapor pressure deficit (or dryness) of the air. These factors are co-correlated, but the differing reactions reveal how this wood reacts when wood fibers are exposed to the atmosphere vs. when internal water content is controlled by living stomata, intact vessel elements, and protective bark tissues.”

The variation in the position of the branches may help the plant regulate the temperature of the soil. The team found, in hot months, the ground beneath a creosote shrub could be over three degrees centigrade cooler than the surrounding soil. This, they argue, could have implications for evaporation and water loss in the soil beneath the shrub.

Hallmark and colleagues state that this motion should be included when researching how plants control their environment. “The assumption that woody plants have static architecture permeates many areas of scientific theory and methodology. We encourage fellow scientists to consider diurnal branch movements in future study designs. Anecdotally, we found that differences in branch position within a single day changed total canopy volume and the resulting biomass estimations of creosote individuals by over 20% when using volume:biomass allometric relationships.”

“Using automated systems to track branch movements over long study periods may help us understand plant physiology and stress adaptation better in a variety of species and habitats. Beyond simply being an interesting phenomenon, these movements may provide insight into daily changes in stress behavior and environmental interactions previously thought to only change over the course of entire seasons or plant lifetimes.”