Listening to the forest breathe: monitoring tree trunk sap flow and size

Water is a polar molecule, a property that accounts for many of its unique physical properties

Water is a polar molecule, a property that accounts for many of its unique physical properties. Photo credit: Jesse Hahm

Trees must pull water from soil and rock, where it is held at tension, to their leaves, where it evaporates to the atmosphere through open stomata. This process, called transpiration, is a major component of the hillslope water budget and can dramatically affect the local climate by lowering air temperature and increasing water vapor concentrations. Understanding how trees use water is a major topic of research at the Angelo Coast Range Reserve. How does the seasonality of transpiration differ between species? At what water content (and potential, or tension) in the subsurface do trees become so stressed that they close their stomata to prevent embolisms (damage to their vascular tissues)? How does rock type influence how hard trees must pull to extract water from the vadose (unsaturated) zone? How do trees access water to continue to transpire throughout the summer drought in Mediterranean climates?

When does this Douglas Fir become water-stressed?

When does this Douglas Fir become water-stressed? Photo credit: Jesse Hahm

To answer these questions, researchers are monitoring tree physiological response to environmental conditions in the Angelo Coast Range Reserve. Sapflow (the vertical ascent of water through xylem) can be measured using a simple sensor consisting of three prongs placed into the sapwood of the tree. The central prong emits a heat pulse, and the two surrounding prongs sense changes in temperature. Flowing sap advects the heat pulse so that a temperature rise is sensed sooner in the upper temperature probe. This principle enables the sapflow velocity to be determined.

Bark must be scraped away to place sensors into the sapwood. The fleshy red bark is characteristic of tan oaks.

Bark must be scraped away to place sensors into the sapwood. This fleshy red bark is characteristic of tan oaks. Photo credit: Jesse Hahm

Sapflow sensors have three metal prongs aligned parallel to the flow of sap. The central prong emits a heat pulse every half hour.

Sapflow sensors have three metal prongs aligned parallel to the flow of sap. The central prong emits a heat pulse every half hour. Photo credit: Jesse Hahm

As the tree ‘pulls’ water up from below, tension increases in the water column in the xylem. This suction can influence the size of the tree trunk, which can be measured with very precise piston dendrometers. These sensors capture the diurnal size variation due to changes in both water content and potential as well as tree growth.

This piston dendrometer senses changes in trunk radius with micrometer-scale precision.

This piston dendrometer senses changes in trunk radius with micrometer-scale precision. Threaded bolts are sunk into the heartwood of the tree to provide a stable reference frame. Photo credit: Jesse Hahm

Turbidites and rip-up clasts in Elder Creek

Elder Creek, part of the South Fork Eel River watershed, lies in the Franciscan Formation found underfoot in most of the Northern Coast Ranges of California. The rocks here were deposited in marine environments when the Farallon slab was still subducting under the North American plate at this latitude. Subsequent uplift following the passage of the Mendocino Triple Junction has elevated these rocks out of the sea.

Clastic sedimentary rocks found in Elder Creek record information about their depositional setting. Grain size, lithology, and shape all provide clues about the energy of the flow and the time spent in transit, sorting and abrading. The vast majority of the rocks in Elder Creek are turbidites, formed from turbidity currents: dense slurries of sediment sloughing off the edge of the continent, rushing off the continental slope to final resting places in deeper, still waters. These currents are thought to be triggered by earthquakes, among other things.

Turbidites contain sand and pebbles that were rounded in terrestrial rivers prior to their arrival at the ocean. They also contain small clay-sized particles that fall out of the ocean water column (the long snowfall, in Rachel Carson’s words). As numerous currents are laid down over time, they create a rhythmic sequence of grain sizes, with a fining upward sequence recording stratigraphic ‘up’ (left to right in the image from the bed of Elder Creek below.)

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Rhythmically bedded turbidite sequence (pebble to sand to clay size), Elder Creek. Photo credit: Jesse Hahm

Sometimes turbidity currents race over clay-sized mud deposits (shale). They pick up bits of the semi-lithified shale and carry them along. These shale bits are called rip-up clasts or intra-formational clasts. They are recognized by their darker color and angular shape, and are often much larger than the terrigenous sediment that surrounds them.

Rip-up (intra-formational) shale clasts in sandy matrix, Elder Creek

Rip-up (intra-formational) shale clasts in sandy matrix, Elder Creek. Photo credit: Jesse Hahm

Measuring stream discharge with the salt dilution technique

Measuring water fluxes in the Eel River Watershed is extremely important. We are in the midst of a multi-year drought and demands on the water supply for agriculture and rural use are only increasing.

An ongoing project at the Eel River Critical Zone Observatory is to improve existing stage-discharge relationships, to better document the amount of water flowing through the watershed. Stage refers to the height of water in the river, and discharge refers to the volume of water that flows by in a given time.

We can measure the stage with an automated system that makes use of pressure transducers, but knowing the discharge is complicated because of the ever-changing geometry of the river bed and the turbulent nature of flowing water. The approach to this problem is to develop an empirical relationship between stage and discharge across a range of stages, from low summer baseflow to high winter floods.

 

Salt dilution technique by David Dralle

Salt dilution technique by David Dralle

Here David Dralle is demonstrating the salt dilution technique to measure discharge on the South Fork of the Eel River, just downstream from Headquarters. A known volume of salt solution is added to a turbulent stretch of the river, and the increase in electrical conductivity is measured downstream, after the salt is well mixed into the flow. The more the salt is diluted, the higher the flow.

Dan Moore has written a very helpful series of articles on the use of the technique. For more information, see the intro to the series, published in Streamline Water Management Bulletin (http://www.siferp.org/sites/default/files/publications/articles/streamline_vol7_no4_art5.pdf)