Comparison of sap flux data from two instrumented tree species in a forested
catchment with different levels of water stress
GC31A-1023
Hartsough, P.C1., E. Roudneva1, A.I. Malazian1, M. Meadows2, A. Kelly3, R. Bales2, M. Goulden3, J.W. Hopmans1
1Dept.
of Land, Air and Water Resources, UC Davis, 2Sierra Nevada Research Institute, UC Merced, 3Dept. Of Earth System Science, UC Irvine Contact: phartsough@ucdavis.edu
INTRODUCTION
Influence of Soil Depth
RESULTS
Forests of the Sierra Nevada are the water towers of California. In an effort to further understand tree/water
relations, two trees were instrumented with heat pulse sap flux sensors in the Southern Sierra Critical Zone
Observatory (SSCZO) within the Kings River Experimental Watershed (KREW) to understand transpiration as it
relates to water availability from a variety of soil depths and substrates. At the first instrumented site, CZT-1, a
White Fir (Abies concolor) was instrumented on a flat ridge with access to deep soil moisture. Extensive
monitoring of shallow and deep soil regions confirm that there is significant soil water available in saprolitic
material from 100-400cm as the tree exhausts water from shallower depths. At the second instrumented site, CZT-2,
a Ponderosa Pine (Pinus ponderosa) was instrumented with a similar suite of sap flux and soil sensors. The CZT-2
site is on a slight slope and is characterized by shallow soils (<90 cm) with extensive cobbles and bedrock outcrops
with limited access to deeper soil or saprolite water. The second site also sits in the open while the first site is more
protected in a closed forest. Total precipitation from WY2009 to 2011 varied considerably in both amount and
type (from 112-211 cm). WY 2010 at the site was mostly snow, while 2011 was a mixture of rain and snow, a clear
indication that this site sits right at the current rain snow transition zone. Comparing these two sites in both time
and space gives a good indication of the variability of approaches to dealing with water stress and drought
conditions in a Sierra Nevada mixed conifer forest.
In the Mediterranean climate of the Sierra Nevada, snow pack
persists well into the spring after precipitation has effectively
stopped. With the onset of summer and continued dry conditions,
snow quickly melts, and soil profiles dry out as shrubs and trees
deplete the available soil water. We compared the dynamics of the
soil profile desiccation at various depths as it transitioned from
saturated to very dry conditions. The two sites—CZT-1, flat,
deeper soils and dense canopy cover; and CZT-2, shallow, sloping
soils, and more exposed—complement each other in capturing this
variability. Tensiometer data within the plot show the cessation of
drainage out of the root zone by early July, leaving an extended
period (3+ months) of soil profile drying through ET only.
Through monitoring of sap flux and periodic leaf water potential
measurements, we tracked the activity of the tree as it responded to
changing available moisture in the root zone. More than 40% of
annual ET takes place during this period where soils <90cm are
extremely dry. Soil moisture removed from this layer accounts for
only a fraction of the total mass loss. At CZT-1, sensors placed at
the soil/saprolite transition show an increase in water depletion
associated with shallow soil water depletion and increasing sap
flux. At CZT-2, when the soil moisture is depleted, the sap flux
also falls off as there is limited storage in the fractured bedrock.
Variability was also seen radially around the stem across the four
sensors. While the magnitude of total flux may contain
considerable uncertainty, the timing is broadly consistent between
the two trees and the 50 m flux tower. All three show
transpiration year round with a peak in mid summer before
diminishing water supplies become a limiting factor. Differences
between the tress can be interpreted as varying access to deeper
water sources as shallow water becomes depleted. Thick
sequences of saprolite exist at the CZT-1 site that appear to
contain considerable water accessible to the tree in late summer.
2009
2010
2011
Total Precipitation (cm)
WY 2009
112
WY 2010
175
WY 2011
211
CZT-1 saprolite core
from 4 m deep
contains up to 15%
porosity
Sap Flux Comparison
Sap flux, 5TE and TDR sensors installed in the trunk
ET was partitioned between different seasons for the instrumented
trees and the flux tower.
CZT-1
CONCLUSIONS
Soil moisture and water
potential sensors
Sap flux sensor
EXPERIMENTAL SETUP
0
Sap flux sensors were responsive to fluctuations in
environmental variables controlling photosynthesis and
values declined along with soil moisture availability. There
was good correspondence between sap flux measured at
individual trees and the spatially averaged (1 ha) values
from the P301 flux tower.
CZT-1
CZT-2
While annual patterns of precipitation and water
storage are similar between the two tree sites,
CZT-2 is left with little plant available water by
the end of the summer.
CZT-2
In August 2008 we instrumented a soil disc (r=5 m, d=1 m)
surrounding CZT-1 and in August 2010, the soil surrounding a second
tree, CZT-2, was instrumented (r=4 m, d=0.75 m). Both trees were
instrumented with heat pulse sap flux sensors and TDR probes in the
tree to measure changing water content of the wood. Precipitation
measurements are taken from a US Forest Service gauge 1 km away
while other meteorological data are measured at the P301 50 m flux
tower. Precipitation type (rain/snow) was determined from image
analysis of photographs taken at the instrumented sites. Soil sensors
were distributed across 14 vertical pits. Additional soil sensors were
added in the deeper soil and saprolite at CZT-1 in summer 2011(Deep
Vadose Zone) to discover deeper soil/root/water dynamics. Sap flux
measurements in the trunk were recorded every half-hour, while leaf
water potential measurements in the canopy were taken every month.
The two sites show differing responses to changes in rain and
snow loading from above as well as soil drainage and water
depletion from below. They also have different thresholds for
transpiration shut down; both due to late season water deficit and
also during winter periods where air temperatures are high
enough to permit photosynthesis. A combination of sap flux and
soil moisture data show different patterns of tree activity and
water stress based on available substrate water resources. While
well developed soils in the region are relatively shallow, the
underlying saprolite contains considerable amounts of moisture
which act as a buffer for dry summer conditions and indeed
longer drought conditions as well.
Acknowledgements
DVP sensors
Change in soil moisture storage and transpiration from the tree can be
simultaneously measured using the sensor array. CZT-2 (right) shows sap flux
decreasing along with shallow (<1 m) soil moisture. CZT-1 (left) shows an
increase in sap flux in midsummer without a corresponding increase in shallow
water storage. Deeper sensors at CZT-1 show the source of moisture switching
to a deeper pool.
A potential new method for measuring tree
water status is a TDR (Campbell Scientific
616) inserted into the tree trunk. This
sensor in CZT-1 (bottom) shows good
correspondence with ET measured at the
tower (top) and air temperature measured at
the tree (middle).
Research is supported by the following National Science
Foundation grants, CZO: Critical Zone Observatory-Snowline
Process in the Southern Sierra Nevada (EAR-0725097) and
Development of a Water Balance Instrument Cluster for
Mountain Hydrology, Biochemistry and Ecosystem Science
(EAR-0619947). We would also like to thank Eric Hoang, as well
as UC Davis, Merced, Irvine and Berkeley students and staff who
helped with lab calibrations, field work, and logistics.