Walnut Orchard Studies at the UC Kearney Agricultural
Center
Estimation
of tree canopy leaf area index by gap fraction analysis
We estimated leaf area index (LAI) in a needle-leaved
forest and a broad-leaved orchard using four instruments which
measure fractional light penetration through the canopy. Gap
fraction data were analyzed by a one-dimensional inversion model
or by using the Beer-Lambert Law. Instrument and analytical technique
both had a strong influence on calculated LAI. There was no consistent
pattern of LAI results among instruments so no simple cross-calibration
can be offered. Three of the four instruments accurately estimated
orchard LAI when used with one or the other analytical technique.
In addition to performance, practical considerations including
cost, sampling, and data analysis were also compared among the
instruments. Because the instruments do not provide consistently
accurate LAI estimates, LAI should be independently corroborated
before use in any particular situation. These instruments may
be most useful for relative LAI comparisons in a specific canopy
type when a single combination of instrument and analytical technique
is employed.
Martens, S.N., S.L. Ustin, and R.A. Rousseau.
(1993). Estimation of tree canopy leaf area index by gap
fraction analysis. Forest Ecology and Management
61: 91-108.
Measurement
of tree canopy architecture
The lack of accurate, extensive geometric data
on tree canopies has retarded development and validation of radiative
transfer models. We have devised a stratified sampling method
to measure the three-dimensional geometry of 16 walnut trees
which had received irrigation treatments of either 100 or 33
percent of evapotranspirational (ET) demand for the previous
two years. Graphic reconstructions of the three-dimensional geometry
were verified by 58 independent measurements. The distributions
of stem- and leaf-size classes, lengths, and angle classes were
determined and used to calculate leaf area index (LAI), stem
area, and biomass. Reduced irrigation trees have lower biomass
of stems, leaves and fruit, lower LAI, steeper leaf angles and
altered biomass allocation to large stems. These data can be
used in ecological models that link canopy processes with remotely
sensed measurements.
Martens, S.N., S.L. Ustin, and J. Norman.
(1991). Measurement and characterization of tree canopy
architecture. International Journal of Remote Sensing 12:1525-1545
Diurnal
change in Trees as Observed by Optical and Microwave Sensors:
The EOS Synergism Study
The EOS Synergism Study examined
the temporal variability of the optical reflectance and microwave
backscatter due to diurnal change in canopy properties of interest
to ecosystem modelers. The experiment was designed specifically
to address diurnal changes in canopy water status (including
water potential and content) that relate to transpiration. The
results demonstrate large diurnal variations in the dielectric
properties of the tree (the stem, or bole, specifically) that
in turn produce significant diurnal changes in the microwave
backscatter. The change in dielectric constant is related to
the diurnal change in water potential, although the specific
physical or physiological relationship has not yet been established.
A diurnal change in optical reflectance could be attributed primarily
to sun angle-view angle change. Based on the results of
this study, optical sensors should be placed in orbits which
minimize the influence of clouds on terrestrial data acquisition.
Way, J.B., J. Paris, M.C. Dobson, K. McDonald,
F.T. Ulaby, J.A. Weber, S.L. Ustin, V.C. Vanderbilt, and
E.S. Kasischke. (1991) Diurnal change in Trees as Observed
by Optical and Microwave Sensors: The EOS Synergism Study.
Transactions in Geoscience and Remote Sensing GE-29(6),
807-821.
Canopy
Architecture of a Walnut Orchard
A detailed dataset describing the canopy geometry
of a walnut orchard was acquired to provide supporting information
to test and compare the predictions of canopy microwave and optical
inversion models. Measured canopy properties included the quantity,
size, and orientation of stems, leaves, and fruit. Eight trees
receiving 100% of estimated potential evapotranspiration water
use and eight trees receiving 33% of potential water use were
measured for these properties. The vertical distributions of
stem, leaf, and fruit properties are presented with respect to
irrigation treatment. Zenith and probability distributions for
stems and leaf normals are also presented. These data show that
after two years of reduced irrigation, the trees receiving only
33% of their potential water requirement had reduced fruit yields,
lower leaf area index, and altered allocation of biomass within
the canopy. Such architectural differences provide an opportunity
to examine the resolution of optical and microwave sensors and
model predictions associated with detecting small changes in
canopy structure associated with modest levels of water stress.
Ustin, S. L., S. N. Martens, and V. C. Vanderbilt.
(1991). Canopy Architecture of a Walnut Orchard. IEEE Transactions
on Geoscience and Remote Sensing, GE-29(6), 843-851.
Diurnal
Water Relations of Walnut Trees: Implications for Remote Sensing
Leaflet water content (WC), relative water
content (RWC), and water potential (Y lf)
were measured as indicators of diurnal change in tree water status
in an experimental walnut orchard receiving two irrigation treatments:
100 and 33% of potential evapotranspiration (PET). Temporal
discrepancies could be caused by cavitation of xylem elements
in the vicinity of the DC probe. Microwave backscatter for L-and
X-bands also measured diurnal variation, but the pattern among
wavelength and polarization signatures was complex. From these
data it appears that radar backscatter can respond to diurnal
changes in plant water status. Interpretation of diurnal patterns
using a satellite sensor will require models that account for
multiple sources of canopy and soil variance, including canopy
architecture and soil moisture and more precise understanding
of the basis of diurnal patterns in backscatter.
Weber, J. A., and S. L. Ustin. (1991). Diurnal
Water Relations of Walnut Trees: Implications for Remote Sensing.
IEEE Transactions on Geoscience and Remote Sensing, GE-29(6),
864-874.
Biophysical
Information in Asymmetric and Symmetric Diurnal Bidirectional
Canopy Reflectance
In this paper we present a new theory for partitioning
the information content in diurnal bidirectional reflectance
measurements in order to detect differences potentially related
to biophysical variables. We collected optical data diurnally
on two orchard canopies at as many as nine view nadir angles
in the principal plane. Even though one of the canopies was permanently
water stressed and the other was stressed each day, the results
of a search for a water stress 'signature' in the reflectance
data proved inconclusive.
Vanderbilt, V. C., S. L. Ustin, K. M. Berger,
W. F. Caldwell, J. Clark, B. D. Ganapol, E. S. Kasischke, S.
N. Martens, R. E. Pettigrew, and R. A. Rousseau. (1991). Biophysical
Information in Asymmetric and Symmetric Diurnal Bidirectional
Canopy Reflectance. IEEE Transactions on Geoscience and Remote
Sensing, GE-29(6), 875-889.
Water
relations of a walnut orchard; Simultaneous measurement with
remote sensing
Water relations of trees vary over time scales
of hours to weeks. The effect of changes in water status on microwave
backscatter measurements from tree canopies was determined. Of
the several parameters measured, leaf xylem water potential showed
the greatest diurnal variation and appeared to co-vary in time
with C-, L-, and X-band microwave measurements. In addition data
collected with L-band dielectric probes embedded in the trunk
of selected trees showed parallel temporal variations. It is
unlikely that water potential, per se, is being sensed; however,
water potential of the leaves is a sensitive measure of overall
plant water status and is a useful parameter for connecting remotely
sensed data to plant activity.
Weber, J. A., and S. L. Ustin. (1988). ìWater
relations of a walnut orchard; Simultaneous measurement with
remote sensing.î IGARSS '88, Edinburgh, Scotland, UK, III,
1749-1752.