1. Stand Structure in Terrestrial Ecosystems
Remote Sensing Instrumentation
for Indirect Methods
Colwell (1983) defines remote sensing as "the measurement or acquisition of information of some
property of an object or phenomenon, by a recording device that is not in physical or intimate contact
with the object or phenomenon under study." Active remote sensing systems, such as laser (light
amplification by stimulated emission of radiation)
and active microwave systems, measure an electromagnetic signal sent by the sensor and returned by
the illuminated surface. Passive sensor systems,
such as film cameras or optical multispectral scanners, measure the brightness of sun-illuminated surfaces or the longwave thermal or microwave emission of surfaces.
Portable Ground Instruments
Rangefinders
Distance measurements are required to layout plot
boundaries as well as for many methods of estimating stand structure. Distance can be measured
by direct comparison (measuring tape or wheel),
optical instruments, or by electromagnetic devices
(Curtis 1995). Because tapes are cumbersome and
impractical over large distances or heights, optical
or electromagnetic instruments are often the preferred alternative. Optical instruments can be based
on angular observation of a reference length at the
targeted distance (e.g., survey stadia) or on focusing of split images created by mirrors and lenses
(coincidence rangefinders). Optical systems are
relatively inexpensive, are usually practical over
distances up to 300 m, and provide precision of
roughly ± 1 %.
Electromagnetic instruments, such as ultrasonic,
infrared, and laser range finders, measure distance
by comparing transmitted and received wave patterns or by measuring the round-trip travel time of
short pulses of radiation. The simplest hand-held
systems allow rapid measurements up to 100 m
with an accuracy of ± 1 %. More-sophisticated infrared or laser surveying systems can measure distances up to several kilometers with accuracies of
± 5 mm ± 0.0001 %. Professional surveying systems also measure inclination and compass bearing
to derive heights, volumes, and three-dimensional
11
position (Curtis 1995). These more-expensive systems can be coupled to global positioning systems
(GPSs) to map absolute locations in three dimensions within a few meters or to submeter accuracy
depending on system configuration. All systems depend on a clear line of sight and are sensitive to
atmospheric effects. Some systems require a special
target, which limits their use for measuring forest
canopies and in rugged terrain, although leaves and
stems serve as suitable reflecting targets for most
laser rangefinders.
Instruments for Gap Fraction Analysis
Several classes of instruments can be used beneath
plant canopies to indirectly measure canopy gap
fraction. Line quantum sensors integrate diffuse
photosynthetically active radiation (PAR) measured
by a linear array of photodiodes, whereas fisheye
light sensors measure diffuse PAR using azimuthally arranged photodiodes. Upward-looking photographs taken through a hemispherical lens can
also be processed to estimate canopy fraction (Rich
1990). Pulse laser systems have also been developed that scan the canopy and estimate the fraction
of pulses intercepted by leaves (e.g., the Decagon
Devices Leaf Laser, Decagon Devices, Inc., Pullman, Washington). As discussed in detail by Welles
(1990), these systems vary considerably in sky condition requirements, calibration, sample size, time
per site, and in the kinds of vegetation that can be
measured. Welles and Cohen (1996) provide a current overview of the design of commercially available systems. Understory light sensors must be
calibrated in the open and perform best under relatively continuous canopies and in diffuse light conditions when the sun is obscured.
Photography
Ground photography has been used for over 150
years to record features of vegetation, such as canopy height, closure, and composition. The major
variables to consider are film format (35-mm cameras are defined as small format and low resolution), film type (panchromatic, normal color, colorinfrared), lens focal length and field of view, speed
of film advance, and scale of reproduction (Philipson 1997). Simple equipment, such as mounted,
downward- or side-looking 35-mm cameras can be
used to acquire mono or stereo color photographs
Remote Sensing Instrumentation
for Indirect Methods
Colwell (1983) defines remote sensing as "the measurement or acquisition of information of some
property of an object or phenomenon, by a recording device that is not in physical or intimate contact
with the object or phenomenon under study." Active remote sensing systems, such as laser (light
amplification by stimulated emission of radiation)
and active microwave systems, measure an electromagnetic signal sent by the sensor and returned by
the illuminated surface. Passive sensor systems,
such as film cameras or optical multispectral scanners, measure the brightness of sun-illuminated surfaces or the longwave thermal or microwave emission of surfaces.
Portable Ground Instruments
Rangefinders
Distance measurements are required to layout plot
boundaries as well as for many methods of estimating stand structure. Distance can be measured
by direct comparison (measuring tape or wheel),
optical instruments, or by electromagnetic devices
(Curtis 1995). Because tapes are cumbersome and
impractical over large distances or heights, optical
or electromagnetic instruments are often the preferred alternative. Optical instruments can be based
on angular observation of a reference length at the
targeted distance (e.g., survey stadia) or on focusing of split images created by mirrors and lenses
(coincidence rangefinders). Optical systems are
relatively inexpensive, are usually practical over
distances up to 300 m, and provide precision of
roughly ± 1 %.
Electromagnetic instruments, such as ultrasonic,
infrared, and laser range finders, measure distance
by comparing transmitted and received wave patterns or by measuring the round-trip travel time of
short pulses of radiation. The simplest hand-held
systems allow rapid measurements up to 100 m
with an accuracy of ± 1 %. More-sophisticated infrared or laser surveying systems can measure distances up to several kilometers with accuracies of
± 5 mm ± 0.0001 %. Professional surveying systems also measure inclination and compass bearing
to derive heights, volumes, and three-dimensional
11
position (Curtis 1995). These more-expensive systems can be coupled to global positioning systems
(GPSs) to map absolute locations in three dimensions within a few meters or to submeter accuracy
depending on system configuration. All systems depend on a clear line of sight and are sensitive to
atmospheric effects. Some systems require a special
target, which limits their use for measuring forest
canopies and in rugged terrain, although leaves and
stems serve as suitable reflecting targets for most
laser rangefinders.
Instruments for Gap Fraction Analysis
Several classes of instruments can be used beneath
plant canopies to indirectly measure canopy gap
fraction. Line quantum sensors integrate diffuse
photosynthetically active radiation (PAR) measured
by a linear array of photodiodes, whereas fisheye
light sensors measure diffuse PAR using azimuthally arranged photodiodes. Upward-looking photographs taken through a hemispherical lens can
also be processed to estimate canopy fraction (Rich
1990). Pulse laser systems have also been developed that scan the canopy and estimate the fraction
of pulses intercepted by leaves (e.g., the Decagon
Devices Leaf Laser, Decagon Devices, Inc., Pullman, Washington). As discussed in detail by Welles
(1990), these systems vary considerably in sky condition requirements, calibration, sample size, time
per site, and in the kinds of vegetation that can be
measured. Welles and Cohen (1996) provide a current overview of the design of commercially available systems. Understory light sensors must be
calibrated in the open and perform best under relatively continuous canopies and in diffuse light conditions when the sun is obscured.
Photography
Ground photography has been used for over 150
years to record features of vegetation, such as canopy height, closure, and composition. The major
variables to consider are film format (35-mm cameras are defined as small format and low resolution), film type (panchromatic, normal color, colorinfrared), lens focal length and field of view, speed
of film advance, and scale of reproduction (Philipson 1997). Simple equipment, such as mounted,
downward- or side-looking 35-mm cameras can be
used to acquire mono or stereo color photographs