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Top-Down Analysis of Forest Structure and Biogeochemistry Across Hawaiian Landscapes 1



Technical and analytical improvements in aircraft-based remote sensing allow synoptic measurements of structural and chemical properties of vegetation across whole landscapes. We used the Carnegie Airborne Observatory, which includes waveform light detection and ranging (LiDAR) and high-fidelity imaging spectroscopy, to evaluate the landscapes surrounding four well-studied sites on a substrate age gradient across the Hawaiian Islands. The airborne measurements yielded variations in ground topography, canopy height, and canopy nitrogen (N) concentration more accurately than they could have been obtained by any reasonable intensity of ground-based sampling. We detected spatial variation in ecosystem properties associated with the properties of different species, including differences in canopy N concentrations associated with the native species Metrosideros polymorpha and Acacia koa, and differences brought about by invasions of the biological N fixer Morella faya. Structural and chemical differences associated with exotic tree plantations and with dominance of forest patches by the native mat-forming fern Dicranopteris linearis also could be analyzed straightforwardly. This approach provides a powerful tool for ecologists seeking to expand from plot-based measurements to landscape-level analyses.
Top-Down Analysis of Forest Structure and Biogeochemistry
across Hawaiian Landscapes
Peter M. Vitousek,
Michael A. Tweiten,
James Kellner,
Sara C. Hotchkiss,
Oliver A. Chadwick,
and Gregory P. Asner
Abstract: Technical and analytical improvements in aircraft-based remote sens-
ing allow synoptic measurements of structural and chemical properties of vege-
tation across whole landscapes. We used the Carnegie Airborne Observato ry,
which includes waveform light detection and ranging (LiDAR) and high-fidelity
imaging spectroscopy, to evaluate the landscapes surrounding four well-studied
sites on a substrate age gradient across the Hawaiian Islands. The airborne
measurements yielded variations in ground topography, canopy height, and can-
opy nitrogen (N) concentration more accurately than they could have been
obtained by any reasonable intensity of ground-based sampling. We detected
spatial variation in ecosyst em properties associated with the properties of dif-
ferent species, including differences in canopy N concentrations associ ated
with the native species Metrosideros polymorpha and Acacia koa, and differences
brought about by invasions of the biological N fixer Morella faya. Structural
and chemical differences associated with exotic tree plantations and with domi-
nance of forest patches by the native mat-forming fern Dicranopteris linearis also
could be analyzed straightforwardly. This approach provides a powerful tool for
ecologists seeki ng to expand from plot-based measurements to landscape-level
Substantial effort has gone into deter-
mining and analyzing ecosystem properties
and processes on a long substrate age gradient
(LSAG) across the Hawaiian Archipelago
(Crews et al. 1995, Kitayama and Mueller-
Dombois 1995, Riley and Vitousek 1995,
Chadwick et al. 1999, Herbert and Fownes
1999, Vitousek 2004), as a model for under-
standing long-term soil and ecosystem devel-
opment. As with other gradient-based studies,
the LSAG research has focused on particu-
lar sites that were selected to represent each
substrate age. This focus on particular sites is
useful because sources of variation other than
substrate age (topography, human distur-
bance, biological invasion) can be controlled
through careful site selection; moreover,
where long-term experiments (Vitousek and
Farrington 1997, Harrington et al. 2001) or
expensive or repeated measurements (Hobbie
and Vitousek 2000, Wiegand et al. 2005) are
carried out, it may also be the only pra ctical
approach. However, this site-based approach
does not account for spatial variation in eco-
system properties on landscapes, whether
that variation arises from systematic chan ges
in landscapes that occur as a consequence of
substrate age or from other processes.
Remote sensing offers a means for evaluat-
ing landscapes synoptically, rather than site
Pacific Science (2010), vol. 64, no. 3:359366
doi: 10.2984/64.3.359
: 2010 by University of Hawai‘i Press
All rights reserved
Research supported by NSF grants DEB-0716852,
-0715593, -0717382, and -0715674; NASA Terrestrial
Ecology Program-Biodiversity grant NNG-06-GI-87G;
and the Carnegie Institution. The Carnegie Airborne
Observatory is made possible by the W. M. Keck Foun-
dation and William Hearst III. Manuscript accepted 1
September 2009.
Department of Biology, Stanford University, Stan-
ford, California 94305 (e-mail:
Department of Botany, Birge Hall, University of
Wisconsin, Madison, Wisconsin 53706.
Department of Global Ecology, Carnegie Institu-
tion of Washington, Stanford, California 94305.
Department of Geography, University of Califor-
nia, Santa Barbara, California 93106.
Corresponding author.
by site. However, until recently most remote
sensing methods have been insensitive to any
but extreme variations in the structure and
biogeochemistry of ecosystems. Recent ad-
vances in light detection and ranging
(LiDAR) (Lefksy et al. 2002) and in high
spectral and spatial resolution imaging spe c-
troscopy (High-Fidelity Imaging Spectros-
copy, or HiFIS) (Smith et al. 2002, Ustin
et al. 2004) have made it possible to measure
certain structural and chemical properties
of ecosystems across landscapes. In previous
studies, we have used LiDAR and HiFIS
separately to evaluate the landscape-level in-
fluence of biological invasions on the three-
dimensional structure and canopy nitrogen
(N) concentrations of Hawaiian forests (As-
ner and Vitousek 2005, Asner et al. 2008),
and of erosion/topography on phosphorus
biogeochemistry in the oldest landscape on
the gradient (Porder et al. 2005a).
An earlier study (Vitousek et al. 2009) used
LiDAR and HiFIS in combination to evalu-
ate aspects of forest structure and biogeo-
chemistry on the landscape level in four
sites across the Hawaiian substrate age gradi-
ent. That study demonstrated that although
the same patterns in canopy height (taller
in the two younger sites) and canopy N con-
centrations (higher in the two intermediate-
aged sites) occur on the landscape scale as
were observed in ground-based rese arch on
the focal sites in each landscape, the long-
term research sites were biased toward areas
with taller-than-average canopies within each
landscape, particularly on the two older
substrates. That study also demonstrated
that stratification of landscapes into cover
classes defined by the influence of invasion,
erosion/topography, human disturbance, and
other processes accounts for a substantial
proportion of the variation in canopy height
and N concentration within some of the
In this paper, we use the same LiDAR and
HiFIS information together with ground-
based sampling to understand aspects of for-
est structure and biogeochemistry in 2 km by
2 km regions around each of the four age-
gradient sites. The earlier paper focused on
landscape-level changes in ecosystem proper-
ties across the substrate age gradient, but here
we concentrate on using the imagery to un-
derstand source s of variation in ecosystem
structure and composition within each re-
gion. We thereby seek to place each of the
long-term research sites into the context of
the landscape that encompasses it.
materials and methods
Study Sites and Landscapes
The 4-million-year LSAG across the Hawai-
ian Archipelago consists of a series of sites
that are matched in elevation (1,200 m), an-
nual precipitation (2,500 mm), topography
(constructional surface of shield volcanoes),
vegetation (>80% basal area is the native
tree Metrosideros polymorpha Gaud), and to
the extent possible distur bance history (none
has been cleared by people). The gradien t is
described in detail in Vitousek (2004), and
the peculiarities of particular sites are de-
scribed in supplemental material accessible
princetonbook.html. Here we focus on 2 k m
by 2 km landscapes surrounding four of the
focal sites on the LSAG: the 0.3-thousand-
yr-old (kyr) Thurston, 20 kyr Laupa
150 kyr Kohala, and 4,100 kyr Ko
ke‘e sites.
The first three sites are on the island of
Hawai‘i; the last is on the island of Kaua‘i.
Remote Sensing
Methods for the remote sensing analyses are
described in Vitousek et al. (2009). Briefly,
we used the Carnegie Airborne Observatory
(CAO), a system designed to determine
chemical and structural properties of vegeta-
tion from the air (
(Asner et al. 2007). The CAO combines three
instrument subsystems into a single airbo rne
package: (1) High-Fidelity Imaging Spec-
trometer (HiFIS), here the Airborne Visible
and Infrared Imaging Spectrometer (AVIRIS)
(Green et al. 1998); (2) Waveform Light De-
tection and Rangi ng (LiDAR) scanner; and
(3) Global Positioning System-Inertial Mea-
surement Unit (GPS-IMU). We use d meth-
ods described by Asner et al. (2007) to match
July 2010
HiFIS and LiDAR data in three-dimensional
(3-D) space. The system was operated from
January to February 2007 at an altitude aver-
aging 3.0 km above ground level, thus pro-
viding spectroscopic measurements at 3.0 m
spatial resolution and LiDAR spot spacing
(postings) of 1.01.5 m, depending upon the
site. The GPS-IMU data were combined
with the laser range data to determine the
3-D location of the laser returns. From the
LiDAR point cloud data, a physically based
model was used to estimate top-of-canopy
and ground digital elevation models (DEM)
using REALM (Optech Inc., Toronto, Can-
ada) and Terrascan/Terramatch (Terrasolid
Ltd., Jyva
, Finland) software packages.
Vegetation height was then calculated as the
difference between the top-of-canopy and
ground DEM.
The HiFIS data were converted to at-
sensor radiances by applying radiometric cor-
rections developed during sensor calibration
in the laboratory. Apparent surface reflec-
tance was then derived from the radiance
data using an automated atmospheric correc-
tion model, ACORN 5LiBatch (Imspec LLC,
Palmdale, California), with settings described
by Asner et al. (2008 ). Upper-canopy leaf N
concentration was estimated from the HiFIS
data using the visible (450690 nm) and the
shortwave-infrared (1,5002,400 nm) wave-
length regions, as described by Asner and
Vitousek (2005).
Field Observations
The processed images resulting from LiDAR
and HiFIS acquisitions provided a synoptic
view of the topography, canopy height, and
upper-canopy N concentration across 2 km
by 2 km landscapes centered on each of the
focal sites. Formal validation of the accuracy
of these products was carried out separately
(see Asner and Vitousek [2005] for canopy
N, Asner et al. [2008, 2009] for vegeta tion
height); here, we sought to use ground-based
observations to understand sources of the
spatial variation detected by remote sensing.
Our field sampling was primarily descrip-
tive; we printed the topographic, tree height,
and canopy N images from each area with
an overlay of GPS coordinates and selected
sample points in advance. For these sample
points, we sought representation of both fre-
quent and unusual combinations of remotely
detected topography, canopy height, and can-
opy N within each landscape. We took the
images into the field and used GPS to locate
the preselected points. Additional sample
points were selected in the field when condi-
tions not encompassed by the preselected
points wer e encountered. In most cases, we
knew what to expect from the imagery; for
example, very tall homogeneous patches of
forest on a consistent slope likely represent
plantations of exotic trees, and our field ob-
servations confirmed those expectations. In
other cases we did not know what to expect
when we reached the selected points, and the
ground observations were essential to under-
standing the remote observations.
At each point, we recorded canopy charac-
teristics (dominant canopy and understory
species, visual estimates of structure), ob-
tained GPS coordinates, and obtained and
archived two digital photographs, one taken
vertically that showed the canopy and one
taken horizontally showing the understory.
This approach allowed us to walk through
the sites both virtually (observing the images)
and literally (with the images in hand); it pro-
vides a powerful way to identify and evaluate
landscape-level variation in forest ecosystems.
Both the remote images and the ground-
based information and photographs are ac-
cessible at
Vitousek/lsaglandscape.htm. We encourage
readers to scroll through the images and pho-
tographs and develop their own sense for
these landscapes.
Patterns in Topography and Forest Canopi es
The LiDAR-derived topographies of the 2
km by 2 km landscapes across the substrate
age gradient a re illustrated in Plates IIV;
the same informat ion is presented in Vitousek
et al. (2009), but the figures are organized and
used differently there, and additional informa-
tion is provided. The topographic images pro-
vide remarkable resolution, even under forest
canopies, and in some cases they enabled us
Aircraft-Based Analysis of Forest Landscapes
Vitousek et al. 361
to identify and explain topographic features
that we had not interpreted correctly in 20
25 yr of fieldwork in and around these sites.
Topography is an important source of
variation in forest ecosystems; fluvial erosion
and deposition influence nutrient availability
and can cause differences in foliar chemistry
among sites (Porder et al. 2005a,b), and to-
pography can shelter stands from wind dis-
turbance, thereby affecting their structure
and chemistry. Also, steep topography can in-
terfere with our analyses of both tree height
and canopy N: the former because the uphill
and downhill sides of a given tree yield dif-
ferent returns to the LiDAR (Su and Bork
2006); the latter because steep slopes create
shadows that make it difficult to derive bio-
chemically meaningful spectra (Roberts et al.
1993, Kupiec and Curran 1995). Accordingly,
we classified steep slopes (>20 degrees) as
erosionally or volcanically derived but then
masked them out of further analyses.
We then evaluated canopy heights from
LiDAR and N concentrations from HiFIS
(Plates IIV ) across the landscapes. In com-
bination with topography, and with our
ground-based observations of a number of
points in each landscape, we defined areas
within each 2 km by 2 km landscape that rep-
resented little-invaded native forest growing
on constructional geomorphic surfaces, and
areas that were influenced substantially by bi-
ological invasions (‘‘substantially’’ defined by
areas in which invaders influenced remotely
detected canopy properties); fluvial erosion
or deposition and steep or recent volcanic to-
pography (from the image of topography);
human disturbance or land use (based on
remotely detected canopy prop erties supple-
mented by ground observations); steep cli-
matic gradients within a landscape; or other
sources of variation (Plates IIV ).
results and discussion
Thurston Landscape (0.3 kyr)
The topography surrounding the youngest
region on the LSAG is constructional; all of
the relief visible in the topographic image
(Plate I) is volcanic in origin. Across most of
this landscape, the substrate is pa
hoehoe lava
overlain with cinder. The major topographic
features are the wall and floor of
lauea Iki
Crater to the west of the focal site (points 14
and 15; the point numbers are shown in the
upper left section of each plate and online as
discussed earlier), and a nearby series of pits
that resulted from the collapse of portions
of the ceiling of the large underlying Thur-
ston Lava Tube. Away from the 50-yr-old
lava lake in
lauea Iki Crater, most of the
area is covered by forests of intermediate
height (1020 m) (Plate I); short er canopies
occur in active and abandoned pastures in
the northern part of the image (34, 37), in
smaller areas of stand-level dieback north and
east of the focal site (6, 8), and in areas with
shallow soils that ground observations show
to be codominated by the mat-forming fern
Dicranopteris linearis Underw. in the south-
western part of the image (18).
The major source of variation in canopy N
across this landscape (Plate I) is biological in-
vasion by the actinorrhizal N-fixing tree Mor-
ella (Myrica) faya (Aiton) Wilbur (points 7,
21, 32, and others). Canopy N levels are in-
creased in invaded stands, but canopy heights
are similar and have the same level of vari-
ability as in intact stands (Plate I). A planted
stand of the native N-fixer Acacia koa (point
35) also created an area of high canopy N.
Overall, biological invasion, human land
clearing, and volcanic activity affect substan-
tial fractions of the Thurston landscape
(Plate I).
hoehoe Landscape (20 kyr)
The focal site for the second-youngest land-
scape is located on small interfluve in deep
volcanic ash soils (Plate II). The area to the
south and east of this site is covered by a
P5,000-yr-old Mauna Kea lava flow (Wolfe
and Morris 1996). Most of the ‘a‘a
lava flow
surface is little dissected by erosion; its topog-
raphy largely reflects lava pressure ridges and
flow channels. However, the portion of this
flow within P300 m of the focal site is man-
tled with tephra tens of centimeters thick, as
can be seen from the subdued flow topogra-
phy there (Plate II) and as is confirmed from
July 2010
ground observations. We wonder if in fact
there are two flows here , with an older one
closer to the focal site partially overridden by
the 5,000-yr-old flow farther to the south and
east, or alternatively if the portion of the flow
nearest the focal site represents an early por-
tion of the flow that was covered by later
tephra from the same eruptive episode. With-
in the northwesterly portion of the landscape
that is underlain by deep er ash soils, the
drainage network is fine but sha llow, with
little more than 5 m of local relief. Older
landscapes have deeper but sparser drainage
networks, as discussed in the next sections.
Tree heights within the Laupa
landscape vary substantially, with larger trees
downslope to the northeast (points 2226)
(Plate II, upper right). The larger trees in
hoehoe are substantially taller than
those at Thurston (>25 m versus <20 m),
and their crowns are broader as well; indi-
vidual trees stand out in the Laupa
canopy-height image (for example, a tall
Acacia koa A. Gray at 32), but they rarely are
distinguishable at Thurston (or the older
sites). At the same time, there are extensive
areas of shorter canopies in the Laupa
landscapemore so than within native forest
at Thurston. The shorter areas reflect human
land use (ongoing koa logging in the adjacent,
partially cleared Waipunalei ahupua‘a [4, 5]
and some past logging in the upper-elevation
portion of Laupa
hoehoe itself ), a few small
hollows and bogs (20), and, most important,
the legacy of canopy dieback (Mueller-
Dombois 1986), especially on the deep volca-
nic ash substrates. Ground observations re-
veal that much of the dieback area has a
canopy of tall Cibotium spp. tree ferns, with
standing dead trees (7, 35).
Canopy N concentrations also vary across
the Laupa
hoehoe landscape (Plate II), with
many value s in excess of those observed in
the Thurston landscape. In part high N con-
centrations result from the presence of Acacia
koa, an N-fixer, within the Laupa
hoehoe for-
est; tall individuals of Metrosideros and Acacia
are readily distinguishable in the imagery
based on their N signature (22, 24), as ground
observations here quickly confirmed. How-
ever, even Metrosideros is substantially higher
in N at Laupa
hoehoe than at Thurston, as
was observed earlier in ground-based sam-
pling (Vitousek et al. 1995). The N image
demonstrates that Acacia reaches its distribu-
tional limit within the Laupa
hoehoe image,
as evidenced by the arc of higher-N canopies
that passes through points 29, 30, and 41; it is
absent from the slightly wetter forests in the
southeastern portion of the image and more
abundant in the drier (and more disturbed)
forests to the northwest. Acacia also responds
strongly to soil scarification, contributing to
its abundance along the road (28, 41) and the
hoehoe-Waipunalei boundary fence
line (5) as well as in logged areas.
Kohala Landscape (150 kyr)
Kohala is a smaller mountain than Mauna
Kea, and the focal LSAG site is located just
400 m south-southeast of a block fault (point
10) that bounds a central graben near the
mountain summit (Plate III). The topo-
graphic image demonstrates that volcanic
features are surprisingly prominent in this
landscape, for a region that has not experi-
enced local volcanism for P150 kyr. The
landscape contains several cinder cones that
erupted 150220 kyr ago and several tra-
chyte lava flows (including a large one west-
northwest of 53) representative of this later
(alkalic) stage of Hawaiian volcanism. The fo-
cal site itself is on an interfluve underlain by
alkalic ‘a‘a
lava; the local drainage netw ork is
coarser and deeper than that in the Laupa
hoehoe landscape.
Canopy heights are relatively short and ex-
tremely variable across the Kohala landscape
(Plate III), reflectin g the influence of a steep
rainfall gradient and multiple additional con-
trolling factors. The southern one-third of
the landscape has been cleared and converted
to pasture (15, 32, 33, 53); trees remain in
that landscape primarily in and near gullies.
Several species of exotic trees were planted
in the 1920s and 1930s at and near the upper
margin of the pasture; some of these (Euca-
lyptus spp. and Cryptomeria japonica (L. f.) D.
Don in particular) (14, 18, 31, 35) represent
the tallest trees in this and the older Ko
Aircraft-Based Analysis of Forest Landscapes
Vitousek et al. 363
The area beyond the block fault north-
northeast of the focal site (4352) receives
substantially more rainfall than the LSAG
site, and it supports areas of short-canopy
Metrosideros forest interspersed with Sphag-
num palustre L. and Metrosideros-covered
bogs with occasional patches of taller Metrosi-
deros (43, 45). Finally, the region to the west-
northwest of the focal site is similar to it
climatically, but canopies are short and co-
dominated by the mat-forming fern Dicranop-
teris linearis. Fern dominance reflects dieback
of the Metrosideros overstory in some porti ons
of the area, with standing dead trees over a
nearly monospecific layer of fern (28, 36,
40); in other areas, the Dicranopteris is climb-
ing into what appears to be a growing, post-
disturbance stand of Metrosideros (27, 41).
Canopy N also varies across the landscape:
with the highest concentrations in the tree
plantations and in postdieback Dicranopteris,
and relatively low concentrations in the pas-
ture and most of the Metrosideros forest (Plate
III). Ground observations revealed that sev-
eral areas near the forest-pasture boundary
with moderately tall canopies and high can-
opy N concentrations (22, 34) are tree plan-
tations dominated by N-fixing Casuarina
equisetifolia L. The wet forest region to the
northeast of the focal site includes a fine-
scale matrix of areas with high and low N
concentrationsa surprising result, because
high rainfall generally is associated with lower
N concentrations in Hawaiian forests (Schuur
and Matson 2001). Following up on these re-
mote observations, we found that the higher
N concentrations reflect in part a greater
abundance of the relatively high-N native
tree Cheirodendron trigynum (Gaud) A. Heller
in the canopy in this region (51, 52), as well as
open areas in which the N-rich alien herbs
Hedychium gardnerianum Sheppard ex Ker
Gawl and Tibouchina herbacea (DC) Cogn.
are present in the canopy (50).
ke‘e Landscape (4,100 kyr)
The Ko
ke‘e area is much older than the other
regions on the LSAG, and more of its topog-
raphy is erosional and depositional (Plate
IV ). However, there is a well-defined area
underlain by a constructional volcanic surface
on which the focal site is located. This area
has a less-pronounced local drainage net-
work (aside from the large and deep valleys
that bound and bisect it) than occurs on the
younger Laupa
hoehoe and Kohala land-
scapes, because it is underlain by thick, flat-
lying caldera-filling deposits of the Olokele
Formation (Sherrod et al. 2007), which are
more resistant to dissection than most Hawai-
ian lavas.
Tree heights generally are short across the
ke‘e landscape, on both constructional and
erosional/depositional surfaces (Plate IV ).
Major exceptions to this pattern are a tall
Cryptomeria plantation along a stream in the
lower-rainfall southern part of the landscape
(29), other pockets of planted Crytomeria
elsewhere (14, 15), and an arc of taller Metro-
sideros forest 500 m east of the focal site
(2325). This taller native forest is in a topo-
graphic depression and so probably has richer
soils than the constructional surface (Porder
et al. 2005b); it is also sheltered from strong
winds, and trees there persisted through hur-
ricanes that passed over this landscape in
1957, 1982, and 1992. One striking feature
of the Ko
ke‘e landscape is an open bog in
the northeast (16); this area is covered by
sedges and very short (P0.5 m) Metrosideros,
as is much of the Alaka‘i Swamp to the east.
The areas of higher canopy N are gener-
ally associ ated with nonnative species: in the
planted Cryptomeria and an area of invasive
Morella and Psidium cattleianum Sabine (30,
31) (Plate IV ) in the southern part of the
area, and in smaller areas associated with
Cryptomeria plantings or invasions by N-
fixing Acacia melanoxylon R. Br. (5) and
Morella faya (13). In addition, ground obser-
vations show that a stream valley on the
northwestern side of the image is dominated
by the high-N invasive herb Hedychium
gardnerianum. On the constructional surface,
concentrations of N in Metrosideros gener-
ally are low; concentrations are higher in the
sheltered, Metrosideros-dominated depression
already described. Mats of Dicranopteris
fern, which cov er large portions of the con-
structional surface and many slopes to the
southwest (9, 20, 21, 32), also have low con-
centrations of N. Overall, erosion and depo-
sition influence more of the Ko
ke‘e landscape
July 2010
Plate 1. Topography and canopy properties in the 2 km by 2 km landscape centered on the 300-yr-old Thurston
site on a substrate age gradient across the Hawaiian Archipelago. (Upper left) Topography derived from LiDAR. The
long-term research site at Thurston is indicated by a circle with 50 m radius; points where ground-based information
on forest structure and composition and canopy photographs were obtained are indicated by numbers. Images that
allow zooming in are accessible online at (Upper right)
Distribution of canopy heights across the Thurston landscape, also obtained via LiDAR. (Lower left) Nitrogen (N)
concentration in the forest canopy, measured with High-Fidelity Imaging Spectrometer (HiFIS). (Lower right) Cover
classes in the Thurston landscape, derived from the remote images and ground observations. Dark green represents
native-dominated ecosystems on constructional volcanic surfaces; dark blue represents sites with invasive trees (here
predominantly the N-fixer Morella faya) at least codominant in the canopy; yellow represents volcanic features, here
the wall and floor of Kõlauea Iki Crater and some lava-tube skylights; and red represents human-disturbed sites, here
active and abandoned pasture in the northern portion of the image and National Park infrastructure (roads and parking
lots) elsewhere. This figure is revised and reorganized from Vitousek et al. (2009).
Plate 11. Topography and canopy properties in the 2 km by 2 km landscape centered on the 20 kyr Laupëhoehoe
landscape, organized as in Plate I. Cover classes include native-dominated ecosystems on constructional surfaces (dark
green); eroded areas (light green); and human-disturbed areas (red), here mostly logging of Acacia koa in what was once
forested pasture.
Plate 111. Topography and canopy properties in the 2 km by 2 km landscape centered on the 150 kyr Kohala land-
scape, organized as in Plate I. Cover classes include native-dominated ecosystems on constructional surfaces (dark
green); eroded and depositional areas (light green); volcanic and tectonic features (yellow), here cinder cones, the steep
sides of trachyte flows, and a fault scarp; human-disturbed areas (red), here mostly pasture; and plantations of intro-
duced trees (light blue). The hatched area to the northeast receives substantially more rainfall than the remainder of
the image, or the other landscapes.
Plate 1v. Topography and canopy properties in the 2 km by 2 km landscape centered on the 4,100 kyr Kýke‘e land-
scape, organized as in Plate I. Cover classes include native-dominated ecosystems on constructional surfaces (dark
green); eroded and depositional areas (light green); invaded areas (dark blue), here mostly Morella faya and Psidium
cattleianum, with some Hedychium gardnerianum and Acacia melanoxylon; human-disturbed areas (red), here mostly a
road and picnic area; and plantations of introduced trees (light blue).
than they do in any of the younger ones
(Plate IV ). Tree plantations and biological
invasions also contribute to landscape-level
variation in canopy structure and chemistry,
particularly on lower slopes and in valley bot-
Aircraft-based remote sensing can provide
highly resolved, synoptic analyses of the to-
pography, canopy heights, and canopy N
concentrations of forested landscapes; espe-
cially when supplemented by ground-based
observations, they can provide far richer
information than can be obtained at com-
parable spatial scales from ground-based
studies alone. These analyses do not depend
on either extrapolations of known sites or
‘‘ground-truthing’’ of remotely sensed im-
ages; rather they are direct, distributed mea-
surements of ecosystem properties on the
landscape scale. Here we used the remote
measurements to provide a regional perspec-
tive on the results of long-term studies in
particular site s; remote observ ations provide
a direct measure of ecosystem properties
on the landscape scale as these vary with
long-term soil and landscape development.
Aircraft-based measurements can also identify
and analyze ecosystem dynamics that might
be difficult to observe from the ground. Fi-
nally, they can be enjoyable, because on-the-
ground measurements designed to pursue the
sources of variance detected by airborne re-
mote sensing require that investigators get
into every unique portion of a study land-
scape. In doing so, new features of the land-
scape, and new biological communities, may
be discovered. For example, the Ko
ke‘e tree-
height imagery (Plate IV ) suggested to us
that there might be an incipient bog like that
at point 16 developing on an adjacent ridge,
in the area near point 9. Ground-based obser-
vations showed that this area is not boglike;
rather, it is dominated by a Dicranopteris
thicket. However, in getting there we found
striking ‘‘barrens’’ of sparse Metrosideros can-
opies over soil surfaces with pedogenic con-
cretions that looked like a rough young ‘a‘a
lava flow at and around point 8. We had not
observed this combination of features previ-
ously and now are attempting to understand
the factors that brought it into being.
We thank T. Varga, D. Knapp, T. Kennedy-
Bowdoin, R. Martin, M. Jones, M. Eastwood,
P. Gardner, S. Lundeen, C. Sarture, and R.
Green for airborne data collection and analy-
sis support. Douglas Turner prepared the
Web-based supplementary material.
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July 2010
... Occupation of native ecosystems by ecosystem-modifying invasive plants (EMIP) has been linked to changes in native biodiversity [1,2], biogeochemical heterogeneity [3][4][5] and ecosystem services [2]. These invaders alter the structure and/or function of the native ecosystems through both habitat degradation, and the development of non-analog communities/ecosystems [6][7][8]. As detailed in Vitousek et al. [8] and Hughes et al. [9], EMIPs both singly and collectively, have the potential to overrun and fragment habitat previously characterized by unique and diverse endemic communities. ...
... These invaders alter the structure and/or function of the native ecosystems through both habitat degradation, and the development of non-analog communities/ecosystems [6][7][8]. As detailed in Vitousek et al. [8] and Hughes et al. [9], EMIPs both singly and collectively, have the potential to overrun and fragment habitat previously characterized by unique and diverse endemic communities. Although some ecosystems are resilient [10,11], continuous degradation of habitat without remediation may alter these diverse native communities to a point beyond recovery. ...
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Occupation of native ecosystems by invasive plant species alters their structure and/or function. In Hawaii, a subset of introduced plants is regarded as extremely harmful due to competitive ability, ecosystem modification, and biogeochemical habitat degradation. By controlling this subset of highly invasive ecosystem modifiers, conservation managers could significantly reduce native ecosystem degradation. To assess the invasibility of vulnerable native ecosystems, we selected a proxy subset of these invasive plants and developed robust ensemble species distribution models to define their respective potential distributions. The combinations of all species models using both binary and continuous habitat suitability projections resulted in estimates of species richness and diversity that were subsequently used to define an invasibility metric. The invasibility metric was defined from species distribution models with <0.7 niche overlap (Warrens I) and relatively discriminative distributions (Area Under the Curve >0.8; True Skill Statistic >0.75) as evaluated per species. Invasibility was further projected onto a 2100 Hawaii regional climate change scenario to assess the change in potential habitat degradation. The distribution defined by the invasibility metric delineates areas of known and potential invasibility under current climate conditions and, when projected into the future, estimates potential reductions in native ecosystem extent due to climate-driven invasive incursion. We have provided the code used to develop these metrics to facilitate their wider use (Code S1). This work will help determine the vulnerability of native-dominated ecosystems to the combined threats of climate change and invasive species, and thus help prioritize ecosystem and species management actions.
... All taxa with a correlation higher than AE 0.63 (equivalent to an r 2 > 0.4) are included in the (Fig. 2). The pervasive distribution of the fallen trees, the high incidence of upper canopy trees rooted in nurse logs, and the synchronized dieback phenomenon in M. polymorpha stands strongly suggest recovery from a large dieback event in the past in this ele-vation zone (Atkinson 1970;Mueller-Dombois 1986;Vitousek et al. 2010). Fallen trees, by providing a new growth substrate, seem to have a greater influence on community composition than does more transient canopy gap formation in Hawaiian montane rain forest (Santiago 2000). ...
QuestionHow do canopy disturbance and soil properties structure vascular plant community species composition and resilience to encroachment by exotic species in a tropical montane wet forest?LocationHawai'i Experimental Tropical Forest (HETF), a tropical montane wet forest, on Mauna Kea, Hawai'i Island, Hawai'i, USA.Methods Previous studies employing airborne LiDAR were used to define three zones across an elevation gradient from 900 to 1500 m. Within each zone, a ~1000-m block transect was selected to cross two different volcanic substrates: one derived from surface lava and one derived from thick ash deposits. Non-metric multidimensional scaling (NMS) scores of vegetation data were related to independently-derived environmental NMS scores and spatial location with generalized additive models (GAM).ResultsVascular plant species composition in all elevation zones consists of three NMS axes, which are best modelled by one of three possible environmental NMS axes or by location. The first NMS axis of species composition in the lowest elevation zone (40% variance explained (VE)) is a function of location on volcanic substrates (61% deviance explained (DE)). The second lowest elevation axis (27% VE) is a function of unexplained spatial heterogeneity (31% DE). The third lowest elevation NMS axis (24% VE) is a function of the spatial mosaic of canopy disturbance (16% DE). In the middle elevation zone, species composition most strongly relates to the interaction between volcanic substrate and the condition of the soil surface for all three NMS axes (41%, 27%, 24% VE; 70%, 16%, 24% DE). The primary axis of species composition in the highest elevation zone (41% VE) corresponds with substrate and soil condition (55% DE) but the second and third axes of species composition (27% and 25% VE) relate to canopy dieback disturbance (36%, 14% DE). Counts of exotic species and 0–2 m height class native tree species respond to the type of volcanic substrate and soil surface condition in all three elevation zones. Lava-derived substrates have a higher incidence of exotic species and less native tree regeneration; whereas ash-derived substrates have higher numbers of native tree species regenerating and many fewer exotic species.DiscussionThe tropical montane forests on Mauna Kea reflect a native-dominated plant community response to disturbance on both lava- and ash-derived volcanic substrates, and a higher propensity for exotic species to occur on the lava-derived substrate. Native plant communities on ash-derived soils may have higher resilience to exotic invasion than communities on lava-derived substrates. Our results indicate resource managers should explicitly account for variation in soils and substrate type when prioritizing, implementing and monitoring management interventions to foster native plant assemblages and control the spread of exotic and invasive species.
... We consider the fate of positions (pixels), and use apparent height changes to infer whether a position was retained by its occupant and associated with vertical height growth or lost to a neighbouring contender and associated with lateral capture. We apply the model to canopy height measurements from airborne LiDAR in a montane rain forest on the windward flank of Mauna Kea on the Island of Hawaii (Vitousek 2004;Vitousek et al. 2010), and we use this framework to test three hypotheses of competition for space in the canopy. The site is dominated by the Hawaiian tree species Metrosideros polymorpha. ...
Trees compete for space in the canopy, but where and how individuals or their component parts win or lose is poorly understood. We developed a stochastic model of three-dimensional dynamics in canopies using a hierarchical Bayesian framework, and analysed 267 533 positive height changes from 1.25 m pixels using data from airborne LiDAR within 43 ha on the windward flank of Mauna Kea. Model selection indicates a strong resident's advantage, with 97.9% of positions in the canopy retained by their occupants over 2 years. The remaining 2.1% were lost to a neighbouring contender. Absolute height was a poor predictor of success, but short stature greatly raised the risk of being overtopped. Growth in the canopy was exponentially distributed with a scaling parameter of 0.518. These findings show how size and spatial proximity influence the outcome of competition for space, and provide a general framework for the analysis of canopy dynamics.
... Despite the environmental controls afforded by working in Hawaii highlighted above, a major source of uncertainty in all studies of ecosystem processes lies in the lack of information on disturbance history. Even in relatively intact forests such as the HETF and HFNWR, disturbance events ranging from gap-phase tree turnover on small scales to weather-related events on larger scales impart variability in forest structure that influences spatial and temporal variation in biogeochemical processes ( Vitousek et al., 2010). To control for disturbance history effects across this MAT gradient, we used airborne light detection and ranging (LiDAR) measurements of forest structure to select seven sites in the HETF at each of six target elevations, where each site represents the maximum aboveground biomass present at a given elevation. ...
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The annual release of CO2 from soils to the atmosphere as soil-surface CO2 efflux (FS; 'soil respiration') follows gross primary production (GPP) as the second largest carbon flux in the global terrestrial carbon cycle. At the stand scale, FS in forest ecosystems can account for 50% or more of GPP and 70% or more of total ecosystem respiration. At the global scale, FS in tropical broadleaf forests accounts for ~30% of the global annual FS budget. Because FS typically increases with temperature, future warming is anticipated to impact terrestrial carbon cycling and atmospheric CO2 concentrations. Representative field estimates of FS are needed to accurately model terrestrial ecosystem metabolism. An obstacle to achieving representative field estimates of FS, however, is its inherent spatial and temporal variability, which remain poorly understood. We quantified FS in nine permanent plots along a 5.2°C mean annual temperature (MAT) gradient (13-18.2°C) in Hawaiian tropical montane wet forest where substrate type and age, soil type, soil water balance, canopy vegetation, and disturbance history are constant. The objectives of this study were to quantify how the (i) magnitude, (ii) plot-level spatial variability, and (iii) plot-level diel variability of FS vary with MAT. To address the first objective, annual FS budgets were constructed by measuring instantaneous FS monthly in all plots for one year. For the second objective, we compared plot-level mean instantaneous FS in six plots derived from 8 versus 16 measurements, and conducted a power analysis to determine adequate sample sizes. For the third objective, we measured instantaneous FS hourly for 24 hours in three plots (cool, intermediate and warm MATs). The magnitude of annual FS and the spatial variability of plot-level instantaneous FS increased linearly with MAT, likely due to concomitant increases in stand productivity. Strong seasonal patterns were evident in daily FS across the measurement period, and the seasonal pattern in FS closely mirrored that in soil temperature. Mean plot-level instantaneous FS from 8 versus 16 measurements did not differ. The number of samples required to estimate instantaneous FS within 10% and 20% of the actual mean increased with MAT. In two of three plots examined, diel variability in FS was significantly correlated with soil temperature, but minimal diel fluctuations in soil temperature (<0.6°C) resulted in minimal diel variability in FS. Our results suggest that as MAT increases in tropical montane wet forests, FS will increase and become more spatially variable if ecosystem characteristics and functioning undergo concurrent changes as measured along this MAT gradient. In line with other tropical forest studies, however, diel variation in FS will remain a minor component of overall plot level variation. Increased FS with future warming warrants detailed attention in the context of potential changes in soil C pools and feedbacks to global climate change.
... Despite the environmental controls afforded by working in Hawaii highlighted above, a major source of uncertainty in all studies of ecosystem processes lies in the lack of information on disturbance history. Even in relatively intact forests such as the HETF and HFNWR, disturbance events ranging from gap-phase tree turnover on small scales to weather-related events on larger scales impart variability in forest structure that influences spatial and temporal variation in biogeochemical processes (Vitousek et al., 2010). To control for disturbance history effects across this MAT gradient, we used airborne light detection and ranging (LiDAR) measurements of forest structure to select seven sites in the HETF at each of six target elevations, where each site represents the maximum aboveground biomass present at a given elevation. ...
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Canopy water content is a dynamic quantity that depends on the balance between water losses from transpiration and water uptake from the soil. Absorption of short-wave radiation by water is determined by various frequencies that match overtones of fundamental bending and stretching molecular transitions. Leaf water potential and relative water content are important variables for determining water deficit and drought effects; however, these variables may only be indirectly estimated from leaf and canopy spectral reflectance. We review the state of understanding in remote sensing measurements of leaf equivalent water thickness and canopy water content. Indexes using different combinations of spectral bands estimate leaf and canopy water contents, albeit sometimes with large errors caused by differences in canopy structure and soil surface reflectance. Inversion of leaf and canopy radiative transfer models, such as PROSPECT and SAIL, or learning algorithms, like artificial neural networks and genetic algorithms trained on radiative transfer models, are promising methods for creating global datasets of canopy water content.
IntroductionHawaiian Ecosystems and Their VulnerabilitiesTechnologies to Detect Ecosystem - Transforming InvasionsAcknowledgementsReferences
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We tested the Walker and Syers (1976) conceptual model of soil development and its ecological implications by analyzing changes in soil P, vegetation, and other ecosystem properties on a soil chronosequence with six sites ranging in age from 300 yr to 4.1 X 10(6) yr. Climate, dominant vegetation, slope, and parent material of all of the sites were similar. As fractions of total P, the various pools of soil phosphorus behaved very much as predicted by Walker and Syers. HCl-extractable P (presumably primary mineral phosphates) comprised 82% of total P at the 300-yr-old site, and then decreased to 1% at the 20,000-yr-old site. Organic phosphorus increased from the youngest site to a maximum at the 150 000 yr site, and then declined to the 4.1 X 10(6) yr site. Occluded (residual) P increased steadily with soil age. In contrast to the Walker and Syers model, we found the highest total P at the 150 000-yr-old site, rather than at the onset of soil development, and we found that the non-occluded, inorganic P
Detailed GIS studies across spatially complex rangeland landscapes, including the Aspen Parkland of western Canada, require accurate digital elevation models (DEM). Following the interpolation of last return lidar (light detection and ranging) data into a DEM, a series of 256 reference plots, stratified by vegetation type, slope and lidar sensor sampling angle, were surveyed using a total laser station, differential GPS and 27 interconnected benchmarks to assess variation in DEM accuracy. Interpolation using Inverse Distance Weighting IDW resulted in lower mean error than other methods. Across the study area, overall signed error and RMSE were +0.02 m and 0.59 m, respectively. Signed errors indicated elevations were over-estimated in forest but under-estimated within meadow habitats. Increasing slope gradient increased vertical absolute errors and RMSE. In contrast, lidar sampling angle had little impact on measured error. These results have implications for the development and use of high-resolution DEM models derived from lidar data.
We determined the consequences of systematic changes in nutrient availability during long-term soil development by measuring foliar nutrient concentrations. Sun leaves of the dominant tree Metrosideros polymorpha and of eight other species were sampled in Hawaiian rain forests developed on substrates that were 0.3 @? 10^3, 2.1 @? 10^3, 5 @? 10^3, 20 @? 10^3, 150 @? 10^3, 1400 @? 10^3, and 4100 x 10^3 yr old. Elevation, annual precipitation, parent material, and dominant species were nearly constant along this gradient. Foliar N and P concentrations in Metrosideros were lowest in the youngest site (0.72% and 0.052% for N and P, respectively), increased to a maximum on 20 @? 10^3 and 150 @? 10^3-yr-old substrates (1.45% and 0.108%), and then declined close to the initial concentrations in the oldest site (0.86% and 0.061%); N:P ratios in foliage varied relatively little across the sites. Most other species followed a similar pattern of variation. On a per unit leaf area basis, foliar N and P contents in Metrosideros also peaked on intermediate-aged substrates. Foliar nutrient concentrations in Metrosideros sun leaves were determined across a parallel but wetter substrate age gradient. The pattern of variation was similar on both gradients, but the magnitude of variation was smaller on the wetter sequence of sites. Overall, the pattern of variation in foliar nutrients with substrate age is consistent with conceptual models for the dynamics of soil nutrient availability during long-term soil development, and with measurements of soil properties along this sequence.
Patterns of nitrogen trace gas emissions, soil nitrogen flux, and nutrient availability were evaluated at five sites that form a chronosequence in Hawaiian montane rain forest. The estimated age of basaltic parent material from which soils developed at the Kilauea site was 200 yr, 6000 yr at the Puu Makaala site, 185000 yr at the Kohala site, 1.65 x 10⁶ yr at the Molokai site, and 4.5 x 10⁶ yr at the Kauai site. Peak net N mineralization and nitrification values were found in soils from the 185000-yr-old Kohala site. Nitrogen content of foliage and leaf litter was highest in the intermediate age sites (Puu Makaala and Kohala) and N and P retranslocation was lowest at the Puu Makaala site. Soil cores fertilized with nitrogen had significantly higher rates of root ingrowth than control cores at the two youngest sites (200 and 6000 yr old) but not in older sites (185000 and 4.5 x 10⁶-yr-old sites) and total fine root growth into control cores was greatest at the Kohala site. The highest NâO emissions were found at the 185000-yr-old Kohala site, while the highest combined flux of NâO + NO was observed at the 4.5 x 10⁶-yr-old Kauai site. While overall NâO emission rates were correlated with rates of N transformations, soil water content appeared to influence the magnitude of emissions of NâO and the ratios of emissions of NO vs. NâO. NâO emissions occurred when water-filled pore space (WFPS) values were >40%, with highest emissions in at least two sites observed at WFPS values of 75%. Among sites, high NâO emissions were associated with high soil N transformation rates. Large NO fluxes were observed only at the Kauai site when WFPS values were
In Hawaiian montane forests, we assessed whether the same nutrients limit decomposition and aboveground net primary production (ANPP) along a soil chronosequence where nutrients demonstrably and predictably limit ANPP. At three sites that vary in parent material age (300, 20000, and 4.1 x 106 yr), we used fertilization to assess whether nitrogen (N) and/or phosphorus (P) limit decomposition. Reciprocal transplants using litter bags allowed us to distinguish limitation by externally supplied nutrients vs. limitation by nutrients within litter. Nutrient limitation of decomposition was not predictable from nutrient limitation of ANPP, in that elevated litter and soil N had only small, if any, effects on decomposition, even at the young site where N limits ANPP. At the oldest site where P limits ANPP, both elevated litter P and increased availability of soil N and P increased decomposition rates. Thus, nutrients may limit decomposition more strongly in low-P than in low-N ecosystems. Fertilization affected litter nutrient dynamics more strongly than it did decomposition, and we observed uptake of both N and P by decomposers that was not always accompanied by changes in decomposition rates. Such nutrient incorporation into decomposing litter may retain nutrients within ecosystems, even when nutrients do not limit decomposition rates.
The remote sensing of foliar biochemical concentration assumes that leaf biochemical absorption features will be manifest in canopy reflectance. This is a reasonable assumption providing the effect of a given change in foliar biochemical concentration has a similar effect on both leaf and canopy reflectance. A comparison between canopy and leaf reflectance was made to determine if canopy effects (composite of leaf area index, biomass, structure, multiple scattering and shadow) could alter the leaf biochemical information in canopy reflectance spectra. Differences in leaf biochemical concentrations and leaf biomass were induced by the application of fertilisers to large plots of slash pine (Pinus elliottii var elliottii) in Florida, U.S.A. The reflectance of plot canopies was measured using the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). The reflectance of samples of leaves drawn from each plot were measured using a laboratory spectrometer. The differences between airborne and laboratory reflectance ratios (fertilised/control spectra) were used to isolate the effects of the canopy in AVIRIS reflectance spectra. From this study it was concluded that the canopy influenced leaf reflectance substantially at wavelengths beyond the water absorption feature at 1400nrn and leaf biochemical information was transmitted virtually unchanged from the leaf to the canopy in near-infrared wavelengths.