Table Of ContentSelbyana 20(2): 350-356. 1999.
LIGHT DISTRIBUTION AND PHOTOSYNTHESIS OF METROS/DEROS
POLYMORPHA FORESTS AT BOTH ENDS OF A NUTRIENT AND
SUBSTRATE AGE GRADIENT IN HAWAII
S. CORDELL* AND G. GOLDSTEIN
Department of Botany, University of Hawaii, 3190 Maile Way, Honolulu, HI 96822, USA
E-mail: cordell@hawaiLedu
ABSTRACT. Patterns of light distribution, leaf area index (LAI), and canopy photosynthesis were studied
in Metrosideros polymorpha forests at both ends of a nutrient and substrate age gradient in the Hawaiian
Islands. The two sites differ in the age of the underlying volcanic substrate, and in being limited by nitrogen
(N) in the young site (Thurston, Hawaii) and phosphorus (P) in the oldest site (Kokee, Kauai). Long-term
fertilization with nitrogen (N), phosphorous (P) and N + P at both sites has resulted in leaf and canopy level
changes. Light transmission through the canopy was lower in the N-addition plot than in the control plot.
Increased LAI was observed at the N-amended plot, also suggesting greater light attenuation with increased
N availability. While light intensity was not measured in the old site, LAI was much lower in the subcanopy
and near ground levels than in the young site. Furthermore, the effects of nutrient amendments on LAI at
the old site were not as large as those observed in the young site. Responses of net CO2 assimilation (A)
to fertilization were different at the young, and old sites. Instantaneous values of A remained constant across
all plots at the young site, but significantly increased with nutrient amendments at the old site. In contrast,
calculated values of canopy carbon gain (annual carbon gain * LAI) at the young site more than doubled
with fertilization. The increase in canopy carbon gain at the young site was the result of larger leaves rather
than higher photosynthetic rates. Variation on leaf area index and in light distribution was correlated to
physiological and morphological traits of M. polymorpha dominated rain forests.
Key words: nutrient limitation, Metrosideros polymorpha, light environment, fertilization, LAI
INTRODUCTION sure of photosynthetic capacity and total leaf ni
trogen. This is based on the understanding that
Resource limitations can affect growth, repro
the photosynthetic machinery accounts for more
duction, and survival in plants. A resource is
than 50% of the nitrogen in a leaf and that pho
considered to be limiting if the addition of that
tosynthesis is strongly affected by nitrogen
resource will result in increased growth or fit
availability (Field & Mooney 1986). Therefore,
ness. Since properties of individual species can
increased availability of nutrients for plant com
control the functioning of a group of co-occur
ponents coupled with increased photosynthetic
ring group of plants, community and ecosystem
capacity of leaves should result in increased
level characteristics will also be influenced by
plant growth. For example, in young substrate
resource limitation (Vitousek 1982). To better
sites Vitousek et al. (1993) documented an in
understand ecosystem processes, it is necessary
crease in the diameter and height of Metroside
to understand how different resource limitations
interact to affect plant growth and in turn eco ros polymorpha with the addition of nitrogen.
system characteristics. Because the coordinated responses of all pho
Nutrients and light availability are the most tosynthetic processes, any environmental stress
limiting resources in montane wet forests (Field that reduces photosynthesis, such as nutrients,
& Mooney 1986, Chazdon 1988, Reich et al. light, or water, will also inhibit leaf morpholog
1992, Fetcher et al. 1994). Although light satu ical traits and biochemical components (Lamb
ration for a single leaf occurs at a radiation far ers et al. 1998). Plants found in infertile sites
short of full sunlight, the arrangement and size tend to acclimate by producing long lived leaves
of leaves within a canopy considerably alter the that have a high leaf mass density (LMA), low
available light in a vertical distribution. In Ha foliar nitrogen concentration, and reduced pho
waiian forests, nitrogen tends to be limiting in tosynthetic capacity (Reich et al. 1992). High
young soils, while phosphorus is limiting on old LMA, in turn, decreases the amount of leaf area
er soils (Vitousek & Howarth 1991, Smith 1992, available for light interception, thereby reducing
Herbert & Fownes 1995). A number of studies the net relative growth rate of a forest stand.
have reported correlations between some mea-
Recent evidence in Hawaiian ecosystems has
shown strong trends in leaf size of Metrosideros
* Corresponding author. polymorpha with substrate age and thus with in-
350
CORDELL & GOLDSTEIN: LIGHT LEVELS AND CARBON GAIN IN HAWAll 351
creasing N availability (Geeske et al. 1994, Cor- M. polymorpha exists in a number of forms in
dell et al. 1998). eluding the pubescent varieties incana and po-
Because Metrosideros polymorpha is the lymorpha and the glabrous variety glaberrima.
dominant canopy tree species found in Hawaiian The pubescent varieties makes up -95% of the
rain forests it is expected that changes in leaf M. polymorpha at the young site with M. poly
morphology and growth would affect the distri- ..m orpha variety glaberrima making up the bal
bution of light through the canopy. Since light ance. Metrosideros polymorpha variety glaber
is a limiting resource that affects growth, sur- ima is the sole variety of Metrosideros at the old
vival and reproduction, any changes in the qual- site (Dawson & Stemmermann 1990). Parent
ity and quantity of the light regime may have an material is also consistent; the chemistry of the
impact on the ecology and physiology of the volcanic material that makes up Hawaii reflects
canopy and understory species (Chazdon 1988, that of a stationary convective plume or "hot
Fetcher et al. 1994). spot" (Moore & Clague 1992), and is relatively
Here, we investigate how nutrient availability constant on both short and long time scales
affects the vertical distribution of light through (Wright & Helz 1987).
the potential alteration of canopy structure by Complete factorial fertilizations have been
nutrient amendments. We will also assess the carried out in the 300 and 4.1 million-year-old
consequences for the efficiency of interception sites. The main treatments were N (100 kglha
of light and how it translates into the canopy per year, half as urea and half as ammonium
carbon balance. nitrate), P (100 kg/ha per year, triple superphos-
phate), and N plus P (100 kg N/ha per year +
100 kg P/ha per year). The field design consists
MATERIALS AND METHODS
of four blocks of eight 15 X 15 m plots in both
Description of Study Site the young (Thurston, Hawaii, hereafter, young)
and old (Kokee, Kauai, hereafter, old) sites (TA
This study was performed in a very well de BLE 1). We conducted intensive measurements in
fined and constrained age sequence of sites in one plot of each of four treatments at both the
the Hawaiian Islands. The sites differ in age of young and old extremes of the gradient (Control,
the underlying volcanic substrate, and in being N, P and N+P) yielding a total of 8 plots. Treat
demonstrably limited by N in young sites and P ment plots were located in a very homogeneous
in the oldest site (Vitousek et al. 1993, Herbert forest stand with similar soil characteristics and
& Fownes 1995). However, all are at the same slopes. Leaves from the upper canopy were ac
elevation (1200 m), annual precipitation (2500 cessed by scaffolding towers (1.6 m X 3.2 m)
mm), topographic position: and all are dominat built to the height of the canopy (10-20 m).
ed by the same species (Crews et al. 1995). Towers were placed in the center of one plot per
Long-term (8-14 year) ongoing forest fertiliza treatment. At least four canopy trees per plot
tion is in place at both the young and old ex were available from each tower. To address the
tremes of the chronosequence. problem of replication, sun leaves were shot
Hawaii is an ideal place for this research; down with a shotgun from three replicate plots
many of the factors that control plants and eco per treatment (not accessible by towers). Mor
systems can be kept constant, while others vary phological information was obtained from these
in wide but well-defined ways (Britten 1962, Vi leaves (area, weight, and LMA), and average
tousek et aI. 1992). Climate varies widely in Ha values of all four replicate plots per treatment
waii as a function of elevation and exposure to were used for calculations of whole leaf physi
the prevailing northeast trade winds, and cli ological and biochemical measurements. Photo
matic variation generally is predictable, contin synthetic gas exchange measurements were only
uous, and occurs on a very fine scale (Vitousek obtained from plots with towers.
et al. 1992). Plant communities can be held con
stant to a striking extent. Hawaii is the most iso Leaf Area Index
lated archipelago on Earth, and the few natural
colonists that have become established have ra Vertical leaf area index (LAI) was estimated
diated to occupy a broad range of environments nondestructively using a LAI-2000 plant canopy
(Carlquist 1980). The myrtaceous tree Metrosi analyzer (LI-COR Inc., Lincoln, NE). Measure
deros polymorpha Gaud. grows from sea level ments were taken from within the upper canopy
to 2500 m, from among the first woody colonists (13.5 m from ground), mid-canopy (6.5 m from
on young lava flows to the oldest substrates in ground), and below canopy (1.5 m from ground)
the islands, and from <400 mm annual rainfall, at the young site and from 10, 5, and 1.5 m
to the wettest place on earth (Dawson & Stem respectively from the old site. Time of day and
mermann 1990). As the species name implies, orientation were held constant for each plot dur-
352 SELBYANA Volume 20(2) 1999
TABLE 1. Characteristics of study sites. Both sites are comprised of control plots, (C) (no added fertilizer) and
plots which received additions of nitrogen (N), phosphorus (P), and a combined treatment containing nitro
gen and phosphorus (N + P) (adapted from Vitousek, 1998).
Site
Characteristic Thurston (young) Kauai (old)
Volcano Kilauea Kauai
Substrate age (years) 300 4,100,000
Annual precipitation (mm) 2500 2500
Elevation (m) ~1200 ~1200
Mean annual temperature caC) ~16 ~16
Treatments used in study C,N,p,N+P C,N,p,N+P
Date fertilization initiated 1985 1991
Limiting nutrients N P
Reference Vitousek et al. 1993 Herbert and Fownes 1995
ing the study. Mid-canopy measurements were Metrosideros polymorpha is below 600 /-Lmol
taken above a distinctive sub-canopy of tree m-2 S-I (Kitayama et al. 1997). Four fully ex
ferns (Cibotium spp.). panded sun leaves (6-9 months old) were mea
sured on each of four trees per treatment. Gas
Light Distribution exchange was carried out between 0900 and
1500 hr to insure maximum rates. Measurements
For the purpose of this study, we constructed were repeated over a three-day period and dur
25 light sensors made of a light sensitive diode ing two different months of the year to account
connected in parallel to a precision resistor. The for seasonal variation (July and January). Values
sensor was connected to a coaxial cable and of annual carbon gain were obtained by aver
sealed in waterproof plastic shrink-wrap tubing aging values of net assimilation of fully expand
to protect the wire cables. The voltage differ ed sun leaves, and oldest non-necrotic leaves
ential measured by the sensors were converted from the same branch leaves then scaling from
into photosynthetic photon flux density (/-Lmol /-Lmol m-2 S-I to mol m-2 yr-I. Canopy carbon
m-2 S-I) by calibrating the voltage difference gain is annual carbon gain * LAI (Kuppers
against a quantum sensor (LI-COR Inc.) at dif 1994). Both estimates of carbon gain assume
ferent light intensities. that all leaves receive the same amount of light,
Light sensors were placed in the N fertilized or are under light saturated conditions.
and control plots ant the young site in the fol
lowing vertical gradient: 1) Within canopy-
RESULTS
13.5 m from the ground, 2) above the sub-can
opy tree fern (Cibotium spp.) layer-6.5 m from
Light environment. Light penetration
the ground, and 3) below the tree fern canopy-
through the canopy was significantly lowered as
1.5 m from the ground. Sensors were attached
a result of nitrogen fertilization. However, max
to 1.6 m long PVC tubing and extended 1.3 m
imum values of photosynthetic photon flux den
from canopy towers. Three diodes per height
sity (PPFD) are similar between the control and
were attached to quantify the light from different
aspects. In addition, a sensor was placed above nitrogen addition plots (FIGURE 1). This was es
pecially evident in the mid-canopy above the
the canopy (15 m from the ground). The diode
tree fern layer.
sensors were attached at ground level to a data
logger (CR-lO, Campbell Scientific, Logan, LA!. Lai increased from above canopy to
UT), which recorded an average light measure the forest floor in all plots (FIGURE 2). In the old
ment per minute. site, LAI was significantly higher in the N + P
addition plot than the control plot (FIGURE 2b).
Photosynthetic Gas Exchange This was especially evident in the mid-canopy
height. In the young, young site measurements
Net CO2 assimilation was determined with a of LAI were significantly higher in the N
portable photosynthesis system (LI-6200; LI amended site than the control taken from both
COR Inc.). A portable light source was used to the mid-canopy and 1.5m from ground positions
insure constant irradiance (1600 /--Lmol m-2 S··I) (FIGURE 2a). Mean values in the N amended plot
during measurements (Qbeam 6200; Quantum were 2.5 versus 0.9 in the control plot at the
Devices Inc., WI). The light saturation point of mid-canopy level, and at the 1.5 m from ground
CORDELL & GOLDSTEIN: LIGHT LEVELS AND CARBON GAIN IN HAW All 353
800
600
400
200
~fh
E
"0
E
.6 800
~
'iii
5i 600
"
x
ci? 400
c
~
£. 200
o
l$
:5 O~~~~~~--~~--~~~~~~
f
800 - control plot
- -nitrogen plot
~ 600
400
Time (hrs)
FIGURE 1. Diurnal variation in photosynthetic photon flux density (PPFD) in a Metrosideros polymorpha
dominated rain forest at: a. canopy level (13.5 m); b, mid-canopy (6.5 m); and c, below the tree fern canopy
(1.5 m).
level, 5.0 and 2.5 respectively (FIGURE 2a). Be DISCUSSION
cause of a very obvious N fertilizer effect on the
sub-canopy tree fern layer (pers. obs.), compar The results of this study suggest that there are
striking contrasts between the consequences of
isons of LAI of Metrosideros polymorpha ap
N additions in young ecosystems and those of P
pear to be most valid at the mid-canopy level
additions in old systems. Our results show lower
(above tree fern layer).
light penetration through the canopy in the N
addition plot than the control at the young site.
Carbon assimilation. Net CO assimilation
2 Increased values of LA! were measured in the
(A) of Metrosideros polymorpha leaves was dif
N-amended plot, also indicating greater light at
ferent for the young, young site, and the older,
tenuation with increased N availability. How
old site, and was differentially affected by fer
ever, in terms of peak values of photosynthetic
tilization at both sites (TABLE 2). At the young
active radiation (PPFD), leaves at the bottom of
site instantaneous values of A and calculated
the trees experienced irradiances similar to those
values of annual carbon gain were essentially measured in the mid-canopy level, and are sim
constant, regardless of nutrient additions, where ilar between the control and nitrogen addition
as at the old site instantaneous values of A and plots. This can be attributed to the frequent oc
calculated values of annual carbon gain in currence of sunflecks through the canopy. Al
creased significantly in the N, P and N+P ad though these sunflecks may occur very briefly,
dition plots. However, calculated values of can their contribution to the total quantum flux can
opy carbon gain show a more than two-fold in be substantial, providing up to 85% of the total
crease in both the young and old sites in the daily photons received (Pearcy 1987, Chazdon
fertilized versus, non-fertilized plots (TABLE 2). 1988). Metrosideros polymorpha forests do not
354 SELBYANA Volume 20(2) 1999
showed a pronounced decrease in light levels
from the canopy to the forest floor. As men
tioned above, peak values of PPFD were consis
tently higher than 300 !-tmol m-2 S-I between
0900 and 1230 in the mid-canopy, and 1000 and
1200 hours in the low canopy. Because the light
saturation point of M. polymorpha is low (250
!-tmol m-2 S-I; Kitayama et al. 1997), understory
M. polymorpha leaves experience light satura
tion for considerable periods throughout the
..... control morning when photosynthesis is most active .
-0-N This is contrary to most studies of understory
~p
-<:J-NP plants which show that carbon assimilation is
strongly dependent on sunflecks, which may ac
count for less that 20 minutes of adequate pho
tosynthetic light per day, with the remainder of
available light being diffuse (10-20 !-tmol m-2
s-I)(Pearcy 1987). While not conclusive, this
data suggests that the low species diversity
among canopy and understory species relative to
10 12
other tropical systems and the effects of canopy
Height from ground (m) structure and distribution, leads to less light in
FIGURE 2. Leaf area index (LAI) at three different terception in Hawaiian rain forests.
heights within Metrosideros polymorpha dominated While the light environment was not studied
forests. a. young site. b. old site. Each point is a mean in the old site, values of LA! were much lower
of nine measurements from each treatment plot + SE.
in the mid-canopy and near ground levels than
values of LA! from the young site. Furthennore,
the effect of nutrient amendments at the old site
was not nearly as substantial as shown in the
typically fonn a closed canopy, and the crowns
of M. polymorpha are not evenly or uniformly young site.
shaped (pers. obs.). It has been speculated that Decreased light attenuation appears to be cou
light availability is higher in Hawaiian forests pled with increased nutrient availability at the
than those in similar mainland tropical habitats young site. While the effect was most obvious
(Pearcy 1983, Chazdon & Fetcher 1984, Chaz in the N-amended plot, significant differences of
don et al. 1996, Pattison et al. 1998), and that increased LA! values associated with P, and
because of the low diversity and disharmonic N + P additions at the same site were observed.
flora in the Hawaiian Islands, there is less selec This result is especially interesting considering
tive pressure to compete for light resources (Pat that foliar N concentrations in the young site do
tison et al. 1998). Results from this study not show a substantial increase upon removal of
TABLE 2. Photosynthetic characteristics, annual carbon gain, and canopy parameters of Hawaiian Metrosideros
polymorpha from fertilized and unfertilized treatment plots at the young and old sites. N, nitrogen; P,
phosphorus. Net CO assimilation is the weighted average value of maximum rates of fully expanded and
2
the oldest non-necrotic leaves on the same branch. Estimates of canopy carbon balance = Annual carbon
gain * leaf area index (LAI) (Kuppers 1994). This estimate assumes each leaf is nnder light saturated
conditions.
Annual
Net assimilation carbon gain Canopy carbon
Site Treatment (I1mol m-2 S-I) (mol m-2 yel) LAI (mol m-2 yel)
Young Control 6.5 20.4 0.96 19.6
(300 years) N 6.4 20.1 2.8 55.5
P 6.1 19.1 2.5 47.6
N+P 6.1 19.3 2.9 56.1
Old Control 3.6 11.3 1.2 13.7
(4.1 million years) N 6.2 19.5 1.4 27.4
P 5.9 18.7 1.3 24.8
N+P 5.3 16.7 2.2 36.7
CORDELL & GOLDSTEIN: LIGHT LEVELS AND CARBON GAIN IN HAWA Il 355
the limitation. While foliar N concentrations do it appears that increased nutrient availability at
not respond to N additions at the young site, the older old site led to an increase in the pho
primary productivity has increased as a result of tosynthetic rate per unit leaf surface area, with
fertilization (Vitousek et al. 1993). no concomitant change in leaf size.
Analysis of canopy carbon gain from this In summary, these data point to a correlation
study provides further evidence of increased between physiological and morphological traits
productivity associated with nutrient amend and enviromnental conditions. Furthermore, leaf
ments. While instantaneous rates of A remain level responses associated with increases in pri
essentially constant despite fertilization at the mary productivity suggest that differential allo
young site, canopy carbon gain increases by cation processes are driven by morphological or
more than 50% upon fertilization. This suggests biochemical acclimation responses. These data
that increased productivity directly correlates demonstrate how variation in leaf area index and
with increased leaf size upon fertilization. At the light distribution correlates with physiological
young site leaf size almost doubled with N ad and morphological traits of Metrosideros poly
ditions [4.8 cm2 (control) to 7.4 cm2 (N-addi morpha in Hawaiian rain forests.
tion)] (results not shown). Larger leaf size in
creases surface area, which directly increases
ACKNOWLEDGMENTS
light interception and CO capture. Rates of net
2
CO assimilation on a whole leaf basis are in The authors wish to acknowledge Heather
2
creased, and the overall effect is an increase in Jeppesen, Ryan Okano, and Randy Senock for
total biomass. Interestingly, additions of P, and their invaluable technical support, and tireless
N + P also reveal substantial increases in canopy assistance with tower building and data collec
carbon gain, despite P being a non-limiting nu tion, and Debbie Carino, Randy Amiscaray, and
trient at the young site. However, leaf size also Stephanie Loo for initiating, constructing, and
significantly increased upon addition of P at this monitoring the light sensor project. This re
site, again directly correlating with increased search was supported by a USDA grant
productivity and decreased light penetration. (#941156365) to P. Vitousek and Guillermo
The influence of crown structure on the vari Goldstein, and a NSF Doctoral Dissertation Im
ability in measured light conditions supports the provement Grant.
conclusion of Chazdon et al. (1988), where Pip
er spp. from different environments were com
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