Net Photosynthesis and Growth Rate of Buckthorn Seedlings and Eight
Native Shrubs at Various Light Intensities
Erica Vandergaast, Calvin College
Methods
Both buckthorn species exhibited high
photosynthesis rates in the sun. Native species
capable of competing with buckthorn on the basis
of photosynthesis rate (within one LSD0.05) are
buttonbush and highbush cranberry.
Full Sun
70% Sun
30% Sun
20
2
R = 0.5621
15
Comparison of Photosynthesis in Sun
10
8
CO2 Exchange
7
6
5
3
2
1
Buttonbush
Fox
Wild black cherry
Redbud
Shade
Sun Edge
Hill
North
Dock
Growth
Highbush cranberry
Maple-leafed viburnum
Observed growth rates were unaffected by differences in net
photosynthesis rate. Other factors driving these measures of
growth rate may include initial age of seedlings or the size of
stored energy reserves before the initiation of shade treatments.
Increase in Leaf Number
Height Increase by Treatment
Height change (cm)
Total leaves gained
0.7
4
3.5
3
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
Bladdernut
Sugar Maple
0.6
0.5
0.4
0.3
0.2
0.1
0
Full Sun
m
M
ap
le
le
af
vi
bu
rn
u
sh
le
be
rr
yb
u
M
ap
ga
r
nn
Conclusions
4
0
Running strawberry bush
Common buckthorn
H
ig
hb
us
h
Photosynthesis rate x environment
There are two species of buckthorn in the
Eastern U.S., Rhamnus cathartica
(common), and Rhamnus frangula (glossy).
Both species are aggressive invaders
introduced from Europe. Buckthorn grows
unchecked, primarily along edges or in early
successional fields, where it out-competes
the native plants and creates a monoculture.
Although some may think buckthorn is
simply a lush, pretty shrub, it out-competes
and kills desirable species, decreasing plant
diversity.
rr
y
R
u
3.5
3
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
Re
db
ud
Net photosynthesis rate also varied by site; some sites had more
natural shade cover from the forest canopy.
Why do we want to get rid of
buckthorn? Why do we want to restore
“buckthorn infested areas?”
S
tr
aw
R
Comparison of photosynthesis in shade
30%
cr
an
be
rr
y
R
ca
th
ar
tic
a
Bu
tt
on
W
bu
ild
sh
Bl
ac
k
Ch
er
ry
R
fr
an
gu
la
Bl
ad
de
rn
ut
Ru
Su
nn
ga
in
r
g
M
St
ap
ra
le
w
be
M
rr
ap
yb
le
us
le
h
af
vi
bu
rn
um
50%
g
R
Both buckthorn species also exhibited fast net
photosynthesis rate in shaded environments.
Several native shrubs are better able to compete in
shaded areas, including: highbush cranberry,
buttonbush, bladdernut, wild black cherry.
7
6
5
4
3
2
1
0
70%
in
fr
an
gu
us
h
nb
tt
o
B
u
In general: the greater amount of shade imposed = the slower
rate of net photosynthesis by the buckthorn seedlings.
Photosynthesis rate depends on
treatment
Native Species
0
PAR
•Growth measurements of the seedling under each
treatment
•Measure CO2 exchange rates of both species of
buckthorn as well as various native plants
2
S
u
2000
C
he
1500
d
1000
4
bu
500
6
B
la
ck
0
8
il d
•Measure CO2 gas exchange using a photosynthesis meter
10
R
ed
-5
12
W
0
14
la
ca
H
th
ig
ar
hb
ti
us
ca
h
cr
an
be
rr
y
B
la
dd
er
n
ut
5
CO2 exchange rate
•One small plant was located in each tent, allowing for 24
plants to be evaluated over an eight week period
Linear (Full Sun)
y = 0.0044x + 2.364
Full Sun
Rationale
50% Sun
CO2 Exchange rate
•Compare photosynthesis rates of the two
invasive buckthorn species with those of
eight native shrubs. This information is also
useful for restoration, to demonstrate which
shrub species might be used to replace
buckthorn and to characterize their ability to
compete with buckthorn.
Net photosynthesis rate is directly related with radiation level,
which was determined by the shade treatment imposed.
CO2 Exchange Rate
•Determine the relationship between sun
exposure, photosynthesis rate, and growth of
glossy buckthorn seedlings. Characterization
of buckthorn growth rates as a function of
light intensity will help those involved in
restoration of buckthorn infestations
determine planting density to retard growth
of young buckthorn seedlings after removing
mature growth.
Native Species
CO2 exchange rate
Objectives
•Built and installed shade tents in the field:
•No tent (full sun)
•Tent that allows 70% sunlight
•Tent that allows 50% sunlight
•Tent that allows 30% sunlight
•These tents were each installed in six locations, three
representing high or low sun exposure
Photosynthesis Rate & Light Intensity
70%
50%
30%
Full Sun
70%
50%
30%
•Higher levels of shade decrease the rate
that buckthorn conducts photosynthesis.
Although further study would be needed to
determine if, in time, this lower rate would
slow growth, it is likely that buckthorn
seedlings can be controlled using shade.
•The fast net photosynthesis rates of
buckthorn in sun exposed environments
provides significant evidence of its ability
to thrive and dominate in these situations.
•Several native species have high enough
CO2 exchange rates that they could provide
significant competition to buckthorn. More
options exist in shade than sun. These
species can be used to restore buckthorn
infested areas as they stand a reasonable
chance of suppressing the growth of
buckthorn seedlings.
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