Evaluating changes in switchgrass physiology, biomass,and light-use efficiency under artificial shade to estimateyields if intercropped with Pinus taeda L.

Janine M. Albaugh • Timothy J. Albaugh •

Ryan R. Heiderman • Zakiya Leggett • Jose L. Stape •

Kyle King • Katherine P. O’Neill • John S. King

Received: 16 July 2013 / Accepted: 8 May 2014 / Published online: 22 May 2014

� Springer Science+Business Media Dordrecht 2014

Abstract There is growing interest in using switch-

grass (Panicum virgatum L.) as a biofuel intercrop in

forestry systems. However, there are limited data on the

longevity of intercropped bioenergy crops, particularly

with respect to light availability as the overstory tree

canopy matures. Therefore, we conducted a greenhouse

study to determine the effects of shading on switchgrass

growth. Four treatments, each with different photosyn-

thetically active radiation (PAR) levels, were investi-

gated inside the greenhouse: control (no shade cloth,

49 % of full sunlight), low (under 36 % shade cloth),

medium (under 52 % shade cloth), and heavy shade

(under 78 % shade cloth). We determined the effect of

shading from March to October 2011 on individually

potted, multi-tillered switchgrass transplants cut to a

stubble height of 10 cm. In the greenhouse, there was a

reduction in tiller number, tiller height, gas exchange

rates (photosynthesis and stomatal conductance), leaf

area, above- and belowground biomass and light-use

efficiency with increasing shade. Total (above- and

belowground) biomass in the control measured

374 ± 22 compared to 9 ± 2 g pot-1 under heavy

shade (11 % of full sunlight). Corresponding light-use

efficiencies were 3.7 ± 0.2 and 1.4 ± 0.2 g MJ-1,

respectively. We also compared PAR levels and asso-

ciated aboveground switchgrass biomass from inside

the greenhouse to PAR levels in the inter-row regions of

a range of loblolly pine (Pinus taeda L.) stands from

across the southeastern United States (U.S.) to estimate

when light may limit the growth of intercropped species

under field conditions. Results from the light environ-

ment of loblolly pine plantations in the field suggest that

switchgrass biomass will be significantly reduced at a

loblolly pine leaf area index between 1.95 and 2.25,

which occurs on average between ages 6 and 8 years

across the U.S. Southeast in intensively managed pine

plantations. These leaf area indices correspond to a

60–65 % reduction in PAR from open sky.

Keywords Panicum virgatum � Bioenergy �Intercropping � Loblolly pine � Light-use

efficiency � Shading

J. M. Albaugh (&) � T. J. Albaugh � J. L. Stape �J. S. King

Department of Forestry and Environmental Resources,

North Carolina State University, Box 8008, Raleigh,

NC 27695, USA

e-mail: Janine.Albaugh@gmail.com

T. J. Albaugh

Department of Forestry, Virginia Polytechnic Institute

and State University, 228 Cheatham Hall, Blacksburg,

VA 24061, USA

R. R. Heiderman

F&W Forestry Services, P.O. Box 3610, Albany,

GA 31706, USA

Z. Leggett

Weyerhaeuser Company, 1785 Weyerhaeuser Rd,

Vanceboro, NC 28586, USA

K. King � K. P. O’Neill

Environmental Studies Program, Roanoke College,

Salem, VA 24153, USA

123

Agroforest Syst (2014) 88:489–503

DOI 10.1007/s10457-014-9708-3

Introduction

There is renewed interest in growing perennial

grasses, such as switchgrass (Panicum virgatum L.),

as dedicated bioenergy feedstocks (Heaton et al. 2004;

Vogel 2004; Albaugh et al. 2012; Blazier et al. 2012).

Perennial grasses are strong biofuel candidates due to

their ability to produce large biomass yields with

relatively few inputs compared to annual crops such as

corn and soybeans. Switchgrass, a perennial warm-

season grass, was chosen by the U.S. Department of

Energy as the model herbaceous species for develop-

ment as a biomass feedstock (Sanderson et al. 1996;

Fuentes and Taliaferro 2002). This species is native to

North America and its favorable characteristics for

bioenergy production have been extensively docu-

mented. These attributes include its perennial nature,

diverse geographic range (Downing et al. 1996), high

biomass potential (Wright 1994; Downing et al. 1996;

Tolbert and Schiller 1996; McLaughlin et al. 1999),

low nutrient demand (Downing et al. 1996), and

environmental benefits such as mitigation of green-

house gas emissions by below-ground carbon seques-

tration (Zan et al. 2001; Sanderson et al. 1996; Vogel

2004; McLaughlin and Kszos 2005).

There is economic risk associated with converting

land to growing a crop dedicated to the relatively new

biofuel market (Blazier 2014). To avoid these

potential risks and to increase yields with optimum

use of available growing space, intercropped manage-

ment systems can be used to integrate traditional forest

production and perennial grasses (Albaugh et al. 2012;

Blazier et al. 2012). In these types of intercropped

agroforestry systems, switchgrass is grown as a biofuel

feedstock between rows of loblolly pine (Pinus taeda

L.). As intercropping allows for annual economic

returns from the grass early in the rotation while

maintaining long-term returns from traditional wood

products, this kind of agroforestry system can con-

tribute to crop diversity and economic viability of

growers (Sanderson et al. 1996; Tolbert and Schiller

1996). Furthermore, intercropping switchgrass on

forest land minimises the ‘food versus fuel’ debate

that has affected other bioenergy ventures such as corn

ethanol (Heaton et al. 2008; Dale et al. 2010).

Although switchgrass biology has been well char-

acterised (e.g. Sanderson 1992; Sanderson and Reed

2000; Parrish and Fike 2005), potential competition

from pine trees in an intercropped agroforestry system

has not been quantified. Early in the rotation, prior to

canopy closure, growing conditions in the inter-row

area of planted pines will be favorable for grass

growth. However, as trees grow taller and crowns

become fuller with increased leaf area, more light will

be intercepted by the pines. Under favorable condi-

tions, biomass production is proportional to the

amount of absorbed radiation (Monteith 1981), a

concept known as light-use efficiency. As increased

shading by pine trees may reduce switchgrass growth,

intercropping may be limited to early in the tree

rotation, the timing of which still needs to be

quantified. Trees planted at wide initial spacing may

allow for long-term intercropping.

The amount of incident light intercepted by a forest

canopy is determined by the density and size of the

tree crowns, i.e. leaf area index (LAI) and canopy

architecture, which is defined by the vertical and

horizontal distribution of foliage and branch mass

(Sampson and Allen 1998; Li et al. 2008). The rate at

which trees shade and impact understory yield

depends on tree species and site productivity, width

of tree rows, row orientation (Burner and Brauer 2003)

and management operations such as fertilisation,

pruning and thinning. In addition to young stands

and stands with wider initial tree row spacing, light

levels in the inter-row area may be sufficient for grass

growth later in the rotation, after heavy pruning or

thinning operations. Studies have been conducted in

the southeastern (se) U.S. on the effect of intercrop-

ping pine trees and grasses, grown mainly for forage

(Clason 1999; Pitman 2000; Burner and Brauer 2003;

Albaugh et al. 2012; Blazier et al. 2012). However,

there has been minimal research investigating specific

effects of light on switchgrass grown as a biofuel

intercrop. As it is critical to understand light avail-

ability and its effect on the longevity of intercropping,

we conducted a greenhouse study to determine the

effects of shading on switchgrass growth. Specifically,

we quantified the effect of varying light levels on

switchgrass growth, physiology, above- and below-

ground biomass production, and light-use efficiency.

We compared light intensities inside the greenhouse to

light levels in the inter-row regions of various aged

loblolly pine stands with different stocking densities to

estimate when light levels may limit the growth of

intercropped species.

490 Agroforest Syst (2014) 88:489–503

123

Materials and methods

Greenhouse experiment

The greenhouse study was conducted at North Caro-

lina State University in Raleigh, NC, USA

(35�4701300N, 78�4104200W) from day of year (DOY)

69 (10 March) to DOY 292 (19 October) 2011. Light

levels inside the greenhouse were varied using shade

cloth constructed of high-density polypropylene

(Greenhouse Megastore, Danville, IL, USA), which

allowed transmittance of varying amounts of photo-

synthetically active radiation (PAR), ranging from 22

to 64 %. Shade structures measuring 1.1 m

(length) 9 1.2 m (width) 9 3.1 m (height) were con-

structed with 1.3 cm diameter PVC pipe and covered

with shade cloth. Three different levels of shade cloth

density, i.e., 36, 52 and 78 %, were used to create low,

medium and heavily shaded environments, respec-

tively. A no-shade cloth (control) treatment did not

have any shade cloth over the switchgrass plants and

allowed full light penetrating the greenhouse roof to

reach the top of the plant canopy. Within each shade

cloth chamber and in the control, photosynthetic light

(PAR) Smart Sensors connected to Micro Station data

loggers (HOBO, Onset Computer Corporation, Pocas-

set, MA, USA) collected PAR data every minute and

logged these as hourly PAR averages (lmol pho-

tons m-2 s-1). These PAR averages were summed on

a daily basis and converted to MJ (1 J of PAR contains

4.6 lmol, Landsberg 1986). As the greenhouse glass

and frame blocked approximately 50 % of incoming

PAR, control plants received 49 % of full sunlight,

and the low, medium, and heavily shaded treatments

received 31, 23 and 11 % of full sunlight, respectively.

Four-month-old switchgrass (cultivar Alamo) seed-

lings were planted into 0.02 m3 black plastic pots (each

pot measured 30 cm diameter and 28 cm height,

covering an area of 0.071 m2, with one seedling/pot)

filled with an artificial potting mix containing Canadian

sphagnum peat moss (50 %), processed pine bark,

perlite, vermiculite, starter nutrients, wetting agent and

dolomitic limestone (Fafard 3B, Conrad Fafard Inc.,

Agawam, MA, USA) in January 2011. The experiment

was laid out as a randomised complete block design with

three blocks: six switchgrass pots were subjected to each

light level, with each treatment replicated three times

(total greenhouse N = 72). Greenhouse temperature

and humidity were not controlled. Six additional

switchgrass pots were placed outside the greenhouse

on DOY 69 (10 March) for the duration of the

experiment. These pots did not receive artificial shading

and provided an estimate of switchgrass biomass that

would be obtained when grown in full sun. The only data

collected on these plants were above- and belowground

biomass. Meteorological data (hourly average air tem-

perature and PAR) for conditions outside the green-

house were obtained from the Reedy Creek weather

station located at 35�4802600N, 78�4403900W, which was

4.9 km from the greenhouse, with a 2 m altitude

difference.

Switchgrass plants were cut to a height of 10 cm

prior to treatment initiation on DOY 69. This initial

aboveground biomass was 0.90 ± 0.04 g pot-1 and

was used for blocking purposes to ensure homogeneity

of experimental units. A height of 10 cm was chosen

to be consistent with field practices (Albaugh et al.

2012). Initial tiller number was 13 ± 4 tillers pot-1 (a

tiller is an individual grass shoot comprised of a

meristem, leaves, stem, roots and latent buds).

Switchgrass plants were watered daily to approximate

pot water-holding capacity. Individual pots were

treated with a surface application of 65 g of 15-9-12

(N P2O5 K2O) Osmocote Plus fertiliser on DOY 84 (25

March) and 241 (29 August) to prevent nutrient

limitation during the study period.

Switchgrass growth and gas exchange measurements

Switchgrass tiller height (cm) was measured monthly

from March to October as the height from the soil to

the collar of the most recent fully expanded leaf blade

per pot. Sanderson (1992) refers to the most recent

fully expanded leaf blade as one where the collar is

fully visible. Tiller height was measured on DOY 111,

139, 165, 199, 240, 258, and 291.

Leaf-level photosynthesis (A, lmol m-2 s-1) and

stomatal conductance to water vapor (gs, mmol m-2 s-1)

were measured monthly on the youngest fully expanded

leaf blade of the tallest tiller per pot with an open portable

photosynthesis system (LI-6400, LI-COR, Inc., Lincoln,

NE, USA) equipped with a 6 cm2 cuvette. Measurement

dates were DOY 110, 143, 166, 199, 235, 258, and 291.

Gas exchange was measured between 10.00 and 14.00 h

to obtain maximum photosynthesis and stomatal con-

ductance rates. Reference CO2 concentration was held

constant at 380 ppm, and temperature and humidity were

maintained at ambient conditions. Photosynthetic photon

Agroforest Syst (2014) 88:489–503 491

123

flux density (PPFD, lmol m-2 s-1) values were set for

each measurement date, calculated as follows: as gas

exchange rates were measured between 10.00 and

14.00 h, we determined the average PAR over these four

hours for each DOY, and then used the average of the

daily PAR values from DOY 69 to DOY 109 as the PPFD

for DOY 110, and the average of the daily PAR values

from DOY 110 to DOY 142 as the PPFD for DOY 143,

etc.

Biomass harvest

The experiment was terminated on DOY 292 (19

October) 2011. Prior to harvesting the switchgrass,

tiller number was quantified, and one representative

leaf was selected from each of three pots per treatment

for the determination of specific leaf area (m2 kg-1),

used for calculating leaf area (m2) per pot. Fresh leaf

area of the selected samples was obtained by scanning

with an Epson scanner (Epson America, Inc., Long

Beach, CA, USA), and the subsequent digital image

was analysed with ImageJ software (http://rsb.info.

nih.gov/ij/docs/). These samples were dried at 65 �C

to a constant mass and specific leaf area was calculated

as fresh leaf area: dry mass. Aboveground material

was harvested at a height of 10 cm. Belowground

material was processed by cutting the remaining

aboveground material at the soil surface and passing

all belowground matter through a 5 mm sieve.

Remaining aboveground material, i.e. between the soil

surface and 10 cm height was not included in any

analyses to be consistent with field practices (Albaugh

et al. 2012). Above- and belowground components

were oven-dried at 65 �C to a constant mass.

Leaf area index and light-use efficiency

Daily leaf area index (LAI, the ratio of switchgrass

leaf area to pot area) was calculated based on the

developmental pattern reported by Albaugh et al.

(2012) for pure switchgrass grown on the Lower

Coastal Plain of North Carolina, USA. Absorbed PAR

(APAR, MJ m-2 day-1) was calculated using the

Beer–Lambert equation (1) as a function of incident

PAR, LAI and the light extinction coefficient (k):

APAR ¼ PAR 1� exp �k � LAIð Þð Þ ð1ÞA value of 0.33 was used for k as measured by Kiniry

et al. (1999) for switchgrass. Daily values of APAR were

summed over the duration of the experiment and light-

use efficiency was calculated as the cumulative above-

ground dry mass per unit of absorbed PAR.

Field PAR measurements

Field PAR data were collected to make inferences

regarding the stage of pine stand development where

light levels may limit growth of intercropped species.

These PAR data were collected from 44 loblolly pine

stands varying in age (clear cut to 37 years), height (m),

stocking density (trees ha-1), basal area (m2 ha-1),

resource availability (through fertilisation), manage-

ment operations (e.g. thinning), and row orientation

from locations in North and South Carolina, and

Georgia, USA (see Table 8 in Appendix). As peak LAI

in loblolly pine occurs in September (Blinn et al. 2012),

PAR was measured in September 2011 and 2012 on

clear days between 10.00 and 14.00 h with an Accu-

PAR LP-80 ceptometer (Decagon Devices, Inc., Pull-

man, WA, USA). Measurements of incoming (above

canopy) PAR were collected in a clearing, and below

canopy measurements were taken in the area between

pine rows (values were averaged from each of the four

cardinal directions). For those sites where it was not

possible to measure incoming PAR in a clearing, PAR

was calculated as 0.5 9 solar radiation (see Landsberg

1986). Solar radiation was calculated from latitude,

longitude, site elevation, daily precipitation and daily

minimum and maximum air temperature (�C; Spokas

and Forcella 2006). Below canopy PAR was summed

over days 69–292 (to correspond to the greenhouse

experimental period) and used to predict potential

switchgrass biomass production (see non-linear mod-

eling in ‘‘Statistical analyses’’ section). We presented

switchgrass biomass as function of peak pine LAI,

which was calculated for each pine stand using the

Beer–Lambert equation:

Peak LAI ¼ � ln � Below canopy PAR

Above canopy PAR

�k ð2Þ

A value of 0.69 was used for the light extinction

coefficient, k, recommended by Sampson and Allen

(1998) for loblolly pine.

Statistical analyses

Repeated measures analyses were performed on tiller

height and leaf-level gas exchange rates using a mixed

492 Agroforest Syst (2014) 88:489–503

123

model analysis of variance (Proc. Mixed; SAS 2000)

to examine each metric over the growing season using

a heterogeneous autoregressive model of the variance/

covariance matrix structure. Treatment, day of year

and treatment 9 day of year interactions were fixed

effects and block was a random effect. An analysis of

variance (ANOVA) using Proc. GLM (SAS 2000) was

performed to test for treatment effects on the final tiller

number, SLA, leaf area per pot, light-use efficiency

and above- and belowground biomass harvested at the

end of the experiment in October 2011. Student’s t

tests (Proc. TTEST; SAS 2000) were conducted to

compare above- and belowground biomass of switch-

grass grown outside the greenhouse in full sun with

each light level inside the greenhouse. Non-linear

models (Proc. NLIN; SAS 2000) were used to fit a

standardised logistic growth curve (3) to the biomass

data to predict switchgrass biomass as a function of

PAR. This growth curve was standardised with

maximum biomass (parameter c, the asymptote) set

to 1. The model form used was:

Biomass ¼ c

1þ exp �a � x�bð Þð Þð Þ ð3Þ

where a = daily rate of biomass accumulation,

b = inflection point, c = asymptote and x = PAR.

Statistical analyses were based on mean values

calculated for each shade structure. In all cases, an

alpha = 0.05 significance level was used. When a

significant difference was observed using the mixed

model ANOVA, least square means were compared

using the Tukey–Kramer adjustment method for

multiple comparisons between treatments. Dependent

variables were checked for normality and homosce-

dasticity and transformed as necessary. Leaf area data

and biomass data analysed using Proc. GLM and Proc.

TTEST were Loge transformed; tiller number data

were square root transformed. All means and standard

errors are presented as untransformed values.

Results

Temperature and photosynthetically active

radiation

Average daily greenhouse temperatures ranged from

12 �C in March to 38 �C in July; corresponding

outdoor temperatures were 3 and 32 �C, respectively.

Shade cloth structures in the greenhouse created a

PAR gradient with mean daily values ranging from 0.8

to 3.7 MJ m-2 day-1 with corresponding total PAR

values of 180 and 826 MJ m-2 over the experimental

period (Table 1). Switchgrass growing outside the

greenhouse received 7.6 MJ PAR m-2 day-1 and

1,699 MJ m-2 total PAR. The greenhouse roof

reduced incident PAR inside the greenhouse by

51 % (Table 1). Shade treatments inside the green-

house reduced PAR by 36 % (low shade), 52 %

(medium shade) and 78 % (high shade) compared to

the control.

Height growth

Switchgrass height increased from a stubble height of

10 cm on DOY 69 to 58 ± 12 cm under 78 % shade

cloth, 104 ± 13 cm under 52 % shade cloth, and

127 ± 3 cm under no shade cloth (control) and 36 %

shade cloth on DOY 291 in October (Fig. 1). There

were significant treatment, DOY and treatment by

DOY interactions (Table 2). There were no height

differences between the control and low-shade treat-

ment throughout the experiment (Table 3). In May and

June (DOY 139 and 165), height differences were

observed between the control and medium shade

treatment, but these differences were not maintained.

In contrast, height differences occurred between the

control and heavily shaded plants from April (DOY

111) until October (DOY 291).

Table 1 Mean (±SE) daily and cumulative photosynthetically

active radiation (PAR, MJ m-2) measured over the experi-

mental period from March to October 2011 (day of year

69–292) for four light levels in the greenhouse (GH) and one

outdoor treatment in a switchgrass shade experiment in North

Carolina

Treatment Mean daily PAR

(MJ m-2 day-1)

Total PAR

(MJ m-2)

Control: GH with no

shade cloth

3.7 (0.1) 826

Low shade: GH and

36 % shade cloth

2.3 (0.1) 525

Medium shade: GH and

52 % shade cloth

1.8 (0.0) 396

High shade: GH and

78 % shade cloth

0.8 (0.0) 180

Outdoor, no shade 7.6 (0.2) 1,699

Agroforest Syst (2014) 88:489–503 493

123

Leaf-level gas exchange rates

There were significant treatment effects on leaf-level

gas exchange rates, a significant DOY effect for

stomatal conductance, and a significant treatment by

DOY effect for photosynthesis (Tables 2, 3, 4). Max-

imum photosynthesis and stomatal conductance rates

were measured in April (DOY 110) in the control, with

values of 24.8 lmol m-2 s-1 and 164 mmol m-2 s-1,

respectively (Fig. 2). Minimum photosynthesis values

(4.1 lmol m-2 s-1) were measured in July (DOY 199)

in heavily shaded switchgrass, and a minimum stomatal

conductance of 41 mmol m-2 s-1 was recorded in the

medium shaded treatment in October (DOY 291).

Photosynthetic rates measured in the control differed

significantly from the other shaded treatments in April

and June (DOY 110 and 166, respectively), and were

significantly higher than heavily shaded switchgrass in

May (DOY 143) (18.7 vs 5.7 lmol m-2 s-1, respec-

tively, Table 3).

Fig. 1 Switchgrass height growth from 10 cm stubble, during

2011, expressed as day of year (DOY) in a greenhouse without

shade cloth (control) and under 36, 52 and 78 % shade cloth in a

shading experiment in North Carolina, USA. Values presented

are means; error bars indicate the standard error of the mean

Table 2 Mixed model analysis of variance results (P [ F) for

switchgrass height, leaf-level photosynthesis (A) and stomatal

conductance (gs) subjected to four light levels in a greenhouse

experiment in North Carolina

Height A gs

Treatment \0.001 \0.001 \0.001

Day of year (DOY) \0.001 0.068 \0.001

Treatment 9 DOY \0.001 0.035 0.063

Table 3 Differences of the least square means using the Tu-

key–Kramer adjustment method to compute P values (Adj. P)

for comparisons between control (None) and low, medium and

heavily shaded treatments (36, 52 and 78 % shade, respec-

tively) for greenhouse-grown switchgrass tiller height from

April (day of year (DOY) 111 to October (DOY 291), and leaf-

level photosynthesis (A) from DOY 110 to DOY 291, 2011

Shade cloth treatment

comparisons

Tiller height A

DOY Adj. P DOY Adj. P

None 36 % 111 1.000 110 0.029

None 52 % 111 0.546 110 \0.001

None 78 % 111 0.007 110 \0.001

None 36 % 139 0.742 143 0.218

None 52 % 139 0.023 143 0.053

None 78 % 139 \0.001 143 0.001

None 36 % 165 0.538 166 0.003

None 52 % 165 0.007 166 \0.001

None 78 % 165 \0.001 166 \0.001

None 36 % 199 0.998 199 0.999

None 52 % 199 0.881 199 0.993

None 78 % 199 0.023 199 0.104

None 36 % 240 1.000 235 0.997

None 52 % 240 0.487 235 1.000

None 78 % 240 0.002 235 0.370

None 36 % 258 1.000 258 0.586

None 52 % 258 0.981 258 0.965

None 78 % 258 0.010 258 0.094

None 36 % 291 1.000 291 0.253

None 52 % 291 0.990 291 0.092

None 78 % 291 0.016 291 0.106

Results presented for repeated measures switchgrass responses

to different light levels were restricted to comparisons between

plants that were not shaded and those subjected to varying light

intensities inside the greenhouse

Table 4 Differences of the least square means using the

Tukey–Kramer adjustment method to compute P-values (Adj.

P) for pairwise comparisons between control (no shade cloth)

and low, medium and heavily shaded treatments for green-

house-grown switchgrass stomatal conductance measured in a

greenhouse experiment in North Carolina

Shade treatment comparison Adj. P

Control Low shade (36 % shade cloth) 0.032

Control Medium shade (52 % shade cloth) \0.001

Control Heavy shade (78 % shade cloth) \0.001

494 Agroforest Syst (2014) 88:489–503

123

Biomass harvest

Tiller number

After seven months growth, control switchgrass had

significantly more tillers than in the other treatments

(Table 5). Switchgrass grown under low and medium

shade did not differ in tiller number and had signif-

icantly more tillers than plants subjected to high shade

levels.

Specific leaf area and leaf area

Specific leaf area increased with increasing levels of

shade (Table 5). The high shade treatment had signif-

icantly higher SLA (34.6 m2 kg-1) compared to the

control and low shade treatments (20.5 and

24.5 m2 kg-1, respectively). Leaf area decreased with

increasing levels of shade, from a maximum of

0.84 m2 pot-1 in the control to 0.12 m2 pot-1 in pots

subjected to maximum shade (Table 5). Switchgrass

grown under low and medium shade did not differ in

leaf area and had significantly greater leaf area than

plants subjected to high shade levels.

Light-use efficiency

The highest light-use efficiency was measured in the

control (3.7 g MJ-1), followed by low and medium

shade treatments (2.2 and 1.6 g MJ-1, respectively)

and the high shade treatment (1.4 g MJ-1) (Table 5).

Above- and belowground biomass

Switchgrass grown outside in full sun, and the control

pots in the greenhouse accumulated significantly more

above- and belowground, and total biomass compared to

the shaded treatments inside the greenhouse (Table 6;

Fig. 3). Switchgrass grown in full sun accumulated

significantly more biomass in all components compared

to switchgrass grown under low, medium and heavy

shading in the greenhouse. There was no difference in

aboveground biomass between switchgrass grown out-

side in full sun compared to the greenhouse control

Fig. 2 a Leaf-level switchgrass assimilation (A) and b stomatal

conductance (gs) rates measured in 2011 under ambient

temperature and relative humidity conditions, with a CO2

concentration of 380 ppm and photosynthetic photon flux

density set at levels measured in the greenhouse under four

light intensities. Gas exchange rates are presented for each

measured day of year (DOY). Values presented are means; error

bars indicate the standard error of the mean

Table 5 Switchgrass tiller number, specific leaf area (SLA, m2 kg-1), leaf area (m2 pot-1) and light-use efficiency (LUE, g MJ-1)

for plants grown under four light levels in a greenhouse experiment in North Carolina

Treatment Percent of full

sunlight measured outside

the greenhouse (%)

Tiller number SLA Leaf area LUE

No shade cloth (control) 49 107 (11)a 20.5 (1.8)a 0.84 (0.07)a 3.7 (0.2)a

Low shade (36 % shade cloth) 31 49 (4)b 24.5 (1.3)a 0.49 (0.08)ab 2.2 (0.2)b

Medium shade (52 % shade cloth) 23 36 (4)b 28.3 (1.6)ab 0.33 (0.07)b 1.6 (0.3)bc

High shade (78 % shade cloth) 11 19 (2)c 34.6 (1.9)b 0.12 (0.02)c 1.4 (0.2)c

Values presented are means and (standard errors). Means with the same letter are not significantly different from each other

Agroforest Syst (2014) 88:489–503 495

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(295 g vs 234 g pot-1, respectively; Table 6). However,

belowground and total biomass (189 g and 484 g pot-1,

respectively) were significantly higher in pots receiving

full sun compared to the greenhouse control (141 g and

374 g pot-1, respectively). For shade treatments in the

greenhouse, aboveground biomass ranged from 6 g

to 68 g pot-1, belowground biomass from 3 g to

33 g pot-1, and total biomass from 9 g to 101 g pot-1.

A comparison of outside PAR (full sun treatments) and

the greenhouse control indicated that a 51 % reduction in

PAR (Table 1) did not result in a significant reduction in

aboveground biomass (Table 6), but a 70 % or more

reduction in PAR resulted in significantly decreased

aboveground biomass.

In addition to light, the other factor that likely

influenced growth in this experiment was temperature.

Lethal temperatures can occur in potting medium

adjacent to the sides of black plastic containers

exposed to direct sunlight, which can effectively

reduce pot-rooting volume.

By DOY 292, the roots of switchgrass plants grown

outdoors had fully utilised the potting medium and it is

likely that high temperatures injured or killed these roots

(see Mathers 2003), which may have reduced above-

ground biomass. For this reason, we were uncertain of

the upper limit of switchgrass biomass production for

plants grown in full sun. Results from the greenhouse

showed that there was a linear increase in aboveground

biomass with an increase in PAR. Therefore, we fitted a

linear relationship to these data to estimate biomass

production under full sun (i.e. 1,699 MJ m-2). Using

this relationship, we estimated an aboveground biomass

of 517 g pot-1 compared with our measured value of

295 g pot-1. We then fit two standardised logistic

growth curves (using Eq. 3); one to the estimated

maximum (517 g), and one to the measured maximum

(295 g) to compare differences in predicted switchgrass

biomass as a function of PAR. The parameter estimates

from these growth curves are presented in Table 7.

A measured maximum biomass of 295 g pot-1 yielded

a daily rate of biomass accumulation (parameter a)

of 0.00828 and 95 % confidence limits (CL) of

0.00574–0.01080. The estimated value of the inflection

point (parameter b) was 666 with 95 % CL of 615–717

(Table 7). An estimated maximum biomass of

517 g pot-1 yielded a daily rate of biomass accumula-

tion (parameter a) of 0.00539 and 95 % CL of

0.00397–0.00681. The estimated value of the inflection

point (parameter b) was 867 with 95 % CL of 815–920.

These two models were then applied to PAR data

collected from field studies to produce a range of

estimates of switchgrass biomass under field conditions.

Field PAR measurements

Field PAR measured in 44 loblolly pine stands across

the se-U.S. and summed over DOY 69 to 292 ranged

from 106 MJ m-2 in a 9-year-old stand with

874 stems ha-1 and 18.1 m2 ha-1 basal area to

2,366 ± 147 MJ m-2 in four recently clear-cut stands

with no trees present (see Table 8 in Appendix).

Table 6 Two sample t test results (P [ |t|) for comparisons

between switchgrass above- and belowground, and total bio-

mass (g pot-1) measured after seven months growth in full sun

versus greenhouse-grown switchgrass subjected to four light

levels

Treatment

comparison

Aboveground

biomass

Belowground

biomass

Total

biomass

Full sun vs. control

(no shade cloth)

0.058 0.007 0.041

Full sun vs. low

shade (36 %

shade cloth)

\0.001 \0.001 \0.001

Full sun vs.

medium shade

(52 % shade

cloth)

\0.001 \0.001 \0.001

Full sun vs. heavy

shade (78 %

shade cloth)

\0.001 \0.001 \0.001

250

150

50

50

150

250

350

Full sunlight Control (noshade cloth)

36% shade cloth

52% shade cloth

78% shade cloth

Sw

itchg

rass

bio

mas

s (g

pot

-1)

Treatment

Belowground mass

Aboveground mass

Fig. 3 Above- and belowground biomass for switchgrass

grown outside in full sunlight, and in a greenhouse without

shade cloth (control) and under 36, 52 and 78 % shade cloth in a

shading experiment in North Carolina, USA. Values presented

are means; error bars indicate the standard error of the mean

496 Agroforest Syst (2014) 88:489–503

123

These PAR data, together with parameter estimates

from the standardised logistic growth curves (Table 7)

and field values of switchgrass biomass were used in

Eq. 3 to predict intercropped switchgrass yields in each

of the 44 loblolly pine stands. A maximum field

switchgrass biomass of 5.20 Mg ha-1 was used,

obtained from Albaugh et al. (2012) for pure switchgrass

grown on the Lower Coastal Plain of North Carolina.

Growth curves were significant (P \ 0.001) for above-

and belowground, and total switchgrass biomass, but we

only present data related to aboveground biomass

estimates (Table 7). When PAR data from the loblolly

pine stands were inserted into Eq. 3, simulated switch-

grass biomass ranged from 0.05 to 5.20 Mg ha-1 using

parameter estimates derived from measured biomass

(295 g pot-1), and from 0.08 to 5.20 Mg ha-1 using

parameter estimates derived from estimated biomass

(517 g pot-1); see Table 8 in Appendix. There was a

rapid decline in simulated switchgrass biomass with an

increase in loblolly pine LAI above 1.95 (Fig. 4b) and

2.25 (Fig. 4a) when using estimated and measured

maximum aboveground biomass, respectively, from

switchgrass plants grown outdoors in full sun.

Discussion and conclusion

Shading had a significant effect on all parameters

measured: with increasing shade, switchgrass was

shorter, had fewer tillers, higher SLA, lower leaf area,

lower light-use efficiency and lower biomass

(Tables 5, 6; Figs. 1, 3). Maximum average daily air

temperatures measured were 32 �C (outside) and

38 �C (inside the greenhouse). In warm-season grasses

such as switchgrass, physiological processes and

growth occur optimally at high temperatures (Balasko

and Smith 1971; Hatch 1992; Long 1999).

Switchgrass tiller height growth followed a sig-

moidal curve, with a rapid increase in height early in

the growing season, followed by slower height growth

from July (DOY 199) until measurements ceased in

October (Fig. 1). This seasonal developmental pattern

for height was consistent with field studies. Tiller

height in the control and low shade treatments

(127 ± 3 cm) were within the range reported from

field trials across the U.S. (van Esbroeck et al. 1997;

Heaton et al. 2008; Albaugh et al. 2012) and Canada

(Madakadze et al. 1998).

Table 7 Parameter estimates of the growth rate (a) and

inflection point (b) derived from a standardised logistic growth

curve of switchgrass grown under varying light levels in a

greenhouse experiment in North Carolina and used to predict

aboveground biomass as a function of photosynthetically active

radiation for switchgrass grown as an intercrop

Parameter Estimate Standard

error

95 % Confidence

limits

Measured maximum biomass (295 g pot-1)

a 0.00828 0.00118 0.00574–0.01080

b 665.9 23.6 615.0–716.9

Estimated maximum biomass (517 g pot-1)

a 0.00539 0.00066 0.00397–0.00681

b 867.2 24.3 814.7–919.6

Parameter estimates were derived from two aboveground bio-

mass values; one for measured maximum biomass (295 g pot-1),

and one for estimated maximum biomass (517 g pot-1)

Fig. 4 Aboveground switchgrass biomass estimated from

incoming photosynthetically active radiation measured in 44

loblolly pine stands across the se-U.S. (Table 7; see Table 8 in

Appendix), plotted as a function of loblolly pine leaf area index

(LAI). Data points in a were plotted using parameter estimates

derived from measured maximum biomass (295 g pot-1) and

data points in b were plotted using parameter estimates derived

from estimated maximum biomass (517 g pot-1). In intensively

managed loblolly pine plantations, peak LAI values of 2 occur

on average between age 6 and 8 years across the se-U.S., but this

will vary based on the level of silvicultural intensity

Agroforest Syst (2014) 88:489–503 497

123

As PAR levels decreased inside the greenhouse,

there was a concomitant decrease in tiller number and

leaf area, but an increase in SLA (Table 5). Increases

in SLA in response to shading have been shown for

many species, including switchgrass (Kephart et al.

1992; Kozlowski and Pallardy 1997; Trocsanyi et al.

2009; Devkota and Jha 2010).

Leaf-level photosynthetic rates ranged from 4.1 to

24.8 lmol m-2 s-1 across all treatments and mea-

surement dates (Fig. 2). These values are consistent

with those measured in field trials across the U.S. (e.g.

Skeel and Gibson 1996; Wullschleger et al. 1996;

Sanderson and Reed 2000; Dohleman et al. 2009;

Albaugh et al. 2014), and the wide range suggests

significant capacity of switchgrass to adjust physiol-

ogy to changes in environmental conditions. Stomatal

conductance ranged from 41 to 164 mmol m-2 s-1

across all treatments, which is within the range of

25–356 mmol m-2 s-1 reported in the literature

(Skeel and Gibson 1996; Dohleman et al. 2009;

Albaugh et al. 2014). Our results are similar to Kephart

et al. (1992) who found that even with a higher SLA,

shaded switchgrass leaves intercepted less light,

resulting in reduced photosynthesis in shaded treat-

ments. However, early differences in gas exchange

between the control (49 % of full sunlight) and the

treatments that received 11–31 % of full sunlight were

not maintained for the duration of the experiment

(Tables 2, 3).

Light-use efficiency decreased from 3.7 g MJ-1 in

plants grown at 49 % of full sunlight to 1.4 g MJ-1 in

plants grown at 11 % of full sunlight (i.e., the heavily

shaded treatment with 78 % shade cloth, Table 5).

Therefore, switchgrass growing at higher light inten-

sities was more efficient at converting PAR into

aboveground biomass compared to plants growing at

lower light levels. Reported light-use efficiency values

for the Alamo cultivar range from 1.6 to 5.3 g MJ-1

(Kiniry et al. 1999) and from 2.2 to 4.3 g MJ-1

(Kiniry et al. 2012).

Forest productivity is driven by light interception

via leaf area (Linder 1987; Cannell 1989). In forestry

operations, the goal is rapid site capture, which entails

increasing tree LAI as quickly as possible. However,

this strategy of increasing tree LAI may be counter-

productive to the viability of an intercropped agrofor-

estry system if light interception by a dense overstory

canopy decreases growth of the intercropped species.

Will et al. (2005) found high stem densities and LAI in

four-year-old loblolly pine stands resulted in higher

levels of intercepted radiation compared to low stem

densities and low LAI. In an intercropping study,

Burner and Brauer (2003) showed that productivity of

tall fescue (Schedonorus arundinaceus (Schreb.) Du-

mort.) was reduced by the fifth growing season, which

they attributed to radiation interception by the loblolly

pine canopy. Further, Bellow and Nair (2003) reported

that LAI had a strong negative linear relationship with

PAR measured beneath a range of tree species in Costa

Rica.

Researchers reporting on intercropping agrofor-

estry studies conducted in southeastern and midwest-

ern U.S. have also cited basal area or light levels as

determinants of intercropping success. For example,

forage grasses were successfully grown between tree

rows in a 26-year-old loblolly pine plantation man-

aged for sawtimber production in Louisiana, USA

(Clason 1999). According to Clason (1999), canopy

shading likely had little impact on forage yields

because residual pine basal area averaged 11 m2 ha-1.

In a thinning study in Missouri, USA, Ehrenreich

(1960) reported that the yield of warm-season grass

grown with 30-year-old shortleaf pine was increased

4–5 times when the pine basal area was reduced from

29.8 to 16.1 m2 ha-1. Furthermore, grass yields were

increased more than 14 times when pine basal area was

further reduced to 11.5 m2 ha-1. In a study of

switchgrass grown for forage in full sun and under a

natural stand of 45–50-year-old longleaf pine thinned

to 11.5 m2 ha-1 in Louisiana, USA, Pitman (2000)

suggested that the pine overstory provided a compet-

itive advantage to switchgrass during establishment,

as weeds dominated the plots in full sun. Light

measurements collected near midday under the pine

canopy ranged from 10 to 100 % of full sunlight with

an average of 65 % throughout the plot area (Pitman

2000). In another study in Louisiana, USA, switch-

grass was successfully established in juvenile and late-

rotation stands, but loblolly pine had a negative impact

on switchgrass biomass in mid-rotation stands (Blazier

et al. 2012). There were negative correlations between

pine basal area and switchgrass biomass in mid-

rotation stands, and even trees planted at a low density

of 250 stems ha-1 suppressed switchgrass growth

(Blazier et al. 2012). These results suggest that

intercropping with switchgrass may be a viable option

late in the rotation, after a thinning operation, provided

residual basal area is less than 18 m2 ha-1 and

498 Agroforest Syst (2014) 88:489–503

123

sufficient leaf area is removed such that incoming

PAR is not reduced by more than 65 %.

Integrated land management regimes that involve a

mix of traditional products and biofuel crops would

likely offer the greatest returns on investment and

provide flexibility to adapt to changing market con-

ditions (Munsell and Fox 2010; Vance 2010). These

kinds of multiple land-use systems have been suc-

cessfully established in the se-U.S. with loblolly pine

intercropped with switchgrass, grown as a biofuel

feedstock (Albaugh et al. 2012; Blazier et al. 2012).

However, the longevity of these systems is unknown,

and key interactions between pines and switchgrass

need to be examined. As we present here, light

interception by the pine canopy and subsequent

shading may negatively affect switchgrass growth.

In addition, success of an intercropped agroforestry

system will also be determined by potential below-

ground interactions between these species, and this

requires further investigation.

Growth rate data collected from this greenhouse

study exhibited the same seasonal trends as has been

observed in field studies. In addition, SLA, gas

exchange and light-use efficiency values were

consistent with field trials. However, there are

limitations of pot studies, and these results should

be confirmed in field experiments. As sunlight

passes through tree canopy plantations, there is a

change in both light quantity and quality. Light

quality is altered because leaves preferentially

absorb light in the 400–700 nm wave band, resulting

in a lower red:far-red ratio under plantations com-

pared to full sun (Wilson and Ludlow 1991; Leight

and Silander 2006). These changes in the red:far-red

ratio are perceived by understory plants through the

phytochrome system that may change morphoge-

netic characters in plants (Smith 1982), such as leaf

extension and expansion, and tiller elongation rate

(Casal et al. 1985, 1987). According to Peri et al.

(2007), it is likely that leaf area is maintained or

increased to maximise light interception at the

expense of leaf thickness. This results in longer,

narrower and thinner leaves compared with those

grown in full sun. In addition, reduced light

intensity and changes in light quality have been

reported to reduce tillering and leaf area index (Peri

et al. 2007). In our study, switchgrass responses to

reduced PAR were quantified under a simulated

shade environment and measuring light quality was

not part of our study. According to Varella et al.

(2011), the red:far-red ratio is not altered by black

shade cloth. Therefore, it is possible that lower

switchgrass biomass yields than our calculated

values could be obtained when intercropped with

loblolly pine. However, this needs to be determined.

Leight and Silander (2006) suggest that plant

responses to changes in the red:far-red ratio may

be species specific. As such, it is important to study

the different ways that plants respond to changes in

the red: far-red ratio to provide a mechanistic

understanding of response processes in a forest

understory.

In addition, our yield predictions were made from

switchgrass growing under shade cloth with a steady

state reduction in light. In pine plantations, the light

environment will likely be more dynamic as light

passes through the canopy as sunflecks. However,

most sunflecks are of a short duration and the

thylakoid apparatus needs to be extensively modified

in order for the leaf to harvest these brief bursts of

photons and store the excitation energy until the

carbon reducing cycle reactions can utilise it (Sharkey

et al. 1986; Pearcy 1990). In C4 plants, two biochem-

ical cycles have to be activated following the onset of

the sunfleck, and these plants will likely have a slower

ability than C3 plants to respond to sunflecks (Sage and

Pearcy 2000). According to Krall and Pearcy (1993),

maize (a C4 plant) has a poor ability to utilise short-

duration (\10 s) sunflecks, possibly because the C4

metabolic cycle is unable to activate at a pace rapid

enough to assimilate the initial influx of CO2 from the

mesophyll. Switchgrass may likely respond to sun-

flecks in a similar way to maize; however, this would

need to be determined.

We predicted switchgrass biomass when grown as a

biofuel intercrop in a range of stands typical of forestry

sites across the se-U.S. Predicted aboveground switch-

grass production was variable, and ranged from

0.05 Mg ha-1 to 5.20 Mg ha-1 year-1 (see Table 8

in Appendix). Our results suggest that switchgrass

yields will not be affected by a 50 % reduction in

PAR, but beyond a threshold reduction in PAR

between 60 and 65 %, or an overstory LAI between

1.95 and 2.25, it is likely that switchgrass yields will

be significantly reduced (Fig. 4). In these field studies,

pine LAIs of 1.95–2.25 corresponded to a basal area

range of 16–21 m2 ha-1 and a 60–65 % reduction in

PAR, suggesting that switchgrass growth will be

Agroforest Syst (2014) 88:489–503 499

123

significantly reduced when overstory canopy reduces

PAR beyond a 60 % full sun threshold.

Our results suggest that switchgrass may still be

highly productive even when half of the incident PAR is

intercepted by an overstory tree canopy. However, it is

unknown how long this would persist and the significant

exponential decrease in belowground biomass with

increased shading has implications for carbon seques-

tration, indicating that the potential of switchgrass to

sequester carbon (Ma et al. 2000; Garten et al. 2010)

may not be realised under low light levels. In intensively

managed loblolly pine plantations across the se-U.S.,

peak LAI values of 1.95–2.25 occur on average between

age 6–8 years (Sampson et al. 2008), but this will vary

with the level of silvicultural intensity based on

practices such as the extent of site preparation,

fertilisation, pruning and thinning. Further, switchgrass

growers need to account for other drivers of economic

viability of growing this species as a biofuel intercrop,

including costs relating to establishment, harvesting and

transport (George et al. 2008), tax incentives, and farm-

gate feedstock cost and conversion efficiency into

ethanol (Mitchell et al. 2008).

Acknowledgments We acknowledge North Carolina State

University, Weyerhaueser Company, Catchlight Energy LLC (a

Chevron and Weyerhaeuser joint venture) and the Forest

Productivity Cooperative for funding and support. We are

grateful to Mr. Sam Brake from the North Carolina Biofuels

Center for advice and for supplying the switchgrass seedlings,

and to Dr. Chris Maier from the Forest Service for access to

unpublished data. Samuel Honeycutt assisted with the biomass

harvest and sample processing. Additional funding for research

was provided by United States Department of Agriculture NIFA-

AFRI Sustainable Bioenergy grant number 2011-67009-20089.

Appendix

See Table 8.

Table 8 Characteristics of loblolly pine stands across the se-U.S. where photosynthetically active radiation (PAR) data were

collected and used to predict aboveground switchgrass biomass when grown as an intercropped feedstock for biofuel

Location Pine age

(years)

Pine

height

(m)

Pine

DBH

(cm)

Pine BA

(m2 ha-1)

Pine

stocking

(stems ha-1)

Pine LAI

(m2 m-2)

PAR

(MJ m-2)

Switchgrass biomass

(Mg ha-1)

(295 g pot-1) (517 g pot-1)

NC 9 12.1 16.1 18.1 874 4.5 106 0.05 0.08

NC 37 23.1 38.5 23.8 199 4.3 125 0.06 0.09

SC 8 14.9 16.3 27.3 1,292 4.2 169 0.08 0.12

GA 14 15.2 18.9 45.8 1,586 4.2 180 0.09 0.13

GA 14 13.0 17.0 38.6 1,654 3.9 233 0.14 0.17

SC 8 14.6 15.9 26.2 1,292 3.6 245 0.16 0.18

GA 14 12.5 16.1 39.7 1,873 3.7 262 0.18 0.19

SC 8 15.4 16.8 28.3 1,270 3.3 307 0.25 0.24

SC 8 15.2 16.5 27.7 1,281 3.3 308 0.25 0.24

SC 8 14.9 16.2 27.0 1,281 3.3 315 0.27 0.25

SC 14 13.2 18.5 34.0 1,236 3.4 316 0.27 0.25

GA 14 13.4 18.4 36.2 1,348 3.3 335 0.31 0.28

SC 14 13.4 18.1 34.3 1,304 3.3 341 0.33 0.29

SC 14 13.5 17.5 33.0 1,338 3.3 345 0.34 0.29

SC 8 15.9 16.4 27.2 1,270 3.1 354 0.37 0.31

SC 14 12.9 17.1 31.9 1,373 3.2 364 0.40 0.32

SC 8 15.0 16.3 27.4 1,292 3.0 374 0.43 0.34

SC 14 13.1 19.5 35.5 1,167 3.1 388 0.47 0.36

SC 8 15.9 16.4 27.5 1,281 2.9 403 0.53 0.39

SC 14 13.4 18.1 31.8 1,201 2.9 427 0.63 0.44

SC 14 13.6 19.2 36.4 1,236 2.9 442 0.70 0.48

SC 8 15.8 16.2 26.8 1,281 2.7 456 0.78 0.51

500 Agroforest Syst (2014) 88:489–503

123

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SC Clear-cut – – – – 0 2,313 5.20 5.20

SC Clear-cut – – – – 0 2,556 5.20 5.20

GA Clear-cut – – – – 0 2,622 5.20 5.20

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