Light distribution in scattered-trees open woodlandsin Western Spain

Marıa Jesus Montero Æ Gerardo Moreno ÆManuel Bertomeu

Received: 18 October 2006 / Accepted: 28 April 2008 / Published online: 10 May 2008

� Springer Science+Business Media B.V. 2008

Abstract We have studied the percentage of radia-

tion transmitted through the tree canopy to the

underlying pasture and crops in dehesas of Southwest

Spain by means of fish-eye photographs taken at

different distances from the tree. Thirty-six trees were

studied covering all the diametric classes (0.1–14 m

canopy width) of two stands, with mean density of 19

mature trees ha-1. Intercepted light decreased with

distance following an logistic curve, indicating a rapid

increase in the light availability with distance from the

tree. For mature trees, radiation was constant beyond

20 m. Applying a multivariable regression light

equation, distance, stem diameter and canopy width

explained more than 88% of the light variability for

each orientation studied. A simple model was built up

from light equations, tree growth curves and allome-

tric relationships. From this model, we have estimated

that radiation available for crops and pasture

decreased up to 21% due to the presence of trees in

a standard dehesa with 24 mature trees ha-1 and 13%

of canopy cover. In addition, we have generated

different radiation maps of virtual dehesas differing in

tree age, density and arrangement, which could be

useful to determine optimal tree planting schemes and

consequent pruning and thinning regimes.

Keywords Allometric relationships � Dehesa �Fish-eye photograph � Light transmittance �Quercus ilex � Radiation map

Abbreviations

FEP Fish eye photograph

I Intercepted radiation

D Distance from the trunk

DBH Diameter at breast height of the tree trunk

Cw Canopy width

Th Tree height

Ch Canopy height

Introduction

Agroforestry systems—tree-pasture or tree-crop

associations—are very efficient in terms of resource

use (Dupraz et al. 1999), and are therefore environ-

mentally friendly and economically profitable

(Gordon and Newman 1997). The dehesa an open,

park-like savannah oak forest, is the most extensive

agroforestry system in the Mediterranean basin

(Eichhorn et al. 2004), located mainly in the

South-Western Iberian peninsula.

M. J. Montero � G. Moreno (&) � M. Bertomeu

Centro Universitario, Forestry School, Universidad de

Extremadura, Plasencia 10600, Caceres, Spain

e-mail: gmoreno@unex.es

M. J. Montero

e-mail: cmontero@unex.es

M. Bertomeu

e-mail: mbergar@unex.es

123

Agroforest Syst (2008) 73:233–244

DOI 10.1007/s10457-008-9143-4

This system of land use, a consequence of the

clearance of oak forests for grazing, has been widely

practiced for as long as 4500 years (Stevenson and

Harrison 1992). Several authors have shown the

positive effects of dehesa trees on the quality and

quantity of pasture (Puerto and Rico 1988, 1989;

Perez-Corona et al. 1995) and understorey crops

(Moreno et al. 2007a). However, during the second

half of the 20th century, increasing mechanization of

agriculture, and livestock density and the lack of

proper oak management have led to a reduction of

dehesa area and a decrease of tree density that

threatens the long-term persistence of trees in many

dehesas (Dıaz et al. 1997). Miguel et al. (2000)

reported a decline of dehesa area of 23% between

1951 and 1981, and a reduction from 2.3 million ha in

1985 to 1.7 million ha in 1998. In the last two

decades thousands of hectares of various oak species

have been planted in southern Europe, primarily in

marginal agricultural land and pastures that replaced

former dehesas of western Spain. Unfortunately, most

reforested areas have been established as forest

plantations without considering any scientific criteria

which would guide decisions on tree density and

arrangement.

The success of different types of agroforestry

systems is based on the existence of complementarity

or facilitative interactions between the upperstorey

and understorey (Ong et al. 1996). Nevertheless,

competition for resources among species in agrofor-

estry systems is more the rule than the exception, and

it increases with tree growth (Jose et al. 2004).

Ideally, before establishing a new agroforestry sys-

tem, informed decisions should be made on stand

structure (i.e. tree density, alley width and orienta-

tion) and tree management (pruning and thinning) in

order to minimize the competitive interactions among

its components.

When soil nutrients, water and temperature are not

limiting, light is the most important factor affecting

crop growth and yield (e.g. Knowles et al. 1999;

Sibbald and Sinclair 1990; Thevathasan and Gordon

2004). For the tropics, there are a number of recent

studies on available understorey radiation considering

tree structural characteristics (Sinoquet et al. 2000;

Bellow and Nair 2003) and the effect that different

levels of pruning have on the amount of light

available for pastures and crops (Miah et al. 1995).

However, few studies on light availability have been

conducted in Mediterranean open oak woodlands.

Etienne (2005) reported that pasture yield is generally

maximised with a Quercus ilex cover around 30%

(Etienne 2005), whereas Moreno (2008) has shown, a

positive and dominant effect of shading on pasture

yield. Further studies on stand structure (i.e. tree

density, alley width and orientation) and tree man-

agement regimes (pruning and thinning) that

minimize the competitive interactions among the tree

and understorey components are therefore needed to

guide future reforestation initiatives in dehesa areas.

The objective of this study is to characterize the

amount of light transmitted through the canopy of

evergreen oak dehesa and its spatial distribution

around trees as a function of tree size. In addition, a

simple allometric-based model has been created to

simulate the effect of tree age, tree density, alley

orientation and alley width on the amount of light

transmitted in Mediterranean Q. ilex stands.

Materials and methods

Study area

The study was performed on two experimental farms:

‘‘Cerro Lobato’’ and ‘‘El Baldıo’’ located in the

province of Caceres, Central-Western Spain

(398410 N–68130 W; elevation 380 m; slope 2%).

The climate is classified as subtropical Mediterranean

with a mean annual temperature of 16.2�C, mean

annual rainfall of 506 mm and a long dry period

covering the three months from June to August. The

soils type is Chromic Luvisols developed over

tertiary sediment with abundant quartzite and more

than 100 cm depth.

Both farms are Holm-oak (Quercus ilex L.), open

woodlands (named dehesas) with a tree density of

19 ± 6 mature trees ha-1. Mean canopy closure is

10 ± 1.5%. Mature trees on the studied plots had

mean diameter at breast height (dbh) of 35 ± 4.1 cm,

mean height of, 6.2 ± 0.3 m, and 8.3 ± 0.9 m of

mean canopy width (±values express the 95%

confidence interval). Canopy radius (Cri) varied

significantly with direction, with mean values of

3.9, 4.0, 4.4 and 4.3 at E, N, W and S sides

(P \ 0.001). The first two to three meters of the stem

are free of branches due to cattle browsing and

mechanization. The understorey consists of native

234 Agroforest Syst (2008) 73:233–244

123

grasses (Trifolium campestre, Medicago polymorpha,

Geranium molle, Erodium cicutarium, Lolium

rigidum) and periodic crops (winter cereal, inter-

cropped every 4 years on average). According to

local knowledge, several reasons are given to perform

periodic pruning of trees (approximately every

10 years): to increase acorn production and light for

crops, and to get fodder (leaves) for livestock in

winter, firewood (large branches) and charcoal (fine

branches).

Data collection

The percentage of radiation transmitted was studied

by means of two sets of fish-eye photographs (FEP).

This technique has been widely used by other authors

to measure the amount of light transmitted through the

tree canopy (e.g., Trichon et al. 1998). The first set

(577 pictures) was taken in spring 2003 in 28 trees

covering all the diametric-classes found in ‘‘El

Baldıo’’ farm (0.5–73.2 cm of DBH). Every tree

was photographed on the four compass directions, as

these four directions are commonly used to study

pasture production and quality (e.g., Maranon 1986;

Perez-Corona et al. 1995) and soil properties (e.g.,

Puerto and Rico 1988; Gallardo 2003) of Iberian

dehesas, and other savanna-like systems (e.g., Ludwig

et al. 2004). At each direction, FEP were taken from

six distances from the tree trunk (0.5, 1, 5, 10, 20 and

30 m). In smaller trees, the furthest distances were

excluded. These FEPs were used for developing the

light equations. A second set of FEPs was taken of 8

trees (DBH from 5 to 70 cm) from a different farm

(Cerro Lobato) for the validation of the equations.

Both sets of trees had been pruned 10 years ago.

The latter set of FEPs (192 pictures from 8 trees)

was taken in November 2003, just before tree

pruning, and were then repeated two months later in

January 2004 just after pruning. These photographs

were used to determine the effect of the pruning

treatment. As holm oaks do not shed leaves, (since

between November and January trees are not losing

leaves or growing), all the variation in canopy

structure can be attributed to pruning.

FEPs were taken with a digital camera (Nikon

Coolpix 995) installed in a platform at 70 cm from

the soil to raise it to the average height of pasture and

cereal crops in the study farms. The platform includes

a function to level the camera before taking the

pictures and a compass to position each photo

towards geographic north, which are both important

in the later calculation of the transmitted light

percentage. Photos were taken on cloudless mornings

at sunrise and evenings prior to sunset, to avoid sun

brightness and thus facilitate the analysis of the FEPs

by capturing a better separation of the tree from

the sky.

In addition, the dimension of trees was character-

ized through the measurement of stem diameter

(DBH), basimetric area (BA), tree height (Th), canopy

height (Ch) and canopy radius at four compass

directions (Cri). These measurements were taken in

360 trees from the study area covering all size classes

and including all photographed trees. Data were used

to obtain different allometric equations and to relate

light transmittance with tree size.

Finally, benefiting from the tree pruning and forest

thinning work, 200 young trees (diameter \10 cm)

were collected to count the number of rings (after

polishing) and to measure the stem diameter. This

data set was combined with DBH and number of

rings data obtained by Plieninger et al. (2003) in 78

trees (DBH [ 10 cm) cut down in the same area.

From these data, we have charted the growth curve

for Q. ilex, the relationship between tree age (number

of rings) and stem DBH and BA.

Data analysis

In every positioned and levelled photo we analyzed

the percentage of total transmitted radiation using the

software ‘‘Gap Light Analyzer’’ (GLA 2.0; Frazer and

Canham 1999). The growing season for grasses and

winter crops (from 1st November until 31st May) was

used as the integration period in the software. In

addition, from the set of 192 FEPs (8 trees 9 4

orientations 9 6 distances), 4 single days were also

analyzed (November, January, March and May) to

determine the seasonal variation of the percentage of

light transmitted. Before running the software all

neighbouring trees were eliminated from the photo to

get the effect of the single tree under study.

Different two-way ANOVAs were carried out to

determine differences in the percentage of light

transmitted as a dependent variable using different

pairs of independent variables: distance, tree size

(4 diametric classes), direction and pruning.

Agroforest Syst (2008) 73:233–244 235

123

Equations and transmitted light map

The calculation of light intercepted for a single tree is

based on three sets of equations:

(1) Light equations: the percentage of light trans-

mitted at each direction is estimated from tree

size and distance from the tree trunk.

(2) Allometric relationships: tree size, characterized

by Cr, Th and Ch, is estimated from the DBH

and BA;

(3) Growth equation: DBH and BA were estimated

from age

Combining the three sets of equations, the per-

centage of transmitted light at each distance and

orientation is estimated for a tree of a given age.

When parameters describing tree size and/or DBH are

known, equations 2 and/or 3 should not be used.

From 577 pictures from the first data set (28 trees),

we obtained light equations by conducting multivar-

iable non-linear regressions of intercepted radiation

(I) with distance from the trunk (D) and tree size

parameters (DBH, BA, Cri, Th and Ch). We adjusted

one equation for each direction (x = N, S, E, W).

Thus, the amount of intercepted light by trees at

specific distances and direction (N, S, E and W) can

be easily calculated using a datasheet. A second data

set (8 trees of the Cerro Lobato farm) was used to

validate the equations.

Once the intercepted radiation for a single tree has

been calculated, the intercepted radiation for a tree

plantation or population can be mapped and calcu-

lated using the interpolation software SURFER 8.0

(Golden Software 1999). The scattered distribution of

trees in dehesas, where trees do not follow the

directions of the light equations (N, W, S and E),

introduce an additional difficulty for the calculation of

transmitted light. We have imposed a grid onto the

study area with one meter by one meter spacing, and we

have determined the amount of light for every grid

node as a consequence of every tree belonging to the

study area. In this way, in each node the shading effect

of each tree is added, obviously with a maximum effect

by the nearest trees. In Fig. 1 we summarized the

calculation process for each node of the grid.

Simulated scenarios

From the aforementioned equations, we simulated

and compared the amount of light transmitted in a 1-

ha plot under different scenarios: varying tree density

(25, 50, 100, 200, and 400 trees ha-1), tree age (5, 10,

20, 50, 75, and 100 years), tree arrangement (trian-

gular versus rectangular spacing), alley width (wide

versus narrow) and orientation (N-S versus E-W)

(Fig. 2), and their respective combinations.

To carry out these comparisons, four different two-

way ANOVAs were applied, with the amount of light

transmitted at plot level as the dependent variable,

and the following pairs of independent variables: (i)

tree age and tree density; (ii) tree arrangement and

tree density; (iii) tree arrangement and tree age; (iv)

alley orientation and tree density (only for 50 and 200

trees ha-1); (v) alley orientation and tree age; (vi)

alley width and tree density (only for 50 and 200 trees

ha-1); and (vii) alley width and tree age.

Results

Effect of distance and orientation

The increase of transmittance with distance followed

a logistic (Transmittance = 100/(1 + A * exp (B *

Distance))), almost exponential trend, indicating a

rapid and significant increase in the light availability

nAA

n

IAA IIII ...%

211

21

4

3

A

2

3

2

3

E

A

N

90-

90

)(_)90(_%

1

NorthEquationEastEquationI A

Fig. 1 Process of

calculation of the amount of

light intercepted at a

specific point by trees in a

current plot (scattered trees)

before the complete

interpolation with SURFER

236 Agroforest Syst (2008) 73:233–244

123

with distance (Fig. 3). At 10 m of distance from the tree,

the available radiation was around 95%, except on the

north side (around 80%).

Mean values of A and B coefficients (estimated for

each single tree and direction) were compared by mean

two one-way ANCOVAs, with A and B as dependent

variables and direction as categorical variable. Canopy

radius was included as a continuous variable to cope

with variation in tree size and crown irregularity

(Cri varied with direction). A coefficient showed values

significantly higher (F3,107 = 9.42; P \ 0.001) at

north side (6.93) than at the other three directions

(4.47, 3.92 and 3.90 at E, W and S, respectively),

indicating the higher transmittance at these latter

directions (without significant differences among

them). B coefficient also showed significant differ-

ences among directions (F3,107 = 17.5; P \ 0.001),

with higher values at north side (-0.329) than at south

side (-0.597), indicating the longer shade projection at

the north side (in spite of its lower Cri). East and west

sides showed intermediate B coefficients, with no

significant differences between them (0.470 and 0.434,

respectively).

The level of transmittance (A coefficient) also

varied significantly with the Cri, with obviously higher

values for smaller trees (F1,107 = 17.5; P \ 0.001).

By contrast, Cri did not affected significantly to

B coefficient (F1,107 = 2.34; P \ 0.115), indicating

that the shape of the logistic curves did not vary with

tree size.

Transmitted light at plot scale

We have mapped and calculated the light transmitted

through the oak canopies in a 1-ha plot of a mature

stand (Cerro Lobato farm). We completed the study

within an outer strip of land of 25 m width around the

REGULAR DESIGN

WIDE ALLEY

Wide Alley (b >> 2a) 50 trees / ha: 4 x 50 m 200 tree / ha: 2.5 x 20 m

Narrow Alley (b = 2a) 50 trees / ha: 10 x 20 m 200 tree / ha: 5 x 10 m

NARROW ALLEY

a

a

a

b

b

a

a

a

a

b

N

SW E

Fig. 2 Different plantation

designs simulated in virtual

scenarios of 1 ha

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Distance, m

% L

ight

tran

smitt

ed

East R2 = 0,911

North R2 = 0,954

West R2 = 0,967

South R2 = 0,936

Fig. 3 Increase of light transmitted with distance from the

mature trees (DBH [ 35 cm and Cw [ 7 m) at four compass

directions. Standard errors are given as vertical bars. Coeffi-

cients of determination (R2) of the logistic curves

(Transmittance = 100/(1 + A * exp (B * Distance))) are also

given

Agroforest Syst (2008) 73:233–244 237

123

main target 1-ha plot to avoid border effects in the

process of interpolation, but we only represented the

light contour map for the inner 1-ha plot. The stand-

level mean of light transmitted through the tree

canopy reached 79% for a stand with 24 mature trees

ha-1 and 13% of canopy cover. A strong heteroge-

neity is observed in the spatial distribution of light in

this dehesa stand, with areas below 30% of light

transmitted (near tree trunks) and other areas with

values near 100% of light (Fig. 4).

Effect of pruning

Pruning significantly increased the percent of light trans-

mitted (F1,137 = 437.83; P\ 0.001); but differences

were significant only in the vicinity of the trunk (Fig. 5).

In fact, at 20 m the effect of pruning on light availability

was not significant (P = 0.834). Considering the normal

density in dehesas (20 tree ha-1 of mature trees), we have

estimated that a normal pruning done every 10 years

increases the light availability for the understorey at plot

scale 10.3% (from 81.7% to 92.0%), and around 40% of

the surface benefited with an increase [10% in the

amount of light received.

Model equations

Light equations

A multivariable non-linear regression, including

distance and the five tree size parameters as inde-

pendent variables was applied to estimate light

transmitted as the dependent variable from the first

data set of 28 trees. Only three variables, distance

(D), DBH, and Cri significantly correlated with the

light interception (I). Given the similarity of the east

and west light curves (Fig.3), we obtained three

equations (N, S and E-W) (Table 1):

Ix ¼ A/ 1 + B � eC�DBH� �� �

� D= 1þ E � eF�Cri� �� �

� G�Distance; R2 [ 88%� �

ð1Þ

x = north, south, and west-east.

The three equations performed well with around

90% of the variability explained. These equations

combine the effects of tree size and distance from

the tree trunk with a Gaussian dependence for light

739180 739190 739200 739210 739220 739230 739240 739250 739260 739270

4397430

4397440

4397450

4397460

4397470

4397480

4397490

4397500

4397510

4397520

0

10

20

30

40

50

60

70

80

90

100

Fig. 4 Distribution of light (% of full shine) in a common

dehesa simulated by SURFER in a current plot of 1 hectare

located at the Cerro Lobato farm

0

20

40

60

80

100

0 5 10 15 20 25

Distance, m

Ligh

t tr

ansm

itted

, %

Pruned

Unpruned

**

*

*

Fig. 5 Comparison of light transmitted in pruned and

unpruned Holm-oaks in dehesas. Results come from averaging

8 trees and four orientations. Asterisks indicate significant

differences between pruned and unpruned trees (P \ 0.05)

238 Agroforest Syst (2008) 73:233–244

123

transmittance on DBH and Cri, and an exponential

dependence on distance.

Allometric relationships and growth equation

Different non-linear regressions were assessed to

estimate Cw from DBH and BA (in m, cm and cm2,

respectively). The highest level of variability

explained was obtained with the following equation

(Fig. 6a):

Cw ¼ 0:368 � BA0:448; R2 ¼ 97:1%; n ¼ 360 ð2Þ

Similarly, we determined the growth equation

(age-BA and age-DBH relationships) by applying a

non-linear regression. The best adjustment was

reached by the following equation, with the highest

level of variability explained (Fig. 6b):

DBH ¼ 98:73 � 1� exp �0.00546 � Ageð Þð Þ;R2 ¼ 91:47%; n ¼ 278

ð3Þ

Both relationships, Cw = f(BA) and

DBH = f(Age) allow us study how tree crowns vary

in size over time. According to differences found for

Cri among directions, Cri are calculated from Cw by

multiplying these coefficients: 0.48, 0.52, 0.47 and

0.53 for N, S, E and W, respectively.

Validation test

From Eqs. 1 and 2 we have estimated the light

available at different distances and orientations for

the second data set of 8 trees, covering all the

diametric classes found in common dehesas (DBH 5–

70 cm). These values were then regressed against

measured values at the same points to validate the

equations. Results indicate a good performance

(R2 = 0.89; Fig. 7) of the process of calculation of

light transmitted through evergreen oak canopies in

studied stands.

Simulated scenarios

Applying Eqs. 1, 2 and 3, we have simulated

different scenarios in a 1-ha plot to analyze the

effect of tree age, tree density and plantation design

(tree arrangement, alley width and alley orientation)

on the amount of the light intercepted by trees.

Effect of tree age and density

The effect of tree age and density was analyzed

irrespective of the tree arrangement. Results showed

Table 1 Light equations

from the multivariable

non-linear analysis based

on distance from the tree

trunk, canopy width and

DBH

Orientation Ix ¼ A1þB�exp C�DBHð Þ � D

1þE�exp F�Crið Þ � G�Distance

A B C D E F G R2

East-West 11.12 101.78 -0.76 11.12 1.85 -0.27 1.29 0.915

North 7.88 30.52 -0.62 12.98 1.45 -0.50 1.17 0.887

South 14.55 136.67 -0.62 2.94 -0.27 0.11 1.41 0.932

y = 0.368x0.448

R2 = 97.1%

0

5

10

15

20

0 1000 2000 3000 4000 5000

Basal Area, cm2

Can

opy

wid

th, m

(a)

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160

Age, years

DB

H,

cm

DBH=-98,73+98,73*Exp(-0,0055*age)

R2=91,47%

(b)

Fig. 6 (a) Allometric relationships between stem diameter

(DBH) and canopy width; (b) Growth curve for Quercus ilexobtained by regressing DBH with number of rings (age)

Agroforest Syst (2008) 73:233–244 239

123

significant differences in light transmittance base

with changing tree age and density (F4,44 = 44.26;

P \ 0.001). Low density plantations (25, 50 trees

ha-1) intercepted significantly less radiation than

high density plantations (100, 200, 400 trees ha-1),

with highly significant differences among the latter.

In addition, the amount of light intercepted increased

significantly with the age of the plantation (F5,44 =

46.80; P \ 0.001). Moreover, the interaction between

both age and density parameters also resulted in a

significant difference (F20,44 = 3.93, P \ 0.001),

indicating that age differences were only significant

for tree densities above 50 trees ha-1, since light does

not vary significantly with tree growth at very low

density (Fig. 8).

Effect of tree design

The analysis of the influence of the tree plantation

scheme (Fig. 2) on the amount of light intercepted by

trees gives the following results: (i) Light interception

was significantly higher with quadrangular spacing

(36.84%) than with triangular spacing (26.8%)

(F1,15 = 12.41; P \ 0.001). The interaction with tree

density was also significant, indicating that differences

from spacing were only significant at the highest tree

density (400 trees ha-1) (F4,15 = 3.11; P = 0.04)

(Fig. 9a). Differences between design were indepen-

dent of the tree age (non significant interaction;

F3,16 = 2.00; P = 0.15); (ii) Trees arranged in north-

south rows intercept significantly less radiation than

trees arranged in east-west rows (F1,14 = 23.47;

P \ 0.001), irrespective of tree density (non- signifi-

cant interaction; F1,14 = 2.59; P = 0.13). In contrast,

y = 0,9996x

R2 = 0,888

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100% Light estimated with equations

% L

ight

mea

sure

d by

FE

Ps Line 1:1

Fig. 7 Relationships between measured (by FEP) and esti-

mated (by the equation sets) values of light intercepted at

different distances from 8 mature trees

Ages

0

20

40

60

80

100

25 a 50 a 100 b 200 c 400 d

Tree density

% L

ight

inte

rcep

ted

5 - a 10 - b 20 - c 50 - c 75 - d 100 - d

Fig. 8 Percentage of light intercepted as a function of tree age

(5–100 years old) and tree density (25–400 tress ha-1).

Different letters indicate the existence of significant differences

(P \ 0.05) among ages or among tree densities

0

20

4060

80

100

120(a)

(b)

Qua

dran

gula

r

Tri

angu

lar

Qua

dran

gula

r

Tri

angu

lar

Qua

dran

gula

r

Tri

angu

lar

Qua

dran

gula

r

Tri

angu

lar

Qua

dran

gula

r

Tri

angu

lar

25 25 50 50 100 100 200 200 400 400

% L

ight

inte

rcep

ted

p < 0,01

0

20

40

60

80

E-W N-S E-W N-S E-W N-S E-W N-S

5 5 10 10 50 50 100 100

% L

ight

inte

rcep

ted

p < 0,01

P < 0,01

Fig. 9 (a) Results of the interaction between tree density and

tree arrangement (quadrangular and triangular designs) on the

percentage of light intercepted; (b) Results of the interaction

between age and orientation of tree rows on the percentage of

light intercepted. Bars indicate standard error. Significant

differences between tree arrangements or tree row orientations

within each tree density are also noted

240 Agroforest Syst (2008) 73:233–244

123

the significance of different row arrangements depends

on age, (F3,12 = 6.79; P = 0.006), indicating that

these differences are only significant for plantations

above 50 years old (Fig. 9b); (iii) There are significant

differences between wide and narrow alleys, as more

light is intercepted on plantations with narrow alleys

(F1,6 = 9.14; P = 0.02). There were no interactions

between alley width and density or age.

Discussion

Tree light transmittance

The decrease of light availability for understorey in

the vicinity of the tree canopy can be seen as a

beneficial or detrimental effect of the tree on

understorey yield (McPherson 1997). In mature

stands of dehesas, the decrease of radiation beneath

the canopy has a positive effect on the microclimate,

as Moreno et al. (2007b) described in the same stand.

Both microclimate and shading patterns in dehesas

could contribute to increased pasture production

beneath the tree canopy and to an enlargement of

the growing season of grasses located beneath the

canopy with respect to those located beyond it, since

the former dries a few weeks later (e.g., Joffre 1987;

Puerto et al. 1990).

Light saturation of photosynthesis is achieved with

around 600 lmol m-2 s-1 for C3 plants (Nobel

2005), and an excessive photosynthetically active

radiation could produce radiation stress in plants

(Larcher 1995). In this sense, Moreno (2008) has

shown a positive effect of the artificial shade (a mesh

giving a 50% of full sunlight) and canopy cover on

pasture yield of Central-Spain dehesas, mostly

explained by the mitigation of extreme temperature

and reduction of the vapor pressure deficit (Moreno

et al. 2007b).

Mean values of daily course of photon flux density

(PFD) in Mediterranean latitudes in spring (when most

of the Mediterranean pastures and winter crops grow)

are around 800 - 1000 lmol m-2 s-1 (Bellot et al.

2004). This means that a tree with a 75% of ligth

transmittance allows to reach the 600 lmol m-2 s-1

threshold of PFD for understorey in that period.

Etienne (2005) pointed out that in a Mediterranean

climate, maximum production of understorey is

obtained with some level of upperstorey cover. He

reported 30% as optimum tree cover for Q. ilex stands

(47 mature trees of 9 m of canopy width), what

according to our results give around 75% of trasnmit-

tance at plot scale (Figs. 6 and 8). With the common

current tree density in dehesa (around 20 trees ha-1),

the mean transmittance at plot scale is around 18%,

which can be considered enough to achieve optimum

understorey production.

Mature Q. ilex trees significantly reduce the

amount of understorey light availability in a wide

surface, specially in north side. However, a high

reduction of light (above 25%) occurs only in the

close vicinity of the trees (first 5 m or near 80 m2

tree-1). Considering 25% reduction as a reference

and a normal stocking density of 20 tree ha-1, it is

estimated that only 15% of the surface could be

significantly affected by shading. Obviously, the

surface affected is much lower in the first decades

of the tree plantations, since Q. ilex is a very slow

growing tree and needs around 80 years to reach a

large size (above 35 cm of DBH, 8 m of canopy

width and 6 m of height).

Traditional tree pruning in dehesa is made the year

before plot intercropping (which is made every

10 years on average) to reduce tree competition

impacts on crops. Indeed, with pruning of mature

trees the surface highly affected by tree shade ([75%

of transmittance) is reduced from 15% to 5% of the

plot surface (only 20 m2 per tree). Hence, although at

the plot scale, the effect of pruning on the increase of

light is rather limited, beneath canopy is very

important, and it seems prudent to recommend the

maintenance of this practice. Besides, PFD values are

low in autumn and winter (maximun values around

600 and 800 lmol m-2 s-1, and mean values around

400 and 500 lmol m-2 s-1, respectively; Bellot

et al. 2004), and with this PDF any decrease in light

availability can become significant in this period.

It must be considered that understorey production

not only depends on understorey species, but also on

the atmospheric conditions and belowground resources

(McPherson 1997; Scholes and Archer 1997). For

instance, Moreno (2008) showed how open-oak forests

favoured understorey forage production through a

direct positive effect of shade and improved soil

fertility, but the potential benefit had a small actual

facilitative effect because the competitive use of soil

water by trees, especially under semi-arid conditions.

In this sense, tree pruning can also contribute to reduce

Agroforest Syst (2008) 73:233–244 241

123

the competition for soil water (see Moreno and Cubera

2008).

Modelling light distribution in dehesas

The set of equations presented here, the growth

equation, allometric relationships and light equations,

allow easy characterization of the amount and

distribution of light transmitted through the canopy

of single scattered Q. ilex trees growing in Iberian

dehesas. From transmittance measured in single trees

of different ages and sizes, the available light for

pasture and crops can be calculated for any specific

point of the stand. As the model is based in the

addition of the shade of single trees, it can be used to

characterize light distribution in whole stands with

either scattered or regularly distributed trees, or with

either even-aged or uneven-aged tree population.

The performance of the calculation process is

satisfactory for whole stands (here we present a 1-ha

plot example; Fig. 4), although for single points,

especially in the crown limits, the precision is not as

high. This is because we considered a regular foliage

distribution in the crown, while oak crowns are

actually irregular in shape and density as a result of

pruning throughout decades. Simplifying the crown

representation results in a reduction in accuracy, but

avoids tedious and time consuming measurements.

With the approach here presented, we account only

for differences caused by the track of the sun and

differences in the Cri (higher in S and W sides), but

possible systematic differences in foliage distribution

are not included.

It is also necessary to point out that Cw tends to a

maximum in biggest trees and that the BA–Cw

relationship worsens with the BA increase (hetero-

scedasticity; Fig. 6a). Hence, if Cw and Cri must be

estimated from DBH or BA, the accuracy of the result

decrease, especially for old stands. Moreover, to have

good simulations for the whole cycle from tree

plantation until tree clear-cut or death, a reliable

growth curve must be obtained for the specific

condition of the site including stand structure and

management.

With these limitations in mind, it is possible to

simulate scenarios in order to optimize the scheme of

tree plantation (density and arrangement) and sub-

sequent thinning as a function of the desired land use

(e.g., silvopastoral system). For instance, the equation

relating transmitted radiation at plot scale (I) with

tree age (A) and tree density (Dens) has been

estimated by means of a multiple regression:

I ¼ 0:14 � Dens0:64A0:62 R2 ¼ 92:89%; n ¼ 30� �

From this equation the optimum tree density at

different ages of a tree plantation can be determined

in order to maintain a specific level of light

availability for the understorey. Based in the thresh-

old of 75% of transmittance above mentioned, an

example of the determination of the tree density as a

function of age is given in Fig. 10.

The application of the light equations to already

existing stands could also be promising due to the

high-quality orthoimages presently available which

allow us to geo-reference and measure canopy width

of single trees in dehesas. In this sense, the imple-

mentation of Eqs. 1 and 2 in a common GIS could be

used to get a rapid estimation of the light distribution

in existing dehesas. A better knowledge of the light

requirements of the common understorey of dehesas

(native grasses and annual crops) and the daily and

seasonal curves of PFD would reinforce the applica-

bility of the results presented here. Light equations

and a better knowledge of light requirements for

pasture together can constitute a useful tool for the

dehesa manager to optimize system productivity.

Although the equations here presented are only

applicable for Iberian dehesas based on evergreen

oaks at low tree density, they could be also valid for

other open woodlands with evergreen oaks with

similar shape. Moreover, with this approach, simple

models of light distribution can be developed for

different agroforestry systems from a limited number

of fish eye pictures taken around single trees.

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100

Age

Tre

e / h

a

25 % light intercepted = 0,14 x Dens^(0,64) x Age^(0,62)

Fig. 10 Curve relating tree density with the age of the

plantation under the condition of 25% of light intercepted

242 Agroforest Syst (2008) 73:233–244

123

Conclusions

Transmittance in mature Q. ilex dehesa stands is very

high at stand level. Values below 75% of transmit-

tance, what could compromise the desired PDF

threshold of 600 lmol m-2 s-1, were found in the

15% of the stand surface. This surface is reduced to

5% of the surface with common pruning practice.

Tree density could be slightly increased to 40 mature

trees ha-1 without intercepting more than 25% of the

light at the plot scale.

Nevertheless, this hypothetical threshold used here

needs to be studied for specific native grasses and

common crops to calculate the optimum tree density.

Moreover, a reliable estimate of the optimum tree

density throughout the whole cycle depends also on

the understanding of the tree-understorey competition

for belowground resources. In this sense, Moreno and

Cubera (2008) have shown that tree-to-tree compe-

tition for soil water become evident in semiarid Q.

ilex dehesas with more than 30 mature trees ha-1.

Here, we have applied a simple model which could

be useful to design a plantation scheme including

alley orientation and tree arrangement, and to

estimate the optimum tree density throughout the

whole cycle of the Mediterranean evergreen oak open

woodlands. The model is sensitive to the performance

of the growth curve, which is dependent on the

growing conditions and management, and to the

allometric relationships, which are dependent on

pruning history. With this in mind, this model can

become a user-friendly tool for agroforesters to

manage systems based in similarly shaped evergreen

oak trees in Mediterranean latitudes, and can help to

characterize light distribution in studies of the

evergreen oak effects on understorey yield.

Acknowledgements This study was sponsored by the E.U.

(SAFE project, QLX-2001-0560), the Spanish government

(MICASA project, AGL-2001-0850) and the regional

government of Extremadura (CASA project, 2PR02C012).

References

Bellot J, Maestre FT, Hernandez N (2004) Spatio-temporal

dynamics of chlorophyll fluorescence in a semi-arid

Mediterranean shrubland. J Arid Environ 58:295–308

Bellow JG, Nair PKR (2003) Comparing common methods for

assessing understory light availability in shaded-perennial

agroforestry systems. Agric For Meteorol 114:197–211

Dıaz M, Campos P, Pulido FJ (1997) The Spanish dehesas: a

diversity in land-use and wildlife. In: Pain DJ, Pienkowski

MW (eds) Farming and birds in Europe: the common

agricultural policy and its implications for bird conser-

vation. Academic press, Cambridge, pp 178–209

Dupraz C, Fournier C, Balvay Y, Dauzat M, Pesteur S, Simorte

V (1999) Influence de quatre annes de culture intercalaire

de ble de colza sur la croissance de noyers hybrides en

agroforesterie. In: Bois et Forets des Agriculteurs. Actes

du colloque de Clermont-Ferrand des 20 et 21 October

1999. Cemagref Editions, France, pp 95–114

Eichhorn MP, Paris P, Herzog F, Incoll LD, Liagre F,

Mantzanas K, Mayus M, Moreno G, Papanastasis VP,

Pilbeam DJ, Pisanelli A, Dupraz C (2004) Silvoarable

systems in Europe—past, present and future prospects.

Agroforest Syst 67:29–50

Etienne M (2005) Silvopastoral management in temperate and

mediterranean areas. Stakes, practices and socio-eco-

nomic contraints. In: Mosquera-Losada MR, Riguero-

Rodriguez A, McAdam J (eds) Silvopastoralism and

sustainable land management. CAB International, Wal-

lingford, pp 299–311

Frazer GW, Canham CD (1999) GLA: gap light analyzer,

Copyright � 1999: Simon Frazer University, Burnaby,

British Columbia, and the Institute of Ecosystem Studies,

Millbrook, New York, USA

Gallardo A (2003) Effect of tree canopy on the spatial distri-

bution of soil nutrients in a Mediterranean dehesa.

Pedobiologia 47:117–125

Golden Software I. (1999) SURFER: 2D and 3D surface

modelling package, Copyright � 1993–1999: Golden

software, Inc. Colorado, USA

Gordon AM, Newman SM (1997) Temperate agroforestry

systems. CAB International, Oxon, p 288

Joffre R (1987) Contraites du milieu et reponses de la vege-

tation herbacee dans les dehesas de la Sierra Norte

(Andalouisie, Espagne). These doctorat (unpublished),

CNRS-CEPE, Montpellier, France, 201 pp

Jose S, Gillespie AR, Pallardy SG (2004) Interspecific inter-

actions in temperate agroforestry. Agroforest Syst 61:

237–255

Knowles RL, Horvath GC, Carter MA, Hawke MF (1999)

Developing a canopy closure model to predict overstorey/

understorey relationships in Pinus radiata silvopastoral

systems. Agroforest Syst 43:109–119

Larcher W (1995) Physiological plant ecology. Springer-

Verlag, Berlin

Ludwig F, de Kroon H, Berendse F, Prins HHT (2004) The

influence of savanna trees on nutrient, water and light

availability and the understorey vegetation. Plant Ecol

170:93–105

Maranon T (1986) Plant species richness and canopy effect in

the savanna-like ‘‘dehesa’’ of SW-Spain. Ecol Mediterr

12:131–141

McPherson GR (1997) Ecology and management of north

American Savannas. University of Arizona Press,

Tucson

Miah MG, Garrity DP, Aragon ML (1995) Ligth availability to

the understorey annual crops in an agroforestry system.

In: Sinoquet H, Cruz P (eds) Ecophysiology of tropical

intercropping systems. INRA, Paris, pp 99–107

Agroforest Syst (2008) 73:233–244 243

123

Miguel E, Pointereau P, Steiner C (2000) Los arboles en el

espacio agrario. Importancia hidrologica y ecologica.

Banco Santander-Central Hispano, Madrid, p 93

Moreno G (2008) Response of understorey forage to multiple

tree effects in Iberian dehesas. Agric Ecosyst Environ

123:239–244

Moreno G, Cubera E (2008) Impact of stand density on water

status and leaf gas exchange in Quercus ilex. For Ecol

Manage 254:74–84

Moreno G, Obrador J, Garcıa A (2007a) Impact of evergreen

oaks on soil fertility and crop production in intercropped

dehesas. Agr Ecosyst Environ 119:270–280

Moreno G, Obrador JJ, Garcıa E, Cubera E, Montero MJ,

Pulido F, Dupraz C (2007b) Driving competitive and

facilitative interactions in Oak Dehesas through manage-

ment practices. Agroforest Syst 70:25–40

Nobel PS (2005) Physiochemical and environmental plant

physiology, 3rd edn. Elsevier, Academic Press, Burlington

Ong CK, Black CR, Marshall FM, Corlett JE (1996) Principles

of resources capture and utilization of light and water.

In: Ong CK, Huxley P (eds) Tree-crop interactions a

physiological approach. CAB International, Wallingford,

pp 73–158

Perez-Corona ME, Garcıa-Ciudad A, Garcıa-Criado B, Vaz-

quez-Aldana B (1995) Patterns of aboveground herbage

production and nutritional quality structure on semiarid

grasslands. Commun Soil Sci Plant Anal 26:1323–1341

Plieninger T, Pulido JF, Konold W (2003) Effects of land-use

history on size structure of Holm oak stands in Spanish

dehesas: implications for conservation and restoration.

Environ Conserv 30:61–70

Puerto A, Rico M (1988) Influence of tree canopy (Quercusrotundifolia Lam. and Quercus pyrenaica Willd.) on field

succession in marginal areas of Central-Western Spain.

Acta Oecol-Oecol Plant 9:337–358

Puerto A, Rico M (1989) Influence of tree canopy (Quercusrotundifolia Lam.) on content in surface soil water in

Mediterranean grasslands. Ecology (CSSR) 8:225–238

Puerto A, Rico M, Matıas MD, Garcıa JD (1990) Variation in

structure and diversity in Mediterranean grasslands related

to trophic status and grazing intensity. J Veg Sci 1:445–452

Scholes RJ, Archer SR (1997) Tree-grass interactions in sav-

annas. Ann Rev Ecol Syst 28:517–544

Sibbald AR, Sinclair FL (1990) A review of agroforestry

research in progress in the U.K. Agrof Abst 3(4):149–164

Sinoquet H, Rakocevic M, Varlet-Grancher C (2000) Compari-

son of models for daily light partitioning in multispecies

canopies. Agric Forest Meteorol 101:251–263

Stevenson AC, Harrison RJ (1992) Ancient forests in Spain: a

model for land-use and dry forest management in south-

west Spain from 4000 BC to 1900 AD. Proc Prehist Soc 58:

227–247

Thevathasan NV, Gordon AM (2004) Ecology of tree intercrop-

ping systems in the North temperate region: experiences

from Southern Ontario, Canada. Agroforest Syst 61:257–268

Trichon V, Walter JN, Laumonier Y (1998) Identifying spatial

patterns in the tropical rain forest structure using hemi-

spherical photographs. Plant Ecol 137:227–244

244 Agroforest Syst (2008) 73:233–244

123