Growth Responses of Trees to Canopy Gaps in Mt. Jumbong in Central Korea

Do-Soon Cho

Department of Environmental Sciences

The Catholic University of Korea

Puchon, Kyonggi-do, 420-743, Korea

Phone: 82-32-340-3357

Fax: 82-32-340-3765

E-mail: dscho@www.cuk.ac.kr

Key words: disturbance, canopy gap, crown architecture,

deciduous hardwood forest, height growth


ABSTRACT

Growth responses and changes in morphological characteristics of trees to canopy gaps were compared among species and between gaps and non-gaps in Mt. Jumbong which lies in the core area of the Mt. Sorak Biosphere Reserve in Kangwondo Province, Korea.

Morphological sensitivity to canopy gap openings was determined by calculating crown asymmetry index and crown plasticity index based on the maps of tree crowns of gap-bordering canopy trees. Differences in asymmetry index among species were not significant. Syringa wolfii has the highest asymmetry index and Fraxinus mandshurica has the lowest. Plasticity index was highest in Cornus controversa, and lowest in Sorbus alnifolia. Asymmetry index and plasticity index did not seem to be closely related.

The lateral expansion growth rate was different in gaps and non-gaps within species; branches toward gaps showed higher growth rate than those toward non-gaps. Height growth of saplings was much higher in gaps than in non-gaps. Bifurcation ratio of branches was higher in saplings growing in gaps than gap-bordering canopy trees, and the difference of bifurcation ratios between gaps and non-gaps was higher in saplings than canopy trees, indicating that saplings were more sensitive to the availability of light than canopy trees.

INTRODUCTION

Canopy gaps formed by disturbances increase the availability of lights (Canham 1988), water and nutrients (Vitousek and Denslow 1986), but generally light is the most important limiting factor in forests. Reception of light is affected by various factors such as branch angle, bifurcation ratio, leaf size and shape, phyllotaxy and nodal length, leaf arrangement, inclination of stem, crown architecture, etc (Horn 1971, Kuuluvainen 1992). However, the ability to respond to gap openings differs among species (Cho 1992, Cho and Boerner 1995). Light absorption and utilization determined by the spatial architecture affect the process of succession in forests. Plants in gaps show some plasticity in the morphology of branches (Voesenek and Blom 1996) in addition to the physiological adaptation to light environment (Bazzaz and Carlson 1982).

Most gap-bordering trees can grow into gaps due to competition with neighboring trees (Trimble and Tryon 1966, Canham 1988), resulting in asymmetric canopy structure (Young and Hubbell 1991). Such growth responses to gaps in the canopy layer together with those in the understory layer can be used for the prediction of future composition of forests and of successional trends (Poulson and Platt 1989).

To examine the ability of tree species in utilizing gap environment, 1) morphological asymmetry and plasticity of tree crown of gap-bordering canopy trees, 2) growth responses of canopy trees to gaps, and 3) growth responses of saplings to gaps were determined and compared between gaps and non-gaps and among different species.

METHODS

(1) Study site:

This study was conducted in Mt. Jumbong (128°22' E and 38°00' N) located in central Korea in 1996-1997. It is a cool temperate hardwood forest at the elevation of about 1,000m.

(2) Crown Asymmetry Index (AsI) and Crown Plasticity Index (PI):

To compare the ability of capturing light in the gaps, I calculated the crown asymmetry index (Young and Hubbell 1991) and crown plasticity index for the canopy tree species found in the gaps studied.

AsI = A/T, where A is the area of crown to the gap side and T is the total crown area, and PI = T/MC, where MC is the area of maximum circle within the crown. To calculate these indices, crown maps were drawn for each canopy tree bordering gaps by marking the crown position of 8 azimuth directions on the graph paper. All the gap-bordering canopy trees were marked with aluminum tags for the determination of future changes.

(3) Growth and morphological characteristics of gap-bordering canopy trees:

Lateral expansion growth, angle between branches, and mean leaf area were measured on branches growing toward gap and toward non-gap in each canopy tree for the 5 abundant species selected: Acer pseudosieboldianum, Acer mono, Quercus mongolica, Fraxinus rhynchophylla and Cornus controversa. Branches in the canopy were cut with a high-limb chainsaw. Bifurcation ratio (BR) was calculated from the Motomura's formula: BR = (N - Ns)/(N - N1), where N is the total number of branches, Ns is the number of branches of the highest order in the sample, and N1 is the number of branches of the first order (Steingraeber et al. 1979).

(4) Growth and morphological characteristics of saplings:

Seven species of trees were selected for comparison of the response to canopy gaps: Acer pseudosieboldianum, Acer mono, Quercus mongolica, Fraxinus rhynchophylla, Cornus controversa, Carpinus cordata and Ulmus laciniata. Five individual saplings were examined for each species in gaps and non-gaps, respectively.

RESULTS

(1) Forest composition

This study was conducted in Mt. Jumbong which is located within the Mt. Sorak Biosphere Reserve in central Korea. It is a deciduous hardwood forest dominated by Quercus mongolica, and Acer pseudosieboldianum, but many other species such as Acer mono, Carpinus cordata, Fraxinus rhynchophylla occur together at the valley bottom (Table 1).

Table 1. Tree species distribution at the study site in Mt. Jumbong. Species

are listed in the order of importance values. Numbers are

in percentages.

Species

acronym

Species

Relative Density

Relative

Cover

Relative

Frequency

Importance

Value

QUMO

Quercus mongolica

10.37

42.47

10.69

21.18

ACPS

Acer pseudosieboldianum

25.49

6.00

14.16

15.22

CACO

Carpinus cordata

13.39

5.28

10.98

9.88

TIAR

Tilia amurensis

10.26

4.42

8.67

7.78

ACMO

Acer mono

6.26

6.35

7.23

6.61

FRMA

Fraxinus mandshurica

4.54

6.86

4.62

5.34

KAPI

Kalopanax pictus

3.13

7.93

3.76

4.94

FRRH

Fraxinus rhynchophylla

4.21

5.01

4.62

4.62

ULLA

Ulmus laciniata

3.56

3.52

4.91

4.00

SOAL

Sorbus alnifolia

3.24

1.17

4.91

3.11

ACTR

Acer triflorum

2.48

2.18

3.18

2.62

ABHO

Abies holophylla

1.94

1.55

3.18

2.22

RHSC

Rhododendron schlippenbachii

2.92

0.04

2.89

1.95

COCO

Cornus controversa

0.97

1.68

1.73

1.46

PRSA

Prunus sargentii

0.86

1.46

2.02

1.45

EUSA

Euonymus sachalinensis

1.30

0.06

2.89

1.41

ACMA

Acer mandshuricum

0.76

0.86

1.16

0.92

PIKO

Pinus koraiensis

0.65

1.18

0.87

0.90

SYWO

Syringa wolfii

0.76

0.24

1.45

0.81

POMA

Populus maximowiczii

0.43

0.37

0.87

0.56

ACAR

Actinidia arguta

0.65

0.04

0.87

0.52

ACTS

Acer tschonoskii

0.32

0.03

0.87

0.41

BECO

Betula costata

0.43

0.20

0.58

0.40

PIDE

Pinus densiflora

0.11

0.41

0.29

0.27

SOCO

Sorbus commixta

0.22

0.01

0.58

0.27

ULDA

Ulmus davidiana

0.11

0.11

0.29

0.17

SAHU

Salix hulteni

0.11

0.05

0.29

0.15

CALA

Carpinus laxiflora

0.11

0.04

0.29

0.15

MASI

Magnolia sieboldii

0.11

0.04

0.29

0.15

AREL

Aralia elata

0.11

0.03

0.29

0.14

SYCH

Symplocos chinensis

0.11

0.00

0.29

0.13

JUMA

Juglans mandshurica

0.11

0.00

0.29

0.13



(2) Crown architecture of gap-bordering canopy trees

Syringa wolfii showed the highest crown asymmetry index (AsI) among the gap-bordering canopy tree species studied, and its crown plasticity index (PI) was also very high. Another species with high AsI was Tilia amurensis. In contrast, Fraxinus mandshurica and Cornus controversa had low AsI (Table 2). PI was in the range between 1.47 and 1.89. Cornus controversa, Ulmus laciniata and Betula costata had higher PI and Sorbus alniflolia, Fraxinus mandshurica, Acer pseudosieboldianum, and Tilia amurensis showed lower PI, although the difference among species was not significant (Table 2). Area of crown, tree height and diameter of breast height (DBH) were higher in Quercus mongolica and Fraxinus mandshurica and lower in Acer pseudosieboldianum and Syringa wolfii (Table 2).

(3) Growth and morphological characteristics of canopy trees:

Rates of lateral expansion growth of the gap-bordering canopy trees were different between branches growing toward gaps and those facing non-gaps, and were different among species, too. Higher lateral growth rates in gaps were found in Fraxinus mandshurica and Quercus mongolica, but the latter showed the biggest difference in lateral growth rates between branches in gaps and non-gaps (Table 3).

Mean angle between adjacent small branches in gap-bordering canopy trees was generally higher for branches facing non-gap than gap, but the difference was small and was not significant. Mean area of leaves was also bigger in branches facing non-gaps than gaps, but the difference was not significant either. Bifurcation ratios for branches of gap-bordering canopy trees were not significant between gaps and non-gaps (Table 3).

(4) Growth and morphological characteristics of saplings:

Rates of height growth of saplings were different among the 7 species examined: Cornus controversa and Fraxinus rhynchophylla showed highest growth rates in gaps and Cornus controversa in non-gaps (Table 3).

Table 2. Comparison of crown architecture among gap-bordering canopy trees in Mt. Jumbong. Same letters within each of the morphological characters are not significantly different (Tukey's test, p>0.05)

Species

Asymmetry

Index (AsI)

Plasticity Index (PI)

Crown Area

(m2/tree)

Height

(m)

DBH

(cm)

SYWO(n=4)

0.93

(±0.08)

1.72

(±0.48)

11.75

(±6.9)

11.3

(±1.71)

21.0

(±6.35)

TIAR(n=4)

0.80

(±0.10)

1.53

(±0.30)

29.5

(±20.34)

11.95

(±2.74)

34.43

(±20.04)

FRRH(n=18)

0.75

(±0.25)

1.67

(±0.37)

17.83

(±15.84)

12.05

(±2.61)

18.96

(±6.82)

ACMO(n=33)

0.75

(±0.25)

1.67

(±0.36)

22.21

(±13.15)

11.6

(±3.11)

26.42

(±11.78)

SOAL(n=4)

0.72

(±0.16)

1.47

(±0.29)

21.0

(±9.42)

14.65

(±3.25)

23.79

(±3.89)

ACPS(n=18)

0.69

(±0.24)

1.53

(±0.34)

16.47

(±7.06)

8.14

(±1.26)

15.13

(±4.81)

ULLA(n=19)

0.68

(±0.18)

1.80

(±0.28)

23.21

(±13.95)

12.53

(±4.74)

26.72

(±18.73)

CACO(n=11)

0.68

(±0.19)

1.56

(±0.36)

36.0

(±7.81)

9.62

(±1.42)

26.71

(±6.5)

ACTR(n=5)

0.67

(±0.13)

1.65

(±0.42)

28.33

(±15.28)

11.59

(±0.97)

28.78

(±1.79)

BECO(n=7)

0.66

(±0.25)

1.79

(±0.51)

26.71

(±38.46)

16.86

(±5.48)

23.63

(±14.88)

QUMO(n=21)

0.66

(±0.27)

1.65

(±0.31)

37.19

(±23.69)

13.31

(±1.88)

38.69

(±17.11)

ACMA(n=7)

0.65

(±0.24)

1.68

(±0.32)

22.43

(±7.02)

10.81

(±1.53)

24.0

(±7.95)

COCO(n=12)

0.62

(±0.19)

1.89

(±0.71)

28.67

(±12.13)

11.73

(±2.15)

27.07

(±8.01)

FRMA(n=15)

0.61

(±0.21)

1.52

(±0.29)

39.36

(±27.68)

14.41

(±3.23)

32.8

(±15.56)

Mean

0.70

(±0.22)

1.66

(±0.38)

25.36

(±18.43)

11.81

(±3.43)

25.94

(±13.54)



Table 3. Comparison of growth responses among species of canopy trees and saplings. Same letters within each growth response are not significantly different (Tukey's test, p>0.05).

Species

Growth

(cm/yr)

Branch angle

(degree)

Leaf area

(cm2)

Bifurcation

ratio

Canopy trees





FRRH

13.13a

54.70b

198.18a

4.90a

QUMO

10.78ab

60.08ab

89.10b

3.49b

COCO

7.64bc

67.44a

60.77c

3.24bc

ACPS

6.66c

57.42b

43.04cd

2.84c

ACMO

4.34c

53.89b

31.98d

3.75b






Saplings





COCO

25.79a

55.28c

39.35c

4.27cd

FRRH

18.45ab

57.61c

222.87a

7.28ab

ULLA

14.99ab

64.21ab

47.71c

8.87a

ACMO

14.64bcd

54.93c

38.85c

3.49c

QUMO

12.10bcd

59.00bc

73.42b

4.89bc

CACO

9.90cd

64.69a

33.49c

7.08b

ACPS

6.46d

56.93c

26.75c

3.00d


Saplings of most species showed significantly higher expansion growth rates in gaps than in non-gaps. Branch angle of saplings was higher in non-gaps than in gaps for all species, although the difference was not significant. In contrast, mean leaf area of saplings was not different between gaps and non-gaps except for Acer pseudosieboldianum. Bifurcation ratio of saplings was generally higher in gaps, especially for Fraxinus rhynchophylla and Quercus mongolica.

DISCUSSION

Plant species can change their canopy architecture for better reception of lights. Crown asymmetry index (AsI) is related to the ability of a tree species to bend its main bole toward the direction of light or gap, and crown plasticity index (PI) is related to the ability of lateral expansion growth of major branches in the canopy layer toward gap. In Mt. Jumbong, AsI was negatively correlated with DBH and crown area, while PI was negatively correlated with tree height (Table 4). It means that smaller trees should bend their bole toward the gap to use sunlight more efficiently while big trees are less affected by the formation of gaps, and that shorter trees should push their branches toward the gap because growing high will take much time in competition with taller trees. However, AsI and PI were not much different among species, and they were not closely related to each other (Table 4). AsI was higher for Tilia amurensis which regenerates from sprouts and whose boles are usually leaning. In contrast, Cornus controversa showed higher PI because its branches are actively expanding and its main stem is growing more or less straight. Species with higher AsI are expected to respond to gaps more strongly while species with higher PI are expected to respond to gaps more quickly.

Table 4. Correlation coefficients between morphological characters: height, DBH, crown area, asymmetry index (AsI), and plasticity index (PI) of gap-bordering trees. *: p<0.05. **: p<0.01, ***: p<0.001


Height

DBH

Crown area

Asymmetry

Index

Plasticity

Index

Height

-





DBH

0.58***

-




Crown area

0.50***

0.82***

-



AsI

-0.14

-0.22**

-1.7*

-


PI

-0.16*

-0.065

-0.14

0.072

-



The rate of lateral expansion growth of gap-bordering canopy trees were different among species, and different between branches facing gaps and non-gaps (Table 3). Shade intolerant species such as Quercus and Fraxinus showed higher growth rate of branches in gaps in this study, and lateral expansion growth was correlated with leaf area and bifurcation ratio (Table 5), meaning shade intolerant trees can produce more and longer branches in gaps to capture sunlight more efficiently.

Height growth of saplings was much higher in gaps than in non-gaps, and was higher than lateral expansion growth of gap-bordering canopy trees. In addition, saplings in gaps showed higher bifurcation ratio and lower branch angle than those in non-gaps as suggested by Whitney (1976) and Horn (1971) (Table 6). Even shade tolerant species such as Acer pseudosieboldianum and Acer mono showed significant growth response in gaps. These results suggest that saplings are more sensitive and more quickly reacting to gaps than canopy trees.

Table 5. Correlation coefficients between growth responses: growth, branch angle, leaf area, and bifurcation ratio of canopy trees and saplings. *: p<0.05. **: p<0.01, ***: p<0.001


Growth

Branch angle

Leaf area

Bifurcation

ratio

Canopy trees





Growth rate

-




Branch angle

-0.16

-



Leaf area

0.39**

-0.23

-


Bifurcation ratio

0.44**

0.28

-0.06

-






Saplings





Growth rate

-




Branch angle

-0.02

-



Leaf area

0.27*

-0.24

-


Bifurcation ratio

0.14

0.003

-0.57***

-



Table 6. Comparison of growth responses of canopy trees and saplings between gaps and non-gaps. Same letters within each growth response are not significantly different (Tukey's test: p>0.05).


Growth rate

(cm/yr)

Branch angle

(degree)

Leaf area

(cm2)

Bifurcation

ratio

Canopy trees





Gap

10.40a

57.14a

84.16a

3.76a

Non-gap

6.63b

60.28a

92.96a

3.53a






Saplings





Gap

20.88a

56.33a

71.20a

6.48a

Non-gap

7.42b

60.81b

69.18a

4.81b




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