RESEARCH ARTICLE

The Janzen-Connell effect on the population dynamics of aFagus engleriana- Cyclobalanopsis oxyodon community in a

subtropical zone of China

Mi ZHANG (✉), Gaoming XIONG, Zhigang CHEN, Zongqiang XIE

Laboratory of Quantitative Vegetation Ecology, Institute of Botany, the Chinese Academy of Sciences, Beijing 100093, China

© Higher Education Press and Springer-Verlag 2009

Abstract The Janzen-Connell (J-C) hypothesis providesa mechanism explaining the high species diversity intropical rainforests. It postulates that predation could causegreater mortality on seeds and seedlings near their parentaltrees. In this study, we tested the hypothesis in asubtropical zone, a mixed evergreen-deciduous broad-leaved forest dominated by the Fagus engleriana andCyclobalanopsis oxyodon. The study area was in theShennongjia region, a key area of biodiversity conserva-tion in both China and the world. The recruitmentprobability index was used to detect the J-C effect onnine species of the community, which were more than 50individuals. Six large adults of each species were selected,and the numbers of saplings and adults were counted at thedistance intervals of 0–5, 5–10, 10–15, 15–20, and 20–25m from each focal tree. Two species in saplings stageand six in adult stage supported the J-C hypothesis, buttheir χ2 was not significant. Three species, the F. engleri-ana, Rhododendron hypoglaucum, and Toona sinensis,showed a strong Hubbell pattern in the adult stage.Because of these results, we reject the J-C hypothesis andconclude that species could recruit near the conspecifictrees in subtropical forest. The reasons why the J-Chypothesis fails to explain the species diversity in thiscommunity are the shortage of seed-consuming agents ofsubtropical forest and the influence of microsite topo-graphic variation.

Keywords density dependence, distance dependence,recruitment, Shennongjia, subtropical forest

1 Introduction

The mechanisms that promote plant coexistence and highspecies diversity is a central question in ecological research(Penfold and Lamb, 1999; Peters, 2003). Many hypotheseshave been put forth to explain biological diversity, such asthe density- and distance-dependent mortality hypothesis(Janzen, 1970; Connell, 1971), regeneration niche theory(Grubb, 1977), intermediate disturbance hypothesis(Grime, 1973; Connell, 1978), and resource heterogeneityhypothesis (Tilman, 1994). The Janzen-Connell (J-C)hypothesis is one of the most important ideas put forth toaccount for the exceptional plant diversity of tropicalforests (Wright, 2002; Nathan and Casagrandi, 2004; Kwitet al., 2004). Janzen (1970) summarized his hypothesiswith a simple graphical model postulating that herbivorousanimals, insects, and pathogens could regulate communitycomposition by inflicting greater mortality on seeds andseedlings near their parental trees, where densities tendedto be higher due to parent-centered dispersal patterns. Thefundamental assumption of the J-C hypothesis includesthat (1) seed fall probability (or density) declines awayfrom the maternal tree, (2) seed and sapling survivalprobability increases away from the maternal tree (becausespecialist enemy density declines away from it), and(3) enemies (seed predators, pathogens, etc.) are species-specific and are the primary agents of progeny mortality(Clark and Clare, 1984). Connell (1971) independentlyproposed a similar mechanism. This theory was generallycalled the Janzen-Connell (J-C) hypothesis. Because therecruitment of seeds to adulthood should be greater forcommon species than for uncommon species, this hypoth-esis has also been called “compensatory death hypothesis”and “escape hypothesis” (Connell, 1984).Hubbell (1980) revised this hypothesis. He argued that

seed densities are extremely high near adults and decrease

Received December 15, 2008; accepted March 10, 2009

E-mail: zhangmi76@126.com

Front. Biol. China 2009, 4(4): 513–522DOI 10.1007/s11515-009-0046-1

with distance at a rate faster than that at which survivalincreases, which would cause a monotonic decline inrecruit density (Hubbell pattern). McCanny (1985) cate-gorized the J-C hypothesis into five patterns. In addition tothe J-C pattern and Hubbell pattern, there are the exact-compensation pattern, invariant survival pattern, andMcCanny pattern (Fig. 1). In all cases, seed densityrapidly declines as distance from the parental plantsincreases. The J-C pattern, Hubbell pattern, and exact-compensation pattern all encompass increased survival ratewith distance. The McCanny pattern was supported by thefact that the survival decreases with distance (Fig. 1e). Theinvariant survival (Fig. 1d) represented the transitionbetween the Hubbell and McCanny patterns (Nathan andCasagrandi, 2004). Nathan and Casagrandi (2004) empha-sized that the outcome of the J-C pattern requires theoperation of enemies to be more limited in space than theseed dispersal to be. If enemies are as wide-ranging asseeds or more wide-ranging than seeds, declining patternsof sapling survival with distance should be observed.To evaluate the J-C pattern, many ecologists have tested

this hypothesis. Most empirical studies were conducted intropical forests (Augspurger, 1984; Schupp, 1988; Schupp,1992; Webb and Peart, 1999; Harms et al., 2000; Blundelland Peart, 2004), while a few studies were in other places,such as subtropical or temperate communities (McCannyand Cavers, 1987; Hiura and Fujiwara, 1999; Packer and

Clay, 2000). The methods to test this hypothesis weregenerally in the population level (Augspurger, 1983;Condit et al., 1994; Cintra et al, 1997) and communitylevel (Condit et al., 1992; Webb and Peart, 1999). Thereare also some studies using matrix or simulated model totest the J-C hypothesis (McCanny, 1985; Hiura andFujiwara, 1999; Nathan and Casagrandi, 2004). Inaddition, this hypothesis has also been directly tested byexamining the spatial patterns of forest tree species (Conditet al., 1992). Although the J-C effect has been tested inmany forests, its role in structuring community still doesnot have a consistent trend. It may be important for somespecies during certain life stages, but its importance inmaintaining species diversity has yet to be proven (Hiuraand Fujiwara, 1999; Matos et al., 1999; Hubbell et al.,2001; Hyatt et al., 2003).In this paper, we tested the J-C hypothesis in a mixed

evergreen-deciduous broadleaved community, which is atypical middle mountain forest in a subtropical zone ofChina. It is the first time that the J-C hypothesis was testedin this kind of community. We compared the differences ofthe J-C effect on tropical and this mixed subtropical forest.In this area, huge mountain covers the entire field, andsteep slopes influence species distribution and their seedsdispersal. We can also test whether the J-C effect works inmountain areas. To detect the J-C effect in population andcommunity levels, all the important species were selected

Fig. 1 McCanny (1985) summarized population recruitment pattern. (a) J-C pattern; (b) exact compensation; (c) Hubbell pattern;(d) invariant survival; (e) McCanny pattern.

514 Front. Biol. China 2009, 4(4): 513–522

and separated into sapling and adult stage in thecommunity. It included the two dominate species andother seven species, occupying 70% of the community interms of significance. The recruitment patterns weredescribed by the Rid curve and classified to the J-C pattern,Hubbell pattern, etc. If, as the J-C hypothesis states,recruitment of conspecific trees are reduced in closeproximity to adults, then the number of conspecific treesnear the focal individuals should be less than that of otherspecies.

2 Materials and methods

2.1 Study site and experimental plot

The study was conducted in an evergreen broadleavedforest in the Longmenhe National Forest Park in theShennongjia area, western Hubei of central China, one ofthe most important areas of biodiversity conservation inboth China and the world (Ying, 2001). The region lies inthe north subtropical and warm temperate climatictransition zone. The vegetation distributes across anelevation gradient that ranges from evergreen broadleavedforests at low elevations to mixed evergreen deciduousbroadleaved forests, mixed conifer and broadleavedforests, coniferous forests, and a Rhododendron-Sinarundinaria nitida shrub community at the highestelevation of 3105 m (Chen and Wang, 1999).In 2001, a 120 m� 80 m (9–600 m2) permanent plot was

established on the south side of the DashengnongjiaMountain (31°19′4″ N, 110°29′44″ E) at an elevation of1750 m (Zhang et al., 2004). The average slope of the plotis 49.5°, the mean annual precipitation is 977 mm, and themean annual temperature is 12°C. The soil is characterizedby a mountain brown soil with an average pH of 5.17 (Wu,1980; Chen and Wang, 1999).This approximately 100-year-old forest community is

dominated by the Fagus engleriana and Cyclobalanopsis

oxyodon. The F. engleriana (Beech) is a multistemmeddeciduous canopy tree (Hong and An, 1993) and is onlyfound in China. The C. oxyodon is a subcanopy evergreentree species. The forest canopy can be classified into threelayers: the F. engleriana and other deciduous tree speciesdominate the first layer, the second layer is mainlycomposed of the C. oxyodon, and the third layer isdominated by other evergreen tree species (Zhang et al.,2003).All stems greater than 1 cm in diameter at breast height

(DBH) were identified, tagged, mapped, and measured.There were 3000 trees and large shrubs in the plot, whichwere comprised of 22 families, 27 genura, and 46 species.In addition to the two dominant species, seven otherspecies with sufficient numbers of individuals (more than50) were included in the analysis. They were theRhododendron hypoglaucum, Lithocarpus glaber, Cornuskousa var. chinensis, Toona sinensis, Acer cappadocicum,Acer griseum, and Lindera obtusiloba (Table 1). The ninespecies contained 70% of all the individuals in thecommunity, and the two dominated species took up 40%(Table 1).

2.2 Seed production and dispersal

The J-C hypothesis suggested that the survival rate ofseedlings will be low where the densities of seeds are high.To test this hypothesis, we measured the relationshipbetween seed density and the number of conspecific trees.Eighteen seed traps were established in the half of thepermanent plots. The seeds of the two dominant species werecollected every 10–15 days from July 10 to November 30 in2002, and the total numbers of seeds of each species werecounted. The numbers of conspecific adults, saplings, andseedlings of the two main species were counted at thedistances of 2.5, 5, 7.5, and 10m from each seed trap(Fig. 2). A correlation index was calculated between the totalnumber of seeds and the number of conspecific seedlings,saplings, and adults at different distances.

Table 1 The characters of selected species

species importance value DBH/cm no.of individuals

life form

average maximal minimal

1 FAEN 16.86 11.89 27.50 1.0 77 first layer deciduous tree

2 CYOX 61.07 6.21 29.9 1.5 729 sublayer evergreen tree

3 RHHY 14.09 7.04 26.5 1.5 209 sublayer evergreen tree

4 LIGL 8.04 14.80 41.7 1.1 64 sublayer evergreen tree

5 COCH 7.39 8.03 16.0 1.1 82 sublayer deciduous tree

6 TOSI 6.01 14.71 34.0 1.2 49 first layer deciduous tree

7 ACCA 5.84 16.12 28.5 3.5 45 first layer deciduous tree

8 ACGR 5.52 16.01 36.0 1.5 39 first layer deciduous tree

9 LIOB 4.94 14.22 29.9 1.7 43 first layer deciduous tree

DBH: diameter at breast height; FAEN: Fagus englerian; CYOX: Cyclobalanopsis oxyodon; RHHY: Rhododendron hypoglaucum; LIGL: Lithocarpus glaber; COKO:Cornus kousa var. chinensis; TOSI: Toona sinensis; ACCA: Acer cappadocicum; ACGR: Acer griseum; LIOB: Lindera obtusiloba.

Mi ZHANG et al. The J-C effect on a Fagus engleriana-Cyclobalanopsis oxyodon community 515

2.3 Neighborhood index

The neighborhood index (W) was used to quantitate theeffect of neighborhood interference of different species anddifferent size classes on the selected sample trees. It wascalculated as

W ¼Xn

i¼1

�Ai

di,

where n is the total number of neighborhood trees, Ai is thesize of adult i, and di is the distance from the trunk base ofthe ith tree to the center of the sample tree (Weiner 1984).The index was calculated for the number of conspecificsaplings and adults, as well as for the number of otherspecies at four distance scales: 0–2.5, 2.5–5, 5–7.5, and7.5–10 m.

2.4 Recruitment probability

Six individuals of each of the nine species were selected asfocal trees, which were 15–41 cm in DBH (Table 1). Thenumber of saplings (1–8 cm in DBH) and adults (> 8 cmin DBH) of all the species were counted and normalizedwithin distance intervals of 0–5, 5–10, 10–15, 15–20, and20–25 m surrounding each of the focal trees.The distribution patterns of saplings and adults of the

species were detected by a normalized recruitmentprobability index (Condit et al., 1992), which estimatesthe probability of recruitment for the focal species andcompares it with the probability of recruitment for the

observed ones. It is defined as

Rid ¼ Sid=Eid ,

where Sid is the recruited number of species i in eachdistance class d from the focal adult. The total number ofrecruits of species i is ΣSid. Eid is the expected distributionof recruits of the focal species. It is defined as

Eid ¼ Aid

XSid=

XAid:

Aid is all recruits in the plot at different distances fromthe nearest adult of the focal species i.If Rid> 1 (Sid>Eid), recruits of the focal species are

overrepresented at distance d. That is, recruits of thefocal species are more abundant than what they havebeen if their distribution is identical to that of allrecruits combined. The observed recruits of the focaltree, Sid, were compared to the expected recruits, Eid,with a χ2 test.

3 Results

3.1 Seed production and dispersal patterns

The correlation analysis of seed production (the totalnumber of seeds collected in seed traps) with the number ofconspecific seedlings, saplings, and adults showed that thenumber of seeds was not significantly correlated withseedlings, saplings, or adults at any of the distances tested(Table 2).

Fig. 2 Placement of seed traps showing concentric circles where trees were enumerated by species and life stage. The symbols representadult (,) and sapling (△) trees of Cyclobalanopsis oxyodon.

516 Front. Biol. China 2009, 4(4): 513–522

3.2 Neighborhood index

The neighborhood index indicates the competitive influ-ence of different species on selected individuals and thedistance where the influence is greater. For saplings of theC. oxyodon, the neighborhood index ranged from thelowest value of 0.83 at 20–25 m to the highest value of7.80 at 0–5 m distance (Fig. 3). The index value of adultsranged from 7.09 to 8.66, but there was little differenceamong distance intervals. The value of neighborhoodindex for other species around the sample trees was greaterthan that around both saplings and adults of theC. oxyodon, indicating that other species had much greaterinfluence on the selected individuals than on theconspecific saplings or adults.

For the F. engleriana, the neighborhood index was lessthan one for both saplings and adults at all distances(Fig. 4). The influence of other species was much greaterthan conspecifics, especially at the 0–5 m distance interval.

3.3 Recruitment probability

The normalized numbers of recruits and adult trees wereillustrated for the two dominated trees in Figs. 3 and 4.Saplings of the C. oxyodon were most likely to occur at adistance of 5–10 m from the adult sample tree (Fig. 5), butdifferences between the distance intervals were notsignificant. The number of adult trees was highest within

5 m of the sample trees, and the combined number ofsaplings and adults was highest at the 0–5 m distanceinterval. The average number of the F. engleriana saplingsaround the sample tree ranged from 0.2 m–2 at distanceintervals of 0–5, 5–10, and 10–15 m to 0.46 m–2 at 15–20m (Fig. 6). The average number of adults ranged from0.36 m–2 to 0.63 m–2 with the highest value at 0–5 mdistance interval, indicating that conspecific adults coulddistribute in close proximity to the sample trees.We presented the recruitment probability index (Rid),

including the observed numbers (Sid), expected numbers(Eid), and their χ

2 test, of the recruits and adults of the nineselected species in Table 3. The Rid value was alsopresented in Fig. 7. The Rid value of the C. oxyodon waslarger than one in 0–5 m distance, suggesting that theobserved individuals were more than what was expected inthat distance interval. The tendency of the Rid curvedeclined with the distance from the conspecific focal trees.Compared with Fig. 1, the CYOX saplings belonged to theHubbell pattern. The Rid value of the F. engleriana,L. glaber, C. kousa var. chinensis, A. cappadocicum,A. griseum, and L. obtusiloba were less than one in anydistance intervals. It did not belong to any pattern thatMcCanny summarized (called no pattern in this paper).The Rid value of the R. hypogl and T. sinensis was high in15–20 m and 5–10 m distance intervals, respectively.

Table 2 Correlation coefficients between adult trees, saplings, and seedlings, and seeds for the two dominant forest species

species item 0–2.5 m 2.5–5 m 5–7.5 m 7.5–10 m

Cyclobalanopsis oxyodon seedlings and seeds – 0.08 – 0.33 – 0.18 – 0.04

saplings and seeds 0.27 – 0.15 0.06 – 0.10

adults and seeds – 0.24 0.34 0.27 0.07

Fagus engleriana all individual and seeds – 0.24 – 0.12 0.13 – 0.30

α0.05 = 0.444, d f = 18.

Fig. 3 The neighborhood index calculated for the cyclobalal-nopsis oxyodon

Fig. 4 The neighborhood index calculated for the Fagusengleriana

Mi ZHANG et al. The J-C effect on a Fagus engleriana-Cyclobalanopsis oxyodon community 517

Because the establishment curve had a peak in somedistance, it belonged to the J-C pattern.In the adult stage, the Rid of C. oxyodon increased with

distance from the focal trees. It indicated that the survival ratewas high in the place far from the conspecific trees. Therewere also some species with a better establishment in aparticular distance from the focal trees, such as L. glaber,C. kousa var. chinensis, A. cappadocicum, A. griseum, andL. obtusiloba. They all supported the J-C pattern. The Rid

curves of the F. engleriana, R. hypogl, and T. sinensis weredifferent from the above species. Their individuals alldecreased with the distance, which belonged to theHubbell pattern.

4 Discussion

Seed density was not related to the distribution of the twospecies, indicating that recruitment did not occur with thebest seed production. These results generally supported theJ-C hypothesis. The results of the neighborhood indexanalysis showed that nonconspecific individuals had abigger influence on the focal tree than conspecificindividuals did. In the community, two species supportedthe J-C hypothesis in saplings stage and six others did inadult stage, but their χ2 was not significant. Three species,the Fagus englerian, R. hypogl, and T. sinensis, showed astrong Hubbell pattern in the adult stage. The F. englerianaand T. sinensis were defoliate canopy trees, and the R.hypoglaucum was a sublayer evergreen species. As the J-Ceffect was not significant in the saplings and adults stage inthis community, we just can conclude that the J-Chypothesis works in seedling stage.

4.1 Seed production

The establishment of seedlings was related not only to seeddistributions but also to the annual fluctuation in seedproduction. In the current study, the seed traps weremonitored for two years. During this period, C. oxyodonhad no mature seeds but still had many seedlings in thesample plot. F. engleriana was monitored over one seedharvest year, but there were only three saplings of thisspecies in the plot. F. engleriana is considered to be adeeply shade-tolerant species (Cao and Peters, 1998), butits recruits (seedlings) still need light or an open canopy toestablish and survive. As a multistemmed species,F. engeriana also could be recruited from sprouts (He et al.,1999). Although there were few seedlings in the plots, theF. engeriana was able to maintain its dominant position inthe community.

4.2 Life stage influence

Many studies have indicated that the J-C effect had itsgreatest impact at the seedling or sapling stage. The J-Ceffect did not influence the two dominant trees at our studysite, which might be related to the characters of thesespecies and the forest community. The C. oxyodon is anevergreen species and, at this elevation, is a subcanopyspecies with a short life span. Neither the J-C effect nor theinterspecific competition reduced the density of individualsof this species near conspecific adult trees. It may suggestthat life stage was not sensitive to the J-C effect in thisforest, perhaps due to the difference between subtropicaland tropical forests.

4.3 Predation influence

The factors postulated to reduce the number of seedlings orsaplings near their parent trees were host pathogens or host

Fig. 6 The numbers of conspecific saplings, adults in the Fagusengleriana. E: saplings; F: adults; G: saplings and adults.

Fig. 5 The numbers of conspecific saplings, adults, and saplings,and adults in the Cyclobalanopsis oxyodon. A: saplings; B: adults;C: saplings and adults.

518 Front. Biol. China 2009, 4(4): 513–522

Tab

le3

Recruitm

entprob

ability

indexforsaplings

andadultsof

nine

tree

species

0–5m

5–10

m10–15

m15–20

m20–25

mpattern

S id

Eid

Rid

χ2S id

Eid

Rid

χ2S id

Eid

Rid

χ2S id

Eid

Rid

χ2S id

Eid

Rid

χ2

saplings

1CYOX

5.8

5.5

1.1

5.9

6.0

6.1

1.0

4.4

5.1

5.9

0.9

1.3

4.7

5.6

0.9

0.8

4.6

5.7

0.8

1.4

Hub

bellpattern

2FA

EN

0.3

0.9

0.4

3.5

0.1

1.0

0.1

4.7

0.1

0.9

0.1

4.5

0.4

0.9

0.4

2.4

0.2

0.7

0.2

2.3

nopattern

3RHHY

1.0

1.6

0.6

3.9

0.6

1.6

0.4

5.1

1.0

1.7

0.6

2.9

2.4

1.9

1.3

7.8

0.6

1.8

0.3

2.1

J-Cpattern

4LIG

L0.2

0.7

0.3

4.3

0.0

0.8

0.0

4.0

0.2

0.8

0.2

3.0

0.1

0.7

0.1

3.5

0.3

0.9

0.3

0.9

nopattern

5COKO

0.5

1.4

0.4

7.3

0.3

1.2

0.3

4.2

0.3

1.0

0.3

3.2

0.2

0.8

0.3

2.7

0.1

0.4

0.4

0.9

nopattern

6TOSI

0.0

0.8

0.0

2.3

1.1

0.8

1.4

2.2

0.5

0.8

0.6

0.8

0.1

0.8

0.2

1.7

0.2

0.9

0.3

1.0

J-Cpattern

7ACCA

0.0

0.8

0.0

4.9

0.0

0.8

0.0

4.6

0.1

0.8

0.1

4.3

0.1

0.7

0.1

3.3

0.0

0.4

0.0

2.4

nopattern

8ACGR

0.2

0.9

0.2

3.3

0.0

0.9

0.0

4.7

0.0

0.8

0.0

4.2

0.1

0.9

0.2

2.1

0.1

0.9

0.1

1.6

nopattern

9LIO

B0.3

1.0

0.3

5.1

0.3

1.0

0.3

3.7

0.4

1.0

0.3

3.4

0.5

1.0

0.5

2.1

0.2

0.9

0.2

1.6

nopattern

adults

1CYOX

0.8

1.3

0.6

3.6

1.4

1.3

1.0

1.6

1.6

1.4

1.1

2.0

1.3

1.3

1.0

0.6

1.6

1.3

1.2

1.8

J-Cpattern

2FA

EN

0.8

0.5

1.6

25.21*

*0.6

0.6

0.9

5.2

0.3

0.6

0.5

2.0

0.6

0.6

1.0

1.0

0.2

0.6

0.4

1.3

Hub

bellpattern

3RHHY

0.7

0.5

1.5

15.41*

*0.6

0.5

1.3

3.0

0.8

0.7

1.2

1.9

1.3

0.8

1.6

1.6

0.6

0.8

0.7

1.7

Hub

bellpattern

4LIG

L0.2

0.2

1.0

6.7

0.1

0.2

0.6

1.1

0.3

0.2

1.4

3.5

0.2

0.2

1.3

0.8

0.2

0.2

0.8

0.4

J-Cpattern

5COKO

0.8

1.3

0.6

6.5

0.2

0.4

0.6

1.3

0.5

0.4

1.3

1.5

0.4

0.3

1.3

1.2

0.1

0.3

0.4

0.4

J-Cpattern

6TOSI

2.0

0.9

2.1

16.04*

*1.1

1.0

1.1

1.8

1.1

1.0

1.0

1.4

0.7

1.0

0.7

1.5

0.3

1.1

0.3

1.2

Hub

bellpattern

7ACCA

0.2

0.4

0.4

1.8

0.2

0.4

0.6

1.6

0.6

0.4

1.4

1.6

0.4

0.4

1.0

1.0

0.4

0.4

0.9

0.1

J-Cpattern

8ACGR

0.0

0.2

0.0

1.0

0.5

0.2

2.5

9.1

0.3

0.2

1.5

0.9

0.1

0.2

0.7

0.3

0.1

0.2

0.3

0.2

J-Cpattern

9LIO

B0.3

0.5

0.7

5.5

0.4

0.4

1.1

3.4

0.5

0.4

1.3

2.4

0.4

0.4

1.0

0.8

0.0

0.3

0.0

0.9

J-Cpattern

χ2 0:01

¼15

:09;

χ2 0:05

¼11

:07;

d f=5.

**:significant

difference.

Mi ZHANG et al. The J-C effect on a Fagus engleriana-Cyclobalanopsis oxyodon community 519

insects (Janzen, 1970; Connell, 1971). Some studies haveshown that pathogens play an important role in themortality of seedlings and saplings (Gilbert et al., 1994).In our community, the seedling and sapling mortality nearparent trees was not high. This might be because there wereless host-specific herbivorous predators in subtropicalforests than in tropical forests. In our study area, there wereno large herbivores. Host-specific insects and pathogenshad an impact, but the impact was not sufficient to reducethe seed population. These results suggested that self-thinning was a more important process than distance- anddensity-dependent mortality as predicted by the J-Cmodel to reduce the numbers of conspecific individualsin the proximity of adults. Janzen (1970) suggested that

host-specificity of seed consumers, pathogens and para-sites, is greater in the tropics than in temperate forests, andthus, it plays a greater role in structuring these commu-nities. These differences may also exist between sub-tropical and tropical forests because predation of seeds wasnot great enough to reduce the dominant position of theF. engleriana and C. oxyodon in this subtropical forestcommunity.

4.4 Environmental heterogeneity influence

Environmental heterogeneity is another factor that influ-ences the density- and distance-dependent effects. In thiscommunity, the most important environmental factor was

Fig. 7 The recruitment pattern of the nine observed species. (a) CYOX: cyclobalalnopsis oxyodon; (b) FAEN: Fagus engleriana; (c)RHHY: Rhododendron hypoglaucum; (d) LIGL: Lithocarpus glaber; (e) COKU: Cornus kousa var. chinensis; (f) TOSI: Toona sinensis;(g) ACCA: Acer cappadocicum; (h) ACGR: Acer griseum; (i) LIOB: Lindera obtusilo.

520 Front. Biol. China 2009, 4(4): 513–522

topography. The landform of the research plot was highlyvariable but in general was quite steep. The steep slopecaused the seeds to travel far down the slope from themother tree. This made it difficult to test the J-C effect.Topographical variation certainly created large differencesin the quality of microsites where seeds were establisheddifferently and had a greater influence than density anddistance did.The role of the J-C effect in maintaining species

diversity has been tested in many forests but no consistenttrends emerged. It may be important for some speciesduring certain life stages, but its importance in maintainingspecies diversity in a community has yet to be proven(Hiura and Fujiwara, 1999; Matos et al., 1999). In thesubtropical mixed evergreen deciduous broadleaved forestwe studied, the patterns we observed do not fully supportthe J-C hypothesis. However, around an individual adulttree, nonconspecific trees at all life stages were much moreabundant than conspecific trees. Although the mechanismfor this pattern is not known, it provides the spacenecessary for the establishment and survival of otherspecies and thus promotes species coexistence.

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