Introduction
Ancient forests are considered as the focus
of conservation because of their complex structure and diverse floristic
composition, but these mature forests are rapidly disappearing as a result of
logging and conversion to agriculture (Schuldt et al. 2010). Therefore,
the protection value of secondary forest is increasing. Successional processes
in secondary forest may restore species composition and diversity to the level
of original forest (Brockerhoff et al. 2008). As we known, succession is
one of the most studied processes in ecology, and it provides highly
predictable. Few researchers have studied the influence of the preceding stages
of succession on subsequent stages of succession to test the assumption that
passing through one stage is essential before entering the next stage. The
succession of the ecosystem processes of each successional stage may be
affected by the diversity of species present, but there are few studies showing
that plant species diversity affects succession. Over time, changes in plant
composition are influenced by the effectiveness of propagules (Bekker et al.
1997), changes in soil fertility (Marrs 1993), above- and below-ground
herbivore activities (Olff and Ritchie 1998), the presence or absence of
mutualistic symbionts (Clay and Holah 1999) and the relationships between plant
species and related soil communities (Westover et al. 1997). The outcome
of all these interactions, such as facilitation and competition (Cavard et
al. 2011), determines which species successfully compete for the available
resources at a certain stage of succession (Putten and Stoel 1998).
Species diversity and
rarity are essential indicators of the value of natural habitats (Locky et
al. 2005). Tree species diversity
is related to ecosystem functions and services, of which productivity and
decomposition are their key features (Hooper et al. 2012; Ammer 2019).
Using ground-sourced data from 777,126 permanent sample plots across 44
countries, Liang et al. (2016) showed that the reduce of species
richness would lead to the decrease of forest productivity. In addition, Ruiz-Benito et al. (2014) used diversity gradient datasets of
~54,000 plots in Spain and showed that species diversity increases carbon stock
and tree productivity. Wood production has also been found to be higher in
Aleppo pine forests in mixed plots than in monospecific plots (Vilà et al.
2003). Similarly, Chamagne et al. (2017) showed that increases in tree
diversity increased forest productivity and increases in tree growth rates
without a cost. Plant diversity and composition have also been indirectly
related to net nitrification through a positive relationship between species
richness and nitrifier abundance (Laughlin et al. 2010).
Understanding plant
biodiversity in different successional periods of subtropical Masson pine
plantations is useful, particularly in the South China subtropical area where
Masson pine is one of the most economically important and
popular tree species grown in monocultures or in mixture with broad-leaved tree
species. Biodiversity
assessments provide opportunities for monitoring community changes over time,
prioritizing areas of conservation concern and developing testable hypotheses
relating patterns of geographic variation in species assemblages to selected
environmental factors (Debinski and Humphrey 1997). Some studies have examined
species-environment relationships to understand the determinants of community
composition (Svenning and Skov 2002). These studies commonly showed a
predominant effect of soil pH on plant diversity, including species richness
and composition (Debinski and Humphrey 1997). In addition, researchers have
shown that interactions between site-specific factors can reduce, facilitate
(Connell and Slatyer 1977), or even divert successional sequences predicted by
the general successional model. Barrufol et al. (2013) showed that
successionally older stands, which had a high plant diversity, also had a
higher total stem basal area.
Successional processes govern the plant
interactions that occur between plant species: interspecific interactions tend
to change with stand development and among species (Cavard et al. 2011).
In this study it was examined the positive and negative interactions in
communities of Pinus massoniana,
plantations and mixed P. massoniana/broad-leaved
forests, which are the different successional stages of Masson pine plantations
in South China, to verify the negative effect of plantations on species
diversity and to assess dissimilarities in species composition. Moreover, by
analyzing the relationships between different forest strata in both forest
types, we aimed to detect general interaction patterns in the Masson pine
monocultures in South China that could either facilitate or counteract the
conversion of Masson pine monocultures into mixed forests. The findings of this
research can be used to guide the management of these forests to enhance plant
species diversity.
Materials
and Methods
Study area
The study
was conducted in the Dinghushan Biosphere Reserve
in a subtropical area of South China (112° 30' 39″ E – 112° 33' 41″
E, 23° 09' 21″ N – 23° 11' 30″ N) with a subtropical moist monsoon climate (Fig. 1). The area
is mountainous, has steep slopes, and has an elevation range of 100–700 m. The annual average
temperature of the study area is 21.4ºC with an annual mean maximum temperature of 28.0ºC and
an annual mean minimum temperature of 12.6ºC and the mean precipitation of the study area
is 1678 mm,
with about 80% of the mean precipitation occurring in the wet
season: April to September. The
study area has ~78% forest cover and is located in the subtropical evergreen
zone, mostly on sand-shale stone. The typical forest types at the study site
are P. massoniana monocultures, mixed
P. massoniana/broad-leaved forests,
and monsoon evergreen broad-leaved forests. The dominant broad-leaved species
in the study area include Schima superba
and Castanopsis chinensis. P. massoniana is the dominant species in
the coniferous forests.
Data sampling
We established six 20 × 20
m plots in two forest types in the Dinghushan Biosphere Reserve. Three plots were located in P. massoniana monoculture, and the other
three plots were placed in mixed P.
massoniana/broad-leaved forest. The study plots were located at 200–300 m
a.s.l. and had similar soil conditions. In late summer, in 2017, a vegetation
survey was conducted in each plot. Species were recorded separately for the
tree layer, which was comprised of woody plants and climbers with
heights of >5 m, shrub layer, which was comprised of woody plants and climbers with a
height of 0.5–5 m and herb layer, which included all herbaceous species and
climbers/woody plants with a height <0.5 m. The moss layer was not considered. Cover values per species and vegetation layer were recorded directly in
percent. The height was
measured by Blume-Leiss Hymsometer (Parde 1955). The diameter at breast height
(DBH) of the tree layer was quantified by measuring the diameter of all trees.
The nomenclature of plant taxa followed the Flora of China (www.efloras.org).
We collected soil using a soil corer with a
diameter of 8 cm to investigate soil chemical characteristics. Ninety-six cores
were bulked by mineral soil depths of 0–5, 5–10, 10–20, 20–40 and 40–60 cm to form 16 composite samples
per depth (Table 1). The organic matter of
the soil in the P. massoniana
plantations and mixed P. massoniana/broad-leaved forests was 29.41 t·ha-1 and 32.04 t·ha-1,
respectively, and the soil pH of the two forests was 2.82 and 2.91,
respectively (Table 1).
Data analysis
The tree layer characteristics, species
richness and abundance of the understory layer and its component shrub and herb
layers were compared for the two forest types P. massoniana monoculture (n
= 3) and P. massoniana/broad-leaved
forest (n = 3) using Wilcoxon
tests. It was further quantified the Shannon diversity index for the understory
layers. Linear regressions were used to quantify the impact of different tree
layer variables, including species richness, height, cover and DBH, on the
understory, with cover and species richness of the understory and its component
shrub and herb layer as dependent variables for each
forest type. The same
method was used to analyze the interactions between the shrub and herb layer variables
and the impact of the tree layer on the number and cover of tree, shrub and
herbaceous species, including graminoids, herbs, and ferns within the
understory, according to the Flora of China.
Table 1: Primary soil chemical
characteristics of the studied mixed and pure Pinus massoniana forests. Superscript letters indicate significant
differences between the pure forest and the mixed forest at each depth (P < 0.05, pairwise Wilcoxon test)
|
Soil Depth (cm) |
Organic Matter (t·ha-1) |
C (t·ha-1) |
N (t·ha-1) |
PH |
||||
|
mix forest |
pure forest |
mix forest |
pure forest |
mix forest |
pure forest |
mix forest |
pure forest |
|
|
0-5 |
32.04 |
29.41 |
18.63 |
1.71 |
1.00 |
0.93 |
2.91 |
2.82 |
|
(2.9)a |
(2.4)a |
(1.7)a |
(1.4)a |
(0.1)a |
(0.1)a |
(0.0)a |
(0.0)a |
|
|
5-10 |
28.74 |
21.70 |
16.71 |
12.62 |
0.98 |
0.74 |
3.11 |
3.04 |
|
(7.6)a |
(0.6)a |
(4.4)a |
(0.4)a |
(0.2)a |
(0.0)a |
(0.0)a |
(0.1)a |
|
|
10-20 |
16.20 |
10.83 |
9.42 |
6.29 |
0.63 |
0.47 |
3.22 |
3.28 |
|
(2.7)a |
(1.0)a |
(1.6)a |
(0.6)a |
(0.1)a |
(0.0)a |
(0.0)a |
(0.0)a |
|
|
20-40 |
9.53 |
7.88 |
5.54 |
4.58 |
0.49 |
0.41 |
3.21 |
3.17 |
|
(1.2)a |
(0.9)a |
(0.7)a |
(0.5)a |
(0.0)a |
(0.0)a |
(0.0)a |
(0.1)a |
|
|
40-60 |
9.97 |
6.25 |
5.80 |
3.64 |
0.44 |
0.38 |
3.32 |
2.81 |
|
(4.3)a |
(0.7)a |
(2.5)a |
(0.4)a |
(0.1)a |
(0.0)a |
(0.0)b |
(0.1)a |
|
The
data figures are means of six replicates
%2031-39_files/image002.jpg)
Fig. 1: Location of Dinghushan Biosphere
Reserve in the City of Zhaoqing in the middle of Guangdong Province, South
China
Detrended correspondence
analysis (DCA; Nesheim et al. 2010) was used to investigate the
gradients of species composition among the six plots and to examine the
vegetation-environment relationships in the two different successional stages.
DCA was used to analyze plant species composition of the shrub and herb layer
and to quantify a potential difference between forest types. Ordinations were
applied to square-root transformed cover values. The cover values of each species
sampled in the shrub and herb layer were fitted to the ordination axes to
investigate which species determined differences in species composition. Then,
indicator species analysis was used to identify possible indicator species for P. massoniana and mixed P. massoniana/broad-leaved forests (Dufrêne and Legendre 1997). Calculated an indicator value (IV) for each species in the shrub and herb layers of the two forest
types as the proportional abundance of the species in the herb or shrub layer
relative to its abundance in both layers multiplied by the proportional
frequency of the species in each layer. The IV
ranges between 0 (no indication) and 1 (perfect indication), which means that a perfect indicator is always present in one of the plant layers. The significance
of each indicator value was tested using Monte Carlo simulation with 1 000 iterations (McCune and Grace 2002). All calculations
and statistical tests were performed using R, version 3.3.1 (QUOTE: R
Development Core Team 2017).
Results
Characteristics of forest types
Within the six study plots, found 57
different plant species, and 48 of these species occurred in the herb and shrub
layers of the understory. Only Eucalyptus
urophylla × E. grandis and S. superba
occurred exclusively in the tree layer in the P. massoniana
plantations and C. fissa occurred
exclusively in the mixed P. massoniana/broad-leaved forests (Table 2). In the P. massoniana
plantations 56 species were recorded, and in the mixed P. massoniana/broad-leaved forest 57 species were recorded.
The investigated P. massoniana plantations were
located at a slightly higher elevation than the mixed P. massoniana/broad-leaved forests (Table 3). The structure and diversity
variables of the tree and understory layers showed no significant differences
between forest types. Separating the understory into its components, the shrub
layer of the P. massoniana plantations had a higher species richness and coverage
than the mixed P.
massoniana/broad-leaved forests,
while the species richness and coverage of the herb layer was higher in the mixed P. massoniana/ broad-leaved
forest (Fig. 2).
Interactions
between vegetation layers
There was a positive relationship between the
coverage of the tree layer and species richness of the understory layers in P. massoniana plantations (Table
4) and a negative relationship between species richness of the tree layer and
species richness of the understory, shrub layer, and herb layer (Table 4).
There was also a negative relationship between the cover of the tree layer and
the species richness of the herb layer in the mixed P. massoniana/broad-leaved forests (Table 4).
There
was no interaction between the shrub and herb layer variables (Table 5).
However, there was a positive relationship between coverage and species
richness of the shrub layer in the mixed P. massoniana/broad-leaved forest and a negative relationship between these
variables in the P. massoniana forests.
Vegetation structure
The DCA ordination revealed a clear
difference in species composition between mixed P. massoniana/broad-leaved forests and P.
massoniana plantations for both the shrub and herb layers along the second
axis (Fig. 3 and 4). S. superba, Desmos chinensis, Schefflera heptaphylla, C.
fissa, and Sapium discolor were characteristic of the shrub
layer in mixed P. massoniana/broad-leaved forests. Correlations between species cover values
and the ordination axes revealed two groups of species communities that were
correlated with the second axis and characterized either mixed P. massoniana/broad-leaved forests or P. massoniana plantations in the shrub
layer (Fig. 3). In the overstory, Ficus
simplicissima, Ixora chinensis,
and Evodia lepta were associated with P. massoniana plantations, whereas Litsea rotundifolia was associated with
mixed P. massoniana/ broad-leaved forests. In
the herb layer, more species were associated with mixed P. massoniana/broad-leaved forests, which had higher herb layer
cover and species richness than P.
massoniana plantations (Fig. 4).
Results of the ordination were largely in accordance with the
assignment of species based on their occurrence frequency and IV. Of
the 32 species in the shrub layer, C.
fissa was identified as a significant indicator of P. massoniana/broad-leaved forests and
Mallotus paniculatus, Embelia ribes, and Melastoma candidum were concentrated in P. massoniana plantations. Of the 40
species present in the herb layer, the indicators for P. massoniana/broad-leaved forests
were Dicranopteris pedata, Mussaenda
pubescens and Blechnum orientale,
and the indicators for P. massoniana
plantations were Melicope pteleifolia and M. paniculatus (Table 6). We also found
that with the increase of succession age, the species number and coverage of
shrub layer decreased: M. paniculatus, E. ribes, M. candidum, Clerodendrum
fortunatum and Alchornea trewioides were
indicators for young stages. In the herb layer, we found that the number and
coverage of herbaceous species increased with the age of succession: D. dichotoma, M. pubescens, and B.
orientale were indicators for old stages.
Alchtrew, A. trewioides; Castfiss, C. fissa; Clerfort, C. fortunatum; Cratcoch, Cratoxylum
cochinchinense; Desmchin, D.
chinensis; Emberibe, E. ribes;
Ficusimp, F. simplicissima; Ixorchin,
I. chinensis; Jatrinte, Jatropha integerrima; Litsglut, L. glutinosa; Litsrotu, L. rotundifolia; Mallpani, M. paniculatus; Melacand, M. candidum; Evodlept, E. lepta; Psycasia, Psychotria asiatica; Rhodtome, Rhodomyrtus
tomentosa; Sapidisc, S. discolor;
Schehept, S. heptaphylla; Scheocto, S. octophylla; Schisupe, S. superba; Smilchin, Smilax china; and Toxisucc, Toxicodendron succedaneum.
The abbreviation in herb
layer: Aspltric (Asplenium trichomanes),
Blecorie (B. orientale), Desmchin (D. chinensis), Dianensi (Dianella ensifolia), Dicrpeda (D. pedata), Emberibe (E. ribes), Eurygrof (Eurya groffii), Gardjasm (Gardenia jasminoides), Ixorchin (I. chinensis), Litsrotu (L. rotundifolia), Lygoflex (Lygodium flexuosum), Melaaffi (M. affine), Meliptel (M. pteleifolia), Miscflor (Miscanthus floridulus), Musspube (M. pubescens), Oplicomp (Oplismenus compositus), Panibrev (Panicum brevifolium), Polychin (Polygonum chinense), Psycasia (P. asiatica), Psycserp (P. serpens), Rhodtome (R. tomentosa).
Table 2: Tree species abundance in P. massoniana plantations and mixed P. massoniana/broad-leaved
forests
|
|
P. massoniana forest |
Mixed P.
massoniana/broad-leaved
forest |
|
C.
fissa |
- |
39 |
|
E. urophylla × E. grandis |
5 |
- |
|
L.
cubeba |
1 |
1 |
|
M.
paniculatus |
1 |
12 |
|
M.
pteleifolia |
2 |
2 |
|
P.
massoniana |
37 |
36 |
|
S.
superba |
7 |
- |
Table 3: The mean values ± standard error of the characteristics of P. massoniana plantations and mixed P. massoniana/broad-leaved forests. There were no significant
differences in values between forest types (P < 0.05 using the pairwise
Wilcoxon test)
|
|
P. massoniana forest |
Mixed P.
massoniana/broad-leaved
forest |
|
Stand Age |
50 - 60 years |
70 - 80 years |
|
Altitude [m asl] |
200 - 300 |
220 - 300 |
|
Tree layer
variables |
|
|
|
DBH [cm] |
24.8 ± 1.6 |
16.9 ± 1.1 |
|
Tree height [m] |
10.1 ± 0.5 |
8.8 ± 0.4 |
|
Cover [%] |
85.3 ± 9.7 |
141.2 ± 3.82 |
|
Species richness/100 m2 |
1.0 ± 01 |
1.0 ± 01 |
|
Understory
variables |
|
|
|
Total Understory |
|
|
|
Cover [%] |
148.43 ± 5.47 |
147.62 ± 4.73 |
|
Species richness/100 m2 |
7.1 ± 0.4 |
6.8 ± 0.4 |
|
Shannon-Index |
4.5 ± 1.7 |
2.8 ± 0.0 |
|
Shrub layer |
|
|
|
Cover [%] |
80.3 ± 5.3 |
53.6 ± 13.9 |
|
Species richness/100 m2 |
8.8 ± 0.4 |
7.3 ± 0.7 |
|
Shannon-Index |
2.3 ± 0.1 |
2.2 ± 0.0 |
|
Herb layer |
|
|
|
Cover [%] |
142.27 ± 4.63 |
142.37 ± 4.07 |
|
Species richness/100 m2 |
7.9 ± 0.3 |
8.2 ± 0.6 |
|
Shannon-Index |
2.4 ± 0.1 |
2.4 ± 0.1 |
%2031-39_files/image004.jpg)
Fig. 2: The mean number of species per subplot for
the three plant layers in each plot: tree layer (n = 3), shrub layer (n = 6),
and herb layer (n = 6). There were no significant differences between the
numbers of species in the three layers using a pairwise
Wilcoxon test
Table 4: Relationships between species richness or
cover and different tree layer variables for the understory, shrub layer and
herb layer. Significance levels are: *P
< 0.1, **P < 0.05
|
A) Cover [%] |
Understory |
Shrub layer |
Herb layer |
|||||||||
|
β (pure) |
R2adj |
β (mix) |
R2adj |
β (pure) |
R2adj |
β (mix) |
R2adj |
β (pure) |
R2adj |
β (mix) |
R2adj |
|
|
Tree layer
variables |
|
|
|
|
|
|
|
|
|
|
|
|
|
Cover [%] |
0.175 |
0.923 |
-0.117 |
0.000 |
1.547 |
0.395 |
2.565 |
0.739 |
0.238 |
0.982 |
-0.353 |
0.000 |
|
DBH(cm) |
0.009 |
0.000 |
-0.017 |
0.000 |
-0.088 |
0.000 |
-0.091 |
0.699 |
0.003 |
0.000 |
-0.010 |
0.000 |
|
Height [m] |
0.002 |
0.000 |
0.005 |
0.898 |
0.035 |
0.700 |
0.000 |
0.000 |
0.003 |
0.000* |
0.005 |
0.991** |
|
Richness |
0.009 |
0.256 |
-0.009 |
0.067 |
0.109 |
0.969 |
0.021 |
0.000 |
0.013 |
0.752 |
-0.011 |
0.614 |
|
B) Richness |
β (pure) |
R2adj |
β (mix) |
R2adj |
β (pure) |
R2adj |
β (mix) |
R2adj |
β (pure) |
R2adj |
β (mix) |
R2adj |
|
Tree layer
variables |
|
|
|
|
|
|
|
|
|
|
|
|
|
Cover [%] |
-5.321 |
0.861 |
14.320 |
0.000 |
-7.712 |
0.815 |
36.650 |
0.430 |
-2.206 |
0.000 |
35.790 |
0.000 |
|
DBH(cm) |
0.050 |
0.000 |
-0.819 |
0.967* |
-0.492 |
0.000 |
-1.528 |
0.917 |
0.534 |
0.308 |
-2.048 |
0.967* |
|
Height [m] |
-0.086 |
0.143 |
0.070 |
0.000 |
-0.046 |
0.000 |
0.050 |
0.000 |
-0.107 |
0.952* |
0.175 |
0.000 |
|
Richness |
-0.321 |
0.929 |
0.000 |
0.000 |
-0.346 |
0.039 |
0.214 |
0.000 |
-0.242 |
0.210 |
0.000 |
0.000 |
Table 5: Relationships between species richness and
cover of the shrub layer and herb layer. There were no significant values
%2031-39_files/image006.png)
Fig. 3: Detrended
correspondence analysis (DCA)-ordination of vegetation
survey data of the shrub layer in the six plots assigned to the two forest types: mixed P. massoniana/broad-leaved forest (Mix1, Mix2, and Mix3) and P. massoniana plantations (Pure1, Pure2, and Pure3). Single species cover values were correlated with the ordination axes
and displayed in the diagram when correlation was significant at P < 0.01
%2031-39_files/image008.png)
Fig. 4: Detrended
correspondence analysis (DCA)-ordination of vegetation
survey data of the herb layer in the six plots assigned to the two forest types: mixed P. massoniana/broad-leaved forest (Mix1, Mix2, and Mix3) and P. massoniana plantations (Pure1, Pure2, and Pure3). Single species cover values were correlated with the ordination axes
and displayed in the diagram when correlation was significant at P < 0.01
|
Shrub layer |
Cover |
Species richness |
||||||
|
β (pure) |
R2adj |
β (mix) |
R2adj |
β (pure) |
R2adj |
β (mix) |
R2adj |
|
|
Shrub layer
variables |
|
|
|
|
|
|
|
|
|
Cover [%] |
|
|
|
|
-2744 |
0000 |
15460 |
0926 |
|
Species richness |
-0144 |
0000 |
0062 |
0926 |
|
|
|
|
|
Herb layer
variables |
|
|
|
|
|
|
|
|
|
Cover [%] |
6856 |
0565 |
-0240 |
0000 |
-3112 |
0688 |
6357 |
0000 |
|
Species richness |
-0301 |
0446 |
0041 |
0494 |
0192 |
0000 |
0714 |
0786 |
|
Herb layer |
Cover |
Species richness |
||||||
|
β (pure) |
R2adj |
β (mix) |
R2adj |
β (pure) |
R2 |
β (mix) |
R2adj |
|
|
Shrub layer
variables |
|
|
|
|
|
|
|
|
|
Cover [%] |
0114 |
0565 |
-0022 |
0000 |
-2406 |
0446 |
1802 |
0494 |
|
Species richness |
-0027 |
0688 |
0002 |
0000 |
0081 |
0000 |
1250 |
0786 |
|
Herb layer
variables |
|
|
|
|
|
|
|
|
|
Cover [%] |
|
|
|
|
-11110 |
0000 |
30500 |
0000 |
|
Species richness |
-0023 |
0000 |
0006 |
0000 |
|
|
|
|
Table 6: The indicator values (IVs) for species in the shrub layer and herb layer. ***P < 0.001, ** P < 0.01, * P < 0.05
|
|
|
Species |
IV |
P |
|
Shrub layer |
P. massoniana/broad-leaved forests |
C.
fissa |
0.816 |
0.002** |
|
P.
asiatica |
0.805 |
0.029* |
||
|
P.
massoniana
plantations |
M.
paniculatus |
0.868 |
0.005** |
|
|
E.
ribes |
0.764 |
0.005** |
||
|
M.
candidum |
0.764 |
0.003** |
||
|
C.
fortunatum |
0.645 |
0.034* |
||
|
A.
trewioides |
0.610 |
0.046* |
||
|
Herb layer |
P. massoniana/broad-leaved forests |
D.
dichotoma |
1 |
0.001*** |
|
M.
pubescens |
0.917 |
0.001*** |
||
|
B.
orientale |
0.913 |
0.001*** |
||
|
|
|
|
|
|
|
P.
massoniana
plantations |
M.
lepta |
0.98 |
0.001*** |
|
|
M.
paniculatus |
0.957 |
0.001*** |
||
|
M.
candidum |
0.764 |
0.004** |
||
|
A.
trewioides |
0.633 |
0.032* |
Discussion
There was a higher species richness of the
understory layer in pure forests (8.8 species/100 m2) than
in mixed forests (7.3 species/100 m2), which is contrary to our
expectations that mixed forests are usually more suitable habitats for forest
species. Duan et al. (2010) who compared the vegetation community in six
plantations in South China and found that mixed forests absorbed more species
than monoculture forests. Song and Yang (2001) showed that species diversity
was much higher in Larix gmelini-Betula
platyphylla forest than in Larix
gmelini forest in the Daxing’an Mountains. In addition, Bruelheide et
al. (2011) tested five successional stages of subtropical broad-leaved
forests in southeastern China and found that species richness raised with
successional stage. While in present study the C. fortunatum, E. ribes, M. candidum and S. octophylla were the leading species and only occurred in the
Masson pine monocultures, which likely corresponds to the lower soil organic
matter and soil pH in the plantations (Table 1). Another factor explaining the
high diversity in Masson pine monocultures in the study area was the proximity
of further natural secondary forests, providing an important seed source (Wang et
al. 2010; Heinrichs and Pauchard 2015). The high coverage of the tree layer
also resulted in the lower species diversity of the understory in the P. massoniana/ broad-leaved forests,
which was 141.2% compare with 85% in the Masson pine plantations (Both et al.
2011).
In the P. massoniana plantations and mixed P. massoniana/broad-leaved forests, the understory should become more similar
in their species composition of seedlings during the course of succession.
However, our result showed that species composition differed largely between
the two forest types, which indicated that tree canopy structure
affected the understory
composition as demonstrated by Wang et al. (2014) and Ou et al.
(2015) in other regions of China. The primary difference between the two forest
types was the higher species richness of the shrub layer in P. massoniana plantations. Legendre et
al. (2009) found that the altitude and terrain convexity have a great
influence on species composition and richness. In the study area, the altitude
of the three P. massoniana plantation
plots was lower than that of the three mixed P. massoniana/ broad-leaved forest plots.
In the mixed P. massoniana/broad-leaved forest plots,
two species were important indicators of the shrub layer and three species were
significant indicators of the herb layer, while in the P. massoniana plantations there were five and four species,
respectively (Table 6). Our finding of an increase in species richness and
cover with successional age is in contract with the findings of Both et al.
(2011), who showed that a decrease in the number and cover of herbaceous
species with successional age. However, such declines in number and cover were
found for the shrub layer woody species M.
paniculatus, C. fissa, which were early-successional pioneer species and indicators for plantations. This result was accordant with the
succession of the plantation to the mixed P. massoniana/broad-leaved forest, in which the tree layer was composed of M. paniculatus, C. fissa, and P. massoniana, suggesting that seed
rain and the seed bank play significant role in providing seeds for plant
recruitment in the understory (Wang et al. 2014).
Forests are structurally
complex, especially the understory vegetation layers, which often account for
the majority of species richness. Furthermore, variations in diversity in the
overstory layer may affect understory diversity due to trees having a
species-specific impact on resource availability and edaphic conditions, which
influence the understory (Ampoorter et al. 2014). Several studies have shown a positive relationship between the species diversity of different
strata (Beatty 2003; Gilliam 2007; Márialigeti et al. 2016; Zhang et
al. 2017), in
which the diversity of the tree layer positively affects the diversity of the
understory through influencing the diversity of tree saplings and creating
heterogeneous environmental conditions. Yet, in our study, increasing tree
species richness reduced the species richness of the understory, shrub layer,
and herb layer in P. massoniana plantations (Table 4B). This may be because
monocultures have a significant impact on resources, while mixtures usually
share the same tree species or species with similar influences on the
environment (Ampoorter et al. 2014). Another explanation is an increase
in specialist herbivore loads, which can reduce plant diversity (Schuldt et
al. 2010). In the P. massoniana
plantations there was higher shrub layer species richness than in the mixed P. massoniana/broad-leaved forests,
which is in contrast to findings by Both et al. (2011) who showed that
there was lighter available in young successional forests thereby increasing
shrub layer diversity. However, in the mixed P. massoniana/broad-leaved forests in this study, there was a higher coverage
(423.75%) than in the P. massoniana
plantations (256%) and the dense cover of the tree layer in the mixed P. massoniana/broad-leaved forests
reduced the species richness of the herb layer (Table 4A), indicating a
potential limitation for the admixture of tree species when tree cover is too
dense. This is because herbaceous
species prefer open areas with sufficient light (Schnitzer et al. 2008; Márialigeti et
al. 2016),
which is evidently influenced by canopy openness.
Conclusion
Present study shows that P. massoniana plantations have a
different species composition to that of mixed forests and they can be
characterized as a very early-successional community. Results also demonstrate a high potential to
convert Masson pine monocultures into more natural and stable mixed stands with
broad-leaved forest species,
especially when other tree species are already present
in the tree layer. The positive relationship between tree species richness in
the overstory and tree species diversity in the understory was probably because
of suitable soil conditions. Our results further indicate a positive overstory-understory diversity
relationship in the early to mid-successional stages and underline the
effectiveness of admixing tree species in Masson
pine plantations to maintain biodiversity in
Chinese subtropical forests. Our findings can be used to manage forests to
enhance species richness in these forests. In
our future research, we measure the plant diversity and composition
continuously, in order to get more accurate information of the plant
development.
Acknowledgments
We acknowledge Dr. Xiaodong Liu for his help in setting up the
experiment plots. Authors thank the Dinghushan Biosphere Reserve for permission
to undertake the fieldwork in the City Zhaoqing, Guangdong Province. This work was supported by funding from the Natural Science
Foundation of Guangdong Basic and Applied Basic Research Foundation, through
the research projects 2021A1515011092, as well the support from Qingyuan
Forestry Bureau and Deqing Forestry Farm. The authors would like to thank all
staff in the Yangmei, Jinji, Yingde, and Xiaolong forest farms. We also thank
the editor and reviewers for their constructive comments on the manuscript.
Author Contributions
Na Lin: Conducted data analysis and writing; Huiyan Xie:
conducted the soil sampling and analysis; Tao Ma: conducted the vegetation
survey; Chunyong Li conducted the tree survey; Demei Huang: conducted the
vegetation survey. Mingxuan Zheng: conducted the vegetation survey; Shiqing
Chen: Conceptualization, Methodology.
Conflicts of Interest
All authors declare that they have no known competing
financial interests or personal relationships that could have appeared to
influence the work reported in this paper
Funding Source
This research was funded by Natural Science Foundation of
Guangdong Basic and Applied Basic Research, grant number "2021A1515011092”
and “Qingyuan Forestry Bureau”
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