Article

Diameter Growth, Biological Rotation Age and

Biomass of Chinese Fir in Burning and Clearing
Site Preparations in Subtropical China
Hua Zhou, Shengwang Meng and Qijing Liu *
Forestry College, Beijing Forestry University, No. 35 Tsinghua East Road Haidian District, Beijing 100083, China;
jeamourvous@163.com (H.Z.); wangzai1220@126.com (S.M.)
* Correspondence: liuqijing@bjfu.edu.cn; Tel.: +86-13911806586

Academic Editors: Jesus Julio Camarero, Raúl Sánchez-Salguero and Juan Carlos Linares
Received: 9 July 2016; Accepted: 11 August 2016; Published: 18 August 2016

Abstract: Sustained forest management of Cunninghamia lanceolata (Chinese fir) plantations in

subtropical China is restricted by the limited availability of quantitative data. This study combines
inventory data and tree-ring analysis of Chinese fir from natural and plantation forests that were
subjected to controlled burning or brush clearing site preparations. Inter-annual variation of Chinese
fir tree-ring widths were measured for the controlled burning, brush clearing and natural forest
sites. The mean annual diametric growth of Chinese fir was 0.56 cm·year−1 for the natural forest,
0.80 cm·year−1 for the brush clearing site and 1.10 cm·year−1 for the controlled burning site. The time
needed to reach the minimum cutting/logging diameter of 15 cm was 14 years in the controlled
burning site, 19 years in the brush clearing site and >40 years in the natural forest. The biological
rotation ages for the burning, cutting and natural forest sites were 15, 26 and >100 years, respectively.
The total aboveground biomasses for the burning and clearing sites were 269.8 t·ha−1 and 252 t·ha−1 ,
respectively. These results suggest that the current 25-year cutting cycle greatly underestimates the
growth rate of Chinese fir plantations.

Keywords: biomass; Chinese fir; rotation age; site preparation; subtropical forests

1. Introduction
Sustainable forest management is a predominant theme in natural resource management because
land managers need to provide current and future generations with the products and services that they
desire [1]. Intensive management through vegetation control (e.g., controlled burning and clearing
ground vegetation) can substantially increase timber volumes, but it can also reduce some aspects of
biological diversity [2–4]. Successful sustainable forest management requires long-term information
on tree growth. Most growth data in tropical forests has been collected in permanent sample plots for
fewer than 20 years [5–7]. Therefore, long-term growth changes have been evaluated using short-term
data that may produce biased conclusions [5,8]. As a tool for sustainable forest management, tree-ring
data have been widely employed to obtain and study the growth pattern, release and suppression of
trees in temperate and tropical forests [9–14].
Site preparation is a very important tool in natural resource management because it promotes the
early establishment and growth of crop trees by reducing competition from ground vegetation [15].
Natural resource managers have contemplated issues related to site preparation treatments.
Several types of harvesting and silvicultural activities may impact plant community and biodiversity in
managed forests, including the harvesting process itself, pre-harvest activities, such as pre-commercial
thinning, and post-harvest site preparation activities (e.g., chemical/mechanical preparation of the
site for planting) [16–19]. Various mechanical/chemical site preparation treatments are applied at

Forests 2016, 7, 177; doi:10.3390/f7080177 www.mdpi.com/journal/forests

Forests 2016, 7, 177 2 of 14

stand initiation to reduce competing vegetation and improve crop tree growth [20]. In subtropical
China, site preparation for planting Cunninghamia lanceolata (Chinese fir) mainly consists of controlled
burning and brush clearing [21]. The long-term effects of brush clearing on the growth and cumulative
biomass of Chinese fir are more complex than those of controlled burning. Compared to controlled
burning, the effects of brush clearing are transient and the soil carbon release is significantly slow.
Controlled burning can impact carbon storage and release not only during the burning process, but
also during the post-burning period [22]. In our burning site, as no other seedlings or broadleaved trees
were recorded, we speculate that nearly 100% of organic materials should have burnt. The long-term
impacts of burning on tree growth and carbon storage have been studied, although the results are
inconsistent [23–25]. These contradictory reports may be caused by differences in burning intensity,
soil moisture, topography and vegetation type [26,27]. In many regions of China, most regulations
for Chinese fir are based on mean growth rates from different stands and geographical areas, and
most forest managers usually ignore biodiversity and growth differences of different site preparation
methods, especially reforestation after logging natural forests. These forest practices are usually aimed
at short-term economic and commercial feasibility, creating doubts on the long-term sustainability of
forests [28,29]. Thus, appropriate site preparation management that may save manpower, costs and
time is essential for the long-term sustainability of Chinese fir plantations.
The subtropical area of South China is vulnerable to climate change, and it is considered to be
an important region for biodiversity and a great natural reserve for endemic plant species [30–32].
Subtropical evergreen broadleaved forests are a typical climax vegetation in the subtropical zone of
South China. In past decades, large areas of natural evergreen broadleaved forests in many regions of
South China might comprise secondary forests, plantations, orchards and arable land [33]. The Chinese
Government has promised that the area of planted forests will increase by 40 million ha by 2020, and
Chinese fir plantations might increase rapidly in this region [34]. The Chinese fir is a fast-growing tree,
up to 30 m tall and 250 cm in diameter; and its wood is desired highly for many purposes, because
it is easy to process, durable, hard and rot-resistant. Its range extends from southern China to the
southern area of Qinling Mountain, and it extends into Vietnam. In China, Chinese fir trees have
been planted for over 1000 years [35], but they are rare in primary forests because of a long history of
anthropogenic influences. It is one of the most important plantation tree species in South China in terms
of commercial value, and its plant area covers over 12 million ha, which accounts for approximately
6.5% of all plantation forests worldwide [36,37]. Therefore, Chinese fir forests play an important role
in the accumulation of biomass carbon.
In this study, we aimed to estimate the rates of diametrical growth, as well as the biological rotation
ages (BRAs) of Chinese fir trees in subtropical China using dendrochronological methods. We also
performed a basic evaluation of the effects of burning and cutting site preparations on Chinese fir
plantation forests 26 and 36 years, respectively, after planting, including the average trees per unit area,
the average tree diameter, the diameter distribution of trees, basal area (BA) and aboveground biomass.

2. Experimental Section

2.1. Study Area

Our study area is located in the Jiulianshan region of the eastern Nanling Mountains, which is
the greatest mountain range in South China and constitutes an important geographic demarcation
line between the northern and southern subtropical zones of China (Figure 1). There are vast areas
of primary evergreen broadleaved forests in the region, and some of them were heavily logged in
the 1970s and 1980s. In 2003, a national nature reserve was established in the region, and felling was
banned thereafter. The area is characterized by a typical subtropical monsoon climate (with a warm,
rainy season and a cool, dry season); the dry season lasts approximately 5 months (October/November
to March). The average annual precipitation at the Xiagongtang meteorological station in our study
region from 1982 to 2014 was 2024.7 mm with over 70% of the precipitation falling from April to
ForestsForests
2016, 2016,
7, 1777, 177 3 of 15
3 of 14

station in our study region from 1982 to 2014 was 2024.7 mm with over 70% of the precipitation falling
from April
September. Thetomean
September.
annualThe mean annual
temperature temperature
ranges from 24.4 ◦ C in
ranges from
the24.4 °C in the
warmest warmest
month to 6.8 ◦ C
month
(July)
in the(July) to 6.8
coolest °C in(January).
month the coolestRelative
month (January).
humidity Relative
remains humidity remains
more or less morethroughout
constant or less constant
the year
(93% in the wet season, 81% in the dry season). The soil pH ranges from 4.2 to 5.5 [38], and thetosoils
throughout the year (93% in the wet season, 81% in the dry season). The soil pH ranges from 4.2
5.5 [38], and the soils are typical acid soils of subtropical or tropical moist lowlands.
are typical acid soils of subtropical or tropical moist lowlands.

Figure 1. Study
Figure region,
1. Study climate
region, climatedata,
data,and
andforest
forest structure. (a)The
structure. (a) Thesampling
sampling sitesite (red)
(red) in subtropical
in subtropical
China;China; (b) Climate
(b) Climate datadata
fromfrom
thethe Xiagongtang
Xiagongtang meteorologicalstation
meteorological station from 2000
2000toto2014;
2014;(c)
(c)Structure
Structure of
of the clearing
the clearing site preparation
site preparation sampling
sampling site 36
site after after 36 years
years and (d)andthe(d)structure
the structure the controlled
of theofcontrolled burning
burning site preparation sampling site
site preparation sampling site after 26 years. after 26 years.

2.2. Site Preparation in the Region

2.2. Site Preparation in the Region
This region was chosen because it represents an important part of the Chinese fir forest resource,
This region was chosen because it represents an important part of the Chinese fir forest resource,
and it contains both pure and mixed stands. Usually, there are two main site preparation methods
and itforcontains both in
afforestation pure
theand mixed
region: one stands. Usually,
is controlled thereand
burning arethe
two main
other is site
brushpreparation methods
clearing/cutting. To for
afforestation
study theinlong‐term
the region: oneof
effects is different
controlledsiteburning and the
preparation other ison
treatments brush clearing/cutting.
Chinese fir, we selectedTotwostudy
the long-term
Chinese fireffects of different
plantation sites, onesite preparation
of which treatments
was prepared on Chinese
by controlled fir, wewhile
burning, selected two Chinese
the other was
fir plantation
prepared sites,
by brushone clearing.
of whichThe wasbrush
prepared by controlled
clearing burning,
site was planted in while the other
the spring wasand
of 1979, prepared
it
developed into a representative mixed broadleaf‐conifer forest of the subtropics,
by brush clearing. The brush clearing site was planted in the spring of 1979, and it developed into while the controlled
burning site was
a representative mixedplanted in the spring offorest
broadleaf-conifer 1989, and it consists
of the of pure
subtropics, Chinese
while fir stands (Figure
the controlled 1). site
burning
Forest tending
was planted in the methods
spring ofwere
1989,implement primarily
and it consists between
of pure 1 andfir5 stands
Chinese years after the initial
(Figure planting.
1). Forest tending
The two
methods weresites, approximately
implement 1000 between
primarily m apart, do not differ
1 and 5 yearsin after
termsthe
of altitude, soil type, The
initial planting. climate
twoorsites,
former land use.
approximately 1000 m apart, do not differ in terms of altitude, soil type, climate or former land use.
2.3. Experimental Design and Data Collection
2.3. Experimental Design and Data Collection
In 2015, 4‐ha site preparation treatments sites were randomly selected in the study region. Two
In 2015, 4-ha site preparation treatments sites were randomly selected in the study region.
sites were planted in a checkerboard design with homogeneous cells of 3 × 3 trees, each of which had
Two asites
50 cmwere× 50planted
cm × 40incm
a checkerboard design
planting pit with with homogeneous
a planting density of 2700cells × 3 1600
of 3 and
trees/ha trees,trees/ha.
each ofAtwhich
had aeach
50 cmsite,×five
50 cm × 40 cm planting
randomly‐located, 50 pit withand
m‐long a planting
20m‐widedensity of 2700
transects weretrees/ha and 1600
established, with trees/ha.
each
At each site, running
transect five randomly-located, 50 m-long
perpendicularly from and
the valley to 20
them-wide transects
upland [39]. Along were established,
each transect, with each
one circular
transect
plotrunning
of 200 m2perpendicularly from
was established 0–25 m the
and valley
25–50 m tofrom
the upland [39].
the valley, for Along each
a total of twotransect,
plots per one circular
transect
plot of 200 m2 was established 0–25 m and 25–50 m from the valley, for a total of two plots per transect
and 10 plots per site. In each plot, species and diameter at breast height (DBH) of all living trees ≥1 cm
DBH were recorded. Physiographic attributes, such as slope, aspect and elevation, were recorded for
each plot. Between July and August 2015, we sampled randomly 34 and 37 increment cores in each
Forests 2016, 7, 177 4 of 14

site. In a nearby natural forest, approximately 800 m from the cutting site, we collected 34 increment
cores. One radial core per tree was extracted at approximately 1.0 m above ground level for growth
analysis and age determination [40]. Because of the wet nature of the study region, wood cores were
frozen for 2 weeks to prevent insect infestation [41].

2.4. Tree-Ring Analysis

All cores were dried, mounted and sanded with progressively finer sandpaper using standard
procedures. Before measurement, all tree-ring boundaries were marked with a pencil under a
stereomicroscope or magnifying glass, and they were visually cross-dated using pointer years [42].
Tree-ring width was recorded to 0.01 mm using the semi-automatic LINTAB 6 measuring system and
the WinTSAP program [43]. The accuracy of visual cross-dating and measurement errors were further
checked using the COFECHA program [44].
If the age corresponded with the planting period, tree rings were considered to be annual [45].
For each size class of a 5-cm width between 0 and 40 cm DBH, the median, minimum and maximum
passage times were calculated as the number of years spent in a size class [46]. A Kruskal–Wallis test
and a Dunn test were used to test for differences between the sites in the passage times for each size
class. To assess the contribution of each size class to the eventual variation in ages at 30 cm DBH,
we analyzed the effect of passage time in each class on age at 30 cm DBH by a multiple regression that
included all size classes, except those causing high collinearity [46].

2.5. Analysis of Growth Changes

Basal area growth (BAG; cm2 ·year−1 ), rather than diameter growth, was used to calculate growth
events, as Chinese fir has a known age trend, especially in plantations [41]. The use of BAG instead of
diameter growth eliminates observing changes in growth that are caused by a negative correlation
between raw ring width and tree circumference [47]. Based on the tree ring-data, current and mean
annual basal area increments (CAI and MAI, respectively) were obtained using the formulae [28]:
 
BA CAIt = π r2t − r2t−1 (1)

and:
BA MAIt = πrt2 /t (2)

where rt is the ring width in year t.

2.6. Bark Thickness

In keeping with the diameter used by the logging company, the diameter of the outside bark (dob )
in any year was calculated from the diameter of the inside bark (dib ) using a bark coefficient. The bark
coefficient (b) is defined as the diameter of the outside bark divided by the diameter of the inside bark.
The b equation is:
b = dob /dib = robt /ribt (3)

where robt and ribt represent ring width with and without bark, respectively, in year t. Note that the
bark coefficient b should be the mean value of many measurements of different samples. In this study,
b for Chinese fir was 1.09.
One way to arrive at the bark thickness (B) of a tree in any year is to use the following equation:

Bt = rt (b − 1) (4)

where Bt is the bark thickness in year t and rt is the ring width in year t.
Therefore, using the bark thickness of a tree in any year, we can calculate the diameter of the
outside bark in any year.
Forests 2016, 7, 177 5 of 14

2.7. Biomass Estimate

The dry weight of Chinese fir in the sampling plots was calculated from previously published
allometric equations [48]:
WT = 0.096D2.410 (n = 15, R2 = 0.99) (5)

where WT (kg) is the dry weight of the total biomass of standing individuals and D is the DBH (cm).
To estimate the aboveground biomass of evergreen broadleaved trees, we harvested 276 woody species
through destructive sampling in a subtropical primary evergreen broadleaf forest to develop the
general allometric equations for tree species group (data not shown). The regression equations for
calculating the dry weight of evergreen broadleaved trees are:

wagb = 0.360 − 0.440D + 0.306D2 (n = 261, R2 = 0.96) (6)

and:
WAGB = 0.0993D2.4897 (n = 15, R2 = 0.99) (7)

where wabg (kg) is the dry weight of individuals whose DBH was ≥1 cm and <5 cm, and WAGB (kg) is
the dry weight of the individuals whose DBH was ≥5 cm and ≤50.2 cm; D is the DBH (cm), and n
represents the sample size.

3. Results

3.1. Structural and Growth Characteristics of Planted and Natural Chinese Fir
In three forest types, tree-ring analyses were completed successfully. Table 1 shows a concise
summary of the growth differences between planted and natural Chinese fir. Both study sites for
planted Chinese fir had normally distributed diameters (Figure 2). The mean diameter of Chinese fir
trees was 18 ± 5.2 cm in the burning site and 19 ± 6.0 cm in the clearing site (Table 1). The average
tree density of Chinese fir was 2655 ± 442 trees·ha−1 in the burning site and 1610 ± 477 trees·ha−1
in the clearing site (Table 2). More detailed structural characteristics are shown in Figure 2. In the
brush clearing site, however, up to 44 woody plants (data not shown) were systematically recorded to
species. Their average tree density was 2295 ± 655 trees·ha−1 for saplings and 695 ± 320 trees·ha−1
Forests 2016, 7, 177 7 of 15
for broadleaved trees (Table 2).

Figure Trees·ha −1 (only for Chinese fir) of each diameter class for the burning and clearing
Figure 2.
2. Trees ha−1 (only for Chinese fir) of each diameter class for the burning and clearing
preparation
preparation sites.
sites. The
The smooth
smooth curves
curves are
are Gaussian
Gaussian fits
fits to
to the
the data.
data.

The diameter growth varied considerably between study sites. The individual diameter growth
curves showed that the change in growth was very clear among the three study sites (Figure 3). The
mean diameter growth varied from 0.56 cm year−1 for the natural site to 1.10 cm year−1 for the cutting
sites. In the controlled burning and brush clearing preparation sites, the mean diameter growth
exhibited similar variations. During the first 10 years of growth, relatively rapid growth occurred in
the two planted Chinese fir forests based on the individual diameter growth. After 10 years of growth,
Forests 2016, 7, 177 6 of 14

Table 1. The means of the mean annual basal area increment (MAI) and the BA-MAI for Chinese fir at
different sites in the Nanling district in southern China.

Plantation
Nature Chinese
Burning Site Clearing Site
Fir Forest
Preparation Preparation
Latitude 24◦ 320 25” N 24◦ 320 09” N 24◦ 320 20” N
Longitude 114◦ 280 19” E 114◦ 270 56” E 114◦ 270 25” E
Altitude (m.a.s.l.) 765 695 793
Planting density (trees·hm−2 ) 2500~3000 1500~2000 No data
Planting pit size (cm) 50 × 50 × 40 50 × 50 × 40 No data
Slope <30◦ <25◦ <25◦
Age of stand 26 36 >GREATER-80
Mean diameter of stand (cm) 18 ± 5.2 19 ± 6.0 No data
No. of increment cores 37 34 34
MAI (cm) 1.10 ± 0.38a 0.80 ± 0.25b 0.56 ± 0.15c
BA-MAI (cm2 ·year−1 ) 4.28 ± 1.37a 3.03 ± 0.95b 2.24 ± 0.95c
Biological rotation age (BRA, year) 15 26 >GREATER-100
Diameter at BRA (cm) 19 23 >GREATER-50
Local management guides for cutting cycle and minimum cutting diameter
Cutting cycle (CC, year) 25 25 No data
Minimum cutting diameter (MCD, cm) 15 15 No data
Different letters (a, b and c) indicate statistically significant differences at a confidence level of 0.05. The number
of selected trees and the biological rotation age (BRA) (years, cm) for Chinese fir are indicated for the three
sampling sites (different site preparation: burning, cutting and natural) in the study area.

Table 2. Trees·ha−1 and total biomass for Chinese fir in the controlled burning and brush clearing
preparation sites.

Stand Characteristics Burning Site Preparation Clearing Site Preparation

Chinese fir biomass (ton·hm−2 ) 269.8 163.4
Sapling biomass (ton·hm−2 ) 0 3.9
Broad leaved trees biomass (ton·hm−2 ) 0 84.7
Total biomass (ton·hm−2 ) 269.8 252.0
Chinese fir density (trees·hm−2 ) 2655 ± 442 1610 ± 477
Sapling density (trees·hm−2 ) (1–5 cm·dbh) 0 2295 ± 655
other tree density (trees·hm−2 ) (>5 cm·dbh) 0 695 ± 320
Total trees per hectare (>5 cm·dbh) 2655 ± 442 2305 ± 539

The diameter growth varied considerably between study sites. The individual diameter growth
curves showed that the change in growth was very clear among the three study sites (Figure 3).
The mean diameter growth varied from 0.56 cm·year−1 for the natural site to 1.10 cm·year−1 for the
cutting sites. In the controlled burning and brush clearing preparation sites, the mean diameter growth
exhibited similar variations. During the first 10 years of growth, relatively rapid growth occurred in
the two planted Chinese fir forests based on the individual diameter growth. After 10 years of growth,
the diameter growth stabilized at approximately 40 years in the clearing site. In the natural Chinese fir
forest, however, the diameter growth exhibited a slow variation for approximately 100 years.
Comparing the mean diameter growth of the three forest types, the Chinese fir trees planted in the
burning preparation site grew significantly faster than those planted in the clearing preparation site
and those in the natural site (Table 2 and Figure 3). However, until approximately 20 years of age, tree
growth seemed to be high and similar in the burning and clearing sites, although it was significantly
higher than in the natural site (Figure 3). For the Chinese fir from the three study sites, the diameters
at the BRA ranged from 19 cm for the burning site to >45 cm for the natural site (Table 1).
 curves showed that the change in growth was very clear among the three study sites (Figure 3). The
mean diameter growth varied from 0.56 cm year−1 for the natural site to 1.10 cm year−1 for the cutting
sites. In the controlled burning and brush clearing preparation sites, the mean diameter growth
exhibited similar variations. During the first 10 years of growth, relatively rapid growth occurred in
the two planted Chinese fir forests based on the individual diameter growth. After 10 years of growth,
Forests 2016,
the7,diameter
177 growth stabilized at approximately 40 years in the clearing site. In the natural Chinese 7 of 14
fir forest, however, the diameter growth exhibited a slow variation for approximately 100 years.

Figure 3. Individual diameter growth curves of Chinese fir at the three study areas. Mean diameter
Figure 3. Individual diameter growth curves of Chinese fir at the three study areas. Mean diameter
growth curves for the natural Chinese fir forest (solid line), the planted Chinese fir forest of the
growth curves for the natural Chinese fir forest (solid line), the planted Chinese fir forest of the
burning
Forestssite
2016,(bold
7, 177 solid line) and the planted Chinese fir of the clearing site (dotted line). Dashed 8 of 15lines
burningindicate
site (bold solid line)
a constant and growth
diameter the planted Chinese
of 1 cm per year.firThe
of the clearing site
dashed‐dotted line(dotted
represents line).
the Dashed
MCD. lines
indicateNote
a constant
that Comparing thediameter
diameter
individual mean diameter
growth ofgrowth
growth cm of
1curves
pertheyear.
for three
the forest types, thefir
The dashed-dotted
natural Chinese Chinese fir trees
line
were truncated planted
at 40 in
represents cm the
in MCD.
the burning preparation site grew significantly faster than those planted in the clearing preparation
Note that
theindividual
natural forest. diameter growth curves for the natural Chinese fir were truncated
site and those in the natural site (Table 2 and Figure 3). However, until approximately 20 years of
at 40 cm in the
natural forest.
age, tree growth seemed to be high and similar in the burning and clearing sites, although it was
significantly higher than in the natural site (Figure 3). For the Chinese fir from the three study sites,
the diameters at the BRA ranged from 19 cm for the burning site to >45 cm for the natural site (Table
3.2. BRAs 1).

Based on the BAIs, we estimated that the average BRAs for the burning and cutting sites were
3.2. BRAs
15 and 26 years, respectively (Table
Based on the BAIs, 1 and Figure
we estimated that the 4). ForBRAs
average the natural site,and
for the burning there was
cutting not
sites a clear reduction
were
in the number of trees
15 and 26 over
years, 80 years, which
respectively (Table 1 may prevent
and Figure 4). Fora the
precise
naturaldetermination
site, there was notof its BRA. However,
a clear
reduction in the number of trees over 80 years, which may prevent a precise determination of its BRA.
we suggest thatHowever,
the mean BRAs are greater than 100 years, according to the trends in the CAI and MAI
we suggest that the mean BRAs are greater than 100 years, according to the trends in the
(Table 1 and Figure
CAI and 4).MAI (Table 1 and Figure 4).

Figure
Figure 4. Current 4. Current
(CAI) and(CAI)
meanand (MAI)
mean (MAI) annual basal
annual basal area increments
area in relation
increments intorelation
tree ages for
tothe
tree ages for the
Chinese fir in the burning (a), clearing (b) and natural forest (c).
Chinese fir in the burning (a); clearing (b) and natural forest (c).
In contrast to other types of site preparation, the growth rate of Chinese fir in the controlled
burning site during the juvenile stages was significantly high, with an average increment of 1.5 cm
year−1. Annual increments increased dramatically at approximately 12 years and declined slightly at
ages over 20 years. For the brush clearing site, the growth rate during the juvenile stage was relatively
slow compared to that of the burning site. In the planted sites, the MAI was 3.03 cm2 for the brush
Forests 2016, 7, 177 8 of 14

In contrast to other types of site preparation, the growth rate of Chinese fir in the controlled
burning site during the juvenile stages was significantly high, with an average increment of
1.5 cm·year−1 . Annual increments increased dramatically at approximately 12 years and declined
slightly at ages
Forests 2016, 7, 177 over 20 years. For the brush clearing site, the growth rate during the juvenile stage9 was of 15
relatively slow compared to that of the burning site. In the planted sites, the MAI was 3.03 cm2 for the
clearing
brush site and
clearing site4.28
andcm4.28 cm2 forcontrolled
2 for the burning
the controlled site. site.
burning In theIn natural site,site,
the natural the the
MAI of Chinese
MAI fir
of Chinese
was
fir 2.24
was cmcm
2.24 2 2
(Table 2 and
(Table 2 andFigure 4).4).
Figure

3.3.
3.3. Biomass
A concise
concise summary
summary of the biomassbiomass at the the two
two planted
planted sites
sites is
is shown
shown in in Table
Table 2.
2. Based
Based on on the
the
allometric
allometric equations
equations for for biomass,
biomass, we we estimated
estimated thatthat the total aboveground
aboveground biomasses
biomasses for
for the burning
burning
and
and clearing
clearing sites
siteswere 269.8t·tha
were269.8 ha−−11 and 252 t·ha− −1,1 ,respectively.
respectively. The
The diameter
diameter classes
classes of the two sites
were
were normally distributed (Figure (Figure 5). 5). In
Inthe
theburning
burningsite, site,the
thebiomass
biomass forfor diameters
diameters ranging
ranging from
from 12
12 cm to 28 cm was 253.8 t · ha −1 , which accounted for over 90% of the total aboveground biomass.
cm to 28 cm was 253.8 t ha , which accounted for over 90% of the total aboveground biomass. In the
−1

In the clearing site, the biomasses of Chinese fir, saplings and broadleaved trees were 163.4 t·ha −1−1,
clearing site, the biomasses of Chinese fir, saplings and broadleaved trees were 163.4 t ha −1, 3.9 t ha
− 1 − 1
andt·84.7
3.9 ha t and
ha−1,84.7 t·ha , respectively.
respectively.

Figure 5.
Figure 5. Biomass
Biomassdistribution
distributionofofeach
each diameter
diameter class
class for for
the the controlled
controlled burning
burning and brush
and brush clearing
clearing sites.
sites.

The distribution ratio of the aboveground biomass per tree diameter class recorded at each site
exhibited a normal distribution (Figure 6a). Therefore, for the burning site, the maximum value of
the aboveground biomass was obtained for the 22 cm-diameter class. The maximum value of the
aboveground biomass at the clearing site was obtained for 26 cm-diameter class. In the cutting site,
however, two separate groups were observed (Figure 6b): broadleaved trees with diameters smaller
than 20 cm or larger than 30 cm. The smaller diameter trees accounted for 9.3% of the total biomass of
the clearing site, while larger diameter trees accounted for 25.8% of the total biomass of the clearing
site. However, it was observed that the trees·ha−1 of the smaller diameter trees were higher than those
of the larger diameter trees (Table 2).

Figure 6. The biomass distribution ratio of each diameter class in the controlled burning and brush
clearing sites (a) and the biomass distribution ratio of each diameter class of Chinese fir and
broadleaved trees in the brush clearing site (b).

The distribution ratio of the aboveground biomass per tree diameter class recorded at each site
exhibited a normal distribution (Figure 6a). Therefore, for the burning site, the maximum value of
the aboveground biomass was obtained for the 22 cm‐diameter class. The maximum value of the
aboveground biomass at the clearing site was obtained for 26 cm‐diameter class. In the cutting site,
ForestsFigure
2016, 7,5.177
Biomass distribution of each diameter class for the controlled burning and brush clearing9 of 14
sites.

Figure
Figure 6.
6. The
The biomass
biomass distribution
distribution ratio
ratio of
of each
each diameter
diameter class
class in
in the
the controlled
controlled burning
burning and
and brush
brush
clearing sites (a) and the biomass distribution ratio of each diameter class of Chinese fir and
clearing sites (a) and the biomass distribution ratio of each diameter class of Chinese fir and broadleaved
broadleaved trees clearing
trees in the brush in the brush clearing site (b).
site (b).

The distribution ratio of the aboveground biomass per tree diameter class recorded at each site
4. Discussion
exhibited a normal distribution (Figure 6a). Therefore, for the burning site, the maximum value of
the
4.1. aboveground biomass
Evaluation of the was obtained
Mean Diameter and BRAfor the 22 cm‐diameter class. The maximum value of the
aboveground biomass at the clearing site was obtained for 26 cm‐diameter class. In the cutting site,
Long-term growth data would be indispensable for sustainable forest management and
however, two separate groups were observed (Figure 6b): broadleaved trees with diameters smaller
planning [49]. However, in the long term, such information might not have been sufficiently assessed
than 20 cm or larger than 30 cm. The smaller diameter trees accounted for 9.3% of the total biomass
through detailed surveys. In the present study, we observed differences in Chinese fir tree growth
rates between the three sites. The planted Chinese fir MAI was 1.1 cm·year−1 and 0.80 cm·year−1 for
the burning and clearing sites, respectively, but it only 0.56 cm·year−1 for the natural site. Based on
these rates, Chinese fir trees in the clearing site would increase to 15 cm DBH during the first 20 years
of growth, whereas Chinese fir trees in the burning site would increase to 22 cm DBH in the same
time frame. Our growth rates are slightly higher than previous estimates for Chinese fir in the same
district [50–52]. For example, Cai et al. reported a MAI of 0.81 cm·year−1 and a CAI of 0.90 cm·year−1
for planted Chinese fir in the first 10 years. The reason for this variation might be partly due to several
factors, including site preparation, site differences or planting density. To the best of our knowledge,
there is no study of the growth rate of planted Chinese fir in our study region, which prevented a
comparison of growth rates.
In the natural site, Chinese fir trees needed over 100 years to reach the BRA (Table 1 and Figure 4).
However, the Chinese fir trees in the clearing site, in which the environmental conditions are probably
very similar to those of natural forests several decades previously, required only 26 years to the BRA.
This implies that the Chinese fir trees in the brush clearing site required 18 years to reach the MCD
established by the local forest regulations. Some local studies of planted Chinese fir forests indicated
that the cutting cycles (CCs) ranged from 18–26 years based on technical maturity, economic maturity
and quantitative maturity [52,53]. This corresponds with our results. However, the 25-year CC for
the burning site was significantly longer than the BRA (Table 1 and Figure 4). Meanwhile, over
30 years should be taken to enhance the proportion of large-diameter trees on high-quality forest
land [52]. The proportion of Chinese fir trees that had a DBH >20 cm 36 years after planting was
50.3% in the clearing site, whereas the same proportion at the burning site 26 years after planting was
40.3%. Several studies indicated that tree growth releases mostly take place during the juvenile phase
because of competition for light [9,54]. In the clearing site, the Chinese fir trees were the dominant
trees because they outcompeted other trees for light. This implies that the growth of Chinese fir in
the burning site would be suppressed, while the growth of Chinese fir in the clearing site would be
released, in the following decade. A similar study in a nearby district also indicated that ecological
Forests 2016, 7, 177 10 of 14

thinning and cutting significantly increased DBH and individual growth volume [55]. Recent studies
also suggested that forest management should be adapted to reduce competition for resources within
stands to enhance tree growth [56–58]. In addition, as one of the most important concepts of sustained
management, the MCD can be calculated according to a dendrochronological analysis of tree growth.
Although most studies in the tropics or subtropics mainly focus on a volume-based MCD [52,53,59–61],
the MCD can also be calculated from BAIs [62–64]. However, the use of volume-based CAI and
MAI is often constrained by height measurements and the existence of significant age-diameter and
diameter-height relationships, as reported by Schöngart [61]. Generally speaking, the MCD or BRAs
determined by BA are lower than those that are determined by volume, although there are only slight
differences in the shapes of the CAI and MAI curves. From a forest regeneration viewpoint, the MCD
and the associated BRA should also consider tree physiology [65]. This implies that the BRA should
be larger than the fructification ages. Many studies of tropical species suggested that logging ages
should be four to five times longer than currently practiced [61,66]. However, it is unclear whether
MCD/BRA based on volume or BA is a better choice. By graphing the CAI and BA MAI curves, the
point at which they intersect is considered by many landowners to be representative of the BRA for
the stand. We estimated that the BRAs of the burning, clearing and natural sites were 15, 26 and
>100 years, respectively. Our results showed that to reach an MCD of 15 cm, the planted Chinese fir
trees in the burning and clearing sites required 14 and 19 years, respectively. These data indicate that
CCs for planted Chinese fir are shorter than those currently practiced by local logging companies or the
government. This implies that the current 25-year CC established by the local forestry administration
is significantly longer than the BRA of the burning site. Without a doubt, the brush clearing site
preparation delayed the cutting rotation age, which will aid the preservation of the structure and
composition of natural forests as sources of wood and biological diversity.

4.2. Evaluation of the Aboveground Biomass

The total aboveground biomass was 269.8 t·ha−1 in the burning site and 252 t·ha−1 in the clearing
site (Table 2). As a result, the total biomass in the clearing site was slightly lower than that in the
burning site. This may be due to the juvenile tending during the first five years after planting Chinese
fir, in which all other seedlings or saplings were cleared. This also explains the small numbers of
broadleaved trees from 20–28 cm DBH in the clearing site (Figure 6b). Another reason may be the
enhanced stand density values of the burning site, which contained 39.4% more crop trees·ha−1 than
the brush clearing site. Many previous studies indicated that transformation of broadleaved forests
to pure Chinese fir plantations would decrease biomass/carbon storage and the production of the
first rotation [33,35,67]. This is not consistent with our results. Chinese fir plantations’ biomass,
nutrient accumulation and allocation have also been studied, although the results have been mixed.
For example, Yang et al. [68] reported that stand age was the main factor affecting biomass and nutrient
accumulation and allocation, and different organs had significant differences in their biomass and
nutrient storage. Yu et al. [69] reported that a 32-year-old Chinese fir plantation had a medium-to-high
level of total biomass. In contrast, Wu et al. [70] found that crop tree release in a Chinese fir plantation
not only promoted stand growth, but also optimized stand structure, which enabled crop trees to
sustain rapid growth and reach larger diameters. Site preparation for reforestation or afforestation is
difficult to implement accurately, although it is easier if topography does not vary greatly. However,
the benefits of site preparation appear to be more obvious in the early years of growth, and they
often decrease over time, with growth rates coinciding with non-prepared sites by the end of the
rotation [71,72]. Furthermore, site preparation by controlled burning could significantly reduce
biodiversity, which might make crop trees more vulnerable to particular pests and diseases. A recent
study indicated that minimizing the disturbance of site preparation would mitigate soil carbon
release [21]; thus using brush clearing site preparation would be more practical and a good choice for
site preparation based on stand biomass estimates.
Forests 2016, 7, 177 11 of 14

5. Conclusions
This study described the main growth characteristics of Chinese fir at a burning preparation
site, a clearing preparation site and a natural forest, and therefore, it considered the different site
preparations for planting Chinese fir in subtropical China. A tree-ring analysis, as well as a biomass
evaluation, is a valuable tool for managing natural and planted forests [54]. Inter-annual changes
in tree-ring widths were measured using tree rings of Chinese fir trees from the burning, clearing
and natural sites. Chinese fir trees showed similar patterns of growth responsiveness to controlled
burning and clearing site preparations at the plot and tree scales, but they grew significantly slower
in the natural site. A comparison of the growth variation and the total aboveground biomass, which
incorporated plot survey data and tree ring data, showed significant differences in density, average
DBH, BA, biomass and BRA between Chinese fir populations at the various site. The controlled
burning site offered more favorable environmental conditions for the early development of Chinese
fir, and the brush clearing site offered suitable natural conditions for the long-term development
of Chinese fir and natural forests. In conclusion, these results are a useful source of information
for the sustainable management and planning of Chinese fir plantation, such as those occurring in
subtropical China.

Acknowledgments: This study was financially supported by the National Hi-tech Research and Development
Plan (Grant No. 2013AA122003). Field work was aided by the Administration Bureau of Jiulian Mountain
National Nature Reserve, which granted us permission to conduct surveys and samplings in the forest. We thank
Zhong Hao, Zhong Yuanchang, Liang Yuelong, Hu Hualin and Fu Qinglin for providing logistical support.
This manuscript was improved by the anonymous reviewers.
Author Contributions: Hua Zhou wrote the manuscript. All authors contributed to study design and manuscript
editing, and Qijing Liu conducted the analyses.
Conflicts of Interest: The authors declare no conflict of interest.

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