Nutrient cycling and distribution in different-aged plantations ofreported
Chinese fir in southern China
Xiangqing Ma a ,*,Kate V .Heal b ,Aiqin Liu a ,Paul G.Jarvis b
a
Forestry College,Fujian Agriculture and Forestry University,Fuzhou,350002Fujian Province,PR China
b
School of GeoSciences,The University of Edinburgh,Crew Building,West Mains Road,Edinburgh,EH93JN Scotland,UK
Received 31August 2005;received in revised form 25January 2007;accepted 7February 2007
Abstract
The distribution in tree biomass and understorey vegetation and annual biological and geochemical cycling of total nitrogen (N),phosphorus (P),potassium (K),calcium (Ca)and magnesium (Mg)were measured in young,middle-aged and mature plantations (8-,14-and 24-years old)of Chinese fir (Cunnin
ghamia lanceolata (Lamb.)Hook.)in southern China.Although >98%of nutrients occurred in the soil,soil nutrient content decreased with plantation age.Nutrient outputs from the soil exceeded inputs in stands of all ages but the net soil nutrient loss increased significantly for N,P and Ca with plantation age.Comparison of nutrient fluxes showed that the smallest (and hence limiting for nutrient cycling)fluxes were litter decomposition in the young plantation in contrast to canopy fluxes (apart from for Mg)in the middle-aged and mature plantations.Nutrient use efficiency,release of nutrients from litter decomposition and nutrient return,particularly in litterfall,increased significantly with plantation age.These results suggest that,as stand age increases,nutrient cycling in Chinese fir plantations is increasingly dominated by biological processes and becomes less dependent on external nutrient sources in rainfall and the soil.It therefore appears that prolonging the rotation length of Chinese fir plantations by approximately 5years could be beneficial for maintaining the soil nutrient status for successive plantings.#2007Elsevier B.V .All rights reserved.
Keywords:China;Chinese fir;Nutrient cycling;Nutrient distribution;Plantation
1.Introduction
Many short-rotation plantations that couple intensive management with rapid growth rates may lead to
high rates of nutrient removal in harvesting,raising concerns about long-term site quality and sustainable production,although it has been difficult to quantify the underlying mechanisms (Wang et al.,1991).In recent decades the rapid extension of plantations has increased the need for a better understanding of the relationship between nutrient cycling and sustainable production in intensively managed plantation systems (Miller,1984).
Chinese fir (Cunninghamia lanceolata (Lamb.)Hook.),a fast-growing,evergreen coniferous tree with high yield and excellent wood quality,is one of the most important tree species for timber production in southern China with a planting history extending over more than 1000years (Wu,1984).Because of its
high commercial value,native broad-leaved and coniferous forests are often transformed into plantations of pure Chinese fir (Lian and Zhang,1998).The total plantation area of Chinese fir in China is around 9.11million ha (Forestry Ministry of China,1994).Because the area of Chinese fir plantations has expanded rapidly it has become necessary to establish plantations on land from clear-cut Chinese fir plantations (rather than the previous practice of planting on land cleared of broad-leaved forest),resulting in successive crops of Chinese fir at the same site.It is common practice in China for plantations of Chinese fir to be managed on a rotation of about 20–25years.In mature Chinese fir stands timber is harvested by clear-cutting,the stems are removed and the residues (foliage,branches
and roots)are either burnt (Yu,1997)or,as has now become the usual practice in southern China,left to decompose on site in preparation for establishment of the next crop.Seedlings are planted in pits (60cm top width,40cm bottom width and 40cm depth)and weeding is conducted twice a year in the first 3years after planting.
Many studies have reported a serious decline in the productivity of successive rotations of Chinese fir utilising
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Forest Ecology and Management 243(2007)61–74
*Corresponding author.Tel.:+8659183568316;fax:+8659183780261.E-mail addresses:mxq@public.fz.fj ,lxymxq@126 (X.Ma).0378-1127/$–see front matter #2007Elsevier B.V .All rights reserved.doi:10.1016/j.foreco.2007.02.018
the management system described above.For example,Fang (1987)found a reduction in height of15-y
ear-old Chinesefir trees of the second and third rotations of7%and33%and Yu and Zhang(1989)reported decreased tree volumes of28%and 69%for the second and third rotations,respectively,compared with thefirst rotation.It is extremely important to understand the reasons for productivity decline in Chinesefir plantations in successive rotations in order to develop sustainable manage-ment of these plantations in China(Yu,1993).Ding and Cheng (1995)and Evans(1999a)suggest that the reported yield declines result more from silvicultural whole tree harvesting,management of organic matter and competition from weeds(grasses and bamboos))rather than from the Chinesefir itself.A review of the topic by Evans(1999a) concluded that on a wider scale the evidence of productivity changes in successive rotations in tropical plantations is limited and often difficult to interpret due to confounding factors.For example,no change in yield in intensively managed Pinus patula plantations in Swaziland was measured between the second and third rotations,despite the occurrence of a severe drought in the latter rotation(Evans,1999b)and in the Amazon basin of Brazil productivity appears to be increasing over successive generations due to evolution of silvicultural practices and genetic improvement(McNabb and Wadouski, 1999).Significant yield declines in second rotation Pinus radiata plantations in South Australia appeared in the1960s but were rectified by improved harvesting and site preparation practices and using genetically superior stock so that by the 1990s no decrease in yield occurred fromfirst to second rotations(Woods,1990).
From the early1980s many studies have investigated biomass production and nutrient cycling in Chinesefir plantations.Pan et al.(1981,1983)reported a total nutrient accumulation of1563–1684kg haÀ1in mature plantation(but crown leaching was not included).A slightly higher nutrient accumulation of1994kg haÀ1was estimated by Feng et al. (1985)for a21-year-old Chinesefir plantation,in which nutrients in crown leaching and litterfall were included in uptake and the ratio of stand uptake to return was reported to be 0.39.Zhong and Hsiung(1993)evaluated the nutritional status of Chinesefir plantations,whilst Nie(1994)compared nutrient dynamics between three Chinesefir stands in different site conditions.The study demonstrated that nutrient cycling varied with site factors and silvicultural practices.Investigations of litterfall and its effect on nutrient return in Chinesefir stands showed that litterfall production and its decomposition rates were low compared with other species(Ma et al.,1997,2002; Xue,1996;Yang et al.,2004).Yang et al.(2005)compared nutrient return in litterfall in29-year-old Chinesefir plantations with natural forests in China and concluded that site productivity may be compromised in the former due to lower nutrient return in litterfall.Chen(1998)investigated net primary productivity and nutrient stocks(but not nutrient cycling)in Chinesefir plantations of different ages.The highest nutrient concentration was found in foliage and the lowest in stemwood.Foliage accounted for41–48%of the mass of individual nutrients in whole trees.From these studies there is now a reasonable understanding of the effect of climate,soil, site conditions and silvicultural practices o
n nutrient cycling in Chinesefir plantations.However it is still unclear how nutrient cycling(nutrient uptake,retention,litter,leaching return and geochemical cycling)varies with stand age and also whether productivity decline in successive Chinesefir plantations is related to nutrient availability(Sheng,1992).
In this study,the objectives were to quantify and compare the distribution and cycling of the nutrients nitrogen,phosphorus, potassium,calcium and magnesium in all compartments of Chinesefir plantations of different ages,including understorey vegetation,under similar site conditions.From the results the contribution of nutrient availability to the reported productivity decline in successive rotations of Chinesefir was assessed and recommendations made for measures to maintain nutrients for successive rotations of Chinesefir.
2.Materials and methods
2.1.Study sites
The plantations studied were at Youxi Ecological Research Station,Youxi County(25.8–26.48N,117.8–118.68E),in central Fujian Province,southern China(Fig.1)which has a subtropical maritime monsoon climate.Mean annual precipita-tion is1600mm,with56%falling in the rainy season from March to June.The highest recorded maximum daily precipitation is131.7mm and mean annual potential evapor
a-tion is1323mm.Mean annual temperature is18.98C and monthly mean temperature ranges from8.98C in January to 27.88C in July(data from meteorological station,located2km from thefield station).Three stands offirst rotation8-,14-and 24-year-old Chinesefir were selected for the investigation, representing the three developmental stages(young,middle-aged and mature).All three sites were on an east-facing hillside at about210m a.s.l.The young plantation was located3km and0.7km from the middle-aged and mature plantations, respectively.Prior to planting of Chinesefir all sites had
been Fig.1.Location of study area in central Fujian Province,southern China.
X.Ma et al./Forest Ecology and Management243(2007)61–74 62
used for one rotation of Pinus massonina plantation (normal rotation length 30years)which has a similar nutrient management regime to Chinese fir.The middle-aged and mature plantations had been thinned under the standard practice for Chinese fir plantations (removal of alternate rows and cutting of the crowns of thinned trees on site).The understorey vegetation in all the plantations studied mainly comprised shrubs and grasses (such as,Oreopteris dichotoma ,Woodwar-dia japonica ,Selaginella doederleinii ,Adinandra millettii ,Miscanthus floridulus )and a few small bamboo.The stand characteristics and selected soil properties are shown in Table 1.The soils were silty loam oxisols developed on sandstone,with a depth of about 80cm.Although the depth of the litter layer on the forest floor increased from the young to the mature plantation,soil organic matter content decreased with planta-tion age.
Measurements of biomass,litterfall,litter decomposition,rainfall,runoff,throughfall and stemflow were made in the young Chinese fir plantation from 1987to 1994,although only litter decomposition results for 1993–1994and other data for 1994when the trees were 8years old are presented here.In the middl
e-aged and mature plantations all the measurements were conducted in 1997–1998,apart from runoff data for the mature plantation which come from Pan et al.(1989).2.2.Biomass
In each of the young,middle-aged and mature stands,twelve 20m Â5m sample plots were selected with the same geology,soil type,gradient and aspect.Litter depth and the height and DBH were measured of every tree in each plot.Since tree DBH and height did not vary much within each stand (Table 1)and it had been previously shown that the biomass of the mean average tree can represent stand biomass in Chinese fir plantations in the same area (Yu,1997),the mean average tree was used to estimate stand biomass.Four trees in each stand that were outside but close to the sample plots were chosen with the same height and DBH as the mean height and DBH of the trees in the plots.These trees were cut down and the roots carefully dug out from the different soil layers and washed with water.The above-ground biomass of each sample tree was divided into stemwood,bark,branches and foliage.The different biomass compartments for each tree were weighed in the field and then sub-sampled for determination of the dry mass and nutrient concentrations.From these sample trees the biomass of each component of the mean tree was calculated.The biomass of each stand was then calculated by multiplying the biomass of the mean tree by tree density.Allometric equations were not derived from the felled trees because the four sample trees in each stand were sel
ected to represent the average tree and consequently the DBH and height of the sample trees did not vary greatly within each stand.The above-and below-ground biomass of understorey vegetation was determined by harvesting eight randomly selected 1m Â1m plots in each stand.All biomass samples were oven-dried at 808C to constant weight and ground to pass through a 1mm stainless steel sieve prior to analysis.After 1
T a b l e 1P r o p e r t i e s o f t h e t h r e e C h i n e s e fir s t a n d s a n d u n d e r l y i n g s o i l s (0–20c m d e p t h )
S t a n d A g e (y e a r s )D e n s i t y (s t e m s .h a À1)D B H (c m )B a s a l a r e a (m 2h a À1)H e i g h t (m )L i t t e r l a y e r (c m )
S B D (g c m À3)p H i n w a t e r S O M (m g g À1)C E C (m e k g À1)A v a i l N (m g k g À1)A v a i l P (m g k g À1)A v a i l K (m g k g À1)Y o u n g 83602Æ198.4Æ1.020.1Æ4.56.4Æ0.77.5Æ1.11.11Æ0.105.6Æ0.835.1Æ0.699.6Æ8.963.59Æ2.13.84Æ0.369.0Æ3.1M i d d l e -a g e d 141950Æ2614.1Æ1.430.5Æ5.99.6Æ0.516.0Æ1.51.07Æ0.075.6Æ0.631.4Æ0.982.3Æ7.859.36Æ1.83.23Æ0.567.5Æ2.7M a t u r e 241775Æ3115.8Æ1.935.0Æ8.012.8Æ0.918.5Æ1.41.09Æ0.095.4Æ0.726.3Æ0.489.2Æ8.254.58Æ1.33.42Æ0.5
66.9Æ2.4
N o t e s .D B H :d i a m e t e r a t b r e a s t h e i g h t ;S B D :s o i l b u l k d e n s i t y ;S O M :s o i l o r g a n i c m a t t e r (i n c l u d i n g t h e o r g a n i c h o r i z o n b u t n o t t h e l i t t e r l a y e r );C E C :c a t i o n e x c h a n g e c a p a c i t y .S e e t e x t f o r d e s c r i p t i o n o f t h e s o i l a n a l y s i s m e t h o d s .D a t a a r e m e a n Æ1s t a n d a r d d e v i a t i o n o f 12p l o t s /s o i l p i t s i n e a c h s t a n d .
X.Ma et al./Forest Ecology and Management 243(2007)61–74
63
year,the sampling and analysis of the trees and understorey vegetation were repeated and the differences in biomass taken as the annual biomass increments.
2.3.Litter
The litter compartment was quantified in each plantation by collecting by hand all the litter on the forestfloor in each of the eight plots used to sample understorey vegetation on the second occasion.The litter from each plot was separated into Chinese fir and understorey litter,weighed in thefi
eld and then returned to the laboratory for determination of dry mass and nutrient concentrations.In addition litterfall was collected monthly for2 years in12litter traps of1mÂ1m surface area in each stand. Because the plantations were on a slope,one side of the litter traps was in contact with the ground surface and the other side was supported by two posts to create a level surface area of 1m2.Litter traps captured all litterfall from Chinesefir and understorey vegetation,apart from some low understorey plants.The litter was sorted into different fractions:understorey and Chinesefir twigs,foliage,cones andflowers,and remainder.The dry masses of the fractions were obtained after oven-drying at808C to constant mass and each fraction was sub-sampled for chemical analysis.To investigate litter decomposition,200g samples of litter were placed in perforated bags with holes of0.2mm.Decomposition of both composite litter samples(comprising Chinesefir foliage,twigs and cones,mixed according to their fractions in litter)and single litter fractions were examined.In each stand48bags of each litter type were placed on the ground and two bags were removed monthly for2years for mass and nutrient analyses. Mass loss from the litter bags over2years averaged51%,56%, 36%and40%for the composite,foliage,twig and cone samples,respectively.Rates of litter decomposition were calculated as k-values from Olson’s equation(1963)which assumes decay with no production,
X¼X0eÀkt(1) where X0is the initial mass of litter in bag and X is the mass of litter after time t.
2.4.Runoff
The twelve20mÂ5m sample plots in each of the young and middle-aged stands were also used for monitoring runoff.The plots were isolated with cement plates placed in the soil to a depth of50cm and runoff was collected in2mÂ1mÂ1.2m tanks. On a weekly basis the volume of runoff and sediment yield were measured and runoff and sediment samples collected for nutrient analysis.When runoff volumes and sediment depth were low in the tanks,sediment yield was estimated from the total suspended solids concentration of the agitated water/sediment mixture. Under conditions of high runoff and sediment yield,sediment yield was determined as the mass of sediment deposited within the tank.It was not possible to establish isolated runoff plots in the mature plantation,because of dense understorey vegetation and Chinesefir roots,so data were used from Pan et al.(1989) who measured runoff and sediment yield using the same methods in1987from four large(20,000m2)isolated plots at a22-year-old Chinesefir plantation in Hunan Province(about500km west of the study site).This plantation was established on a site clear-cut of natural forest and was underlain by yellow mountain earth soils above a slate geology.
2.5.Rainfall,throughfall,stemflow
Rainfall was measured for2years outside but close to the plantations with two tilting siphon raingauges.The raingauges siphoned into a bottle,which was sampled weekly for chemical analysis.Throughfall was collected in eight barrels(surface area706cm2),randomly placed on the ground in each stand.In each stand stemflow was measured on10trees(two average trees withinfive DBH classes)using semicircular plastic ducts, wrapped around and sealed to the stem,which connected to a collecting vessel.Throughfall and stemflow volumes were measured weekly for2years and sampled for chemical analysis.Total stemflow for each stand was calculated as the sum of the mean stemflow of each DBH class multiplied by the number of trees within that DBH class in the stand.
2.6.Soil
Soil was sampled at12locations randomly selected in each stand.At each sampling location,a soil pit was dug and three samples were taken at20cm depth intervals until the parent material was reached(about80cm depth).Stone-free soil bulk density was determined by collecting samples involumetric rings (100cm3),drying to constant weight and removing stones (>2mm diameter)before reweighing.In preparation for analysis,soil samples were air-dried in the laboratory and then passed through a2-mm mesh sieve to remove stones and large roots before oven-drying at658C.The oven-dri
ed samples were then crushed and passed either through a0.25mm-mesh sieve for determination of organic matter content,total N,P,K,Ca and Mg and cation exchange capacity or through a1mm-mesh sieve for determination of pH and available N,P and K.Soil pH was measured with a pH meter in a1:1mixture of soil and CO2-free water which had been agitated for1min.Soil organic matter was determined by wet oxidation using potassium dichromate and cation exchange capacity by extraction with1N ammonium acetate.The moisture content of soil samples was also measured after oven-drying at1058C in order to express the soil nutrient analysis results per dry weight of soil.
2.7.Nutrient analyses
All plant,soil and water samples were analysed to determine the concentrations of total nitrogen(N),phosphorus(P), potassium(K),calcium(Ca)and magnesium(Mg).Total N in the soil was extracted with H2SO4,P with H2SO4–HClO4,and K, Ca and Mg with Na2CO3.Available N,P and K were also determined in the soil samples according to standard methods for acid soils(Nanjing Institute of Soil Science,1978).Available N
X.Ma et al./Forest Ecology and Management243(2007)61–74 64
was extracted with 1.8N NaOH and 1g FeSO 4,available P with 0.03N NH 4F–0.025N HCl and availabl
e K with 1N ammonium acetate.All plant samples were digested with HNO 3–HClO 4–H 2SO 4.In the plant digests and soil extracts N was determined by distillation,P with the molybdenum blue method,K by flame photometry,and Ca and Mg by atomic absorption spectro-photometry (Allen et al.,1974;Bartram and Ballance,1996;Nanjing Institute of Soil Science,1978).All chemical analyses were carried outin triplicate.It should be noted that the soil results reported here focus on total nutrients and that in most cases there is very little relationship between total and available nutrients.2.8.Data analysis
2.8.1.Calculation of nutrient stocks,fluxes and cycling coefficients
The stand nutrient content was obtained by multiplying each biomass fraction by the respective nutrient concentration and finally summing the contents of the individual fractions.The mass of nutrients in the soil was calculated by multiplying the mean concentration of each nutrient in each layer by the corresponding mean soil bulk density.The terms used to assess nutrient cycling and distribution in the stands are defined below.It should be noted that canopy flux as defined here may overestimate nutrient uptake because crown leaching and deposition captured by the canopy cannot be separated in the data available (Nie,1994;Barnes et al.,1998;Vitousek,1982,1984;Wang et al.,1991).
Nutrient uptake ¼annual nutrient increment
þnutrient return
(2)
Annual nutrient increment
¼X ðbiomass of each component
Ânutrient concentration at end of year Þ
ÀX ðbiomass of each component Ânutrient concentration at start of year Þ(3)Nutrient return ¼ðChinese fir litterfall biomass
ÂChinese fir litterfall nutrient content Þ
þðunderstorey litterfall biomass Âunderstorey litterfall nutrient content Þþcanopy flux
(4)Canopy flux ¼½ðthroughfall volume
Âthroughfall nutrient concentration Þþðstemflow volume
Âstemflow nutrient concentration Þ Àðrainfall volume
Ârainfall nutrient concentration Þ
(5)Cycle coefficient ¼
nutrient return
nutrient uptake
ðNie ;1994Þ
(6)Net accumulation
¼nutrient input in rainfall
Ànutrient output in runoff ðwater and sediment Þ(7)
Nutrient use-efficiency ¼
dry biomass of stand nutrient content of stand
ðBarnes et al :;1998;Vitousek ;1982;1984;Wang et al :;1991Þ
(8)
2.8.2.Statistical analysis
t -Tests,one-way analysis of variance (ANOVA)with Tukey’s multiple comparison tests,and F -test comparison of means and least significant difference test statistics were used as appropriate to determine whether nutrient stocks and fluxes differed significantly (P <0.05)between plantations of different ages.Many of the nutrient stores and fluxes were calculated from measurements with different sample sizes (for example,annual biomass increment comprised biomass increment in Chinese fir (n =4per stand)and understorey vegetation (n =8per stand)).When conducting statistical comparisons of calculated nutrient stores and fluxes between plantations of different ages the smallest sample size was used to calculate the degrees of freedom and critical test statistic to minimise the probability of incorrectly identifying significant differences between stands (type II errors).3.Results
3.1.Nutrient distribution in plantation ecosystems
The stores of nutrients in soil and different biomass fractions of the plantation ecosystems studied are shown in Table 2.In plantations of all ages more than 98%of each nutrient was in the soil compartment,although as stand age increased,the nutrient content of the soil decreased significantly.A
bove-and below-ground biomass and stocks of all nutrients in Chinese fir trees increased significantly with stand age along with nutrients stored in the litter layer from both Chinese fir trees and understorey vegetation.Above-and below-ground understorey biomass showed bimodal patterns with stand age.Below-ground biomass decreased between the young and middle-aged plantations and then increased significantly in the mature plantation.The above-ground biomass of the understorey vegetation decreased significantly between the young and middle-aged plantations and then increased in the mature plantation (but not significantly)to similar values as in the young plantation.These changes are probably due to the response of under-storey vegetation to changes in light resulting from canopy closure of the Chinese fir and thinning in the middle-aged and mature plantations.Nutrient stocks in above-ground under-storey vegetation varied significantly with plantation age:N,P and K generally decreased,whilst Ca and Mg increased with plantation age.
X.Ma et al./Forest Ecology and Management 243(2007)61–7465
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