全 文 ：5-；
第19卷第4期
1999年 7月
生 态 学 报
ACTA ECOI OGICA SINICA
Vo1 1 9，No．4
July，1 999
沙棘对中国亚湿润干旱区的杨树人工林养分
分布及生物循环影响
(中国林业科学研究院森林生态环境与保护研究所 ，北京 100091)
了
- I、I、
摘要 通过将沙棘(Hippophae rhamnoldes L )与三种杨树品种 叫、黑畅(Populus‘Xiaohei’)一昭林6号扬(P．‘Zaolin00’)
和欧美畅64号，(P．⋯ 卅 ⋯ N3016’))的凡工韩分别按株馄和行混两种方式进 行混交宴验，研究了固氮植物沙
棘对亚湿润干旱 区的杨树人工林养分分布和循环的影响 连续3a的观测研究得到 下结果：沙棘叶片的氟，磷和钾的元
素裱度显著地高于杨树叶片，混空林中杨树各器官的养分浓度大于纯林 中的橱树．但是养j}浓度随元素和扬树品种而
异{混交林中的土壤的总氮含量和有效氟含量分别比纯#高86 ～164 和19％～36 I混交林中扬树生物量增加不总
是与叶片的氯素裱度正相关，但是与氟素年吸收量的增加有关 ；由于混变林中养分的归还与吸收比率大干纯林 ，所以杨
树混交林 中养分的年归还量增加-这意味着扬树与沙棘混交之后有大量的养分进入再循环过程。研究结果表明，沙棘在
扬树人工林中作为氯泵起到了维持土壤中稳定的氮素有教 以礴足杨树迅速生长的作用
关键调；亚湿润干旱区；沙棘；杨树凡工林 ·养分j}布 生物循环
Effects of seabuckthorn(Hippophae rhamnoides L．)on nutrient dis—
tribution and biological cycling of poplar plantations in dry subhu—
mid area of China
I IU Shi—Rong (Re drch lnslitute of F。，- EcOlogy．Envim⋯ ， Ⅱ月d Protecti⋯ Chinf5F Academy 0，Fore try．
Beijlng，100091·China)
Abstract：Effects of seabuckthorn (Hippophae rharanoides L．)on nutrient distribution and nutrient cycling
of poplar plantations in dry subhumid area of China were examined over 3 years by introducing seabuck—
thorn into three poplar stands of different varieties(Populus ‘Xiaohei’，P． ‘Zaolin06 and P．eurameri一
P cv． ‘N3016’)t and in tWO different mixing designs(individual—or strip—mixing patterns)．The concert—
trations of N，P and K were found tO be significantly higher in the seabuckthorn leaves than in the poplar
eaves．Nutrient in poplar tissues were frequently larger in the mixed stands than in the
pure startds，but the nutrient concentrations varied with elements and varieties．The concertirations of soil
total N and available N were 86 ～ 164％ and 19 ～ 36 higher in the mixed stands than in the pure
stands．The enhanced growth was not always correlated with foliar N concentration，but related to the in-
creased annual amount of N uptake．The annual nutrient returns in the poplar mixed stands~,-gere en—
haneed，as the ratios of annual nutrient return tO uptake were higher in mixed stands than irt pure stalds．
This implied that there was a large amount of nutrients involving in the nutrient recycling processes in the
poplar stands when mixed with seabuckthorn．The results indicated that seabuckthorn。acting as a N
pump，plays art important role in maintaining sufficiently stable N ava lahil ty in concert with rapid poplar
tree growth．
Key words：dry subhumid are日~seabuekthorn~popular plantation~nutrieixt distribution；biological cycling
文章编号 ；1000 0933(1999)04—0534—09 中圉分类号 ：$718 95 4 2 文献标诅码：A
Reeel*ed dlite l998—03—16
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4期 刘世荣 ：沙棘对 中国亚湿润干旱区的杨树^工林养分分布及生物循环影响 53o
1 Introduction
Seabucktbo rn(H ippophae rhamnoides L．)has long been recognized as an important nitrogen—fixing
and drought—resistant tree species．In northwest of China，seabuckthorn was widely used to stabilize and
enrich infertile soils，while to provide fruits for drink making industry Although it has falen out of favor
a5 a plantation species，seabuckthorn is considered as a good companion of many broad—leaved tree species．
The presence of seabuckthron within a forest may result in significant changes to the internal cycling of ni—
trogen-
Poplar is a fast growing and high—yielding plantation species in dry subhumid area of China，but the
growth is affected by nutrient and water stresses．This problem is increasingly concerned under successive
rotations with continuous cropping 。’ ．Introduction of the seabuckthorn into pure poplar plantations by
mixing approach may lead to significant changes in organic matter decomposition and nutrient recycling，
and hence to an increase in tree growth and biomass production．Little is known，however，about bo w the
presence of seabuckthorn affect soil nutrient availability，foliar nutrient concentrations-and nutrient hip
logical cycling in poplar plantations，and how these effects vary with mixing patterns or poplar varieties．
Early studies reported only growth responses and Ioliar nutrient status of young poplar trees after mixed
with seabucktbo rn，and few published experiments have formaly addressed long—term effects of seabuck
thorn on biomass production and nutrient cycling of poplar plantations ．
The objectives of the study were：to determine responses of soil nutrient availability and nutrient cy—
cling processes of poplar plantations after mixed with seabuckthorn{The study was expected to provide sci
entitle information for maintenance of sustainable site productivity of po plar plantations under successive
rotations with continuous cropping．
2 M Itterials and methods
2．1 Site description The study area is located on fluvial plain sites in Heishui，Jianping county，Liaon—
ing Province，in the northwest of Chitin (40。17 ～ 42。20 N，I1O。10 ～ 120。02 E)．The typical dry subhumid
continental climate is overwhelmingly dominated in the study area． Annual mean temperature is 7．9 C．
Annual average precipitation is 470．5ram，faling mainly in July through August·and annual mean evapo
ration is 21 13．7ram．The mean relative air humidity is only 52．3 ．Soils are a nutrient—poor sandy Loam
at fluvial plains，with relatively scarce organic matter and low soil fertility．The sites are considered of low
productivity in a landscape ，which suffers~-ater shortage and nutrient deficiency．The experiment was de—
signed with three poplar varieties(Populus ’Xiaohei’．P．‘Zaolin06’and P cv． ‘N30l6’)and
in the two mixing patterns (individual—or strip—mixing)，i．e． treatment I：P． “rdm cv．‘N3016’
mixed with seabuckthorn in strips at spacing of抽 X 3m for both po pular and seabuckthorn-and the con
trol of P cv． ‘N3016 pure stand at a spacing of 2m X 3m ；treatment I：P ‘Zaol[n06
mixed with seabuckthorn in strips at spacing of 2m X 3m and 1．5mX 3m for poplar and seabuckthorn，re—
spectively；and the control of P． ‘Zaotin06’pure stand at a spacing of 3m × 4m； treatment I：P．
‘Xiaohei’mixed with seabuckthorn in indivIdual—tree mixture at spacing of 3m X 4m for both poplar and
seabuckthorn(one poplar tree together with two seabuckthorn plants were planted in a group)，and the
control of P． ‘Xiaohei’pure stand at a spacing of 3m ×4m．The staitds of treatment I and 1 were estah—
lished in 1987．and that of treament 1 was established it 1985．Ir-treatment Ⅲ，seabuckthorn plants were
removed in 1990 from the mixed poplar stand by cutting at ground level with minimum soil disturbance，
and thereafter regenerated naturally．
2．2 Growth and biomass M easurements of the stand growth were made annualy during
1993～ 1996 in the pred etermined six plots of 300m each，including the three poplar mixed stands wi th
1 ●j
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536 生 态 学 报 19卷
seabucktbo rn and the three controls．By applying the method of stand survey，all trees grown in sampling
plots were measured and recorded for tree heJ．ght and diameter at breast height(DBH )．Destructive har-
vests for above-ground hiomass measurement wel~e made at the end of August of 1993 and 1996，respee—
tively．Trees were selected based on diameter dimension and number percentage in each diameter class for
above—ground harvest．The total~resh weight of trunk，bark，branches and leaves were directly measured
by weighing．Biomass of various components or organs in expressed in dry mass．Biomass date of the or—
ge ns from the destructive sampling were used to estahllsh alfometric relationships with DBH and tree
height．The allometric equations were used to quantify the same variables of the trees for whole investiga—
tion plots．Biomass of seahuckthorn，herb and litter was directly measured by ha rvesting and weighing a11
plant tissues respectively from three 5 X 5m and 1 X lm subplots．
2．3 Litter fall measurement Ten litter tal traps of 1×lm。were rand omly placed in an 300m plot with—
in each treatment to quailfy litter input to the[orest floor，and litter tal was collected twice a month．A|-
let separation into diferent components by species，tissues and site origin，the litter fall was weighed and
then dried at 40"C on oven for 48h．The dry nines of litter tal was weighed and analyzed for nutrients．
of litter[al were co nd ucted simultaneously on all plots in 1993 and were repeated {or
tw o years-
2．4 Primary productivity The net primary production (NPP)of the stand components was calculated by
the folowing formulae：NPP=G+L+ A[~]，Where G is annual net blomass production．L is annual litter
[al1 production of the current biomass production，and A is herbivore co nsumption which is assumed to be
neglglhl~and not considered in the calculation．The annual abo ve-ground net hiomass production was cal—
eulated by an average of the total net biomass production during 1993~ 1996．
2．5 Soil sampling For the strip mixing stands·27 frames were taken in each plot and divided into 3
groups of 9 frames each．Th group 1，2 and 3 were selected within the rows of po plar trees．within the
space between poplar strip and se abucktbo rn sirip，and within the rows of seabuckthorn，respectively．
Sampling underneath the biter layer was by depth increments of O～ 5，5～ 20．20~ 50cm ．using soil dril1．
Within each group，nine co res fro r~each depth were collected systematically and po oled in 3 single sam—
pies·For ind ividual mixing stands，9 frames Were rand omly taken in each plot·and pooled into 3 sing le
samples．So il samples were collected at three subLayers by the same depth horizons as described abo ve．
2．6 Laboratory ana lysis So il samples were air—dried，and ground to pass through No．4O stainless mesh
screen．The total N content was determined by the Kjeldahl method，P by contiouous flow method(molyb—
denum—vana dium method)，and concentrations of Ca，M g and K were measured by atomic absorption spa c．
trophotometry．Exchangeable NH and combined NO；+NOf were deternd ned from 2 M KCl extracts(1‘
4v／v)by alkaline steam distillation，and exchangeable K and soluble P were determined using 1．0 M am—
monium acetate extraction(pH 4．65 I 1 I lOv／v)．Plant samples were dry-ashed at 550"C for 5 hours，dis．
solved in 5％(v／v)HN0j，and were．analysed for tota1 N，P，K，ca and Mg with the same method as de—
scribe d above[ ．Chemical analysis of plant and soil samples were cond ucted at Chemical Analys is Center．
Chinese Academy of Forestry．
2-7 Nutrient calculation The nutrient standing stock was calculated by multiplying the standing crop by
the correspo nd ing mean elemental co nc entration．The abo ve—ground an nual nutrient uptake was defined as
the sum ofthemaximum nutrlep．tpool sizeof currev．t stem and branch，andfoliar biOW．~,Sfl,plusthe呲 Bn．
ntis．1increraent in the nutrient poo1 of stems and branches older tha n current year．The annua1 nutrient re—
tention was de[ined by the annual net increment in nutrient p0o1 of perennial tissues．Al these estimates of
those of po l size of hiomass and nutrient co ntent of different compo nents are expressed on al oven-dry
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4期 刘世荣：沙棘对中国亚湿润十旱 区的杨耐人工林养分分布＆生物循环影响 537
weight basis and on an annual basis as weight per hm
3 Results
3．1 Nutrient concentrations Ln plant tissues Nutri—
ent concentrations in plant tissues were shown in fig．
1．Nutrient concentrations varied with organs and va—
r[eties， hut nutrient concentrations were frequently
greater in the leaves than in the other organs． The
concentrations of N ，P and K were significantly higher
in the seabuckthorn leaves than in the poplar leaves．
Nutrient in poplar tissues~*ere fre
quently larger in the mixed stands than in the pure
stands in most cages of this study，but the nutrient
concentratl0ns varied with elements and treatments．
For example，the foha r N and P concentrations of
poplar trees in the mixed stands were 1．2 and 1．2，1．7
and1．2。and 1．4 and1．1times higherfortreatment1．
I and I，respectively，compared to that in the pure
stands．Regardless of poplar variety，the nutrient con一
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：图图图 图国
国酉图国
+ 8 p P -_s " + s Px
Fig．1 Nutriem conceatradons of plant tissues
centrations of woody tissues were relatively lower for po plar trees in the mixed．stands than in the pure
stands．
3．2 Nutrient concentrations in soil Nutrient concentrations in soils varied with soil depth，and mixing
patterns(seetable 1)．Thetotal N，Ca andMg，and available N·P andK concentrationswere significant—
ly decreased with soil depth，but total P and K were found to increase slightly with soil depth．Nutrient
concentrations of soils were frequently higher in the mixed stand s than in the pure stands．These increase
in the t0tal N and available N concentration were 164 and 36 for the treatment I，132 and 37 for
the treatment 1，and 86 and 19 for treatment I ，respectively．
3．3 Nutrient standing crop In the strip—mixing patterns．the total above ground standing crop of nutri
ents in tr~e-e layers was found to be larger in the pure stands than in the mixed stands，but only difference in
N standing crop were not statisticaly significant．In addition，the standing crop of nutrients in the woody
tissues was less in the mixed stands than in the pure stands·while the standing crop of nutrients in the
leaves was larger in the mixed stands than in the pure stand．This was due largely to either the relatively
higher nutrient concentrations or the larger biomass of woody tissues in the pure stands than in the mixed
stands．In treatment I ，however，the total above—ground standing crop of nutrients in the tree layer was
significantly higher in the mixed stand than in the pure stand，regardless of dement or organ(see Tahable
2)．The total foliar N standing crop of tree layers increased by 18 ，32 and 269 after poplar trees
mixed with seabuckthorn for treatment I，I and Ⅱ．
The tota[N and P standing crops of stands were larger in the mixed stands than in the pure stands，Jr-
respective of poplar variety or mixing pattern，but there was no statisticaly significant differences in the P
standing crop between the mixed stands and the pure stands in treatment I and I．In treatment l and _·
the total K ．Ca and M g amount of stands weFe frequently less in the mixed stands than in the pure smnds，
while they were significantly larger in the mixed stands than in the pure stands in treatment I．The stand—
ing crop )f herb Iu this case comprised a negligible proportion of total above—ground nutrient pool size，but
the．ida-tnt of nutrients accumulated in seabuckthorn plants made a great contribution to the total nutrient
m
on ong 。
2 c0 山
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,538 生 态 学 报 1 9卷
Table 1 Total element conlents and exch~ eable nutrients In soils The smls were sampled at the three soil depths 0f O-
5cm·5~ 20~m 珥^d 2O～ 5 m． ，pz and px represent Ihree exgerimenlal stands composed of PoputusX o Poputus
× Z日0Iin0‘and Pop．us×euramerieane cv． ‘N3061’·respectively S indica tes Po plin-mi*~d stands with seabuckthe ru．
The士 values are the s．e 0f the meal 0f three replicates
standing crop of the stands．For example，
the N amount in seabuckthorn plant ac．
counted for 31 ，56 and 22 of the to．
tBl N amount ot star~ds for treatment I， 1
and g，respectively (see table．2)．
3．4 Nutrient biological cycling Regard—
less of poplar variety or mixing pattern。
the annual amounts ot N，P，K。Ca and
M g uptakes and returns were larger in the
mix stand s then in pure stands．For ex—
ample，the annual amount of N uptake was
increased by 56 ，15l 。and 268 ，re—
spe ctively，for treatment I，Ⅱ and g，af-
ter poplar stands mixed with seabuck-
thorn． In the mixed stands。 the relative
c0Ⅱtributi0ns of seabuckthorn to the total
annual amount of above—ground N uptake
accounted|or 29％ ，5B and 23 。re-
spectively，for the treatment I+I and g，
and the relative contfibutions of seabuck—
Tahle 2 EITeet~of seabuekihorn 0n natrient standing~1．0~05 of stand$
(kg．／hm )． pc，pz and px represent thre expc~nlental stands~OlD-
posed of POlA~Igt× Xlaohei，populu~x Zat~ln 06 and Popnlus× e
，4膨 nl删 cv． ‘N3061’． respectively． S l~ tes poplar mixed
s n凼 with seahe cklhorn．Valu~ are the IIleaD~of three replicates
thorn to the total annual amount of above-ground N return accounted for 28％ ，55 and 20％ ，respective．
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4期 刘世荣 ：沙棘对中国亚湿润 F旱区的扬树 人工林养分分布及生物循环影响 539
lv，for treatment I，I and I (Table 3)
Table 3 Effects or seabucktborn ㈨ nutrient biological cycUn8 or poplar plantations(kg／hm ·a】．pe．pz and px repre—
sent three experlmeatal stands eo=t~ d of Pol~las× Xlaohel·Pandas× Zlmolln 06 and Pogulus× earametqcane cv
‘N3Q61’-respectively S indicates poplar mixed staads with seabackthora Values r^e tke melllIS of thr* l es
The annual amount of nutrient return by leaf litter fal comprised a relatively larger proportion of the
total annual amount of nutrient uptake in the mixed stands than in the pure stands，and thus the rarios 0f
nutrient return to uptake were frequently higher in the mixed stands than in the pure stands
． The increase
in the ratios of nutrient return to uptake was more pronounced for the mixed stand with the individua1．
mixing pattern than for the mixed stand with the strip—mixing pattern．
4 DlSCUSS10n
As known that seabuckthorn is a N—fixing plant with root nodules by which they fix N?from the air
into soils，and consequently leads to an increase in availability of soil N．Zhang el a1
． ： ：reported that the
activity of N-fixing enzyme in the root nodule was 36．23~ 470．4 acetylene pmo1．fresh mass ．min．。1．
and the activity of N—fixing en2yme was higher in the fluvial plains than in the hill slope sitesC ．In this
study，the substantial increases in total soil N and exchangeable N concentrations occurred in the mixed
stands，irrespectiveof poplar variety ormixing pattern。andthisincreasew&s accompanied hythe concomi—
rant increase in the poplar leaf N concentrations．Because of a positive correlation between photosynthesis
and nitroge n at current ambient atmospheric CO2 condition[ ，the increase in leaf N may be
due partially to the increased soil N availability after seabuckthorn introduction and this could provide a
sufficien t available N in concert with the increasing demand of rapid tre growth．However。leaf N concen—
trations from poplar trees in the treatment 1 were not statistically significant different，thereby tree growth
may not highly related to the lear N co ncentrations in this case ．due to the differences in tree growth rates
and the resultant dilution effects． This sugge sted that increased soil N availability may enhance tree
growth but leaf N concentration may not necessarily increase in compatible with growth enh ancement．The
similar phenomena was hypothesized by Tschaplinski and NorbyI ．and Kim et a1．m]that leaf N ∞ ncen—
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540 生 态 学 报
tration was a poor indicator of response to N fertilizer or increased soil N availability．and therefore it was
not closely related to plant growth response．In addition．this phenomena also occurred in many imp㈣t ex
periment studies that leaf N concentration has been decreased due to the dilution eEeeta resuIted from f0
liar starch accumulation while rapid plant growth was frequently observed in response to elevated CO ：．
Positive effects of seabuekthorn plants on poplar tree growth were observed during the study period
regardless of poplar variety or mixing pattern． Although the contribution of seabuckthorn to the total
biomass of the stands comprised relatively small proportions(1 2 ．23 and 9 for treatment I． I and
1，respectively)一its contribution to the total nutrient pool size increased dramatically．For example．the
N standing crops of seabuekthorn were acces~tO 30 t 56 and 22 of the total N pool size for treatment
I， I and _·respectively．Similarly·they occupied relatively larger proportions of the total annual VaSCU—
lar biomass producfion (20 一41 and 23 fortreatment1t I and I ，respectively)and oftotal annual
vascular N uptake．
The seabuekthorn played 8n important role in nutrient cycling through accelerated N recycling pro—
cesses and increased soil available N．The annual N uptake by seabuckthorn accounted for 29 ．56 and
23 of the total annual N uptake for treatment I一 1 and I —respectively，as a result of higher N concen—
trations in its tissues．In addition，the seabuckthorn returned 82 ．8O and 76％ of its total annual N
uptake to forest floor for treatment I．日 and I ，respectively，by its litter fal which is easily decomposed
by soil microbes with rapid biomass turnover and N recycling．At the same time，the seabuckthorn also
enhanced nutrient biological cycling of poplar trees by either the increased nutrient concentrations of poplar
leaf tissues or the increased annual amount of nutrient uptake and return．The increases in a删 a1 N up-
take of poplar trees after mixed with seahuckthorn were l11 ．110 and 283 for treatment 1， l and
I —respectively，while the annual N returns were increased by 1l4 ，1l3 and 352 for treatment 1，
_ and I ，respectively．As a result，the rata1 annua1 N uptake of the mixed stands were increased bv
156 ，251 t 368 for treatment I． I and I．respectively．In addition to the increased annual nutrient
uptake．the annual nutrient returns of the mixed stands were also enhanced as the ratios of annua1 nutrient
return to uptake were higher in the mixed stands than in the controls．This implied that there was relative．
1y larger amount of nutrient elements involving in nutrient recycling processes in the mixed stands than in
the pure stands．Although poplar is referred as a deciduous tree that returns a large amount of litter fall by
annua1 leaf senescence，the quality of its litter fal1 was poor than that of seahuckthorn due to the relatively
lower nutrient concentratioDs of its litter tissues in contrast to seabuckthorn．This was rnanifesled in tr t
ment l，especialyfor N，i．e．the annual N return by seahuckthornlitterfal1 was123 highertha nthat
by poplar litter fal1．
Compared with other forests，the amount of annual N uptake in the mixed poplar plantations ranged
from 65 to 102 kg／hm ·a，which was less than 123 kg／hm ·a in oak forests reported hy Duvigneaud and
DermeyerDeSmet[ ，257．7kg／hm ·a in tropical mountain rain forests reported hy Zeng and Liu[】】1．but
larger than 56kg／hm ·a in temperate larch plantation reported by Liu⋯：．41．9kg／hm ·a in subtropical
China fir plantation reported by Feng et a1． ，and 38．2kg／hm ·a iF· ：rm—temperate Chinese pine planta—
lion reported by Nie et a1．[ ．The significantly larger annual N uptake in mixed poplar stands than in the
pure po plar stands indicated that the annual N uptake was highly dependent on responses of net productivi一
y of the stands to the increased soil N availabihty，1．e．the higher stand productivity，the larger annual N
uptake．The increased annual N uptake was contrihuted to meet with the increasing N demand of higher
net productivity．Overall，seahuckthorn as a understory minor vegetation is particular significance in view
of the ecological role of N pump species in the nutrient—poor poplar plantation[ ，especialy under the
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4期 刘世荣 ：沙棘对中国亚馒润干旱医的杨树 ^工林养分分布及生物循环影响 541
drought condition resulted from dry suhhumid climate．Even though seahuckthorn does not have an impor．，
tant commercial value of timer production．it plays an indisputable role in the contribution to efficiency of
energy flow -nutrient conservation and rapid nutrient recycling(especially N)in man-made polar plantation
due to its higher ratio of leaf to stem ，relatively repid leaf turnover，and high tissue nutrient concentra—
tlons，as welI as its important function of N—fixation
In addition to effects of increasing nutrient availability and recycling，seabuckthorn also plays an im—
portant role in protecting soil and water from erosion due to its thick canopy cover and wide spread shallow
root system．The measurement made of rain fall interception rate was 40 ～ 49 ，resulting in reduction
of run off by 8o and of sand and suspended matter by 75 ，while leading to an increase in soil moisture
by 2．2 ～ 6．6 ”】．The improved environmental conditions associated with rapid litter decomposition
and the increased nutrient availability were contributed to the enhanced growth of poplar plantations after
mixed with seabuckthorn．
Although the timber production was found be to relatively lower in the strip—mixing stands than in the
pure stands，the strip-mixing stands could maintain the long—term stability of a high soil N availability and
thereby achieve the long—term sustainability of stand hiomass production．This should be considered as a
sound silvicultural strategy in view of long—term interests．rather than short term interests1] 。 ．Recent
survey made by Sun et a1． ：indicated that the reductions of stand growth in terms of Illean height and
DBH werefound by 24％ and 11 inthe secondarygeneration of poplar pureplantation underthe contin—
UOUS cropping in China，due to the associated decreases in soil available nutrient elements·especially N．
This discovery and similar others" ’ ’ 《supported the silvicultural strategy proposed by this study and
the dense，pure po plar stands should be cha nged into the mixed plantations with an appropriate N—fixing
species like seabuckthorn．A mixed po plar stand with seahuckthorn either in an individual—mixing pattern
or a strip—mixing pattern。both can obtain a good balance be tween short—terra economic interest from timer
production and long—term site stability from maintenance of rational normal nutrient availability and recy—
cling processes．and furthermore ensure the resultant high level of sustainable plantation productivity，in
particular，under successive rotations with continuous cropping．
5 Conelu~lons
A growth response to the occurrence of seabuckthorn was found in poplar plantations under the nutrl—
eat—deficient and seasonal drought stress in the dry suhhumid climate of China．The growth enhancement
of po plar trees mixed with seabuckthorn was largely attributed to the increased soil nutrient availability，
especially N ，and the increased N availability was the result of one or more of the followings：the increased
N inputs into the soil through N—fixation by root nodule，good—quality litter fal1．rapid nutrient recycling．
or improved site environments．The enha nced growth was not consistently correlated with foliar N concert—
tration，but related to the increased annual amount ol N uptake and accelerated N recycling．Seahuckthorn
acts as a N pump and plays an important role in maintaining sufficiently stable N availability in concert
with rapid pophr growth．Seabuckthorn was characterized by its high ratio of leaf to stem 。relatively rapid
leaf turnover．and high tissue nutrient concentrations．as well as its important function of N—fixation．The
importance of seabuckthorn in modifying soil chemical processes and elemental cycling would be signified in
view of long-term site pro6uctivhy and sustainable productivity in po plar plantations．in particular，under
successive rotations with continuos cropping on the same sites．
Acknowledgm ents
This study was supported by the International Foundation for Science(IFS grant agreement No．D／
2052一1)．The author would like to extend his sincere thanl【s to Dr．Sabine Bruns and all other staff mere—
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542 生 态 学 报 19卷
hers from IFS for their information assistance and administrative services durinG the project period．The
field asslatanee and site maintenance of M r．Limin Niu，Zuoyi Dong and Li Chen are gratefully acknowl—
edged．The author also are grateful to the project advisor·Dr．Lars一()we Nilsson of the Swedish Univers[一
ty of Agricultural Sciences for providinG technical suggestions 0n the experimental design．
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