全 文 ：Received 19 Feb. 2004 Accepted 21 May 2004
Supported by the Knowlege Innovation Program of The Chinese Academy of Sciences (KSCX1-08-03) and the State Key Basic Research
and Development Plan of China (G2000018603).
* Author for correspondence. E-mail: .
http://www.chineseplantscience.com
Acta Botanica Sinica
植 物 学 报 2004, 46 (9): 1032-1039
Effects of Simulated Grazing Pattern and Nitrogen Supply on Plant
Growth in a Semiarid Region of Northern China
YUAN Zhi-You1, LI Ling-Hao1, HAN Xing-Guo1*, JIANG Feng-He2, LIN Guo-Hui2, ZHAO Ming-Xu2, REN Li-Yun3
(1. Laboratory of Quantitative Vegetation Ecology, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China;
2. Grassland Management Station of Duolun County, Duolun 027300, Nei Mongol, China;
3. Animal Husbandry Bureau of Taipusi, Taipusi 027000, Nei Mongol, China)
Abstract: Grazing in grassland ecosystems affects plant growth by removing biomass and depositing
excretal nutrients. However, grazing is not uniformly distributed in space. The spatial pattern of defoliation
and excretion deposition by herbivores across vegetation mosaics has been frequently discussed, but
rarely spatially quantified. A 60-day field experiment in a native semiarid grassland community was con-
ducted to examine the responses of plant growth to simulated grazing pattern and varying nitrogen levels.
Plants were subjected to five defoliation treatments determined by circularly clipped patches of different
size (0, 10, 20, 40, 80 cm in radius), and four nitrogen supply levels in soils (0, 5, 10, 20 g N/m2). It was
detected that defoliation had reduced primary productivity by 41.5% whereas fertilization had increased it
by 57.8%. The negative effect of defoliation was greater in the smallest, fertilized patches. N addition had
been found to have altered the effect of defoliation, as plants growing at higher nitrogen levels were more
negatively affected by defoliation than plants with no supplementary application of nitrogen. These results
indicated that the magnitude of defoliation response for an individual plant was modulated by not only
defoliation itself, but also other factors, such as nutrient availability. The increase in the ratio of live to
dead plant parts suggested that urine deposition delayed the senescence of plants. The results also
showed that (1) the effect of defoliation on primary productivity was affected by the patch size, and (2)
nitrogen addition (simulated urine deposition) could increase primary productivity and affect the response
to defoliation more obviously in the smaller patches than in the larger ones.
Key words: simulated defoliation; nitrogen fertilization; grazing patch; relative growth rate
Grazing animals affect primary productivity mainly by
the joint action of tissue removal and nutrient return
through dung and urine deposition (Matches, 1992;
McNaughton et al., 1997). The response of plants to tissue
removal has important implications for their survivals, while
the effect of tissue removal on plant growth can range from
damage to benefit depending on the change in the relative
growth rate (RGR) in response to defoliation (Oesterheld
and McNaughton, 1991). Negative impacts on plant growth
are known as damage, whereas positive responses, that
may include partial, full, and over compensation for the
removal of biomass to grazers, are known as compensatory
responses (Hilbert et al. , 1981; Oesterheld and
McNaughton, 1991; Ferraro and Oesterheld, 2002).
Grazers also affect grazing ecosystems by influencing
the nutrient recycling rate. Nutrient cycling via grazing ani-
mals can be important in enhancing or maintaining soil fer-
tility (Floate, 1981). Herbivores recycle about 75%-85% of
the nitrogen that they uptake from vegetation (Afzal and
Adams, 1992; Russelle, 1992). Cycling of nutrients through
grazers may help keep a pool of readily mineralizable or-
ganic nutrients near the soil surface where they are more
accessible to plants and microbes (Botkin and Wu, 1981),
thereby bypassing the rate-limiting step of the release of
nitrogen from litter (Day and Detling, 1990; Seagle et al.,
1992).
In grazed pastures, however, there is seldom a uniform
distribution of these two components of grazing, i.e. defo-
liation and nutrient return. The heterogeneous matrix, which
was generated as a result of grazing defoliation and excre-
tal return (Adler et al., 2001; Augustine and Frank, 2001),
plays an important role in ecological processes (Kotliar and
Wiens, 1990). Ungrazed, semi-desert grasslands, including
agro-pastoral ecotone, were more heterogeneous than
grazed grasslands (Bock et al., 1984; Aguiar and Sala, 1999).
Although considerable researches have addressed the ef-
fects of defoliation and excretion deposition in pastures
and grasslands over the world, little has been done in
YUAN Zhi-You et al.: Effects of Simulated Grazing Pattern and Nitrogen Supply on Plant Growth in a Semiarid Region of
Northern China 1033
native semiarid grasslands of China, where species with
both contrasting growth forms and photosynthetic path-
ways coexist. Furthermore, the effect on the primary
productivity, of grazing coupled with nitrogen fertilization
from grazers’ excretion in grasslands has not been com-
pletely understood yet although the interactions clearly
play a vital role in the plant compensatory response
(McNaughton, 1990). The properties of patches created by
either defoliation or excretion deposition have been sepa-
rately studied previonsly (Day and Detling, 1990; Afzal and
Adams, 1992; Jaramillo and Detling, 1992; Semmartin and
Oesterheld, 2001), but to our knowledge the properties of
patches formed along the gradient of nitrogen supply have
not been studied. Plant tissue nitrogen, an important vari-
able that may determine plant responses to defoliation, had
not been quantified in the previous studies also
(Coughenour et al., 1985). In this study, we investigated
the coupled effects of the grazing-derived spatial pattern,
simulated by artificially defoliated patches differing in size,
and nitrogen addition on the primary productivity. We also
examined the interactions between multiple nitrogen levels
and defoliation treatments to determine to what extent the
gradient of nitrogen supply accounted for these effects.
We propose that the effects of the spatial pattern of
defoliation on the primary productivity are mediated by
shifts in light availability. Within grazed patches, light in-
tensity at canopy base is expected to be lower at the edge
than at the center. As the size of the grazed patch increases,
the perimeter of contact between grazed plants and their
ungrazed neighbors decreases in relation to the total size
of the grazed patch (Semmartin and Oesterheld, 1996).
Therefore, light intensity at the entire grazed patch will be
greater and plant growth will be more likely increased, or
less reduced, by grazing. Furthermore, we propose that ni-
trogen availability will affect the response of plant growth
to the spatial pattern of defoliation. We predict that greater
nitrogen availability will increase the effect of patch size
because, as nitrogen availability increases, shoot competi-
tion for light becomes more critical for the biomass produc-
tion (Wilson and Tilman, 1995).
1 Materials and Methods
1.1 Study site
The experiment was conducted in Duolun County
(115o50-116o55 E，41o46-42o36 N), which is located on
the southern edge of the Hunshandake Sandland in the
central part of Inner Mongolia Autonomous Region, China.
This area belongs to a typical agro-pastoral ecotone. The
terrain is relatively flat, with total relief of <10 m. The cli-
mate belongs to semiarid monsoon climate of moderate tem-
perate zone. Mean annual precipitation is around 385.5 mm
and mean annual temperature is 1.6 ℃, with mean monthly
temperature ranges from –18.3 ℃ in January to 18.5 ℃ in
July. The frost-free period averages 150 d. Frequently, there
is strong wind in winter and spring (Yuan et al., 2004a;
2004b).
The field survey was conducted in a 6-year-old, 100-
hm2 exclosure of a native grassland community, 40 km north
to the Duolun town. This exclosure, one of the few sites in
the region resembling natural grasslands, provided a well
developed and nearly ungrazed canopy. The site charac-
teristics of the study are summarized in Table 1.
1.2 Experimental design
A two-way factorial experiment was designed, includ-
ing defoliation and urea application. There were four repli-
cates for each treatment. Defoliation treatments had five
levels determined by different sizes of circularly clipped
patches (0 (i.e. control), 10, 20, 40, 80 cm in radius). These
patch sizes were selected to represent a wide range of graz-
ing patches commonly observed in field situations. Plants
were clipped at 10 cm height above the ground, which re-
sulted in a 50% removal of the total aboveground standing
biomass. We used these defoliation patches of different
size to simulate the grazing pattern by herbivores (Day and
Detling, 1990; Semmartin and Oesterheld, 1996; Hamilton
et al., 1998). The initial biomass of the patches was esti-
mated from the difference between the aboveground biom-
ass of control plots (clipped to ground level) and the clipped
biomass of the patches. Urea treatments had four levels: 0
(control), 5, 10, 20 g N/m2, applied uniformly to the plots.
Table 1 Site characteristics in this study
Variables Values or description
Dominant plant species Leymus chinensis, Stipa spp., Agropyron cristatum, Artemisia frigida
Soil type Coarse rocky chestnut soil
pH (soil:water 1:5) 7.25
Organic C (mg/g) 10.16
Total N (mg/g) 0.60
Bulk density (g/cm3) 1.44
Site management Fenced for six years, free from grazing
Acta Botanica Sinica 植物学报 Vol.46 No.9 20041034
Control plots received the same amount of water. Both ni-
trogen addition and defoliation treatments were conducted
on the same day. Urea was applied only one time during the
study period.
1.3 Harvest
The field experiment started in June and ended in
August, 2002, a period covering the approximate length of
the growing season in this area. Initial and final
aboveground biomass was determined at the beginning
and the end of the experiment separately. The recovery
time for defoliated plants was 60 d. Harvested biomass was
further divided into green and dead parts. Each part was
oven-dried at 60 ℃ for 72 h, and weighed. Dried samples
were ground through a 40-mesh screen in a Wiley mill for N
analysis using the standard Kjeldahl acid-digestion method
with an Alpkem autoanalyzer. The primary productivity was
estimated as the difference between the final and initial
biomass divided by the length of the recovery period. Thus,
the primary productivity of the defoliated patches repre-
sents the rate of production after defoliation and does not
include the biomass removed by defoliation. The relative
growth rate (RGR) was calculated as dry weight increment
per day per unit initial plant biomass.
The photosynthetically active radiation (PAR) at the
canopy base, in both the center and the edge of patches,
was assessed 30 d after the beginning of treatments, a time
most likely to represent the average conditions experienced
by the plants in different treatments. Photosynthetic pho-
ton flux density (PPFD) was measured with a Li-Cor 6400
portable photosynthesis measurement system (Li-Cor,
Lincoln, NE, USA) in a horizontal position, at noon, when
light was perpendicular to the canopy. PPFD was measured
in two locations within the patch, i.e. the center and the
western edge. Similarly, the soils of both the center and the
edge of patches were sampled at 0-30 cm depth and
analysed for water content. Percentage soil water content
was determined by mass loss upon drying at 105 ℃ for 24
h.
1.4 Statistical analysis
Statistical tests were performed using SPSS version
10.0 (SPSS Inc., Chicago, IL, USA). The effects of defolia-
tion and N supply on the aboveground net primary
productivity, relative growth rate and plant N concentra-
tion were tested by two-way ANOVA. Tukey-Kramer tests
were used to make multiple comparisons among treatment
means. Unless otherwise specified in the text, the level of
statistical significance was 0.05.
2 Results
Photosynthetic photon flux density (PPFD) data showed
that light at the center of the defoliated patches was inde-
pendent of the patch size and was higher there than in the
control (Fig.1a), while PPFD recorded at the edge of the
defoliated patches was similar to that of the control for
smaller patches and greater for larger patches. It could be
noticed that smaller patches not only had a larger propor-
tion of edge but also had a more shaded edge. In contrast,
soil water content at the center of the defoliated patches
was lower than that of the control, while soil water content
at the edge of the defoliated patches was similar to that in
the control for small patches and lower for large patches
(Fig.1b).
Defoliation and N addition had significant effects on
the primary productivity (Table 1). Figure 2 shows that de-
foliation had reduced the primary productivity by 41.5%,
while N fertilization increased it by 57.8%. Defoliation tended
Fig.1. (a) Photosynthetic photon flux density (PPFD) and (b) soil water content recorded at the center and the edge of the defoliated
patches with different size 30 d after treatment. Vertical bars indicate SE. Dotted fine lines above and under the control line indicate SE
of the control patches.
YUAN Zhi-You et al.: Effects of Simulated Grazing Pattern and Nitrogen Supply on Plant Growth in a Semiarid Region of
Northern China 1035
to have had more negative effects on the smaller and fertil-
ized patches which were the only ones that had not re-
sponded to fertilization. Thus, as we had proposed, the
effect of the spatial pattern of tissue removal was more
evident under high nutrient availability.
Primary productivity increased with the increasing level
of nitrogen addition. The effect of N level was greatest in
larger and intermediate defoliated patches, which was re-
flected by the significant defoliation ´ N level interaction
(Table 1; Fig.2). Plant growth at high nitrogen levels was
more negatively affected by defoliation than at standard
levels of nutrient availability, which indicated that nitrogen
availability had also altered the magnitude of the defolia-
tion effect.
Our determination of the initial biomass allowed us to
calculate the relative growth rate (RGR) and, thus, to evalu-
ate the amount of compensatory growth. The results showed
that defoliation, N level, and their interactions had signifi-
cant effects on RGR (Table 1; Fig.3). Defoliation had re-
duced RGR by 34.9%, while N fertilization increased it by
60.1% (Fig.3). Defoliation had much more negative effects
on the smallest, fertilized patches. The effect of N level on
RGR was more obvious in larger and intermediate defoli-
ated patches.
Defoliation and N supply affected the ratio of live to
dead biomass. The ratio of live to dead biomass was higher
in N-addition patches than in the control, but only in the
larger patches (Fig.4). Defoliation reduced standing dead
biomass by 35.98%, while it increased green biomass by
56.21%. The ratio of live to dead biomass was higher in
larger defoliated patches.
Nitrogen concentrations were 1.22% and 0.74% in green
biomass and standing dead biomass respectively. N con-
centration increased by 16.64% with defoliation and by
15.09% with N supply, though the increases were not sig-
nificant (Fig.5; Table 2).
3 Discussion
Our results showed that the response of plant growth
to defoliation was affected by the patch size (Fig.2). It ap-
peared that compensatory responses of plants to the graz-
ing pattern (in terms of patch sizes) depended on the out-
come of the competition between grazed plants and their
neighboring ungrazed plants and this outcome, in turn,
Fig.2. Effects of defoliation and urea-N application on the
aboveground net primary productivity (ANPP). Bars are means
with SE.
Fig.3. Effects of defoliation and urea-N application on the
relative growth rate (RGR). Bars are means with SE.
Fig.4. Effects of defoliation and urea-N application on the green
to dead biomass ratio. Bars are means with SE.
Fig.5. Effects of defoliation and urea-N application on the plant
nitrogen concentration. Bars are means with SE.
Acta Botanica Sinica 植物学报 Vol.46 No.9 20041036
depended on the size of the grazed patch. We found that
the productivity pattern of the fertilized patches matched
the irradiance pattern (greater productivity in larger and
intermediate patches) (Fig.1). This suggested that shifts in
productivity along a patch size gradient might be governed
by shifts in light availability, provided that N was not a
limiting factor. Therefore, under high N availability, the
negative effects of the ungrazed neighbors decreased as
the patch size increased beyond 20 cm radius. The spatial
pattern of defoliation affected the amount of light available
to defoliated plants. The removal of aboveground biomass
increased the irradiance at the base of plants, but the mag-
nitude of this effect depended on the spatial pattern of
defoliation. It has been reported that irradiance increased
with the patch size until reached a constant value at a inter-
mediate patch size (Semmartin and Oesterheld, 2001). This
was a combined result of the decreasing importance of edge
effects. Edges became more luminous as the patch size
increased. Thus, in a defoliated patch, there is a perimetric
interface where defoliated plants suffer a more constrained
light environment. As the patch size increases, the impor-
tance of that interface decreases and the constraint is less
intense. More data will be necessary for a definite under-
standing of the plant response to canopy structure changes.
Our results also showed that competition for light may
modify the interaction between grazing and N availability,
which could be partly reflected by that, in small patches,
the effect of defoliation was more negative under high N
availability, whereas in intermediate and large patches the
effect of defoliation was independent of N availability.
N fertilization had increased the primary productivity as
a whole in our study (Fig.2). The negative effect of defolia-
tion on N fertilization was greater in the smallest (10 cm)
patches, indicating that grazing will have a negative impact
on the productivity only when grazing patches are fine-
grained. This suggested that positive responses in plant
growth are more likely to be found if grazing patches are
coarse-grained. However, when the grazing pattern is very
fine-grained, the positive effect of N fertilization on the
defoliated patch is counteracted by the undefoliated
neighbors. The high variation in the primary productivity
indicated that, besides nutrients availability and grazing,
environmental factors such as water availability are also
important. Primary productivity increased significantly af-
ter nitrogen fertilization (Fig.2), indicating that nitrogen was
a limiting nutrient in this agro-pastoral ecotone (Yuan et
al., 2004c). A number of studies have reported that the ad-
dition of soil N via animal excretions could increase the
primary productivity in a wide variety of grasslands (Day
and Detling, 1990; Clark and Woodmansee, 1992; Seagle et
al., 1992; Zhang et al., 2004; Yuan et al., 2004d). Consistent
with the result reported by Semmartin and Oesterheld (2001),
our results also showed that defoliation had a negative
effect at any level of urea addition. Patches that were both
defoliated and fertilized with urea, resembling a grazing
condition, had produced the same amount of biomass as
undefoliated and unfertilized patches which resembled a
non-grazing condition, except when the defoliation pattern
was very fine-grained. Defoliation experiments on individual
plants usually tend to deal with only one aspect of the
manifold influences of grazing, i.e. the removal of leaf
biomass. Other concomitant and more positive effects, such
as nutrient returns or competition release, are usually ab-
sent from these controlled experiments which may be im-
portant in the field.
It can be seen that defoliated patches had negative ef-
fects on the plant growth rate per unit of biomass (Table 1;
Fig.3). This is in good contrast with the results of another
study in the Flooding Pampas grasslands (Argentina),
where defoliated patches had part of their biomass removed
without any positive effect (and negative in some cases)
(Semmartin and Oesterheld, 2001). The effects of grazing
on plant growth may vary among as well as within species
(Matches, 1992). This depends on the ability for the spe-
cies (or genotypes) to change allocation and compensate
for the removed biomass. However, not all species show
the same response to defoliation. In some species, alloca-
tion is very plastic and drastically shifted from belowground
to aboveground plant parts after defoliation. Other species,
in contrast, are more conservative in terms of allocation
Table 2 ANOVA for the effects on the primary productivity, relative growth rate (RGR), and nitrogen (N) concentration (%) in plants
Source
Primary production (g.m-2.d-1) RGR (d-1) N concentration (%)
df Mean F-ratio P-value Mean F-ratio P-value Mean F-ratio P-value
square square square
Defoliation 4 10.499 37.813 0.000 4.369×10-4 18.164 0.000 0.123 2.439 0.057
N addition 3 6.578 23.691 0.000 1.372×10-3 57.064 0.000 0.135 2.208 0.096
Defoliation × N addition 12 0.957 3.446 0.001 6.486×10-5 2.697 0.006 0.011 0.189 0.998
Error (SE) 60 0.278 2.405×10-5 0.056
YUAN Zhi-You et al.: Effects of Simulated Grazing Pattern and Nitrogen Supply on Plant Growth in a Semiarid Region of
Northern China 1037
and do not change their allocation patterns in such a con-
spicuous fashion (Richards, 1984). Furthermore, the way
the growth of grasses is affected by grazing is regulated to
a large extent by the way the resources are distributed within
plants after defoliation (Oesterheld, 1992). Therefore, it is
not surprising that our results were not in correspondence
well with some previous studies. In N-limited systems, fer-
tilization primarily enhances the growth of graminoids and
deciduous plants (Coley et al., 1985). This may be respon-
sible for the increases in the primary productivity and RGR
by N addition in our study in which the community was
dominated by graminoids and deciduous plants.
Simulated urine deposition apparently had delayed the
senescence when the live to dead ratio was greater in urine
patches, although only in larger patches (Fig.4). Similar re-
sults have been obtained with grasses (Day and Delting,
1990; Jaramillo and Detling, 1992), and the herb Glechoma
hederacea L. (Slade and Hutchings, 1987). Delayed leaf
senescence, increase in N uptake and in biomass produc-
tion suggested that urine deposition in this semiarid grass-
land may result in patches consisting of better quality and
greater quantity of forage for grazing animals. Leaf nitro-
gen concentration usually increases after defoliation. This
is especially common in species from frequently grazed
sites, such as the Serengeti plains in Tanzania (Ruess, 1988).
In our results, however, N concentration in biomass was
not affected by fertilization (Fig.5), suggesting that the N
addition was diluted as a consequence of the greater pro-
ductivity of fertilized patches. These findings agreed with
those of Aerts et al. (1995) who reported that in N-limited
sites, primary productivity increased with N addition while
the N concentration in tissues remained constant.
In conclusion, this experiment demonstrated that the
interaction between grazing and N availability was modi-
fied by the competition for light. In small patches, the effect
of defoliation was more negative under high N availability,
whereas in intermediate and large patches the effect of de-
foliation was independent of N availability. Our results also
showed that the effect of defoliation on the primary pro-
ductivity was affected by the size of defoliated patches,
and this effect was correlated with light availability. Nitro-
gen addition (simulated urine deposition) could increase
primary productivity and affect the response to defoliation
in the smallest patches.
Acknowledgements: We are grateful to the staff of the
Grassland Management Station of Duolun County for their
help and hospitality during our field survey and laboratory
work. Dr. WAN Shi-Qiang and Prof. CHENG Wei-Xin had
provided valuable comments on the manuscript.
References:
Adler P B, Raff D A, Lauenroth W K. 2001. The effect of grazing
on the spatial heterogeneity of vegetation. Oecologia, 128:
465–479.
Aerts R, van Logtestijn R, van Staalduinen M, Toet S. 1995.
Nitrogen supply effects on productivity and potential leaf
litter decay of Carex species from peatlands differing in nutri-
ent limitation. Oecologia, 104: 447–453.
Afzal M, Adams W A. 1992. Heterogeneity of soil mineral nitro-
gen in pasture grazed by cattle. Soil Sci Soc Am J, 56: 1160–
1166.
Aguiar M R, Sala O E. 1999. Patch structure, dynamics, and
implications for the functioning of arid ecosystems. Trees, 14:
273–277.
Augustine D J, Frank D A. 2001. Effects of migratory ungulates
on spatial heterogeneity of soil nitrogen properties in a grass-
land ecosystem. Ecology, 82: 3149–3162.
Bock C E, Bock J H, Kenny W R, Hawthorne V M. 1984. Re-
sponses of birds, rodents and vegetation to livestock exclosure
in a semidesert grassland site. J Range Manage, 37: 239–242.
Botkin M J M, Wu L S. 1981. How ecosystem processes are
linked to large mammal population dynamics. Fowler C W,
Smith T D. Dynamics of Large Mammal Populations. New
York: John Wiley. 373–387.
Clark F E, Woodmansee R G. 1992. Nutrient cycling. Coupland R
T. Ecosystems of the World. Natural Grasslands. Introduc-
tion and Western Hemisphere. Amsterdam: Elsevier. 137–146.
Coley P D, Bryant J P, Chapin F S Ⅲ. 1985. Resource availabil-
ity and plant anti-herbivore defense. Science, 230: 895–899.
Coughenour M B, McNaughton S J, Wallace L L. 1985. Responses
of an African graminoid (Themeda triandra Forsk.) to fre-
quent defoliation, nitrogen, and water: a limit of adaptation to
herbivory. Oecologia, 68: 105–110.
Day T A, Detling J K. 1990. Grassland patch dynamics and
herbivore grazing preference following urine deposition.
Ecology, 71: 180–188.
Ferraro D O, Oesterheld M. 2002. Effect of defoliation on grass
growth: a quantitative review. Oikos, 98: 125–133.
Floate M J S. 1981. Effects of grazing by large herbivores on
nitrogen cycling in agricultural ecosystems. Clark F E, Rosswall
T. Terrestrial Nitrogen Cycles. Ecol Bull, 33: 585–601.
Hamilton E W, Giovannini M S, Moses S A, Coleman J S,
Acta Botanica Sinica 植物学报 Vol.46 No.9 20041038
McNaughton S J. 1998. Biomass and mineral element re-
sponses of a Serengeti short-grass species to nitrogen supply
and defoliation: compensation requires a critical [N].
Oecologia, 116: 407–418.
Hilbert D W, Swift D M, Detling J K, Dyer M I. 1981. Relative
growth rates and the grazing optimization hypothesis.
Oecologia, 51: 14–18.
Jaramillo V, Detling J. 1992. Small-scale heterogeneity in a semi-
arid North American grassland. Ⅱ. Cattle grazing on simu-
lated urine patches. J Appl Ecol, 29: 9–13.
Kotliar N B, Wiens J A. 1990. Multiple scales of patchiness and
patch structure: a hierarchical framework for the study of
heterogeneity. Oikos, 59: 253–260.
Matches A G. 1992. Plant response to grazing: a review. J Prod
Agr, 5: 1–7.
McNaughton S J. 1990. Mineral nutrition and seasonal move-
ments of African migratory ungulates. Nature, 345: 613–615.
McNaughton S J, Banykwa F F, McNaughton M M. 1997. Pro-
motion of the cycling of diet-enhancing nutrients by African
grazers. Science, 278: 1798–1800.
Oesterheld M. 1992. Effect of defoliation intensity on aboveground
and belowground relative growth rates. Oecologia, 92: 313–
316.
Oesterheld M, McNaughton S J. 1991. Effect of stress and time
for recovery on the amount of compensatory growth after
grazing. Oecologia, 85:305–313.
Richards J H. 1984. Root growth response to defoliation in two
Agropyron bunchgrasses: field observations with an improved
root periscope. Oecologia, 64: 21–25.
Ruess R W. 1988. The interaction of defoliation and nutrient
uptake in Sporobulus kentrophyllus, a short-grass species from
the Serengeti Plains. Oecologia, 77: 550–556
Russelle M P. 1992. Nitrogen cycling in pasture and range. J Prod
Agr, 5: 13–23.
Seagle S W, McNaughton S J, Ruess R W. 1992. Simulated effects
(Managing editor: HAN Ya-Qin)
of grazing on soil nitrogen and mineralization in contrasting
Serengeti grasslands. Ecology, 73: 1105–1123.
Semmartin M, Oesterheld M. 2001. Effects of grazing pattern
and nitrogen availability on primary productivity. Oecologia,
126: 225–230.
Semmartin M, Oesterheld M. 1996. Effects of grazing pattern on
primary productivity. Oikos, 75: 431–436.
Slade A J, Hutchings M J. 1987. The effects of nutrient availabil-
ity on foraging in the clonal herb Glechoma hederacea. J Ecol,
75: 95–112
Wilson S D, Tilman D. 1995. Competitive responses of eight old-
field plant species in four environments. Ecology, 76: 1169–
1180.
Yuan Z-Y, Li L-H , Han X-G. 2004a. Nitrogen use efficiency of
competing individuals in a dense stand of an annual herb,
Chenopodium album. Acta Phytoecol Sin, 28: 294-299. (in
Chinese with English abstract)
Yuan Z-Y, Li L-H, Han X-G. 2004b. The effect of plant size on
the nitrogen use strategy in an annual herb, Helianthus annuus
(Sunflower). Acta Bot Sin , 46: 889-895.
Yuan Z Y, Li L H, Han X G, Wan S Q. 2004c. Variation in nitrogen
use traits of two Stipa species in a semi-arid region of northern
China. J Arid Environ, 54 (in press).
Yuan Z Y, Li L H, Han X G, Zhang W H, Wan S Q. 2004d.
Nitrogen resorption from senescing leaves of 28 species in a
semi-arid region of northern China. J Arid Environ, 54 (in
press).
Zhang L-X, Bai Y-F, Han X-G. 2004. Differential responses of
N:P stoichiometry of Leymus chinensis and Carex korshinskyi
to N additions in a steppe ecosystem in Nei Mongol. Acta Bot
Sin , 46: 259-270.