Effects of Topography, Soil Moisture, Wind and Grazing on Festuca Seedlings in a
Patagonian Grassland
Guillermo E. Defossé; Mónica B. Bertiller; Ronald Robberecht
Journal of Vegetation Science, Vol. 8, No. 5. (Nov., 1997), pp. 677-684.
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Mon Dec 3 12:11:06 2007
Journal of Vegetation Science 8 : 677-684,1997
O IAVS; Opulus Press Uppsala. P~intedin Sweden
Effects of topography, soil moisture, wind and grazing
on Festuca seedlings in a Patagonian grassland
Defosse, Guillermo E.l*, Bertiller, Monica B.l & Robberecht, Ronald2
'Centro Nacional Patagdnico, Boulevard Brown sin, 9120 Puerto Madryn, Chubut, Argentina; Tel. +54 965 51024; 2Department of Range Resources, University of Idaho, Moscow, ID 83844-1135, USA; * Corresponding author;
Fax +54 965 51543; E-mail defosse@cenpat.edu.ar Abstract. The effects of topography, soil moisture, wind and
grazing on the emergence and survival of seedlings of Festuca
spp. were examined in the steppe zone of Patagonia, Argentina. Ungrazed and grazed field treatment plots were established on a plain and a north-facing slope at the Media Luna
Ranch (43" 36' S, 71" 25' W). On the leeward and windward
sides of each of 15 Festuca plants, 0.1 m x 0.4 m quadrats
were censused bimonthly for seedling emergence and survival
over three growing seasons. Three categories were distinguished: recently germinated and up to the first leaf, two to
four leaves, and from five leaves up to one tiller. Soil moisture
content, litter cover and frost heaving effects were also determined for each treatment at each sampling date. Festuc,a spp.
showed two emergence peaks, one in late fall and the other in
early to mid-spring. Seedling emergence was significantly correlated with soil moisture content in the 0-5 cm of the soil during
the three growing seasons. Seedlings that emerged in the fall
had higher survivorship than those that emerged in spring.
Seedling emergence and survival was significantly ( p < 0.01)
lower on slopes, in the grazing treatment, and on windward
sides of adult plants. In this grassland, an increase in the
availability of safe sites for seedling emergence and survival
might be achieved by protecting vegetation from grazing,
particularly on north-facing slopes.
Keywords: Fesrnc,a pallerc~ens;Grassland steppe.
Nomenclature: Correa (1969- 1988)
Introduction
Arid and semi-arid ecosystems occupy the majority
of the Patagonian region of Argentina. This area evolved
with few native herbivores, which exerted a very low
pressure on native vegetation (Soriano 1983). Since the
introduction of sheep around the final decades of the
1800s these ecosystems have been subjected to disturbance caused mainly by sheep grazing (Soriano 1956a,
1983; Ares et al. 1990). The effects of continued grazing
and trampling have resulted in substantial changes in the
structure and composition of pristine Patagonian vegetation (Ares et al. 1990). In many areas, grazing has
reduced the vegetation cover and facilitated the development of bare soil patches where wind and water
remove litter, plant disseminules, and in some instances
the upper layer of the soil (Soriano 1956a; Ares et al.
1990). The recovery of native vegetation in these patches
is negligible, while the number of adventitious species
that colonize these areas is increasing.
One of the most important grasslands of Patagonia
with regard to productivity and forage value occurs in
the Province of Chubut as a narrow belt along the
foothills of the Andes, from 43" 25' S to 46" 15' S, and
continues further south from 5 I" 05' S in the west and
widening toward the east as it approaches the Atlantic
Ocean. This grassland, which also covers some areas of
the southern oriental Andes in Chile (Montaldo 1976),
has been classified as the Sub-Andean Floristic District
of the Patagonian Phytogeographic Province (Soriano
1956a; Cabrera 1976).
The dominant species is Festuca pallesc,ens (St.
Yves) Parodi which produces 50 - 90 C/c of the aerial
biomass (Soriano 1956a; Ares et al. I 990; DefossC et al.
1990; Aguiar et al. 1996). Festuca pallesc,ens, which
reproduces strictly from seed (Soriano 1960; Bertiller
1992), produces a high percentage of viable seeds. In
laboratory experiments, these seeds can reach more than
95 % germination when soil temperatures are above
10 "C and soil moisture content is above 8 C/c (DefossC
et al. 1995). The soil seed bank for this species, which is
replenished after seed dispersal during the fall, steadily
diminishes during the growing season to a minimum at
the end of the summer (Bertiller & Coronato 1994).
Festuca pallescens is considered one of the most
valuable forage grasses native to Patagonia (e.g. Parodi
1953). However, this bunch-grass species has been exposed to long-term sheep grazing and trampling. As a
consequence, the interspaces among bunch-grasses have
increased. The open spaces created between bunch-grasses
are subject to wind and water-caused erosion, and exposed to higher degrees of frost heaving. These events are
aggravated on north-facing slopes, where sheep tend to
concentrate earlier in the growing season. The combined
DefossC, G.E. et al.
effects of grazing, wind and water erosion have created
suitable microhabitats or 'safe sites' (Harper 1977) that
have been colonized by native species including Mulinum
spinosum, Carex patagonica, and adventitious species
such as Apera interrupta and Rumex acetosella instead
of the native grass F. pallescens. Although competition
from adult F.pallescens has been shown to be detrimental for F, pallescens seedling establishment (DefossC et
al. 1997), other reasons of minimal establishment of this
dominant are unknown.
The objectives of this study were to determine (1)
the dynamics of seedling emergence and survival of
Festuca spp. under natural conditions; (2) the principal
environmental factors affecting these dynamics; (3) the
combined effects of grazing, topography, and wind on
the availability of safe sites for F e ~ t u t a .
Material and Methods
Field study site
This study was conducted in an area representative
of the Sub-Andean Floristic District of the Patagonian
Phytogeographic Province (Soriano 1956a) at the Media Luna Ranch (45" 36' S, 71" 25' w), ca. 700 m a.s.1.
The site is characterized as a Festucu pallest~ensgrassland steppe, in which this species comprises ca. 4 5 4 of
the vegetation cover (Defosse et al. 1990; Bertiller &
Coronato i 994). The soils have a uniform, coarse texture, are well-drained and accumulate organic matter at
the surface and down to ca. 30 cm. The soils tend to be
slightly acid to neutral (A.M. Beeskow int. report 1987).
This area has a cold, wet winter (June to September) and
a warm, dry summer (December to March). The growing season extends from September to April and seed
dispersal occurs from mid- to late-summer (Bertiller et
al. 1991). Mean annual temperature is 4.7 "C, and July
(- 1.9 "C) and January (10.7 "C) are the coldest and
warmest months, respectively. Mean precipitation is
374 mm/yr, with 67 % of the precipitation falling in
winter and early spring in the form of either rain or
snow. There is no frost-free period and strong westerly
winds blow continuously throughout the year, especially during spring and summer.
Influent e o f envi~.onmenta/jufho~-s
on ~eedlingdynamics
In a relatively uniform paddock of 700 ha two
exclosures were built in 1983 on a plain and on northfacing slopes, two of the most common topographic
positions found in the grass steppe zone (1 ha and 0.5 ha
for the plain and slope, respectively). The areas selected
on the plain and north facing slopes were representative
of this grassland (Bertiller & DefossC 1990a, b; Bertiller
1992). The grazed area outside the exclosures has been
subjected to grazing by sheep every year from May to
December at a stocking rate of ca. 1.5 sheeplha for at
least the past 25 yr (Bertiller 1992). We acknowledge,
however, that the experimental design used has limitations and inferences drawn from our results are valid
only within the study area (Hurlbert 1984; Brown &
Waller 1986; Collins 1987; Dutilleul1993). Generalizations could only be made after comparing our results
with similar studies carried out in other areas of the
same floristic district (see Abadie 1967; DefossC et al.
1990; Bertiller 1992).
On east-west transects in the exclosures and on the
adjacent grazed areas, 15 similarly-sized adult plants of
Festuca pallescens were selected on the basis of basal
area and height. Two (0.10 m x 0.40 m) quadrats were
marked in the leeward and windward direction of these
15 plants. These quadrats followed the prevailing west
to east wind direction. The dynamics of seedling emergence and survival of Festuca spp. and other perennials
and annuals were followed in these quadrats for three
growing seasons. Sampling was performed bimonthly,
except during winter when the soil was covered by
snow. Seedlings of Festucxz spp. were censused according to three categories: (1) recently emerged, composed
of seedlings with one leaf and less than two months old;
(2) seedlings that survived from the previous census,
those with two to four leaves and more than two months
old; and (3) seedlings with more than four leaves and up
to one tiller. The latter group of seedlings had survived
for more than two censuses. Since F. palle.st~ens is
associated with F. pyr.ogca in the study area, and it is
very difficult to differentiate between species at the
seedling stage, they were censused together. However,
the contribution of F. pxr.ogea to the above-ground
phytomass of this grassland is less than 1 %, which is
negligible compared to F. pallescens, which provides
more than 90 C/c (DefossC et al. 1990).
Other perennials and annuals were censused together
and grouped as 'other species'. This group of species
contained adventitious species such as Ape1.a interrupta,
E~.odiumt.it.utarium, and Rumes acetosella, and native
shrubs and herbs such as Acaena spp., Ca~.e.rpatagonica,
Mulinum spinoxurn, Nassauvia ahhreviata, Erophila
verxa, Gilia grac.ilis, Myosurus apetalus and Cerastium
arvense. Litter cover was estimated within the quadrats
on a five-point scale:
1 : < 2 0 % , 2 : 2 1 - 4 0 % ; 3 : 4 1- 6 0 % ; 4 : 6 1 - 8 O % a n d 5 : > 8 0 % .
Frost heaving (cryoturbation) inside the quadrats was
estimated and ranked as: 1 : no signs of cryoturbation; 2:
< 30 % of the quadrat's surface with signs of the phenomenon; 3: 30 - 100 C/c of its surface with signs of the
phenomenon.
- Emergence and survival of Festuca spp. seedlings in a Patagonian grassland -
Environmental data: Soil moisture, temperature and
wind
Soil moisture content was evaluated gravimetrically
from five replicates for each of the leeward and windward sites near adult F. pallescens plants on plain and
sloped sites in grazed and ungrazed areas. Samples were
taken at depths 0 - 5, 5 - 10, 10 - 20 and 20 - 40 cm at
every sampling date. Precipitation, daily maximum and
minimum temperatures and wind speed at 1.5 m aboveground were monitored for the entire study period at the
nearest climate station, which was 1 krn away from the
study site. Wind speed was also registered with cup
anemometers (SIAP AM 9, 130 mm radius) at specific
sampling dates during the spring, summer and fall seasons. Wind speed was sampled simultaneously at 5 cm
and 1.5 m above the soil surface on the leeward and
windward sides of a typical Festuca plant.
E,perimental design and statistical analyses
The effects of grazing use (exclosure versus grazing),
topographic position (plain versus slope), wind protection (leeward vs windward sides of a typical F.pallesc.ens
plant) and sampling dates (from September 1989 to April
1992) on Festuca spp. seedlings of the three categories,
and seedlings of other species were analyzed in factorial
type designs using analysis of variance techniques (Norusis
1993). The analyses were based on the square-root transform of seedling density data to homogenize variances.
When appropriate, within-treatment differences were analysed as one-way analysis of variance using the Least
Square Differences procedure (Norusis 1993).
Significant effects of grazing use, topographic position and wind protection on soii moisture content, its
variation in the soil profile and sampling date were
analysed as a five-way analysis of variance (Norusis
1993). Analysis of variance was also used to determine
significant differences in wind speed at 1.5 m and 5 cm
above-ground, on the leeward and windward sides of
typical F. palle~censplants during spring, summer and
fall. Correlation analysis was used to determine significant relationships between seedling emergence and survival of Festuca spp. and litter cover, and between
seedling emergence and soii moisture content at 0 5 cm soil depth. A cross-tabulation procedure was used
to determine the intensity of the effects of frost heaving
during the different sampling dates (Norusis 1993).
Unless stated otherwise, the level of significance used
throughout this study was set at p < 0.01.
Results
Dynamics of seedling emergence and survival
During the three growing seasons considered and for
the entire population, Festuca spp. showed emergence
peaks during the spring and mid-fall, with a steady
decline toward the summer (Fig. la). Seedling emergence was significantly correlated ( r = 0.87, p < 0.05)
with soil moisture content in the 0 to 5-cm soil depth
during the entire study period. The dynamics of seedlings with two to four leaves followed a similar pattern
as those recently emerged, except that their peak of
survival was delayed by about two months relative to
recently emerged seedlings (Fig. lb). In contrast, seedlings with more than four leaves showed higher densities in mid- to late-spring seasons of the three growing
seasons considered (Fig. lc). During the summer, when
soil moisture content was at a minimum, seedling emergence was reduced to nearly zero and mortality rates
increased for seedlings with up to four leaves. During
this period, seedlings with more than four leaves and up
to one tiller, showed the highest survival rate of the three
groups considered.
The dynamics of seedling emergence for other species showed a peak during early spring of the first
growing season, and a decline to nearly zero during late
--
k
60
1
60
0
Seedlings recently emerged
I
Seedlings with two to four leaves
Seedlings with five leaves and up to one tiller
/
C
SONDJFMAMJ JASONDJFMAMJ J A S O N D J F M A
&
Summer
Wlnter
Summer
W~nter
Summer
Growing season
Fig. 1. Population dynamics of (a) recently emerged, (b)
more than two months old, and (c) five leaves and up to one
tiller seedlings of Festucu spp. during three growing seasons
at the Media Luna Ranch in Patagonia. Each point represents
a mean value for 120 quadrats (+ 1 SE).
DefossC, G.E. et al.
Summer
W~nter
Summer
Winter
Summe,
Summer
Winter
Summer
Wlnter
Summer
Growing season
Growing season
Fig. 2. Dynamics of recently emerged seedlings of Fesrrrca
spp. during three growing seasons as affected by (a) grazing,
(b) topography, and (c) wind protection. Each point represents
a mean value for 60 quadrats (f 1 SE).
Fig. 3. Dynamics of more than two-months old seedlings of
Festuca spp. (two to four leaves) during three seasons as af-
spring and up to the next fall. During the second growing
season, seedling emergence rate for this category remained relatively low, with less than 20 seedlings/m2.
Their emergence peaked again in spring of the third
growing season, and maintained high emergence rates
for the rest of the season.
wind protection (Table lc). A significantly higher number
of seedlings of this category was found in the exclosure
as compared to the grazing treatment, and also on the
plain as cornpared to the slope (Fig. 4). Additionally,
reduction in seedling survival was intensified in microsites exposed to wind on slopes, as indicated by a
The dynarnics of seedling emergence were also affected by grazing, topography, wind and the combined
effects of grazing and topography (Table la). Significantly higher seedling emergence was found in the
exclosure as compared to the grazing treatment, in the
plain as compared to the slope and on leeward quadrats
as compared to windward quadrats (Fig. 2). The cornbined effects of topographic position and grazing treatment intensified the negative effects of these two factors
on seedling emergence.
In the case of seedlings with two to four leaves, all
main factors except topography (plain versus slope) had
a significant influence on seedling survival (Table lb).
Significantly higher seedling survival was observed in
the exclosure as cornpared to the grazed area, and also
on leeward cornpared to windward quadrats (Fig. 3). No
significant interaction was found between either grazing
and topography, grazing and wind protection, and topography and wind protection.
Seedlings with more than four leaves were affected
by grazing and slope and not by the individual effect of
fected by (a) grazing, (b) topography, and (c) wind protection.
Each point represents a mean value for 60 quadrats (k 1 SE).
Summer
Winter
Summer
Winter
Summer
Growing season
Fig. 4. Dynamics of seedlings of Fesrrrc,a spp. with five leaves
and up to one tiller during three growing seasons as affected by
(a) grazing, (b) topography, and (c) wind protection. Each
point represents a mean value for 60 quadrats (+ 1 SE).
68 1
- Emergence and survival of Festuca spp. seedlings in a Patagonian grassland -
Table 1. Analysis of variance for the effects of topographic position (plain versus slope), grazing use (grazing versus exclosure), wind
protection (leeward versus windward sides of adult Fesruca pallescens plants), and sampling dates on emergence (a), survival of
seedlings of Fesruca spp. with two to four leaves (b), and survival of seedlings of Fesruca spp. with five leaves and up to one tiller (c).
Source of variation
a
Mean square
F
Mean square
F
Main effects
Topography
Grazing
Wind protection
Sampling dates
14.04
9.45
31.00
4.21
13.83
41.57**
27.96**
91.76**
12.46**
40.94**
7.33
0.05
10.64
3.62
7.93
31.11**
2.1411s
45.19**
15.40**
33.66**
Two-way interactions
Sampling dates - Topography
Sampling dates - Grazing
Sampling dates - Wind protection
Topography - Grazing
Topography - Wind protection
Grazing - Wind protection
1.70
2.18
2.54
0.53
2.21
0.83
0.01
5.01**
6.46**
7.51*"
1.57115
6.51 *
2.4711s
0.02115
1.00
0.92
1.45
0.78
0.14
0.86
0.21
4.26**
3.92**
6.16"*
3.35*"
0.5811s
3.65 ns
0.87 ns
Three-way interactions
Sampling dates - Topography - Grazing
Sampling dates - Topography - Wind protection
Sampling dates - Grazing - Wind protection
Topography - Grazing - Wind protection
0.46
0.97
0.21
0.20
0.58
1.3811s
2.88"*
0.62115
0.6111s
1.7111s
0.45
0.56
0.63
0.13
0.58
1.91**
2.40**
2.68*"
0.57 ns
1.71 ns
0.14
0.14
0.4211s
0.42115
4.11
4.1 1
1.7411s
1.74 ns
df
Four-way interactions
Sampling dates - Topography - Grazing - Wind protection
Significance:
"* p< 0.01; * p <
12
12
b
Mean square
F
0.05; n.s. not significantly different.
significant two-way interaction found between topographic situation and wind protection (Table 1c).
Seedling emergence of other species was also affected by grazing and topographic position, and not by
their position in windward or leeward quadrats. Significantly higher emergence for seedlings in this group was
found in the exclosure as compared to grazing and on
the slope as compared to the plain.
Litter cover varied seasonally and showed no particular pattern in its distribution. No significant correlation was found among seedling emergence or survival
of any age and litter cover. Exclosures, leeward sides of
a Festuca bunch-grass and slopes tended to have significantly higher litter cover as compared to areas that were
exposed to grazing, windward sides of a F e ~ t u c ubunchgrass or plain areas.
Frost heaving occurred mostly during the spring
months of September and November of the three growing seasons considered. No frost heaving was evident
during the summer, fall and winter seasons (Fig. 5).
This phenomenon occurred with similar intensity on
slopes and plains, in exclosures and grazed areas, and
also in windward and leeward quadrats. The only noticeable difference was that frost heaving ceased earlier
in November on slopes, in the grazing treatment as
compared to the exclosures, and at windward sides of
adult Festuc,a plants as compared to leeward sides.
En] ironmental fat tors. soil nloisture and M rnd
Soil moisture content was higher during the winter
and early spring, with a steady decline during the summer and a recharge of soil moisture during the fall. Soil
moisture content was significantly different at different
sampling dates, and reflected the long-term rainfall pattern. The upper soil profiles had the greatest seasonal
variation in soil moisture content, while this variation
diminished with greater depth into the soil profile. Soil
moisture content was significantly higher in the plain area
as compared to the slope, but was not significantly different between exclosed and grazed areas, or between leeward and windward sides of established F. pallescens
plants. However, soil moisture in the first 0 - 5 cm of the
soil significantly differed between plains and slopes, and
between leeward and windward sides of F. pallesc.ens
plants. In this soil profile, plain and leeward sides of a
Festuca plant showed consistent and significantly higher
soil moisture availability throughout the seasons as compared to slopes and windward quadrats, while there were
no significant differences in regard to grazing or exclosures
(Fig. 6).
At the Media Luna Ranch field site, the long-term
mean annual wind speed was 15.49 km/h, with maximum wind speeds of 20.60 km/h during the summer and
minimum wind speeds of 9.54 km/h during the winter.
During the study period, mean wind speed followed the
same trend as the long term mean, with maxima during
the summer of 20.4, 21.8, and 20.0 km/h for the three
growing seasons. Minima during the winter months were
Defosse, G.E. et al.
682
Discussion
Summer
Wtnter
Summer
Winter
Summer
Growing season
Fig. 5. Intensity of the occurrence of frost heaving during the
three growing seasons considered at the Media Luna Ranch in
Patagonia.
10.3, 8.2 and 6.8 km/h for the three seasons. The wind
profile for a typical F . pullescens plant showed significantly higher wind speed at a windward side as compared to leeward side of the plant during a typical
growing season. At 0.05 m above-ground, the figures
were 6.54 and 4.15 km/h in spring, 10.93 and 5.1 1 km/h
in summer, and 5.44 and 2.99 km/h in autumn, for
windward and leeward sides, respectively. At 1.5 m
above-ground, wind speed was 22.1,35.9, and 17.2 km/h
for spring, summer, and fall, respectively.
Exdosure 30 20
a
h
$?
10 w 0
r
0
I
I
Leeward
Summer
Wlnter
Summer
Wlnter
Summer
Growing season
Fig. 6. Water dynamics on 0 to 5 cm soil depth as affected by
(a) grazing, (b) topography, and (c) wind protection (bottom)
at the Media Luna Ranch in Patagonia. Each point represents
the average of n = 10 samples (*I SE).
In the windswept grassland steppe of Patagonia, the
natural course of seedling emergence and survival for
Festuca spp. and other species has been significantly
affected by the interaction of several factors. The combined effects of long-term grazing by introduced ungulates, and wind and water erosion has created relatively
large bare patches between Festuca tussocks. These
bare patches are detrimental for the emergence and
survival of Festuca spp. seedlings and are more favourable for the establishment of adventitious species.
Of all environmental factors examined in the present
study, the availability of moisture in the first 0 to 5 cm of
the soil was the most important for emergence and
survival of young seedlings of Festirca spp., i.e. those
with up to four leaves. Top-soil moisture was less relevant for survival of older seedlings of Festirca spp. or
for seedlings of other species. The importance of topsoil moisture for the emergence and survival of younger
seedlings was suggested by the strong relationship found
between top-soil moisture content and seedling emergence, and was consistent with another study performed
in the area (Defosse et al. 1997) and also with studies on
seedling emergence in other semi-arid (Juhren et al.
1956; Heady 1958; Mack & Pyke 1984; Soriano & Sala
1986) and humid grasslands (Bertiller & Ares 1978).
Seedling emergence, however, also depended on several other factors. Wind, for example, was shown to
differentially affect seedling emergence and survival. In
the study area, wind blows at high speed even near the
soil surface, and is significantly higher on the windward
side as compared to the leeward side of mature Festuca
plants. This had a desiccating effect on the top-soil,
which might have reduced emergence and survival of
young seedlings of Festuca spp. Older seedlings of
Festuca spp. and seedlings of other species were less
affected. This was probably due to a more developed root
system in the case of Festucu spp., and to differences in
root phenology and moisture demand in the case of other
species. Seedlings grown up in windy environments,
such as the Patagonian grassland studied, can also be
particularly vulnerable to abrasive damage caused by
wind-driven particles (Fryrear et al. 1973). This might
have affected seedling survival in the Festucu grassland
steppe. Furthermore, in windy environments, differences
in topography and the characteristics of the soil surface
(Chambers et al. 1990, 1991) can also affect seedling
emergence and survival.
The detrimental effect on seedling dynamics of
Festuca spp. for all seedling age categories, were significantly accentuated on sites with north-facing slopes.
These areas showed significantly higher seed numbers
of Festuca spp. per unit of area as compared to plains
- Emergence and survival of Festucu spp. seedlings in a Patagonian grassland -
after seed dispersal, but seed loss on slopes was greater
than on plain areas during the rest of the year (Bertiller
& Coronato 1994). Sloped areas represented particular
microhabitats in which the action of granivores (Price &
Reichman 1987; Kerley 1991), sheet wash (Rostagno
pers. comm.) and lower moisture availability in the
upper soil profile can significantly affect seedling dynamics. The interaction of these factors may explain
why significantly less seedling emergence and survival
of Festirca spp. were found in those areas as compared
to the plain during most of this study. In contrast,
seedlings of other species appeared to be better adapted
to these disturbances, and showed higher emergence on
slopes as compared to plain areas.
The lower seedling emergence and survival found in
grazed areas might be attributed to the effects of grazing
and trampling and not to a lower number of seeds in the
soil or to a lower top-soil moisture availability. In a
similar semi-arid grassland of North America, Balph &
Malecheck (1985) showed that significant disturbance
in bare soil patches in grazed areas was due to the action
of grazers, thereby reducing seedling establishment. In
the Patagonian grassland steppe, continuous grazing is
practised from May to December (Bertiller 1992), which
included most of the seedling emergence period. Thus,
the combination of defoliation and trampling by sheep
may have been responsible for the lower seedling emergence and survival of the grazed areas compared to
exclosures in this Fe.rtuca grassland in Patagonia. Furthermore, the combined effects of grazing and slope
were found to be highly significant, which indicated that
the effects of grazing on slopes were detrimental for
seedling emergence and survival.
Litter cover was found not to be a good indicator of
seedling success, and the correlation between litter cover
and seedling emergence and survival of all age categories was very poor. Litter tended to increase during the
winter months and to decrease during late spring and
summer. High winds redistribute it within the environment during the spring and summer months (DefossC et
al. 1990). The mobility of litter during these months
may explain the poor correlation found between litter
cover and seedling emergence and survival.
Frost heaving occurred in the study area mostly
during early to mid-spring, immediately after snowmelt and coinciding with the peak of seedling emergence. This effect has been observed to physically push
seedlings out of the ground, thereby exposing their root
system to the combined effects of desiccating high
winds and low soil moisture availability during the
summer (Soriano 1956b). In the present study, frost
heaving might have contributed to the high mortality
observed in recently emerged and young seedlings during the summer. This phenomenon has been considered
683
one of the main causes of failure in seedling establishment in some North American semi-arid rangelands
(Mack & Pyke 1984), where frost heaving can account
for 75% of seedling mortality during some years (Biswell
et al. 1953).
During the three growing seasons considered, older
seedlings were less affected by frost heaving or top-soil
moisture content than younger seedlings, i.e., those with
up to four leaves. Seedlings that emerged during the fall
were those that reached the stage of five leaves by late
spring. These seedlings had a higher probability of
surviving the summer drought than those emerging in
spring. The seedlings that emerged in spring, by contrast, only reached the two to four leaf stage by summer.
Consequently, survival during the summer drought period was greatly reduced. Excavation of the root system
of seedlings in the field during preliminary studies
showed that the root systems of older seedlings had
elongated considerably toward deeper soil horizons compared to younger seedlings with fewer leaves. This
explains the higher survival rate of older seedlings during the summer drought, when most of the seedlings of
the other two age categories died. This is consistent with
studies in other arid and semiarid ecosystems, where
seedlings have been shown to be highly vulnerable to
moisture stress (Wilson & Briske 1978; Mack & Pyke
1984). Of the initial number of emergent seedlings, few
survived past the one-tiller stage to become adult plants.
This was the case for grazed and non-grazed areas, and
is consistent with another study carried out in a similar
floristic district in northern Chubut (Abadie 1967). This
suggests that the survival of seedlings to adult plants
may be related to a complex of factors, of which
intraspecific competition has been shown to be particularly important (Defosse et al. 1997).
Acknowledgements. This research was supported by a grant
(PID 3058100/88) from the Consejo Nacional de Investigaciones Cientificas y TCcnicas (CONICET), Repliblica Argentina. We wish to thank the Ayling family, owners of the
Media Luna Ranch, for their continued and generous support
during the course of this study. Special thanks are due to F.
Coronato, who helped with the field work and provided some
environmental data.
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Effects of Topography, Soil Moisture, Wind and Grazing on Festuca Seedlings in a
Patagonian Grassland
Guillermo E. Defossé; Mónica B. Bertiller; Ronald Robberecht
Journal of Vegetation Science, Vol. 8, No. 5. (Nov., 1997), pp. 677-684.
Stable URL:
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References
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Jeanne C. Chambers; James A. MacMahon; Ray W. Brown
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http://links.jstor.org/sici?sici=0012-9658%28199008%2971%3A4%3C1323%3AASETIO%3E2.0.CO%3B2-4
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