UNIVERSIDADE FEDERAL DE SANTA MARIA
CENTRO DE CIÊNCIAS NATURAIS E EXATAS
PROGRAMA DE PÓS-GRADUAÇÃO EM AGROBIOLOGIA
GRASSLAND COMMUNITIES CHARACTERIZATION
UNDER DIFFERENT GRAZING FREQUENCIES
DISSERTAÇÃO DE MESTRADO
Fernando Forster Furquim
Santa Maria, RS, Brasil.
2016
GRASSLAND COMMUNITIES CHARACTERIZATION
UNDER DIFFERENT GRAZING FREQUENCIES
Fernando Forster Furquim
Dissertação apresentada ao Curso de Mestrado do Programa de
Pós-Graduação em Agrobiologia, da Universidade Federal de Santa Maria
(UFSM, RS), como requisito parcial para obtenção do grau de
Mestre em Agrobiologia
Orientador: Prof. Dr. Fernando Luiz Ferreira de Quadros
Santa Maria, RS, Brasil.
2016
Universidade Federal de Santa Maria
Centro de Ciências Rurais
Programa de Pós Graduação em Zootecnia
A comissão examinadora, abaixo assinada,
aprova a Dissertação de Mestrado
GRASSLAND COMMUNITIES CHARACTERIZATION UNDER
DIFFERENT GRAZING FREQUENCIES
elaborada por
Fernando Forster Furquim
Como requisito parcial para obtenção do grau de
Mestre em Agrobiologia
Santa Maria, 15 de Agosto de 2016.
DEDICATÓRIA
Ao meu vô, Mario Cardoso Furquim (in memorian), pelas virtudes que me transmitiu e,
principalmente, pelo Amor que me foi dado. Sou eternamente grato pelos nossos anos de
convivência que, com absoluta certeza, foram incríveis. Te dedico, Vô, mais esta conquista!
(Que saudade de ti!)
AGRADECIMENTOS
Agradeço, primeiramente, àqueles que são o meu maior motivo de
felicidade: meus pais – Mario e Isabel – e minha irmã – Gabrielli. Obrigado por
todo amor, carinho e incentivo! Vocês são tudo para mim! Amo vocês!
À minha Vovó Nair, por todo amor, carinho e zelo!
À Ana Carolina Bankow Mayer, por todo carinho, paciência e
compreensão para comigo. É uma alegria imensa te ter ao meu lado! :F
Ao professor Fernando Quadros, pela oportunidade proporcionada em
mais esta etapa da minha vida profissional.
Ao doutor José Pedro Pereira Trindade, pelo suporte logístico e pelas
valiosas conversas científicas-filosóficas.
Ao Clodoaldo Leites Pinheiro, pela ajuda em prol da plena execução do
experimento. Meu amigo, muitíssimo obrigado!
À Gabriela Machado Dutra, minha parceira de POT30 e de tantas outras
indiadas. Muito obrigado por toda ajuda, parceria e boa vontade! Que nossa
parceria perdure por infinitos outros experimentos!
Aos professores Gerhard Ernst Overbeck e Ilsi Iob Boldrini, pelas valiosas
contribuições para com a condução do experimento.
Ao professor John Derek Scasta, por toda atenção dispensada para com a
meu trabalho. Tuas sugestões, comentários e correções foram importantíssimas!
Aos doutores Stéphane Dray, Pedro Higuchi e Peter Borchardt, pelo
auxílio nas análises estatísticas.
À “Ramiro Família”, por toda atenção e cuidado durante o meu mestrado.
Ao amigo Augusto - conhecedor das potencialidades dos “campos de
Deus” -, pelos mates e pelo compartilhamento de vivências de vida e de campo
madrugadas adentro.
Às plantas! Seres incríveis e fascinantes que me provêm inúmeras
oportunidades de aprendizado.
The lunatic is on the grass
The lunatic is on the grass
Remembering games and daisy chains and laughs
Got to keep the loonies on the path
(Brain Damage – Pink Floyd)
RESUMO
Dissertação de Mestrado
Programa de Pós–Graduação em Agrobiologia
Universidade Federal de Santa Maria
GRASSLAND COMMUNITIES CHARACTERIZATION UNDER DIFFERENT
GRAZING FREQUENCIES
AUTOR: FERNANDO FORSTER FURQUIM
ORIENTADOR: FERNANDO LUIZ FERREIRA DE QUADROS
Data e Local da Defesa: Santa Maria, 15 de Agosto de 2016.
O bioma Pampa é um ecossistema natural com uma grande biodiversidade e recurso
forrageiro para os rebanhos. O manejo do pastoreio é uma ferramenta através da qual nós
podemos integrar produção animal e conservação de recursos provendo benefícios para todos
os participantes do sistema produtivo. Nesse contexto, os objetivos deste trabalho foram:
(identificar um padrão espacial de distribuição das comunidades de plantas; (2) identificar
diferença entre a composição de espécies vegetais; (3) identificar espécies de plantas com
potencial para caracterizar diferentes manejos de pastoreio; e (4) analisar a co-ocorrência
entre espécies indicadoras e outras espécies de plantas numa pastagem do bioma Pampa.
Foram analizados os efeitos de três diferentes métodos de pastoreio no padrão de distribuição
das espécies de plantas. Foi observado um padrão bem definido de distribuição de
comunidades de plantas apenas no manejo sem pastejo. Foram identificadas diferenças na
composição de espécies de plantas entre todos os manejos. As espécies indicadoras e suas coocorrências com outras espécies de plantas podem ser úteis para a compreensão sobre a
interação planta-herbívoro e planta-planta.
Palavras-chave: Campos Sulinos. Composição de espécies. Co-ocorrência de espécies.
Espécies indicadoras.
ABSTRACT
Dissertação de Mestrado
Programa de Pós–Graduação em Zootecnia
Universidade Federal de Santa Maria
GRASSLAND COMMUNITIES CHARACTERIZATION UNDER DIFFERENT
GRAZING FREQUENCIES
AUTHOR: FERNANDO FORSTER FURQUIM
ADVISOR: FERNANDO LUIZ FERREIRA DE QUADROS
Date and place of the defense: Santa Maria, August 15th, 2016.
The Pampa biome is a natural ecosystem with a large biodiversity and forage resource to
livestock production. Grazing management is a tool through which we can integrate livestock
grazing and resource conservation providing benefits for all participants of production system.
In this context, the objectives of this work were: (1) to identify a spatial pattern of plant
communities’ distribution; (2) to find differences between plant species composition; (3) to
identify plant species with potential to characterize different grazing managements; and (4) to
analyze co-occurrence between indicator plant species with others plant species in a Pampa
biome grassland. Three grazing managements’ effects on plant species patterns were
analyzed. A defined pattern of plant communities’ distribution was only observed in ungrazed
management. Difference on plant species composition between all managements was
identified. Indicator plant species and its co-occurrence with others plant species seems useful
for comprehension about plant-grazer and plant-plant interactions.
Keywords: Indicator species. Southern Campos. Species composition. Species co-occurrence.
SUMMARY
INTRODUCTION .................................................................................................................... 10
METHODS ............................................................................................................................... 12
Experimental area and land use history ................................................................................ 12
Grazing management ............................................................................................................ 13
Vegetation sampling ............................................................................................................. 14
Data analysis ......................................................................................................................... 15
Principal Coordinates Analysis ......................................................................................... 15
Plant species composition .................................................. Erro! Indicador não definido.
Cover of species life forms and Eragrostis plana; and canopy height ............................. 16
Indicator species analysis .................................................................................................. 16
Co-occurrence species analysis ......................................................................................... 17
RESULTS ................................................................................................................................. 18
Vegetation sampling ............................................................................................................. 18
Principal Coordinates Analysis ............................................................................................. 18
Plant species composition ..................................................................................................... 19
Indicator species analysis ...................................................................................................... 21
Co-occurrence species analysis ............................................................................................ 21
DISCUSSION........................................................................................................................... 23
CONCLUSIONS AND IMPLICATIONS ............................................................................... 28
REFERENCES ......................................................................................................................... 29
APPENDIX A.......................................................................................................................... 42
CONCLUSÕES....................................................................................................................... 43
10
1
INTRODUCTION
2
3
The Pampa is a natural ecosystem with a large biodiversity (Bilenca &
4
Miñarro, 2004) and forage resource to livestock production (Carvalho and
5
Batello, 2009). The successful use and conservation of this ecosystem is
6
straight related with sustainable land-use, which could provide an equilibrium
7
between biodiversity and forage production (Jacobo et al., 2006; Overbeck,
8
2007). Grazing management is a tool through which we can integrate livestock
9
grazing and resource conservation providing benefits for all participants of
10
production systems (Bailey, 2005; Da Trindade et al., 2012).
11
Briske et al. (2008) define as goal of grazing systems the increase
12
production by providing conditions (i.e. capture of light and nutrients) to plant
13
species growth and by enabling livestock to select forage more efficiently.
14
Frequency and intensity of grazing alters plant species composition (Altesor et
15
al., 2005; Jacobo et al., 2006; Oesterheld and Semmartin, 2011; Overbeck,
16
2014; Da Trindade et al., 2016), which reflects on forage quality and animal
17
performance (Searle et al., 2007).
18
Distribution of plant communities along different grazing managements:
19
(1) can evidence changes on species composition (Boldrini and Eggers, 1996;
20
Altesor et al., 2006; Jacobo
21
nutritional acquirement decision of grazers given spatial arrangement of
22
palatable and unpalatable species (Barnes et al., 2008; Laca et al., 2011);
23
and, (3) can be used as predictor of degradation processes (Carvalho and
24
Batello, 2009). Considering these items, identification and description of
et al., 2006; Overbeck, 2014); (2) affects
11
25
grazing disturbances’ effects by plant descriptors are needed to develop
26
management tools that combine biodiversity maintenance and forage
27
efficiency (Ewald, 2003; Paruelo et al., 2004; Lezama et al., 2006).
28
Our objectives in this article were: (1) to identify a spatial pattern of plant
29
communities’ distribution; (2) to find differences between plant species
30
composition; (3) to identify plant species with potential to characterize different
31
grazing managements; and (4) to analyze co-occurrence between indicator
32
plant species with others plant species in a Pampa biome grassland.
12
33
METHODS
34
35
Experimental area and land use history
36
The experiment was conducted in a natural grassland of Pampa biome
37
belonging to Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA) –
38
Pecuária Sul Unit, located at Bagé city (31° 18' S, 53° 57' W) in Rio Grande do
39
Sul, the southernmost Brazilian state. The area is situated in an ecotonal
40
transition zone between the Campanha and Serra do Sudeste physiographic
41
regions and the soils are Vertisols and Luvisols (STRECK et al, 2008). The
42
altitude is 214.2 m and the climate is classified as temperate and humid
43
according to the Köpen classification with the 30 year historical mean
44
precipitation of 1446.2 mm and mean temperatures of 18.7 ºC (INMET, 2015).
45
This vegetation survey was conducted from December 2014 to February 2015.
46
During this period the mean temperature was 23.4 ºC and total rainfall
47
accumulation was 448.6 mm (INMET, 2015).
48
The area has no history of mechanized agriculture or tillage. During the
49
last 40 years, the land has been used primarily for cattle and sheep grazing at
50
low stocking rates (< 0.5 animal unit ha-1). Before experiment installation, the
51
vegetation was dominated by Acanthostyles buniifolius (Hook. ex Arn.) R.M.
52
King & H. Rob., Eragrostis plana Ness, Eryngium horridum Malme and
53
Saccharum angustifolium (Ness) Trin..
13
54
55
Grazing management
56
From June of 2012, the experimental area was completely excluded
57
from grazing of large herbivores and was subdivided into five separate
58
paddocks that were managed with three treatments: two grazing methods
59
(continuous stocking (CONT) and rotational stocking (ROT)) and an excluded
60
area (EXCL). Two paddocks were managed with CONT (4.9 ha each) and two
61
with ROT (5.6 ha each). Each paddock managed with ROT was subdivided
62
into eight sub-paddocks of 0.7 ha each. The EXCL paddock was 3.0 ha.
63
Grazing in CONT and ROT started in February of 2013.
64
In ROT management, the rotational criteria for grazing intervals were the
65
accumulative thermal sum of 375 degree-days (degree Celsius per day; DD).
66
This criteria is based on the requirements for for leaf elongation duration of 2.5
67
leaves per tiller of grasses for two functional groups: functional group A (e.g.
68
Axonopus affinis Chase) and functional group B (e.g. Paspalum notatum
69
Flügge), both characterized as resources capture grasses (Cruz et al, 2010).
70
To define the rest interval in ROT management, mean phyllochron (time in DD
71
for complete leaf elongation) of functional groups A and B (150 DD) was
72
multiplied by the number of expanding leaves per tiller, generating the rest
73
periods of each sub-paddock. The number of expanding leaves of the grasses
74
in the functional groups is related to plant genetic traits and defines the time of
75
rest intervals (Cruz et al., 2010). Thus, the occupation period was defined by
76
dividing rest intervals (in thermal sum) of ROT management by the number of
14
77
sub-paddocks minus one (sub-paddock under occupation), resulting in the
78
time, in Celsius degrees, of occupation of each sub-paddock. In all grazing
79
treatments we used 18 month old Brangus heifers and in CONT management
80
heifers remained in the same paddock for the duration of the study.
81
A forage allowance (FA) of 12 % of the total available forage biomass
82
available was allocated to the heifers (FMa; kg DM ha-1). The stocking rate
83
(SR) was adjusted considering the following equation: SR = ((FMa / N) + FAR)
84
/ FA. Where, FMa is the forage mass available; N is the number of days in a
85
paddock; FAR is the forage accumulation rate; FA is the forage allowance of
86
12 %. In CONT management, the N used was the period between two
87
evaluations. In ROT management, the N used was the average number of
88
occupation days in each sub-paddock, during each period.
89
For estimative of forage accumulation rate (FAR, kg DM ha-1 day-1), we
90
allocated three exclusion cages from grazing in each paddock and used the
91
equation proposed by Campbel (1966), as following described: FAR = (FMin –
92
FMout) / n. Wherein, FMin (kg DM ha-1) is the forage mass inside the exclusion
93
cage; FMout (kg DM ha-1) is the forage mass outside the exclusion cage; n is
94
the number of days between evaluations.
95
96
Vegetation sampling
97
Fifty transects with 1.25 m² of area (0.50 m × 2.50 m) were demarcated
98
in the experimental area: 20 in ROT, 20 in CONT and 10 in EXCL. Each
15
99
transect was subdivided into five plots with 0.25 m² of area (0.50 m × 0.50 m).
100
Each vascular plant species in the plot was identified to the lowest possible
101
taxonomic level and unknown specimens were collected and later identified
102
using several regional taxonomic keys. We used the modified Londo-scale
103
(LONDO, 1976) cover classes (< 1 %; 1-3 %; 3-5 %; 5-15 %; 15-25 %; 25-35
104
%; 35-45 %; 45-55 %; 55-65 %; 65-75 %; 75-85 %; 85-95 %; 95-100 %) to
105
estimate abundance and canopy cover of each vascular plant species. We
106
also measured canopy height of each plot using a graduated ruler (Barthram,
107
1985).
108
109
Data analysis
110
111
Principal Coordinates Analysis
112
We performed an ordination, via Principal Coordinates Analysis (PCoA),
113
aiming to identify and interpret distribution patterns by projecting onto the
114
ordination diagram the species’ cover distribution per transect (i.e. plant
115
community). The ordination was based on chord distance (Podani, 2000)
116
between transects with the vegdist and cmdscale functions of the ‘vegan’ and
117
‘stats’ packages in R statistical software (Oksanen et al., 2016; R
118
Development Core Team 2015). Species were selected to include in the
119
diagram according to its cover values and correlations with the ordination
120
axes.
121
16
122
123
Cover of species life forms, palatable species and Eragrostis plana; and
124
canopy height
125
We grouped cover of species according to: (1) its life form based on
126
classification used by Altesor et al. (2005); and (2) its palatability based on
127
adaptions from Rosengurt (1976) and our field observations (e.g. animal
128
grazing preference). We also included cover of Eragrostis plana (the main
129
invader species on Pampa biome) and canopy height.
130
131
Indicator species analysis
132
In order to reveal indicator species that characterize the subsequent
133
effects of different grazing treatments, we conducted Indicator Species
134
Analysis (Dufrêne and Legendre, 1997) using ‘indicspecies’ package (De
135
Cáceres and Jansen, 2012; R Development Core Team 2015). We selected
136
as candidates those species occurring in at least 10 % of the sampling units
137
(i.e. transects) to reduce the number of combinations being explored. From the
138
candidate species, we allowed combinations of up to five species, meaning we
139
considered as potential indicators all singles and possible pairs, triplets,
140
quartets and quintets. The indicators function was used to calculate indicator
141
specificities (A) and sensitivities (B) according to De Cáceres and Legendre
142
(2009). Indicators were considered valid when A > 0.5 and B > 0.5 (De
143
Cáceres et al. 2012; Bachand et al. 2014). For parsimony to further reduce the
144
possible indicators, the pruneindicators function was used to: (1) discard final
17
145
indicators whose occurrence patterns were nested within other final indicators,
146
and (2) evaluate coverage (i.e., the proportion of sites in a target reference
147
group where at least one of the final indicators is present) of the remaining
148
indicators by progressively increasing numbers of indicators until reaching a
149
subset with the same coverage as the complete set, up to a maximum of five
150
indicators (De Cáceres et al. 2012).
151
152
Co-occurrence species analysis
153
Relative cover data were used to obtain the probability of pairwise
154
species co-occurrence in order to find evidence of whether the communities
155
organizing processes were due to random or structured patterns. We used the
156
methodology proposed by Veech (2013) which has lower Type I and II errors
157
than null models and was appropriate to our matrix given the large number of
158
vascular plant species in our study (Griffith et al., 2014). For the co-occurrence
159
analysis, we used only the pairs of species with expected co-occurrence
160
greater than 1. This analysis was performed for CONT, EXCL and ROT
161
grazing managements using ‘cooccur’ package (Griffith et al., 2016; R
162
Development Core Team 2015) and its pair function (which extracts results for
163
a single species from co-occurrence analysis) for indicator species of each
164
management.
165
18
166
RESULTS
167
168
Vegetation sampling
169
In CONT transects, we recorded 116 species; in EXCL, 85 species; and
170
in ROT, 108 species. In all three grazing managements, most numerous
171
botanical genus belong, respectively, to Poaceae and Asteraceae families
172
(Appendix A).
173
174
Principal Coordinates Analysis
175
176
The first and second axes of Principal Coordinates Analysis explained,
177
respectively, 22.90 % and 17.82 % of total variation of plant communities (Fig.
178
2).
19
179
Figure 2 - Principal Coordinates ordination diagram showing first two axes of
180
plant communities’ distribution and the spatial position of grazing
181
managements’ dominant species. Legends: CONT (Δ), EXCL (×)
182
and ROT (○). Cover dominant species (●). Legends: Acbu: A.
183
buniifolius; Anlan: A. lanata; Axaf: A. affinis; Axar: A. argentinus;
184
Ereb: E. eburneum; Erho: E. horridum; Erpl: E. plana; Mnse: M.
185
selloana; Pano: P. notatum; Papu: P. pumilum; Paqu: P.
186
quadrifarium; Saan: S. angustifolium.
187
188
It was possible to identify a well-defined pattern in communities’
189
distribution mainly in EXCL management, which was positioned along positive
190
portion of Axis 2. It was obtained a pattern that most of the units of CONT
191
management was positioned at the right of Axis 1 and the majority of ROT was
192
at the left of the same axis. Although it could not be as well-defined as EXCL
193
as we could observe some (i.e. seven) units of the other grazing
194
managements dispersed along the diagram. This could be attributed to other
195
site effects not linked with the grazing gradients evaluated.
196
197
Plant species composition
198
199
Mean cover of life forms was presented in Table 2. Cover of forbs was
200
higher in EXCL and lesser, but similar, between CONT and ROT
201
managements. The cover of graminoids was higher and similar in CONT and
20
202
ROT; and lesser in EXCL. Shrubs cover was greater in EXCL than CONT and
203
ROT; the latters were similar. Cover of palatable species was higher in ROT,
204
intermediate in CONT and lower in EXCL. Cover of Eragrostis plana was
205
similar in ROT and EXCL but different between those and CONT, which had
206
the highest value. Canopy height was similar between CONT and ROT, and
207
higher in EXCL.
208
209
Table 2
Mean cover values and standard deviation of forms of life (forbs,
210
graminoids and shrubs), palatable species and Eragrostis plana
211
cover; mean height (cm) and standard deviation of canopy in
212
different managements
213
CONT
EXCL
ROT
Forbs
19.02 ± 0.60
25.20 ± 1.30
16.12 ± 0.57
Graminoids
77.49 ± 4.59
48.94 ± 3.08
80.31 ± 4.67
Shrubs
3.49 ± 0.63
25.87 ± 6.58
3.56 ± 0.83
Palatable
47.36 ± 2.65
9.31 ± 0.35
71.36 ± 4.82
Eragrostis plana
30.20 ± 0.22
1.56 ± 0.04
6.35 ± 0.07
Canopy height
13.65 ± 7.49
44.72 ± 25.41
11.38 ± 6.82
21
214
Indicator species analysis
215
We obtained two single indicator species for each management
216
totalizing six different species. Four of them were from Poaceae family and two
217
were from Asteraceae family (Table 3).
218
219
Table 3 Indicator species analysis, where A is specificity, B is sensitivity, sqrtIV
220
is square root of the indicator value and Cover is pooled coverage
221
(%).
Management
CONT
Final indicators
A
B
sqrtIV
Eragrostis plana*
0.81
0.95
0.88
Aspilia montevidensis†
0.55
0.80
0.66
Achantostyles buniifolium†
0.81
0.80
0.81
Saccharum angustifolium*
0.73
0.70
0.71
Axonopus affinis*
0.76
0.90
0.83
Cover
100
EXCL
100
ROT
100
*
Paspalum notatum
222
* Poaceae
223
† Asteraceae
0.57
0.95
0.74
family
family
224
225
Co-occurrence species analysis
226
The species co-occurrence analysis showed that, in CONT, Aspilia
227
montevidensis had positive co-occurrence with Axonopus argentinus (p =
228
0.0433). In EXCL, it was observed a positive co-occurrence of Acanthostyles
229
buniifolius with Baccharis crispa Spreng. (p = 0.0222) and Paspalum
22
230
plicatulum Michx. (p = 0.0222). In ROT management, Axonopus affinis had
231
negative co-occurrence with Baccharis coridifolia DC. (p = 0.0158) and
232
Eryngium nudicaule Lam. (p = 0.0158).
233
Other management’s indicator species (i.e. Eragrostis plana (CONT),
234
Saccharum angustifolium (EXCL) and Paspalum notatum (ROT)) did not
235
showed neither positive nor negative co-occurrence with other species.
236
23
237
DISCUSSION
238
239
In EXCL management, due to the great efficacy in light capture, tall-plants (i.e.
240
Acanthostyles buniifolius, Anthaenantia lanata, Eryngium eburneum, Eryngium
241
horridum, Paspalum quadrifarium and Saccharum angustifolium; Fig. 1) were
242
dominant while short-plants were subdued to shading (Sala, 1988; Boldrini and
243
Eggers, 1996; Altesor et al., 2006). The latter disfavors the full maintenance of the
244
vital activities of short plant species, causing replacement of these species by few
245
species with larger, horizontal and vertical, size (Sala, 1986; Sala, 1988; Rambo and
246
Faeth, 1999; Kuijper et al., 2008). This provide a close proximity between plant
247
communities of EXCL management along the positive portion of Axis 2 (Fig. 1),
248
reducing the heterogeneity of this management.
249
which can be observed in our results through: (1) highest values of canopy
250
height (Table 2); (2) dominance of tall-size species; (3) proximity between plant
251
communities along the positive portion of Axis 2; and (4) indicator species (Table 3)
252
(Acanthostyles buniifolius (shrub) and Saccharum angustifolium (caespitous-grass).
253
The EXCL management had higher shrub cover when compared to grazed
254
managements. Similar results were obtained by Cabral et al. (2003), Altesor et al.
255
(2006), De Villalobos and Zalba (2010), Lezama et al. (2014), where areas excluded
256
from grazing of large herbivores experienced a shrub encroachment. This process
257
creates a microenvironment under shrub’s canopy, reducing solar radiation, air
258
temperature and wind speed when compared with an open site (Holmgren et al.,
259
1997). Possibly, due these changes, Acanthostyles buniifolius provided ideal
260
conditions to establishment of Baccharis crispa and Paspalum plicatulum under its
261
canopy.
24
262
The cover of palatable species in EXCL decreased due absence of grazing
263
and therefore dominance of unpalatable species (i.e. majority of tall-size species). It
264
was also observed a low cover of E. plana in EXCL management. According to Focht
265
and Medeiros (2012), the development of a high and dense canopy structure difficult
266
the spread of this invasive species, which could avoid its expansion.
267
In grazed sites (i.e. CONT and ROT), according to Lemaire (2001), the
268
presence of defoliation events, at community level, reduces the importance of
269
competition for light because: (1) it does not affect only a plant species but also its
270
neighbors; and (2) it can be different according to intensity and frequency of
271
defoliation provided by adopted management criteria. Thus, grazing dynamics can
272
increase
273
(McNaughton, 1983; Adler, 2001) and, in our case, this was confirmed through
274
ordination diagram, wherein it is possible to detect difference between spatial pattern
275
of grazed and ungrazed communities (Fig. 1).
heterogeneity
of
grazed
communities
at
different
spatial
scales
276
Even in a short time of a specific grazing management (about two-years), all
277
sites (CONT, EXCL and ROT) showed different species composition. We were not
278
expecting major differences between CONT and ROT managements. Probably due
279
to different frequency and intensity of defoliation and trampling events (Belsky, 1992;
280
Olff and Ritchie, 1998), which were more severe in CONT management than ROT,
281
the ordination diagram showed a slightly different pattern. Table 1 reinforced this
282
difference. Thus, as we made for EXCL management, we ran an indicator species
283
analysis in order to evidence, with more reliability, management effects through
284
indicator plant species (Lavorel et al., 1998; Lawton and Gaston, 2001; Cousins and
285
Lindborg, 2004).
25
286
Through this approach, first indicator species of CONT management was
287
Eragrostis plana (Table 3), which have the higher cover in this management (Table
288
2). According to estimates of Medeiros and Focht (2007), around 10 years ago, this
289
specie occupied more than 1 million of hectares in Pampa biome and its expansion
290
rate was estimated as 14,000 ha per year (Carvalho and Batello, 2009). The CONT
291
management can accelerates the biological invasion of E. plana through several
292
trampling of animals (Meeuwig and Packer, 1999) and higher biomass removal of
293
native plants with higher nutritional value; furthermore, decreasing vegetation cover,
294
leading to a soil exposure and, consequently, providing conditions to spread of E.
295
plana (Lonsdale, 1999; Davis et al., 2000; Carvalho and Batello, 2009).
296
Low forage quality of E. plana avoids intense defoliation and, as consequence,
297
the production of panicles is increased from November to March (Medeiros and
298
Focht, 2007; Medeiros et al., 2009; Medeiros et al., 2014). This event occurs when
299
there is a decrease in biomass of high forage quality native species thus, in order to
300
compensate their nutritional requirements, animals turns to consume E. plana, whose
301
less unpalatable part, when it became an adult plant, is its panicle (Medeiros et al.,
302
2009).
303
There are three main possibilities to seed destination of E. plana (we adapted
304
from Medeiros et al., 2009 and Medeiros et al., 2014): (1) one fraction accumulates in
305
the soil seed bank; (2) other fraction is ingested by large herbivores, scarified in
306
rumen and distributed locally through feces, with a high concentration of organic
307
matter; and (3) another fraction can be transported, by animals, to other
308
areas/regions. All of these destinations can aggravate or even start a new invasive
309
process, which could be frequently observed in continuous grazing - specially, in
310
overgrazed areas.
26
311
The second indicator species of CONT, Aspilia montevidensis, is a short plant
312
with some coarse green biomass (i.e. low senescent leaves), raised leaves (higher
313
upper leaf’s density) (Blanco et al., 2007). This species is a heliophytic perennial
314
herb that contains xylopodium (Takeda and Farago, 2011) and stages of flowering
315
and fruiting over all year (Biondi et al., 2007). These characteristics allow A.
316
montevidensis to increases its performance with the increase in grazing intensity
317
(Altesor et al., 1998; Blanco et al., 2007). In addition, this species, being unpalatable
318
for cattle, seems to establish an associational avoidance for defoliation with
319
Axonopus argentinus (palatable grass) (Milchunas and Noy-Meir, 2002). All of these
320
species’ characteristics and associations seem to be according to grazing dynamics
321
of CONT management.
322
The ROT management was dominated and indicated by two palatable species
323
of functional groups A and B, respectively: Axonopus affinis and Paspalum notatum.
324
Interaction between characteristics/mechanisms to avoid and/or tolerate grazing
325
disturbance and rest interval criteria, based on physiological characteristics of these
326
functional groups, provides conditions to maintenance and spread of these (Briske
327
and Heidschidt, 1991; Olff and Ritchie, 1998; Loreti et al., 2001; Laca, 2009;
328
Lemaire, 2011; Funk et al., 2016).
329
We obtained, through co-occurrence species analysis, a negative relationship
330
between Axonopus affinis with two species that have physical and chemical
331
characteristics to evade from grazing: Baccharis coridifolia (species with unpalatable
332
secondary compounds; Altesor et al., 1998) and Eryngium nudicaule (species with
333
rosette form, fibrous tissues, unpalatable compounds, leaves almost in ground level;
334
Diaz et al., 1992). Negative co-occurrence of A. affinis and these species seems to
335
be attributed to competition for cover space in grazing-rest intervals existing in ROT
27
336
management. With cover dominance of A. affinis, we could expect that toxic diseases
337
of cattle will be decreased through reduction of toxic species B. coridifolia.
338
Furthermore, some results are important to be highlighted: (1) the higher cover
339
of palatable species in ROT management; and (2) the lower cover of E. plana. The
340
latter, according to Focht and Medeiros (2012), is limited by rotational stocking
341
management through a maintenance of natural grassland residual biomass (with a
342
height near 10 cm), which limits the availability of resources (i.e. water and nutrients).
343
Then, it is possible to observe that barrier to spread of E. plana invasion is
344
dependent on both grazing intensity and frequency.
28
345
CONCLUSIONS AND IMPLICATIONS
346
347
Different grazing managements cause changes on plant communities’
348
composition even in a short time (i.e. about two years). On natural grasslands
349
of this region, grazing exclusion defines a new pattern of vegetation dynamics
350
leading to a shrub encroachment and tall-size species dominance, while
351
grazed managements leads to a decrease of shrubs and to a dominance of
352
short-plant species. ROT management favors palatable species, can be used
353
to toxic species control (i.e. Baccharis coridifolia) and, together with EXCL, can
354
be used as ecological management to barrier invasive spread of Eragrostis
355
plana. CONT management, however, aggravates this invasive process
356
causing degradation of natural vegetation.
357
For this region, indicator plant species and species co-occurrence
358
analysis were time and cost efficient. Their results represent reliably effects of
359
different grazing managements and provide us a well-detailed description
360
about plant-grazer and plant-plant interactions.
361
29
362
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APPENDIX A
43
CONCLUSÕES
642
643
Diferentes métodos de pastoreio causam mudanças na composição das
644
comunidades de plantas mesmo em um curto período de tempo (i.e. dois
645
anos). Nas pastagens naturais dessa região, a exclusão do pastejo acarreta
646
um novo padrão de vegetação favorecendo a expansão de subarbustos e de
647
plantas de grande porte. O manejo rotativo favorece espécies palatáveis,
648
pode ser usado para o controle de espécies tóxicas (i.e. Baccharis coridifolia)
649
e, junto com a exclusão do pastejo, pode ser usado como uma manejo
650
ecológico para barrar a expansão do Eragrostis plana. O manejo de pastoreio
651
contínuo, entretanto, agrava o processo invasivo do E. plana degradando,
652
assim, a vegetação natural.
653
Para esta região do bioma Pampa, a análise de espécies indicadoras e
654
a co-ocorrência de espécies foram eficientes tanto em custo quanto em
655
tempo. Os seus resultados representaram fidedignamente os efeitos dos
656
diferentes métodos de pastoreio e nos forneceram uma descrição bem
657
detalhada acerca das interações entre planta-herbívoro e planta-planta.