ARTÍCULOS ORIGINALES
Effect of Prosopis flexuosa on understory species and its importance to pastoral management in woodlands of the Central Monte Desert *
Efecto de Prosopis flexuosa sobre las especies del sotobosque y su importancia para el manejo pastoril de los algarrobales del desierto del Monte Central
Erica M. Cesca 1, Carlos Passera 2, 3 ,Pablo E. Villagra 1, 3, Juan A. Alvarez 1
* This study was supported by Agencia Nacional de Promoción Científica y Tecnológica (Argentina)
(PICT 2007 01222) and Universidad Nacional de Cuyo.
1 Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. IANIGLA, CCT CONICET.
C. C. 330. (5500) Mendoza, Argentina. ecesca@mendoza-conicet.gob.ar.
2 Instituto Argentino de Investigaciones de Zonas Áridas. IADIZA, CCT CONICET. C. C. 507.
(5500) Mendoza, Argentina.
3 Facultad de Ciencias Agrarias. Universidad Nacional de Cuyo. Alte Brown 500. Chacras de Coria,
Mendoza, Argentina. M5528AHB.
Originales: Recepción: 28/10/2011 - Aceptación: 09/10/2012
ABSTRACT
In the Monte Biogeographic Province, located in the arid region of Argentina, the presence of Prosopis flexuosa DC. produces spatial heterogeneity through edaphic modifications and microclimate changes. This results in vegetation patches differing in species composition and abundance. However, this interaction can be modified by the occurrence of gradients of biotic stress or disturbance intensity. In particular, grazing has been observed to enhance or reduce vegetation heterogeneity. Such complex of interactions could determine forage availability for cattle in one of the driest areas of the Monte Desert. We assessed the effect of Prosopis on understory species and analyzed whether the outcomes of this interaction differed with distance to watering points, as a proxy of grazing intensity, in the Northeast of Mendoza Province, Argentina. We used a two-way factorial design including the following factors: 1) microsite (under the cover of P. flexuosa trees and in intercanopy microsites) and 2) distance to watering points ("near the watering point", 500-700 m away, and "far from the watering point", 3-4 km away). Cover of each species, total cover, bare soil, and litter were recorded, and plant diversity, richness, and evenness were estimated with the modified Point Quadrat method. Results showed that P. flexuosa cover, distance from watering points, and the interaction between them determined species composition, abundance and spatial distribution of understory species, and were, consequently, a determining factor for forage availability. The presence of P. flexuosa enhances carrying capacity by supporting higher abundance of grasses under its canopy. Near watering points, high grazing intensity appears to disrupt the patches formed under P. flexuosa canopies, reducing the differences between microsites.
Keywords: Spatial heterogeneity; Patchiness; Facilitation; Distance from watering points; Forage availability
RESUMEN
En la Provincia Biogeográfica del Monte, ubicada en la zona árida de Argentina, la presencia de Prosopis flexuosa DC. genera heterogeneidad espacial a través de modificaciones edáficas y cambios microclimáticos, lo que genera parches de vegetación con distinta composición florística y abundancia de especies. Sin embargo, esta interacción puede ser modificada por la presencia de gradientes de estrés biótico o de intensidad de disturbio. En particular, se ha observado que el pastoreo puede incrementar o disminuir la heterogeneidad espacial. Este complejo de interacciones puede determinar la disponibilidad de forraje en una de las zonas más áridas del Monte. Este estudio analiza la influencia de P. flexuosa sobre el patrón espacial del resto de las especies de la comunidad y cómo esta interacción es modificada según la distancia a la aguada, tomada como un indicador de la intensidad de pastoreo, en el Noreste de la Provincia de Mendoza, Argentina. Fue realizado un diseño bifactorial, incluyendo los siguientes factores: 1) micrositio ("bajo la cobertura de P. flexuosa" y "áreas expuestas") y 2) distancia a la aguada ("cerca de la aguada", desde 500 hasta 700 m, y "lejos de la aguada", entre 3 y 4 km). Se registró la cobertura de cada especie, cobertura total, suelo desnudo y mantillo, y se estimó la diversidad, riqueza y equitatividad a través del método de Point Quadrat modificado. Los resultados muestran que la cobertura de P. flexuosa, la distancia a la aguada y la interacción entre ambas determinan la composición, abundancia y distribución espacial de las especies en la comunidad y, consecuentemente, la disponibilidad de forraje. La presencia de P. flexuosa aumenta la capacidad de carga incrementando la abundancia de pastos bajo su dosel, aportando por lo tanto a la cobertura forrajera. Cerca de la aguada, la alta intensidad ganadera disminuye el efecto de P. flexuosa al disminuir la diferencia entre los micrositios.
Palabras clave: Heterogeneidad especial; Parches; Facilitación; Distancia a la aguada; Disponibilidad de forraje
INTRODUCTION
In arid and semiarid environments, with scarce and patchy plant cover, it is usual
to observe vegetation growing mostly around trees and shrubs (3, 13). Woody plants
can have positive, negative or neutral effects on other species (21, 44, 51). Tree cover
influences the likelihood of seedling establishment in woodlands by modifying physical
environmental conditions (8, 9, 44). Some species are able to increase local soil fertility
through accumulation of carbon and nitrogen and modification of the decomposition
rate (34, 42, 56); through increased infiltration rate (29, 44), and through protection
against high temperatures and radiation (16, 29, 44).
Thus, it has been proposed that positive interactions among plants are an essential
mechanism to maintain diversity at regional scale (29, 33, 52, 53). However, the
presence of woody plants can also negatively influence other species through light
reduction, allelopathy, or certain mechanical and chemical effects caused by litter (22) and, in environments where water is the major limiting factor, there may be competition
for water and nutrients (18).
The final effect of environmental changes produced by woody species on
the spatial pattern of the plant community depends on the different environmental
requirements of each understory species and on its ability to adjust to such changes.
Therefore, the effect of the presence of woody species on the community is the result of the balance between positive (facilitation) and negative effects (competition,
inhibition) on the different species (10, 17, 44, 51). This balance may change for each
species according to physical environmental conditions. Positive interactions seem
to be more important with higher abiotic stress, whereas negative interactions, such
as competition, seem to be dominant when abiotic stress is relatively low (10, 44).
However, this relation seems to be modified by the occurrence of gradients in biotic
stress or disturbance intensity, e. g. increasing grazing pressure (52). The pattern
of disturbance can interact with other causes of heterogeneity and alter vegetation
patchiness (10, 43) or change its scale of expression (2, 11, 47). This interaction has
strong consequences in the functional processes of ecosystems and, therefore, in
their potential management. In particular, grazing has been observed to enhance or
reduce vegetation heterogeneity by modifying water and nutrient cycles (2, 12), by
affecting the facilitative capacity of woody plants through reduction of their crowns (52) or by fragmentation of large shrub clumps into smaller units and differential use of
microsites (41). However, the final effect of grazing on positive plant interactions
depends on grazing intensity, on the palatability of interacting species, and on the
mechanisms involved in plant-plant interactions (47).
Herbivores select their food according to palatability of the plant species, and therefore
species can be classified into preferred, indifferent or avoided (7). Overgrazing decreases
the proportion of highly palatable species, increasing the proportion of grazing-tolerant
and invading species (50). However, the presence of tolerant species can act as physical
protection for preferred species, thereby contributing to their persistence (47). In addition,
the grazing effect includes other action mechanisms such as trampling and fruit dispersal.
The Monte Biogeographic Province occupies a large expanse of the arid region of
western Argentina (14). The major activity carried out in the Monte is extensive livestock
production, particularly goat breeding (55). Prosopis flexuosa DC. is the main tree species
and, because of the capacity of its roots to reach the water table, it forms woodlands
in areas where groundwater occurs (30, 54), and is a major contributor to community
structure. The edaphic and microclimatic changes caused by this species cover produce
spatial heterogeneity and, consequently, modify species distribution (5, 45). Studies have
been conducted on the effect of grazing on vegetation structure (23, 38, 39), on diversity
and abundance of annual plants (48) and on the distribution of grasses in response
to grazing gradients (27). However, the relation between spatial heterogeneity and
disturbance is not yet well understood. In particular, the effects of grazing on the patchy
pattern produced by woody species have been scarcely analyzed in the Monte Desert.
In the Southern Monte, it has been observed that, besides reducing plant cover, grazing
by domestic livestock can affect the structure and dynamics of vegetation patches by
preventing litter accumulation or mineralization of organic matter (11, 12).
Therefore we hypothesized that the interaction between the facilitative effects of Prosopis flexuosa and grazing determine the composition, abundance, and spatial
distribution of species in the community and, consequently, forage availability for
cattle. The postulated mechanisms were: a) the facilitation by P. flexuosa determine
the growth of vegetation mostly under its canopy; b) grazing reduce plant cover and c) grazing modify the structure of vegetation patches generated by the presence of
P. flexuosa through the reduction of the facilitative capacity of this species and/or the
differential use of each microsite.
Typically, grazing intensity and its environmental effects decrease as distance to
the watering point increases (11, 12, 50). Therefore, the distance to the watering point
can be used as a good surrogate of grazing intensity in studying the effect of grazing
on plant interactions. In this work, we assessed the effect of Prosopis on understory
species and analyzed whether the outcomes of this interaction differed with distance
to watering points. We expected an increased difference between microsites near
watering points in vegetation structure and richness if the prevalent mechanism is
the differential uses of microsites by cattle, or a reduced difference if the lost of the
facilitative capacity of P. flexuosa is the prevalent mechanism.
MATERIALS AND METHODS
Study area
The study area is located in the Northeast of Mendoza Province and includes
20,700 ha in the Telteca Reserve and surrounding areas. The study area lies entirely
within the Monte Biogeographic Province, which is characterized by shrub steppe
dominated by species of the family Zygophyllaceae (Larrea spp. and Bulnesia retama)
and, in areas with higher water availability, by woodland of P. flexuosa (35, 46). The zone
has a wide daily and annual temperature range; the absolute maximum temperature is
50°C and the absolute minimum is -10°C. Annual precipitation varies between 50 and
200 mm, with an average of 120 mm. The landforms of the area are predominantly
aeolian, with sand dunes up to 20 m high (1). The Prosopis woodland is the community
with highest carrying capacity (0.6-1.4 goat unit.ha-1), which is several times higher
than in Larrea divaricata shrubland or sand-dune communities (0.1-0.4 GU ha-1) (6).
We set up sampling plots in homogeneous areas of the most representative P. flexuosa woodlands. Mean density of this woodland is 207 individuals ha-1, and the modal height
of adult trees is between 4 and 6 m, modal basal diameter classes are between 15
and 25 cm, and modal crown diameter is between 5 and 6 m (4).
Experiment design
We used a two-way factorial design with microsites and distance from livestock
watering points as factors. Two different microsites were considered: 1) under
P. flexuosa canopy (under-Prosopis microsites) and 2) in areas outside the influence of
this species (intercanopy microsites). Because the concentric effect of grazing around
watering points reaches a 2 km radius (23), two levels were considered for the second
factor: 1) 500-700 m from the watering point (referred to as "near the watering point")
with high grazing pressure; and 2) 3-4 km from the watering point (referred to as "far
from the watering point") with low grazing pressure. In the study area, watering points
consist of different types of wells dug by local people to extract water for their livestock.
Within the homogeneous areas, we chose the watering points of two typical settlements:
Los Rosales (32°17'S; 67°55'W) and El Jagüel (32°23' S; 67°58' W). These watering
points were far enough from each other to allow us to set up sampling plots on the
"far from the watering point" sites in areas without influence from any other watering
point. In all, twelve sampling sites were selected at random: seven near watering
points and five far from watering points. At each site, five trees representative of the
most common classes (15-25 cm basal diameter, 4-6 m height, and 4-6 m crown
diameter) were randomly selected. Five intercanopy microsites were selected 5-10 m
away from the tree canopy to make sure they were outside the influence of the tree.
The sampling technique used was the Modified Point Quadrat method (20, 37). On
under-Prosopis microsites, the sample unit consisted of two transects of 50 points, set
4 cm apart. Each transect was set up at 1.5 m from the trunk, one in the Northern part
of the canopy and the other in the southern part. Because intercanopy microsites are
more extended and heterogeneous than under-Prosopis microsites, the sample unit
consisted of two transects of 100 points, set 4 cm apart. Sampling was performed in
May 2002 and in February 2003. No significant differences between dates were found
(P > 0.05), so we considered both dates as the same data population.
Floristic composition, cover of each species, and percentage of bare soil and of
litter-covered soil was recorded. Species richness was estimated as the number of
species present at each site, and total plant cover was estimated as the percentage
of points where at least one species was found. We estimated diversity and evenness
using the Shannon index (31).
Species were classified into the following growth-forms that are present in the area: trees
(phanerophytes more than 3 m tall and stems reaching 10 cm in basal diameter), shrubs
(phanerophytes less than 3 m tall and several stems less than 5 cm in basal diameter),
perennial grasses (hemicryptophytic grasses), annual grasses (therophytic grasses), forbs
(non grassy hemicryptophytes and therophytes), and vines (creepers). We estimated cover
and richness for the most abundant life forms: hemicryptophytic grasses, therophytic grasses
and shrubs. In addition, we estimated fodder cover as the percentage of points where we
found at least one fodder plant species. We defined fodder species in function of their specific
quality index estimated in previous studies taking into account the acceptability, growth form,
season of consumption, and nutritional value of each species (28, 36).
We considered each sampling site as only one entry representing a replication,
and we considered the mean value of all five sampling units at each microsite.
Data analysis
Data were subjected to a two-factor analysis of variance. As the design was unbalanced,
type III sum of square was used to test the hypothesis. When interactions between factors
were significant, we used an a-posteriori mean comparison test (Tukey test) to compare
pairs of the four possible treatment combinations (57). Because the data on species cover
did not fit the assumptions of the variance analysis, they were transformed using rank
transformations (19).
RESULTS
Under-Prosopis microsites showed a higher percentage of total cover (independent of the tree cover) and a lower percentage of bare soil than intercanopy microsites. The difference in the percentage of total cover between microsites seemed to increase in areas far from watering points, but interaction between microsite and distance from watering points was only marginally significant. The percentage of litter-covered soil was higher on under-Prosopis microsites than on intercanopy microsites, and it was similar for distance to watering points. Species richness and diversity were significantly greater far from watering points than near them, but there was no difference between microsites (table 1).
Table 1. Plant community parameters on the two microsites in relation to distance
to watering points.
Tabla 1. Parámetros de la comunidad de plantas en dos micrositios en relación con
las distancias a la aguada.
Both microsite and distance to watering points affected floristic composition and relative abundance of species (table 2, page 213).
Table 2. Floristic and growth-form composition and percent cover of each species for the two distances to watering points and the two microsites.
Tabla 2. Composición florística y porcentaje de coberturapor forma de vida y especie para dos distancias a la aguada y dos micrositios.
Some species occurred preferentially on under-P. flexuosa microsites, such
as Aristida mendocina, Pappophorum caespitosum (only far from watering points), Trichloris crinita, and Capparis atamisquea. Other species showed preference
for intercanopy microsites, such as Tricomaria usillo and Sporobolus phleoides (table 2, page 213). In turn, we were able to detect species that were indicative
of stocking rates. For example, some species grew exclusively or preferentially
far from watering points, such as Eragrostis cilianensis, Aristida adscencionis,
Ximenia americana, Senna aphylla, Plectrocarpa tetracantha (only in intercanopy
microsites), Aristida mendocina, Pappophorum caespitosum, Setaria leucopila,
and Larrea divaricata. In contrast, Lycium tenuispinosum occurred preferentially
near watering points (table 2, page 213). Most species preferring sites far from
watering points showed high specific quality index, except Larrea divaricata (table 2, page 213).
The interaction between microsite and distance to watering points was significant
only for Pappophorum caespitosum and Prosopis flexuosa (renewals), although
Capparis atamisquea and Lycium chilense showed marginally significant probability
values (table 1, page 212).
With respect to functional groups, hemicryptophytic grasses showed higher cover
on under-Prosopis than intercanopy microsites, with this difference increasing far from
watering points (interaction between microsite and distance to the watering point was
significant) (table 2, page 213). Therophytic grasses showed a higher percentage
of cover and richness on sites far from watering points, and showed no significant
differences between microsites. No significant differences were observed for shrubs,
either between microsites or between distances from watering sites.
Percent cover of fodder plant species was higher on under-Prosopis microsites
than on intercanopy microsites (figure). The difference in cover between these two
microsites was greater in areas far from watering points (interaction between microsite
and distance to the watering point was significant).
Figure. Effect of the cover of Prosopis flexuosa on forage cover in relation to distance
to watering points and microsites.
Figura. Efecto de la cobertura de Prosopis flexuosa sobre la cobertura forrajera en
relación con las distancias a la aguada y los micrositios.
DISCUSSION
In accordance with the postulated hypothesis, both the presence of P. flexuosa and grazing determined the composition and structure of plant associations in the
Central Monte. Prosopis flexuosa modified the spatial pattern of associated species by
generating vegetation patches with different floristic composition and relative abundance
of their component species. However, no differences between microsites were found
in richness and diversity. Besides, distance to the watering point also affected species
abundance and composition, as well as the patch structure generated by P. flexuosa.
We found some evidence indicating an interaction between both factors studied, which
suggests that grazing alters the structure of vegetation patches produced by the cover
of P. flexuosa. In general we observed that, near watering points, patch structure under
Prosopis cover was disrupted and became more similar to that in intercanopy areas. This
evidence supports Saiz & Alados proposals (47) suggesting a decreased association
among species under high grazing intensity because of the lower protection effectiveness.
Nevertheless, given that such evidence was true for only some of the studied variables,
new research is needed to broaden our understanding of this relationship.
Because species richness and diversity were not affected by P. flexuosa cover,
we attributed the change in floristic composition induced by P. flexuosa to species
replacement. This is consistent with previous observations for Ñacuñán, a more humid
area in the Monte (45). However, a higher percent of total cover under the canopy of P. flexuosa than in intercanopy microsites was found in Telteca, whereas no differences
were found in total plant cover between both microsites in Ñacuñán (45). These results
seem to indicate that the contribution of P. flexuosa to the productivity of understory
species is higher in Telteca than in Ñacuñán. In addition, hemicryptophytic grasses
occurred preferentially under the canopy of P. flexuosa in Telteca. On the other hand,
it was observed that the same species occur preferentially in intercanopy microsites in Ñacuñán (45). Considering the higher water deficit in Telteca, the difference between
both sites appears to support the stress-gradient hypothesis, which predicts that positive
biological interactions become more important in relation to negative interactions when
environmental stress increases (10, 43, 44). At this point, it is necessary to clarify
that Ñacuñán is a natural reserve excluded from grazing and, therefore, could be an
additional conditioning factor affecting the spatial distribution of grasses (40).
No differences were found between microsites or between distances to the
watering point in total shrub cover, but species replacement was apparent, with some
species, such as Capparis atamisquea, preferring under-Prosopis microsites, and
other species, such as Tricomaria usillo, preferring intercanopy microsites. In arid
zones, forage shrubs are key elements in livestock feeding (7), representing 35% of
total forage intake and over 50% (of it) during the critical period of fodder shortage
in the dry season (26). The species replacement that we observed may be important
because of the resulting increased variability in the diet of livestock. In arid zones,
availability of each species is variable depending on season. In the NE of Mendoza,
in the autumn, at least 13 species were available for consumption by goats, whereas
only nine were available in the winter (7).
In areas near watering points we observed higher proportion of bare soil and lower
total cover, in addition to lower number of species and lower cover of hemicryptophytic and
therophytic grasses. Thus, our findings concur with other studies for the Monte (11, 25) and other arid lands (50). Most of the species with a preference for areas far from watering
points were grasses with a high specific quality index and a high degree of selectivity, such
as Pappophorum caespitosum, Setaria leucopila or Eragrostis cilianensis. In contrast,
Lycium tenuispinosum, with a preference for areas near watering points, shows the lowest
specific quality index. In addition, the total percent cover of fodder species was lower
near watering points. These findings suggest that intake by cattle is the main mechanism
involved in the effect of grazing on vegetation structure as has been proposed by previous
studies (27, 50). However, the decrease in the cover of Larrea divaricata, Geoffroea
decorticans and Senna aphylla, species with the lowest specific quality index, suggests
that other mechanisms, like trampling, should be involved in limiting the establishment or
increasing the mortality of young plants in the area near watering points.
Several authors suggest that grazing affects interspecific interactions by modifying
patch structure (2, 10, 11, 43, 47, 52). One of the most notorious effects of woody plants
is the protection against herbivores they provide to fodder species, which is why, on
grazed sites, it is usual for fodder species to show a denser distribution in vegetation
patches, enhancing vegetation heterogeneity. As a result, in the most grazed areas,
the most palatable species tend to group together under shrubs (15, 40, 47, 49). In
this study, we found that the increase in grazing intensity reduces heterogeneity by
reducing the differences between both microsites. This finding suggests that P. flexuosa,
a tree with a more open growth form than shrubs, does not provide an effective barrier
against goat grazing and trampling, at least on sites with high grazing intensity, and
that the facilitating effect on fodder species is mainly a result of the edaphic and/or
microclimate changes under the canopy.
The environmental heterogeneity created by P. flexuosa and the consequent facilitation
effect on fodder species could explain the high stock carrying capacity observed in Prosopis
woodlands (0.63-1.33 goat unit ha-1) compared to the other communities in the region (6) and with that estimated by Guevara et al. (24) for the area (0.1 GU ha-1) using data on rainfall
use efficiency (32). This explanation is consistent with the proposal that heterogeneous
vegetation in arid lands is more productive than homogeneous systems (3).
We can conclude that the carrying capacity of woodlands is enhanced by the presence
of P. flexuosa, which provides a greater abundance of grasses under its canopy and,
therefore, contributes to an increased forage cover. This concept is reinforced by the fact
that the tree itself represents a major fodder contribution in the form of fruit and litter (7), a
contribution that was not considered in this study. This conclusion suggests that recovery
of degraded areas in the region through afforestation with Prosopis trees can yield benefits
in terms of silvopastoral management. Therefore, our results contribute not only from a
theoretical point of view to understanding the functional aspects of spatial heterogeneity
and its relationship with disturbance, but also from a practical point of view to understanding
the connections between two important activities in the economy of arid lands of the world,
extensive grazing and the use of forest products.
Acknowledgements
We thank José A. Boninsegna, Ricardo Villalba, and Alejandro Bisigato for their valuable suggestions.
We thank Gualberto Zalazar, Germán Berra, and Belén Lana for their cooperation in the field work.
We thank Nelly Horak and Judy Boshoven for improving the English version of the manuscript.
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