Studies on Structure in Plant Communities. VII. Field and Experimental Analyses of Atriplex vesicaria Populations From the Riverine Plain of New South Wales

Malik, AR; Anderson, DJ; Myerscough, PJ

doi:10.1071/bt9760265

“…Deviations from this general sequence have been observed particularly in arid and semi-arid environments. Malik et al (1976) and Szwagrzyk and Czerwczak (1993) observed random spatial patterns and suggested that these patterns are associated with heterogeneous micro-topography or availability of resources in space. Fowler (1986) suggested that reduction in densitydependent effects, and hence the possible formation of random patterns, may be attributed to a decrease in the intensity of competition due to low precipitation.…”

Section: Discussionmentioning

confidence: 99%

Intraspecific facilitation: a missing process along increasing stress gradients ? insights from simulated shrub populations

Malkinson¹,

Jeltsch²

2007

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In recent years many field studies have been conducted to assess the relative importance of facilitation and competition in structuring vegetation communities in different environments. Herein, we present a simulation model which systematically explores the relative importance of intra-specific facilitation and competition between adult shrubs and seedlings for spatial pattern formation. A grid-based simulation model was constructed and calibrated using data collected in the field from Sarcopoterium spinosum populations in Israel to simulate population dynamics along a rainfall gradient. A series of simulation experiments was conducted in which manipulations of seedling survival probabilities were carried out to assess the relative importance of these processes in generating spatial patterns. Increased survival probabilities of first-year shrubs in open areas were used to simulate competition effects, while increased survival probabilities in the vicinity of shrubs were used to simulate facilitation effects.Simulation results were then compared to shrub spatial patterns observed in the field. The results indicate that facilitation is not an important process in generating intra-specific spatial patterns. Rather, in mesic environments with high precipitation, competition is the dominant process generating spatial patterns, resulting in regular spacing of shrubs, similarly to the patterns observed in the field (L(h) values B0). In arid sites, where precipitation values are lower, and stress conditions are higher, the dominant process generating spatial patterns was random mortality due to drought conditions. The resulting spatial patterns in this case are random (L(h) Â0), whereas observed field populations exhibited clumped patterns (L(h) !0). We conclude that as stress conditions increase, the importance of intraspecific neighborhood interactions decrease whereas the importance of environmental factors increase in dictating intra-specific spatial pattern formation. Consequently in mesic environments intra-specific competition among adults determines the emerging patterns, while intraspecific facilitation is a negligible process.

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“…This variation in growth rates chiefly reflected differences between census intervals. Variation between grazed plots within a site probably arose from small-scale variation in microtopography and edaphic conditions (Malik, Anderson & Myerscough 1976), as defoliation of A. vesicaria was fairly uniform within sites ( L.P. Hunt, unpublished data).…”

Section: Transition Matricesmentioning

confidence: 99%

Heterogeneous grazing causes local extinction of edible perennial shrubs: a matrix analysis

Hunt

¹

2001

Journal of Applied Ecology

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Summary1. Population modelling and field measurements of births, growth and deaths were used to investigate the long-term change in abundance of Atriplex vesicaria (Chenopodiaceae), a long-lived, palatable, perennial shrub, under sheep grazing. Of particular interest was whether A. vesicaria is at risk of being eliminated throughout grazed paddocks when the recommended practice of continuous grazing at conservative stocking rates is employed. 2. Time-invariant matrix population models indicated that the A. vesicaria population was in decline over much of the study paddock, but the rate of decline was greatest nearer to the water point (population growth rate λ ≈ 0·8). Time-varying stochastic matrix models projected that the A. vesicaria population would become locally extinct at most sites up to approximately 2200 m from water, occurring first closer to water (within 12-29 years). The population was stable (i.e. λ ≥ 1) at sites greater than 2200 m from water over the projection period of 100 years. 3. Decreases in adult survival and recruitment made the largest contributions to reductions in the population growth rate. However, there were spatial patterns centred on the water point in the degree to which particular demographic processes contributed to these reductions, because of a grazing gradient and the differential sensitivity of demographic processes to grazing. Thus decreases in recruitment contributed to reductions in the population growth rate at greater distances. Such responses, together with the sensitivity of the population growth rate to these processes, determined the spatial pattern in population growth. 4. The results suggest that piospheres (i.e. the zone of impact) continue to expand over many years under set-stocking so that the area around the water point that is devoid of A. vesicaria becomes larger. The process of expansion appears to first involve the inhibition of recruitment, followed by eventual mortality of established shrubs. 5. The large contribution of adult survival to the population growth rate in A. vesicaria suggests that minimizing the mortality of established adults should be a priority for management. This is likely to involve resting from grazing at critical times such as during extended dry periods. This may also permit increased levels of recruitment during subsequent moister periods.

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“…Leps and Kindlmann (1987) obtained such succession of patterns from clumped to regular in a simulation model. Deviations from this general sequence have been observed particularly in arid and semi‐arid environments Malik et al (1976). and Szwagrzyk and Czerwczak (1993) observed random spatial patterns and suggested that these patterns are associated with heterogeneous micro‐topography or availability of resources in space.…”

Section: Discussionmentioning

confidence: 98%

Intraspecific facilitation: a missing process along increasing stress gradients – insights from simulated shrub populations

Malkinson¹,

Jeltsch²

2007

View full text Add to dashboard Cite

In recent years many field studies have been conducted to assess the relative importance of facilitation and competition in structuring vegetation communities in different environments. Herein, we present a simulation model which systematically explores the relative importance of intra-specific facilitation and competition between adult shrubs and seedlings for spatial pattern formation. A grid-based simulation model was constructed and calibrated using data collected in the field from Sarcopoterium spinosum populations in Israel to simulate population dynamics along a rainfall gradient. A series of simulation experiments was conducted in which manipulations of seedling survival probabilities were carried out to assess the relative importance of these processes in generating spatial patterns. Increased survival probabilities of first-year shrubs in open areas were used to simulate competition effects, while increased survival probabilities in the vicinity of shrubs were used to simulate facilitation effects.Simulation results were then compared to shrub spatial patterns observed in the field. The results indicate that facilitation is not an important process in generating intra-specific spatial patterns. Rather, in mesic environments with high precipitation, competition is the dominant process generating spatial patterns, resulting in regular spacing of shrubs, similarly to the patterns observed in the field (L(h) values B0). In arid sites, where precipitation values are lower, and stress conditions are higher, the dominant process generating spatial patterns was random mortality due to drought conditions. The resulting spatial patterns in this case are random (L(h) Â0), whereas observed field populations exhibited clumped patterns (L(h) !0). We conclude that as stress conditions increase, the importance of intraspecific neighborhood interactions decrease whereas the importance of environmental factors increase in dictating intra-specific spatial pattern formation. Consequently in mesic environments intra-specific competition among adults determines the emerging patterns, while intraspecific facilitation is a negligible process.

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Studies on Structure in Plant Communities. VII. Field and Experimental Analyses of Atriplex vesicaria Populations From the Riverine Plain of New South Wales

Cited by 20 publications

References 19 publications

Intraspecific facilitation: a missing process along increasing stress gradients ? insights from simulated shrub populations

Intraspecific facilitation: a missing process along increasing stress gradients ? insights from simulated shrub populations

Heterogeneous grazing causes local extinction of edible perennial shrubs: a matrix analysis

Intraspecific facilitation: a missing process along increasing stress gradients – insights from simulated shrub populations

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