Population viability in plants , Population viability in plants , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Population viability in plants

A coupled energy and water balance model is used to simulate the effects of large tree canopies on soil moisture and water stress across a series of sites spanning a regional moisture gradient in southern Africa The model tracks evapotranspiration from five components of the land surface at each site—the tree canopy, the grass under and between tree canopies, and the bare soil under and between tree canopies The soil moisture dynamics are simulated at daily time steps and driven by a stochastic model of storm arrivals and storm depth Evapotranspiration is modeled using the Priestley-Taylor approach, with potential evapotranspiration scaled by soil moisture availability The soil moisture under tree canopies is compared to the soil moisture between tree canopies, and differences in average annual soil moisture stress conditions are analyzed at each site The spatial distribution of large trees has important consequences for small-scale soil moisture dynamics across the rainfall gradient The results indicate that tree canopies serve to reduce soil moisture stress of under-canopy vegetation in the middle of the rainfall gradient At the dry end of the rainfall gradient, the effect of tree canopies on soil moisture is dependent on the amount of rainfall received in a given growing season

Tree Canopy Effects on Simulated Water Stress in Southern African Savannas

A track across a burnt grass swale was used intensively on the Jabiluka Mineral Lease (located adjacent to Kakadu National Park in the seasonally wet tropics of the Northern Territory, Australia) for a short time period during the 1998 dry season. Repeated vehicle passes over the burnt grass increased soil bulk density and locally disrupted the root and algal mat, lowering the critical shear stress for sediment transport. Overland flow during the next wet season was above average and eroded eleven discontinuous, flow-aligned scour holes in the wheel ruts where the track crossed grassed sandy swales. Although the site was burnt again during the next dry season, the scour holes did not coalesce during the second wet season, which was wetter than the previous one, because infrequent traffic bypassed the eroded section allowing grass to re-establish. Scour holes on vehicle tracks in the Kakadu region are an intermediate but reversible stage in the development of gullies in grassed swales. Treatment of scour holes by soil conservation works may prevent gully formation.

Gully Initiation and Implications for Management of Scour Holes in the Vicinity of the Jabiluka Mine, Northern Territory, Australia

The essential qualities of grasses and grasslands that have made them prominent in the biosphere and have made them central to human history emerged as grasses adapted to tolerate drought and to exploit full-sun habitats. Grass progenitors are believed to have developed their basic features in wooded habitats, initially as subsidiary species, then perhaps as codominants in savanna-like situations. The drama of a good grass fire encourages the belief that every feature of grasses and grasslands must have been selected to deal with this awesome force of nature. A careful study of the response of grasses to fire suggests a more nuanced view, which is addressed in this chapter. Some grasses and grasslands seem to thrive on fire and to languish if left long unburned. In general, these are to be found on the wetter end of the grassland climatic gradient. Although there is a wealth of information, it is difficult to extract guidance from it in specific situations. More refined understanding of direct and indirect fire effects and how these interact with all the other influences that cause change in grassland ecosystems is thus required.

13 – Fire Effects on Grasslands

Population modeling, and especially the concept of population viability analysis (PVA), has played a fundamental role in the development of conservation biology as a discipline (Chap. 1, this Vol.). Early applications of PVA largely focused on the use of demographic models to identify minimum viable population sizes, but the term has since broadened to encompass a much wider range of approaches (Beissinger and Westphal 1998; Chaps. 6,9, this Vol.). Typically, PVA involves the development of models to address questions about the extinction risks facing one or more populations or to explore the factors influencing population persistence. What role PVA can or should play in conservation and management is a question that continues to stimulate a great deal of debate. Some authors argue that PVA is an effective and important conservation tool (Schemske et al. 1994; Brook et al. 2000), while others caution that PVA methods can be easily misused or may provide unreliable guidance to managers (Beissinger and Westphal 1998). In spite of this debate, there is no question that both the number of PVA approaches available to practitioners and their application to conservation problems is growing rapidly.

Approaches to Modeling Population Viability in Plants: An Overview

The native annual Sorghum populations of the Australian wet-dry tropics are highly resilient to dry season fires. During the early wet season, however, fires that occur after the new grass population has emerged can cause catastrophic population crashes. We examined savanna plots that had been burnt in this way, and compared them with adjacent unburnt plots. We found that Sorghum densities in the burnt plots were lower on average by a factor of 10, but that some fires had reduced the density only to one-third of the unburnt plots. It is not clear whether these differences relate directly to site or seasonal factors, or to differences in the way the burning was carried out. Other vegetation components responded to the fires differently: forbs (dicotyledonous herbs) increased in cover, while perennial grasses, woody plants, and overall species richness, were not significantly affected. The amount of leaf litter declined. A population model for Sorghum based on the demography of unburnt populations predicted that they should recover from a wet season burn, taking 7–16 years to return to normal densities. However, the actual field populations did not seem to be recovering, suggesting that wet season fires not only lower densities, but may also fundamentally change population processes in these annual grasses.

Modelling the recovery of an annual savanna grass following a fire-induced crash

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Modelling the recovery of an annual savanna grass following a fire-induced crash