Fire and plants Bond, W.J; Wilgen, B.W. van (Department of Botany, University of Cape Town (South Africa))
1996, 1995, 1996., Volume:
14
eBook, Book
Large regions of the world are regularly burnt either deliberately or naturally. However, despite the widespread occurrence of such fire-prone ecosystems, and considerable body of research on plant ...population biology in relation to fire, until now there have only been limited attempts at a coherent conceptual synthesis of the field for use by students or researchers.
Ancient grasslands at risk Bond, William J.
Science (American Association for the Advancement of Science),
01/2016, Volume:
351, Issue:
6269
Journal Article
Peer reviewed
Highly biodiverse tropical grasslands are at risk from forest-planting efforts
Concerns over deforestation have led to attempts to identify suitable areas for reforestation around the world (
1
). ...The most ambitious effort to date is the World Resources Institute (WRI) Atlas of Forest and Landscape Restoration Opportunities (
1
). This map is linked to a global plan to reforest degraded lands to offset anthropogenic CO
2
emissions. The immediate target is the reforestation of 1.5 million km
2
by 2020 (
2
,
3
). Vast areas of open grassy vegetation have been identified as suitable for reforestation. But are all these grasslands secondary products of deforestations? Recent research shows that grasslands are often ancient and highly biodiverse, but it remains difficult to distinguish between primary and secondary grasslands on a large scale. Reforestation efforts thus risk converting ancient tropical grasslands to plantations.
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Though the distribution of global vegetation can generally be predicted from climate, grasslands are an exception. C₄ grassy biomes cover vast areas that are warm enough and wet enough to support ...closed forests. The extent of this climate mismatch has been revealed by physiologically based global vegetation simulations and by large empirical data sets. Reasons for the existence of grassy biomes have long been debated, polarized into bottom-up (resources) or top-down (fire, herbivory) arguments. Recent studies indicate that both are important, especially in suppressing woody recruits. Grasses are formidable competitors belowground, create highly flammable fuels, and can support large herbivore densities. The net effect on trees is rare and episodic recruitment of adults in tree-fall gaps. The implication is that ecosystem structure and function depend on demographic transitions. Tree cover is increasing and grass/forest boundaries are changing. These changes can have large feedbacks to the earth-atmosphere system. Though progress has been made, there is still great uncertainty in predicting the future of C₄ grassy biomes.
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Savannahs are a mixture of trees and grasses often occurring as alternate states to closed forests. Savannah fires are frequent where grass productivity is high in the wet season. Fires help maintain ...grassy vegetation where the climate is suitable for woodlands or forests. Saplings in savannahs are particularly vulnerable to topkill of above-ground biomass. Larger trees are more fire-resistant and suffer little damage when burnt. Recruitment to large mature tree size classes depends on sapling growth rates to fire-resistant sizes and the time between fires. Carbon dioxide (CO 2 ) can influence the growth rate of juvenile plants, thereby affecting tree recruitment and the conversion of open savannahs to woodlands. Trees have increased in many savannahs throughout the world, whereas some humid savannahs are being invaded by forests. CO 2 has been implicated in this woody increase but attribution to global drivers has been controversial where changes in grazing and fire have also occurred. We report on diverse tests of the magnitude of CO 2 effects on both ancient and modern ecosystems with a particular focus on African savannahs. Large increases in trees of mesic savannahs in the region cannot easily be explained by land use change but are consistent with experimental and simulation studies of CO 2 effects. Changes in arid savannahs seem less obviously linked to CO 2 effects and may be driven more by overgrazing. Large-scale shifts in the tree—grass balance in the past and the future need to be better understood. They not only have major impacts on the ecology of grassy ecosystems but also on Earth—atmosphere linkages and the global carbon cycle in ways that are still being discovered.
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There is growing interest in the application of alternative stable state (ASS) theory to explain major vegetation patterns of the world. Here, we introduce the theory as applied to the puzzle of ...nonforested (open) biomes growing in climates that are warm and wet enough to support forests (alternative biome states, ABSs). Long thought to be the product of deforestation, diverse lines of evidence indicate that many open ecosystems are ancient. They have also been characterized as ‘early successional’ even where they persist for millennia. ABS is an alternative framework to that of climate determinism and succession for exploring forest/nonforest mosaics. This framework explains not only tropical forest–savanna landscapes, but also other landscape mosaics across the globe.
There are many ancient open vegetation formations worldwide that maintain a high diversity of shade-intolerant species where the climate is suitable for forests.Fire and herbivores are ancient consumers of plant biomass that maintain open ecosystems and shape shade-intolerant species.Therefore, open ecosystems are not necessarily either produced by deforestation or early successional, but have been maintained by consumers as ABSs to forests.ABSs are not only found in tropical environments, but also in temperate and Mediterranean conditions.
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Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found ...that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.
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•Tropical grassy biomes are extensive and critical to human livelihoods.•The definition of these grassy systems is inconsistent and misleading.•We discuss novel threats linked to problems with biome ...definition.•The assumption that ‘more trees are better’ does not hold for tropical grassy biomes.
Tropical grassy biomes (TGBs) are globally extensive, provide critical ecosystem services, and influence the earth–atmosphere system. Yet, globally applied biome definitions ignore vegetation characteristics that are critical to their functioning and evolutionary history. Hence, TGB identification is inconsistent and misinterprets the ecological processes governing vegetation structure, with cascading negative consequences for biodiversity. Here, we discuss threats linked to the definition of TGB, the Clean Development Mechanism (CDM) and Reducing Emissions from Deforestation and Forest Degradation schemes (REDD+), and enhanced atmospheric CO2, which may facilitate future state shifts. TGB degradation is insidious and less visible than in forested biomes. With human reliance on TGBs and their propensity for woody change, ecology and evolutionary history are fundamental to not only the identification of TGBs, but also their management for future persistence.
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Megafaunal extinctions and a lack of suitable remote sensing technology impede our understanding of both the ecological legacy and current impacts of large mammal herbivores in the Earth system. To ...address this, we reconstructed the form and intensity of herbivory pressure across sub-Saharan Africa ~1000 years ago. Specifically, we modeled and mapped species-level biomass for 92 large mammal herbivores using census data, species distributions, and environmental covariates. Trait-based classifications of these species into herbivore functional types, and analyses of their biomass surfaces, reveal four ecologically distinct continental-scale herbivory regimes, characterized by internally similar forms and intensities of herbivory pressure. Associations between herbivory regimes, fire prevalence, soil nutrient status, and rainfall provide important insights into African ecology and pave the way for integrating herbivores into global-scale studies.
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9.
Humboldt and the reinvention of nature Pausas, Juli G.; Bond, William J.; Sankaran, Mahesh
The Journal of ecology,
20/May , Volume:
107, Issue:
3
Journal Article
Peer reviewed
Open access
Alexander von Humboldt is a key figure in the history of ecology and biogeography who contributed to shape what is today ecology, as well as the environmentalist movement. His observation that the ...world’s vegetation varies systematically with climate was one of his many contributions to science.
Here, we question to what extent Humboldt’s view biased our vision of nature. The current emphasis on the role of climate and soils in ecological and evolutionary studies, and the emphasis on forests as the potential and most important vegetation, suggests that we still view nature through the eyes of Humboldt.
Over the last 20 years, diverse studies have shown that many open non‐forested ecosystems (savannas, grasslands, and shrublands) cannot be predicted by climate and are ancient and diverse systems maintained by fire and/or vertebrate herbivory. Paleoecological and phylogenetic studies have shown the key role of these plant consumers at geological time scales. This has major implications for how we understand and manage our ecosystems.
Synthesis. We need to consciously probe the long‐standing idea that climate and soils are the only major factors shaping broad‐scale patterns in nature. We propose to move beyond the legacy of Humboldt by embracing fire and large mammal herbivory as additional key factors in explaining the ecology and evolution of world vegetation.
Foreign Language
Resumen
Alexander von Humboldt es una figura clave en la historia de la ecología y la biogeografía, y contribuyó a dar forma a lo que hoy es la ecología y el movimiento ecologista. Su observación de que la vegetación del mundo varía sistemáticamente con el clima fue una de sus muchas contribuciones a la ciencia.
En este artículo cuestionamos hasta qué punto la visión de Humboldt ha sesgado nuestra visión de la naturaleza. El actual énfasis en el papel del clima y el suelo en los estudios ecológicos y evolutivos, y la idea de que los bosques son la vegetación potencial y más importante de muchos lugares, sugiere que aún observamos la naturaleza a través de los ojos de Humboldt.
Durante los últimos 20 años, diversos estudios han demostrado que el clima no puede predecir muchos de los ecosistemas no forestales (sabanas, praderas y matorrales), y que son sistemas antiguos y diversos mantenidos por fuego y por herbívoros vertebrados. Los estudios paleoecológicos y filogenéticos han demostrado el papel clave de estos consumidores de vegetación a la escala geológica. Todo esto tiene importantes implicaciones sobre cómo entendemos y gestionamos actualmente nuestros ecosistemas.
Síntesis: Necesitamos cuestionarnos la idea tradicional de que el clima y el suelo son los únicos factores importantes que generan patrones ecológicos a gran escala. Proponemos ir más allá del legado de Humboldt, y considerar el fuego y la herbivoría por grandes mamíferos como factores clave para explicar la ecología y la evolución de la vegetación a escala global.
One of the contributions of Alexander von Humbondt to science was the description of altitudinal vegetation patterns. And this is his description (1807) for the Chimborazo, the highest mountain in Ecuador. In this paper we question to what extent Humboldt’s view biased our vision of nature, and we propose to move beyond his legacy by embracing fire and large mammal herbivory as key factors in explaining the ecology and evolution of world vegetation.
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Tropical savannas have a ground cover dominated by C4 grasses, with fire and herbivory constraining woody cover below a rainfall-based potential. The savanna biome covers 50% of the African ...continent, encompassing diverse ecosystems that include densely wooded Miombo woodlands and Serengeti grasslands with scattered trees. African savannas provide water, grazing and browsing, food and fuel for tens of millions of people, and have a unique biodiversity that supports wildlife tourism. However, human impacts are causing widespread and accelerating degradation of savannas. The primary threats are land cover-change and transformation, landscape fragmentation that disrupts herbivore communities and fire regimes, climate change and rising atmospheric CO2. The interactions among these threats are poorly understood, with unknown consequences for ecosystem health and human livelihoods. We argue that the unique combinations of plant functional traits characterizing the major floristic assemblages of African savannas make them differentially susceptible and resilient to anthropogenic drivers of ecosystem change. Research must address how this functional diversity among African savannas differentially influences their vulnerability to global change and elucidate the mechanisms responsible. This knowledge will permit appropriate management strategies to be developed to maintain ecosystem integrity, biodiversity and livelihoods.
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