This paper is the first global study of the extent to which fire determines global vegetation patterns by preventing ecosystems from achieving the potential height, biomass and dominant functional types expected under the ambient climate (climate potential). To determine climate potential, we simulated vegetation without fire using a dynamic global-vegetation model. Model results were tested against fire exclusion studies from different parts of the world. Simulated dominant growth forms and tree cover were compared with satellite-derived land- and tree-cover maps. Simulations were generally consistent with results of fire exclusion studies in southern Africa and elsewhere. Comparison of global 'fire off' simulations with landcover and treecover maps show that vast areas of humid C(4) grasslands and savannas, especially in South America and Africa, have the climate potential to form forests. These are the most frequently burnt ecosystems in the world. Without fire, closed forests would double from 27% to 56% of vegetated grid cells, mostly at the expense of C(4) plants but also of C(3) shrubs and grasses in cooler climates. C(4) grasses began spreading 6-8 Ma, long before human influence on fire regimes. Our results suggest that fire was a major factor in their spread into forested regions, splitting biotas into fire tolerant and intolerant taxa.

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The global distribution of ecosystems in a world without fire

Summary

1 Savanna ecosystems are characterized by the codominance of two different life forms: grasses and trees. An operational understanding of how these two different life forms coexist is essential for understanding savanna function and for predicting its response to future environmental change.



2 The existing model, which proposes that grasses and trees coexist by a separation of rooting niches, is not supported by recent empirical investigations. Our aim was to define an alternative mechanism of grass–tree coexistence in savanna ecosystems. The model we have built concentrates on life history–disturbance interactions between grasses and trees.



3 The model demonstrates coexistence for a wide range of environmental conditions, and exhibits long periods of slow decline in adult tree numbers interspersed with relatively infrequent recruitment events. Recruitment is controlled by rainfall, which limits seedling establishment, and fire, which prevents recruitment into adult size classes. Decline in adult tree numbers is the result of continuing, but low levels, of adult mortality. Both aspects of the dynamics are consistent with an established non-equilibrium mechanism of coexistence (the storage effect).



4 A sensitivity analysis indicated that data on tree resprouting ability, stem growth rates and the relationship between seedling establishment and wet season drought are essential for predicting both the range of conditions for which coexistence is possible and the response of savanna ecosystems to environmental change.



5 Our analysis suggests that understanding grass–tree interactions in savanna requires consideration of the long-term effects of life history–disturbance interactions on demography, rather than the fine-scale effects of resource competition on physiological performance.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2745.2000.00435.x

Fire, resprouting and variability: a recipe for grass–tree coexistence in savanna

Disturbances from fire, wind-throw, insects and other herbivores are, besides climate, CO 2 , and soils, critical factors for composition, structure and dynamics of most vegetation. To simulate the influence of fire on the dynamic equilibrium, as well as on potential change, of vegetation at the global scale, we have developed a fire model, running inside the modular framework of the Lund-Potsdam-Jena Dynamic Global Vegetation Model (LPJ-DGVM). 2 Estimated litter moisture is the main driver of day-to-day fire probability. The length of the fire season is used to estimate the fractional area of a grid cell which is burnt in a given year. This affected area is converted into an average fire return interval which can be compared to observations. 3 When driven by observed climate for the 20th century (at a 0.5 ° longitude/latitude resolution), the model yielded fire return intervals in good agreement with observations for many regions (except parts of semiarid Africa and boreal Siberia). We suggest that further improvement for these regions must involve additional process descriptions such as permafrost and fuel/fire dynamics.

The role of fire disturbance for global vegetation dynamics: coupling fire into a Dynamic Global Vegetation Model

A comprehensive and quantitative evaluation of the effects of fire on eco- system nitrogen (N) is urgently needed for directing future fire research and management. This study used a meta-analysis method to synthesize up to 185 data sets from 87 studies published from 1955 to 1999. Six N response variables related to fire were examined: fuel N amount (FNA) and concentration (FNC), soil N amount (SNA) and concentration (SNC), and soil ammonium (NH4 1 ) and nitrate (NO3 2 ) pools. When all comparisons (fire treatment vs. control) were considered together, fire significantly reduced FNA (58%), increased soil NH4 1 (94%) and NO3 2 (152%), and had no significant influences on FNC, SNA, and SNC. The responses of N to fire varied with different independent variables, which were vegetation type, fire type, fuel type, fuel consumption amount, fuel consumption percentage, time after fire, and soil sampling depth. The response of FNA to fire was significantly influenced by vegetation type, fuel type, and fuel consumption amount and percentage. The reduction in FNA was linearly correlated with fuel consumption percentage (r 2 5 0.978). The response of FNC to fire was only affected by fuel type. None of the seven independent variables had any effect on SNA. The responses of SNC, NH4 1 , and NO3 2 depend on soil sampling depth. The responses of both NH4 1 and NO3 2 to fire were significantly affected by fire type and time after fire but had different temporal patterns. The soil NH4 1 pool increased ap- proximately twofold immediately after fire, then gradually declined to the prefire level after one year. The fire-induced increase in the soil NO 3 2 pool was small (24%) immediately after fire, reached a maximum of approximately threefold of the prefire level within 0.5- 1 year after fire, and then declined. This study has identified the general patterns of the responses of ecosystem N that occur for several years after fire. A key research need relevant to fire management is to understand how the short-term responses of N to fire influence the function and structure of terrestrial ecosystems in the long term.

/pdf/fire-effects-on-nitrogen-pools-and-dynamics-in-terrestrial-50ynx0ojfq.pdf

Fire effects on nitrogen pools and dynamics in terrestrial ecosystems: a meta-analysis

Disturbance is a dominant factor in many ecosystems, and the disturbance regime is likely to change over the next decades in response to land-use changes and global warming. We assume that predictions of vegetation dynamics can be made on the basis of a set of life-history traits that characterize the response of a species to disturbance. For crown-fire ecosystems, the main plant traits related to postfire persistence are the ability to resprout (persistence of individuals) and the ability to retain a persistent seed bank (persistence of populations). In this context, we asked (1) to what extent do different life- history traits co-occur with the ability to resprout and/or the ability to retain a persistent seed bank among differing ecosystems and (2) to what extent do combinations of fire- related traits (fire syndromes) change in a fire regime gradient? We explored these questions by reviewing the literature and analyzing databases compiled from different crown-fire ecosystems (mainly eastern Australia, California, and the Mediterranean basin). The review suggests that the pattern of correlation between the two basic postfire persistent traits and other plant traits varies between continents and ecosystems. From these results we predict, for instance, that not all resprouters respond in a similar way everywhere because the associated plant traits of resprouter species vary in different places. Thus, attempts to generalize predictions on the basis of the resprouting capacity may have limited power at a global scale. An example is presented for Australian heathlands. Considering the com- bination of persistence at individual (resprouting) and at population (seed bank) level, the predictive power at local scale was significantly increased.

/pdf/plant-functional-traits-in-relation-to-fire-in-crown-fire-27e5rmw011.pdf

Plant functional traits in relation to fire in crown-fire ecosystems

1 Characteristics of South African Biomes.- 2 Fire Regimes in the Biomes of South Africa.- 3 Man's Historical and Traditional Use of Fire in Southern Africa.- 4 An Historical Review of Research on Fire in South Africa.- 5 Fire in Fynbos.- 6 Fire in Grassland.- 7 Fire in Savanna.- 8 Fire in Forest.- 9 Fire Behaviour.- 10 Effects of Fire on Vegetation Structure and Dynamics.- 11 The Effect of Fire on Forage Production and Quality.- 12 Effects of Fire Regime on Faunal Composition and Dynamics.- 13 The Responses and Survival of Organisms in Fire-Prone Environments.- 14 The Effect of Fire on Soil and Microclimate.- 15 The Effect of Fire on Water Yield.- 16 The Use of Fire as a Management Tool.- Concluding Remarks. Fire Research - a Perspective for the Future.- References.- Index of Plants and Animals (Scientific).- Index of Plants and Animals (Common).

P. de V. Booysen

Papers

Ecological effects of fire in South African ecosystems