Fire and open ecosystems

Can fire account for the widespread occurrence of open ecosystems? This chapter explores fire as a major consumer shaping vegetation in diverse regions worldwide. The concept of fire regime helps explain the diverse influences of fire on vegetation structure. Fire regimes select compatible growth forms from the species pool. These, in turn, create the fuel which in large part determines the fire regime. Experimental evidence can show whether fire is a major determinant of vegetation structure or merely an emergent property of ecosystems determined by climate and soils. Whether fires consume closed forests or stop at their margins will determine the dominant vegetation in mosaic landscapes. A mechanistic framework for analysing processes influencing fire effects on the boundary is introduced with examples. Pyrophilic open systems in mosaics with pyrophobic closed forests have been considered as examples of Alternative Stable States. Recent evidence for the patterns and processes expected by ASS theory are discussed.

Uncertain ecosystems: the conceptual framework

Climate sets the potential biomass of trees and physiologists have made considerable progress in understanding and predicting that potential and applying it in global vegetation models. The problem is in understanding and predicting tree cover where it is far from the climate potential. Vast areas of non-forested vegetation occur where climates are suitable for forests. Arguments over why forests are absent, ongoing for over a century, are generally polarized between favouring bottom-up factors (resource constraints) or top-down factors (herbivory, predation, fire). There is increasing support for hypotheses invoking the interaction between the two. This chapter introduces the key hypotheses, their assumptions and predictions. Trophic ecology is a useful framework for exploring departures from the climate potential for trees, focussing explicitly on regulation by consumers, including fire. Alternative stable state theory is emerging as particularly appropriate for explaining forest/non-forest mosaics with each state maintained by positive feedbacks to the preferred environment.

Soils and open ecosystems

Boundaries between open and closed ecosystems often coincide with soil differences of greater or lesser degree. It has long been argued that these soil differences explain the striking differences in vegetation structure. But the nature of the vegetation itself feeds back on soil properties so that it is far from trivial to determine whether soil differences are a cause or consequence of the vegetation growing at a site. This chapter reviews soil physical and chemical characteristics and their potential to determine open and closed vegetation mosaics. The chapter also explores competitive exclusion of forest trees by grasses, herbs, and shrubs.

Introduction to open ecosystems: a global anomaly and a local example

This book is about the light side of ecology, the non-forested open ecosystems of the world. More than a quarter of the world’s land area is dominated by open, non-forested ecosystems in climates which can support closed forests. They are particularly common in the tropics, making up grasslands and savannas, but also occur in other climate zones. Open ecosystems have been widely attributed to human deforestation. While deforestation is widespread and increasing in many regions, open ecosystems include ancient vegetation, in species, with traits divergent from closed forests. Using Cape fynbos, the world’s richest temperate flora, as an example, the ideas and explanations for these anomalously low biomass systems are introduced. The aim of this book is explained as introducing a wider readership to the still poorly known biology of open ecosystems on the light side. The structure and content of chapters is outlined.

The origins of closed and open ecosystems

Explanations for vegetation change in the past, including the ‘deep past’ (many millions of years ago) are deeply rooted in the idea that climate determines major vegetation patterns. But other factors have also changed, including large fluctuations in atmospheric CO2, influencing plant growth, and atmospheric oxygen, altering fire activity. Vertebrate herbivores have changed from gigantic dinosaurs, to small forest mammals, to the giant beasts of the Pleistocene. Plant growth forms dominating current biomes are relatively recent; broadleaved tropical and temperate forests only became common 50 million years ago (50 Ma), and C4 savannas began to sweep over the tropics from ~7 Ma. This chapter describes the changing fortunes of uncertain ecosystems and the forces that drove those changes. Researching the deep past exposes the antiquity of fire and large vertebrate consumers as processes creating open ecosystems. The past is also a test of our understanding of uncertain ecosystems in the present.

The nature of open ecosystems

If open ecosystems were of recent anthropogenic origin, linked to human activity in the last millennia, they should support an impoverished biota assembled in large part from forest-dwelling species. Yet several of the world’s biodiversity hotspots are open ecosystems, rich in species and rich in endemics. This chapter introduces the diversity of open ecosystems and the distinction between old growth and secondary, early-successional vegetation. The functional traits of species in open ecosystems can be highly informative as to the dominant consumers maintaining their structure. Traits adapted to different fire regimes and those adapted to vertebrate herbivory are considered and compared. Faunal differences between open and closed vegetation are beginning to emerge from both invertebrate and vertebrate studies and indicate the importance of vegetation structure for habitat choice.

The future of open ecosystems

What is the future of open ecosystems, the ancient savannas, grasslands, shrublands, and woodlands that are the central topic of this book? Their trajectories under current and future climate change are difficult to project since their dominant growth forms are only indirectly determined by climate. Rising CO2 is changing the balance so as to favour trees. Woody encroachment is widespread in open ecosystems globally, though the causes are complex, including fire suppression, changes in herbivore densities and composition, and CO2 effects on plant growth. Increasing drought is promoting large fires in woody fuels. The net effect on forest advance or retreat is uncertain. The biggest threat for untransformed open ecosystems is conversion to forests, whether by invasion of native or alien trees, or afforestation schemes targeting their assumed potential for carbon sequestration. This chapter considers the threats to open ecosystems, the consequences of their loss, and changes in policy and management needed to ensure their future.

Vertebrate herbivory and open ecosystems

Can herbivores account for the widespread occurrence of open ecosystems? Some suggest that Pleistocene megafauna did so, and large mammal herbivory is still important in some regions today. Exclosure studies have been widely used to test herbivore impacts on trees, but global patterns of the ‘brown world’ are not readily seen from satellites. Areas of mammal consumer dominance occur in cool temperate/boreal regions (e.g. Tibetan montane grasslands) and savannas in Africa, but not in those in Australia or South America. Herbivores vary in their impact on openness of vegetation because of differences in body size, feeding mode, predator avoidance behaviour while plants also differ in their defences and accessibility. Unlike fire, proxies are lacking for how extinct herbivores, even giant sauropods, impacted vegetation. Very few studies deal explicitly with how vertebrate herbivores help create and maintain open ecosystems where climates are suitable for forests, and there is an urgent need to find out more.

The pattern of open ecosystems and the climates in which they occur

Climate has long been considered the prime factor determining the distribution of major vegetation based on climate–vegetation correlations. These correlations underlie the common assumption that there is a single stable vegetation state for a given climate. Thus tropical forests are characteristic of climates that are warm and wet, deserts where climates are dry. But the assumption of ‘one climate = one vegetation’ is not true for large parts of the world. These have strikingly different vegetation states, such as forests and grasslands, occurring in the same landscapes and sharing the same climate. Correlative approaches are being challenged by process-based biogeographic models which reveal the extent of the vegetation–climate mismatch. For most of the twentieth century the non-forested ecosystems were thought to be secondary vegetation produced by deforestation and anthropogenic burning. While deforestation has occurred and is increasing at an alarming rate, there is also growing evidence for ancient origins of many naturally non-forested ecosystems.