Introduction

There is a major planet-wide experiment under way. Anthropogenic changes to the atmosphere–biosphere system mean that all ecosystems on Earth are now affected by our activities. While outright deforestation is physically obvious, other subtler processes, such as hunting and surface fires, also affect forests in ways that are less evident to the casual observer (cf. Estes et al. 2011; Lewis, Malhi & Phillips 2004a; Malhi & Phillips 2004). Similarly, anthropogenic atmospheric change is intensifying. By the end of the century, carbon dioxide concentrations may reach levels unprecedented for at least 20 million years (e.g. Retallack 2001) and climates may move beyond Quaternary envelopes (Meehl et al. 2007). Moreover, the rate of change in these basic ecological drivers may be unprecedented in the evolutionary span of most species on Earth today. Additionally, these atmospheric changes are coinciding with the greatest global upheaval in vegetation cover and species’ distributions since at least the last mass extinction at ~65 million years ago (Ellis et al. 2011). Collectively, the evidence points to conditions with no clear past analogue. We have entered the Anthropocene, a new geological epoch dominated by human action (Crutzen 2002; Steffen et al. 2011).

In this chapter we focus on the changes occurring within remaining tropical forests. Most forest vegetation carbon stocks lie within the tropics. Tropical forests store 460 billion tonnes of carbon in their biomass and soil (Pan et al. 2011), equivalent to more than half the total atmospheric stock, and annually process 40 billion tonnes (Beer et al. 2010). They have other planetary influences via the hydrological cycle, and emit aerosols and trace gases, and they are also characterised by their exceptional variety and diversity of life.