Scattered trees are keystone structures

doi:10.1016/j.biocon.2006.04.023

Biological Conservation

Volume 132, Issue 3, October 2006, Pages 311-321

https://doi.org/10.1016/j.biocon.2006.04.023 Get rights and content

Abstract

Scattered trees are prominent features in many landscapes worldwide, including natural landscapes, cultural landscapes, and recently modified landscapes. The ecological importance of scattered trees is widely acknowledged in natural landscapes, but has not been sufficiently appreciated in human-modified landscapes. This paper shows that scattered trees are keystone structures in a wide range of landscapes. At the local scale, ecological functions of scattered trees include: provision of a distinct microclimate; increased soil nutrients; increased plant species richness; increased structural complexity; and habitat for animals. At the landscape scale, ecological roles include: increased landscape-scale tree cover; increased connectivity for animals; increased genetic connectivity for tree populations; and provision of genetic material and focal points for future large-scale ecosystem restoration. Furthermore, in disturbed landscapes, scattered trees often are biological legacies that provide ecological continuity through time. In combination, these ecological functions support the argument that scattered trees are keystone structures. That is, their contribution to ecosystem functioning is disproportionately large given the small area occupied and low biomass of any given tree, and the low density of scattered trees collectively. Because scattered trees fulfill unique functional roles in a wide range of scattered tree ecosystems, their loss may result in undesirable ecological regime shifts. A key management challenge in all landscapes with scattered trees is to maintain a balance between recruitment and mortality of trees in an appropriate spatial pattern. Meeting this challenge may represent an important step towards the genuine integration of conservation and production in human-modified landscapes.

Introduction

Ecosystems with scattered trees occur throughout the world. The origins and ecological roles of scattered trees in natural ecosystems have been intensively studied in many parts of the world, including in the Brazilian Cerrados (Furley, 1999), Venezuelan Trachypogon savanna (San José et al., 1991), African savannas (Belsky, 1994), arid rangelands in South Australia (Facelli and Brock, 2000), oak savannas in North America (Nuzzo, 1986) and the forest-tundra transition zone of the boreal forest (Sirois, 1992). Scattered trees are also prominent features of many human-dominated landscapes, including recently cleared landscapes in Central America (Guevara et al., 1992), Africa (Duncan and Chapman, 1999) and temperate Australia (Ozolins et al., 2001), well-established cultural landscapes such as the dehesas in Spain and Portugal (Díaz et al., 1997) or British wood-pastures (Peterken, 1996), and severely disturbed forest landscapes (Gibbons and Lindenmayer, 2002). In this paper, these systems are collectively referred to as “scattered tree ecosystems”. This definition is intended to be broader than that of “savanna” (sensu Bray, 1960), and includes natural, cultural and recently modified, as well as disturbed and undisturbed ecosystems (Fig. 1). The key defining feature of scattered tree ecosystems is the dispersed pattern of the trees. Scattered trees are referred to by various synonyms in different areas, including isolated trees (Dunn, 2000), pasture trees (Otero-Arnaiz et al., 1999), paddock trees (Law et al., 2000), and remnant trees (Guevara et al., 1986).

In this paper, scattered tree ecosystems are categorized into three groups for the purposes of discussion: (1) natural (such as savannas), (2) cultural (such as wood-pastures), and (3) recently modified (such as remnant paddock trees in south-eastern Australia) (Fig. 1). The distinction between cultural and recently modified scattered tree ecosystems is that the former have a long-term history of manipulation by humans and have been sustained by cultural systems over a long period of time. In contrast, the latter are recently modified, and often highly modified, and levels of tree cover are often declining. In reality, the distinction between the three categories will be blurred, and both natural and cultural scattered tree ecosystems can be highly modified. Similarly, levels of human modification and natural disturbance often interact, and scattered tree ecosystems therefore occur on a continuum from natural through to recently modified.

Despite large differences in climate and origin, scattered trees in natural, cultural and recently modified landscapes share many key ecological roles as well as several threats to their continued existence. However, especially in modified landscapes, the ecological value of scattered trees has rarely been recognized. The aims of this paper are to:

(1)
demonstrate the keystone role of scattered trees;
(2)
synthesize key ecological functions of scattered trees and highlight parallels between natural, cultural and recently modified ecosystems;
(3)
establish common threats to scattered trees, especially in human-dominated landscapes; and
(4)
outline ways in which scattered trees might serve as a landscape management tool to integrate conservation and production in human-modified landscapes.

By outlining the similarities between scattered trees in natural, cultural and recently modified ecosystems, this paper aims to facilitate increased recognition of the importance of scattered trees in modified landscapes. It is argued that native scattered trees exert a disproportionate effect on ecosystem function in a wide range of ecosystems, and that their loss therefore may lead to the deterioration of important ecosystem functions.

Section snippets

A large amount of evidence demonstrates a wide range of important ecological functions of scattered trees in many natural, cultural, and recently modified landscapes (reviewed in detail below). In various different ecosystems, several authors have independently noted the “keystone” role of scattered trees, for example, in the Negev desert in Israel (Munzbergova and Ward, 2002), and in dehesas in Spain (Plieninger et al., 2003). Tews et al., 2004a, Tews et al., 2004b considered scattered trees

Functions of scattered trees

In natural, cultural and recently-modified landscapes, scattered trees fulfill many functions. The following sections consider: (1) the local-scale ecological functions of individual trees, (2) their role as biological legacies in modified landscapes, and (3) landscape-scale ecological functions of multiple scattered trees. In addition, examples of direct benefits from scattered trees to humans are given in Table 1.

Threats to scattered trees

Scattered trees in natural, cultural and recently modified landscapes face some similar threats, as well as some threats that are unique to particular ecosystems. The most direct threat to all scattered trees is clearing by humans. For example, the legal and illegal removal of scattered trees is widespread in Australian grazing landscapes (Gibbons and Boak, 2002). Similarly, widespread land clearing continues in some Central American landscapes (Aguilar and Condit, 2001).

A slower, but equally

Landscape management approaches

One of the great challenges in landscape management is the trade-off between meeting short-term human needs and maintaining the capacity to provide ecosystem services in the long term (Foley et al., 2005). There is a growing debate about the best way to manage landscapes in the face of growing human populations and associated demand for food. This debate has recently been framed as a trade-off between two approaches: “land sparing” versus integrated landscape management (Green et al., 2005,

Acknowledgments

The authors are grateful to L.W. Braithwaite, L. Edenius, A. Hamblin, K. Kirby, J.I. Nassauer, V. Nuzzo, H. Shugart, L. Sirois, H. Tyndale-Biscoe, and P. Werner for helpful discussions and/or supplying literature and information. Thanks to P. Werner for comments on an earlier draft. The authors are very grateful to D. Gilmour and IUCN Publications and R. Heinsohn for permission to use photos.

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Scattered trees are keystone structures – Implications for conservation

Abstract

Introduction

Section snippets