identified, especially with respect to the large numbers of coffee pests and their
natural enemies in complex coffee ecosystems.
The most rustic coffee plantations, where the tree crops are grown under a
dense, highly diversified shade canopy, form part of a group of agroforestry sys-
tems that are collectively called complex agroforests or simply agroforests because
of their diversity, structural complexity, and similarity to natural (secondary)
forests. Chapter 10 reviews the conservation potential of such agroforests, which
have evolved in several parts of the humid tropics, distinguishing between systems
based on canopy tree crops, such as rubber and damar trees, and shade-tolerant
understory tree crops, such as cocoa and tea. Because of their extensive manage-
ment, these systems are characterized by a substantial amount of spontaneous
vegetation and associated fauna. Because agroforests are able to provide a range of
timber and nontimber forest products and maintain high plant and animal diver-
sity, they are particularly valuable as landscape conservation tools.
The aforementioned agroforestry systems occur in tropical landscapes as
patches or blocks of variable shape and size. However, many tropical land-
scapes that have been cleared of forest to establish fields or pastures do not
contain such distinct agroforestry patches but may still retain trees in the form
of live fences, windbreaks, or isolated trees, which may either be remnants of
the original forest vegetation, planted, or retained from spontaneous regener-
ation. Although these agroforestry elements may cover only a very small per-
centage of the landscape, they may still offer some habitat and increase the
permeability of the agricultural matrix for many organisms that are not
adapted to wide, treeless areas, thereby increasing connectivity between islands
of natural vegetation. Chapter 11 discusses the roles of live fences, isolated
trees, and windbreaks as habitats, corridors, and stepping stones and identifies
knowledge gaps that must be filled before the potential of these agroforestry
elements for landscape-scale conservation strategies can be fully appreciated.
Chapter 12 focuses on a role of trees in agricultural landscapes that is even
less obvious: their effect on gene flows between trees in forest fragments. In
the past, the knowledge that most tropical trees are outcrossing led to the
assumption that many trees that occur in small numbers in forest fragments
or even individually as remnants in agricultural areas are “living dead,” or
unable to produce viable offspring because of the lack of nearby mating part-
ners. However, recent studies show that agroforestry trees that occur in agri-
cultural areas near forest fragments may contribute significantly to gene flow
across landscapes, which occurs over larger distances than had been assumed.
However, problems may arise when agroforestry trees come to dominate the
pollen pool of certain species in adjacent forests, thereby narrowing the
genetic base of subsequent generations. Although these aspects warrant further
study, the chapter shows clearly the potential of tree management, especially
of remnants and natural regeneration of forest trees in agricultural areas, for
the conservation of viable tree populations in fragmented forest landscapes.
152 III. The Biodiversity of Agroforestry Systems