[Lapg] The Overstory #149--Live Fences, Isolated Trees, and Windbreaks...

[Wesley Roe and Marjorie Lakin Erickson]

THE OVERSTORY #149--Live Fences, Isolated Trees, and Windbreaks: Tools
for Conserving Biodiversity
By Celia A. Harvey, Nigel I.J. Tucker, and Alejandro Estrada


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THE OVERSTORY #149--Live Fences, Isolated Trees, and Windbreaks: Tools
for Conserving Biodiversity
By Celia A. Harvey, Nigel I.J. Tucker, and Alejandro Estrada



Contents:

: INTRODUCTION
: DEFINITIONS
: IMPORTANCE OF LIVE FENCES, WINDBREAKS, AND ISOLATED TREES IN TROPICAL
   REGIONS
: FARMER MANAGEMENT AND USE OF LIVE FENCES, WINDBREAKS, AND ISOLATED TREES
: FLORISTIC AND STRUCTURAL DIVERSITY OF LIVE FENCES, ISOLATED TREES, AND
   WINDBREAKS
: --> Floristic and Structural Diversity of Live Fences
: --> Floristic and Structural Diversity of Isolated Trees
: --> Floristic and Structural Diversity of Windbreaks
: FAUNA ASSOCIATED WITH LIVE FENCES, ISOLATED TREES, AND WINDBREAKS
: --> Fauna Associated with Live Fences
: --> Fauna Associated with Isolated Trees
: --> Fauna Associated with Windbreaks
: CONCLUSIONS
: ORIGINAL SOURCE
: ABOUT THE AUTHORS
: WEB LINKS
: RELATED EDITIONS OF THE OVERSTORY
: PUBLISHER NOTES
: SUBSCRIPTIONS


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This article is excerpted from the original. See "Original Source" below
for a full citation.


INTRODUCTION

At first glance, many deforested tropical landscapes appear to be simple
mosaics of forest patches, interspersed with pastures and crop fields.
However, closer examination reveals that many of the agricultural areas
retain abundant and conspicuous tree cover, whether as individual
isolated trees, live fences, windbreaks, or clusters of trees. Some of
these trees are relicts of the original forest that were left standing
when the area was cleared; others have regenerated naturally or been
planted by farmers. Often, the isolated trees, live fences, and
windbreaks form part of agroforestry systems that the farmers manage to
obtain a wide array of goods and services. Although this on-farm tree
cover is often overlooked or ignored in surveys of land use (FAO 2000;
Kleinn 2000), analyses of forest fragmentation patterns, and
conservation efforts, it may be critical to maintaining biodiversity in
the fragmented landscapes that characterize many tropical regions
(Guevara et al. 1998; Gascon et al. 1999; Harvey et al. 2000).

The presence of live fences, isolated trees, windbreaks, and other
agroforestry elements in deforested regions could help conserve
biodiversity by serving as habitats, corridors, or stepping stones for
plant and animal species, adding structural and floristic complexity to
the agricultural landscape and enhancing landscape connectivity. Whereas
the importance of these agroforestry elements for conservation efforts
has been studied in great detail in temperate regions (Forman and Baudry
1984; Baudry 1988; Capel 1988; Burel 1996), little attention has been
focused on their ability to help conserve species in deforested regions
in the tropics. Until recently, even the ample literature on the effects
of forest fragmentation on the survival of plant and animal populations
in the tropics has largely ignored the ability of the surrounding
agricultural matrix to support species diversity and enhance species
persistence.

In this article, we examine the potential role of three common
agroforestry elements - live fences, windbreaks, and isolated trees - in
helping to retain plant and animal species and maintain the continuity
of species populations and ecological processes in fragmented tropical
landscapes. We focus on these elements because they are conspicuous in
many regions of the tropics, are easily integrated into farm practices,
and appear to hold potential for conservation efforts.


DEFINITIONS

"Live fences" refers to narrow lines of trees or shrub species planted
on farm boundaries or between pastures, fields, or animal enclosures
whose primary purpose is to control the movement of animals or people
(Westley 1990; Budowski and Russo 1993). Live fences usually are
composed of a single row of trees or shrubs that are closely planted at
uniform distances and may support barbed wire (Sauer 1979; Westley
1990), although sometimes they arise from natural regeneration
underneath fence lines.

"Windbreaks" refers to linear plantings of trees and shrubs (usually
several rows wide) and linear strips of remnant vegetation whose primary
function is to protect crops, livestock, and homes from wind damage
(Finch 1988; Wight 1988). Although we focus on windbreaks, many of the
generalizations about the relationships between windbreak structure and
species composition and biodiversity conservation also hold for hedges.

"Isolated trees" refers to trees that are scattered in pastures, in
fields, or around homes, occur in varying densities and spatial
arrangements, and have variable origins (e.g., relicts of the original
forest, naturally regenerated, or planted by farmers; Harvey and Haber
1999).


IMPORTANCE OF LIVE FENCES, WINDBREAKS, AND ISOLATED TREES IN TROPICAL
REGIONS

With the exception of commercial crops grown in large expanses (e.g.,
sugar-cane, pineapple, and banana), most tropical agricultural
landscapes contain at least some trees, although the density, diversity,
and spatial arrangement vary greatly between sites. Although an
individual tree, live fence, or windbreak is likely to have little
impact on landscape structure and be insignificant to conservation
efforts, the presence of several agroforestry elements in the
agricultural landscape may greatly enhance tree cover and structural
heterogeneity and provide complementary habitats and resources to the
remaining forest remnants, thereby contributing to biodiversity
maintenance. In addition, by connecting forest patches and other patches
of remnant vegetation and forming complex, integrated networks of trees
across agricultural landscapes, live fences may reduce the isolation
between suitable habitats and influence animal movement patterns
(Estrada et al. 1993, 1998; Guevara et al. 1998).

The prevalence of these agroforestry elements in many regions suggests
that they may have a significant impact on conservation efforts. For
example, in Central and South American landscapes, 60 - 95 percent of
the cattle farms have live fences and 25 - 93 percent of the farms have
scattered, isolated trees in pastures. In a study in Veracruz, Mexico,
isolated trees covered approximately 3.3 percent of the total area in a
5,509-ha landscape and created a fragmented, discontinuous canopy that
nevertheless enhanced biotic connectivity (Guevara et al. 1998).


FARMER MANAGEMENT AND USE OF LIVE FENCES, WINDBREAKS, AND ISOLATED TREES

In any particular region, the abundance and distribution of live fences,
windbreaks, and isolated trees reflect the history of deforestation and
land use as well as the management of farm tree resources (Browder 1996;
Arnold and Dewees 1998; Janzi et al. 1999). When farmers clear forests
to create agricultural lands, they often retain some forest patches,
strips of trees along rivers or streams, and remnant forest trees as
sources of future products and services, although in some tropical
regions such as the Mata Atlantica of Brazil and parts of the Wet
Tropics of northeastern Australia farmers have extensively cleared the
land and left little tree cover.

Isolated trees typically are retained in pastures and agricultural areas
because of their value as sources of timber, fence posts, firewood, and
fruits, as shade and forage for cattle, and as sources of organic matter
for improving soil fertility or because their cutting is prohibited by
law (Pezo and Ibrahim 1988; Marmillod 1989; Harvey and Haber 1999;
Cajas-Giron and Sinclair 2001). They may also be retained or planted to
beautify the farm landscape and increase its economic value (Wight 1988;
Bird et al. 1992). Windbreaks are maintained or planted primarily to
provide wind protection and prevent soil erosion, although they may
provide additional functions and services (Baldwin 1988; Drone 1988;
Wight 1988). In contrast, live fences usually are established to
delineate borders with adjacent properties, divide pastures into smaller
sections for cattle rotation, and prevent animals and humans from
trespassing.

When choosing which trees to retain on their farms, farmers generally
select healthy trees that have valuable timber or firewood, provide
fruits for humans, or serve as cattle forage (Paap 1993; Barrance et al.
2003). Farmers may also carefully determine the distribution of trees
within the farm, as is the case in Honduras where maize farmers tend to
limit trees to field edges to minimize shading of associated crops
(Barrance et al. 2003). This contrasts with tree distributions in
pastures, where trees often are widely scattered across the entire
pasture to offer shade and supplementary fodder to cattle while they are
grazing. Farmers may protect individual trees by clearing around the
stem when they are saplings while weeding fields and pastures. To
minimize competition between the trees and agricultural crops or
pastures, farmers not only regulate tree densities and arrangements but
also prune the lower branches of trees to reduce shade, taking care not
to affect tree development (Kowal 2000; Barrance et al. 2003). Thus,
tree management by farmers is likely to influence the potential of the
land to conserve biodiversity.


FLORISTIC AND STRUCTURAL DIVERSITY OF LIVE FENCES, ISOLATED TREES, AND
WINDBREAKS

The value of individual agroforestry elements for conservation depends,
to a large degree, on their floristic composition and structural
diversity. In general, the greater the floristic and structural
diversity, the greater the ability of the agroforestry element to
provide habitat and resources for wildlife.


--> Floristic and Structural Diversity of Live Fences

When planted by farmers, live fences tend to be simple linear plantings
of trees (usually of only a single species) that are evenly spaced and
periodically pollarded and trimmed (Sauer 1979; Budowski 1987). Although
numerous tree species may be used, a few species account for most live
fences. For example, although more than 100 species are used in live
fences in Costa Rica, only 8 species account for 95 percent of the posts
(Budowski and Russo 1993). In the humid zones of Central America,
northern South America, and several Caribbean countries, live fences
generally consist of Erythrina spp. and Gliricidia sepium, whereas in
dry areas they usually consist of Bursera simaruba, Spondias purpurea,
and Leucaena leucocephala (Budowski 1987). Over time, some of the
planted live fences are colonized by other plant species whose seeds are
dispersed to the site by birds or other animals (Molano et al. 2002).
However, because of the small area below the live fences, the open,
exposed conditions, and the frequent disturbance by cattle and humans,
only a limited number of plant species establish.


--> Floristic and Structural Diversity of Isolated Trees

In contrast to planted live fences, isolated trees may represent a
higher floristic and structural diversity depending on the tree origin
(relict, regenerated, or planted), density, distribution within the
landscape, and management by farmers. Although the floristic diversity
represented by isolated trees is highly variable, in some regions these
trees may represent a significant portion of the original tree species
present in the forest. For example, isolated trees in pastures of
Monteverde, Costa Rica, represented 60 percent of the species present in
the study area (Harvey and Haber 1999), whereas isolated trees in
pastures in Veracruz, Mexico, represented 33 percent of the total
rain-forest tree flora, albeit at greatly reduced densities (Guevara et
al. 1998). In the traditional agricultural systems where farmers pollard
or cut trees to provide mulch for crop production, tree diversity within
the system can be quite high because many trees survive despite being
pollarded and resprout in subsequent years (Wilken 1977; Hellin et al.
1999; Garcia Rodriguez et al. 2001; Barrance et al. 2003). However, in
other regions where deforestation has been more complete and there are
few isolated trees, the floristic diversity may be minimal: for example,
in Rondonia, in the southwest of the Brazilian Amazon, 10-year-old
pastures retained only 20 of the 326 plant species present in the
original forest and only 6 of the 196 tree species in the current forest
(Fujisaka et al. 1998).


--> Floristic and Structural Diversity of Windbreaks

Planted windbreaks generally consist of a limited number of species
carefully selected for their rapid growth, ability to provide adequate
wind protection, and suitability for a given climatic zone. For example,
windbreaks in the highlands of Costa Rica tend to consist of primarily
exotic species such as Cupressus lusitanica, Alnus jorullensis,
Casuarina equisetifolia, and Croton niveus (Combe 1981; Harvey et al.
2000). In Mexico, windbreaks are dominated by Cupressus sp. on the
Pacific coast, Tamarix sp. and Casuarina sp. in the semiarid areas,
Casuarina sp. in the Golfo, and Erythrina sp. in the highlands of
Chiapas (Wilken 1977). Windbreaks are also common features of African
countries, with the genera Eucalyptus, Senna, Leucaena, Prosopis,
Casuarina, Azadirachta, and Acacia being used in dry areas
(Krishnamurthy and Avila 1999). In tropical Australia, windbreaks are
generally composed of Eucalyptus spp., hoop pine (Araucaria
cunninghamiana), and the exotic conifer Caribbean pine (Pinus caribea
var. hondurensis; Chapter 18, this volume). Unfortunately, many of these
common windbreak species offer little in terms of resources for wildlife
(Crome et al. 1994).

Despite the fact that the floristic diversity of planted windbreaks
usually is quite limited, they can potentially facilitate natural
regeneration in their understories by serving as perching and seed
deposition sites for birds and other animals and providing a modified
microclimate that enhances the establishment of some forest trees. A
study in Monteverde, Costa Rica, found that windbreaks (consisting of
Montanoa guatemalensis, Cupressus lusitanica, Casuarina equisetifolia,
and Croton niveus) received 40 times as many tree seeds and more than
twice as many species of seeds as adjacent pastures due to increased
bird visitation, indicating the potential for windbreaks as foci for
regeneration (Harvey 2000b). Surveys of the understories of windbreaks
found a total of 91 tree species (including primary and secondary forest
species) occurring as seedlings, just 5 - 6 years after the windbreaks
were established (Harvey 2000a). Interestingly, windbreaks connected to
forests had significantly higher numbers of tree species and higher
densities of tree seedlings than those that were isolated from forests
by 20 - 50 m (Harvey 2000a). This pattern probably reflects the greater
activity of frugivorous birds in connected windbreaks (DeRosier 1995;
Tucker 2001). Planted windbreaks consisting of Eucalyptus camaldulensis,
Tecoma stans, and Leucaena leucocephala in Le6n, Nicaragua, similarly
appeared to serve as habitats for plant regeneration, although the
density and species richness of trees (33 species) in windbreak
understories were low, probably because of the frequent use of fire in
adjacent agricultural lands (Alvarado et al. 2001). Although it is not
clear how many of the regenerating seedlings will survive and grow into
mature trees, there is at least a strong potential for the windbreaks to
be colonized by native species. To a large degree, the fate of the
seedlings depends on windbreak management practices, especially the
exclusion of cattle (Capel 1988; Johnson and Beck 1988).


FAUNA ASSOCIATED WITH LIVE FENCES, ISOLATED TREES, AND WINDBREAKS

A variety of animal species may take advantage of agroforestry elements
in fragmented landscapes, using them as habitats, foraging sites,
corridors, or stepping stones to cross open areas. Here we review the
available information on fauna using live fences, isolated trees, and
windbreaks and identify factors that influence the value of these
agroforestry elements for fauna conservation.


--> Fauna Associated with Live Fences

Live fences in tropical landscapes provide perching sites, cover, and
foraging sites for some animals, including birds, bats, beetles, and
nonflying mammals. For example, a total of 98 bird species (representing
54 percent of the bird species detected in adjacent forest fragments)
were detected in a 6-km-long live fence consisting of Bursera simaruba
and Gliricidia sepium (with a few naturally regenerated species) in
Veracruz, Mexico (Estrada et al. 1997). Similarly, in naturally
regenerated live fences in Colombia, a total of 105 bird species of 45
families were found, with older, more structurally complex live fences
having more bird species and more birds typical of forest borders and
secondary growth (Molano et al. 2002). Although live fences often are
dominated by bird species typical of edge or open habitats, a few forest
interior resident species, including some that rarely leave the forest,
also visit them (Estrada et al. 2000). The visiting bird community
includes granivores, frugivores, and insectivores that use the fences as
perches and foraging sites.


--> Fauna Associated with Isolated Trees

Like live fences, isolated trees may provide habitats, perching and
foraging sites, and stepping stones for a variety of animal species,
particularly birds. For example, a study in Veracruz, Mexico, recorded
73 bird species visiting four isolated fig trees (Ficus yoponensis and F
aurea) in pastures (Guevara and Laborde 1993), and isolated trees in
Costa Rican pastures were visited by at least 27 frugivorous bird
species (Holl et al. 2000). Some of the frugivorous birds are resident
species that nest in pastures, whereas other birds nest elsewhere and
use the trees as perching or feeding sites (Guevara and Laborde 1993;
Slocum and Horvitz 2000). Similarly, isolated Eucalyptus trees in sheep
paddock of New South Wales, Australia, appear to be important for a
large range of bird taxa, with 31 bird species observed using paddock
trees (Fischer and Lindenmayer 2002a, 2002b). Although many of these
birds are open-country birds, several birds considered to be woodland
species were also observed visiting the trees (e.g., striated pardalote
[Paradalotus straitus], scarlet robin [Petroica multicolor], grey
shrike-thrush [Colluricinla harmonica], and crested shrike-tit
[Falcunculus frontatus]; Fischer and Lindenmayer 2002a, 2002b).


--> Fauna Associated with Windbreaks

Numerous detailed studies from temperate regions have shown that,
depending on their floristic diversity, structural complexity, and
management, windbreaks may help conserve a large number of plant and
animal species, including a limited number of forest-dependent species,
by providing food, cover from predators, refuge, and travel lanes (e.g.,
Arnold 1983; Osborne 1983; Fournier and Loreau 2001). Windbreaks tend to
have the greatest conservation value if they contain a variety of native
plant species and life forms, connect to intact forest or other natural
vegetation, are wide (so that they contain some interior habitat), and
are protected from grazing cattle (Arnold 1983; Capel 1988; Johnson and
Beck 1988; Fritz and Merriam 1993, 1996; Burel 1996). In general, the
greater the structural and floristic diversity, the more ecological
niches are available for other plants and animals. When windbreaks
connect forest fragments or other remnant vegetation, they may also
serve as corridors for some animal species (Yahner 1983; Haas 1995). The
modified microclimatic conditions in the windbreaks may be more
favorable than those in the open pastures or fields and provide
protection from weather extremes; however, these microclimatic
conditions are likely to be spatially and temporarily variable
throughout the length of the windbreak. Most of the species that benefit
from the presence of windbreaks are edge species that are capable of
using highly modified habitats; few forest interior species appear to
take advantage of windbreak habitats (Burel 1996; Corbit et al. 1999)


CONCLUSIONS

The emerging data show that live fences, windbreaks, and isolated trees
may contribute to biodiversity conservation and suggest that retaining
or establishing trees in agricultural lands may be a critical component
of conservation efforts in fragmented landscapes. The floristic
diversity conserved in these agroforestry systems can be high, and a
substantial number of animal species may exploit these habitats for
feeding, movement, and in some cases reproduction, although the value of
each agroforestry element depends on its structure, composition,
management, and position in the landscape. Many species that benefit
from agroforestry systems are generalist species, but some forest
specialist species usually are also present. By forming networks of
natural habitats, live fences, windbreaks, and isolated trees may also
enhance landscape connectivity and contribute biodiversity conservation
at different scales.

However, it should be emphasized that although these agroforestry
elements are useful additions or complements to the conservation of
natural habitats, they are not substitutes for the original vegetation.
Live fences, windbreaks, and isolated trees are not complete ecological
units and cannot provide the full array of habitats or services of the
original habitat; consequently, the organisms in them are likely to
depend, at least to some degree, on nearby remnant habitats. Efforts to
conserve biodiversity in fragmented landscapes therefore should focus on
developing landscape-scale strategies that integrate the retention and
establishment of windbreaks, live fences, isolated trees, and other
agroforestry elements with the conservation of forest fragments, the
retention of riparian vegetation, the maintenance of connectivity in the
agricultural landscape, and other conservation strategies (Vandemeer and
Perfecto 1997; Harvey et al. 2000; Tucker 2000; Daily et al. 2001).


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::::::::::::::
ORIGINAL SOURCE

This edition of The Overstory was adapted with the kind permission of
the publisher and authors from the original:

Harvey, C.A., N.I.J. Tucker, and A. Estrada. 2004. Live Fences, Isolated
Trees, and Windbreaks: Tools for Conserving Biodiversity. In: G.
Schroth, G.A.B. da Fonseca, C.A. Harvey, C. Gascon, H.L. Vasconcelos,
and A-M.N. Izac. Agroforestry and Biodiversity Conservation in Tropical
Landscapes. Island Press, Washington, DC.

To learn more about this publication, visit:
http://islandpress.org/books/detail.html?SKU=1-55963-357-3

Copies of this publication can be purchased in the US from:

Island Press DISTRIBUTION CENTER
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Distribution Center
11030 South Langley Avenue
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Phone: 1-800-621-2736; Fax: 1-800-621-8476
Email: custserv at press.uchicago.edu

For distributors outside the US, visit
http://www.islandpress.org/contact.html.


::::::::::::::
ABOUT THE AUTHORS

Celia A. Harvey is an Associate Professor in the Department of
Agriculture and Agroforestry at the Tropical Agricultural Research and
Higher Education Center (CATIE), Costa Rica, and coordinates its Masters
Program on Tropical Agroforestry. She has a Ph.D. in Ecology, with a
concentration in Conservation and Sustainable Development, from Cornell
University. Her research focuses on understanding patterns of
biodiversity in fragmented landscapes and the roles of agroforestry
systems in conserving both local and regional biodiversity. Contact
Celia Harvey at: Department of Agriculture and Agroforestry, CATIE, Apdo
7170, Turrialba, Costa Rica, email: charvey at catie.ac.cr .

Alejandro Estrada is a Senior Research Scientist at the field station
Los Tuxtlas, Universidad Nacional Aut6noma de Mexico, located in
southern Mexico. His main research interest is in documenting the
demographic and behavioral responses of mammals to landscape changes in
the wet tropics of Northern Mesoamerica. Contact Alejandro Estrada at:
Ph.D. Investigador Titular C TC;  Estacisn de Biologma Los Tuxtlas
Instituto de Biologma, UNAM Apdo. Pos. 176, San Andris Tuxtla, Veracruz,
Mexico 95700; Tel/Fax: + 200 + 125-5405 Web site:
http://www.primatesmx.com; E-mail: aestrada at primatesmx.com .

Nigel I. J. Tucker is a Director of Biotropica Australia and a member of
the Faculty of Biological Sciences at James Cook University, north
Queensland, Australia. His research interests include tropical forest
restoration and landscape ecology.


::::::::::::::
WEB LINKS

World Resources Institute, Biodiversity and Protected Areas:
<http://www.wri.org/biodiv/>

Biodiversity Support Programme: <http://www.BSPonline.org/>

Biodiversity Planning Support Programme:
<http://www.undp.org/bpsp/>

The Nature Conservancy: <http://nature.org>


::::::::::::::
RELATED EDITIONS OF THE OVERSTORY

The Overstory #109 -  Cultural Landscapes
The Overstory #105 -  Complex Agroforests
The Overstory #93 -  Trees, Forests and Sacred Groves
<http://www.agroforestry.net/overstory/overstory93.html>
The Overstory #88 - Revegetation Planning for Farm Forestry
<http://www.agroforestry.net/overstory/overstory88.html>
The Overstory #77--Tropical Forest Conservation
<http://www.agroforestry.net/overstory/overstory77.html>
The Overstory #76--Ethnoforestry
The Overstory #73--Buffers, Common-Sense Conservation
<http://www.agroforestry.net/overstory/overstory73.html>
<http://www.agroforestry.net/overstory/overstory76.html>
The Overstory #64--Homegardens
<http://www.agroforestry.net/overstory/overstory64.html>
The Overstory #51--Expanding Traditional Agroforests
<http://www.agroforestry.net/overstory/overstory51.html>
The Overstory #49--Traditional Pacific Island Agroforestry Systems
<http://www.agroforestry.net/overstory/overstory49.html>


::::::::::::::
PUBLISHER NOTES

Publisher: Permanent Agriculture Resources
Editor: Craig R. Elevitch

Distributor:
The Overstory is distributed by Agroforestry Net, Inc., a nonprofit
501(c)(3) organization based in Hawaii.
Address: P.O. Box 428, Holualoa, Hawaii 96725 USA
Email: overstory at agroforestry.net ; Web site:
http://www.overstory.org

Past editions of The Overstory: http://www.overstory.org

This publication is Copyright 2005 Permanent Agriculture Resources.
All Rights Reserved Worldwide. For Conditions of Use please contact
overstory at agroforestry.net or write to Agroforestry Net, Inc. at the
address above.

This journal is designed to provide agricultural information, but
is sent with the understanding that the editors and publishers are
not engaged in rendering consultation. If expert assistance is
required, the services of a professional should be sought.


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Dear Wes,

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