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Abstract

The loss of large old trees in many ecosystems around the world poses a threat to ecosystem integrity.
DOI: 10.1126/science.1231070
, 1305 (2012);338 Science et al.David B. Lindenmayer
Global Decline in Large Old Trees
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PERSPECTIVES
Large old trees are among the biggest
organisms on Earth. They are keystone
structures in forests, woodlands, savan-
nas, agricultural landscapes, and urban areas,
playing unique ecological roles not provided
by younger, smaller trees. However, popula-
tions of large old trees are rapidly declining in
many parts of the world, with serious implica-
tions for ecosystem integrity and biodiversity.
The definition of “large and old” trees
depends on the ecosystem, tree species, and
environmental conditions under consid-
eration. Both the size and the age of a tree
affect characteristics such as the large inter-
nal cavities, complex branching patterns,
and idiosyncratic canopy architectures that
distinguish large old trees from younger and
smaller trees ( 1).
Large old trees (see the fi gure, panels A to
C) play critical ecological roles. They provide
nesting or sheltering cavities for up to 30%
of all vertebrate species in some ecosystems
( 2). Large old trees also store large quantities
of carbon, create distinct microenvironments
characterized by high levels of soil nutri-
ents and plant species richness, play crucial
roles in local hydrological regimes, and pro-
vide abundant food for numerous animals in
the form of fruits, fl owers, foliage, and nec-
tar. In agricultural landscapes, large old trees
can be focal points for vegetation restoration,
facilitate ecosystem connectivity by attracting
mobile seed dispersers and pollinators, and
act as stepping stones for many animals.
Younger and smaller trees cannot provide
most of the distinctive ecological roles played
by large old trees ( 3). For instance, large old
trees in Mountain Ash (Eucalyptus regnans)
forests of mainland Australia provide irre-
placeable shelter and nesting sites for more
Global Decline in Large Old Trees
ECOLOGY
David B. Lindenmayer,
1
William F. Laurance ,2 Jerry F. Franklin
3
The loss of large old trees in many ecosystems
around the world poses a threat to ecosystem
integrity.
1Fenner School of Environment and Society, The Austra-
lian National University, Canberra, ACT 0200, Australia.
2Centre for Tropical Environmental and Sustainability Sci-
ence, and School of Marine and Tropical Biology, James
Cook University, Cairns, Queensland 4878, Australia.
3School of Environmental and Forest Science, University
of Washington, Seattle, WA 98195, USA. E-mail: david.
lindenmayer@anu.edu.au
cal regions of the world, but is rare in large
areas of central and western Africa where
many individuals lack Duffy-antigen recep-
tor expression on red blood cells. Thus, this
“Duffy-negative” phenotype appears to have
evolved as an innate resistance mechanism to
P. vivax infection.
McMorran et al. extend previous work that
demonstrated an important role for platelets in
resistance to malaria ( 8) by identifying plate-
let factor 4 (PF4) as a key molecule in plate-
let-mediated killing of P. falciparum. PF4 is
released from α granules in activated plate-
lets to promote blood coagulation ( 9). It binds
the Duffy-antigen receptor, along with sev-
eral other chemokines ( 10). McMorran et al.
found that a functional Duffy-antigen receptor
is required for the antiparasitic activity of PF4.
The implications of lacking this antipara-
sitic mechanism for Duffy-negative individ-
uals living in P. falciparum malaria endemic
regions are not yet clear. One might predict
that these individuals will be more prone to
episodes of severe malaria. Indeed, mortality
among African children with malaria-induced
coma is higher than in children with the same
condition from Papua New Guinea, where
Duffy-negative individuals are less common
( 11). However, further evidence is required
to support this proposition. Alternatively,
compensatory antiparasitic mechanisms may
have evolved in Duffy-negative individuals
to help control parasite growth and/or reduce
pathology following infection. The identifi ca-
tion of other such mechanisms will offer fur-
ther insights into innate immune responses to
infection, and potentially identify vulnerable
aspects of parasite biology.
Platelets decrease in number (thrombocy-
topenia) during acute malaria. McMorran et
al. suggest that this is not to the host’s advan-
tage, limiting this innate form of resistance.
However, other data show that platelets can
contribute to cerebral malaria, a major cause
of mortality. Platelets at normal physiologi-
cal concentrations cause clumping of para-
sitized red blood cells from African chil-
dren, a phenomenon associated with cerebral
malaria ( 12). Thrombocytopenia may there-
fore reduce pathology by protecting the host
against cerebral malaria, which may explain
in part why there has been less pressure to
maintain platelet-associated parasite killing
mechanisms in Africans. The Duffy-negative
phenotype to prevent P. vivax invasion of red
blood cells seems to have been under stron-
ger selective pressure than the maintenance
of a PF4-dependent antiparasitic mechanism
in central and western Africa. However, given
the potentially different origins and timelines
of P. falciparum and P. vivax adaptations to
humans ( 13, 14), another possibility is that
the Duffy-negative phenotype has simply
been under selective pressure in this part of
Africa for longer. In addition, nonmalaria
pressures may also have infl uenced this selec-
tion over time.
Cells of the innate immune system—mac-
rophages, natural killer cells, dendritic cells,
and γδ T cells—play an important role in
defending against parasites, often providing a
rst line of defense and augmenting acquired
(adaptive) immunity. By understanding how
innate mechanisms of protection against
malaria have been under strong selective pres-
sure during evolution, we may better under-
stand how to protect people from malaria. For
example, how PF4 kills P. falciparum is not
yet clear, but when this knowledge is avail-
able, vulnerable features of parasites will be
identifi ed that could be targeted with appro-
priate drugs. Understanding new antiparasitic
mechanisms selected by evolution will enable
us not only to complement existing cellular
and molecular approaches to identifying drug
targets to kill parasites, but also to select safer
targets that have less effect on the host.
References
1. World Health Organization, “World Malaria Report” WHO
Press, Geneva, 2011.
2. B. McMorran et al., Science 338, 1348 (2008).
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5793 (1995).
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6. J. D. Haynes et al., J. Exp. Med. 167, 1873 (1988).
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10.1126/science.1232439
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7 DECEMBER 2012 VOL 338 SCIENCE www.sciencemag.org
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PERSPECTIVES
than 40 species of cavity-using vertebrates
( 4). For many dependent species, the keystone
roles of large old trees continue for decades
or even centuries after tree death, when they
become standing dead trees or large logs ( 1).
The loss of large old trees is a recognized
concern in many ecosystems worldwide.
For example, populations of large old trees
are plummeting in intensively grazed land-
scapes in California, Costa Rica, and Spain,
where such trees are predicted to disappear
within 90 to 180 years ( 5). In southeastern
Australia, millions of hectares of grazing
lands are projected to support less than 1.3%
of the historical densities of large old trees
within 50 to 100 years ( 6).
Large old trees are declining in forests at
all latitudes. Larger trees (>45 cm in diameter)
throughout southern Sweden have declined
from historical densities of ~19 per hectare
to 1 per hectare ( 7). In California’s Yosemite
National Park, the density of the largest trees
(see the figure, panel A) declined by 24%
between the 1930s and 1990s ( 8). Large old E.
regnans trees—Earth’s tallest fl owering plants
(see the figure, panel B)—are predicted to
decline from 5.1 in 1997 to 0.6 trees per hect-
are by 2070 ( 4). Fragmented Brazilian rain-
forests are likely to lose half of their original
large trees (60 cm diameter) in the fi rst three
decades after isolation ( 9).
Large old trees are exceptionally vulnera-
ble to intentional removal, elevated mortality,
reduced recruitment, or combinations of these
drivers (see the figure, panel C). They are
removed during logging, land clearing, agri-
cultural intensifi cation, fi re management, and
for human safety. Droughts, repeated wild-
res, competition with invasive plants, edge
effects, air pollution, disease, and insect attack
( 10) can all increase tree mortality. The likeli-
hood of new trees growing into large old trees
can be diminished by overgrazing or browsing
by native herbivores ( 11) and domestic live-
stock ( 6), by competition with exotic plants,
and by altered fi re regimes.
Drivers of large old tree loss often inter-
act to create ecosystem-specifi c threats ( 12).
In agricultural landscapes, chronic livestock
overgrazing, excessive nutrients from fertil-
izers, and deliberate removal for firewood
and land clearing combine to severely reduce
large old trees ( 6). Populations of large old
pines in the dry forests of western North
America declined dramatically in the last cen-
tury because of selective logging, uncharac-
teristically severe wildfi res, and other causes,
although efforts are now made to reduce the
density of the stands so that high-severity fi res
do not occur and large trees are saved (see the
gure, panel D). Salvage logging is equally
damaging, whereby natural disturbances,
such as fi re or insect attack, are followed by
removal of all remaining live and dead trees
(see the fi gure, panel E). In certain tropical
savannas and temperate forests, interactions
among drivers occur over vast areas and result
in entire landscapes supporting few large old
trees ( 13). Modeling suggests that even mod-
est increases in adult mortality can seriously
erode populations of long-lived organisms
such as large old trees ( 14).
Although large old trees are declining
across much of the planet, not all ecosystems
are losing such trees. Elevated plant-growth
rates in tropical forests, possibly in response
to rising concentrations of atmospheric car-
bon dioxide, might result in larger numbers
of large old trees, at least where such forests
escape other human disturbances.
Large old trees are more likely to per-
sist in particular parts of landscapes such
as disturbance refugia. Research is needed
to determine the locations and causes of
such refugia and to devise strategies to pro-
tect them ( 15). For example, timber or other
commodity extraction (e.g., cropping) in
managed landscapes might be concentrated
where large old trees are least likely to per-
sist or develop. Maintenance of appropriate
population age structures can help to ensure
the perpetual supply of large old trees. This
requires policies and management practices
that intentionally grow such trees and reduce
their mortality rates ( 5).
Just as large-bodied animals such as ele-
phants, tigers, and cetaceans have declined
drastically in many parts of the world, a grow-
ing body of evidence suggests that large old
trees could be equally imperiled. Targeted
research is needed to better understand their
key threats and devise strategies to counter
them. Without such initiatives, these iconic
organisms and the many species dependent on
them could be lost or greatly diminished.
References
1. R. van Pelt, Identifying Mature and Old Forests in Western
Washington (Washington State Department of Natural
Resources, Olympia, WA, 2007).
2. J. Remm, A. Lohmus, For. Ecol. Manage. 262, 579 (2011).
3. D. B. Lindenmayer, Forest Pattern and Ecological Process:
A Synthesis of 25 Years of Research (CSIRO Publishing,
Melbourne, 2009).
4. W. F. Laurance, New Sci. 213, 39 (2012).
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(2010).
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Manage. 257, 2296 (2009).
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nclimate1635 (2012).
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14. S. L. Lewis et al., Nature 457, 1003 (2009).
15. B. Mackey et al., Ecol. Appl. 22, 1852 (2012).
A B C
DE
Global decline. (A) Over 95% of California’s majestic coastal redwoods have been lost to logging and for-
est clearing ( 8). (B) Large old Mountain Ash (E. regnans) trees in mainland southern Australia are critical
habitats for many elements of the biota but are also readily killed and often consumed by wildfi res ( 4). (C)
Baobab trees, like this giant in Tanzania, are under threat from land clearing, droughts, fungal pathogens,
and overharvesting of their bark for mat-weaving ( 3). (D) Efforts to conserve large old Ponderosa Pine (Pinus
ponderosa) trees include reducing the risk of stand-replacing fi re by removing small trees and applying low-
severity prescribed fi re. (E) During post-insect attack salvage logging operations in British Columbia, Canada,
all large trees are removed.
10.1126/science.1231070
PHOTO CREDITS: (A) R. BUTLER; (B) W. INCOLL; (C) W. LAURANCE; (D) R. NOSS; (E) K. HODGES
Published by AAAS
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... These trees have great potential to become large old trees in the future [3]. Old trees are keystone structures in forests, woodlands, and agricultural ecosystems, playing unique ecological roles that are not provided by younger trees [4]. Some studies have shown that large and big-sized trees are key elements for carbon storage [5][6][7], and the rate of tree carbon accumulation increases continuously with tree size [7]. ...
... Some studies have shown that large and big-sized trees are key elements for carbon storage [5][6][7], and the rate of tree carbon accumulation increases continuously with tree size [7]. In addition, old trees can create microclimate and microhabitat heterogeneity, such that epiphyte species richness and abundance increase with tree size [4,8]; provide irreplaceable habitats; and act as stepping stones for many animals [4]. Moreover, old trees are an important part of cultural heritage and can provide people with aesthetic, symbolic, religious, and historical cues [9]. ...
... Some studies have shown that large and big-sized trees are key elements for carbon storage [5][6][7], and the rate of tree carbon accumulation increases continuously with tree size [7]. In addition, old trees can create microclimate and microhabitat heterogeneity, such that epiphyte species richness and abundance increase with tree size [4,8]; provide irreplaceable habitats; and act as stepping stones for many animals [4]. Moreover, old trees are an important part of cultural heritage and can provide people with aesthetic, symbolic, religious, and historical cues [9]. ...
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Standing dead trees, or snags, serve myriad functions in natural forests, but are often scarce in forests managed for timber production. Variable retention (VR), the retention of live and dead trees through harvest, has been adopted globally as a less intensive form of regeneration harvest. In this study, we explore how two key elements of VR systems — level (amount) and spatial pattern of live-tree retention — affect the carryover and post-harvest dynamics of natural and artificially created snags. We present nearly two decades of data from the DEMO Study, a regional-scale experiment in VR harvests of Douglas-fir-dominated forests in the Pacific Northwest. Snag losses to harvest were greater at 15 than at 40% retention (67 vs. 47% declines in density) and greater in dispersed than in aggregated treatments (64 vs. 50% declines). Densities of hard and tall (≥5 m) snags were particularly sensitive to low-level dispersed retention, declining by 76 and 81%, respectively. Despite these losses, post-harvest densities correlated with pre-harvest densities for most snag size and decay classes. In contrast to initial harvest effects, snag densities changed minimally over the post-harvest period (years 1 to 18 or 19), with low rates of recruitment offsetting low rates of loss. Post-harvest survival of snags was greater at 15 than at 40% retention (79 vs. 69%), as were rates of decay (68 vs. 52% of hard snags transitioned to soft). However, pattern had no effect on either process. Snag recruitment did not vary with retention level or pattern at the scale of the 13-ha harvest unit, but was several-fold greater in the 1-ha aggregates (14.3–27.8 snags ha⁻¹) than in the corresponding dispersed treatments (4.2–5.3 snags ha⁻¹). Snag size (diameter) distributions showed greater change in dispersed than in aggregated treatments, reflecting greater loss of smaller snags and recruitment biased toward larger snags. Created snags showed uniformly high survival (97%), irrespective of treatment, but rates of decay were greater at lower retention. If a goal of VR is to sustain snag abundance and diversity through harvest, emphasis should be placed on minimizing initial losses, either by reducing the intensity of felling in areas of dispersed retention or locating forest aggregates in areas of greater initial snag density, diversity, or incipient decay.
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Heavily exploited for its reddish, decay-resistant heartwood, the tallest conifer, Sequoia sempervirens, is a major component of coastal forests from extreme southwestern Oregon to California’s Santa Lucia Mountains. Primary Sequoia forests are now restricted to < 5 % of their former distribution, and mature secondary forests with trees over 60 m tall are even scarcer due to repeated logging. Leveraging allometric equations recently derived from intensive work in both forest types, we climbed, measured, and core-sampled 235 trees in 45 locations distributed across the species range to examine growth trends and understand how tall Sequoia are responding to recent environmental changes. Paired samples of sapwood and heartwood collected along the height gradient were used to quantify Sequoia investment in decay resistance. During the 20th century, trees in most locations began producing more wood than expected for their size with this growth surge becoming pronounced after 1970 and ending around 2000. Radial increments—ring widths—correlate with climatic variables related to water availability, and these relationships are strengthening as temperatures rise. Sensitivity to drought increased from north to south along a 6° latitudinal gradient of decreasing precipitation and summer fog frequency. Sequoia trees north of 40° were least sensitive to drought, producing similar biomass annually during dry and wet years, whereas trees farther south produced less biomass during individual drought years. Hotter 21st century drought barely affected Sequoia growth efficiency (biomass increment per unit leaf mass) north of 40° until the fourth consecutive year (2015), when growth efficiency dropped precipitously, recovering within two years. South of 40°, Sequoia trees exhibited steadily declining growth efficiency during the multi-year drought followed by recovery, but recovery did not occur south of 37° despite ample precipitation in 2017. Sequoia growth efficiency is currently highest in secondary forests north of 40°, where trees produce relatively small amounts of heartwood with the lowest decay resistance (least fungicide) while receiving the most nocturnal summer fog. Increasing sink limitations, whereby rising temperatures, drier air at night, and extreme tree height collectively lower turgor pressure to inhibit cambial activity, may reduce Sequoia growth efficiency while contributing to more durable biomass production. Heartwood and fungicide increments are higher in primary than secondary forests across the species range. Crown structural complexity promotes development of vascular epiphytes and arboreal soil habitats in Sequoia forests with sufficient moisture availability. These habitats are lacking in secondary forests and rare in primary forests south of 40°. After logging, restoration of tall Sequoia forests can be achieved via silviculture that maximizes height increments during early stand development and then retains some dominant trees in perpetuity, allowing them to gain full stature, produce increasingly decay-resistant heartwood, and support significant arboreal biodiversity.
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One‐third of the world's trees are at risk of extinction, with large, old, long‐lived trees among the most vulnerable. Long‐lived trees in arid and semi‐arid biomes are particularly at risk, including Australian sandalwood (Santalum spicatum, Santalaceae), which is experiencing substantial population decline due to a suite of natural and anthropogenic drivers, with no appreciable recruitment estimated for more than 80 years. To contextualize this range‐wide collapse and quantify regional variation in population dynamics across Australia's western rangelands, we investigated the size‐class profiles of 12 sandalwood populations in a 1,500‐kilometre arc between Shark Bay and the Gibson Desert in central Western Australia including Indigenous Protected Areas, pastoral leases and public and private conservation parks and reserves. Stem diameters, indicative of age using known growth rates, were recorded for 1,355 sandalwood plants, along with a set of another plant structural and ecological parameters. Using size‐class profiles and associated demographic data, we estimated the population age structure and trajectory to determine whether each population was increasing, stable or declining. Our surveys revealed sandalwood populations are declining and are composed almost entirely of very old trees in advanced states of senescence. Of 1,355 plants sampled, 1,198 (88.4%) individuals were large (old) trees. A total of 23 seedlings and 21 saplings were recorded across all sites, almost all of which (22 and 19, respectively) were in one population, and located under the canopies of parent trees where they would not be expected to survive to maturity. Our findings reinforce the urgent need to list Santalum spicatum as a threatened species in Western Australia (where wild plants are still being commercially harvested) and to initiate effective conservation actions to secure the species' continued existence across its natural range. A field survey of 12 populations of Australian sandalwood across a 1,500‐kilometre arc from the Gibson Desert to Shark Bay in Western Australia has revealed that populations are dominated by very old plants undergoing senescence. This ground‐truthing field study reinforces recent claims that the species is on a trajectory towards becoming extinct in the wild, and the recent listing of sandalwood as a vulnerable threatened species on the IUCN Red List.
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Trees are the most important landscape architects of our planet, not only in forests but also in countless other ecosystems, including human-fabricated habitats. Due to their significance in the majority of terrestrial ecosystems, trees play an important role in maintaining biodiversity and providing food and habitat for countless microorganisms, fungi, climbers, invertebrates, and vertebrates. Trees are also indispensable for the development of human societies and are important for our survival today and in the future. Trees therefore have an inestimable scientific, economic, social, cultural, and aesthetic value. In addition, they were and are playing an essential role in myths, rituals, and cultures of nearly all indigenous and modern societies. Despite these facts, the protection of trees is insufficient, both globally due to climate change and deforestation, but also locally, for example in the big cities through deterioration of soils or improper care. In industrialized countries, only recently has the need for targeted protection efforts for tree species or even for individual trees been recognized. Our review starts with the differentiation and definitions of forests and trees. Furthermore, we present the main categories and subcategories of trees, each of them possessing different functions in their ecosystems and for human societies and thus needing specific legal protection measures. The second part of our review presents the most important tools for improving the protection of trees. On the one hand, there exist a series of international initiatives, conventions, and agreements, and on the other hand, there are numerous legal tools, such as red lists, lists of protected species, and legislation for the protection of monument and habitat trees. The main challenge of the 21st century is to find a solution to make the development and growth of modern human societies compatible with the protection of natural resources such as forests. The large number of tree species, as well as the large proportion of threatened tree species, makes this problem even larger. Finally, the most recent and controversial approach of attributing trees the rights as legal persons is discussed.
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Forests provide innumerable ecological, societal and climatological benefits, yet they are vulnerable to drought and temperature extremes. Climate-driven forest die-off from drought and heat stress has occurred around the world, is expected to increase with climate change and probably has distinct consequences from those of other forest disturbances. We examine the consequences of drought- and climate-driven widespread forest loss on ecological communities, ecosystem functions, ecosystem services and land–climate interactions. Furthermore, we highlight research gaps that warrant study. As the global climate continues to warm, understanding the implications of forest loss triggered by these events will be of increasing importance.
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Forest Pattern and Ecological Process is a major synthesis of 25 years of intensive research about the montane ash forests of Victoria, which support the world's tallest flowering plants and several of Australia's most high profile threatened and/or endangered species. It draws together major insights based on over 170 published scientific papers and books, offering a previously unrecognised set of perspectives of how forests function. The book combines key strands of research on wildfires, biodiversity conservation, logging, conservation management, climate change and basic forest ecology and management. It is divided into seven sections: introduction and background; forest cover and the composition of the forest; the structure of the forest; animal occurrence; disturbance regimes; forest management; and overview and future directions. Illustrated with more than 200 photographs and line drawings, Forest Pattern and Ecological Process is an essential reference for forest researchers, resource managers, conservation and wildlife biologists, ornithologists and mammalogists, policy makers, as well as general readers with interests in wildlife and forests. 2010 Whitley Certificate of Commendation for Zoological Text.
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A 135-kD parasite protein, a minor component of the Plasmodium knowlesi malaria radiolabeled proteins released into culture supernatant at the time of merozoite release and reinvasion, specifically bound to human erythrocytes that are invaded and carry a Duffy blood group determinant (Fya or Fyb), but did not bind to human erythrocytes that are not invaded and do not carry a Duffy determinant (FyFy). Specific anti-Duffy antibodies blocked the binding of the 135-kD protein to erythrocytes carrying that specific Duffy determinant. Purified 135-kD protein bound specifically to the 35-45-kD Duffy glycoprotein on a blot of electrophoretically separated membrane proteins from Fya and Fyb erythrocytes but not from FyFy erythrocytes. Binding of the 135-kD protein was consistently greater to Fyb than to Fya both on the blot and on intact erythrocytes. The 135-kD protein also bound to rhesus erythrocytes that are Fyb and are invaded, but not to rabbit or guinea pig erythrocytes that are Duffy-negative and are not invaded. Cleavage of the Duffy determinant by pretreating Fyb human erythrocytes with chymotrypsin greatly reduced both invasion and binding of the 135-kD protein, whereas pretreating Fyb erythrocytes with trypsin had little effect on the Duffy antigen, the 135-kD protein binding, or on invasion. However, instances of invasion of other enzyme-treated erythrocytes that are Duffy-negative and do not bind the 135-kD protein suggest that alternative pathways for invasion do exist.
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Herbivores cause treefalls in African savannas, but rates are unknown at large scales required to forecast changes in biodiversity and ecosystem processes. We combined landscape‐scale herbivore exclosures with repeat airborne Light Detection and Ranging of 58 429 trees in Kruger National Park, South Africa, to assess sources of savanna treefall across nested gradients of climate, topography, and soil fertility. Elephants were revealed as the primary agent of treefall across widely varying savanna conditions, and a large‐scale ‘elephant trap’ predominantly removes maturing savanna trees in the 5–9 m height range. Treefall rates averaged 6 times higher in areas accessible to elephants, but proportionally more treefall occurred on high‐nutrient basalts and in lowland catena areas. These patterns were superimposed on a climate‐mediated regime of increasing treefall with precipitation in the absence of herbivores. These landscape‐scale patterns reveal environmental controls underpinning herbivore‐mediated tree turnover, highlighting the need for context‐dependent science and management.
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We explored the main factors affecting the global distribution of tree cavities – a habitat component of mostly biotic origin that is crucial for many animal species. We considered the influence of eight environmental variables (ranging from the single-tree to the biogeographic-region scale) on cavity density in a meta-analysis of 103 published studies. The global median density of cavities was 16ha−1, with densities highest in Australasia and lowest in the Palaearctic region. Two major factors influencing density were identified: cavity density was positively related to the amount of precipitation, and was higher in natural than in managed forests. These effects suggest that the distribution of tree cavities largely reflects the incidence of fungal heart-rot in trees, and that forest management, by affecting wood decay processes, can have a broad-scale impact on tree microhabitat availability. Although air temperature, forest composition and wood hardness had suggestive univariate effects, neither these variables nor biogeographic region explained any additional variation in multifactor models. In regions where woodpeckers are present there was an upper limit to the density of woodpecker-excavated cavities (approximately 10–20cavitiesha−1) that was considerably lower than the highest total cavity densities encountered (up to 140ha−1). This indicates that primary cavity-nesters are particularly important keystone species in cavity-poor forests where wood decay processes are suppressed either climatically or by forest management.
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In response to climate change and other threatening processes there is renewed interest in the role of refugia and refuges. In bioregions that experience drought and fire, micro-refuges can play a vital role in ensuring the persistence of species. We develop and apply an approach to identifying potential micro-refuges based on a time series of remotely sensed vegetation greenness (fraction of photosynthetically active radiation intercepted by the sunlit canopy; fPAR). The primary data for this analysis were NASA MODIS 16-day L3 Global 250 m (MOD13Q1) satellite imagery. This method draws upon relevant ecological theory (source sink habitats, habitat templet) to calculate a micro-refuge index, which is analyzed for each of the major vegetation ecosystems in the case-study region (the Great Eastern Ranges of New South Wales, Australia). Potential ecosystem greenspots were identified, at a range of thresholds, based on an index derived from: the mean and coefficient of variance (COV) of fPAR over the 10-year time series; the minimum mean annual fPAR; and the COV of the 12 values of mean monthly fPAR. These greenspots were mapped and compared with (1) an index of vascular plant species composition, (2) environmental variables, and (3) protected areas. Potential micro-refuges were found within all vegetation ecosystem types. The total area of ecosystem greenspots within the upper 25% threshold was 48 406 ha; around 0.2% of the total area of native vegetation (23.9 x 10(6) ha) in the study region. The total area affected by fire was 3.4 x 10(6) ha. The results of the environmental diagnostic analysis suggest deterministic controls on the geographical distribution of potential micro-refuges that may continue to function under climate change. The approach is relevant to other regions of the world where the role of micro-refuges in the persistence of species is recognized, including across the world's arid zones and, in particular, for the Australian, southern African, and South American continents. Micro-refuge networks may play an important role in maintaining beta-diversity at the bio-region scale and contribute to the stability, resilience, and adaptive capacity of ecosystems in the face of ever-growing pressures from human-forced climate change, land use, and other threatening processes.
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Questions: Can small and isolated high-conservation value forests (e.g. designated woodland key habitats) maintain old-growth forest characteristics and functionality in fragmented landscapes? To what extent have past disturbances (natural and anthropogenic) influenced the development of old-growth characteristics of these forests? How long does it take for selectively cut stands to attain conditions resembling old-growth forests? Location: Southern boreal zone of central Sweden. Methods: We linked multiple lines of evidence from historical records, biological archives, and analyses of current forest structure to reconstruct the forest history of a boreal landscape, with special emphasis on six remaining core localities of high-conservation value forest stands. Results: Our reconstructions revealed that several of these stands experienced wildfires up to the 1890s; all had been selectively harvested in the late 1800s; and all underwent substantial structural and compositional reorganization over the following 100-150 years. This time interval was sufficient to recover considerable amounts of standing and downed dead wood (mean 60.3 m3 ha−1), a range of tree ages and sizes (mean basal area 32.6 m2 ha−1), and dominance of shade-tolerant spruce. It was insufficient to obtain clearly uneven tree age structures and large (>45 cm diameter) living and dead trees. Thus, these forests contain some, but not all, important compositional and structural attributes of old-growth forests, their abundance being dependent on the timing and magnitude of past natural and anthropogenic disturbances. Our landscape-level analysis showed marked compositional and structural differences between the historical forest landscape and the present landscape, with the latter having a greater proportion of young forests, introduction of non-native species, and lack of large trees and dead wood. Conclusions: The remnant high-conservation value stands were not true representatives of the pre-industrial forests, but represent the last vestige of forests that have regenerated naturally and maintained a continuous tree cover. These traits, coupled with their capacity for old-growth recovery, make them valuable focal areas for conservation.
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The vegetation dynamics of the savanna ecosystem are driven by complex interactions between biotic and abiotic factors, and thus are expected to exhibit emergent properties of biocomplexity. We explore the relative importance of static and dynamic drivers in explaining the patterns of mortality of large trees in the Kruger National Park, South Africa. Data on large trees were collected from 22 transects in April 2006, and these transects were re-sampled in November 2008. Of the 2546 individually-identified trees that were re-sampled, 290 (11.4%) died in the interim. We tested several competing hypotheses with varying levels of complexity, and found that mortality of large trees was affected mainly by both static (geophysical and landscape characteristics) and dynamic (elephant damage and fire) factors that were either additive or interactive in their effects. Elephant damage was the main predictor of tree mortality, but fire also played an important role depending on the landscape type. Other static variables such as position-on-slope, height below canopy, and altitude had weak effects in explaining tree mortality. These results indicate that keystone features such as large trees, show differential vulnerability to mortality that is landscape-specific. For conservation managers, this implies that the dynamic drivers (elephant and fire) of tree mortality have to be managed at the specific landscape-level. We suggest that this emergent biocomplexity in the spatial and temporal patterns of large tree mortality is not unique to the African savannas, but is likely widespread across heterogeneous landscapes.
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Studies of forest change in western North America often focus on increased densities of small-diameter trees rather than on changes in the large tree component. Large trees generally have lower rates of mortality than small trees and are more resilient to climate change, but these assumptions have rarely been examined in long-term studies. We combined data from 655 historical (1932–1936) and 210 modern (1988–1999) vegetation plots to examine changes in density of large-diameter trees in Yosemite National Park (3027 km2). We tested the assumption of stability for large-diameter trees, as both individual species and communities of large-diameter trees. Between the 1930s and 1990s, large-diameter tree density in Yosemite declined 24%. Although the decrease was apparent in all forest types, declines were greatest in subalpine and upper montane forests (57.0% of park area), and least in lower montane forests (15.3% of park area). Large-diameter tree densities of 11 species declined while only 3 species increased. Four general patterns emerged: (1) Pinus albicaulis, Quercus chrysolepis, and Quercus kelloggii had increases in density of large-diameter trees occur throughout their ranges; (2) Pinus jeffreyi, Pinus lambertiana, and Pinus ponderosa, had disproportionately larger decreases in large-diameter tree densities in lower-elevation portions of their ranges; (3) Abies concolor and Pinus contorta, had approximately uniform decreases in large-diameter trees throughout their elevational ranges; and (4) Abies magnifica, Calocedrus decurrens, Juniperus occidentalis, Pinus monticola, Pseudotsuga menziesii, and Tsuga mertensiana displayed little or no change in large-diameter tree densities. In Pinus ponderosa–Calocedrus decurrens forests, modern large-diameter tree densities were equivalent whether or not plots had burned since 1936. However, in unburned plots, the large-diameter trees were predominantly A. concolor, C. decurrens, and Q. chrysolepis, whereas P. ponderosa dominated the large-diameter component of burned plots. Densities of large-diameter P. ponderosa were 8.1 trees ha−1 in plots that had experienced fire, but only 0.5 trees ha−1 in plots that remained unburned.