Download Palms at Inotawa

INTRODUCTION
Palms and the Tropical Landscape
Tropical forests are biodiversity hotspots and contain the most diverse plant
communities on the planet (Givinish 1999). Neotropical rainforests are the most
extensive of all global tropical forests; around half of the global total, 4 x 106 km2
in area, and one-sixth of the total broad-leaf forest of the world (Whitmore 1998).
Tropical forest habitats can support a host of different tree species even in small
areas; up to 283 species per hectare (Phillips et al. 1994).
Palms are among the dominant vascular plants in many tropical forests,
and are important components of the forest structure in Amazonian forests
(Scariot 1999). Palms (Arecacea family) belong to the class of monocotyledons;
they form a distinctive crown of feathery or fan-shaped leaves which are always
alternate, often very large and pinnately incised to a varying degree (Bruggeman
1962). There number of palm species is estimated at 2500 – 3500 in
approximately 210 – 236 genera (Jones 1995). Montufar & Pintaud (2006) have
estimated the number of native palms in Western Amazonia to be about 121
species and 33 genera (roughly two-thirds of the Amazon palms and one-fifth of
the New World palms).
The richness and abundance of palm taxa, as well as their occurrence in
all strata of the forest, and their importance as a vital food source for wildlife
make this family a primary target for conservation (Scariot 1999). The sedentary
habitat and patchy distributions of plants make them susceptible to habitat
1
destruction, which may cause changes in taxa composition and population sizes
(Schemske et al. 1994). And despite the importance of plants, particularly palms,
in the forest structure and function, their response to habitat fragmentation has
been studied less than animals (Laurence & Bierregaard 1997).
There are 165 individual species registered on the 2006 IUCN Red list, 9 of those
are critically endangered, 14 individuals are endangered and 67 species are
registered as vulnerable. Almost a third of the species on the list are data
deficient (45 individuals), where there is inadequate information to make an
assessment on the extinction risk of an individual species. Only one species,
Amorphophallus preussii (endemic to Ecuador), has a recorded population trend;
which is declining (Darbyshire 2004), all other individuals have not had their
population trends studied. The major threat for all of the individuals is ongoing
human induced habitat loss/degradation (ICUN 2006), whilst other major threats
include large-scale wood plantations, livestock and crop agriculture, fires,
infrastructure growth and in rare occasions; natural disasters such as volcano
eruptions (Benavides & Pitman 2003).
2
Factors Influencing Species Abundance
Land Use in Tropical Habitats
Much of the Amazon Basin is a mosaic of forest types produced by
erosion/deposition cycles of the major rivers (Clarke et al. 1995) and variation in
terre firme parent material and geochemistry (Guillaumet 1987). Another source
of small to medium scale spatial heterogeneity within tropical forests is the
historical or current impact of human activity, from agriculture, silviculture and
selective harvesting, even within stands considered to be old growth (Clark et al.
1995; Bush & Colinvaux 1994).
Agriculture is the biggest land use of tropical forests and serves the main
purpose for which rain forests are cleared (Whitmore 1998). Conversion of
tropical rainforests to pasture for cattle husbandry is particularly widespread in
the neotropics. Shifting agriculture, commonly known as ‘swidden’, is a
sustainable low-input form of cultivation which can continue indefinitely on the
infertile soils underlying most tropical rainforests, provided the carrying capacity
of the land is not exceeded. Dufour (1990), claims that under some
circumstances, shifting agriculture based on long fallow periods can be an
ecologically and an economically sustainable practice in tropical forests. Not all
shifting agriculture is practised in a sustainable manner, in some circumstances
farmers fell and burn the forest and grow crops on the released nutrients for
several years in succession, continuing until coppicing potential and the soil seed
3
bank are exhausted, invasive species take hold and soil nutrients are depleted.
They then move onto a new patch of virgin forest. In western Amazonia,
peasants from the Andean plateau are moving into the forest with no previous
experience of forest agriculture, after growing a few crops they sell-out to a
pastoralist who raises cattle, creating a wave of cultivation and poor pasture
sweeping western from the foothills of the Andes (Whitmore 1998).
On a more commercial scale, tropical forests are frequently used as a
source of timber and non-timber products, and are an important and widespread
forest land use in much of the tropics (Collins et al 1991). Clear felling results in
greater levels of habitat loss and degradation, and it is believed that many palms
do not regenerate in open areas and are therefore threatened in areas with
extensive deforestation (Pedersen, 1994; Moraes et al., 1995), whereas selective
logging; the periodical extraction of commercially valuable trees from forests
(Johns 1988), has a much less detrimental effect on the forest ecosystem
(Grieser Johns 1997; Asner et al 2004).
The rate at which habitats are disturbed influences the species
assemblages and diversity within a plant community. At very low rates of
disturbance, such as infrequent clear-felling, competitive exclusion would occur
and species richness would be low, species richness would be greatest at
moderate levels of disturbance, because dominance is prevented, and the pool
of potential colonists is relatively large (Armesto & Pickett 1985).
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Habitat Associations
The distribution of individuals within a population of plants is rarely random
across a landscape (Harms et al. 2001). Tropical trees and shrubs often display
distributional biases with respect to environmental variables such as soil type and
water availability, across spatial scales of several ha to many km2.
The association of species with physical habitat variables generates some
of the most obvious patterns in the distribution and abundance of organisms, and
its study has a long history (Cowles 1899; Whittaker 1956). On a smaller scale,
microhabitat heterogeneity (referring to environmental conditions that vary at
scales less than 10m2, e.g. treefall gaps or local topographic variations
(Svenning 1999)), has been suggested to play an extensive role in determining
species distributions (Grubb 1977; Ricklefs 1977). Whereas several authors
(Molofsky & Augspurger 1992; Nicotra et al. 1999) have shown heterogeneity in
environmental conditions that clearly influence seed and seedling survival, such
as light, litter and soil moisture, have been shown to occur at spatial scales of
less than 1m. Webb & Peart (2000) suggest that the most influential hypothesis
for habitat partitioning in rain forest trees relies on not on any underlying
heterogeneity in the physical environment, but on the endogenous, local
heterogeneity created by canopy openings. A tropical landscape scattered with
small indigenous communities practising traditional agricultural methods would
create a mosaic of habitat types, and increasing heterogeneity on a landscape
and microhabitat scale.
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Only one group of tropical forest trees, the pioneer or early-successional
species, has been clearly and consistently associated with a specific habitat type;
disturbed areas such as large canopy openings (Whitmore 1989). Among nonpioneers species, little evidence has been compiled to link any differences in
individual-level response to environmental conditions to differences in distribution
along environmental gradients (Baraloto & Goldberg 2004). This shows a critical
gap in current knowledge, as non-pioneer species form a large part of the tropical
forest ecosystem, and in the case of many species such as palms, are
fundamental keystone species, components for the present and future of the
tropical forests globally. Consequently, it is important to identify any strong
habitat associations resulting from human influenced habitat loss/degradation
and their effects on the abundance and diversity of palms.
Niche Width & Occupation
Several authors (Grubb 1977; Denslow 1987) believe that the exceptional
diversity within a tropical forest is maintained through niche differentiation with
respect to resources. The niche is a multidimensional description of a species’
resource needs, habitat requirements and environmental tolerances (Hutchinson
1957). Where the niche structure of complex plant communities has been
investigated in some detail, niche differences have been found in germination
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behaviour, root depth, temperature thresholds, grazing tolerance, phenology and
many other factors (Crawley 1997).
The niche width of a species refers to the area which a species could
physically inhabit; the niche width often differs from the area that a species
actually inhabits, or its realized niche width. The niche concept has rarely been
used in plant ecology due to the difficulty in defining a species’ niche breadth, in
zoological terms it is fairly straightforward to define an individual’s niche in
relation to diet and range size, yet it is much more difficult to apportion resources
such as light, water or nitrogen between species. Within plant communities, a
species with a ‘broad niche’ will be found growing abundantly over a wide range
of niche conditions, and are known as generalists, and species with a ‘narrow
niche’ , a specialist, will only be found under a restricted range of conditions.
Specialist species with a narrow realized niche width will eventually become
extinct from the landscape due to competition from more abundant species with a
broader niche width. Crawley (1997) suggests that in order to maintain diversity,
species-rich plant communities may be composed of: (i) species with narrower
niches; (ii) species with more broadly overlapping niches; (iii) habitats providing
‘longer’ niche axes; or (iv) a combination of these.
The ability for species to coexistence rather than out-compete in plant
communities is driven by niche specialisation along gradients or resource
availability (Grubb 1977; Crawley 1990). The niche diversification hypothesis
created by Connell (1978) is an important equilibrium hypothesis stating that
species coexist by occupying different niches. There are many authors that
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discredit such as simple hypothesis, and a much more complex version proposes
that coexistence is a result of habitat or microhabitat specialisation (Denslow
1987; Gentry 1988; Welden et al. 1991; Clark et al. 1993) and that much of the
tropical plant diversity therefore depends on habitat and microhabitat
heterogeneity.
Aims & Objectives
The aim of this study is to investigate possible species associations
between palms and balsas across a range of habitat types, displaying different
degrees of anthropogenic alteration and supporting a range of microhabitat
variables. Specifically the questions addressed are (i) What is the palm density
and abundance in the study area? (ii) Are there any habitat associations between
palm species in natural and anthropogenically altered tropical forest? (iii) What
are the species associations between balsa trees and palm species? The
objectives derived from these aims are (i) Obtain the number of palms and
balsas present in the study area and calculate species density and abundance
through plot sampling. (ii) Identify differences in habitat type and land use and
record microhabitat variables across the study site. (iii) Identify differences in
species abundance and assemblages between natural and anthropogenically
altered habitats.
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METHODS
Study Site
The study took place during July and August 2007, conducted in the native
community of La Torre, within the buffer zone of the Tambopata Nature Reserve,
27km southwest of Puerto Maldonado in south-eastern Peru (12°49’S, 69°17’W).
The area is at the moist tropical/subtropical vegetation boundary (Brightsmith
2004). Rainfall is on average 2810 mm per year (Pearson & Derr 1986). The dry
season starts in April and ends in October (Pearson & Derr 1986). The
surrounding area is made up of a mix of floodplain and terra firme forests
(Brightsmith 2004). There are no large-scale deforestation or agricultural areas in
the La Torre community, although along the river edge and close to tourist and
community residence there are small cleared sections for subsistence agriculture
(less than 3 hectares) and homegardens (R. Harris, pers. Obs.). The dominant
habitat is secondary floodplain forest, with fragments of understorey Guadua
bamboo (Lloyd 2004) and remnants of primary forest. A seasonal palm swamp
(12°42’S, 69°20’W), which has not been subjected to anthropogenic disturbances
or land use change, located approximately 5km downstream of the La Torre
community was also sampled.
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Figure 1. Location of study site within South-eastern Peru.
10
Figure 2. Location of transects within study site. 1= 1250m, 2 = 1750m, 3 = 600m, 4 = 250m, 5 = 250m, 6 = 700m (Palm
swamp), 7 = 1500m. Total length surveyed = 6.3km
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Habitat Sampling
A total of 136 survey plots were sampled laid out along transects following
existing pathways, Figure 2 shows the location and length of the transects. A
100m2 plot was sampled every 50m along the transect, a pilot study using 25m
intervals between plots was initially trailed but the method was deemed too timeconsuming for the limited study period. Plots were sampled at alternate sides of
the transect to include more habitat types and ensure a more random sampling
strategy. In total an area of 1.36ha was surveyed. Each plot was categorized at
the time of the survey according to habitat type or past/present land use. The
habitat categories were:
Regenerated secondary forest with no visible anthropogenic alteration
(SEC). Regrowth of secondary forest after large scale disturbances both
natural and anthropogenic develops rapidly in tropical habitats, but
species richness often accumulates quite slowly. Even regrowth over a
century old does not contain all the species present in primary forest.
(Corlett & Turner 1997).
Garden, either abandoned or receiving lowest level of management such
as occasional fruit harvesting (GARD). Home gardens usually contain
small numbers of useful plant species grown such as trees for fruit and
firewood, specialised vegetables and medicinal plants. Small populations
of animals such as poultry and pigs are sometimes raised in homegardens
(Kellman & Tackaberry 1997).
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Residential (RES); either tourist residence or member of the La Torre
Community, including the land immediately surrounding the buildings of a
distance of less than 10m and not including land managed for agriculture
purposes (managed land within the vicinity of residential property classed
as home gardens).
Agricultural land (AGRI); land recently (within last 20 years) >1ha used for
the growth of crop species cleared completely of the original forest species
present and may or may not of been subjected to burning. Agricultural
land includes small-scale shifting agriculture and larger monoculture
plantations.
Dominant bamboo understorey within secondary forest (BAM). Bamboo
grows in monotypic stands, which is unusual in tropical plant communities.
The structure of bamboo stands differs from other habitats in that the
relatively thin-stemmed plants are densely packed and have a thick
subcanopy of similarly shaped leaves (Kratter 1997).
Periodic palm swamp (SWA). Soils tend to be richer in plant nutrients due
to seasonal flooding (Whitmore 1998). Yet flooded forests are usually less
floristically diverse than non-flooded forests and contain more specialised
species (Kellman & Tackaberry 1997).
Habitat measurements were taken at each plot, the canopy openness was taken
from the centre of the plot with an acetate sheet, the sheet had a 10cm x 10cm
grid split into 100 cm squares, the number of squares which the sky was visible
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through when held directly above at arms length, equalled the percentage of
canopy openness.
The five tallest trees within the survey plot were selected and their heights
estimated by sight alone, the estimation of tree heights was based on previous
knowledge of vegetation sampling in the same habitat, using an automatic range
-finder and clinometer to record tree heights. The tree architecture of the five
tallest trees was taken to give an indication of the recent history of the forest
(Torquebiau 1986; Jones et al. 1995). The categories are:
BA – ‘branching above’, if the first or major branch was above half of the tree
height, indicative of tree growth under a closed canopy (undisturbed primary
forest) or;
BB – if it was ‘branching below’ half of the tree height, suggesting growth under
an open canopy due to disturbance or treefall
BASB – ‘branching above scars below’, where the first or major branch is above
half height of the tree but there is evidence of dropped branches below half the
tree height, caused by tree growth under a regenerating, closing canopy
BBVG – ‘branching below vegetated growth’, the branches are growing vertically
from close to its base, suggesting tree growth in a heavily altered habitat with
frequent tree-cutting.
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Palm and Balsa sampling
Previous field work within the same study region and a pilot study revealed that
Balsa (Ochroma pyramidale) was present along with five palm (Araceae)
species.
Ochroma pyramidale (Balsa) is harvested for its light
timber which is produced when growth has been
adequately rapid (Whitmore & Wooi-Khoon 1983).
Ranges from southern Mexico to Bolivia and is found
in the West Indies. Ochroma reaches heights of 20 –
30m and is an extremely fast grower; 5 -6 m per year
(Whitmore 1998).
Figure 3. Ochroma Pyramidale
Socratea exorrhiza (Walking Palm) has wellseparated stilt roots (Fig. 2) which can grow up to
4m and are covered in small spines (Henderson et
al 1995). Socratea exorrhiza can reach a height of
20m and is present from Nicaragua to Brazil
(Chiquita 2006).
Figure 4. The stilt roots of a Socratea exorrhiza palm.
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Attalea butyracea (Shebon) can grow up to
30m tall (Wright & Duber 2001). The plant
initially develops a basal rosette of fronds
which may remain for 20years before trunk
growth (Wright & Duber 2001). Attalea
butyracea
are
found
across
Central
America, Northern South America, Western
South America (IPGRI 2007).
Figure 5. Attalea butyracea
Astrocaryum murumuru are shade –
tolerant species (Losos 1995) and can
grow
up
to
20m
tall.
Astrocaryum
murumuru has black spines along the
stem and leaf midrib (Fig. 3) and is
distributed across western South America
(IPGRI 2007).
Figure 6. The underside and trunk of Astrocaryum murumuru
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Euterpe edulis (Wasai Palm) can reach a height
of 25m (Silva Matos & Watkinson 1998). Occurs in
coastal forests on steep slopes and forest patches
inland (Henderson et al 1995), typically on wetter
soils, often in swampy areas (Dias et al 1988).
Euterpe species are commonly harvested for their
palm hearts (Silva Matos & Watkinson 1998);
although Euterpe edulis within the study area are
Figure 7. Euterpe edulis
not
being
extracted.
Distributed
throughout
Northern South America & Western South America
(IPGRI 2007).
Iriartea deltoidea (Erotic Palm) is the only species in
the Iriartea genus. Clearly identified by clustered roots
at the base of the stem, which can grow 1.5m above
ground (Henderson et al. 1995). Characteristic canopy
components of premontane tropical forests up to
1000m (Kessler 2000), individuals can reach 35m in
height (Gentry 1993). Iriartea deltoidea have a wide
Neotropical distribution across South America.
Figure 8. Root structure of Iriartea deltoidea
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The number of Ochroma pyramidale individuals with a diameter at breast height
(d.b.h) ≥300mm were recorded, whereas with the palm species, the diameter of
the palm was recorded at 500mm above ground level rather than the d.b.h, this
would include the diameter of the root stilts in Iriartea and Socratea rather than
the main stem, as the diameter of stilt roots can be used as an indicator to the
age of an individual (A. Lee, pers. Comm.). The palm species were identified to
species level and all palm and Ochroma species surveyed with a diameter ≥
300mm had their heights recorded and whether the tree was fruiting, flowering or
neither.
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RESULTS
Species Abundance
There were 195 individuals recorded over all 7 transects at an overall abundance
of 144 individuals per ha, 171 of those were palm species, 24 were Ochroma
pyramidale. The most abundant species was Iriartea deltoidea, recorded at an
abundance of 53 individuals per ha (Table 1 shows all abundances), the least
abundant species was Socratea exorrhiza, recorded at 4.4 individuals per ha,
Euterpe edulis also had a very low abundance, 6.6 individuals per ha. Transect 1
was the only transect to have all species present, whereas in transect 4 and 5,
only one species was recorded, both these transects had the lowest abundance,
33.3 individuals per ha for both. Transect 6, located in the palm swamp, had the
highest abundance of palms recorded, 214.3 per ha, but no Ochroma pyramidale
individuals. No species was recorded on every transect, although Attalea
butyracea occurred most frequently, on 6 out of the 7 transects, Socratea
exorrhiza and Euterpe edulis occurred the least frequently, on only 3 out of the 7
transects.
Table 2 shows the abundance of individuals based on habitat type at each plot.
Secondary forest was the only habitat with all species present, agricultural
habitats supported the lowest number of species, only Ochroma pyramidale and
Attalea butyracea were present there. Agricultural habitats also had the lowest
abundance, 20 individuals per ha, the highest abundance within a habitat was
within the palm swamp, 214 individuals per ha. The palm swamp also had the
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highest abundances of 3 palm species over all habitats; Iriartea deltoidea,
Euterpe edulis & Socratea exorrhiza (129, 36 & 29 individuals per ha
respectively). Garden habitats had the lowest abundance for palm species (119
individuals per ha), but the highest abundance for Ochroma pyramidale (81
individuals per ha).
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Table 1. Abundance of individuals by transect. Key to habitats; SEC – secondary forest, GARD – garden, RES –
residential including homegardens, AGRI – agricultural land, BAM – dominant bamboo understorey, SWA – seasonal
palm swamp
Transect
1
2
3
4
5
6
7
ALL
31
36
12
6
6
14
31
136
Area surveyed (Ha)
0.31
0.36
0.12
0.06
0.06
0.14
0.31
1.36
Dominant habitat type
SEC
SEC
SEC
SEC
SEC
SWA
SEC
Number of plots
GARD /
RES / AGRI
/ BAM
Other habitats
AGRI
RES
Abundance (individuals per Ha plus number recorded in parentheses)
Ochroma pyramidale
9.7 (3)
50 (18)
16.7 (2)
0
0
0
3.2 (1)
17.6 (24)
Astrocaryum murumuru
41.9 (13)
11.1(4)
25 (3)
33.3 (2)
0
0
16.1 (5)
19.6 (27)
Iriartea deltoidea
74.2 (23)
22.2 (8)
0
0
0
128.6 (18)
74.2 (23)
52.9 (72)
Attalea butyracea
48.4 (15)
16.7 (6)
33.3 (4)
0
33.3 (2)
21.4 (3)
9.7 (3)
24.3 (33)
Euterpe edulis
6.5 (2)
0
16.7 (2)
0
0
35.7 (5)
0
6.6 (9)
Socratea exorrhiza
3.2 (1)
0
0
0
0
28.6 (4)
3.2 (1)
4.4 (6)
183.9 (57)
100 (36)
91.7 (11)
33.3 (2)
33.3 (2)
214.3 (30)
106.5 (33)
125.7 (171)
All palm species
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Table 2. Abundance of individuals per ha in different habitats including number of
individuals recorded in parentheses. Key to habitats; SEC – secondary forest,
GARD – garden, RES – residential including homegardens, AGRI – agricultural
land, BAM – dominant bamboo understorey, SWA – seasonal palm swamp.
Number of plots
Area surveyed (Ha)
SEC
86
0.86
GARD
16
0.16
RES
6
0.06
AGRI
5
0.05
BAM
8
0.08
SWA
14
0.14
Abundance (individuals per ha plus number recorded in parentheses)
Ochroma pyramidale
Astrocaryum murumuru
Iriartea deltoidea
Attalea butyracea
Euterpe edulis
Socratea exorrhiza
All palm species
5.8 (5)
26.7 (23)
52.3 (45)
27.9 (24)
3.4 (3)
2.3 (2)
81.2 (13)
12.5 (2)
0
6.2 (1)
0
0
50 (3)
16.6 (1)
16.6 (1)
0
0
0
20 (1)
0
0
20 (1)
0
0
25 (2)
12.5 (1)
62.5 (5)
12.5 (1)
0
0
0
0
128.5 (18)
21 (3)
35.7 (5)
28.5 (4)
112.7 (97)
18.7 (3)
33.3 (2)
20 (1)
87.5 (7)
214.2 (30)
A non-parametric test was carried out to investigate differences in species
abundances between habitats. Ochroma pyramidale & Iriartea deltoidea were
significant to the P <0.01 level (df = 5, H = 21.59 & 21.34 respectively), and
Euterpe edulis & Socratea exorrhiza were significant to the P < 0.05 level (df = 5,
H = 11.42 & 14.19 respectively). A post-hoc test, Dunn’s multiple comparison test
showed that for all species except Iriartea deltoidea, there was no significant
difference between habitat type and species abundance. For Iriartea deltoidea,
Dunn’s multiple comparison test showed that the mean rank of the garden habitat
is lower than the mean rank of the swamp habitat, and that all other habitats did
not differ significantly from each other (P < 0.05, nGARD = 16, nSWA = 14).
22
Habitat Sampling
In the initial pilot study there were seven habitat types identified, secondary
forest, garden, residential, agricultural, bamboo understorey, palm swamp and
slash and burn; there was only one plot located within a slash and burn habitat,
and for all analyses this plot has been omitted from the data. Secondary
regenerated forest was the most dominant habitat type, while agricultural land
was the rarest (Table 3). The highest average canopy height and lowest canopy
openness was recorded in the palm swamp, and the lowest canopy height and
greatest canopy openness recorded in agricultural land. The palm swamp
showed higher numbers of trees with a ‘Branching Above’ architecture, typical of
undisturbed forests, the garden and agricultural habitats had the highest numbers
of trees with ‘Branching Below Vegetated Growth’ architecture, associated with
frequent and extreme disturbance.
Table 3. Number of plots within each habitat type and their mean micro-habitat
variables with standard deviations in parentheses; tree architecture as a mean %
of plot, average canopy height and % of canopy openness. Key to habitats; SEC
– secondary forest, GARD – garden, RES – residential including homegardens,
AGRI – agricultural land, BAM – dominant bamboo understorey, SWA – seasonal
palm swamp.
Number
of plots
SEC
GARD
RES
AGRI
BAM
SWA
86
16
6
5
8
14
Mean
Canopy
Height (m)
BBVG
Canopy
openness
(%)
Tree Architecture (mean % of plot)
BA
BB
BASB
11.75 (3.26)
12.4 (17)
42 (27.4)
40.2 (28.2)
2.8 (6.8)
4.13 (7.03)
8.99 (1.63)
0
67.6 (30.8)
17.4 (21.6)
8.8 (16.2)
5.13 (4.42)
10.26 (2.57)
3.4 (8.2)
20 (29.4)
56.6 (29.4)
3.4 (20)
16 (33.8)
6.65 (0.44)
4 (8.8)
48 (43.8)
4 (8.8)
8 (10.8)
61 (53.43)
10.7 (0.74)
0
40 (21.2)
52.6 (30)
5 (9.2)
2.25 (1.28)
12.32 (0.81)
30 (15.2)
27.2 (23)
42.8 (23.2)
0
1.79 (1.57)
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The heights for each species within the plots were also recorded and a mean
height was calculated for each habitat type (Table 4). A one-way ANOVA was
then used to test for significance within the means. Euterpe edulis and Socratea
exorrhiza were excluded due to low occurrence rates within a large proportion of
the different habitat types. The highest averages were for Ochroma pyramidale
and Iriartea deltoidea. Ochroma pyramidale was recorded at a greater mean
height within residential habitats, and lower average heights within agricultural
land, the average heights recorded for Ochroma pyramidale were the only
significant results.
Table 4. Mean (± standard deviation) average heights of selected species within
different habitat types including significance. Key to habitats; SEC – secondary
forest, GARD – garden, RES – residential including homegardens, AGRI –
agricultural land, BAM – dominant bamboo understorey, SWA – seasonal palm
swamp.
GARD
RES
AGRI
BAM
7
0
11.5
0.7
F = 7.956
P < 0.05
9
0
F = 1.056
P > 0.05
Ochroma
pyramidale
Mean
Std. Dev.
12.2
1.64
10
1.58
14.67
1.52
Astrocaryum
murumuru
Mean
Std. Dev.
8.3
4.01
11.5
0.7
3
0
Attalea
butyracea
Mean
Std. Dev.
7.07
3.85
8
0
Iriartea
deltoidea
Mean
11.1
Std. Dev.
3.11
7
0
4
1.73
F = 0.494
P > 0.05
16
12.2
11.83
F= 1.092
0
2.16
3.33
P > 0.05
6
0
24
SWA
Significance
SEC
Habitat and Species Associations
The influence of micro-habitat variables across all habitat types on the presence
or absence of all species is shown in Table 5. Only 2 species showed significant
associations with any micro-habitat variable. Iriartea deltoidea was the only
species to show significance associations with all variables. The openness of
canopy had a negative effect (P <0.05) on the presence of Iriartea deltoidea, the
greater the canopy openness, the lower the incidence rate of species presence
within a plot (mean canopy openness when species present = 9.02 ±21.7; mean
canopy cover when species absent = 3.02 ±5.14). The mean average canopy
height had a positive effect on Iriartea deltoidea, the greater the average canopy
height, the greater the chance of species presence (mean average canopy height
when species present = 12.7 ±2.8; mean average canopy height when species
absence = 10.8 ±4.7). The tree architecture had influence the presence of two
species, Iriartea deltoidea and Attalea butyracea. Attalea butyracea was
positively correlated with habitats with a tree architecture consisting of ‘Branching
Above’, Iriartea deltoidea was positively correlated with habitats containing trees
with ‘Branching Above’ and ‘Branching Above Scars Below’ architecture. There
were higher rates of Iriartea deltoidea absent in a plot when the tree architecture
was described as ‘Branching Below’ or ‘Branching Below Vegetated Growth’.
None of the other species showed any significant correlation to micro-habitat
variables.
25
Table 5. Influence of micro-habitat variables on species presence or absence
across all habitat types including significance. (NS = no significance)
Tree architecture
Canopy
openness
%
BA
BB
BASB
BBVG
Average
canopy
height
Ochroma pyramidale
NS
NS
NS
NS
NS
NS
Astrocaryum murumuru
NS
NS
NS
NS
NS
NS
Iriartea deltoidea
Negative
P<0.05
Positive
P<0.01
Negative
P<0.01
Positive
P<0.01
Negative
P<0.01
Positive
P<0.01
Attalea butyracea
NS
Positive
P<0.05
NS
NS
NS
NS
Euterpe edulis
NS
NS
NS
NS
NS
NS
Socratea exorrhiza
NS
NS
NS
NS
NS
NS
Table 6. Association between species across all habitat types based on Chisquared test: + +percentage of occasions when both species were present, + percentage of occasions when only one of the two species were present,
including P value.
Attalea
butyracea
Ochroma
pyramidale
Attalea
butyracea
Astrocaryum
murumuru
Iriatea
deltoidea
Euterpe
edulis
Socratea
exorrhiza
+ + 0.7%
+ + 1.4%
+ + 0%
+ + 0%
+ + 0%
+ - 27.9%
P > 0.05
+ - 23.52%
P > 0.05
+ - 40.4%
P < 0.05
+ - 14.7%
P > 0.05
+ - 13.9%
P > 0.05
+ + 5.1%
+ - 25%
P > 0.05
+ + 5.7%
+ - 34.5%
P > 0.05
+ + 5.1%
+ - 36.02
P > 0.05
+ + 2.9%
+ - 17.6%
P < 0.05
+ + 0%
+ - 20.5%
P > 0.05
+ + 2.9%
+ - 28.6%
P > 0.05
+ + 0%
+ - 22.7%
P > 0.05
+ + 1.4%
+ - 16.9%
P > 0.05
+ + 2.2%
+ - 29.4%
P > 0.05
Astrocaryum
murumuru
Iriartea
deltoidea
+ + 0%
+ - 8.1%
P > 0.05
Euterpe
edulis
26
There were only two significant correlations between species (Table 6), Ochroma
pyramidale and Iriartea deltoidea not once occurred in the same plot (X2 = 6.74,
P < 0.05), and Euterpe edulis and Attalea butyracea only occurred in the same
plot on 2.9% of the time (X2 = 9.17, P <0.05). There were 5 other occasions of
negative correlation, Euterpe edulis & Ochroma pyramidale, Socratea exorrhiza
& Ochroma pyramidale, Socratea exorrhiza & Attalea butyracea, Astrocaryum
murumuru & Euterpe edulis, Socratea exorrhiza & Euterpe edulis. None of the
species have strong positive correlations with other species, Euterpe edulis has
the most negative correlations with 4, and Astrocaryum murumuru and Iriartea
deltoidea have the least negative correlations, 1 each. The proportion of plots
where one species was absent and another present was greatest with Ochroma
pyramidale and Iriartea deltoidea, 40.4%, and lowest between Euterpe edulis and
Socratea exorrhiza.
27
DISCUSSION
Microhabitat Heterogeneity
The microhabitat variables recorded in this study reflect only a small
proportion of potential parameters of which can directly affect species
distributions on a spatial and temporal scale. Microhabitat variables not recorded
in this study that may influence species distributions within communities include
topography/elevation (Vazquez & Givinish 1998; Keating 1999), soil type
(Newbery et al. 1986; Sabatier et al. 1997) and climate conditions (Overpeck et
al. 1990; Bongers et al. 1999). The selection of variables recorded was chosen to
reflect the land use and possible influences of anthropogenic alterations or
disturbances. Most of the results were consistent with the expected outcomes of
varying degrees of disturbance, the palm swamp, which has not been subjected
to major anthropogenic disturbances, displayed the most pristine habitat, with the
greatest mean canopy height and the lowest percentage of canopy openness, on
the other end of the scale; agricultural land which has been subjected to the most
severe forms of disturbance within the area suffered from the lowest canopy
height and the greatest percentage of canopy openness.
The main inconsistencies in microhabitat variables in accordance to land
use and expected forest structure was the tree architecture. Recording the tree
28
architecture of a plot is said to give an indication of the recent history of the forest
(Torquebiau 1986; Jones et al. 1995), the majority of the habitat types had
predictable tree architecture, such as secondary forest, which mainly consisted of
trees grown under an open canopy due to disturbance or treefall. The palm
swamp however, did not have tree architecture corresponding to the previous
habitat variables recorded or to the known history of the site. The plots surveyed
were mainly composed of trees signifying growth under a regenerating closing
canopy, not the type of tree structure found within an undisturbed primary forest.
So based on these results it would appear that compared to the other habitats
used in this study, the palm swamp represents a more preserved habitat,
although it cannot be classed as primary or pristine forest, and like most of the
forest in the La Torre region, it is a secondary old growth forest. Regardless of
this, the habitats chosen for this study show microhabitat variations according to
land use and disturbance, and therefore are adequate sites for basing species
and habitat associations on.
29
Species Abundance
Iriartea deltoidea was the most abundant species recorded across the
whole of the study area, and the dominant species in 4 out of 7 transects.
Although it was the most common species within the La Torre region of south
eastern Peru; a study by Montufar & Pintaud (2006), investigating the distribution
of palm communities across western Amazonia found that Iriartea deltoidea was
the most abundant aborescent palm species on terra firme within Ecuador, but in
north-east Peru it becomes patchily distributed or rare over large areas. Iriartea
deltoidea has a wide range across the neotropical forests of South America, but
the distribution of individuals across landscapes on a countrywide scale is
obviously not even, but could be dependent on randomness or more likely;
relying on the macro and micro scale environmental heterogeneity discussed in
previous and subsequent sections.
The most frequently occurring species was Attalea butyracea, which was
found in 6 out of 7 transects and in 5 out of the 6 habitats. Attalea butyracea has
proven in this study to be the best species in dispersal and recruitment. The
stony endocarps of Attalea butyracea can persist on the forest floor for several
years before decomposing and are distributed by several species including
primates, rodents and small mammals (Harms & Dalling 1995). The mean
heights of individuals were less than that of other species, suggesting that
although dispersal mechanisms for Attalea butyracea are more effective than in
30
other palm species, individuals are easily out competed by existing and pioneer
species, resulting in numerous stunted individuals.
Species Abundance by Habitat Type
Secondary forest had the greatest number of palms present, whereas the
palm swamp had the greatest density of palms; the palm swamp supported the
largest communities of Iriartea deltoidea, Euterpe edulis and Socratea exorrhiza,
although Euterpe edulis and Socratea exorrhiza had the lowest occurrence rates
within all the transects. Euterpe edulis typically occurs on wetter soils (Dias et al.
1988) due to the fact that germination is higher on such soils (Silva Matos &
Watkinson 1998), and can probably explain why the largest number of Euterpe
edulis individuals was found in the palm swamp. In opposition of this explanation,
Clark et al. (1995), found that Euterpe edulis was closely associated with steep
topography and less fertile soils, contradicting their high abundance within the
palm swamp, which tends to be rich in nutrients due to seasonal flooding
(Whitmore 1998). Rather than suggesting Euterpe edulis is more successful in
the palm swamp than other habitats, the low abundance within habitats outside of
the palm swamp could be a result of past over-extraction of palm hearts, which
results in death of the individual tree. Although there is no current harvesting of
palm hearts in the La Torre region studied, past harvesting may have depleted
the seed bank, meaning any further regeneration of Euterpe edulis communities
31
will require seed dispersal from habitats such as the palm swamp or by
management prescriptions such as seedling transplants.
The palm swamp was the only habitat in which Ochroma pyramidale was
absent from, Pearson et al. (2003) suggests that Ochroma pyramidale has a high
demand for nutrients, and larger gaps are required for release from root
competition, the palm swamp had the lowest canopy openness and the highest
density of palms, resulting in unfavourable conditions for Ochroma pyramidale to
thrive, the garden habitat had the highest density of Ochroma pyramidale, which
had the lowest number of palms and a high percentage of canopy openness. The
pioneer or early-successional species (such as Ochroma pyramidale), has been
clearly and consistently associated with a specific habitat type; disturbed areas
such as large canopy openings by Whitmore (1998), and follows the same trend
here, with the largest communities present in the most disturbed habitats.
Dransfield (1978) suggests that stilt roots in palms favour the exploitation of
canopy gaps similar to pioneer species; the development of the root cone early in
the life cycle permits a rapid height increase without loss of stability (Swaine
1983). Socratea exorrhiza and Iriartea deltoidea are both stilt root palms, yet
neither of these species was found in abundance in any of the disturbed habitats,
Socratea exorrhiza was only found within the palm swamp and secondary forest
habitats, suggesting that these two stilt root species do not favour canopy gaps,
and could not be described as pioneers.
There are clear differences in palm species assemblages in the range of
habitats sampled here, the density of palm species also varies between habitats.
32
The habitats that have been subjected to anthropogenic disturbances supported
fewer species and fewer individuals than the more pristine habitats. The seedling
stage and mechanisms driving dispersal and germination are the most affected
by disturbances attributed to anthropogenic influences, both positively and
negatively. Wright & Duber 2001 found that anthropogenic disturbance indirectly
reduced seed dispersal and seed predation; this concentrated seeds near
reproductive trees and enhanced seed survival. As a consequence, seedling
recruitment increased with the intensity of anthropogenic disturbance, with
disproportionate increases near seed-bearing trees, and a reduced dispersal of
new recruits to other habitats. This would result in concentrations of species
within disturbed habitats, which was not apparent within the habitats sampled in
this study. In a study of Amazonian palms, Scariot (1999) established that the
seedling stage was most affected by habitat disturbances and fragmentation, and
that this possible lower seedling recruitment may lead to taxa composition in
small forest fragments diverging from continuous forest, which is what has
occurred within the La Torre region, with each habitat having a unique
assemblage of palm species that are derived from the list of species found within
the secondary forest habitat.
33
Microhabitat and Species Associations
There have been many authors that have studied a range of microhabitat
variables such as topography, soils, climate and canopy height, and found
significant
relationships
between
the
distributions of
individuals across
landscapes according to the mosaic of microhabitats (Webb & Peart 2000;
Phillips et al. 2003; Svenning 1999), whereas other studies have found little
evidence for microhabitat partitioning among non-pioneer tree species (Welden
et al. 1991; Clark & Clark 1992; Clark et al. 1993).
In this study there were only two species to show significant microhabitat
associations: Iriartea deltoidea and Attalea butyracea. Iriartea deltoidea displayed
significant associations with all variables; a higher mean canopy height and lower
mean canopy openness had a positive effect on the species presence, tree
architecture reflecting regenerating closed canopies and undisturbed forest also
had a positive effect on species presence, whereas tree architecture indicating a
frequently disturbed forest structure had a significant negative effect on species
presence. Regarding canopy height; Welden et al. (1991) found that canopy
height was unimportant for the two palm species included in their study whereas
a study by Svenning (1999) showed that canopy height influenced the distribution
of some palm species, but was of much less importance than topography. The
only other species to show significant associations with microhabitat variables
was Attalea butyracea, which was positively associated with tree architecture
indicative of undisturbed primary forest. These results suggest that Iriartea
34
deltoidea are significantly associated with forest structures containing tall, closed
canopies and have limited anthropogenic disturbance or alteration. Due to the
fact that Attalea butyracea was the most frequently occurring species across all
habitat types, it is difficult to say with conviction that Attalea butyracea is
associated with a particular forest structure. It has been discussed previously that
Attalea butyracea has the best dispersal mechanisms seen in this study, and is
therefore able to establish communities in most habitats. Consequently, the
microhabitat association seen here merely suggests that Attalea butyracea has a
preference to undisturbed primary forests, and that although it is able to colonise
and exploit canopy gaps and openings through dispersal and recruitment, it is a
non-pioneer species, with a habitat preference for undisturbed forests.
A study by Svenning (1999) investigating microhabitat specialisation in
Ecuadorian palm communities found that 20 out of 31 taxa analysed had
significant microhabitat relationships, and that most of the palm species were
distributed according to the microhabitat variables at scales of tens to hundreds
of metres. Wider research into tropical tree communities, including palm species,
has also found examples of microhabitat specialisation; Webb & Peart (2000)
discovered that there were significant associations of species with variation in
physiography, within an area that is limited in spatial and elevational range,
although, all habitat associated species occurred at least once in a non-preferred
habitat. A study by Phillips et al. (2003) found that a large fraction of tree species
have a significant tendency to be favoured by one habitat or another, and that the
proportion of species with significant habitat associations with their study
35
approaches 80%. There have been studies in the neotropical tree communities
that have found no microhabitat associations, particularly amongst non-pioneer
species; Clark & Clark (1992) found that four of six non-pioneer species at La
Selva, Costa Rica did not differ in sapling distributions with respect to canopy
structure, and a study by Baraloto & Goldberg (2004) found that among nonpioneer species few indications of differences in the microhabitat conditions
permitting seedling bank establishment.
The difference between pioneer and non-pioneer species in respect to
habitat and niche partitioning has been noted by several authors before (Grubb
1986; Pacala & Rees 1998; Rees et al. 2001), the biggest single influential
environmental factor driving niche occupation is light availability; variation in light
within the forest provides colonisation-based niches. There was no strong
evidence within the results to suggest that any of the species were specialists
and restricted by niche width, although there were clear habitat preferences
towards the palm swamp (Euterpe edulis, Socratea exorrhiza, Iriartea deltoidea).
Tilman & Pacala (1993) suggest that habitat association alone does not provide
strong support for the hypothesis of niche differentiation as a mechanism for
maintaining species diversity. Vormisto (2002; 2004) suggests that palms tend to
be widespread and habitat generalists, and that palm communities are more
influenced by dispersal across broader spatial scales than by environment
heterogeneity. However, in this study; in which only a small selection of palms
were identified and sampled, there was one species for which
36
a clear
association with all microhabitat variables recorded has been identified, with one
other species showing a significant association with one variable.
Ochroma pyramidale had the most negative associations with other
species. Ochroma pyramidale is a pioneer species, which typically exploit canopy
gaps and areas of recent disturbance. Ochroma pyramidale are very fast growing
(Whitmore & Wooi-Khoon 1983), and are able to out-compete palm seedlings
within newly created canopy openings, therefore when Ochroma pyramidale are
present within a plot, it is unlikely there will be established communities of palm
species.
Based on the strong microhabitat associations shown by Iriartea deltoidea
with undisturbed forests with a closed canopy, it is perhaps unsurprising that
Ochroma pyramidale was never found within the same plot. Ochroma pyramidale
is a pioneer species; its habitat preferences are recently disturbed habitats and
canopy gaps, which were negatively associated with Iriartea deltoidea. Although
there is no evidence in the results to directly link microhabitat associations to
species associations; Iriartea deltoidea and Attalea butyracea both had the same
significant positive association with primary forest tree architecture, but there
were no significant positive or negative species associations. Harms et al. (2001)
found that in a study investigation habitat associations in a neotropical forest;
their results did not support the hypothesis that habitat specialisation is among
the principal mechanisms of species coexistence and associations, which
apparently maintains a large fraction of the diversity within a tropical plant
community. A study in the tropical forest of Costa Rica by Clark et al. (1995)
37
found that Socratea exorrhiza and Iriartea deltoidea had a low probability of
interspecific encounter, and that possible forms of interaction between the two
are cited as competition for pollinators or indirect effects mediated by specific
mycorrhizal fungi or natural enemies; but there were no significant negative
associations between the two species in the La Torre region, the two species
coexisted in both habitats in which Socratea exorrhiza was present, and were
found to have their highest abundances in the same habitat, the palm swamp;
displaying a higher probability of interspecific encounter than Clark et al. (1995)
stated. Again, Iriartea deltoidea had a different abundance characteristic in the La
Torre region compared to another part of South America where its range extends
to. Previously we discussed how Iriartea deltoidea was more common in La Torre
than in North Peru, and now it is apparent that Iriartea deltoidea is able to coexist
with other palm species within the region that it would normally be negatively
associated with.
38
Conclusions
The first biogeographical hypothesis regarding palm distributions in Amazonia
may be attributed to de Candolle (1857); he suggested that Amazonian palms
were ‘almost equally diffused throughout the tropics’. Extensive research into the
tropics and particularly palm communities has disproved this hypothesis.
Amazonia is a highly heterogeneous region, in which climate, soils, landscape,
dispersal and geological history can only partially explain the distribution patterns
of palms (Montufar & Pintaud 2006). Even in a small area, such as the La Torre
region sampled in this study, there are differences in palm species abundance
and assemblages based on the varying levels of anthropogenic alteration and the
high microhabitat heterogeneity present. The palm swamp is clearly a important
patch habitat within the matrix of secondary forest and anthropogenically altered
habitats, it is therefore crucial that this habitat is conserved to prevent the onset
of succession and encroachment of invasive pioneer species seen in other parts
of the La Torre region. The palm swamp supports the highest density and
diversity of palm species, and crucially, supports the largest community of Iriartea
deltoidea, which in this study has been identified as the most important species.
It is highly abundant in the La Torre region, whereas in north-east Peru it
becomes patchily distributed or rare over large areas, it was the only species to
show significant habitat preferences and microhabitat associations, and coexists
with Socratea exorrhiza when other studies have found there to be negative
species associations between the two species. in the La Torre region the primary
39
target for conservation within the palm community would be Iriartea deltoidea,
and the habitats and species it is closely associated with, due to its patchy
abundance across Peru, and its unpredictable distribution characteristics within
La Torre.
The family of palms have been described as keystone species, crucial to
the forest structure and ecosystem, this study highlights the sensitivity of palm
communities to relatively low impact anthropogenic disturbances over a small
area. Based on this, it would be fair to assume that high impact anthropogenic
disturbances such intensive agriculture, commercial timber extraction and
urbanisation could potentially result in the breakdown of palm communities,
facilitate the extinction of specialist species from a region, and encourage the
recruitment of invasive and pioneer species.
40
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