Factors recorded by both means of transects and

Factors affecting plant species diversity and distribution of native and
nonnative species along the Wa’ahila Ridge Trail (Oahu, Hawaii) were
investigated in ten different elevational plots.  All plant species within each of the ten
plots were recorded by both means of transects and quadrat sampling with
estimates of their biomass and canopy cover. 
Plant species diversity was later calculated using Simpson’s indexA1 .
The hypothesis that increased canopy cover would increase plant species
diversity was supported by the data and observations of plant distribution
trends along the ten different elevation plots. 
A decrease in canopy cover was found primarily within the lower
elevation plots and were all dominated by nonnative grass.  This relationship was concluded to be
attributable to a large-scale forest fire in 2007.  The secondary succession that took place
within these plots promoted the growth of invasive grasses such as Megathyrsus maximus and out competition with other species may have reason for
the low plant species diversity within these plots.  Nonnative species appeared to be better
adapted to these open canopy climates and are not easily displaced by native
species or another species found along the trail. The relationship between
precipitation and plant species diversity was insignificant to examine
distribution trends of but observational analysis of each plot revealed that
increasing sample size might produce significant data for analysis. 

 

Introduction

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By observation alone, a visual assessment of climate factors and
vegetation may support a strong correlation between the two.  On a global scale, vegetative communities are
strongly correlated with different regions of temperature climate-moist tropics
are associated with tropical forests, dry subtropics with deserts, etc.  At the root of plant ecology lies the
relationship between climate and spatial distribution among different
vegetative communities and even various physiographic factors such as canopy
cover, precipitation levels, and soil type.

Ko?ppen’s climate classification system defined boundaries between
climates correspondingly to vegetative zones (Ko?ppen, 1936).  Its categories are based on annual and
monthly means of temperature and precipitation among five climatic
regions-tropical moist climates, dry climates, moist mid-latitude climates with
mild winters, moist mid-latitudes with cold winters, and polar climates.  Ko?ppen’s climate classification system was
one of the first quantitative establishments of defining the relationship between
climate and vegetation on a global scale (Ko?ppen, 1936).  Similarly, Prentice et al. (1992) identified 13 different vegetative types in relation
to five different climatic factors-the daily sum of degrees, mean temperatures,
and the ratio of annual evapotranspiration to annual potential
evapotranspiration in what they called the BIOME model.  Their findings of the BIOME model allowed for
extensive use in reproductions investigating the equilibrium response of
vegetative communities to changes in climate. 

The
phenology of alien species varies in cycle with the native Hawaiian plant
species due to differences in the adapted climates. In response, nonnative
species that overtake native populations remain latent during the wettest
months, resulting in increased surface runoff and erosion.  Andropogon virginicus is an invasive species that
has impacted Hawaii’s ecosystem through such mechanisms (Smith 2007).  Rejmarek (1989) found that nonnative species
exhibit greater coverage in proportion to native species within early mesic
ecosystems.  Over 30 dominant species and
their year of maximum coverage were recorded over 20 years of vegetative
succession in Hutcheson Memorial Forest, NJ. 
The data revealed that both proportion and number of alien species are
greatest during the initial stages of mesic succession and tended to decrease
in numbers at both ends of the moisture gradient. He noted that native species
colonized more successfully on either extreme of the moisture gradient. It was
concluded that nonnative species within the experiment were not well adapted to
the native environment and thus could not successfully colonize among either
sides of the moisture extremes as the natives could (Rejmarek, 1989).

A study conducted
by Egler (1992) confirmed that locations among the Ko’olau Mountains exhibit
patterns between vegetative zones and particular species that inhabit
them.  Egler (1992) investigated
vegetative zones of the Ohia forests in the arid and semiarid lowland of Oahu,
Hawaii.  Part of the study area resided
along the Ko’olau mountain range.  Mt.
Tantalus, residing along the windward range of the Ko’olau Mountains, was
identified as one of the climatically distinct vegetative zones.  The dominant species exhibiting the highest
biomass was the observed native Metrosideros collina.  This occurrence
was attributed to the moist regions of Mt. Tantalus where precipitation
exceeded 50 inches per year. Eglers study on vegetative zones confirmed that
there are climatically distinct zones along the Ko’olau Mountains relative to
the distribution of different plant species and their ability to adapt better
to Hawaii’s native climate. 

Using such findings, it may be
expected to see an immediate relationship and spatial distribution between
vegetative communities along the various elevations of the Wa’ahila Ridge Trail
(Oahu, Hawaii).  After the fire in 2007 along
the lower elevation plots, a drier, more herbaceous vegetative community may be
expected within successional areas -as has been documented previously in the
literature.  At higher elevation plots,
where precipitation was expected to be greater, larger plant growth and canopy
coverage can also be expected.  However,
by limiting sun exposure, it may alter the vegetative community and we can predictA2  to see a wider
range of species inhabiting the area.   This investigation focused primarily on
identifying such distribution patterns within the ten different plots along the
Wa’ahila Ridge TrailA3 .  General observations were made in terms of
differences in species richness, vegetative coverage, and biodiversity.  Abiotic factors such as temperature, soil
depth, and precipitation levels were recorded. One hypothesis tested is
that there is no correlation between plant species diversity and precipitation.  Another hypothesis tested was that there is a
positive correlation between canopy cover and species diversity.

 

Materials and Methods:

 

The vegetation and physiographic
data was recorded using transect and quadrat sampling methods.  A total of 10 plots, that were 20×20 meters
each, were covered A4 alongside
the Wa’ahila Ridge Trail on O’ahu, HawaiI, October 2017. Each quadrat consisted
of 1×1 meter dimensions and each transect was analyzed 15 meters in
length.  Data collected over the 10 plots
at various elevations was compiled and analyzed. All plant species within each
conical quadrat station area and transect line were recorded and
identified.  Individuals that fell within
a transect line were identified and recorded in meter length using a transect
tape.  Physiographic features such as
light exposure (%), rainfall, and soil depth were recorded. The following
community structures were also recorded: dominate species, population density,
and % coverage.

    Mechanical devices were used to provide
readings for most of the physiographic data. An iButton was used to record air
temperatures within the surveyed area and a rain gauge was used to measure
rainfall.  Correlation analyses were
conducted on the relationship between plant species diversity and precipitation
levels, as well as between plant species diversity index and estimated % canopy
cover. Plant species diversity was calculated by Simpson’s index using a
Microsoft Excel spreadsheet.

 

Results

This
investigation focused primarily on plant species and ecological distribution
patterns along climatically different plots along the Wa’ahila Ridge Trail on
the island of O’ahu, Hawaii. Transect sampling of plant species within ten
different elevations along the trail revealed few statistically significant
patterns and several more observations between the various plots along the
trail. The relationship between plant species diversity (Simpson’s index) and %
canopy cover was considered statistically
significant A5 (Correlation:
=; =2.64; df=18; and p=0.05).
There was a positive relationship between plant species diversity and % canopy
cover along the ten plots on the trail-as canopy cover increased, plant species
diversity increased as well (Fig. 1). 
Plots at higher elevations (Plots 8-10) were observed to have greater
absolute canopy cover and thus a greater species diversity value for each
corresponding plot (Table 1). The highest % canopy cover was recorded at plot 9
at 21.775% and with plot 8 just following at 20.525%.  All species found within these two plots were
similar in richness and diversity, with plot 9 containing several extra species
not found in plot 8 such as Citharexylum
caudatum (fiddle wood), Heteropogon
contortus (pili grass) and Pimenta
dioica (all spice). The highest
species diversity index (Simpson’s index) was recorded for plot 10 at
3.19.  The lowest plant species index
value was recorded at plot 5 with a value of 1.23.

 According to Fig. 2, there was no significant
relationship between plant species diversity and precipitation levels
(Correlation; =; =3.18; df=18; p=0.327). 
With a P-value of greater than 0.05, the null hypothesis that there is
no correlation between plant species diversity and precipitation levels failed
to be rejected.  There were no graphical
trends that could be observed for further analyzing the relationship between
plant species diversity and precipitation levels (Fig. 2). 

 

 

 

Figure 1A6 : Relationship between plant species
diversity and % canopy coverage. Plant species diversity was measured by
Simpson’s index (D). The correlation analysis for the given data is provided
below: (Correlation; =; =2.64; df=18; p=0.05)

 

 

Figure 2: Relationship between plant species diversity
and precipitation levels. Plant species diversity was measured by Simpson’s
index (D). The correlation analysis for the given data is provided below:
(Correlation; =; =3.18; df=18; p=0.327)

 

 

Figure 3: 
Canopy Closure along the ten elevational plots on the Wa’ahila Ridge Trail
was measures as a percent estimate within each plot and increased as the plots
progressed in elevation.  A transition
point appeared to be present after plot 7 where possible environmental or
climatic shifts may have caused a change in canopy cover.

Discussion

                     The first
hypothesis, which was valued as a framework for this investigation is repeated
as follows:  Plant species diversity
increases with increasing canopy coverage because a more suitable habitat is
available for species to propagate. Compared to the dry and high sun exposure
at lower elevation plots, a more humid environment with sufficient sun exposure
and shade will provide the right environment for a wider range of species to
inhabit.  Plant species diversity was
also hypothesized to increase with increasing precipitation levels, since
native plants are better adapted to such conditions in Hawaii, it was expected
to observe or record several native species only at the higher elevational
plots. These two hypotheses have been developed to investigate distribution
trends along climatically different regions within the Wa’ahila Ridge Trail.

    The analysis of plant species
diversity and % canopy cover authenticates the prediction that plant species
diversity increases within conditions of higher canopy cover. Contrary to the
expected results, nonnative plants dominated the upper elevational plots,
exhibiting similar biomass coverage as the lower elevation plots.  One indigenous species, Waltheria indica (uhaloa), and one native Hawaiian species, Furcraea foetida (agave) were
recorded along the 10 plots on the trail. Although several other native plants
were observed, these were the only two species that were represented in the
population. This was contrary to what was expected since these two species were
surveyed within the lower elevational plots. W. indica was identified within plots 1-6 while F.foetida) was recorded in plots
1,2,4,5, and 7. It is possible that these species are better adapted to drier climatesA7 
and do not propagate well within shadier understories consistent with plots
above plot 7.  Any native individuals
previously prevalent along the upper plots would have been outcompeted by
nonnatives such as the frequent Leucaena
leucocephala or Senna surattensis
associated with the upper elevational plots with increased canopy cover.  The distribution patterns of this species
were demonstrated over a wide range of plots, prevalent from plot 1 all through
plot 10. The climates within both elevations seemed to be well suited for this
species.

Megathyrsus maximus dominated both lower and upper elevational plots.  This species is considered a highly
successful invader in tropical and warmer climates and competes highly with
native flora.  M. maximus is highly fire resistant and spreads quickly to invade
open land during the initial stages of succession after a fire (Amondt &
Litton 2011).  Secondary succession
following the forest fire of 2007 along the Wa’ahila Ridge Trail attributed
greatly to the observed distribution trends within the plots. Plant species
diversity decreased within the areas of succession following the fire. It was
concluded that nonnative species such as M.
maximus dominated the open land and would outcompete both native and
another nonnative species.

The physiographic data
collected at each station denoted possible explanations for such trends.  Within the lower elevations (plots 1-7),
there was a much lower level of canopy closure and a lower mean precipitation level.  It may be concluded from this data that the
given conditions support a smaller range of species (primarily nonnative
grasses and shrubs such as Megathyrsus
maximus) and thus confirming reduced species diversity. The fire that took
place among the lower half of the plots was concluded to have a great effect on
such trends.  According to Ainsworth
(2007), primary succession following a fire may facilitate an environment
favored by most nonnative weeds and shrubs. Megathyrsus
maximus showed to dominate the lower elevational plots, yet minimal
additional species were able to inhabit the hot and dry conditions along plots
1 through 7 allowing M. maximus to
dominate the community.  Within the
canopies present along the upper plots (plots 7-10), the shadier and cooler
environment may have provided an environment suitable for a greater diversity
of species to grow-thus confirming the increase in recorded species along these
plots.

Differences in canopy
closure may have attributed greatly in explaining the observed trend of native
and nonnative prevalence between the various communities.  Among the lower elevation plots where canopy
closure was observed as very scattered (less than 5%), plant species diversity
showed to decrease. It was concluded that the high prevalence of nonnative
species within the community outcompeted much of the land available for native
and another competing species. With fewer nonnative species to overtake the
vegetation within the plant community, a greater number of individuals are
given the opportunity to grow-possibly increasing species richness and in this
case exhibiting reduced species diversity.

Although it would
be expected for native species to be found in greater numbers within the higher
elevation plots, the data supported otherwise. 
Plot 10 displayed few native species, yet was dominated by nonnative trees
such as Grevillea robusta (silk oak) and
Megathyrsus maximus.  It was concluded that the arid climate that
provided greatest sun exposure and minimal % canopy cover were favored by the
nonnative species-as they were better
adapted to the physiographic properties similar to their mainlandA8 . With little to no native predators,
introduced species such as the lower elevation-dominant Megathyrsus
maximus and
Leucaena leucocephala had the opportunity to overtake much of the
vegetation and land-as seen with their distinctly high prevalence and biomass
along the community.  

The relationship
between plant species diversity and precipitation levels was statistically
insignificant.  There was a slight negative relationship A9 between the two factorsA10 
but the data collected was unsupportive to reject the null hypothesis (fig.2).  A greater sample size may have solved this
insignificance.  Inconsistencies with
data collection at additional plots may also have affected the trend, such as
identification of various species along the trail. If one species was recorded
as another within another plot (or vice versa), the plant species diversity may
change in relation to abiotic factors being testedA11 .

 A1Good

 A2Good
J Predictions are
supported

 A3I’m a little
confused on what your 2 hypotheses are

 A4Awkward
wording. Maybe you could say, A total of 10 plots that were 20x20m each on the
Wa’ahila Ridge Trail on O’ahu, Hawaii were used in data collection in October
2017.

 A5good

 A6captions
go below graphs and above tables

 A7you
gave reasoning, good J

 A8good
explanation/reasoning

 A9good
observable trend

 A10(fig.
x)

 A11Overall,
I thought this was really good! The wording in some areas was awkward and hard
to follow, but for the most part I understood what the point of the study was
and what conclusions you ended up with. Also be careful with grammar! 

x

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