Download Rainfall has an effect on the length of the boab`s (Adansonia gregorii

Cygnus (2013) 1: 12-18
DOI [21294131, 21114466, 21320437] RESEARCH ARTICLE__________________________________
Rainfall has an effect on the length of the boab’s (Adansonia
gregorii) flowering and leafing period.
Madeleine Brockman • Sean Smith • Maria Harris
Received: 20 May 2013 / Accepted: 29 May 2013
Subject Editor: Christopher Jones, Manuscript Editor: Susan Barker
Abstract The genus Adansonia commonly known as the boab, undergoes seasonal leaf and
flower growth and removal. The aim of this paper was to investigate whether the trees change
the start time and length of these phenophases in response to rainfall in the previous and
present seasons to allow proper management of water. It was expected that a drier wet season
will cause the boabs in the next season to grow new leaves and flower sooner and that a lower
amount of rainfall in a present wet season will cause flowers and leaves to be present on the
trees for a shorter period of time. ClimateWatch data, which uses the reports of ‘citizen
scientists’ to plot the timing and duration of phases in the boab lifestyle was analysed to
determine the relationship in Australia while a separate study on African boabs was used to
analyse African data. The results did not show any distinct change in the timing of any
phenophase, though the conclusions were hampered by incomplete, brief and sometimes
contradictory data.
Keywords – boab, phenology, rainfall, Adansonia, leafing, flowering, climate change
1 Introduction
Phenology is the influence of seasonal changes on the annual events of flora and fauna, for
example flowering times, migration and mating. A study of phenology through data sets can
lead to an evaluation of the effect of climate change on a particular species and the effect that
this species has within its community and food web, and is essential to present day problems
such as conservation of threatened species.
Climate change is a variation in weather patterns over time, changes including temperature,
rainfall, solar radiation, and oceanic processes or circulation. While these changes are caused
in part by natural cycles, the speed at which some of the changes have escalated is mostly
12 agreed to be caused by human activity, i.e. the greenhouse effect (IPCC 2007).The impact
that climate change is having on rainfall is more unpredictable than most other weather
patterns as the trends in rainfall include changes in distribution as well as abundance
(Trenberth 1998). The increased global temperature means that the atmosphere is able to hold
more moisture, and the increase of infrared radiation on the Earth’s surface contributes to
evaporating surface moisture, and combined this means that actual moisture within the
atmosphere should increase. While this means that more rain should fall in certain weather
systems, the causal basis of the changing pattern is still not fully understood (Trenberth
1998). Rainfall trends in the Northern Territory of Australia have slightly increased from
1900 to 2012 (Australian Bureau of Meteorology 2013) and have slightly decreased in
Zimbabwe from 1900 to 2000 (Doyle 2007).
The Australian and African Boab, Adansonia gregorii and A. digitata respectively, are part of
a genus with eight extant species that rely upon the timing of rainfall events to survive in
their environment. This report analyses the extent to which rainfall events influence
phenology in order to make predictions about what effect climate change will have on the
trees in the future. For the purposes of having a large enough bank of data any difference
between species which would affect the timing of phenological events has been deemed
negligible, especially considering that the Australian species is theorised either to have been
the same direct ancestor species when the continents were joined or else, brought to Australia
from Africa through human migration (Bowman 1997).
The boab is categorised as a deciduous tree, losing its leaves in the dry season and producing
new growth at the start of the wet season (note that the wet season in the tropic areas runs
from around November to April, and the wet season in Perth occurs from June to August).
Deciduous growth patterns have emerged in plants that evolved in climates where plants must
live without water or in near drought conditions for several months a year. Leaves have many
small openings called stomata and they facilitate gas exchange for photosynthesis. Through
transpiration these openings (stomatal pores) stay saturated with water and as wind blows
past the water vapour slowly diffuses away. Thus even with the ability to open and close the
pores throughout the day, some water is always being passively lost.
Boab trees are characterised by a large, swollen, shiny trunk with a disproportionally large
circumference to height and large taproots for absorbing water in the lowest levels in the
water table. The trees do an excellent job of collecting and storing water to safeguard against
drought and provide the water to create new leaves, or for leaf flushing (Chapotin et al.
2006). Boab’s survive during the dry season on the vast amounts of stored water in their
swollen trunks, and photosynthesise from the trunk cells, until the next wet season. This
adaption must happen correctly during the critical months for initial leaf-flush (Sanchez et al.
2011), perhaps to maximise the efficiency of photosynthesis during the short and highly
variable wet seasons (Chapotin et al. 2006).
The initial flushed leaves do not actually begin to respire until the onset of rain but are
present so as to be ready for the start of the wet season (Sanchez et al. 2011). The
13 phenological event of leaf flushing can thus be hypothesised to be determined by the previous
wet seasons’ total rainfall as, in theory, the tree will wait until its water stores are almost
depleted to flush leaves. The timing of flowering is also deemed to be influenced by the onset
of rainfall as it requires a ready supply of water and the animal vectors for transmission of
boab pollen are becoming abundant during this same period. For the trees to have flowers
outside of this period would create a trophic mismatch (Mitchell 2013).
This report will be looking at data from ClimateWatch (www.climatewatch.org.au) that was
collected by citizen scientists. The data will be analysed to determine when and how long the
two phenophases of interest occur. The ClimateWatch data are observations of recently
replanted baobabs in Kings Park, Perth and several specimens in the Kimberleys, Western
Australia. The data will be compared with those from a previous study that included 126
African Boabs in Zimbabwe (Swanepoel, 1993).
This investigation seeks to evaluate the relationship between the amount of rainfall as a
determining factor for the timing of the new leafing and flowering events. It is an expectation
that a drier wet season from one period will cause the boabs to grow new leaves sooner in the
next and that a lower amount of rainfall in the present season will cause them to be present on
the trees for a shorter period of time.
2 Materials and Methods
Data collected from Climate Watch (2013) consisted of sightings of the Australian Boab tree
by citizens that were then uploaded to the Climate Watch website. The information uploaded
included the location of the boab and what phenophase was seen. Options people could
choose were various stages in the flowering, leafing, and seed presence (ClimateWatch
2013).
Climate Watch data was provided in Microsoft Excel format. We edited the data to remove
very inconsistent sightings that were outnumbered by the exact opposite of other sightings
around it. A table was constructed to show the flowering and leafing periods of the boab tree.
This made it clear that the boab tree has a distinct phenophase, which slightly differs between
years.
The Australian Bureau of Meteorology (www.bom.gov.au) enabled access to the temperature
and rainfall data for Perth, where most sightings were made. An average monthly temperature
across the range of the ClimateWatch dataset was determined. This allowed a connection to
be seen between the phenophases of the Australian Boab with the rainfall (Fig 1) and
temperature (Fig 2) of Perth (Australian Bureau of Meteorology, 2013).
Researching the boab trees in Africa led to information being gathered on the phenophases of
this tree. All data for the rainfall and seasonal events for the African Boab are presented as
described by Swanepoel (1993). A comparison between the boab phenophases in Africa and
Australia was conducted.
14 3 Results
Over 90% of the ClimateWatch data was on the boab trees found in Kings Park, Perth. These
sightings showed that the Australian Boab leafing began in August of 2011, and September
of 2012. The flowering of this species started in October of 2011 and September of 2012.
This is shown in Table 1. It can be seen that the Australian Boab responded to the
temperature and rainfall of the climate, leafing and flowering when the temperature increased
after a large quantity of rain as shown in Figs. 1 and 2.
Table 1 ClimateWatch data has been analysed to show the different phenophases of the Australian Boab
observed from August of 2011 until March of 2013 (ClimateWatch 2013)
Year
(month/year)
Start of
Leafing
End of
Leafing
Leafing
period
Start of
Flowering
End of
Flowering
Flowering
period
2011-12
8/11
5/12
9 months
10/11
5/12
7 months
2012-2013
9/12
After 3/13
>6 months
9/12
After 3/13
>6 months
Fig. 1
The average rainfall received in Metropolitan Perth during the years of 2010, 2011 and 2012. Distinct
high and low rain periods can be observed. Data from Australian Bureau of Meteorology (2013)
15 Fig. 2
The average temperature Metropolitan Perth received during the period of 2010 to 2012 (Australian
Bureau of Meteorology 2013). This graph shows a close relationship to the rainfall received during this
period.
Table 2 The data for African Boabs in Zimbabwe, (Swanepoel 1993). Leaf loss and rainfall from 1983 – 1986
are shown.
Date of leaf loss
Rainfall total of
previous season
Rainfall of
current
season
October 1983
13/5/1984
375 mm
399 mm
7
October 1984
8/7/1985
399 mm
883 mm
9
November 1985
3/7/1986
883 mm
891mm
8
October 1986
27/5/1987
891 mm
363 mm
7
Date of leaf flush (more
than 50% of trees)
Time trees spent
with leaves
(months)
4 Discussion
Considering the data from ClimateWatch (2013) and the previous research in Africa
(Swanepoel 1993), the hypothesis that the length and timing of the leafing and flowering
period of boabs is directly correspondent to the rainfall is inconclusively supported. In years
of greater rainfall, the boab was shown to both have a longer leafing and flowering period,
therefore in part supporting the hypothesis. However, the amount of rainfall in the previous
year had no detectable influence on the timing of the leafing and flowering period, in part
disproving the hypothesis. This conclusion however, is inconclusive on account of the
limitations of the data available. Furthermore, there are no previous studies that support our
conclusions historically apart from the data obtained in Africa (Swanepoel 1993), so no
comparison of data sets can be made to verify the reliability of the data that are available.
16 The quality and reliability of the data is affected by several data entries in the ClimateWatch
website that are incomplete or in direct contradiction to the rest of the data set, such as the
boab having no leaves in the middle of the leafing period. The reliability of the data from
Africa (Swanepoel 1993) was also affected by additional environmental disruptions, as
during the study of the African Boabs, elephants damaged many of the trees. It is unknown
how this influenced the trees’ flowering and leafing time, or whether it influenced them at all.
The large boab in Perth was transplanted from the Kimberleys in 2008, at which time it
experienced a complete shift in weather pattern from the timing of the wet and dry seasons,
from tropical to Mediterranean climates. Though this happened three years before the data
from ClimateWatch was collected, the influence of this traumatising change could have
affected results for the flowering and leaf growth of that particular tree.
The data from ClimateWatch only covers two reproductive seasons of the Australian Boab,
which is an inadequate amount of time to properly analyse and find trends relating to
anychange in phenology. Data was generalised for our study, and as a result of this the exact
timing of the leafing and flowering could not be obtained, severely limiting the accuracy of
our conclusions. The African data (Swanepoel 1993) covers only four reproductive seasons
of the boab, and while a co-relationship is apparent, trends cannot be attributed solely to
rainfall, as there are other factors that may influence this correlation.
Other environmental factors that might influence the phenology of boabs include temperature
(Table 1) which is known to have an effect on seasonal events of other native Australian
species (Gallagher et al. 2009). While the overall temperature of 2011 in Australia was lower
than in 2012 and the leafing period went for nine months and the flowering period went for
seven months compared to the six months for leafing and flowering in 2012, there is not
enough data to determine a trend. In comparison there is a clear relation to the date of leaf
loss of the African Boab and the temperature and rainfall of that year; higher temperatures
and lower rainfall correlated with leaf drop earlier in the year (Swanepoel 1993). From
analysis of the ClimateWatch data, it can be determined that lower rainfall correlated with the
leafing and flowering period being shorter in duration (Fig. 1 and Table 1). Further studies
with a larger and more reliable data set are recommended to determine the strength of the
relationships between temperature, rainfall and phenology of the Australian Boab. The
expected trend of the length of the leafing and flowering period could be predicted by an
evaluation of the rainfall trends of a particular area. From this can be extrapolated a
prediction of the effects on ecosystems of which they are a part and the fauna that rely on the
boab as a food source, for example hawkmoths in Australia (Bowman 1997).
The relationship between the boab and animals that rely on them will be affected if the length
of the leafing and flowering times of the boab change. The boab will then be further affected
as animals that are vectors for pollen and fertilisation change their feeding habits. From this it
can be inferred that a change in rainfall may have a massive effect on the whole ecosystem,
and that the different rainfall levels in the Mediterranean (Perth) and tropical (Kimberleys
and Zimbabwe) climates will affect how the boab interacts with its ecosystem.
17 It is because of this predicted relationship that the boab is an important indicator species for
climate change. However, because of the unreliability of the ClimateWatch data set, further
research and study must be conducted in order to better understand the effect of climate
change on boabs as well as the effect of climate change on Australia’s ecosystems and
biodiversity. While the temperature cannot accurately be said to affect the times or duration
of leafing and flowering of the boab, and the previous seasons’ rainfall total can be seen to
have no affect on the commencement time of the leafing and flowering, it can be seen that the
rainfall of the present season can affect the duration of the leafing and flowering periods of
the boab.
Acknowledgements We would like to acknowledge ClimateWatch and C.M. Swanepoel for
providing the data on boabs that was used in this study.
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