Survey data analysis
The survey data were analysed using a multivariate approach.
Specifically, the polythetic divisive classification procedure Two-Way
Indicator Species Analysis (TWINSPAN:
Hill
1979; Gauch 1982) was applied to simultaneously group samples, and
species present in the samples. This strategy allowed classifying
samples based on similar vegetation types and identified species that
tend to occur in the same sample, producing a species classification
that identifies the main assemblages present in the dataset. Moreover,
the analysis distinguished indicator species which characterize
sample-groups produced by the classification
(Ali
et al. 2000; Al-Shehabi & Murphy 2017).
Then, ANOVA tests with mean separation by Tukey’s least significant
difference test (significant outcomes only: P<0.05) were
applied to examine differences between plant and environmental
variables, both between geographical areas, and between sets of samples
making up sample-groups identified by TWINSPAN classification. Datasets
of both the plant-related data, and environmental data were tested for
normality (Ryan-Joiner test) and log10-transformed if
necessary prior to the use of ANOVA. In cases that the data could not be
normalized by transformation, the non-parametric Kruskal-Wallis test was
applied instead of ANOVA.
Field experiment: growth of Haloxylon
salicornicum in oil-polluted
soils
A controlled field experiment was carried out in in 2013 with the aim
to: (1) determine the tolerance of H. salicornicum to desert soil
contaminated with petroleum hydrocarbons, (2) assess factors important
for growing native plant species in contaminated desert soils such as
ease of handling for operational purposes, and (3) determine
requirements for environmental amendment(s) to assist plant
establishment and growth.
Experimental plots were located at two of the areas where oil-polluted
soils were found during an earlier vegetation survey. One site was
located in north of Kuwait (Bahra) and the other in south (Burgan).
Two-year old H. salicornicum plants (grown in a greenhouse
of the Public Authority of Agriculture and Fisheries Resources (PAAF)
Kuwait) was utilized as a representative native desert plant species. In
both locations, plants were grown according to an identical experimental
setup that comprised ten randomly located plots
(Hurlbert
1984) within a fenced area of weathered contaminated soil (in an old dry
oil lake). Three replicates were used per plot. As a control, a second
set of plants were similarly planted at a nearby similar-sized fenced
area of clean soil at both locations. Plants were planted in a 20-30 cm
deep hole (depending on specimen size). Fencing was required to keep out
grazing animals, especially camels and goats. Each plot was watered
prior to planting, and a 1.0 litre supply of “Driwater™”
(http://driwater.com/what-is-driwater/)
was installed per plot. “Driwater™” is a cellulose gum source of water
that dissolves at a constant rate and when installed into the soil
adjacent to a plant. It releases water to the root system for up to 70
days.
Soil samples were collected from both oil-polluted plots (n = 10 per
sampling area) and were analysed using a standard GC analytical
procedure, which was carried out at the Kuwait University Laboratory for
Bioremediation Research. The field experiment began 31 January 2013 in
Bahra and ended 28 March 2013 with 9 field trips to collect data. In
Burgan the start date was 5 February 2013, and ended 3 April 2013 after
10 sampling trips. In both cases, sampling was undertaken during 7 - 9
day intervals and plant height was measured to determine incremental
growth rate. At the end of the experiment, the plants were carefully
removed from the soil and transported to the lab for measuring their
biomass (shoot, root and total dry weight per plant). The TPH in plant
tissues were quantified using a procedure described above.
Results
The distribution of plants and hydrocarbon contamination
The semi-quantitative Oil Damage Score (ODS), shown in Fig. 2, indicates
substantial variations between the examined sites in pollution
(Kruskal-Wallis test, P<0.004). Um Al-Aish, Sabriya, Burgan,
and Um Ghadair oil fields were highly impacted by massive oil pollution,
whereas Bahra and Sabah Al-Ahmad Protected Area are less influenced, and
Um Al-Rros Military Base was generally unaffected. The variation in
hydrocarbon contamination was strongly associated with the geographical
occurrence of the three dominant species (Haloxylon salicornicum,
Cyperus conglomeratus, and Rhanterium epapposum), seen in the
survey. All three species were found in both hydrocarbon contaminated
areas (often in oil-polluted soils) and undamaged areas (Table. 1, Fig
3).