The Role of Moss in Facilitating Natural Revegetation of Metal-Contaminated Sites During Primary Succession
By: Davin Ringen
Mentor: Dr. Catherine Zabinski
Abstract
The purpose of this study was to examine mechanisms of facilitation by primary succession species for later succession on heavy-metal contaminated areas. This project specifically looked at moss growing on three mine tailings sites. pH levels, moisture content and percentages of C and N were the four factors tested. Soil samples were taken from four plots on each of the three sites. The four factors were compared between moss-covered tailings and bare tailings at two different levels (surface layer and deep layer). There were significant increases in moisture content, C and N in the surface layers of the moss-covered tailings but no differences in the deep layers. There were no statistical differences in pH between the layers. Therefore the hypothesis which stated that the presence of moss would alter the pH of mine tailings was not supported. The hypotheses which stated that the presence of moss would affect moisture content and increase C and N percentages was supported
Introduction
Primary succession is the process of vegetation establishing in an area which has never been colonized, or on a site where a disturbance has removed all plants (Connell and Slatyer 1977). Examples of sites devoid of plants are cliffs and rock outcrops (Shure and Ragsdale 1977), and newly exposed glacial till, in areas where glaciers have receded (Jumpponen et al. 1998). Sites affected by disturbances include areas of erupting volcanoes where lava and ash have been deposited (Delgadillo and Cardenas 1995) and mine waste sites which consist of toxic, metal-contaminated soils (Lepp 1981).
Primary
succession begins with the establishment of an individual plant species
(Jumpponen et al. 1998; Connell and Slatyer 1977). Characteristics which enable a species to colonize include good
dispersal abilities, the ability to remain dormant upon arrival at a site, and
quick growth-maturation (Connell and Slatyer 1977). Nitrogen fixing
characteristics are also important in primary successors because nitrogen is
limiting during early stages of community development (Vitousek and Walker
1989). Pioneer species may be “nurse
species” if they facilitate the colonization of new species in later succession
periods (Connell and Slatyer 1977; Pugnaire and Haase 1996; Jumpponen et al.
1998). This facilitation occurs through
modifications in microclimate as well as through changing physical and chemical
properties of the soil (Jumpponen et al. 1998). Studies have shown that in some instances nurse species alter the
pH of a site, and add organic matter to the soil (Connell and Slatyer 1977;
Callaway 1995).
My research focuses
on the role of moss as a colonizing species on mine tailings. Mine tailings range in texture and pH, but
are characterized by high concentrations of heavy metals. Succession on these sites is very slow and
difficult because metals inhibit the growth and establishment of plants in many
ways. Metal toxicity has also been
shown to interfere with the uptake of other nutrients necessary to the
plant. High levels of toxicity also
affect internal biochemical processes of vascular plants. Metals decrease the permeability of plant
membranes and decrease enzymatic activities (Lepp 1981).
We observed on
several abandoned mine sites that grass became established on tailings piles in
areas that had first been colonized by moss.
Bryophytes are common pioneer species because they enhance nitrogen
fixation by association with cyanobacteria (During and Van Tooren 1990). This is important because nitrogen
deficiency is expected in disturbed sites (Vitousek and Walker 1989). Mosses are also resistant to high levels of
copper and other heavy metals (Tyler 1990), and are able to accumulate metals
in their tissues (Lee et al. 1984: Tyler 1990). A number of mosses are most common on substrates with high
concentrations of heavy metals (Shaw 1987), and are able to endure extremely
acidic soil conditions (Tyler 1990).
To better
understand the effect of moss on mine tailings, I set up a study, during the summer of 1999, to test
the following hypotheses:
H1: The presence of moss will alter the pH of
mine tailings.
Prediction 1: On acidic mine site, the pH will be more neutral directly beneath the moss layer and more acidic at deeper levels.
Prediction 2: The pH of bare tailings will be more acidic than moss covered tailings, and will not vary between depths.
H2: The presence of moss will affect soil moisture.
Prediction 3: Soil moisture will be higher under the moss layer at both depths when compared to the adjacent bare tailings.
H3: Moss will increase carbon and nitrogen in tailings substrate.
Prediction 4: Carbon and nitrogen will be in greater quantity directly under the moss.
Prediction 5: Carbon and nitrogen will be lower at the deeper level under the moss and will be low in both of the levels of the bare tailings.
The three sites used for this research were the Fool Hen, Gray Lead and Forest Rose mining sites, all located in western MT. All of these sites consisted of an adit, a mill and a tailings area. The Fool Hen mine is located southeast of Lincoln, MT off Stemple Pass Rd. The Gray Lead mine is located west of I-15 between Helena and Butte, MT. The Forest Rose is located south of Drummond, MT. off of I-90. On all of these sites, moss patches grew on the tailings and more dense vegetation of grasses grew on top of these mosses (personal observation).
At
each mine site, four plots were delineated.
Each plot consisted of a moss-covered tailings and adjacent bare
tailings. Eight soil cores (2.2cm dia
x 12 cm l) were aseptically collected from both the moss-covered tailings and
the bare tailings in each plot, for a total of
sixteen cores per plot. The
surface six centimeters was the depth to which seedling roots penetrated (pers.
observation), and also the layer that
would be most affected by moss. The soil
cores were stored in zip lock bags, with the surface 6 cm separate from the
lower 6 cm.
Soil
cores were brought back to the lab where they were homogenized by hand for four
minutes, before analysis. Two g of each sample was mixed with 4 mL of deionized
water and stirred into solution. The pH
was taken with a pH meter and recorded. The
moisture content of the samples was determined by gravimetric means. The soil
samples were weighed, and placed in a pre-weighed soil tin, and dried at 60o
Celsius until mass remained constant.
Carbon
and nitrogen analysis was completed in the Murdock Environmental Lab at the
University of Montana. Samples were
ground mechanically for five minutes, until they reached a common consistency. 0.4 mg of the ground sample was placed in
the elemental analyzer, where it was combusted and gas content analyzed. The results were copied onto a spreadsheet.
Paired
comparison t-tests were used to test the hypotheses listed above. We tested for differences between
moss-covered tailings and bare tailings in both the surface and deep
layers. A two-tailed t-test was used to
test for differences in pH and moisture, and a one-tailed test was used to
compare C and N levels. ANOVA could not
be used, because the data could not be transformed to meet the assumptions of
normality and homogeneity of variances.
Results
pH
Analysis of the
pH for the three sites showed that there were no significant difference in pH
between the zone of influence under the moss patch and the zone of influence of
the tailings (Table 1). The average pH
under the moss was 6.3508 and the average pH of the bare tailings was
5.6092. There were also no significant
differences between the pH of the deep layers.
The averages were 6.1042 and 5.6408 respectively
The soil
moisture was significantly higher (P= 0.022) in the zone of influence under the
moss compared to the zone of influence of the bare tailings (Table 1). The soil moisture of the deep layers was not
significantly different (Figure 2).
The
percentage of carbon in the zone of influence under the moss patches was
significantly higher (P= 0.023) than the percentage of carbon in the zone of
influence of the metal-contaminated soil (Table 1). The percentage of carbon of the deep layers was not significantly
different (Table 1, Figure 3).
The
percentage of N in the two layers mirrored the C results. Nitrogen was significantly higher in the
zone of influence under the moss patches (P= 0.05) (Table 1). The deep layers
were not significantly different (Table 1,
Figure 4).
These results suggest that moss has an effect as a primary successional species on metal-contaminated soils. The pH did not differ between moss-covered and bare tailings. Differences in pH between sites (Fool Hen average = 8.7; Grey Lead = 4.5; Forest Rose = 4.6) may mask the effects of moss on acidic sites. The results at the Forest Rose suggest that moss may have a pH effect at some sites. We also recognize that without moss removal experiments, we cannot exclude the possibility that moss colonized in less acidic areas, so that differences in pH are not due to moss effects, but correlate with moss colonization.
Moss
has an effect on soil moisture in the surface layer only. This is important for seedling establishment
in open tailings areas, since desiccation due to limiting amounts of moisture
is a cause of seedling mortality.
Both
C and N increased with the presence of moss.
While C increased significantly in the surface layer, the quantity of C
in the soil was on the order of 0.01%, an extremely low level. Soil C is important for soil bacterial and
fungal populations, some of which may be important for seedling establishment
and growth (Tate 1995).
Soil
N existed in low percentages in all layers, but there is a significant increase
in the percentage of N in the surface
layer of the moss-covered tailings compared to the bare tailings.
The results suggest that moss alters the soil environment where it grows. The soil moisture, and percentages of C and N are all important environmental characteristics for recovering sights and all significantly increased. The pH of the soil is also an important characteristic of recovering sites, but may be more site-specific. Differences in pH were not significant between the surface and deep layers of the moss-covered tailings and bare tailings but the graphs show the trend that moss is neutralizing the pH beneath it.
The
results of this project are important to suggest that moss may be an effective
primary colonizer on harsh environments.
We did not account for temporal scales in this study, and had no
information about how long moss had been established on a site. We recognize that differences between
moss-covered and bare tailings could reflect the amount of time moss has bee
established on a site. Moss may have a
more significant effect on a site the longer it has been established. Also, we treated the moss in this project as
a single taxon for the sake of simplicity, but recognize that different species
of moss may have different effects.
Further studies could compare species effects, and effects of length of
establishment.
The conclusions
drawn from these projects are beneficial to the state of Montana. Thousands of mine sites are spread across
this state and widely dispersed in the National Forests. The revegetation of these mining areas is a
very important issue that needs to be addressed. Any study that increases our understanding of the revegetation of
mine tailings benefits the state of Montana.
I would like to thank the Howard
Hughes Medical Institute and IBS-core for accepting my project to be funded and
executed during the summer of 1999. I
also would like to thank my mentor Dr. Catherine Zabinski for all the help and
support she gave me for this project and the past two years. Finally I would like to thank October
Seastone Moynahan, Dawn White, Heather Tone, Zach Franz, and Sara Barth for
their laboratory and field assistance on this project.
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