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BIO-INTRUSION BARRIER MADE OF PLANTS WITH ALLELOPATHIC
EFFECTS TO IMPROVE LONG TERM PERFORMANCE OF COVERS
WITH CAPILLARY BARRIER EFFECTS
Evgeniya Smirnova1, Bruno Bussière1, Yves Bergeron2, Francine Tremblay2,
Nelson Thiffault3, Abdelkabir Maqsoud1, Réal Marcotte4
1
Chaire CRSNG-Polytechnique-UQAT en environnement et gestion des rejets miniers, Université du
Québec en Abitibi-Témiscamingue (UQAT), 445 boul. de l'Université, Rouyn-Noranda, QC J9X 5E4;
2
Chaire industrielle CRSNG- UQAT-UQAM en aménagement forestier durable, Université du Québec
en Abitibi-Témiscamingue (UQAT), 445 boul. de l'Université, Rouyn-Noranda, QC J9X 5E4;
3
Direction de la recherche forestière, Ministère des Ressources naturelles et de la Faune du Québec, 2700
Einstein, Québec, QC G1P 3W8
4
Corporation minière Northern Star, 153 A, ave. Perreault, Val d'Or, QC J9P 2H1
Contact: eugenia.smirnova@uqat.ca
Allelopathic effects (AE) are the inhibition of the germination and growth of certain species of
plants by other plant species through the release of chemical compounds in the environement. In
this project, the use of plants with AE is suggested as a bio-barrier to protect Covers with
Capillary Barrier Effects (CCBE) against undesirable plant colonisation. CCBE covers are used
to reduce oxygen migration from the atmosphere to the tailings and are constructed to reclaim
acid generating mine sites. Trees represent a particular threat for covers’ performance: their roots
may reduce the ability of the cover to limit oxygen migration (and consequently acid generation)
and can increase the risk of physical damage to the CCBE. To test the ability of plants with AE
to reduce the colonisation of trees on covers, a field experiment was initiated in 2008. The study
area is an existing acid-generating tailings impoundment, located in the Abitibi region, Quebec,
reclaimed by CCBE in 1996. Species common in the boreal zone with evidence of AE were
selected as potential bio-barrier: Bluejoint reedgrass (Calamagrostis canadensis Michx. Deauv.),
sheep laurel (Kalmia angustifolia L.) and bog Labrador tea (Rhododendron groenlandicum
[Oeder] Kron & Judd. formerly Ledum groenlandicum Oeder). These species with AE were
planted in 3m x 3m experimental plots together with trees found in the forest surrounding the
tested area: trembling aspen (Populus tremuloides Michx.), black spruce (Picea mariana Mill.),
willows (Salix spp.), speckled alder (Alnus rugosa DuRoi.) and balsam poplar (Populus
balsamifera L.). Overall, 120 test plots were built. Different approaches will be used to evaluate
the impact of plants with AE on the target tree species: the development of morphological
characteristics of the trees, root architecture, chemical content and concentration of plant tissues,
soil and water. This paper describes the field study in detail.
Key words: Covers with Capillary Barrier Effects (CCBE), bio-barriers, Allelopathic effect,
secondary metabolites, tree roots, acid mine drainage, water budget
1
INTRODUCTION
Problem
The Canadian mining industry produces approximately 6 billion tons of mining wastes per year
that are stored at the soil surface in mine waste disposal sites. Many of these mine sites contain
acid–generating materials (tailings and waste rock) that can have a significant impact on the
surrounding environment; acid mine drainage (AMD) is usually characterised by low pH and
high metal concentrations (e.g. Aubertin et al., 2002). To avoid AMD generation, different
control methods have been proposed (MEND, 2001) such as flooding (water cover), oxygen
consuming barriers, and impermeable covers. To limit oxygen migration (oxygen is the main
oxidant of sulphide minerals at near-neutral pH), multilayered covers that use capillary barrier
effects can also be used; these covers will hereinafter be called Covers with Capillary Barrier
Effects (CCBE). The main role of a CCBE in a humid climate is to limit the oxygen diffusion
flux reaching the reactive minerals. To do so, the cover must maintain a high degree of saturation
in one (or more) of its layers. The diffusion of gas through a nearly saturated porous media (such
as a fine-grained soil) can be low enough to limit the oxygen flux, thereby reducing the rate of
oxidation to a negligible value (e.g. Nicholson et al., 1989; Rasmuson and Erikson, 1986; Aachib
et al., 1993). CCBEs usually contain three to five layers, made of different materials. The lower
three layers aim to create the oxygen barrier (by using capillary barrier effects) while the upper
layers protect against erosion and bio-intrusion the CCBE. More information on CCBE can be
found in Bussiere et al. (2003) and MEND (2001). Tree establishment on disposal sites represent
a particular threat for CCBE performance: their roots may reduce the ability of the cover to limit
oxygen migration and they increase the risk of physical damage to the CCBE.
Bio-Intrusion barriers
Many approaches are suggested (Cooke and Johnson, 2002) to limit bio-intrusion impacts on
cover performance, such as herbicides, asphalt, soil compaction, basaltic gravel, salt water, etc.
However, most of them have serious drawbacks, such as high implementation costs, short-term
impacts on vegetation or severe environmental impacts. Recently, it has been suggested that the
establishement of species with strong allelopathic effects could be used as bio-intrusion barriers
(DOE, 1990; Vyvyan, 2002; Cooke and Johnson, 2002). Allelopathic effects (AE) are defined as
the inhibition of the germination and growth of a certain species of plant, by other plant species
through the release of chemical compounds in the environement. Allelopathic effects can be due
to the production of biochemical compounds that directly affect target plant mechanisms, or to
the creation of ecological conditions that exert a negative effect on competitors (Wardle et al.,
1998; Siciliano and Germida, 1998). Allelopathic effects are complex and involve numerous
classes of chemical compounds (more than 4000) (Waterman and Mole, 1994, Kruse et al.,
2000), and different plant organs, litter, roots, and shoot exudates can exert substantial
allelopathic influences on surrounding species (Mallik and Indetjit, 2001; Vivanco et al., 2002).
Thus, the production and release of secondary metabolites (considered as indirect competition),
in addition to direct resource competition, are two major factors justifying the use of AE species
as a bio-intrusion barrier against incompatible plants.
Bluejoint reedgrass, sheep laurel and bog Labrador tea are species that have been shown to have
AE on some tree species (species present in the Abitibi region). The last two species are known
to successfully reproduce vegetatively (Titus et al., 1995, Jobidon, 1995). Once established on2
site, they form a dense monospecies cover; therefore, overseeding would seldom be required if
they were used as a barrier. The open-sky conditions of tailings impoundments and the hydraulic
conditions in the CCBE could also represent a suitable habitat for Bluejoint reedgrass, sheep
laurel and bog Labrador tea; however, these species have never been tested as a barrier to biointrusion on CCBEs. There is also a lack of knowledge about their inhibiting effects on the
undesirable tree species, and about the mechanisms involved in any inhibition. In particular, no
information is available on the influence of AE species on tree-root systems. It is important to
take into consideration that the AE species themselves can eventually alter the performance of
CCBE.
Different approaches have been used to identify AE and the significance of direct resource
competition on target vegetation. For example, AE could be revealed through the study of
morphological and biochemical characteristics of neighbouring plants (Wardle et al., 1998). Rate
of plant growth and morphological characteristics mirror these changes. Nutrient concentration
of different organs of plants in the vicinity of inhibitors (Thiffault et al., 2004) and root-shoot
ratio are parameters that also allow evaluation of phyto-toxicity stress i.e. allelopathic effect.
Plant root systems consist of individual components, which are sensitive to edaphic and nutrient
conditions (Waisel and Eshel, 2002). Changes in soil physical and nutrient properties alter
individual root component characteristics: dimensions, lateral development, ramification etc.
Masle (2002) synthesized numerous results indicating rapid (within minutes to < 24 hours) root
physiological response to edaphic stress. Root biomass, volume and root density per soil volume
are the most common methods to assess root system distribution. Based on root architecture
modeling, it has been reported that the character of the response in root architecture to
environmental conditions varies greatly among species (Godin et al., 1999; Danjon and Reubens,
2008). Depending on the concentration and the type of AE compounds present, both a
stimulatory and inhibitory effect on root growth and distribution have been observed (Rao, 1990;
Neumann and Römheld, 2002).
Impact of bio-intrusion barriers on CCBE water budget
The ability of a CCBE to limit oxygen migration is mainly related to its capacity to maintain a
nearly-saturated moisture-retaining layer. Therefore, all parameters that can modify the hydraulic
behaviour (or water balance) of the site must be taken into account at the design stage. For the
site under study (LTA), the hydraulic behaviour of the CCBE was estimated assuming that there
is no vegetation (e.g. Bussière et al. 2006). If a vegetative cover is installed as a bio-intrusion
barrier, it is necessary to evaluate its impact on the water budget.
The water balance of a covered site can be expressed in the following way:
ET P R Pr
S (1)
Where ET is evapotranspiration, P is precipitation, R is runoff, Pr is percolation, and S is soilwater storage. Vegetation on a CCBE will influence its hydraulic behaviour mainly by increasing
ET and reducing R in inclined zones. The roots can also extract some of the water (in other
words, influence S) retained in the moisture-retaining layer. Different studies have used direct
measurements and predictions from numerical tools to evaluate the water budget of vegetated
(e.g. Khire et al., 1999; Ogorzalek et al. 2008) and unvegetated (e.g. Woyshner and Yanful,
1995; Swanson et al., 2003) covers. The work performed on vegetated covers was for CCBEs
used in relatively dry climates to limit water infiltration while the evaluation of the water balance
3
for CCBEs used to limit oxygen migration was limited (to the knowledge of the authors) to the
case of unvegetated covers (or vegetation was ignored, if present).
Objectives and Hypothesis
The main objective of this study is to evaluate the capacity of bio-intrusion barrier to limit long
term deterioration of CCBE (in the Quebec context) caused by tree-root invasion. The specific
objectives are: 1) to introduce the selected bio-barrier species with known allelopathic effects
[Bluejoint reedgrass, sheep laurel and bog Labrador tea] to an existing CCBE; 2) to evaluate two
factors: the production of phyto-toxins and resource limitation of bio-barrier species on the target
tree species; 3) assess the impact of the AE species on the root system characteristics of target
tree species [trembling aspen, balsam poplar, black spruce, willows and speckled alder]; and 4)
to evaluate the effect of AE species on CCBE water balance. The main long-term objective is to
integrate bio-intrusion barriers in the design of CCBEs to improve their long-term performance.
The main hypothesis of the study is that direct resource competition and allelopathic interference
of potential bio-barrier species inhibit target tree species’ growth. More specifically, we assume
that compared to control (trees planted without AE species), tree species planted with AE species
will cause: an increase in root-shoot ratio in target tree seedlings and a decrease in the growth
rate of target tree seedlings. The impact of AE species on seedling roots may result in a decrease
in the number and length of lateral roots and overall root distribution depth; other changes in root
spatial and dimensional characteristics may also be found. Due to toxicity and direct resource
competition, the foliar nutrient concentrations of target trees should decrease. A second
hypothesis is that the presence of an efficient bio-barrier will not detrimentally affect the water
budget of the CCBE and, consequently, its capacity to limit oxygen migration (and AMD
generation).
Since the study has started in summer 2008 and, consequently, no significant results are available
yet, the present paper focuses on the site description, the design of the experimental plots, and on
the research strategy proposed to reach the main objectives.
THE LTA MINE SITE
The LTA site located in the Fourniere and Dubuisson Townships near Malartic in Québec is
chosen as the study area (see Figure 1). The tailings impoundment covers an area of
approximately 60 ha and contains 12 m of sulphide tailings placed over 5 m of non acidgenerating material (Bussière et al., 2006). In 1995-1996, the LTA site was rehabilitated using a
CCBE. The cover design configuration consists of three layers. From bottom to top (see Figure
1), they are (Ricard et al. 1997): a 0.5 m layer of sand used as support and a capillary break
layer; a moisture-retaining layer with a thickness of 0.8 m (this layer is made of a non-plastic
silty material); a 0.3 m sand and gravel layer on top used as protective layer. Hydroseeding was
performed on the sloping areas of the site (estimated at approximately 15 to 20% of the total
area) in 1997. Vegetation was not planted or seeded on the top (nearly flat) area of the site,
which represents nearly 80 to 85% of the total area of the site. Nevertheless, because the site is
surrounded by forest, some trees and shrubs were observed on the site shortly after the end of the
cover construction. To avoid the potentially detrimental long-term influence of uncontrolled
4
vegetation on the performance of the CCBE, plants and trees were eliminated via exfoliation on a
regular basis (every two to three years) on the top surface.
The last 10 years of monitoring have shown that two different zones are present on the LTA site
(see Bussière et al. 2006 for details): a wet zone, where the water table is high (in or near the
CCBE) and the water content in the layers are relatively high (near saturation in the bottom
capillary break layer and in the moisture-retaining layer, and usually greater than 12% in the top
sand and gravel layer), and a dry zone, where the water table is more than a meter below the
bottom of the CCBE (the water content in the layers is lower than in the other ecotope). Study
areas have been delimited in both ecotopes (dry and wet) since the implementation of the
vegetation could differ depending on edaphic conditions.
Figure 1: a) Aerial picture of the LTA site before construction of the CCBE and cover
configuration (modified from Bussière et al. 2006); b) plan view of the LTA site with the
location of the experimental plots (blocks B1, B2 and B3 are in the dry ecotope, while B4, B5
and B6 are in the wet ecotope).
METHODS AND WORK PERFORMED IN 2008 AND 2009
Implementation of bio-barrier species on the LTA site
In the summer of 2008, 6 blocks, containing 120 experimental plots of approximately 9 m2, were
delimitated (Fig. 1b). Each set of experimental plots formed a block consisting of 20
experimental plots and were replicated three times per ecotope. Before planting, the thickness of
the top CCBE layer was measured. The surface of the experimental plots was raked, big cobbles
were removed, and plots were manually weeded. The planting substrate (15 cm thick) was then
deposited on the experimental plots. The substrate deposited in 2008 was exceedingly sandy and
contained an insufficient concentration of organic matter (less than 15%). Therefore at the
beginning of the growing season of 2009, an organic substrate (approximately 90% organic
matter) was added (5 to 10 cm thick).
5
Sheep laurel and bog Labrador tea reproduce mainly vegetatively (Titus et al., 1995; Jobidon,
1995). In order to accelerate their establishment, grafts of these species were transplanted on the
LTA site from the adjacent forest. Titus et al., (1995) reported a density of 1.8 sheep laurel
clumps, with 12 stems per square meter, on cutovers where mineral soil was exposed. Based on
these values, 40 grafts per square meter were planted. A similar strategy will be applied for the
bog Labrador tea propagation.
Aspen
Poplar
Spruce
Alder
Willows
Fig.2 a) One block of the experimental plots: control (only target trees)
Target tree + Bog
Labrador tea
Bluejoint reedgrass
Sheep laurel
; b) dispositive of the experimental
plot: trees and AE species.
Hydroseeding was used to sow Bluejoint reedgrass. Approximately one gram of seeds was sown
per 1 m2 plot. The sowing norm was calculated based on the agricultural cultivation
recommendations.
Introduction of the target tree species in the LTA experimental plots
Trembling aspen, balsam poplar, black spruce, speckled alder, and willow were identified as the
most problematic species for the long term stability of CCBEs located in Quebec (see Trépanier
et al. 2006). Two-year seedlings of speckled alder and black spruce were purchased in a nursery,
while the three other species were excavated directly on the LTA site and replanted in the
experimental plots. The height of the excavated seedlings was controlled. At the end of the first
growing season, the seedlings were planted on the experimental plots at 50 cm x 50 cm intervals
between the rows with AE species. Four targeted trees were planted in each experimental plot
(Fig. 2). Plots where trees were planted without AE species were considered as control plots.
Before planting, several measurements of seedlings were taken. Stem height, basal diameter,
number of tap and lateral roots, and their diameters and aspects were measured. Coarse roots
6
were cut to a length of 15 cm (approach used by Collet et al. 2006). Abundance of fine roots was
estimated visually.
The experimental plots are equipped by thermocouples at depths of 10 to 30 cm. Water quality
can provide information on phenolic metabolite concentrations; therefore, percolation water will
be recovered in some plots. Also, as one of the objectives was to investigate the impact of a biobarrier on CCBE water budget, five weighing lysimeters will be installed during the summer of
2009. A weighing lysimeter is a scale that measures the change in mass of soil volume due to
water loss by evapotranspiration (Maidment, 1993). To validate these measurements, the
evapotranspiration will be also estimated using digital software (see Ogorzalek et al., 2008 for a
list of software that can be use to predict evapotranspiration). The programs usually use different
parameters such as root density and depth, percent bare area, growing season and the leaf area
index (LAI) to estimate evapotranspiration (e.g. Khire et al. 1999; Ogorzalek et al. 2008).
In July 2009, shoot exudates (in rain water) collectors were installed below the AE species
within each set of experimental plots. To assess AE compounds in the root exudates, the microcollectors (micro-centrifuge Eppendorf tubes) will be installed directly on the active roots of AE
species. The tubes filled with distilled water will remain on the roots for five hours. After
sampling, shoot and root exudates will be analysed in the laboratory to measure allelopathic
compound concentrations.
Monitoring strategy on the plots
To assess the direct and indirect effects of AE species on the tree seedlings’ morphological
characteristics, stem height, basal diameter, and number and length of branches will be measured
at the end of each growing season for three years. Subtraction of initial dimensions from final
ones will allow the assessment of three-year diameter and height increments, as well as relative
growth rates. Starting in the second growing season, three seedlings per experimental plot will be
excavated every year for a root architecture study. Hand tools will be used, starting at the stem
and progressing downwards along the roots. The main quantitative physical properties that
characterize the plant rhizosphere will be measured: root biomass, volume and architecture (i.e.
root topology and geometry). The study of root system architecture is based on a set of
quantitative parameters (Collet et al., 2006, Danjon and Reubens, 2008) representing the root
system architecture: root axes, dimensions, branching, spatial position of branches, root life
status etc. In this study, the Polhemus fastrack digitizer will be used for root classification and
digitization. The next step will be to model root systems using PiafDigit software (developed by
research group in INRA, Clermon-Ferrand, France). Three-year measurements should allow for
the assessment of root temporal dynamics. Digital root parameters will be analyzed statistically
to assess competitor impacts on root components. After each root excavation, shoot and root
biomass will be measured to assess the root-shoot ratio.
To assess the effect of AE species on target tree nutrition, foliar nutrient concentrations (N, P, K,
Ca, Mg) of seedlings will be evaluated at the end of the experiment. Allelopathic compounds
concentrations (phenolic acids and/or condensed tannins) in the foliar (year production) and
foliar-litter tissue of Bluejoint reedgrass, sheep laurel and bog Labrador tea will be identified
once a year. The soil below AE species will be sampled with the same frequency. The water
from shoot and root exudate collectors will be sampled three times per growing season.
7
The mass variation of the weighing lysimeters can be related to the change in water storage.
Knowing the precipitation and the percolation through the lysimeter, the real evapo-transpiration
can be deducted using the water budget euqtion. To compare the results with numerical
predictions (see Ogorzalek et al. 2008 for examples), meteorological data are necessary. A
transportable weather station will be installed on-site during strategic periods. The station will
measure precipitation, wind speed and direction, relative humidity, temperature, and net
radiation.
CONCLUSION
No study dedicated to the influence of vegetation on CCBEs has been conducted previously in
Quebec or Canada. Because CCBEs have proven to be an efficient measure to limit AMD (in the
short-term), a strategy to protect CCBEs against bio-intrusion must be developed. The approach
based on a bio-intrusion barrier consisting of native species with AE properties seems to be
promising since it would use nature (existing plants) instead of engineered measures (physical
barriers) to control a natural phenomenon (tree invasion). The project will also allow us to
advance the existing knowledge of allelopathic compound effects on trees. Therefore, the
proposed project aims to answer some fundamental biological questions about allelopathic
compounds and root property interactions, as well as developing a practical approach for biointrusion barrier on CCBEs. The influence of the bio-intrusion barrier made of plants on CCBEs
water budget is also an important part of the project. If the results are positive, guidelines will be
drafted to establish an efficient bio-intrusion barrier made of AE plants in the Quebec context.
The methodology proposed could be transposed to other Canadian context.
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