Poplar trees can also be grown to provide environmental benefits. For example, one option for managing sewage sludge, which is an unwanted by-product of the water purification industry, is its application on agricultural land as a soil amendment. Recently, because of the food chain contamination risk, there has been a tendency for banning agricultural sludge application in many countries. Therefore, it seems that non-food, non-feed energy crops could provide an alternative application site for sewage sludge from municipal water treatment plants. A high fertilizer value of sewage sludge was noted by Moffat et al. [25] in studies designed to evaluate the effect of sewage sludge application and wastewater irrigation on biomass production of two poplar genotypes. The three-year experiment showed that irrigation affected biomass yield more than sewage sludge application and that waste application at the rates used did not pose any risk for nutrient pollution of groundwater.

The special importance of riparian forest or stream buffer zones is understandable and, therefore, establishing buffer zones in forest or agricultural space is treated as a stan­dard environmentally friendly practice in many countries [26]. Growing poplar in these systems is, therefore, a logical option for combining biofuel production with surface and groundwater protection. Furthermore, this would also increase biodiversity near water courses and their banks. Poplars, because of their physiological properties, are very well suited to have an important role in establishing riparian buffer zones. Henri and John­son [26] suggested that social debate is needed to determine if riparian zones should be left as a “no touch” area or should be managed. They also evaluated options for man­aging such buffers and found that harvesting 50% of the area and selling biomass could provide both economic and environmental benefits. Fortier et al. [27] studied a multi­functional system of hybrid poplar riparian buffer in southern Quebec, Canada, and also found effective environmental and economic aspects. They stated that biomass produced in riparian buffers can be harvested for different purposes, especially with a multiclonal structure where some clones could be harvested for energy and some for pulp. When biomass productivity in buffers is considered, it is possible to achieve yields comparable to SRP poplar plantations and, since mineral nitrogen is often a limiting factor, the poplars also provide a very effective way to control nutrient flow to groundwater and surface water resources.

Agro-ecological zones have been used for global, national and regional evaluation of agricultural practices [28]. Recently this methodology was enhanced with digital geographic databases. This advanced technology was used to evaluate agricultural areas in Eastern Europe as well as North and Central Asia for their suitability to produce dedicated energy crops. A large variation in the potential for biofuel production was found among these countries, with the highest potential for poplar production being in the Czech Republic and Georgia, due to good soil conditions and a favorable climate. European energy use was estimated at 111 GJ per capita, with Latvia, Lithuania, Hungary and Estonia having the potential to produce more than 140 GJ per capita of bioenergy. The studies also identified some technical and non-technical barriers for bioenergy utilization, thus emphasizing the necessity for future research programs.

The economic soundness of poplar plantations for energy was also evaluated by Yemshanov and McKenny [29]. They constructed two scenarios: (1) “business-as-usual,” where only the biomass has value; and (2) a “fibre + carbon” scenario, where benefits from sequestering carbon in silvicultural systems are included. Many factors were considered, with transportation costs appearing to play a very important role. When burnt for energy, the cost for 1 GJ from biomass ranged from $4 to $5 for scenario 1 and started at $3 for scenario 2. Obviously, adding the benefits of carbon sequestration helped but, as the anal­yses show, biomass cost was still higher than the price of low-quality coal currently being used by power plants. Assuming the option of producing bioethanol from poplar biomass becomes feasible, the economics of biomass production will be substantially improved.

Several authors point out that the most important environmental effect of SRP poplar is the perennial nature, which promotes increased diversity and frequency of many soil organisms and the beneficial impact on soil organic matter [30]. The use of SRP poplar as a vegetative filter was also studied by Coyle et al. [31], who concluded that coppicing poplar was suitable for this purpose because of the extensive root system and high evapotranspiration rate. Poplar clones in their study were irrigated with leachate from municipal landfill and compared with control treatments receiving mineral fertilizers. Effects on soil meso and microfauna were also compared. They reported that microfauna (i. e., soil nematodes) as well as mesofauna (mainly insects) were more abundant in control treatments, while with the leachate, biodiversity among soil organisms was much higher. Based on these findings, the authors concluded that introducing phytoremediation technologies did not always lead to higher sustainability within the soil environment.

Studies on growth, biomass distribution and nutrient use by eight poplar (Populus bal — samifera L., P. trichocarpa Hook) and hybrid poplar (P. trichocarpa Hook x P. deltoides Bartr.) clones in Sweden were conducted by Karacic and Weih [32]. The clones were cho­sen from Canada because its latitude is similar to that of Sweden. The objective was to evaluate genotype by environment interactions with a special focus on phytoremediation. All studied clones showed a high and positive response to irrigation. The results helped identify clones that were better suited for phytoremediation, which involves the application of as much water and nutrients as possible with minimum leaching from the system.

In California, U. S.A., irrigation water can have bad quality because of high selenium, boron, and/or sodium chloride concentrations. Research being conducted by Banuelos et al. [33] is, therefore, focused on identifying plants that are resistant to elevated levels of these contaminants. Trees have an advantage over vegetative plants because they transpire large amounts of water, produce high amounts of biomass, live longer, have deeper roots, and, for many species, can re-grow after being cut. Poplar is one species that has all of these features and, therefore, this genus is widely used for phytoremediation. However, because of the wide genetic variation among species, hybrids and clones of this genus, screening experiments focused on the tolerance of the various genotypes are essential. Among the findings of this research were differences in the chloride and boron concentrations of both lower and upper leaves in poplar genotypes classified as susceptible or resistant to high concentrations of these micronutrients. The mechanism of resistance to high salt concentrations in the irrigation water was also identified as being early abscission of lower leaves containing a high concentration of chloride. Although the physiology of boron tolerance or toxicity remains to be determined, it appears that boron uptake is inhibited when irrigation waters contain elevated chloride concentrations although other resistance mechanisms may exist within the Populus genus.

Poplar grown in SRPs was also able to effectively degraded ethylene glycol, which is present in the environment because of its use as a coolant and deicing agent. Two mechanisms for removal of ethylene glycol (microbial degradation in the rhizosphere and uptake by the trees through evapotranspiration) have been identified [34, 35]. Based on these results, it is very probable that similar mechanisms can be effective for removal of other organic compounds. This was verified by Jordahl et al. [36], who reported that hydrocarbon degrading microorganisms were more common in the rhizosphere of poplar trees than in bulk soil.

Growth and survival of poplar clones at sites contaminated with hydrocarbons and at sites polluted by long-lasting industrial activity near Lake Michigan were investigated by Zalesny et al. [37]. In some spots, the pollution level exceeded 1% hydrocarbons per kilogram of soil. The average poplar survival rate was 67%, with the variation ranging from 56 to 100% and losses being higher for 60-cm cuttings than for 20-cm cuttings. The growth rate was the highest for commercial clones bred for SRP energy production.

To minimize bioaccumulation of toxic trace elements, Wang and Jia [38] proposed growing poplar or larch on contaminated soils. The reason for selecting these tree species for phytoremediation was the fact that deep roots are able to create microenvironments in the soil where immobilization or uptake of the metals can occur. The growth of two tree species in soil spiked with a mixture of cadmium, copper and zinc was investigated by Wang and Jia [38], who found that poplar could remove 56.2 g ha-1 of cadmium, 196 g ha-1 of copper and 1170 g ha-1 of zinc. Heavy metal transferring capacity from roots to aboveground organs was higher in poplar than larch, leading the authors to propose growing poplar on contaminated soils.

Poplar cannot be considered as a cadmium hyperaccumulator because it is able to take up only 10 mgCd kg-1, whereas the known hyperaccumulator Thlaspi caerulescens can accumulate 100 mg kg-1. However, because of the high biomass production in poplar plantations the total accumulation of cadmium is considerably higher per hectare and can actually reach 1000gCdha-1 for poplar compared to just 250 gCd ha-1 for T. caerulescens. Pietrini et al. [39] reported the results of studies on cadmium phytoremediaton potential of several poplar clones. They found high genetic variation among the 15 Italian clones that were studied. The most promising clones showed three desired strategies that could positively affect phytoremediation. Firstly, a relatively high cadmium accumulation level in wood parts; secondly, high leaf tolerance when measured as photosynthetic activity; and, thirdly, a very fast juvenile phase growth rate. The authors concluded that the best indicators of suitability of given poplar genotype for phytoremediation would be some chlorophyll fluorescence parameter.

Finally, a Life Cycle Assessment (LCA) approach was used to quantify the environmen­tal impact of Italian poplar plantations [40]. Two types of short-rotation plantations (a 1- to 2-year cutting frequency and a medium cutting frequency of >5 years) were distinguished. All energy inputs and outputs were taken into account as well as other environmentally important aspects (acidification potential, eutrophication potential, global warming poten­tial, ozone layer depletion, human toxicity potential, ecotoxicty potential, photochemical oxidation formation potential) in a life span of poplar plantations grown for energy purposes, from field preparation (in the first year) to field recovery (in 25th year). It was concluded that from the environmental aspect the best solution is to replace industrial fertilizers with cattle manure; this can reduce total energy use by 19.8%. The authors also concluded that future environmental soundness can be improved the by breeding of high-yielding clones of different poplar hybrids.