3.1. Very High Wood and Biomass Production in Mature Hybrid Poplar Riparian Buffers Located on Small Farms of Southern Québec
Yield results from this study highlight the very high potential of riparian buffers to produce substantial wood volumes and biomass during a short time period, even if the study sites are located in extensive farmland (pasture, hayfield). Across the four riparian buffer sites, hybrid poplar productivity after nine years ranged from 116 to 450 m
3ha
−1, for stem wood volume (
Figure 3a), and from 51 to 193 Mg ha
−1, for woody biomass (
Figure 3b,
Table 5). Consequently, the Site effect was by far the largest effect detected by the ANOVA followed by the Clone effect (
Figure 3,
Table 5), while the Site × Clone interaction for biomass and volume production was not statistically significant. As it was the case after six years of growth [
1], the highest mean annual yields were obtained at the fertile site of Bromptonville after nine years, reaching 49.9 m
3ha
−1yr
−1 and 21.4 Mg ha
−1yr
−1 (
Table 5).
Very high mean annual yields were also observed at Roxton Falls (26.3 m
3ha
−1yr
−1 and 11.4 Mg ha
−1yr
−1) and at St-Isidore-de-Clifton (30.7 m
3ha
−1yr
−1 and 13.3 Mg ha
−1yr
−1) (
Table 6). With yields above 25 m
3ha
−1yr
−1 on three of the four study sites, it is clear from this study that hybrid poplar riparian buffers are generally more productive than other hybrid poplar plantation systems established in the province of Québec [
48]. Yields reported in the literature for Québec (excluding short rotation coppice) are generally between 15 and 25 m
3ha
−1yr
−1 for plantations established on high soil fertility sites in agricultural areas [
22,
49], while yields are generally below 5 m
3ha
−1yr
−1 for plantations established on clear-cut forest sites [
48]. In fact, volume yields within the range measured in this study are generally observed in fast-growing plantations in tropical countries [
50], which have much more favorable climates than southern Québec. For example, yield observed in eucalyptus and acacia operational plantations can reach 40 and 30 m
3ha
−1yr
−1, respectively [
50]. There are several factors that can contribute to the high yield of hybrid poplar riparian agroforestry systems: (1) The high water availability in the riparian zone soils; (2) nutrients are continuously migrating from the adjacent agricultural land; (3) silt deposits that are periodically observed at the tree bases following flooding events, improve soil fertility, and (4) light availability is much higher than in large plantations because of the narrowness of the buffer strips.
Figure 3.
Site and Clone effects for (a) total stem wood volume production (m3ha−1) and (b) total woody biomass (Mg ha−1) production after nine years in hybrid poplar riparian buffer strips Total wood volumes and total woody biomass include trees of clone MxB-915311 that had broken at the end of the eighth and during the ninth growing seasons and that were harvested by the landowner. Vertical bars represent SE.
Figure 3.
Site and Clone effects for (a) total stem wood volume production (m3ha−1) and (b) total woody biomass (Mg ha−1) production after nine years in hybrid poplar riparian buffer strips Total wood volumes and total woody biomass include trees of clone MxB-915311 that had broken at the end of the eighth and during the ninth growing seasons and that were harvested by the landowner. Vertical bars represent SE.
Table 5.
Total aboveground dry biomass production (Mg ha−1) at the four hybrid poplar riparian buffer sites and for the three poplar clones after nine years. Percent (%) of each tree compartment versus total woody biomass is indicated.
Table 5.
Total aboveground dry biomass production (Mg ha−1) at the four hybrid poplar riparian buffer sites and for the three poplar clones after nine years. Percent (%) of each tree compartment versus total woody biomass is indicated.
Sites and clones | Stem biomassa (Mg ha−1) | % | Branch biomassa (Mg ha−1) | % | Woody biomassa (Mg ha−1) |
---|
Sites | | | | | |
Bromptonville | 142.1 | 74 | 50.7 | 26 | 192.8 |
St-Isidore-de-Clifton | 88.4 | 74 | 31.3 | 26 | 119.7 |
Roxton Falls | 76.1 | 74 | 26.7 | 26 | 102.8 |
Magog | 37.7 | 74 | 13.5 | 26 | 51.2 |
SE | 5.7 | | 2.1 | | 7.8 |
P< | 0.001 | | 0.001 | | 0.001 |
Clones | | | | | |
MxB-915311 | 103.8 | 71 | 41.7 | 29 | 145.5 |
DNxM-915508 | 92.6 | 74 | 33.1 | 26 | 125.7 |
DxN-3570 | 61.8 | 79 | 16.8 | 21 | 78.7 |
SE | 5.0 | | 1.8 | | 6.8 |
p< | 0.001 | | 0.001 | | 0.001 |
Even the riparian buffer located on the poor, imperfectly drained, stony and unfertilized pasture site of Magog (
Figure 1,
Table 1,
Table 2), which had earlier been considered marginal for wood production (only 4 m
3ha
−1yr
−1 after 6 years) [
1], produced an interesting volume yield at the end of the ninth growing season (12.8 m
3ha
−1yr
−1) (
Table 6). The Magog site now has a yield that falls within the desired mean annual increments for a short rotation woody crop (10–30 m
3ha
−1yr
−1) [
51]. Therefore, even riparian zones of marginal or very extensive (unfertilized) agroecosystems might be interesting to generate relatively high wood volume in temperate regions. Further studies are needed to evaluate the yield potential of hybrid poplar buffer strips bordering intensively managed annual row crops (soy and maize) located in the St. Lawrence Valley Lowlands, where the best soils and mildest climate of the province of Québec are found. Yields are expected to be even higher in such systems.
Table 6.
Mean annual volume yield (m3ha−1yr−1) and mean annual woody dry biomass yield (Mg ha−1yr−1) increases from the sixth year to the ninth year at the four hybrid poplar riparian buffer sites (three clones mean) and for the three clones (four sites mean).
Table 6.
Mean annual volume yield (m3ha−1yr−1) and mean annual woody dry biomass yield (Mg ha−1yr−1) increases from the sixth year to the ninth year at the four hybrid poplar riparian buffer sites (three clones mean) and for the three clones (four sites mean).
Sites and clones | Volume yield (m3ha−1yr−1) | Increase (m3ha−1yr−1) | Increase (%) | Biomass yield (Mg ha−1yr−1) | Increase (Mg ha−1yr−1) | Increase (%) |
---|
| 6 years | 9 years | | | 6 years | 9 years | | |
---|
Sites | | | | | | | | |
Bromptonville | 37.8 | 44.4–49.9a | 6.6–12.1a | 15–32a | 16.1 | 18.9–21.4a | 2.8–5.4a | 15–33a |
St-Isidore-de-Clifton | 15.6 | 30.7 | 15.2 | 98 | 6.6 | 13.3 | 6.7 | 100 |
Roxton Falls | 11.3 | 26.3 | 15.0 | 132 | 4.9 | 11.4 | 6.5 | 133 |
Magog | 3.9 | 12.8 | 9.0 | 230 | 1.8 | 5.7 | 3.9 | 218 |
Clones | | | | | | | | |
MxB-915311 | 19.7 | 30.7–34.9a | 11–15.2a | 56–77a | 9.2 | 14.3–16.2a | 5.1–7.0a | 55–76a |
DNxM-915508 | 17.4 | 30.9 | 13.5 | 78 | 7.5 | 14.0 | 6.4 | 85 |
DxN-3570 | 14.3 | 24.0 | 9.7 | 68 | 5.3 | 8.7 | 3.4 | 65 |
The very high yield increases that occurred from the sixth to the ninth year (
Table 6), suggest that after six growing seasons, none of the four riparian buffers had reached their maximal productivity in terms of mean annual increment. Even the buffer strip at Bromptonville, which already had a mean annual volume yield of 37.8 m
3ha
−1yr
−1 after six years (three clones mean), increased its yield by 12.1 m
3ha
−1yr
−1 to reach 49.9 m
3ha
−1yr
−1 after nine growing seasons (
Table 6). However, the mean annual volume yield increase during this 3-year period was much lower (6.6 m
3ha
−1yr
−1) when yield calculations were done only with live standing trees (
Table 5). The largest yield increases were observed at the intermediate sites of St-Isidore-de-Clifton and Roxton Falls, where mean annual volume yield increased by an average of 15 m
3ha
−1yr
−1 from year six to year nine (
Table 6).
Finally, despite the very high relative yield increase at the Magog site (230%), the productivity gap, in terms of mean annual yield, between this less fertile site (
Figure 1,
Table 1,
Table 2) and the three other sites, has widened (
Table 6). This highlights the important economic advantage, in terms of productivity gain, that can be made simply by growing hybrid poplars in the most fertile riparian zones. From the landowner’s economic perspective, selecting high quality sites is the main factor to consider. As shown for intensive white spruce plantations in Québec, the first factor affecting plantation profitability for the private landowner is site quality, followed by the use of improved genotypes and silvicultural treatments, respectively [
24]. Site quality, in terms of soil fertility and climate, was by far the most important factor affecting hybrid poplar yields in southern Québec upland farm sites [
22].
3.2. Some Clones Reached Their Biomass Production Limit after 9 Years
A significant Clone effect (
p < 0.001) was detected for volume and woody biomass production after nine years, with total volume and woody biomass production ranging from 216 to 314 m
3ha
−1 and from 78.3 to 146 Mg ha
−1 (
Figure 3,
Table 5). Clone ranking in terms of yield remains similar from year six to nine, with clone MxB-915311 being the most productive, and clone DxN-3570 being the least productive (
Table 5). Yet, if broken / harvested trees are not included in calculations, the production of clones MxB-915311 and DNxM-915508 were not significantly different after nine years (
Figure 3,
Table 6). We suggested earlier that clone MxB-915311 might not be suitable for the production of solid wood products over long rotation in riparian buffers [
1]. The allometric relationships presented in this study, along with field observations, provide additional evidence supporting this recommendation.
Between 5 to 20 cm DBH, allometric relationships between DBH and stem volume or stem biomass are similar for the three clones (
Figure 2a, b). However, for larger trees (DBH > 20 cm), clone MxB-915311 accumulated much more stem volume and biomass for a given DBH than the two other clones. This may be related to its forking habit, which generates multiple main stems (
Figure 4). Furthermore, at equivalent DBH, clone MxB-915311 and clone DNxM-915508 had much more branch biomass than clone DxN-3570 (
Figure 2c). This particular tree architecture of clone MxB-915311 is consistent with its inherent fragility when planted in windy environments, such as riparian buffer strips. Several trees of this clone had broken at the end of the eighth and during the ninth growing seasons (
Figure 4). These broken trees accounted for 150 m
3 of wood at the Bromptonville site. Clearly, this clone had reached its physical limit to produce biomass after 9 years on the best site.
Thus, in riparian buffers designed for production, clone MxB-915311 should only be planted to produce biomass or pulp wood on short rotations, given its high productivity at a young age [
1] and its high susceptibility to mechanical breakage when it reaches larger diameters (DBH > 20 cm). These observations are consistent with the fact that
P.
deltoides and DxN hybrids are now favored over balsam poplar hybrids for long term uses such as shelterbelt plantings in the northern North American Prairies [
52].
Given the high volume of broken woody biomass that has been produced by clone MxB-915311 at Bromptonville after only nine years, it is clear that this clone may be used to provide large amounts of coarse woody debris in riparian zones within a short time frame. These coarse woody debris are key structural attributes for both aquatic and terrestrial biodiversity [
53,
54], but they also have important water quality functions, as reviewed by Dosskey
et al. [
10]. Since growth of natural mature riparian forest and production of coarse woody debris often takes decades, even centuries following forest removal [
55], hybrid poplar planting with clones such as MxB-915311 may be used to rapidly restore these key structural attributes. Conversely, the use of clones that are susceptible to mechanical breakages in buffer design may result in important gaps in the canopy over the years, which will increase the quantity of light reaching the understory. This situation may negatively affect native plant communities given the strong positive relationship between canopy openness and richness or abundance of exotic plants in poplar buffer understories [
15].
With its low branch biomass and straight bole, clone DxN-3570 might be a good candidate for riparian agroforestry systems that are designed for the production of solid wood products on longer rotations (
Figure 2c,
Figure 4). Although clone DxN-3570 was the least productive across the four study sites, very high yields were obtained at the fertile Bromptonville site after 9 years (44.3 m
3ha
−1yr
−1). In addition,
Populus deltoides ×
P.
nigra (DxN) hybrids generally have higher wood density and better mechanical proprieties than hybrids related to the
Tacamahaca (balsam poplars) section [
29,
56]. Clone DNxM-915508 also produced a straight bole. However, its high branch biomass (
Figure 2c) might increase labor costs or time associated with pruning operations, a silvicultural treatment often recommended for the production of knot free wood [
28].
Figure 4.
On the left, straight bole of clone DxN-3570 planted in riparian buffer strips. On the right, mechanical damage to clone MxB-915311 at the Bromptonville site following strong winds at the end of the eighth growing season.
Figure 4.
On the left, straight bole of clone DxN-3570 planted in riparian buffer strips. On the right, mechanical damage to clone MxB-915311 at the Bromptonville site following strong winds at the end of the eighth growing season.
3.3. Which Soil Testing Method can be Used to Assess Riparian Soil Fertility for Hybrid Poplar Agroforestry?
Results from the stepwise regression suggest that the three soil testing methods used in this study (nutrient supply rates measured with ion exchange membranes, nutrient stocks in the 0–20 cm soil depth range, and nutrient concentrations in the 0–20 cm soil depth range) gave similar models for predicting hybrid poplar volume yield across the four study sites (
Table 7). Independently of the soil testing method used, available soil P, in terms of P supply rate, available P stock or available P concentration, was always the first soil factor explaining hybrid poplar volume yield in this study (
Table 7). This trend was also observed for NxM, MxB and DNxM hybrids across a gradient of climate and soil fertility in abandoned farmland of southern Québec [
22].
The three soil testing methods used in this study may be useful to understand relationships between poplar productivity and riparian soil fertility since soil fertility variables measured with the different methods are highly correlated (
Table 8). Strong correlations were observed for a given nutrient when nutrient supply rates were plotted against nutrient stocks (
r = 0.68–0.83,
p < 0.001) or nutrient concentrations (
r = 0.57–0.67,
p < 0.001) (
Table 8). Strong correlations between NO
3 supply rate measured with PRS-Probes, soil NO
3 concentration, or nitrification, have also been observed in hybrid poplar buffers [
57]. Still, the best correlations were generally observed when nutrient supply rates were plotted against nutrient stocks. This indicates that nutrient supply rate measured with ion exchange membrane (PRS-probes) may better reflect soil nutrient stocks, which are a function of bulk density, stoniness and nutrient concentrations, than nutrient concentrations alone (
Table 8). The weaker model in the stepwise regression, in terms of determination coefficient (
R2), was also the one developed with nutrient concentrations as predictor variables (
Table 7). It has also been shown in agricultural studies that nutrient supply rates measured over a wide range of soil types with ion exchange membranes were a better index of nutrient availability than the use of nutrient concentrations obtained from chemical extractions [
31].
Table 7.
Results of stepwise regressions between nutrient supply rates, nutrient stocks or nutrient concentrations in the 0–20 cm soil depth range (predictor variables), and hybrid poplar volume yield (m3ha−1yr−1) (response variable) measured at the end of the ninth growing season (n = 48). Volume yield calculations include trees of clone MxB-915311 that had broken at the end of the eighth and during the ninth growing seasons and that were harvested by the landowner. All models and predictor variables are significant at p < 0.05.
Table 7.
Results of stepwise regressions between nutrient supply rates, nutrient stocks or nutrient concentrations in the 0–20 cm soil depth range (predictor variables), and hybrid poplar volume yield (m3ha−1yr−1) (response variable) measured at the end of the ninth growing season (n = 48). Volume yield calculations include trees of clone MxB-915311 that had broken at the end of the eighth and during the ninth growing seasons and that were harvested by the landowner. All models and predictor variables are significant at p < 0.05.
Nutrient supply rates (µg 10cm−2 20d−1) | Parameter estimate | R2 | Nutrient stocks (kg ha−1) | Parameter estimate | R2 | Nutrient concentrations (mg kg−1) | Parameter estimate | R2 |
---|
P | 2.53 | 0.41 | P (available) | 0.098 | 0.45 | P (available) | 0.25 | 0.36 |
K | 0.03 | 0.67 | K | 0.042 | 0.65 | Mg | −0.042 | 0.51 |
Mg | −0.023 | 0.75 | Ca | 0.0049 | 0.73 | K | 0.079 | 0.67 |
Intercept | 21.8 | | Mg | −0.023 | 0.79 | Intercept | 21.4 | |
| | | Intercept | 12.4 | | | | |
Table 8.
Correlation coefficient (r) obtained from pairwise correlations between nutrient stocks (kg ha−1) or nutrient concentrations (mg kg−1) in the 0–20 cm soil depth range and nutrient supply rates in the 0–10 cm soil horizon measured with PRS-probes. All correlations are significant at p < 0.001.
Table 8.
Correlation coefficient (r) obtained from pairwise correlations between nutrient stocks (kg ha−1) or nutrient concentrations (mg kg−1) in the 0–20 cm soil depth range and nutrient supply rates in the 0–10 cm soil horizon measured with PRS-probes. All correlations are significant at p < 0.001.
Nutrient stocks (kg ha−1) vs nutrient supply rates (μg 10cm−2 20d−1) | r | Nutrient concentrations (mg kg−1) vs nutrient supply rate (μg 10cm−2 20d−1) | r |
---|
Available P stock vs P supply rate | 0.78 | Available P concentration vs P supply rate | 0.72 |
Ca stock vs Ca supply rate | 0.77 | Ca concentration vs Ca supply rate | 0.57 |
K stock vs K supply rate | 0.83 | K concentration vs K supply rate | 0.73 |
Mg stock vs Mg supply rate | 0.68 | Mg concentration vs Mg supply rate | 0.67 |
Beyond result accuracy, numerous advantages and disadvantages are associated with the use of the different soil testing methods. From a practical point of view, it is clear that the more convenient soil testing method is to assess only soil nutrient concentrations. With this method soil samples are easily collected in the field without specific equipment and, once dry, soil samples can be sent directly to a soil analysis laboratory. Therefore, a landowner could easily collect soil samples by himself from different areas of his fields, and soil analysis results could be used to identify the more fertile areas. The same approach could be used at a regional scale to identify high quality riparian sites for hybrid poplar agroforestry. Although very convenient, the sole use of nutrient concentrations as indicators of soil fertility also has its disadvantages. Depending on the chemical extraction solution used to process soil samples, nutrient concentration measurements for a particular nutrient will vary greatly [
58], which makes standardization very difficult among results obtained from different extraction methods. In addition, accurate evaluation of soil NO
3 is complicated by the fact that the NO
3 concentration in a soil sample can change significantly if the sample is not handled properly once collected [
37]. Therefore, soil samples should be dried immediately after sampling, a procedure that is not always logistically possible [
37].
The use of PRS-probes ion exchange membranes in long term burials (20 days in this study) has the advantage of providing information, in undisturbed conditions, on the dynamics of nutrient supply, which are affected by processes such as mineralization and dissolution, but also by factors such as soil temperature and moisture content [
31]. PRS-probes are easy to install in most soil types, although probe breakage may occur in stony soils, as was the case at the Magog site (
Figure 1a). Furthermore, we have seen, on rare occasions, wildlife disturbance and trampling of the probes. A source of distilled or deionised water is also required to wash the probes when they are removed from the soil. One of the challenges with the use the PRS-probe technology, is that multiple site assessments (with long time burials) requires that ion exchange membranes be buried simultaneously and for the same time period. This can be troublesome if there are several distant sites to assess.
From a practical perspective, the PRS-probes can be a useful tool to rank potential sites for hybrid poplar riparian agroforestry within a region given the strong relationship between poplar yield and nutrient supply rates [
1] (
Table 7). However, a standardized approach needs be developed for hybrid poplar site assessment because soil nutrient status in riparian buffer strips may evolve with time and with ongoing upland agricultural activities. While NO
3 supply rate best predicts hybrid poplar growth in the same riparian buffer strips after 6 years [
1], P supply rate was the best predictor variable for volume yield after the 9th growing season (
Table 7). This discrepancy might be related to the change in NO
3 and P supply rates measured at Bromptonville and St-Isidore-de-Clifton. While NO
3 and P supply rates were similar after 6 years at these two sites [
1], a significantly higher NO
3 supply rate was measured at St-Isidore during the ninth year, while P supply rate was significantly higher at Bromptonville (
Table 2). The higher NO
3 supply rate measured during the ninth growing season at St-Isidore-de-Clifton may be related to the application of inorganic N fertilizer in the adjacent pasture (N application rate = 18 kgha−1 per 5 years), one month prior to the installation of PRS-probes in the soil (
Table 1).
For the moment, Western Ag provides a service to crop producers in western Canada and North Dakota only. The service is delivered through field service representatives, who obtain soil samples that are incubated with PRS-probes for 24 hours under standardized conditions and, more importantly, used with a computer model (PRS-probe Nutrient Forecaster) to assist with the planning of which crops to grow and how to fertilize them (Eric Bremer, pers. com., Western Ag). A similar tool could be developed regionally for hybrid poplar plantation site selection.
Concerning the use of nutrient stocks, we have identified many drawbacks with this approach for practical application to soil testing in riparian agricultural zones. First, the determination of nutrient stocks, as it is also the case for determining carbon stocks, requires that soil bulk density be assessed adequately. The core method has become ecologists’ favored method for bulk density measurements [
39]. However, since bulk density estimation with the core method can be done using three different methods, which reflect how coarse fragments (>2 mm) are handled in calculations, inconsistencies in nutrient or carbon stock calculations may occur [
39]. To increase the precision of bulk density measurements, Throop
et al. [
39] suggest removing coarse fragments from cores by sieving, and then calculate bulk density as the soil dry mass divided by the core volume. Consequently, this approach is more time-intensive than the common determination of core bulk density obtained from the dry mass of the entire core divided by core volume [
39]. Furthermore, despite the widespread use of the core method to quantify bulk density, it is clear that under many circumstances it is inappropriate. For example, the core method will underestimate nutrient stocks when coarse fragments are larger than the size of the corer [
59]. Consequently, in soils with a high volume of coarse fragments, a very large volume of soil needs to be excavated to properly measure the soil volume occupied by stones [
60], a procedure that is very time consuming, labor intensive and costly.
In short, for practical reasons, we would recommend the use of both soil nutrient concentrations obtained from chemical extractions and soil nutrient supply rates obtained from ion exchange membranes (PRS-probes) to assess soil fertility in hybrid poplar riparian agroforestry systems.
It is also important to mention that nutrients stocks, concentrations and supply rates reported in this study were only measured once in the ninth growing season. Therefore, relationships between nutrient availability and hybrid poplar productivity should be interpreted with caution given the uncertainties associated with the snapshot approach used to measure nutrient availability. Nutrient availability in a riparian buffer may fluctuate between growing seasons and during a single growing season as it is influenced by management practices in the adjacent agricultural land use, local climate (precipitation and temperature), natural disturbances such as flooding, water table level fluctuations and biological processes such as nutrient uptake by trees, organic matter mineralization, denitrification and bacterial immobilization [
61,
62,
63,
64,
65]. Repeated measures of nutrient availability within and between growing seasons should be done in further studies to have a more complete picture of the causal relationship between site fertility and poplar growth in agricultural riparian zones. Nevertheless, one-time soil nutrient measurements have been shown to be strong predictors of hybrid poplar yields over eight years of growth [
22], and are therefore very useful for the selection of new plantation sites, without the need for multiple soil nutrient measurements over an entire season and over several years.
3.4. Agricultural Riparian Zones as Prime Areas for Sustainable Poplar Production: Some Considerations for Landowners
With volume and woody biomass yields ranging from 26.3 to 49.9 m
3ha
−1yr
−1 and from 11.4 to 21.4 Mg ha
−1yr
−1 after nine years, obtained at the three most productive sites (
Table 6), it is clear that hybrid poplar riparian buffers can produce very high quantities of wood and biomass, when compared to other poplar plantation systems in Québec [
48], while increasing nutrient accumulation, carbon sequestration and habitat quality for native plants [
14,
15]. It is also important to mention that the hybrid poplar riparian buffer strips studied had received very minimal silvicultural treatments; there was no site/soil preparation and there was a single local (1 m
2/tree) herbicide application early during the first growing season. It has been argued that improving the sustainability performance of bioenergy systems can be achieved by minimizing emissions to air, water and soil, and by developing systems that maintain or improve biodiversity [
66]. With that in mind, hybrid poplar riparian buffers can contribute to improve the sustainability of biomass and timber production in temperate regions.
Some considerations concerning tree harvesting in riparian zones could also improve the sustainability of biomass production in agricultural riparian zones. To maintain benefits for biodiversity, wood or biomass harvest in riparian buffers should be planned at both farm and landscape levels, in order to continuously maintain a proportion of unharvested patches, which can be achieved by rotational and/or selective harvests [
67,
68,
69]. Rotational or selective harvests will also be important to maintain other functions, such as the nutrient accumulation potential of the buffer [
70]. Rotational harvest could also reduce impacts on soil erosion [
71]. Heavy machinery, which is often used to harvest biomass or timber, can cause soil compaction; a problem that can be overcome if harvest only occurs when the ground is frozen [
72].
This study also provides evidence that trees growing in agroforestry systems such as riparian buffers may have different biomass allocation patterns compared to trees growing in forests or in plantations (
Figure 5). A significant decline in the proportion of branch biomass is generally observed with increasing tree size for both conifer and hardwood species growing in natural forests [
73]. This trend has also been observed along a productivity gradient in eight year-old hybrid poplar plantations of southern Québec [
22] (
Figure 5). However, in this study the proportion of branch biomass of nine year-old poplars remains the same across the four sites (26%), although aboveground woody biomass accumulation varies considerably (
Table 5) (
Figure 5). The same trend was observed when the riparian buffers were six years old (
Figure 5). This suggests that hybrid poplars grown in high light environments, such as riparian buffer strips, will produce more branch biomass than hybrid poplars grown in large conventional plantations. Consequently, landowners who wish to produce veneer lumber in buffer strips may have to dedicate more resources to pruning operations than in conventional plantation systems.
Figure 5.
Significant relationship (
p < 0.001) between mean aboveground woody biomass measured at each site and branch biomass expressed as a percentage of aboveground woody biomass in eight year-old hybrid poplar plantations of southern Québec (data obtained from Truax
et al. [
22]). No significant relationship links the two variables at the four hybrid poplar buffer sites (at six or nine years).
Figure 5.
Significant relationship (
p < 0.001) between mean aboveground woody biomass measured at each site and branch biomass expressed as a percentage of aboveground woody biomass in eight year-old hybrid poplar plantations of southern Québec (data obtained from Truax
et al. [
22]). No significant relationship links the two variables at the four hybrid poplar buffer sites (at six or nine years).
Another important consideration is related to the marketing of timber and biomass produced in riparian zones. As observed in the US cornbelt region, markets for biomass are presently lacking and market-pull will be required to organize harvesting, processing, storage and transport of woody biomass [
74]. Foresters interviewed in the same region noted that one of the biggest constraints to woody biomass production on privately owned agricultural land would be the size of individual plots, but also the weak return on investments [
74]. The same constraints may be associated with the production of woody biomass and timber in farmland riparian zones of southern Québec. In a recent provincial report, it was stated that local markets for biomass for bioenergy are lacking in agricultural areas of Québec, that provincial regulations are overly restrictive and that economic benefits are uncertain [
75]. Another important logistical constraint lies in the linear configuration of riparian corridors, which results in a dispersed resource at the landscape level. Therefore, longer average hauling distances to lumber and/or biomass facilities characterize linear plantation systems when compared to large-scale plantations, which can be concentrated near the facility. This is an important issue since break-even costs of biomass production in riparian agroforestry systems is largely dependent on transportation distance to transformation centers [
76].
In that context, the best alternative might be to use hybrid poplar biomass directly on the farm, as firewood (or chips) for heating farm buildings and houses, as it was the case at the Bromptonville site (
Figure 6). This would in turn help reduce harvesting pressures for firewood in the last few remaining natural forests in the agricultural landscape. Establishing wide riparian buffers (10–15 m) might also be a way to reduce hauling distance to biomass facilities, while increasing the quality of other ecosystem services (non-point source pollution control, habitat for biodiversity, soil stabilization, flood control,
etc.) [
77].
Figure 6.
Wood harvested in hybrid poplar riparian buffer strips can be used as firewood for farm buildings and houses, as it was the case following a partial harvest (one in nine trees) in 2008 (sixth growing season) at the Bromptonville site.
Figure 6.
Wood harvested in hybrid poplar riparian buffer strips can be used as firewood for farm buildings and houses, as it was the case following a partial harvest (one in nine trees) in 2008 (sixth growing season) at the Bromptonville site.
To increase the economic feasibility of hybrid poplar buffer implementation on farmland, ecosystems services, such as water quality and habitat protection, erosion and flood control, carbon sequestration,
etc., should no longer be considered as externalities [
78]. An appropriate valuation of these ecosystem services [
79] is needed, because this added value might be the only way to offset the economic loss associated with the conversion of some areas of agricultural systems into riparian agroforestry systems, especially in the current context of high annual crop value [
20].