Study sites and experimental design
This study took place in the southern region of the province of Québec, Canada. At the four study sites (Bromptonville, Magog, Roxton Falls and St-Isidore-de-Clifton) three types of riparian land uses were studied for root biomass and soil C stocks distribution: (1) hybrid poplar riparian buffer, (2) herbaceous riparian buffer and (3) natural riparian woodlot.
Three of the study sites (Bromptonville, Magog and Roxton Falls) are located in a hilly landscape (Sherbrooke unit), which is characterised by gentle slopes and a continental subhumid moderate climate (Robitaille and Saucier 1998). Land use in this landscape unit is 71% natural and managed forest (mostly private), 28% agriculture and 1% urban. Agricultural activities are concentrated in larger valley bottoms; pastures are frequently found on the poorer hillside soils. The St-Isidore-de-Clifton site is located in the Mont Mégantic landscape unit, which is characterised by continental subhumid-subpolar climate, higher elevation, steeper hillside slopes and lower agricultural land use (9% of land use) (Robitaille and Saucier 1998). Both landscape units are covered by a thick surface deposit of till and share a similar precipitation regime (1000–1100 mm). St-Isidore-de-Clifton, Magog, and Bromptonville sites are located in the St-François River watershed, while the Roxton Falls site is in the Yamaska River watershed. These watersheds both drain into the St. Lawrence River.
At each site, hybrid poplar riparian buffers where planted in spring 2003 at a density of 2222 stems per hectare on both sides of the streams for a total length of 90 m and a width of 4.5 m on each stream bank. Bare-root hybrid poplar plants were 1 year-old when they were planted. In the year of the study (2011), the buffers were in their 9th growing season. No site preparation was done prior to planting and tending operations consisted in a single localised herbicide treatment (1 m2/tree) in June 2003. Information regarding hybrid poplar buffer management, aboveground biomass and volume growth, aboveground nutrient and C accumulation, and understory biomass and diversity can be found in previous studies (Fortier et al. 2010a, 2010b, 2011; 2012;2013).
At each site, unmanaged (free-growing) herbaceous buffers were located within 100 m upstream or downstream of the hybrid poplar buffers. These herbaceous buffers generally consist of a mixture of native and exotic ruderal species that have naturally colonised the riparian zone, or that have been sown as pasture forage (Fortier et al. 2011). The dominant species (in percent coverage) in such buffers are Phleum pratense, Agropyron repens, Agrotis spp., Vicia cracca, and Solidago spp. The unmanaged herbaceous buffers were protected by a fence for at least two years at the three pasture sites to prevent livestock grazing.
At each site, a natural riparian woodlot, located as close as possible from both hybrid poplar and herbaceous buffers, was selected. These woodlots were located 1 km or less upstream of the poplar buffers. The 4 riparian woodlots were very different among the sites: (1) a 200 year-old eastern hemlock (Tsuga canadensis) dominated stand at Bromptonville; (2) a 73 year-old eastern white cedar (Thuja occidentalis) stand where livestock have complete access at Magog; (3) a 27 year-old grey birch (Betula populifolia) stand at Roxton Falls, and (4) a 54 year-old sugar maple (Acer saccharum) stand at St-Isidore-de-Clifton. The age of these stands was estimated by coring the dominant trees. Riparian land use characteristics are summarized in Table 5.
In the hybrid poplar buffer land use, a randomized block design was used at each of the 4 sites, with 4 blocks (replicates) and 3 hybrid poplar clones: (1) P. deltoides × nigra (DxN-3570; also named P. x canadensis); (2) P. canadensis × maximowiczii (DNxM-915508); and (3) P. maximowiczi × balsamifera (MxB-915311). A total of 48 hybrid poplar riparian buffer experimental plots were sampled in this study. These plots are 4.5 m wide and 9 m long (40.5 m2). Each plot contains 9 trees from a single clone (3 rows; 3 trees per row).
At each site, four herbaceous buffer plots were sampled (n = 16, 4 sites × 4 plots/site). The size of the herbaceous buffer plots was 4.5 m (poplar buffer width) × 9 m (40.5 m2). At each site, there were 4 riparian woodlot plots (n = 16, 4 sites × 4 plots/site). The size of these woodlot plots was 4.5 m × 9 m (40.5 m2).
In this study, the entire experimental design contains 80 experimental plots covering three types of riparian land uses: (1) 48 hybrid poplar riparian buffer plots; (2) 16 unmanaged herbaceous riparian buffer plots; (3) 16 natural riparian woodlot plots.
Coarse and fine root sampling
Root sampling was done from mid-June to mid-July 2011. In each plot (n = 80), coarse root biomass (diameter > 2 mm) samples were obtained by excavating pits (50 × 50 cm by 60 cm deep) and harvesting all coarse roots in the pits. During the excavations, coarse root distribution was also measured for three soil depth ranges: (1) 0–20 cm, (2) 20–40 cm and (3) 40–60 cm. Coarse root samples where washed with water and air dried. Coarse root subsamples were collected to determine dry weight. In the hybrid poplar buffer and woodlot plots, the pits were located 25 cm away from a representative tree, so that coarse root samples did not include stump biomass. The representative tree was the closest to the average diameter at breast height (DBH) of all trees in the plot. Diameter at breast height ranges of the sampled trees for the hybrid poplar buffer and the woodlots at each site are given in Table 5. In the particular case of herbaceous buffers, roots having a diameter greater than 2 mm were herbaceous plant rhizomes and they will be considered as coarse roots in this study.
In each plot (n = 80), fine root biomass (diameter < 2 mm) samples were obtained by extracting two soil cores (core diameter = 5.3 cm, core length = 10 cm, volume of both cores = 220.6 cm3) from pit walls for each of three depth ranges (10–20, 20–40 and 40–60 cm). In each plot, two additional soil cores were randomly extracted vertically from the soil surface (0–10 cm), and combined with the two cores extracted from pit walls between 10–20 cm depth, in order to obtain a single fine root sample for the 0–20 cm depth range. For the 20–40 and 40–60 depth ranges, the two soil cores were combined to produce a single fine root sample per depth. Fine root biomass samples, which contained both live and dead fine root biomass, were separated from soil by hand picking, washed and dried at 65°C to determine dry weight.
Mineral soil characteristics and carbon stocks and distribution
Soil sampling was done from mid-June to mid-July 2011. In each plot (n = 80), soil characteristics and carbon stocks were obtained by extracting two soil cores (core diameter = 5.3 cm, core length = 10 cm, volume of both cores = 220.6 cm3) from pit walls for each of three depth ranges (0–20, 20–40 and 40–60 cm). For each depth range, the two cores were combined to produce a single soil sample. In woodlot plots, the sampling protocol was slightly modified for the 0–20 cm layer in order to properly sample the A horizon, which was relatively thin at some sites. In woodlot plots, one core was extracted vertically from the soil surface (0–10 cm) and combined with another core extracted from pit walls between 10–20 cm depth. Soil samples were air dried and sieved (2 mm). Soil C concentrations were determined by the combustion method at high temperature (960°C) followed by thermal conductivity detection. These analyses were done by the CEF lab (Dr. R. Bradley and Dr. W. Parsons) at the University of Sherbrooke. Soil pH and texture were determined by the Agridirect Inc. soil analysis lab in Longueuil (Québec). Methods used are those recommended by the Conseil des productions végétales du Québec (1988). The determination of soil pH was made using a 2:1 ratio of water to soil. For particle size analyses, the Bouyoucos (1962) method was used. However, due to high analysis costs, particle size analysis was done on composite soil samples. In the poplar buffers, soil samples where pooled at the block level at each site (4 samples were analysed per site). For the herbaceous buffers, one composite soil sample was made at each site by combining soil samples collected in each replicate. The same procedure was used in woodlot plots.
Soil bulk density was determined by drying sieved soil at 105°C and dividing the soil dry mass by the volume of soil cores, as recommended by Throop et al. (2012). Stoniness was assessed visually, by at least two persons, from the soil pit excavation. For each sampling depth range, stones (larger than the core diameter) that were removed by excavation were replaced in the pit to estimate pit volume (in %) that was occupied by stones. In each plot and for each depth, C stocks and nutrient stocks were calculated by multiplying soil C and nutrient concentrations with soil mass, with respect to soil bulk density and stoniness.
Forest floor sampling
In each woodlot plot that had a LFH Horizon (O Horizon), three LFH samples (50 × 50 cm) were collected at the end of July 2011. These LFH samples consisted essentially of dead tree leaves, and excluded fine and coarse woody debris. Subsamples were collected to determine dry weight and C concentrations and contents of the LFH layer.
Statistical analyses
For data analysis related to hybrid poplars, ANOVA tables were constructed in accordance with Petersen (1985), and degrees of freedom, sum of squares, mean squares and F-values were computed. When a factor was declared statistically significant (Site, Clone and Site × Clone interaction), the standard error of the mean (SE) was used to evaluate differences between means for three levels of significance (p < 0.05, p < 0.01 and p < 0.001). All of the ANOVAs were run with the complete set of data (4 sites, 3 clones, 4 blocks = 48 experimental plots).
Given that no Clone effect and no Site × Clone interaction were detected by the ANOVA on root biomass and soil variables in the hybrid poplar experimental design, we have averaged root and soil variables of the 3 clones within a block, in order to produce data at the block level. Consequently, for statistical analysis, the number of plots in the hybrid poplar buffer land use type was reduced from 48 to 16 plots, which is equivalent to the number of plots found in the two other riparian land uses (herbaceous buffer and woodlot). Thereafter, a series of ANOVAs was used to evaluate the riparian Land use and Site effects and Land use × Site interaction on root biomass and soil C variables. The model for each ANOVA included 3 Land uses (hybrid poplar buffer, herbaceous buffer and woodlot) and 4 sites (Bromptonville, Magog, Roxton Falls and St-Isidore-de-Clifton) and 4 replicates of each riparian land use at each site (3 Land uses × 4 Sites × 4 replicates = 48 plots).
For the presentation of results in figures, abbreviations of the names of plantation sites were used (Bromptonville = Bro, Magog = Mag, Roxton Falls = Rox, St-Isidore-de-Clifton = Sti). Root biomass and soil carbon stocks data were scaled up to the hectare for comparison purposes with other studies.