Results and Discussion
Results from a dbRDA (Figure 13), NMDS (Figure 14), PCA (Figure 15), and perMANOVA (Table 3) of the data are presented below.
There were no trends observed in the distance-based redundancy analysis (dbRDA) of the vegetation species and environmental variables based upon underplanting treatments (Figure 13). The first axis (CAP1; eigenvalue=2.33) explained 6.5% of the variation in the species data and the second axis CAP2; eigenvalue=1.51) explained 6.3% of the variation in species data. All other axes have eigenvalues below 1 and are not present here.
There were no trends observed in the distance-based redundancy analysis (dbRDA) of the vegetation species and environmental variables based upon underplanting treatments (Figure 13). The first axis (CAP1; eigenvalue=2.33) explained 6.5% of the variation in the species data and the second axis CAP2; eigenvalue=1.51) explained 6.3% of the variation in species data. All other axes have eigenvalues below 1 and are not present here.
Fig. 13: Relationship of understory species composition to understory environmental variables determined using a distance-based Redundancy Analysis. Points are plots sampled; colors indicate plot location and shape indicate year underplanted. Vectors represent environmental variables (FHDep: FH layer depth; Ca: soil calcium; NO3N: soil nitrate; LitDep: litter depth; TotN: total soil nitrogen; Decomp: decomposition rate; Mg: soil magnesium; NH4N: soil ammonium; K: soil potassium; Moist: soil moisture).
No trends were observed in non-linear multidimensional scaling (NMDS) ordination of vegetation cover type based upon underplanting treatmeats (Figure 14). Only the first two dimensions (X1 and X2) are present here because of the low stress (0.118).
No trends were observed in the principal component analysis of the environmental variables based upon underplanting treatments (Figure 15). The proportion of the variance explained by the first axis (Comp.1) is 0.229 and by the second axis (Comp.2) is 0.178.
Fig. 15: Principal Component Analysis (PCA) plot of the first and second components based upon understory environment. Points are plots sampled; colors indicate plot location and shape indicate year underplanted. Vectors represent environmental variables (FHDep: FH layer depth; Ca: soil calcium; NO3N: soil nitrate; LitDep: litter depth; TotN: total soil nitrogen; Decomp: decomposition rate; Mg: soil magnesium; NH4N: soil ammonium; K: soil potassium; Moist: soil moisture).
A dbRDA of the environmental and vegetation species data, a NMDS of the vegetation cover types, and a PCA of the environmental data were conducted using only the 1994 data (not shown) to have a better look at only the oldest underplanted stands. Figures did not show any trends based upon the different plot locations.
There were no significant differences in understory environment and vegetation from plot location (base, adjacent, control; p=0.4942) and year underplanted (0.6744) based upon a permutation MANOVA of the data (Table 3). The only significant difference indicated by the perMANOVA was in the stands within year (p=0.0002). This can be attributed to differences in the ecosites of the aspen stands which were studied. For example, some of the stands had an aspen/rose d1.6 ecosite classification while others had an aspen/beaked hazelnut d1.3 ecosite classification (Beckingham and Archibald 1996). In the nine stands studied, six different ecosites were observed; this can account for some of the variation observed in Figures 13-15. Control plots were established within each stand to take this variation due to ecosite into consideration.
Underplanting white spruce in aspen-dominated stands does not change the understory environment or vegetation by 15 years after underplanting. White spruce is not an ecosystem engineer by this point in time (as seen by no separation among the different treatments by environmental variables in the PCA). Since we know that differences exist in the understory environment and vegetation between aspen, aspen-white spruce mixedwoods and white spruce stands, this suggests that by 15 years after underplanting, the white spruce have not been present in the stands long enough and/or are large enough to be an ecosystem engineer. As the white spruce grow larger over time, creating more shade and contributing more needles to the litter, the white spruce may start to act as an ecosystem engineer. Further research will be conducted as part of this project during the 2010 summer field season to look at the influence of older underplanted white spruce on the forest understory.
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