The lowest number of all the leaves was observed at the 1st (May/June) sampling term, while the highest was at the 2nd (June/July) sampling term. Consequently, the number of all leaves decreased, till, in September and October, this decrease was significant (2nd June/July versus 4th September p < 0.05; 2nd June/July versus 5th October p < 0.001; 3rd August versus 5th October p < 0.01). The weight of the all leaves was the highest in the 2nd (June/July) sampling term. Consequently, no distinctions were observed between the separate sampling terms. However, the average weight of the single leaf was significantly higher at first, in comparison to other of the sampling terms. The ratio between the entire plant´s weight and all leaves´ weight was more than 3.5, in general, as well as in every category of sampling stands. The lowest number of the assimilating leaves was noticed in May. Consequently, their number significantly increased from June to August, and repeatedly significantly decreased in October. A similar pattern was observed concerning the weight of assimilating, green, leaves; however, the distinctions between the particular sampling terms were not significant. In opposition, the number, as well as the weight, of non-assimilating, brown, leaves were the highest just at the end of the growing season. Since the average weight of the single assimilating leaf decreased, those of the non-assimilating increased from the beginning to the end of the growing season. The mutual ratio of the assimilating and non-assimilating leaves changes on behalf of the non-assimilating leaves as the growing season came to the end.

Fully developed inflorescences were noticed in the 3rd sampling term in the first half of August. In general, neither the length nor the weight of the inflorescence changed significantly during the monitored period.

The highest relative water content of the entire plant, stem, and all the leaves was noticed in the 1st (May/June) sampling term and in inflorescence at the beginning of the blooming period. Consequently, relative water contents decreased, and the lowest were noticed in October.

2.2. Morphological Traits—Different Degree of Invasion

In general, the plants from the stands with the mild degree of invasion (25–50%) were significantly lower (A versus C p < 0.001; B versus C p < 0.01) and lighter (A versus C p < 0.05). The differences between the sampling stands in the plant´s height and the entire plant´s weight are also shown in the dendrograms (

In general, the plants from the stands with the mild degree of invasion (25–50%) were significantly lower (A versus C p < 0.001; B versus C p < 0.01) and lighter (A versus C p < 0.05). The differences between the sampling stands in the plant’s height and the entire plant’s weight are also shown in the dendrograms (

Figure 1a,b), where clusters, consisting of stands with the identical degree of invasion (A1–4, B5–8, C9–12), are clearly recognizable. The separated position of the group of stands with a heavy degree of invasion (A category) is obvious.

a,b), where clusters, consisting of stands with the identical degree of invasion (A1–4, B5–8, C9–12), are clearly recognizable. The separated position of the group of stands with a heavy degree of invasion (A category) is obvious.

Figure 1. Dendrograms of the sampling stands classification according to Solidago canadensis (a) height and (b) entire plant’s weight. Stands with the different degree of invasion were classified using Cluster analysis, Ward’s method. Abbreviation and notes: A1–4 = heavily invaded stands with 75–100%, B5–8 = medial invaded stands with 50–75%, C9–12 = mildly invaded stands with 25–50% relative abundance of Solidago canadensis within the vegetation cover.

The plants with the longest stems were noticed within the stands with 75–100% S. canadensis relative abundance. The stem´s average length significantly decreased simultaneously with the decrease of invasion degree (A versus C p < 0.001; B versus C p < 0.001). The stem weight was, in general, significantly lowest in the plants from the C category of sampling stands (A versus C p < 0.001; B versus C p < 0.001).

The plants with the longest stems were noticed within the stands with 75–100% S. canadensis relative abundance. The stem’s average length significantly decreased simultaneously with the decrease of invasion degree (A versus C p < 0.001; B versus C p < 0.001). The stem weight was, in general, significantly lowest in the plants from the C category of sampling stands (A versus C p < 0.001; B versus C p < 0.001).

The lowest number of all the leaves was observed in the plants from the mildly invaded stands (25–50% S. canadensis representation; A versus C p < 0.05; B versus C p < 0.001). Plants from the stands with mild degree of invasion had significantly lower weight of all leaves (A versus C p < 0.01; B versus C p < 0.01), as well as the single leaf’s weight, in comparison to plants from the A category (A versus C p < 0.01).

The lowest number of all the leaves was observed in the plants from the mildly invaded stands (25–50% S. canadensis representation; A versus C p < 0.05; B versus C p < 0.001). Plants from the stands with mild degree of invasion had significantly lower weight of all leaves (A versus C p < 0.01; B versus C p < 0.01), as well as the single leaf’s weight, in comparison to plants from the A category (A versus C p < 0.01).

In addition, researchers did not notice the distinctions in the number of assimilating, green, leaves between the A, B, and C categories. However, assimilating leaves from the plants of mildly invaded stands showed significantly lower weight (A versus C p < 0.01; B versus C p < 0.05). Non-assimilating, brown, leaves from the plants of the C category showed a significantly lower count, as well as weight, in comparison to those of A and B categories (number: A versus C p < 0.05; B versus C p < 0.01; weight: A versus C p < 0.01; B versus C p < 0.05).

In addition, we did not notice the distinctions in the number of assimilating, green, leaves between the A, B, and C categories. However, assimilating leaves from the plants of mildly invaded stands showed significantly lower weight (A versus C p < 0.01; B versus C p < 0.05). Non-assimilating, brown, leaves from the plants of the C category showed a significantly lower count, as well as weight, in comparison to those of A and B categories (number: A versus C p < 0.05; B versus C p < 0.01; weight: A versus C p < 0.01; B versus C p < 0.05).

The plants from the heavily invaded stands had the longest and heaviest inflorescences. However, neither the length nor the weight was significantly different, according to the degree of the invasion.

Generally, no distinctions in the entire plant´s nor stem´s relative water contents were observed between the A, B, and C sampling stands categories. Only all the leaves and inflorescences from the plants of the heavily invaded stands had a significantly higher relative water content in comparison to those from the mildly invaded stands (leaves A versus C p < 0.05; inflorescences A versus C p < 0.05).

Generally, no distinctions in the entire plant’s nor stem’s relative water contents were observed between the A, B, and C sampling stands categories. Only all the leaves and inflorescences from the plants of the heavily invaded stands had a significantly higher relative water content in comparison to those from the mildly invaded stands (leaves A versus C p < 0.05; inflorescences A versus C p < 0.05).

2.3. Relative Daily Growth Rate

Relative daily growth rate concerning selected morphological traits was, in general, the highest in the plants from the heavily invaded stands. The highest growth rate of the plant’s height, the entire plant’s weight, and the stem’s weight was noticed in the plants of the A and B categories between 1st/2nd sampling terms (May–-beginning of July). However, in comparison, the plants from the stands with just the mild degree of invasion (25–50%) showed their highest plant’s height, the entire plant’s weight, and stem’s weight daily growth rates to be belated, i.e., between the end of June and the first half of August (2nd/3rd sampling terms) (Figure 2). Belated daily growth rate was also noticed in the inflorescence, when, in plants of the A and B categories, flower weight relative daily growth was the highest between August and September, while, between September and October, it was highest concerning C category plants.

Figure 2. Differences in the relative daily growth rate of the selected morphological traits in relation to degree of Solidago canadensis invasion. Abbreviation and notes: A = heavily invaded stands with 75–100%, B = medial invaded stands with 50–75%, C = mildly invaded stands with 25–50% relative abundance of Solidago canadensis within the vegetation cover; ½—between 1st/2nd sampling terms (May–beginning of July), 2/3—between 2nd/3rd sampling terms (end of June and the first half of August).Figure 2. Differences in the relative daily growth rate of the selected morphological traits in relation to degree of Solidago canadensis invasion. Abbreviation and notes: A = heavily invaded stands with 75–100%, B = medial invaded stands with 50–75%, C = mildly invaded stands with 25–50% relative abundance of Solidago canadensis within the vegetation cover; ½—between 1st/2nd sampling terms (May–beginning of July), 2/3—between 2nd/3rd sampling terms (end of June and the first half of August).

The average daily growth rate of all the leaves’ weight was the highest between May and the beginning of July (1st/2nd sampling terms) and from side of the degree of invasion.

3. Essential Oil Productivity Rate

Mean values and standard deviations (+SD) of the EO yields are listed in Table 1, Table 2 and Table 3.

The hydro-distillation of the S. canadensis dry plant material yielded pale yellow oil. In general, the mean EO yield increased continuously till the 4th sampling term (September), when it was the highest, and significantly higher in comparison with every of the other sampling terms. A similar pattern was observed by evaluating the EOs´ yields from the separated plant organs, i.e., stems, leaves, and inflorescences, as well as according to degree of invasion. Generally, the highest average EO yield was obtained from the plants of the heavily invaded stands, while the lowest was obtained from those of the mildly invaded stands (

The hydro-distillation of the S. canadensis dry plant material yielded pale yellow oil. In general, the mean EO yield increased continuously till the 4th sampling term (September), when it was the highest, and significantly higher in comparison with every of the other sampling terms. A similar pattern was observed by evaluating the EOs’ yields from the separated plant organs, i.e., stems, leaves, and inflorescences, as well as according to degree of invasion. Generally, the highest average EO yield was obtained from the plants of the heavily invaded stands, while the lowest was obtained from those of the mildly invaded stands (

Figure 3a). The mean EO yield obtained from the plants of the A category was significantly higher in comparison to those of the C category (p < 0.01). The highest EOs´ yields were, in general, extracted from the inflorescences, followed by leaves and stems (

a). The mean EO yield obtained from the plants of the A category was significantly higher in comparison to those of the C category (p < 0.01). The highest EOs’ yields were, in general, extracted from the inflorescences, followed by leaves and stems (

Figure 3b). The inflorescences´ mean EO yield was, in comparison, also significantly higher (I versus L p < 0.001; I versus L p < 0.001). The same pattern was equally observed within the separated A, B, and C categories.

b). The inflorescences’ mean EO yield was, in comparison, also significantly higher (I versus L p < 0.001; I versus L p < 0.001). The same pattern was equally observed within the separated A, B, and C categories.

Figure 3. Differences in the essential oil productivity rate according to different Solidago canadensis (

 Differences in the essential oil productivity rate according to different Solidago canadensis (

a

) degree of invasion and (

b) plant organs. Abbreviations and notes: A = sampling stands´ category with 75–100%, B = 50–75% and C = 25–50% relative abundance of Solidago canadensis within the vegetation cover; S—stems, L—leaves, I—inflorescences. Boxplot outliers are shown as * and °.

) plant organs. Abbreviations and notes: A = sampling stands’ category with 75–100%, B = 50–75% and C = 25–50% relative abundance of Solidago canadensis within the vegetation cover; S—stems, L—leaves, I—inflorescences. Boxplot outliers are shown as * and °.

4. Correlations

The plant´s height and overall plant´s weight positively and significantly (p < 0.05) correlated with all other morphological traits. The exception concerned number of all leaves and relative water content of the entire plant. The number of all leaves positively and significantly (p < 0.05) correlated with the stem´s length and weight. Length and weight of the inflorescence positively and significantly correlated with the stem´s weight. The entire plant´s relative water content did not correlate with any morphological traits evaluated.

The stems´ and leaves´ EO yields were mutually correlated with each other.

The stems´ EO yield was negatively and significantly correlated with the entire plant´s relative water content (p < 0.05). The leaves´ EO yield positively and significantly (p < 0.05) correlated with the stem´s length.

Categories of sampling stands did not mutually differ in the variables used for their characterization. However, researchers observed a significant positive correlation (p < 0.01) between the degree of invasion and soil reaction. Average pH was the highest within the stands with the highest S. canadensis % representation. On the other side, the significant negative correlation (p < 0.05) between the goldenrod relative % abundance and the overall area of sampling stand was detected. The stands with the lowest area had the highest goldenrod coverage, and vice versa.

The plant’s height and overall plant’s weight positively and significantly (p < 0.05) correlated with all other morphological traits. The exception concerned number of all leaves and relative water content of the entire plant. The number of all leaves positively and significantly (p < 0.05) correlated with the stem’s length and weight. Length and weight of the inflorescence positively and significantly correlated with the stem’s weight. The entire plant’s relative water content did not correlate with any morphological traits evaluated.

The stems’ and leaves’ EO yields were mutually correlated with each other.

The stems’ EO yield was negatively and significantly correlated with the entire plant’s relative water content (p < 0.05). The leaves’ EO yield positively and significantly (p < 0.05) correlated with the stem’s length.

Categories of sampling stands did not mutually differ in the variables used for their characterization. However, we observed a significant positive correlation (p < 0.01) between the degree of invasion and soil reaction. Average pH was the highest within the stands with the highest S. canadensis % representation. On the other side, the significant negative correlation (p < 0.05) between the goldenrod relative % abundance and the overall area of sampling stand was detected. The stands with the lowest area had the highest goldenrod coverage, and vice versa.

Concerning relations of morphological traits and EO productivity rate to variables used for the sampling stands characterization, plant’s height, stem’s length, and stem’s weight, as well as inflorescence’s humidity, were significantly (p < 0.01) and positively correlated with relative % abundance of S. canadensis within the vegetation cover, i.e., degree of invasion (Figure 4a). CCA analysis confirmed this ascertainment (Figure 5). The same pattern was observed concerning the soil reaction. The tallest plants, with the longest and heaviest stems, were noticed within the stands with the highest pH, which, concurrently, means those with the highest goldenrod abundance. Conversely, the aforementioned morphological traits significantly and negatively (p < 0.05) correlated with the overall stand’s area (Figure 4b). Only the leave’s weight was correlated with soil humidity, and none of the evaluated morphological traits was correlated with the measure of interventions, i.e., number of mowings.

Figure 4. Simple linear regression between Solidago canadensis plant height and two variables. Models display simple linear regression between (a) plant height and the degree of invasion and between (b) plant height and overall area of sampling stands.

Figure 5. Ordination plot of the selected Solidago canadensis morphological traits and variables used for the sampling stands characterization. CCA ordination triplot shows association of 8 morphological traits to degree of invasion, soil reaction, soil moisture, stand area, and measure of interventions. The cumulative percentage variance of morphological traits-variables relation explained on Axis 1 = 70.26% and on Axis 2 = 20.92%, with eigenvalues for Axis 1 = 0.003 and for Axis 2 = 0.001. Abbreviations and notes: P.H.-plant´s height, P.W.-entire plant´s weight, S.L.-stem´s length, S.W.-stem´s weight, L.N.-all numbers of leaves, L.W.-all leaves weight, I.L.-inflorescence´s length, I.W.-inflorescence´s weight, r.a.-Solidago canadensis relative % abundance within vegetation cover, pH-soil reaction, W-soil moisture, area-overall area of sampling stands, int.-measure of interventions (number of mowings).

 Ordination plot of the selected Solidago canadensis morphological traits and variables used for the sampling stands characterization. CCA ordination triplot shows association of 8 morphological traits to degree of invasion, soil reaction, soil moisture, stand area, and measure of interventions. The cumulative percentage variance of morphological traits-variables relation explained on Axis 1 = 70.26% and on Axis 2 = 20.92%, with eigenvalues for Axis 1 = 0.003 and for Axis 2 = 0.001. Abbreviations and notes: P.H.-plant’s height, P.W.-entire plant’s weight, S.L.-stem’s length, S.W.-stem’s weight, L.N.-all numbers of leaves, L.W.-all leaves weight, I.L.-inflorescence’s length, I.W.-inflorescence’s weight, r.a.-Solidago canadensis relative % abundance within vegetation cover, pH-soil reaction, W-soil moisture, area-overall area of sampling stands, int.-measure of interventions (number of mowings).

Stem and leaf EO yields positively and significantly correlated with the soil reaction (p < 0.05). Moreover, leaf EO yield was positively correlated with the S. canadensis relative % abundance.