There is growing recognition (e.g., Cornelissen et al. 2003) that classifying terrestrial plant species based on their function (into „functional types “) rather than their taxonomic identity only is a promising way forward to tackling important ecological questions at the scale of ecosystems, landscapes or biomes. Since data concerning the functional traits of medicinal plants are scarce, our results significantly contribute to this topic.
Variation in leaf functional traits, especially in SLA, have guided many studies of functional ecology, which have addressed important ecological correlations. At the organ level, specific leaf area (SLA) is well known to positively associate with the plant’s relative growth rate. A high leaf SLA represents a resource-acquisitive plant strategy, while a high leaf dry matter content (LDMC) represents a resource-conservative strategy (Lambers and Poorter 1992). Acording to Poorter and de Jong (1999) large difference in SLA between species from nutrient-poor and nutrient-rich habitats, but no systematic difference in the construction costs (the amount of glucose required to construct 1 g biomass). Species from highly productive habitats had higher SLA than those from sites of low productivity, although individual species sometimes deviated substantially from the general trend. Construction costs were similar for plants from different habitats. This was mainly due to the positive correlation between an expensive class of compounds (proteins) and a cheap one (minerals). For a single leaf, SLA is negatively correlated with light intensity (Ratjen and Kage 2013). Based on the above-mentioned findings, we first divided the studied species into the functional groups whereby SLA was considered. In general, SLA values found in our experiments (10.3 m2 kg−1–66.3 m2 kg−1) (Fig. 1a) corresponded with the values published by Cornelissen et al. (2003): SLA=2–80 m2 kg-1.
Lambers and Oliveira (2019) stressed that plants growing in shady conditions invest relatively more of the products of photosynthesis and other resources in LA. Leaves of shade plants are relatively thin and have a high SLA and low leaf mass density. However, certain genotypes have characteristics that are adaptive in a shady environment (shade-adapted plants). In addition, all plants have the capability to acclimate to shade, to a greater or lesser extent, and form a shade plant phenotype (shade form). The term shade plant may therefore refer to an ‘‘adapted’’ genotype or an ‘‘acclimated’’ phenotype. Plants that tolerate shade do not respond with increased stem elongation; instead, they increase their LA. Their leaf thickness is reduced to a smaller extent than in shade-avoiding species, and their chlorophyll concentration per unit LA often increases. According to Wilson et al. (1999), specific leaf area is found to suffer from a number of drawbacks; it is both very variable between replicates and much influenced by leaf thickness. Leaf dry-matter content (sometimes referred to as tissue density) is much less variable, largely independent of leaf thickness and a better predictor of location on an axis of resource capture, usage and availability. However, Villar et al. (2013) considered over 26 broad-leaved angiosperms, that leaf thickness and density were equally important to explain the variation in leaf mass per area (LMA), but the class difference between deciduous and evergreen species was mainly determined by thickness, whereas variation within each group was largely due to density. Evergreens had thicker leaves, predominantly caused by a larger volume of mesophyll and air spaces, whereas the higher leaf density within each group was due to a lower proportion of epidermis and air spaces, and overall smaller cells.
As it has already been mentioned, a high leaf SLA represents a resource-acquisitive plant strategy, while a high leaf dry matter content (LDMC) represents a resource-conservative strategy (Lambers and Poorter 1992). Cornelissen et al. (2003) emphasized that leaf dry matter content is related to the average density of the leaf tissues (it is also related to leaf thickness) and tends to scale with 1/SLA. It has been shown that LDMC correlates negatively with potential relative growth rate (Lambers and Poorter 1992) and positively with leaf life-span, but the strengths of these relationships (between LDMC and potential relative growth rate and between LDMC and leaf life-span) are usually weaker than those involving SLA. Species with low LDMC tend to be associated with productive, often highly disturbed environments. In cases where leaf area is difficult to measure (see above), LDMC may give more meaningful results than SLA, although the two traits may not capture exactly the same functions (Cornelissen et al. 2003). In general, the LDMC values found in our experiments (0.12–0.38 kg kg-1=120–380 mg g-1) (Fig. 1b) corresponded with the values published by Cornelissen et al. (2003): LDMC=50–700 mg g-1.
Results published by Majeková et al. (2014) confirmed the connection between plant functional traits and population temporal stability, whereby population temporal stability, measured as a coefficient of variation of species’ biomass over time, was related to plant traits (including SLA and LDMC) covering different functional trade-offs. Plant functional traits linked to the leaf economic spectrum are important predictors of population stability regardless of both the abiotic and biotic conditions in which plants grew and species phylogenetic relatedness. High values of LDMC are associated with greater temporal stability, indicating that slow-growing species with more conservative economics are generally more stable over time. Leaves with high LDMC tend to be relatively tough and are thus assumed to be more resistant to physical hazards (e.g., herbivores, wind, hail) than leaves with low LDMC (Cornelissen et al. 2003).
Wild medicinal plant species as a substantial component of natural plant communities contribute to the biodiversity and stability of natural ecosystems. Since medicinal plants are frequently exposed to various environmental stresses in their natural conditions, they have evolved physiological, biochemical, and molecular mechanisms to respond to harmful effects of these stresses (for details, see Masarovičová et al. 2019). The present results of SLA and LDMC (Fig. 1-3) characterized studied medicinal plants from the perspective of leaf functional traits that contribute to our understanding of their position in natural plant communities.
The next step in our research was development of plant functional groups using SLA and LDMC values (Fig. 1). Considering the importance of ecological effects on plants and vegetation, the summer period (June – August) 2018 in Slovakia could be characterized as relatively warm, compared with long-term records (Anonymous 2022): the mean daily temperature was 21.9°C. In the warm half-year (April – September) 2018 a record number (135) of summer days (days when maximal air temperature ≥ 25°C) was registered (Faško and Pecho 2018). During the growing season (March – September) 2018, sunshine duration was 1787 h. The Sunshine duration data were provided by the Slovak Hydrometeorological Institute (Anonymous 2021). Similarly, the summer period (June – August) 2019 was relatively warm compared with long term records (Anonymous 2022), with the highest temperature of 37.6°C (Beránek and Faško 2020). During the growing season (March – September) 2019, the sunshine duration was 1646 h. The sunshine duration data were provided by the Slovak Hydrometeorological Institute (Anonymous 2021). Eliáš et al. (1989) analyzed stand microclimate in an oak-hornbeam forest for the fully-leaved season and found that approx. 50% of PAR was absorbed by the upper 4 – 5 m layer of leaves and only approx. 5% penetrated to the forest floor. This is important for the herb layer of the forest. These authors found that vertical gradients of air temperature and relative humidity were generally low, but large differences in diurnal ranges of air temperature and relative humidity were observed between vertical levels. From the meteorological perspective, sunny and warm weather was favourable for photosynthesis and growth of the studied medicinal plant species grown in both the botanical garden and natural communities. However, sunny and warm weather is usually accompanied by drought, especially under natural conditions, like in our experiments.
The ordination diagram (Fig. 1a) shows that in two functional groups with the lowest SLA values includes species growing on the control site (BG) under the highest PAR values (1927 µmol m−2 s−1) except H. helix that occurred at 1178 µmol m−2 s−1 PAR. Ratjen and Kage (2013), for a single leaf, found that SLA is negatively correlated with light intensity as shown in our results. According to Lambers and Poorter (1992), at the organ level, specific leaf area (SLA) is well known to positively associate with the plant’s relative growth rate. SLA of a species is in many cases a good positive correlate of its potential relative growth rate or mass-based maximum photosynthetic rate. Species in resource-rich environments tend to have larger SLA than those in environments with resource stress, although some shade-tolerant woodland understory species are known to have remarkably large SLA as well (Cornelissen et al. 2003). Lambers and Oliveira (2019) stressed that plants growing in shady conditions invest relatively more of the products of photosynthesis and other resources in LA. Their leaves are relatively thin and have a high SLA and low leaf mass density.
Species with the medium values of SLA were assigned to the third and fourth functional group (Fig. 1a) and occurred at sites (BG), (R), (ST), and (PB). According to Lambers and Poorter (1992), these species would have intermediate potential relative growth rates.
The fifth and sixth functional groups presented species with the highest SLA values (Fig. 1a) that occurred mainly on site (PB). These results correspond with the findings of Cornelissen et al. (2003) that species growing in the herb layer of a forest are shade-tolerant with high values of SLA. These species also have a high potential relative growth rate. To these functional groups belong, in addition to species from site (PB), two species from site (BG) – G. odoratum and G. hederacea. These species were grown under a lower PAR level than the other species at the (BG) site. A. podagraria occurred on the (R) site with the lowest measured PAR value (2.7 µmol m−2 s−1 (11:50 h), which was manifested in relatively high SLA values (Fig. 1a). I. glandulifera plants grown at site (R) with a mean value of PAR 366 µmol m−2 s−1.
Species from site (BG) had, in general, lower values of SLA in comparison with plants from other sites. The highest values of this parameter were shown by G. hederacea (PB), indicating the best adaptation of this species to the low irradiance level. This finding agrees with results in Lambers and Oliveira (2019) that plants growing in shade invest relatively more resources into LA, and these leaves are thin with high SLA values.
We divided the studied species into the functional groups according to the LDMC (Fig. 1b) parameters and their 95% confidence intervals in accordance with bootstrapping values.
In the first functional group were assigned species with the lowest LDMC values (Fig. 1b). Since LDMC is negatively correlated with potential relative growth rate (Lambers and Poorter 1992), these species would possess the highest potential relative growth rate of all studied species. Leaf dry matter content is related to the average density of the leaf tissues (it is also related to leaf thickness) and tends to scale with 1/SLA. It has been shown that LDMC correlates negatively with potential relative growth rate and positively with leaf life-span, but the strengths of these relationships (between LDMC and potential relative growth rate and between LDMC and leaf life-span) are usually weaker than those involving SLA (because 1/SLA combines leaf tickness and leaf density) (Cornelissen et al. 2003; Lambers and Poorter 1992). This finding was not confirmed for the first functional group because to this group belong four species with summer leaves – P. lanceolata, T. farfara, I. glandulifera, and A. podagraria and only two species with evergreen leaves – G. hederacea, and G. odoratum (Klotz and Kühn 2002). Species with low LDMC tend to be associated with productive, often disturbed environments.
The second functional group comprised four species: G. odoratum (BG), G. urbanum (PB), and P. vulgaris (R) with evergreen leaves and U. dioica (PB) with summer leaves (Klotz and Kühn 2002). Results published by Majeková et al. (2014) documented a connection between plant functional traits and population temporal stability. Plant functional traits linked to the leaf economic spectrum are important predictors of population stability regardless of both abiotic and biotic conditions in which species grow and species phylogenetic relatedness. High values of LDMC are associated with greater temporal stability, indicating that slow-growing species with more conservative economics are generally more stable over time.
The third functional group included species with summer leaves (A. podagraria, H. perforatum, S. gigantea, and U. dioica), and four species with evergreen leaves (P. vulgaris, H. helix, F. vesca, and G. urbanum) (Klotz and Kühn 2002). According to McIntyre (2008), a conservative strategy of plant species for low-productive undisturbed habitats relates to low SLA and high LDMC in contrast to fertile disturbed habitats, which select for high SLA and low LDMC. Leaf characteristics are useful in quantifying the links between vegetation change and ecosystem function that will be a vital part of ecosystem value assessments.
The last functional group included two species: H. helix, and F. vesca, having the highest values of LDMC (Fig. 1b). According to Cornelissen et al. (2003), leaves with high LDMC tend to be relatively tough and are thus assumed to be more resistant to physical hazards (e.g., herbivores, wind, hail) than leaves with low LDMC. Additionally, leaves of H. helix are scleromorphic, and leaves of F. vesca are mesomorphic (Klotz and Kühn 2002). Since LDMC is negatively correlated with relative growth rate, we suppose that the above-mentioned species, compared with all studied species, has grown with the slowest rate. Since leaves of these species are evergreen, LDMC showed a positive correlation with the lifespan of the leaves (Klotz and Kühn 2002).
Since seasonal plasticity may enable plants to cope with adverse environmental conditions and/or resource variability (Zunzunegui et al. 2011), we studied the plasticity of the plants using MDS (Fig. 2). More plastic species (e.g., U. dioica, and G. urbanum) might show a greater ability for adaptation to ecological conditions. Similarly, G. odoratum, I. glandulifera, and S. gigantea are plastic species under given environmental conditions.
We performed a more complex differentiation of the examined species (Fig. 3). The first functional group includes P. lanceolata (BG), and T. farfara (BG). According to Klotz and Kühn (2002), the species of this first functional group belong to C-S-R plant strategists. Plant strategies are groupings of similar characteristics among species or populations and cause them to exhibit similarities in ecological relationships. The habitats that are exploited by primary strategists: competitors (C), stress-tolerators (S) and ruderals (R) represent extremes in the range of conditions available to plants (Grime 2001). Plants with C (competitor) strategy are characterized by high SLA, low LDMC and generally grown at habitats with rich resources and less interference; plants with S (stresstolerator) strategy are characterized by low SLA, high LDMC and usually found in environments with poor resources and great interference; plants with R (ruderal) strategy are characterized by low SLA and LDMC (Lambers and Poorter 1992; Wigley et al. 2016). Indicator values for the environmental factors (nutrients and irradiance) affecting plant occurrence are expressed on ordinal scales defined by Ellenberg (1979) and Ellenberg et al. (1991). The values for individual taxa were modified and extended for the Czech flora by Chytrý et al. (2018, 2021). According to these authors, P. lanceolata ranks amongst plants that can tolerate partially sunny places; in most cases, it grows under full-sun conditions, but it also grows in the shade, up to 30% of scattered radiation on the soil surface. This species is present in areas that are moderately nutrient-rich. It is less often observed in areas that are quite nutrient-poor or quite nutient-rich – generalist (taxa with wide ecological amplitude, for example, in relation to irradiance). Bednářová (2017) reported that P. lanceolata is abundant in the Czech region: grasslands, fields, roadsides. According to Chytrý et al. (2018, 2021), T. farfara is a plant present in sunny places; only in some exceptional cases, it grows in areas with less than 40% of scattered radiation on the soil surface. T. farfara represents a crossing-point between (i) species present in areas that are moderately nutient-rich, less often in areas that are quite nutient-poor or quite nutient-rich (generalist) and (ii) species with more frequent occurrence in areas that are nutient-rich than in areas with average availability of nutrients, and only exceptionally in places that are quite nutient-poor (generalist). In the Czech region, this species is commonly found in wet clay soils, embankments, glades, grasslands, ruins, and quarries (Bednářová 2017). With regard to (BG), both plants had analogous conditions – humus content (3.2%) and average value of PAR (1927 µmol m−2 s−1). Compared with the specimens collected at the experimental sites, they had a relatively low average value of SLA. Given the fact that the above-mentioned species are included within the indication value of nutrients by a generalist, the average value of PAR likely had a more significant impact on them.
The species of the second functional group were: I. glandulifera (R), G. hederacea (PB), G. hederacea (BG), A. podagraria (R), G. odoratum (PB), G. odoratum (BG), G. urbanum (PB), and U. dioica (PB). However, these species have different strategies. G. urbanum, and G. hederacea follow C-S-R strategies. I. glandulifera is C-R, A. podagraria, and U. dioica are C-strategists, and G. odoratum follows the S-strategy (Klotz and Kühn 2002). Chytrý et al. (2018, 2021) reported that I. glandulifera only in some rare cases grows in areas with less than 20% of scattered radiation on the soil surface. It is more frequent in areas that are nutient-rich than in areas with average availability of nutrients and only exceptionally in places that are quite nutient-poor. According to Beerling and Perrins (1993), I. glandulifera is an invasive species. In terms of PAR, this species had better conditions on site (BG), but higher humus content were recorded in terms of (R). G. hederacea is present in half-shady places; it exceptionally grows under full-sun conditions, but it generally develops in places with more than 10% of scattered radiation on the soil surface (generalist). It is more frequent in areas that are nutient-rich than in areas with average concentrations of nutrients, and only exceptionally in places that are quite nutient-poor (Chytrý et al. 2018, 2021). The above-mentioned species is widely distributed in the Czech region, from plains to mountainous regions. It prefers shady zones, e.g., riparian woodland or moist fields (Bednářová 2017). With regard to humus content, this species showed better conditions in terms of growth at site (PB), manifested notably in its SLA values.
A. podagraria is seldom present in places with less than 20% of scattered radiation on the soil surface (generalist). This species is an indicator of nutrients (Chytrý et al. 2018, 2021). With regard to humus content, this species showed more advantageous conditions in terms of growth at site (R), although we have measured the lowest average value of PAR – which was consequently manifested in the measured values of leaf characteristics. According to Chytrý et al. (2018, 2021), G. odoratum ranks amongst plants that can tolerate shady areas, with occurrence both in places with less than 5% of scattered radiation on the soil surface and in more sunny places. It also represents a crossing-point between (i) species present in areas that are moderately nutient-rich, less often observed in areas that are quite nutrient-poor or quite nutient-rich, and (ii) species with more frequent occurrence in areas that are nutient-rich than in areas with average availability of nutrients, and only exceptionally in places that are quite nutient-poor. This species is relatively abundant in the Czech country, especially in shady broad-leaved forests (Bednářová 2017). Both in terms of (BG) and (PB), G. odoratum was observed in environments with insufficient PAR. As for humus content in the soil, it showed better conditions for growth at site (PB).
According to Chytrý et al. (2018, 2021), the species G. urbanum ranks amongst plants that can tolerate half-shady places; it exceptionally grows under full-sun conditions, but it generally develops with more than 10% of scattered radiation on the soil surface (generalist). It is more frequent in areas that are nutient-rich than in areas with average availability of nutrients and only exceptionally in places that are quite nutient-poor. With regard to humus content in the soil, this species showed better conditions for growth at site (R). Chytrý et al. (2018, 2021) reported that U. dioica is a plant present in half-shady places; it exceptionally grows under full-sun conditions, but it generally develops with more than 10% of scattered radiation on the soil surface (generalist). This species is observed in riparian woodland, along with watercourses across Europe (Bednářová 2017). With regard to soil specimens from (PB) and (BG), we determined value of humus content that reached 10.7% in (PB) and 21% in (BG). Thus, we confirmed that the occurrence of this species is concentrated in places with high humus content. Bednářová (2017) also states that it is a species that prefers forest soil with high humus content. It is also observed around watercourses and water surfaces, near inhabited areas, especially in places with higher N availability in soil caused through fertilization.
P. lanceolata (ST), T. farfara (R), and I. glandulifera (BG) are included in the third functional group. We described the strategies of these species in the previous functional groups.
The fourth functional group includes: P. vulgaris (R), S. gigantea (BG), S. gigantea (R), H. helix (PB), H. perforatum (BG), U. dioca (BG), and H. perforatum (ST). P. vulgaris has a C-S-R strategy, H. helix follows a C-S strategy, whereas S. gigantea, H. perforatum, and U. dioica exhibit a C-strategy (Klotz and Kühn 2002). Chytrý et al. (2018, 2021) reported that P. vulgaris is a plant present in partially sunny places; in most of the cases it grows in full sun, but also in the shade, up to 30% of scattered radiation on the soil surface. P. vulgaris is present in moderately nutrient-rich areas. It is less often observed in areas that are quite nutrient-poor or quite nutient-rich (generalist). With regard to PAR, this species has less favourable conditions (lower PAR values – 44.1 µmol m−2 s−) for growth at (R).
According to Chytrý et al. (2018, 2021), S. gigantea is a plant present in partially sunny places; in most cases, it grows in full sun but also in the shade, up to 30% of scattered radiation on the soil surface. S. gigantea is quite frequent in nutient-rich areas, rather than in areas with average concentrations of nutrients, and only exceptionally in places that are nutient-poor. At site (R), this species had less favourable conditions for growth than at site (BG). Nevertheless, this was only minimally reflected in the examined leaf characteristics (no statistically significant differences were recorded between the above-mentioned sites with respect to SLA and LDMC). S. gigantea is an invasive species (Herr et al. 2007). Weber and Jakobs (2005) reported that this species can tolerate a wide range of irradiance, temperature, soil humidity, and chemical properties of soil. Güsewell et al. (2006) studied features of naturally growing individual plants and artificially introduced individuals of this species. It is interesting that these authors determined the same SLA value for individuals sampled in the USA and in Europe: SLA= 21.2 m2 kg−1. These values are also similar to the present SLA values (Fig. 1a).
In terms of indicative values of irradiance, H. helix represents a crossing-point between (i) plants that can tolerate shady conditions, occurring in places with less than 5% of scattered radiation on the soil surface and in more sunny places, and (ii) plants growing in half-shady places, with exceptional growth under full-sun conditions, but with general development with more than 10% of scattered radiation on the soil surface. It also represents a crossing-point between (i) species present in areas that are moderately nutrient-rich, less often in areas that are quite nutient-poor or quite nutient-rich (generalist), and (ii) species with more frequent occurrence in areas that are nutient-rich than in areas with average availability of nutrients, and only exceptionally in places that are quite nutrient-poor (generalist) (Chytrý et al. 2018, 2021). A specific feature of this species is presence of scleromorphic leaves (Klotz and Kühn 2002). H. helix at site (BG) compared with site (PB) was adapted to higher PAR values throught the remarkably low average value of SLA.
H. perforatum is a plant present in partially sunny places; in most cases, it grows under full-sun conditions, but also in the shade, up to 30% of scattered radiation on the soil surface. With regard to nutrients, the above-mentioned species represented a crossing-point between (i) species more likely to be present in areas that are quite nutient-poor than in areas with average availability of nutrients, or exceptionally in areas quite nutient-rich, and (ii) species with more frequent occurrence in areas that are moderately nutrient-rich, less often in areas that are quite nutient-poor or quite nutient-rich (Chytrý et al. 2018, 2021). This species is present in relatively arid and sunny places, e.g., fields, grasslands, glades, and hillsides (Bednářová 2017). From the aspect of both, humus content and PAR, it had better conditions in (BG). No statistically significant differences were found between the studied sites with regard to SLA and LDMC. U. dioica has already been described in the second functional group.
The fifth functional group includes only one species – F. vesca (R). This species has a C-S-R strategy (Klotz and Kühn 2002). F. vesca is rarely present in places with less than 20% of scattered radiation on the soil surface (generalist). F. vesca is present in areas that are moderately nutrient-rich, less often in quite nutient-poor or quite nutient-rich areas (Chytrý et al. 2018, 2021). With regard to humus content in the soil, F. vesca showed better conditions for growth at (BG). We have included the following species in the sixth functional groups: H. helix (BG) and F. vesca (BG). In the previous functional groups, we already described the strategies of these species and also those of the species of the seventh functional group. According to Lambers and Oliveira (2019), low SLA values and a long-life span of leaves are related to structural properties necessary for tolerance of unfourable environmental conditions. The above-mentioned species with high LDMC values are likely more resistant against unfourable conditions than other examined species.