Medicinal plants are an important part of the world's flora. They are located on every continent and are still collected and used in folk medicine. Unfortunately, there is little scientific work on medicinal plants. Even if some aspects of medicinal plants are processed, they are mostly about using medicinal plants in current pharmacy or about folk medicine (e.g. Petrovska 2012, Gurib-Fakim 2006, Jamshidi-Kia et al. 2018, Van Wyk and Wink 2018). 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 and LDMC, have guided many studies of functional ecology, which have addressed important ecological correlations (Adler 2014, Pérez-Ramoz et al. 2019). At the organ level, specific leaf area (SLA) is well known to positively associate with the plant’s relative growth rate. Based on the above-mentioned findings, we first divided the studied species into the functional groups whereby SLA was considered.
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). 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 (Smart et al. 2017).
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 (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 and 2019 as well, in Slovakia could be characterized as relatively warm. 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.
Plant communities differ in light, nutrients, and water availability, which are important factors in the selection and diferentiation of which leaf traits should be used as a indicators of change in the environment (Amaral et al. 2021). Based on the relationships between SLA and LDMC we focused on the two ecological factors: photosynthetically active radiation (PAR) and soil organic matter (SOM). SLA values increase with decreasing PAR and nutrient availability, which have been confirmed by the results of our research. There is a strong link between these two abiotic factors, while leaf blade thickness and mesophyll thickness increase with increasing PAR and nutrient availability without interaction. Photosynthetically active radiation is a vital source of energy for plants, and plants compete for this source, especially in dense communities. Plants have a variety of photosensory receptors through which they can detect the presence of competing species and subsequently adapt their growth and development strategies (Fiorucci and Fankhauser 2017). The availability of solar energy is a major ecological factor determining the convergence of leaf characteristics in the plant community (Gitelson et al. 2021), with no apparent effect of soil moisture on leaf characteristics (at the stand level), despite the importance of water in the drought-prone ecosystems (Ackerly 2003). The content of organic matter is an important parameter that indicates the overall quality of the soil, as well. It is influenced by many factors (vegetation, climatic conditions, soil type) (Scharlemann et al. 2014, Wan et al. 2019, Taghizadeh-Mehrjardi et al. 2020). In general, it depends mainly on the physical, chemical and microbiological properties (Rasmussen et al. 2018, Massaccesi et al. 2020) of the soil and has a positive effect on plant growth and biomass production (Lal 2020, Prommer et al. 2020, Anacker et al. 2021).
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 Hedera 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.
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) – Galium odoratum and Glechoma hederacea. These species were grown under a lower PAR level than the other species at the (BG) site. Aegopodium 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). Impatiens 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 Glechoma 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). 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) (Smart et al. 2017) 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 (Cornelissen et al. 2003) and net primary production of aboveground biomass (Wang 2007), but the strengths of these relationships are usually weaker than those involving SLA (because 1/SLA combines leaf tickness and leaf density) (Lambers and Poorter 1992, Smart et al. 2017). This finding was not confirmed for the first functional group because to this group belong four species with summer leaves – Plantago lanceolata, Tussilago farfara, Impatiens glandulifera, and Aegopodium podagraria and only two species with evergreen leaves – Glechoma hederacea, and Galium 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: Galium odoratum (BG), Geum urbanum (PB), and Prunella vulgaris (R) with evergreen leaves and Urtica 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 (Aegopodium podagraria, Hypericum perforatum, Solidago gigantea, and Urtica dioica), and four species with evergreen leaves (Prunella vulgaris, Hedera helix, Fragaria vesca, and Geum 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: Hedera helix, and Fragaria vesca, having the highest values of LDMC (Fig. 1b). Leaves with high LDMC tend to be relatively tough (Cornelissen et al. 2003, Smart et al. 2017) and are thus assumed to be more resistant to physical hazards (e.g., herbivores, wind, hail) than leaves with low LDMC. Additionally, leaves of Hedera helix are scleromorphic, and leaves of Fragaria vesca are mesomorphic (Klotz and Kühn 2002). Since LDMC is negatively correlated with relative growth rate (Lambers and Poorter 1992), 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 (Pérez-Ramoz et al. 2019, Stotz et al. 2021) and/or resource variability (Zunzunegui et al. 2011, Wang et al 2019), we studied the plasticity of the plants using MDS (Fig. 2). More plastic species (e.g., Urtica dioica, and Geum urbanum) might show a greater ability for adaptation to ecological conditions. Similarly, Galium odoratum, Impatiens glandulifera, and Solidago gigantea are plastic species under given environmental conditions.
We performed a more complex differentiation of the examined species, when we used the metric multidimensional scaling (MDS) (Fig. 3). The first functional group includes Plantago lanceolata (BG), and Tussilago farfara (BG). According to Klotz and Kühn (2002), the species of this first functional group belong to C-S-R plant strategists. The values for individual taxa were modified and extended for the Czech flora by Chytrý et al. (2018, 2021).
The species of the second functional group were: Impatiens glandulifera (R), Glechoma hederacea (PB), Glechoma hederacea (BG), Aegopodium podagraria (R), Galium odoratum (PB), G. odoratum (BG), Geum urbanum (PB), and Urtica dioica (PB). However, these species have different strategies. Geum urbanum, and Glechoma hederacea follow C-S-R strategies. Impatiens glandulifera is C-R, Aegopodium podagraria, and Urtica dioica are C-strategists, and Galium odoratum follows the S-strategy (Klotz and Kühn 2002).
Plantago lanceolata (ST), Tussilago farfara (R), and Impatiens glandulifera (BG) are included in the third functional group.
The fourth functional group includes: Prunella vulgaris (R), Solidago gigantea (BG), S. gigantea (R), Hedera helix (PB), Hypericum perforatum (BG), Urtica dioca (BG), and Hypericum perforatum (ST). Prunella vulgaris has a C-S-R strategy, Hedera helix follows a C-S strategy, whereas Solidago gigantea, Hypericum perforatum, and Urtica dioica exhibit a C-strategy (Klotz and Kühn 2002).
The fifth functional group includes only one species – Fragaria vesca (R). This species has a C-S-R strategy (Klotz and Kühn 2002).