The anatomical, morphometric and leaf biomass differences between these two species allow us to infer contrasting adaptations due to exposure to the particular conditions of their ecological niches. On the other hand, the results also show that there is a differentiation between the microenvironments where each of these species are found, which could explain the spatial segregation between E. standleyana and E. santanderensis.
Microenvironmental characteristics of the two species sites
The paramos are very heterogeneous in both biotic and abiotic characteristics, creating very marked micro and macroscale gradients, causing mosaics of species with differential characteristics over short distances (Azócar & Rada, 1993; Abadín et al. 2002; Llambí & Rada 2019). Cárdenas et al. (2018) found diurnal temperature amplitudes between 10 and 30°C in the Romerales and Belmira paramos in the central mountain range of the Colombian Andes. In the García rural area, air temperature oscillations remained in the range of 1–32°C for E. standleyana and between 1–22°C for E. santanderensis. The RH also had different ranges for the microsites of the two species, broader for E. standleyana compared to E. santanderensis, which demonstrates that these species are subject to different environmental conditions. It is important to note that that these microclimatic characteristics were measured in an extremely short wet season period, and fluctuation ranges are, most likely, greater during the drier season.
Cárdenas et al. (2018) found water saturation in deeper layers of the soil while in the most superficial ones the volumetric water contents were high in the dry season. In our work, the gravimetric humidity values remained above 50% and organic matter was high for the E. standleyana microsite at a depth of 10 cm. However, for E. santanderensis, lower values were found for both gravimetric soil moisture and organic matter. Llambí et al. (2012) state that paramo soils can have from 3 to 44% of the volume in organic matter and this has a strong relationship with soil humidity. As organic matter increases, the substrate becomes more porous, allowing a large amount of water to be stored. As reported by Zhao et al. (2023) for an alpine grassland, soil organic matter alleviates drought stress by positively affecting the water holding capacity through the improvement of soil structure and cation exchange capacity. Because the substrate of E. santanderensis is more inorganic and rockier, more water may infiltrate and be lost from the topsoil. The soil pH of both species is acidic, although higher for E. santanderensis. According to Moreno et al. (1994) and Estupiñán et al. (2009), the pH of paramo soils is usually acidic due to the high amount of aluminum compounds, exchangeable aluminum and organic matter. For the E. standleyana site the pH is lower due to a high content of organic matter with high amounts of humic acids. According to Denney et al. (2020), the heterogeneity of a site can be observed at distances below 2 m due to topography, nutrient levels and available water, creating contrasting microhabitats in very short distances. This in turn results in differential morpho-physiological traits/responses in plants, i.e., water stress can reduce stomatal conductance and limit photosynthesis.
Leaf and peduncle anatomy
The sunken stomatal crypts in E. standleyana correspond to a more suitable anatomical character that allows it to buffer the adverse effect of microclimatic variables (Roth 1973; Roth-Nebelsick et al. 2009), especially temperature and relative humidity, variables that determine water loss through transpiration (López Rhenals 2021; FAO 2022). These two variables showed greater fluctuations in this species´ microsite, which could indicate that it is a response to the greater variation and more extreme conditions. Other authors mention that it may be a scleromorphic character, given by an increase in the thickness of the mesophyll, so that the stomatal crypts increase in depth, with the aim of increasing the efficiency of gaseous diffusion, as has been described in species of the genus Banksia (Hill 1998; Jordan et al. 2008; Hassiotou et al. 2009). This study agrees with these reports, since E. standleyana, having greater thickness of the mesophyll, was the species with much more sunken stomata.
The greater thickness of the mesophyll for E. standleyana would indicate greater photosynthetic tissue, mesophyll conductance and therefore greater photosynthetic capacity, as has been reported in species of the genus Rhododendron (Cai et al. 2014).
According to Alemán Sancheschúltz (2019), the number of conductive elements per unit area is related to the efficiency of hydraulic conductivity. E. standleyana, being a taller species, requires conducting large amounts of water towards larger structures. Therefore, the greater number of vascular bundles and the greater diameter of the xylem vessels favors water distribution to all organs. However, E. santanderensis, having a smaller size and being in a drier soil environment, increases the safety of water conduction with a smaller diameter of the xylem vessels to avoid embolisms. This has been recorded in woody grass species in the genus Chusquea (Ely et al. 2019) and the perennial herb Senecio formosus (Araujo et al. 2023), characteristic species of the paramo. Furthermore, the diameters of the small xylem vessels indicate that both E. standleyana and E. santanderensis aim for safety in water conduction, avoiding the risk of embolisms (Davis et al. 1999; Pittermann and Sperry 2003). The efficiency of water conduction in E. santanderensis would be determined by greater safety in the conduction of water from the soil to all organs due to a low water content of the soil; while E. standleyana would rely on a quicker movement of water to all compartments of the plant, due to a higher soil water content.
Morphometry
Taller plants are subject to suffering from embolisms, because they tend to have larger xylem vessels to transport large amounts of water to higher structures. Therefore, in the dry season and under freezing temperatures, these are more vulnerable to injury by cavitation (Olson et al. 2018). While smaller xylem vessel diameters prioritize safety over efficiency in the conduction of water. The larger size of E. standleyana could be associated to more favorable water conditions in its microsite. Furthermore, during the dry season, where the availability of water would be affected, its large water reservoir in the stem pith, would allow it to withstand conditions of water stress (Goldstein et al. 1984). On the other hand, E. santanderensis, an acaulescent rosette, does not have a pith to store water, but at the same time, water travel shorter distances, increasing the safety of conduction to all organs by their small xylem vessel diameters.
The greater number of leaves and the smaller leaf area in E. santanderensis could be an adaptation for the dissipation of thermal energy, regulation of water loss and thermal insulation of the juvenile leaf bank and, therefore, the apical meristem (Murcia & Mora 2003). According to Leigh et al. (2017), smaller leaves have a higher rate of heat convection per unit area between the leaf and the air, which allows them to maintain a high leaf area in a small coverage to capture photosynthetically absorbable light. However, these characters are opposite in E. standleyana, fewer leaves and greater leaf area. E. standleyana regulates leaf temperature through an increase in the boundary layer caused by greater pubescence on the leaf surface, which also allows greater light reflectance, as has been reported in other caulescent rosettes (Baruch & Smith 1979; Meinzer & Goldstein 1985).
Morphophysiological and leaf biomass traits
Strong positive relationships were found between %Scl and relative air humidity, and negative relationships with succulence and water content (Berdugo Lattke 2020). Here we find consistency with our results, E. standleyana presents a substrate richer in water and leaves with large water percentages of up to 70%, it presents a lower %Scl. In contrast, E. santanderensis has higher %Scl, lower gravimetric soil humidity and lower leaf water percentages. Furthermore, a conservative use of resources could be indicated, due to the lower availability of organic matter in the substrate.
Specific leaf area is a key functional trait in the study of functional responses in plants. Differences in SLA have been linked to niche separation between species, e.g., low SLA values are associated with strategies related to slow growth rates and nutrient poor soils, especially nitrogen (Poorter et al. 2009; He et al. 2018). He et al. (2018) find that soil nitrogen and organic carbon are the factors that best explain changes in SLA in a study of 207 woody species, i.e., lower SLA under conditions of lower nitrogen and organic carbon in the soil. Other species of Espeletiinae such as Ruilopezia atropurpurea and Espeletia schultzii, the first with glabrous leaves and the second very pubescent, showed low SLA, 36 and 67 cm2/g, respectively, in a Venezuelan paramo (Rada & Navarro 2022). Cabrera & Duivenvoorden (2020) indicate that, in the paramos of Colombia, plant communities are dominated by species with low SLA values, reinforcing the idea that in these environments, plants rely on strategies for the conservative use of nutrient resources (Wright et al. 2004). In our case, low values were also reported for the two species, however, the lower SLA in E. standleyana does not coincide with what was expected in terms of its relationship with poorer soils as suggested by previous authors. This could be due to the fact that this species shows greater pubescence and/or a greater thickness of the mesophyll, which would increase the dry mass of the leaf (Molina Montenegro 2008; Villar et al. 2013; Pompelli et al. 2019; Rada et al. 2021).
The low LAI for the two species agrees with what was reported for E. schultzii and R. atropurpurea; furthermore, for these species it was positively related to the availability of water in their microenvironments (Baruch & Smith 1979). According to Bucci et al. (2008), LAI decreases when water availability becomes limited. The highest availability of water found in our study was obtained by E. standleyana, likewise, it is the species that presented the highest LAI, which is consistent with these studies. On the other hand, LAI has been proposed as a good predictor of leaf biomass (Parker 2020). In this work, a strong correlation between these two variables was found for both species, supporting Parker's results. Despite the marked differences in terms of sizes, there are certain individuals of E. santanderensis that can reach leaf biomasses as high as that of some individuals of E. standleyana, which proposes a compensation for these individuals due to the greater number of leaves in the acaulescent rosette.
The ellipsoids shown in the PCA agree with a segregation of all the measured characters, determining completely contrasting expressions of the traits for the two species. The differential characteristics in the microenvironment correspond to the responses of the two species, the features of E. standleyana mainly aim to solve problems of water aspects related to the leaf-air interface, greater pubescence, deeper stomatal crypts, rosette with a large stem pith, a greater number of vascular bundles and a smaller diameter, which allows it to withstand these more fluctuating and extreme conditions. While E. santanderensis faces a more favorable environment, but the edaphic conditions are more extreme compared to the microhabitat of E. standleyana. For this reason, the species responds to these conditions with smaller leaves, fewer and larger diameter vascular bundles, small size allowing water to reach all organs more quickly, and tougher leaves, which supports the idea that sclerophylly occurs in environments where nutrients are limiting.