The functional traits of plants explain their species response to changes in environmental gradients (Gouveia & Freitas et al. 2009, Sterck et al. 2011, Kichenin et al. 2013, Salgado-Negrete et al. 2013, Valladares et al. 2014), which can be used to predict the responses of communities to environmental change (Paine et al. 2011, Violle et al. 2012, Lohbeck et al. 2013).
Differences in phenotypic and physiological responses are associated with the geographical locations of populations at local or regional scales (Albert et al. 2010, Nicotra et al. 2011, Fajardo & Piper 2011). Leaves are organs that are exposed to different environmental factors, and it is reasonable to expect that their morphology and structure represent the responses of the plants to local conditions, such as water availability or light intensity, as well as intra- and interspecific interactions (Castro-Díez et al. 1997, Bruschi et al. 2003, Lambrecht & Dawson 2007, Cuevas-Reyes et al. 2018).
Rainfall deficit and seasonality are important determinants of the structure, composition and physiognomy of vegetation (Moles et al. 2014). Low precipitation rates promote changes in foliar traits that in turn reduce water loss and increase photosynthetic capacity, and thus high leaf densities impart dehydration tolerance (Niinemets 2001, McLean et al. 2014). Plants that inhabit seasonal forests have mechanisms that allow them to remain unharmed by extended periods of drought, reducing carbon absorption as a result of the strong control of stomatal conductance and canopy deciduousness (Rossatto et al. 2013). On the other hand, temperature also affects the energy balance of leaves as well as the metabolic rate of plants (Moles et al. 2014). High temperatures associated with high solar incidence promote an increase in leaf thickness and a low specific leaf area, which reduce damage to leaf tissue from the sun (Leigh et al. 2012, McLean et al. 2014). Specific leaf area is related to the photosynthetic efficiency of plants and is considered to be very important for the biogeochemical cycles of forests (Meier & Leuschner 2008). It is also related to changes in leaf density and thickness (Gouveia & Freitas 2009), which, in turn, are affected by both precipitation and temperature gradients.
Different ecological and evolutionary studies have suggested that temperature and precipitation are the main determinants of plant morphological features on a global scale (Moles et al. 2014). However, leaf phenotypic variability can also be explained by latitudinal and elevational gradients (Tang & Ohsawa 1999). Previous studies have documented the importance of geographic and environmental factors as regulators of plant structure and morphology and species colonization and establishment in different habitats (Bruschi et al. 2003, Díaz & Cabido 2001). In particular, the variation in foliar traits in oak species has been correlated with specific environmental factors such as temperature and precipitation at different spatial scales, suggesting plastic responses or adaptive genetic differentiation within and among populations (Balaguer et al. 2001, Uribe-Salas et al. 2008, Aguilar-Romero et al. 2016, Rodríguez-Gómez et al. 2018, Albarrán-Lara et al. 2019).
Oak species (Fagaceae, Quercus) occur in a wide variety of habitats ranging from temperate to tropical forests and in both humid and dry conditions along a wide geographical range in the Northern Hemisphere, which has promoted remarkable morphological variation. The high morphological diversity of oaks has been of great interest in taxonomic (Valencia-A 2004, Rodríguez-Rivera & Romero-Rangel 2007, Martínez-Cabrera et al. 2011) and ecological studies analyzing the patterns of variation in morphological traits in relation to environmental gradients (González-Rodríguez & Oyama 2005, Uribe-Salas et al. 2008).
In this study, we analyzed the leaf morphological variation in populations of Quercus elliptica Née (Fagaceae) to determine the degree of population differentiation across its geographic distribution in Mexico and Central America. Quercus elliptica occurs in very diverse habitat types from tropical deciduous forests at low altitudes to humid temperate forests at higher altitudes. Therefore, this red oak species represents an ideal system for testing the effects of environmental variables on morphological and functional traits along latitudinal and altitudinal gradients. We evaluated population differentiation using ecological niche analysis and projection with distribution models.
Material and methods
Study species. Quercus elliptica Née is a red oak species widely distributed in Mexico ranging from southern Sinaloa to Nayarit, Jalisco, Guerrero, State of Mexico, Oaxaca, Veracruz and Chiapas, and into Central America (Figure 1). Quercus elliptica occurs under many types of environmental conditions in a wide elevational range between 500 and 3,100 m in canyons to plains and in pine-oak, oak, cloud and deciduous forests (Romero-Rangel et al. 2002, Valencia-A 2004). This tree reaches a height of up to 20 m with a trunk diameter of up to 60 cm. Quercus elliptica is a deciduous species with mature coriaceous leaves and elliptical-narrow or elliptical-wide shapes (Rodríguez-Rivera & Romero-Rangel 2007). The petiolate leaves have fasciculate trichomes with 5-7 rays and a short stipe, sometimes with crushed hairs, mainly near the primary veins (González-Villarreal 1986, Rodríguez-Rivera & Romero-Rangel 2007) and sometimes stellate trichomes on the middle vein.