Variation in phenotypic traits is a fundamental attribute of natural populations and often reflects the linking of traits with geographical and climate variables across the range of a species (McKnow et al. 2014). In tree species, variation among populations in several fundamental functional traits is considerably influenced by water availability, which in turn depends on the amount and distribution of rainfall, soil physical properties, and the relationship between evaporation and transpiration (Souto et al. 2009, Ramírez-Valiente et al. 2010, Cooper et al. 2018). Therefore, differences in water availability along the distribution of a species are expected to set contrasting selection pressures leading to local adaptation of populations, as has been shown in many tree species (Ramírez-Valiente et al. 2010, Aranda et al. 2015, Lind et al. 2107). However, the extent of local adaptation at fine spatial scales in long-lived tree species characterized by high rates of gene flow has not been sufficiently studied yet (Cavender-Bares & Ramírez-Valiente 2017). Alternatively, such species may face environmental challenges by maintaining high levels of within-population genetic variation or through phenotypic plasticity, particularly when single individuals may experience a range of conditions throughout the course of a long lifespan (Cavender-Bares & Ramírez-Valiente 2017, Meireles et al. 2017).
The genus Quercus (oak species) is a very diverse group of tree species distributed in temperate, subtropical and tropical regions of the northern hemisphere, where it is considered one of the most important taxa of trees (Cavender-Bares 2016). This genus, along with others in the Fagaceae family are emerging as non-classical models to study adaptive variation, integrating ecology and evolution (Petit et al. 2013, Cavender-Bares 2019). Mexico is an important diversification center for the oaks, with about 161 species in total (32-40 % of the diversity in the world) and more than 100 endemics (Valencia-A 2004). Oaks constitute a very important group for understanding the factors that determine phenotypic variability, due to their particularly high levels of variation at different levels, that is, among species, among populations, within populations and within individuals (Uribe-Salas et al. 2008, Hernández-Calderón et al. 2014, García-Nogales et al. 2016).
Plant functional traits are measurable characteristics that influence performance or fitness and are assumed to reflect evolutionary responses to external conditions and can frequently refer to ecological factors changing along a gradient (Lavorel et al. 2007). Research on different oak species has revealed functional variation related to leaf traits, phenology, and growth rates associated to altitudinal, latitudinal, water availability or temperature gradients (Bruschi 2010, Hernández-Calderón et al. 2013, Ramírez-Valiente et al. 2017). This variation has been explained as resulting from strategies to avoid or tolerate different types of environmental stress, allowing oak species to develop under a wide range of environmental conditions and resource availability. However, most of these studies have been conducted at broad geographical scales, and little is known about how functional traits vary among tree populations at finer spatial scales such as the landscape level.
Quercus castanea is a Mexican red oak with a wide geographical and altitudinal distribution that occupies contrasting environments, with populations in the Sierra Madre Occidental, the Central Plateau, the Trans-Mexican Volcanic Belt, and the Sierra Madre del Sur (Valencia-A 2004). It is a dominant element in temperate forests and mountain cloud forests and is frequently found in perturbed areas with a xerophytic shrub type of vegetation (Valencia-Cuevas et al. 2015). Therefore, Q. castanea represents a suitable species to study patterns of phenotypic variation in response to environmental factors at a landscape scale, despite potentially very high gene flow even in a fragmentation scenario (Herrera-Arroyo et al. 2013, Oyama et al. 2017).
In this study, we hypothesized that gradients in temperature, precipitation and soil characteristics across the distribution of Q. castanea within the Cuitzeo basin promote variability in functional traits related to the adjustment to differential water availability. We focused on leaf traits that have been considered easy and inexpensive to measure in a standardized manner (leaf chlorophyll concentration, leaf area, leaf mass per area, leaf thickness and the Huber value) but that capture the general patterns of plant functional responses to environmental gradients (Wright et al. 2004, Lavorel et al. 2007). According to these previous generalizations, we expected Q. castanea individuals to have a higher Huber value and smaller, thicker leaves with a higher mass per area in the parts of the basin with higher temperature and lower precipitation. Particularly, the objectives were: i) to evaluate the amount and patterns of variation in this set of functional traits across Q. castanea populations at a landscape level and ii) to determine significant associations between functional traits and environmental variables.
Materials and methods
Study site and sampling procedure. This study was conducted in the basin of Lake Cuitzeo. It has an area of 4,026 km2 and is located at 19° 30’ - 20° 05’ N and 100° 35’ - 101° 30’ W in the Trans-Mexican Volcanic Belt in the northern part of Michoacan state and the southern part of Guanajuato state (Figure 1). Climate in the basin is temperate with a marked rainy season during summer months (June to September). There is a significant climate gradient, with precipitation increasing and temperature decreasing from north to south and with altitude (Mendoza et al. 2006). The topography and soils of the study area are product of the volcanic activity of the Quaternary. The dominant soil groups are vertisols, luvisols, leptosols, acrisols and andosols (Mendoza et al. 2006, Chávez-Vergara et al. 2014).