Leaf morphological and anatomical variations of paper birch populations along environmental gradients across Canada
Leaf hair density
Pioneer tree and climate
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Leaf morphology and anatomy have been found to vary considerably among tree species, and leaf characteristics have widely been used for analyzing plant growth and resource use strategies because of their structural adaptation to withstand environments. Considering the changing climate projections, early-successional, broad niched species like paper birch (Betula papyrifera Marsh.) are expected to increase dominance due to a zonal shift of natural vegetation and/or open gaps within the current vegetation zones. Hence, it is important to understand factors such as leaf characteristics that enable these pioneer species to inhabit a wide geographic range and their increasing dominance. Paper birch is a pioneer tree species in North America that inhabits wide climatic and geographic gradients; in addition, the species has developed different leaf morphology and anatomy that have allowed paper birch to adapt to diverse habitats. This study examines how the leaf characteristics of paper birch vary under uniform and stressed environments. The major objectives were (a) to investigate leaf characteristics variations in paper birch populations grown in uniform environmental conditions as in a greenhouse and a common garden; (b) to correlate between leaf characteristics and paper birch’s environment of origins; (c) to investigate leaf characteristic variations in paper birch populations grown under different carbon dioxide concentrations [CO2] and soil water levels to determine the relationship between leaf characteristics and individual or interacting effects of [CO2], water levels and populations; and (d) to analyze the relationship within and between leaf morphology and anatomy of the birch populations. The study found significant differences among paper birch populations in leaf morphological characteristics under a uniform environment at the greenhouse and the common garden. The leaf characteristic variations in the uniform environment may be related to the different genotypes of the birch inhabiting a wide environmental gradient. In paper birch populations grown in the common garden, significant differences in stomatal density, stomatal area, pore area and guard cell width were identified. As expected, the birch populations in greenhouse and common garden environments showed significant correlations of leaf characteristics, namely specific leaf area (SLA), leaf maximum width index and petiole area to latitude, longitude, elevation, temperature, precipitation and aridity index of origin. Correlation between leaf characteristics of paper birch in the greenhouse showed that populations originated in limited precipitation (during growing season) had low hair density on leaf adaxial surface, with larger leaf width and petiole area. Birch populations grown in the common garden revealed that populations originated in higher mean annual precipitation had less hair density on leaf adaxial surface with smaller leaf area and higher stomatal density. Relationships within the leaf characteristics revealed significant correlations within and between leaf morphology and anatomy as populations with larger leaf area had larger petiole area and less adaxial hair density in greenhouse. The larger petiole in larger leaf reflects the need for mechanical strengthening to support, whereas inverse relationship between leaf area and hair density possibly showed a strategy of the birch to balance water loss. In common garden, the birch populations with larger leaf area had larger specific leaf area and higher adaxial hair density but low stomatal density. All these features in paper birch populations provide a structural basis for reducing water loss through leaves and increasing water use efficiency. There was no consistency in leaf characteristics when the paper birch populations were grown in uniform environments as in the greenhouse and the common garden. Analysis of the leaf characteristics in the birch showed significant differences due to the interaction and/or main effects of [CO2], water levels and populations. Paper birch had decreased leaf area and increased stomatal density under elevated [CO2] which might have reduced stomatal conductance and increased water-use efficiency. Under low soil water level, paper birch populations studied had smaller stomatal area, pore area and guard cell width. Contrasting with the expectation neither stomatal area was larger nor stomatal density increased under low water level. A trade-off between stomatal area and density in this study showed that stomatal area per unit leaf area remained the same. Hence, smaller stomatal area and guard cell width under low water level must have improved [CO2 ] diffusion and decreased water loss compared to larger stomatal area and guard cell width. The results of this study confirmed significant genotypic difference in leaf characteristics of paper birch populations irrespective of a uniform growing environment. The characteristics, namely leaf area, maximum width, SLA, stomatal density and stomatal area, appear related to the environment of origin; however, these relationships were not consistent in the birch populations grown in the greenhouse and common garden. Paper birch populations acclimated to the uniform environments; differences in leaf area, stomatal density and stomatal area in paper birch populations under different [CO2] and soil water levels prove the birch’s ability to acclimate to environmental changes. Lastly, integration of leaf morphology and anatomy enhanced paper birch’s ability to balance between [CO2] gain and water loss.