Leaf morphological and anatomical variations of paper birch populations along environmental gradients across Canada

Leaf morphology and anatomy have been found to vary considerably among tree
\nspecies, and leaf characteristics have widely been used for analyzing plant growth and
\nresource use strategies because of their structural adaptation to withstand environments.
\nConsidering the changing climate projections, early-successional, broad niched species
\nlike paper birch (Betula papyrifera Marsh.) are expected to increase dominance due to a
\nzonal shift of natural vegetation and/or open gaps within the current vegetation zones.
\nHence, it is important to understand factors such as leaf characteristics that enable these
\npioneer species to inhabit a wide geographic range and their increasing dominance.
\nPaper birch is a pioneer tree species in North America that inhabits wide climatic
\nand geographic gradients; in addition, the species has developed different leaf
\nmorphology and anatomy that have allowed paper birch to adapt to diverse habitats. This
\nstudy examines how the leaf characteristics of paper birch vary under uniform and
\nstressed environments. The major objectives were (a) to investigate leaf characteristics
\nvariations in paper birch populations grown in uniform environmental conditions as in a
\ngreenhouse and a common garden; (b) to correlate between leaf characteristics and paper
\nbirch?s environment of origins; (c) to investigate leaf characteristic variations in paper
\nbirch populations grown under different carbon dioxide concentrations [CO2] and soil
\nwater levels to determine the relationship between leaf characteristics and individual or
\ninteracting effects of [CO2], water levels and populations; and (d) to analyze the
\nrelationship within and between leaf morphology and anatomy of the birch populations.
\nThe study found significant differences among paper birch populations in leaf
\nmorphological characteristics under a uniform environment at the greenhouse and the
\ncommon garden. The leaf characteristic variations in the uniform environment may be
\nrelated to the different genotypes of the birch inhabiting a wide environmental gradient.
\nIn paper birch populations grown in the common garden, significant differences in
\nstomatal density, stomatal area, pore area and guard cell width were identified. As
\nexpected, the birch populations in greenhouse and common garden environments
\nshowed significant correlations of leaf characteristics, namely specific leaf area (SLA),
\nleaf maximum width index and petiole area to latitude, longitude, elevation, temperature,
\nprecipitation and aridity index of origin. Correlation between leaf characteristics of
\npaper birch in the greenhouse showed that populations originated in limited precipitation
\n(during growing season) had low hair density on leaf adaxial surface, with larger leaf
\nwidth and petiole area. Birch populations grown in the common garden revealed that
\npopulations originated in higher mean annual precipitation had less hair density on leaf
\nadaxial surface with smaller leaf area and higher stomatal density. Relationships within
\nthe leaf characteristics revealed significant correlations within and between leaf morphology and anatomy as populations with larger leaf area had larger petiole area and
\nless adaxial hair density in greenhouse. The larger petiole in larger leaf reflects the need
\nfor mechanical strengthening to support, whereas inverse relationship between leaf area
\nand hair density possibly showed a strategy of the birch to balance water loss. In
\ncommon garden, the birch populations with larger leaf area had larger specific leaf area
\nand higher adaxial hair density but low stomatal density. All these features in paper
\nbirch populations provide a structural basis for reducing water loss through leaves and
\nincreasing water use efficiency. There was no consistency in leaf characteristics when
\nthe paper birch populations were grown in uniform environments as in the greenhouse
\nand the common garden.
\nAnalysis of the leaf characteristics in the birch showed significant differences due
\nto the interaction and/or main effects of [CO2], water levels and populations. Paper birch
\nhad decreased leaf area and increased stomatal density under elevated [CO2] which might
\nhave reduced stomatal conductance and increased water-use efficiency. Under low soil
\nwater level, paper birch populations studied had smaller stomatal area, pore area and
\nguard cell width. Contrasting with the expectation neither stomatal area was larger nor
\nstomatal density increased under low water level. A trade-off between stomatal area and
\ndensity in this study showed that stomatal area per unit leaf area remained the same.
\nHence, smaller stomatal area and guard cell width under low water level must have
\nimproved [CO2
\n] diffusion and decreased water loss compared to larger stomatal area and
\nguard cell width.
\nThe results of this study confirmed significant genotypic difference in leaf
\ncharacteristics of paper birch populations irrespective of a uniform growing
\nenvironment. The characteristics, namely leaf area, maximum width, SLA, stomatal
\ndensity and stomatal area, appear related to the environment of origin; however, these
\nrelationships were not consistent in the birch populations grown in the greenhouse and
\ncommon garden. Paper birch populations acclimated to the uniform environments;
\ndifferences in leaf area, stomatal density and stomatal area in paper birch populations
\nunder different [CO2] and soil water levels prove the birch?s ability to acclimate to
\nenvironmental changes. Lastly, integration of leaf morphology and anatomy enhanced
\npaper birch?s ability to balance between [CO2] gain and water loss.

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