Changes in leaf structure in relation to crown position and tree size of Betula papyrifera within fire-origin stands of interior cedar-hemlock

Abstract

Dimensions of anatomical and morphological attributes of leaves can influence physiological response to changes in environment over time. Linking structural attributes of leaves to crown position and tree size within naturally developing cohorts of trees can provide a clearer understanding of changes in crown morphology for a species. This study examined leaf anatomy and morphology of Betula papyrifera Marsh. growing in two stages of stand development of interior cedar-hemlock forest, northern British Columbia. Anatomical and morphological measurements of leaves located at six different positions within the crown were made on trees selected from stands that originated 15 years (small tree size) and 145 years (large tree size) after catastrophic fire. Leaf area and mass were measured in the field. Microscopic measures were made in the laboratory of thicknesses of leaves, cuticle, upper and lower epidermis, palisade, and spongy mesophyll. Stomatal density and stomatal aperture lengths were also determined. Leaf anatomy and morphology varied significantly with position in the crown and among size-classes of trees. Changes in leaf anatomy observed among positions within the crown reflect the changing availability of light and moisture experienced during crown development. For both size-classes of tree the largest anatomical dimensions of leaves were at the outer and uppermost parts of the crown, whilst the smallest were at the lower and innermost parts. Foliage of large trees (145 years) exhibit leaf attributes characteristic of the sun-shade dichotomy reported in the literature, but this was not shown for foliage of young saplings (15 years). For the small trees (15 years) the largest leaves were located at the top of the crown while the smallest were located at the bottom. This has been reported for many tropical pioneers but is in contrast to the usual sun-shade dichotomy of temperate pioneers. The large trees (145 years) followed the typical pattern with the smallest leaves at the top of the crown. Information from this study contributes to our understanding of foliar development of tree crowns by demonstrating how leaf structure changes with crown position and tree size under stand competition.Key words: Betula papyrifera, British Columbia, crown development, cuticle thickness, leaf area, palisade mesophyll, paper birch, stand dynamics.