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

Dimensions of anatomical and morphological attributes of leaves can influen ce physiological response to changes in environment over time. Linking stru ctural attributes of leaves to crown position and tree size within naturall y developing cohorts of trees can provide a clearer understanding of change s in crown morphology for a species. This study examined leaf anatomy and m orphology of Betula papyrifera Marsh. growing in two stages of stand develo pment of interior cedar-hemlock forest, northern British Columbia. Anatomic al and morphological measurements of leaves located at six different positi ons within the crown were made on trees selected from stands that originate d 15 years (small tree size) and 145 years (large tree size) after catastro phic fire. Leaf area and mass were measured in the field. Microscopic measu res were made in the laboratory of thicknesses of leaves, cuticle, upper an d lower epidermis, palisade, and spongy mesophyll. Stomatal density and sto matal aperture lengths were also determined. Leaf anatomy and morphology va ried significantly with position in the crown and among size-classes of tre es. Changes in leaf anatomy observed among positions within the crown refle ct the changing availability of light and moisture experienced during crown development. For both size-classes of tree the largest anatomical dimensio ns 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 (14 5 years) exhibit leaf attributes characteristic of the sun-shade dichotomy reported in the literature, but this was not shown for foliage of young sap lings (15 years). For the small trees (15 years) the largest leaves were lo cated 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 t he usual sun-shade dichotomy of temperate pioneers. The large trees (145 ye ars) followed the typical pattern with the smallest leaves at the top of th e crown. Information from this study contributes to our understanding of fo liar development of tree crowns by demonstrating how leaf structure changes with crown position and tree size under stand competition.