Background and aims – The leaf size/number trade-off has been recently established as a wide-spread and highly predictable relationship associated with between-species leaf size variation. In this study, we examine whether this trade-off relationship also applies at the between-plant (within-species), and at the between-shoot (within-plant) levels associated with spatial variation in incident light availability within tree canopies.
Methods – Replicate current-year shoots were sampled from north-facing (shaded) and south-facing (sun-exposed) canopy sides of sixteen broadleaf tree species in eastern Ontario, Canada. For each shoot, measurements were recorded for mean individual leaf dry mass, number of leaves, number of side branches, and stem length, girth, and tissue dry mass. Leafing intensity was calculated as the number of leaves produced per unit of supporting stem tissue dry mass.
Key results – All of the direct trait measurements had generally larger values for shoots collected from south-facing canopy sides (as expected). However, negative isometric relationships between leaf size and leafing intensity were found at the between-plant level (for Acer saccharum) and the between-shoot (within-tree) level for at least some individuals of most species. The predominant trend at the within-tree level, however, was allometric – i.e. north-facing (light-limited) shoots generally had lower individual leaf dry mass but disproportionately higher leafing intensity compared with south-facing shoots.
Conclusions – The results confirm that there is a fundamental leaf size/number trade-off at the betweenplant (within-species) level and also at the between-shoot (within-plant) level, as previously reported at the between-species level. But more specifically, the results reveal distinctly different leaf deployment strategies in response to spatial light variability within tree canopies: Under high light exposure, larger leaves are favoured (with lower leafing intensity imposed as a trade-off), but in deeply shaded portions of the canopy, smaller leaves result, we suggest, for two reasons: (i) they are favoured directly (because they minimize overlap of closely spaced adjacent leaves); (ii) they are imposed as a trade-off of selection favouring high leafing intensity, which in turn maximizes the size of the reserve bud bank (number of axillary meristems per unit of supporting stem tissue) available for initiating continued growth or reproduction in the following year.
Deciduous angiosperm trees were sampled from natural populations near Kingston, Ontario (7º25’N 39º17’W), Whitney, Ontario (10º27’N 29º11’W), and the Queen’s University Biological Station, near Elgin, Ontario (44º33’N 76º21’W). Species with very large leaves (e.g. Juglans nigra, Carya cordiformis) and very small leaves (e.g. Populus tremuloides, Prunus serotina) were deliberately sampled, ensuring that a wide range of species leaf sizes were included. The remaining species (table 1) were sampled based on ease of availability at the study sites, but were otherwise selected without bias. Sample sites consisted of south-facing woodland edges opening into old-field vegetation. This ensured that the canopies of sampled trees were exposed to a sharp gradient of light intensity, with one side (facing south into the open field) exposed to direct open sky for most of the day, while the opposite side (facing north) would be intensely and permanently shaded by the contiguous woodland. Three replicate reproductively mature trees (i.e. not saplings) were sampled for each of fifteen study species, while twenty replicate trees were sampled for Acer saccharum (all from the same population). All samples were collected during the summer, beginning in July, after shoot and leaf growth for the season were complete. Following Kleiman & Aarssen (2007), a shoot sample was defined as all of the current year’s ‘leaf-bearing’ growth emerging from all meristems (buds) produced on a single leader (‘twig’) that had been produced in the previous year – see Fig. 2 in Kleiman & Aarssen (2007). This includes the new terminal leader emerging from the terminal (apical) meristem, plus any new lateral branches emerging from the subtending axillary meristems on the previous year’s leader. From each tree, ten shoots with minimal herbivore damage were sampled: the five highest (most sun-exposed) shoots on the south-facing side of the canopy (to the maximum limit of the tree pruner - 24 ft), and the five lowest (most shaded) shoots on the north-facing side. For each sampled tree, the height of attachment was recorded for the highest collected shoot on the south-facing side, and for the lowest collected shoot on the north-facing side. Incident light intensity was also measured from the north-facing collection point, using a Licor L1-250, while simultaneously measuring light intensity with a similar light meter positioned at 3 m above the ground within the adjacent open field (an approximation of the incident light intensity at the highest, south-facing sample point). In total, thirty shoots were sampled for each of fifteen study species, while two hundred shoots were sampled for A. saccharum. After being clipped from the trees, sampled shoots were returned to the lab and placed in cold storage (1ºC) until processing.