Size‐dependent variation in shoot light‐harvesting efficiency in shade‐intolerant conifers

Shoots and foliage elements of shade-intolerant conifers strongly vary in size, but the effects of size on shoot light-harvesting efficiency have not been quantified. We investigated shoot adaptation to seasonal average integrated quantum flux density in gymnosperms Pinus palustris Mill., P. patula Schlect. & Cham., and P. radiata D. Don. and angiosperm Casuarina glauca Sieb. ex Spreng. In addition, P. sylvestris L., sampled from infertile and fertile sites, and P. taeda L. were included to test for general correlations among shoot architecture and size. All studied species possess needle-like photosynthetic elements. A shoot model was fitted to the data to separate the determinants of shoot light-harvesting efficiency. The model estimated shoot light harvesting on the basis of angular distribution of foliage surface areas, degree of spatial clumping, foliage area density in shoot volume, and beam path length in shoot volume. Increases in irradiance primarily led to greater foliage aggregation, greater foliage area density in shoot volume, and to a minor degree, to changes in foliage area angular distribution. Greater foliage aggregation resulted in lower efficiency of light harvesting but greater investment of foliar biomass in high light where the photosynthetic returns are greater. The species behave similarly except that the light-harvesting characteristics of P. patula, which has long, strongly bending needles, were independent of light. In all species, the shoots were larger in higher irradiance, and the fraction of biomass in shoot axis increased with increasing irradiance, indicating greater costs for light harvesting in high light. There were further significant species differences in light-harvesting efficiency that were linked to differences in foliage and shoot size. Foliage element length varied between 1.1 and 31.4 cm and shoot axis length between 1.2 and 36.5 cm among the species, leading to 4 orders of magnitude variation in shoot cylinder volume and three orders of magnitude variation in foliage area density (ρ); ρ decreased with increasing foliage element length and shoot volume. The degree of foliage clumping scaled positively with ρ and negatively with foliage element length. Foliage clumping was positively associated with foliage dry mass to shoot silhouette area ratio, signifying a trade-off between efficient light harvesting and large photosynthetic biomass accumulation. These data demonstrate a general increase of light-harvesting efficiency with increasing length of foliage elements, but they also demonstrate that this increase is limited by enhanced bending of longer foliage elements and by augmented support costs