Abstract
Whole-plant energy capture depends not only on the photosynthetic response of individual leaves, but
also on their integration into an effective canopy, and on the costs of producing and maintaining their
photosynthetic capacity. This paper explores adaptation to irradiance level in this context, focusing on
traits whose significance would be elusive if considered in terms of their impact at the leaf level alone.
I review traditional approaches used to demonstrate or suggest adaptation to irradiance level, and outline
three energetic tradeoffs likely to shape such adaptation, involving the economics of gas exchange,
support, and biotic interactions. Recent models using these tradeoffs to account for trends in leaf
nitrogen content, stornatal conductance, phyllotaxis, and defensive allocations in sun v. shade are evaluated.
A re-evaluation of the classic study of acclimation of the photosynthetic light response in Atriplex,
crucial to interpreting adaptation to irradiance in many traits, shows that it does not completely support
the central dogma of adaptation to sun v. shade unless the results are analysed in terms of whole-plant
energy capture. Calculations for Liriodendron show that the traditional light compensation point has
little meaning for net carbon gain, and that the effective compensation point is profoundly influenced
by the costs of night leaf respiration, leaf construction, and the construction of associated support and
root tissue. The costs of support tissue are especially important, raising the effective compensation point
by 140 pmol m- s - ' in trees 1 m tall, and by nearly 1350 pmol m - s - ' in trees 30 m tall. Effective
compensation points give maximum tree heights as a function of irradiance, and shade tolerance as a
function of tree height; calculations of maximum permissible height in Liriodendron correspond
roughly with the height of the tallest known individual. Finally, new models for the evolution of canopy
width/height ratio in response to irradiance and coverage within a tree stratum, and for the evolution
of mottled leaves as a defensive measure in understory herbs, are outlined