Trade-offs in seedling survival, growth, and physiology among hardwood species of contrasting successional status along a light-availability gradient

To better understand the regeneration ecology of northern red oak (Quercus rubra L.) in mesic forests, we compared its seedling growth and physiology with those of two other hardwoods, differing in successional status, along a gradient in light availability ranging from forest understories (2.6% of full light) to small clearings (69% of full light). Oak's relative growth rate (RGR) closely resembled that of shade-tolerant sugar maple (Acer saccharum Marsh.), and the positive response of both to increasing light was modest, especially beyond a relative light availability of 15%. Intolerant trembling aspen (Populus tremuloides Michx.) outgrew the others when relative light availability exceeded 5%, and its RGR increased more or less linearly with increasing light. However, there was a rank reversal of RGR in deep shade, where maple and oak had a higher RGR than aspen. This reversal was mirrored by seedling demography, as aspen survival was comparatively high in small clearings but negligible in deep shade. Aspen's low RGR and poor survival in low light were associated with a high rate of shoot dark respiration and minimal allocation to starch reserves. Aspen's high RGR in openings was attributed primarily to a high photosynthetic capacity per unit leaf mass. Thus, differential growth and survival among species could be explained in part by trade-offs in attributes that promoted rapid growth in high light at the expense of a favorable carbon balance in low light, or vice versa. Oak's suite of traits facilitated a positive carbon balance in perhaps all but the darkest understories. We suggest that the lack of persistence of oak seedlings in many understory environments may center around factors (e.g., vulnerability to biotic stresses) not directly related to carbon gain.