1. To test hypotheses concerning adaptation and acclimation of tree species to shaded habitats we determined the growth, biomass partitioning and morphology of seedlings of nine near‐boreal tree species in high‐ and low‐light greenhouse environment (25 and 5% of full sunlight, respectively), comparable to sunlit gap and shaded microsites in boreal forests. The species differ widely in shade tolerance, seed size and leaf life span. 2. In low light, all species allocated proportionally more biomass to stems and less to roots, but the same to foliage, compared with the high‐light environment. At a common size, all species had finer leaf morphology (higher specific leaf area, SLA) but coarser root morphology (lower specific root length, SRL) in low than high light. From a whole plant perspective, all species enhanced leaf area per unit plant mass (leaf area ratio, LAR) in low light and root length per unit plant mass (root length ratio, RLR) in high light. 3. Shade‐intolerant deciduous species had higher RGR, SLA and SRL than larger seeded evergreens: ranking from Populus, Betula and Larix spp., then to five evergreen Pinus, Picea and Thuja spp., which were generally comparable in these traits. There were no changes in growth rankings of species between high‐ and low‐light environments, nor consistent differences among species in biomass partitioning. Hence, species differences in leaf and root morphology (SLA, SRL) drove whole plant patterns, such as Populus, Betula and Larix had greater total leaf area and root length per unit plant mass (LAR and RLR, respectively) than the evergreens. Interspecific variation in RGR in both high and low light was positively correlated (r≈ 0·9) with SLA, SRL, LAR and RLR, and negatively correlated (r≈–0·9) to seed mass and leaf life span. 4. These data suggest that SLA, SRL, NAR and RGR are closely associated with variation in life‐history traits and that variation in leaf and root structure more strongly influences patterns of RGR among species and light environments than does biomass partitioning.

1. To test several hypotheses about acclimation and adaptation of photosynthesis and respiration to differing light conditions, we investigated the interspecific relationships between leaf and root metabolism, chemistry and morphology in high and low light conditions for young seedlings of nine boreal tree species that differ in relative growth rate (RGR). 2. Light‐saturated net photosynthesis (Asat), whole‐plant nitrogen (N) uptake rates, leaf and root respiration and morphology, and RGR all varied in parallel among the nine species when grown in both 5% and 25% of full sunlight. RGR, Asat, leaf and root respiration (Rd), and N uptake rate per unit root mass or length differed significantly among species, ranking (from high to low): Populus, Betula and Larix spp. (all deciduous) and then to five evergreen conifers (Pinus, Picea and Thuja spp.), which were generally comparable in these measures. 3. A sat, leaf and root Rd and N uptake rates were all correlated (r≈ 0·8 to 0·9) with species traits, such as seed mass, leaf life span and shade‐tolerance rankings. Mass‐based Asat was greater in conifer seedlings raised in low than high light. In contrast, area‐based Asat was higher for plants grown in high than low light, especially in the deciduous species. Once adjusted for differences in plant mass, leaf or root respiration rates did not differ for plants grown in low vs high light. 4. Interspecific variation in RGR was positively correlated (r≈ 0·9) with rates of photosynthesis, respiration and N uptake. Leaf photosynthesis and respiration rates were correlated to specific leaf area and leaf N concentrations (r≈ 0·9). Root respiration rates, N uptake rates, specific root length (root length per root dry mass) and root N concentrations were all highly correlated with each other (r≈ 0·8 to 0·9). These data suggest a close coupling of tissue‐level metabolism, chemistry and structure with whole‐plant performance and species ecophysiological and life‐history traits.

There is increasing interest in understanding the costs and benefits of increased size and prolonged lifespan for plants. Some species of trees can grow more than 100 m in height and can live for several millennia, however whether these achievements are obtained at the cost of some other physiological functions is currently unclear. As increases in size are usually associated with ageing, it is also unclear whether observed reductions in growth rates and increased mortality rates are a function of size or of age per se. One theory proposes that reduced growth after the start of the reproductive phase is caused by cellular senescence. A second set of theories has focussed instead on plant size and the increased respiratory burdens or excessive height. We report on experimental manipulations to separate the effects of extrinsic factors such as size from those of intrinsic factors such as age for four tree species of contrasting phylogeny and life history. For each species, we measured growth, gas exchange and leaf biochemical properties for trees of different ages and sizes in the field and on propagated material obtained from the same genetic individuals but now all of small similar size in our common gardens. For all species, evidence indicated that size, not cellular senescence, accounted for the observed age-related declines in relative growth rates and net assimilation rates. Two species exhibited evidence of genetic control on leaf characters such as specific leaf area, although size also exerted an independent, and stronger, effect. We found partial support for the theory of hydraulic limitations to tree growth. The lack of a marked separation of soma and germline, an unlimited proliferation potential of meristem cells and the exponential increase in reproductive effort with size all help explain the lack of a senescence-induced decline in trees. It is possible that trees much older than the ones we sampled exhibit senescence symptoms.