The mean labor time of a leaf (hour/day -1 ) is defined as the ratio of mean daily photosynthetic rate of a leaf (D a ; mol m -2 day -1 ) to the mean value of potential hourly photosynthetic rate (60 ◊ 60A max mol m -2 h -1 ) of the leaf. A model was proposed to estimate mean labor time of leaves. Mean labor time was obtained as the product of 24 (hours/day -1 ) and the four effects, each of which reduces production of a leaf: diel change in light (Diel Effect), reduction in light during cloudy and rainy days (Cloudy Effect), shading on the focal leaves (Shading Effect), and midday and afternoon depression in photosynthesis (Depression Effect). These four effects were estimated for open grown saplings of alder ( Alnus sieboldiana ), by measuring instantaneous photosynthetic rate and photon flux density above each leaf. The potential daily photosynthetic rate calculated from diel light condition in a clear day was 46.5% of hypothetical daily photosynthetic rate where maximum instantaneous photosynthetic rate was assumed to last throughout the life of the leaf (Diel Effect). The average of the daily photosynthetic rate considering clear, cloudy and rainy days was 79.7% of the clear day (Cloudy Effect). The photosynthetic rate estimated from light condition on the leaf was 85.6% of that in the open site (Shading Effect). Midday depression reduced the daily photosynthetic rate to 72.1% of the potential daily photosynthetic rate (Depression Effect). The product of the four effects multiplied by 24 h gave the estimate of mean labor time of leaves to be approximately 5.5 (h/day -1 ).

Research Highlights: We demonstrate the first quantitative evidence that the shoot shedding of fast-growing species growing in a high-light environment is part of the process of shoot redeployment into better-lit outer parts of the crown. Background and Objectives: Light foraging by redeploying organs from shaded regions of a tree crown into better-lit regions is considered to apply to both leaves and shoots. To date, however, this hypothesis has never been tested for shoots. Materials and Methods: We investigated the shoot dynamics of saplings of five deciduous woody species. We included fast-growing and slow-growing species (Alnus sieboldiana Matsum., Castanea crenata Siebold & Zucc., Betula ermanii Cham., Acer distylum Siebold & Zucc., and Fagus crenata Blume). Results: Shoots in the shaded regions of the crowns of the fast-growing trees showed higher mortality rates than those at better-lit positions. Because of the selective shedding of the shaded shoots, at the end of the growth period the light environment experienced by the shoots that survived until the following spring was similar to that at the early stage of the same growth period. By contrast, the slow-growing trees displayed slow and determinate growth, with a very low mortality rate of shoots at all positions in the crown. Conclusions: The rapid shoot turnover of the fast-growing species resulted in the redeployment of shoots into better-lit positions within the tree crown in a manner similar to the redeployment of leaves.

To explain why the composition of evergreen and deciduous forests changes along air temperature gradients, we measured several traits of single leaves from temperate deciduous and evergreen broadleaf trees with simultaneous and successive leaf emergence growing at different altitudes in the field. The parameters included seasonal net photosynthetic rate, longevity, mass per area, nitrogen content, and photosynthetic nitrogen‐use efficiency. With decreasing altitude, the leaf longevity of deciduous broadleaf trees increased, whereas the maximum net photosynthetic rate decreased. In contrast, leaf longevity of evergreen broadleaf trees decreased, whereas the minimum net photosynthetic rate in winter increased. Along the air temperature gradient, the annual production of deciduous trees with simultaneous leaf emergence may be constant, because the integrated lifetime net photosynthetic rate (ILNPR) of a single leaf changed little. In comparison, deciduous trees with successive leaf emergence may show enhanced annual production with increasing air temperature, by increasing the total leaf number per branch and tree under an extended growing season. Temperate evergreen broadleaf tree species may also show increased annual production with increasing air temperature by sufficiently raising the number of the first‐year leaves to the total leaves of branch and tree, which is accelerated by raising the integrated first‐year net photosynthetic rate of the single leaf, despite little change in the ILNPR. With increasing air temperature from cool‐temperate to warm‐temperate zones, evergreen broadleaf tree species gain an advantage of the annual production over deciduous broadleaf tree species with simultaneous leaf emergence.

Leafing pattern has long been considered as an important element characterizing the growth strategy of tree species; however, the consequences of leafing pattern for tree-crown formation have not been fully understood. To address this issue, the dynamic events (growth, birth, and death) of current-year shoots and leaves were investigated together with their location in saplings of a pioneer tree, Alnus sieboldiana. The leafing pattern was characterized by successive emergence and shedding of short-lived leaves. The combination of successive leafing and within-crown variation in leaf production brought about characteristic outcomes in crown morphology. In the outer crown, because of continuous leaf production, the shoots achieved great extension and enormous daughter shoot production, resulting in rapid expansion of the crown. In contrast, in the inner crown, due to early termination of leaf production, the shoots completely lost their leaves early in the growing season and consequently themselves died and were shed within the season. Such quick shedding of shoots caused ''crown hollowing'', i.e., the interior crown consisted of primary branches with little secondary development or foliage. These dynamic features are an effective adaptive strategy in early succession but also may be a disadvantage to maintaining foliage for longer period. Crown maintenance associated with the longevity of structural components is thought to play an important role in survival strategy of tree species.