Seasonal nitrogen cycling in temperate trees: Transport and regulatory mechanisms are key missing links.

Nutrient accumulation, one of the major ecosystem services provided by forests, is largely due to the accumulation and retention of nutrients in trees. This review focuses on seasonal cycling of nitrogen (N), often the most limiting nutrient in terrestrial ecosystems. When leaves are shed during autumn, much of the N may be resorbed and stored in the stem over winter, and then used for new stem and leaf growth in spring. A framework exists for understanding the metabolism and transport of N in leaves and stems during winter dormancy, but many of the underlying genes remain to be identified and/or verified. Transport of N during seasonal N cycling is a particularly weak link, since the physical pathways for loading and unloading of amino N to and from the phloem are poorly understood. Short-day photoperiod followed by decreasing temperatures are the environmental cues that stimulate dormancy induction, and nutrient remobilization and storage. However, beyond the involvement of phytochrome, very little is known about the signal transduction mechanisms that link environmental cues to nutrient remobilization and storage. We propose a model whereby nutrient transport and sensing plays a major role in source-sink transitions of leaves and stems during seasonal N cycling.