Nitrogen storage and remobilization by trees: ecophysiological relevance in a changing world

Abstract

The role of carbon (C) and nitrogen (N) storage by trees will be discussed in terms of uncoupling their growth from resource acquisition. There are profound differences between the physiology of C and N storage. C storage acts as a short-term, temporary buffer when photosynthesis cannot meet current sink demand and remobilization is sink driven. However, the majority of C allocated to non-structural carbohydrates such as starch is not reused so is in fact sequestered, not stored. In contrast, N storage is seasonally programmed, closely linked to tree phenology and operates at temporal scales of months to years, with remobilization being source driven. We examine the ecological significance of N storage and remobilization in terms of regulating plant N use efficiency, allowing trees to uncouple seasonal growth from N uptake by roots and allowing recovery from disturbances such as browsing damage. We also briefly consider the importance of N storage and remobilization in regulating how trees will likely respond to rising atmospheric carbon dioxide concentrations. Most studies of N storage and remobilization have been restricted to small trees growing in a controlled environment where (15)N can be used easily as a tracer for mineral N. We highlight the need to describe and quantify these processes for adult trees in situ where most root N uptake occurs via ectomycorrhizal partners, an approach that now appears feasible for deciduous trees through quantification of the flux of remobilized N in their xylem. This opens new possibilities for studying interactions between N and C allocation in trees and associated mycorrhizal partners, which are likely to be crucial in regulating the response of trees to many aspects of global environmental change.

Interactions between leaf nitrogen status and longevity in relation to N cycling in three contrasting European forest canopies

“…Thereafter, the N concentrations remained constant until the onset of senescence, when N became remobilised from the leaves to perennial woody parts, leading to a rapid decrease in both the N concentrations and chlorophyll concentrations (Figs. 3-A1 and 2-A1; see also Bauer et al, 1997;Millard and Grelet, 2010). The intensive N re-translocation is necessary to conserve N for leaf establishment in spring independently of soil N availability (Santa-Regina and Tarazona, 2001).…”

“…For instance, in deciduous trees, leaf N tends to be translocated into the woody roots and/or the trunks before leaf fall, to support tree growth in the following spring. In evergreen trees, N stored in previous-years leaves can nourish new leaf growth and, especially in most conifers, the youngest age class of needles are the main sites of N storage (Millard and Grelet, 2010). In N-limited forests, trees face a relatively high selection pressure to balance the N economy with acquisition, retention and remobilisation (Pearson et al, 2002).…”

Alteration of plant primary metabolism in response to insect herbivory

“…In addition to making plant tissue less nutritious, herbivore-induced reallocation of primary metabolites could be a tolerance strategy that allows later regrowth after the threat of herbivory has passed (Millard and Grelet, 2010). For instance, carbohydrate was transported away from maize roots infested with western corn rootworm (Diabrotica virgifera) into stem tissue, thereby allowing better regrowth of crown roots after herbivory (Robert et al, 2014).…”

Temperate and boreal forest tree phenology: from organ-scale processes to terrestrial ecosystem models

“…The seasonal pattern of N reserves is less continuous than that of C reserves in deciduous trees. After the spring mobilization of N reserves (Millard and Grelet 2010;El Zein et al 2011;Bazot et al 2013), N reserves remain at low levels throughout the leafy season (Bazot et al 2013), likely reflecting the balance between N requirements (for growth and maintenance) and supplies from the current root absorption. N storage is limited to autumn because it is strongly dependent on leaf senescence processes.…”

“…A remobilization of N from older shoots (Salifu and Timmer 2003) or the needles from the previous year (Millard and Proe 1992) provides the N necessary for the growth of newly formed tissues. Fluctuations of N reserves in evergreen species have not been well studied for the rest of the year but appear to be less variable than those of C reserves (Millard and Grelet 2010).…”