Internal cycling of nitrogen has been shown to be a major source of nitrogen used for the seasonal growrh of both evergreen and deciduous trees providing up to 90% of N used for leaf growth of some species. The processes of internal cycling comprise seasonal nitrogen storage. followed by remobilisation during either periods of growth (e.g. in the spring) or during leaf senescence. The ecophysiology of these processes is reviewed. along with the methods used to quantify their contribution to tree growth.Nitrogen budget studies have been widely used to estimate internal cycling, panicularly in relation to soil fertility. These studies have shown that as trees develop their rate of N uptlke decreases, but as they grow their storage capacity incnsxs. However, budget studies are imprecise and have not always quantified rcmobilisation adequately. An alternative approach has been the use of I5N to quantify N uptake and partitioning, allowing precise measurements of N storage and remobilisation to be made. The use of isotopes has allowed experiments to be run which have shown that environmental factors 6uch as soil fertility influence the amount of N stored, but have no direct influence upon the amount of N nmobiliscd. These methods are &scussed in light of recent research on N remobilisation. which has provided an understanding of the processes of storage and remobilisation which potentially allows direct measurements to be made in field p w n trees for tht first time.

Plant growth is usually constrained by the availability of nutrients, water, or temperature, rather than photosynthetic carbon (C) fixation. Under these conditions leaf growth is curtailed more than C fixation, and the surplus photosynthates are exported from the leaf. In plants limited by nitrogen (N) or phosphorus (P), photosynthates are converted into sugars and secondary metabolites. Some surplus C is translocated to roots and released as root exudates or transferred to root-associated microorganisms. Surplus C is also produced under low moisture availability, low temperature, and high atmospheric CO 2 concentrations, with similar below-ground effects. Many interactions among above-and belowground ecosystem components can be parsimoniously explained by the production, distribution, and release of surplus C under conditions that limit plant growth. What Drives Carbon Allocation in Plants? Highlights Plant growth is normally constrained by nutrients, water or temperature, not photosynthesis, and plants often have surplus carbohydrates. Secondary metabolites are produced in N-limited plants primarily to dispose of surplus carbon, although they may subsequently help reduce browsing damage. Surplus carbohydrates are translocated from leaves and below ground some are discharged via exudates and mycorrhizal fungi. Root exudates contain more of the elements that plants have in surplus, and less of those in short supply. The abundance and type of mycorrhizal fungi is influenced by the amount and composition of surplus carbon in roots. Surplus carbon provides an alternative lens though which to view interactions between plants and soil organisms.

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The paper develops a Markov model in continuous time for the length of stay of elderly people moving within and between residential home care and nursing home care. A procedure to determine the structure of the model and to estimate parameters by maximum likelihood is presented. The modelling approach was applied to 4 years' placement data from the social services department of a London borough. The results in this London borough suggest that, for residential home care, a single-exponential distribution with mean 923 days is adequate to provide a good description of the pattern of the length of stay, whereas, for nursing home care, a mixed exponential distribution with means 59 days (short stay) and 784 days (long stay) is required, and that 64% of admissions to nursing home care will become long-stay residents. The implications of these findings and the advantages of the proposed modelling approach in the general context of long-term care are discussed. Copyright 2005 Royal Statistical Society.

The empirical distribution of length of stay of patients in departments of geriatric medicine is fit extremely well by a sum of two exponentials. Most of the patients in a geriatric department are rehabilitated and discharged or they die within a few weeks of admission, but the few who become long-stay patients remain for months or even years. A model is presented for the flow of patients through a geriatric department, which has analogies to models of drug flow in pharmacokinetics. The theoretical model explains why the empirical pattern of length of stay in the occupied beds fits a sum of two exponentials; conversely, the empirical distribution, obtained from the midnight bed state report, can be used to study the effect of various policy decisions on both immediate and future admission rates for the department, and shows the benefits of policies which reduce long-stay patient numbers by improving long-stay rehabilitation.