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We have measured the rates of root production and death and of root respiration in situ under two grasslands along an altitudinal gradient in the northern Pennines, UK, represented by a lowland site at 171 m in an agricultural setting, and three upland sites between 480 and 845 m. One grassland was dominated by Festuca ovina and was on a brown earth soil; the other was dominated by Juncus squarrosus and Nardus stricta and occurred on a peaty gley. The natural altitudinal gradient was extended by transplantation. Although root biomass and root production (estimated using minirhizotrons) both showed pronounced seasonal peaks, there was no simple altitudinal gradient in either variable, and neither root production nor root death rate was a simple function of altitude. Increased root accumulation in summer was a function of change in the length of the growing season, not of soil temperature. Root populations in winter were similar at all sites, showing that increased production at some sites was accompanied by increased turnover, a conclusion confirmed by cohort analyses. Respiration rate, measured in the field by extracting roots and measuring respiration at field temperature in an incubator, was unrelated to temperature. The temperature sensitivity of respiration (expressed as the slope of a plot of log respiration rate against temperature) showed no simple seasonal or altitudinal pattern. Both root growth (under Festuca) and respiration rate were, however, closely related to radiation fluxes, averaged over the previous 10 days for growth and 2 days for respiration. The temperature sensitivity of respiration was a function of soil temperature at the time of measurement. These results show that root growth and the consequent input of carbon to soil in these communities is controlled by radiation flux not temperature, and that plants growing in these upland environments may acclimate strongly to low temperatures. Most carbon cycle models assume that carbon fluxes to soil are powerfully influenced by temperature, but that assumption is based largely on short-term studies and must be reassessed.

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Summary1. This account presents information on all aspects of the biology of Urtica dioica that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation. 2. Urtica dioica is a tall, usually dioecious, rhizomatous, perennial herb with numerous stinging hairs, probably native in fens and semi-natural ancient woodlands, but widely naturalized in a range of habitats and abundant throughout the British Isles. British material is mainly ssp. dioica which also extends throughout Europe, and locally ssp. galeopsifolia (without stinging hairs) which is also found in western, central and eastern Europe. 3. Urtica dioica is a moderately shade-tolerant species, which occurs on most moist or damp, weakly acid or weakly basic, richly fertile soils. 4. A highly competitive ruderal species, Urtica dioica often forms monospecific stands which are not infrequently the product of a single individual that has spread by means of horizontal rhizomes. 5. Urtica dioica has frequently been described as a nitrophile, but there are many soils in which the supply of inorganic nitrogen is adequate for growth. However, there are other soils in which growth is checked with symptoms of severe phosphorus deficiency unless soluble phosphate is added to the soil. This is especially so in the long-established deciduous woodlands, and where there has been no addition of fertilizers to the soil. The growth responses of U. dioica to the availability, source and utilization of nitrogen and phosphorus have been examined experimentally in some detail. 6. Urtica dioica with its stems and leaves densely covered with stinging hairs, which release potential pain-inducing toxins when brushing contact is made with them, is rarely eaten by cattle and rabbits, but is palatable to some species of snail. It is the food plant of the larvae of a number of attractive butterflies and other phytophagous insects. 7. Male clones of U. dioica flower in advance of female clones. The pollen grains are extremely small. It is usually wind-pollinated, but occasionally insect-pollinated. 8. The low seed mass of U. dioica enables the production of vast numbers of seeds. Few of the seeds germinate in the period immediately following dispersal, and the species maintains a seed bank that changes little in size with season and is large in relation to annual seed production.Key-words: communities, conservation, ecophysiology, geographical and altitudinal distribution, germination, herbivory, nutrients, reproductive biology, soils Urticaceae, Urtica dioica L. (stinging nettle, common nettle) is a perennial herb with an extensive sympodial system of rhizomes and stolons, rooting at the nodes and giving rise in spring to aerial shoots up to 1.5-2 m or (rarely) 3...

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