Ecological significance of plant biomass turnover

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In climates that have favourable and unfavourable seasons, plant behaviour shows seasonal patterns of growth and senescence. Responses to the seasons may vary from species to species, a well-known example being the contrasting habits of deciduous and evergreen trees with either autumn-senescing or overwintering leaves. There are indications that roots of wetland plants show a similar dichotomous choice of overwintering behavior: the entire root system either senesces in the autumn, or it largely survives the winter. But while the adaptive value of leaf turnover pattern in different environments is well understood, for roots less information is available. Not even the timing of root production and root senescence is well known. Nevertheless, recent research has shown that root behaviour can considerably differ from turnover patterns of the above-ground plant parts.***This research program will address the issue of root turnover in a two-fold manner. The first approach is to describe in detail patterns of root production and senescence over the year, and to understand how these patterns respond to variation in soil temperature and growing season length. This will be investigated in outdoor experiments in an experimental garden by manipulating soil temperature, and in semi-controlled field experiments in different climate zones, the locations ranging from the James Bay coast to the southeastern US. The second approach will be to investigate the functional consequences of variation in seasonal patterns of growth and senescence of roots. Roots take up mineral nutrients from the soil, the availability of which varies depending of the season. We will investigate how the timing of root production and senescence affects nutrient uptake over a year, using the stable isotope technique. We will also investigate how the residence time of nitrogen in the plant varies among plant species with different root turnover patterns. ***The warming climate will lead to changes in growing season length and in seasonal patterns of plant behavior. Understanding these processes is essential to predict effects of climate change on ecosystem processes such as carbon cycling. Current models are mostly based on above-ground data, as data on below-ground processes is scarce. The research proposed here will contribute to a better understanding of below-ground processes in wetlands in response to temperature and growing season length. The expected results will help to make accurate predictions of root turnover, and the resulting carbon cycling and sequestration in these wetlands, which cover large areas in Canada's North.