Ecological significance of plant biomass turnover

在有有利和不利季节的气候中，植物的行为表现出生长和衰老的季节性模式。对季节的反应可能因物种而异，一个众所周知的例子是落叶和常绿树木的不同习性，它们的叶子要么是秋天衰老的，要么是越冬的。有迹象表明，湿地植物的根系表现出类似的越冬行为的二分选择：整个根系要么在秋季衰老，要么在冬季基本存活下来。但是，虽然叶片更替模式在不同环境下的适应价值已经得到了很好的理解，但对根系的适应价值却知之甚少。甚至连根系产生和根系衰老的时间都不为人所知。然而，最近的研究表明，根系的行为与地上植物部分的周转模式有很大的不同。***这个研究项目将从两个方面解决根系周转问题。第一种方法是详细描述一年中根系生产和衰老的模式，并了解这些模式如何响应土壤温度和生长季节长度的变化。这将在一个实验花园的室外实验中通过操纵土壤温度进行调查，并在不同气候带的半控制现场实验中进行调查，地点从詹姆斯湾海岸到美国东南部。第二种方法将是研究根系生长和衰老的季节性模式变化的功能后果。根系从土壤中吸收矿物质养分，其可用性因季节而异。我们将使用稳定同位素技术研究根系生产和衰老的时间如何影响一年内的养分吸收。我们还将研究氮在植物体内的停留时间在不同植物种类和不同根系周转模式下的变化。气候变暖将导致植物生长季节长度和季节性行为模式的变化。了解这些过程对于预测气候变化对生态系统过程（如碳循环）的影响至关重要。目前的模型大多基于地上的数据，因为地下过程的数据很少。这里提出的研究将有助于更好地理解湿地地下过程对温度和生长季节长度的响应。预期的结果将有助于准确预测这些覆盖加拿大北部大片地区的湿地的根系周转，以及由此产生的碳循环和固存。

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.