The role of silicon in temperate and boreal forest ecosystems in a changing environment

气候变化预计会增加干旱的发生率和延长生长季节，这可能会改变森林的养分供应和需求。硅（Si）是土壤中最丰富的元素之一，现在被认为是某些物种的营养物。最近，它在减轻二氧化碳浓度增加的影响方面的作用吸引了很多人的兴趣。我最近的研究表明，美国东北部的两种主要硬木糖枫和山毛榉可能是硅的活跃蓄积者。如果得到证实，这将表明它需要在最佳水平上生长，研究人员一直低估了它在这些物种中的作用。

Climate change with its predicted higher incidence of droughts and longer growing seasons is likely to change the nutrient supply and requirements of forests. Silicon (Si), one of the most abundant element in soils, is now considered a nutrient for some species. It has attracted lots of interests lately for its role in mitigating the effects of increasing CO2 concentrations. My recent research indicates that sugar maple and beech, two dominant hardwoods in northeastern America, may be active accumulators of Si. If confirmed, this would suggest that it is required to grow at optimum levels and that researchers have been underestimating its role in these species. My research will therefore focus on the role of Si in the physiology/nutrition of sugar maple and beech with the primary objective of getting a more comprehensive understanding of their nutrition and physiology, and of the way they drive soil fertility in forests. The first step will consist in revising the nutritional norms for both species. To get a complete measure of the nutritional balance, we will include Si in the nutrient indices/ratios. A set of at least 200 trees per species growing on contrasting soil conditions will be sampled. Optimal nutrient concentrations and ratios will be determined based on boundary-line regressions. The second step will consist in assessing the physiological role of Si. I hypothesize that the energy saved by producing a "cheap" silicate structure will increase the production of defensive chemicals e.g. phenolics, tannins. For this study, seedlings will be grown in hydroponics with and without Si. A series of morphological variables of the roots and stems/leaves, and leaf chemistry will be monitored and compared. As active Si accumulators, one would expect sugar maple and beech to have developed specialized mechanisms to access Si and associated nutrients from minerals/rocks. I therefore propose to test their capacity to extract nutrients from three rocks used in ecological agriculture: wollastonite (Ca), apatite (P), and greensand (K). Testing would be done in controlled conditions (hydroponics) and in the field. With a greater capacity to absorb soil Si, I also expect a faster cycling and a greater accumulation of plant-derived silicates and silicate occluded C under sugar maple and beech. To test for these effects, we will make use of a plantation designed to maximize species interactions. With 19 species of trees grown in pure and mixed plots, we will be able to test the full spectrum of Si absorption potential of tree species commonly grown in eastern Canada. These studies will provide some new insights on how sugar maple and beech can become dominant in old forests of eastern Canada while likely playing a significant role in mitigating the negative effects of the increase in atmospheric CO2. Two novel and simple approaches will also be developed to assess soil Al toxicity and estimate the rate of transpiration in trees.