Evaluating tree mortality and regeneration processes at multiple scales

古老的北方森林最近被证明是世界上许多地方的普遍现象，而不是例外。这是因为火灾虽然壮观，但并不经常影响大面积的森林，特别是在北美东部。相反，这些森林受到其他干扰的影响。这些局部干扰（如昆虫暴发、风吹和与老年有关的死亡）释放的资源将允许不同物种进行招募。干扰频率、大小、被杀树木的数量和种类的差异会影响树木的补充和多样性。然而，干扰本身可能不是影响北方混交林树木多样性的唯一因素。预计在干扰和现场生产力之间会有相互作用。换句话说，当土壤生产力低时，很少有物种能够招募，而当生产力高时，干扰将允许不同物种根据它们对土壤资源或地面资源（如光）的需求进行招募。了解这些相互作用非常重要，因为北方混交林受到气候变化和人类活动（如木材采伐）的巨大压力。因此，我的研究将集中在沿着站点生产力梯度的不同程度和频率的局部干扰下树种组成的变化。我假设，不同严重程度的干扰频率的变化将为具有特定建立需求的长寿物种的招募创造暂时的生态位，否则这些物种将被排除在完整的森林之外。预计这种干扰介导的多样性将在高生产力场地上最大。尽管在温带和热带森林中进行了此类研究，但在北方地区，尽管这种森林很重要，但基本上被忽视了。我的工作将集中在过渡北方混交林，因为这个地区包含了最大的树木多样性，作为一个过渡区，也面临着气候因素变化的最大风险（即在干扰制度等）。本研究将结合航空摄影分析、树木生态学分析和建模。

Old boreal forests have recently been shown to be the rule rather than the exception in many parts of the world. This is because fires, although spectacular do not regularly affect large areas of the forest, especially in eastern North America. Instead these forests are affected by other disturbances. These partial disturbances (such as insect outbreaks, windthrow, and old-age related mortality) release resources that will allow different species to recruit. Differences in disturbance frequency, size and the number and species of trees killed will affect recruitment and tree diversity. However, disturbances alone will probably not be the only factor affecting tree diversity of boreal mixedwoods. It is expected that there will be an interaction between disturbances and site productivity. In other words, when soil productivity is low few species should be able to recruit whereas when productivity is high then disturbances will permit different species to recruit depending on their needs for soil resources or above ground resources such as light. Understanding these interactions is important as boreal mixedwood forests are under great pressure from climate change and human activities such as timber harvesting. My research will thus focus on how tree species composition changes with different severity and frequency of partial disturbances along a site productivity gradient. I hypothesise that a varying frequency of different severity disturbances will create temporal niches for the recruitment of long-lived species with specific establishment needs that would otherwise be excluded from intact forests. It is also expected that such disturbance mediated diversity will be greatest on high productivity sites. Although such research has been conducted in temperate and tropical forests it has basically been ignored in boreal regions despite the importance of this forest. My work will focus on the transitional boreal mixedwoods as this region encompasses the greatest tree diversity and being a transition zone is also under the greatest risk of change (i.e. in disturbance regimes, etc) to climatic factors. This research will be based on a combination of aerial photographic analysis, dendroecological analysis and modeling.