Evaluating tree mortality and regeneration processes at multiple scales

在世界上的许多地方，古老的北方森林最近被证明是规则，而不是例外。这是因为大火虽然壮观，但并不经常影响大片森林，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、风灾和老年相关死亡)释放的资源将允许不同的物种进行招募。干扰频率、大小以及被杀死的树木数量和物种的差异将影响招募和树木多样性。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。预计干扰和现场生产力之间将会存在相互作用。换句话说，当土壤生产力低时，很少有物种能够补充，而当生产力高时，干扰将允许不同物种根据其对土壤资源或地面资源(如光)的需求进行补充。了解这些相互作用很重要，因为北方混交林承受着气候变化和人类活动(如木材采伐)的巨大压力。因此，我的研究将集中在树种组成如何随不同强度和频率的局部干扰沿立地生产力梯度而变化。我假设，不同严重程度的不同干扰的不同频率将为招募具有特定建立需求的长期生存物种创造临时利基，否则这些物种将被排除在完好的森林之外。预计在高生产率的地点，这种干扰介导的多样性将是最大的。尽管这类研究是在温带和热带森林中进行的，但在北方地区基本上被忽视了，尽管这片森林很重要。我的工作将集中在过渡的北方混交林，因为这个地区包含了最大的树木多样性，作为过渡地带也面临着最大的气候因素变化的风险(例如，在干扰制度等)。这项研究将基于航空摄影分析、树生态分析和建模相结合的基础上。

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.