Doctoral Dissertation Research: Factors Determining Geographical Range Limits of Boreal and Temperate Tree Species

关于预期气候变化对树木和其他植物物种地理分布的影响的预测因缺乏足够的机械基础而受到批评。 为了帮助解决这个缺点，这个博士论文研究项目将比较气候对北方树种和温带树种的生长和生理的影响。 通过使用从北方威斯康星州延伸到密西西比中部的纬度梯度，将树木暴露于未来可能面临的气候（由于预测的变暖），从而完成该比较。 根据沿着约1,800公里长的样带收集的初步数据，博士候选人将测试两个主要假设：（1）在目前的范围以南，北方物种在高度和生物量方面比温带特有种生长得更慢。 (2)北方树种的净生长相对较慢，主要是由于温度对碳利用效率（植物利用同化碳产生生物量的效率）的负面影响，这是由呼吸代谢的明显温度敏感性介导的。 在四个共同的花园沿着建立样带在两年的时间内，博士生将监测目标的北方和温带树种，颤抖的白杨（山杨）和郁金香，杨树（鹅掌楸）的树苗。 以下变量将在花园和物种之间进行比较（以及物种内的地理种源之间）：生长及其生理决定因素（即光合特性，生物量分配，叶片形态，暗呼吸和碳利用效率）;组织碳水化合物储备的休眠季节动态。 该研究项目将解决长期存在的问题，内在和外在的限制，对当前和未来的物种分布，并最终在校准的生理子程序在景观模型模拟森林对气候变化的反应，陆地植物物种的地理范围限制预计将转移相当大的气候变暖的反应。 例如，在北美东部，北方树种预计将向北撤退，而温带树种预计将填补它们的后面。 控制一个特定物种的南部范围边界的因素可能会决定它向北撤退的程度。 然而，到目前为止，形成南部范围界限的具体因素仍然不清楚。 该项目将直接解决这一问题，从而为预测树种分布对全球气候变化的反应的模型提供投入。 该项目进行过程中收集的数据也将提供洞察竞争的相互作用发生在新的物种组合，可能会导致树木迁移。 该项目将帮助科学界提高能力，就全球变化对北美和其他地方陆地生态系统结构和功能的影响作出可靠的预测。 这些预测是州、区域、国家和国际各级决策者使用的重要产品，用于应对全球变化影响以及潜在的减缓战略。 作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立的研究生涯。

Forecasts regarding influences of anticipated climate change on geographical distributions of trees and other plant species have been criticized for lacking a sufficiently mechanistic basis. To help address this shortcoming, this doctoral dissertation research project will compare influences of climate on the growth and physiology of boreal versus temperate tree species. This comparison will be accomplished by exposing trees to climates they may face in the future (owing to forecasted warming) through use of a latitudinal gradient extending from northern Wisconsin to central Mississippi. Based on preliminary data gathered along a roughly 1,800-km-long transect, the doctoral candidate will test two major hypotheses: (1) South of their current ranges, boreal species grow more slowly in terms of height and biomass than temperate endemics. (2) Comparatively slow net growth of boreal species is attributable primarily to negative influences of temperature on carbon-use efficiency (the efficiency with which plants use assimilated carbon to produce biomass) mediated by the pronounced temperature sensitivity of respiratory metabolism. In each of four common gardens along the established transect over a two-year period, the doctoral candidate will monitor saplings of target boreal and temperate tree species, trembling aspen (Populus tremuloides) and tulip-poplar (Liriodendron tulipifera). The following variables will be compared across gardens and species (and among geographic provenances within species): growth and its physiological determinants (i.e. photosynthetic traits, biomass allocation, leaf morphology, dark respiration and carbon-use efficiency); and dormant season dynamics in tissue carbohydrate reserves. This research project will address long-standing questions regarding intrinsic and extrinsic constraints on current and future species distributions and will culminate in the calibration of physiological subroutines in a landscape model simulating forest responses to climate change.The geographical range limits of terrestrial plant species are expected to shift considerably in response to climate warming. In eastern North America, for example, boreal tree species are predicted to retreat northward, while temperate species are expected to fill in behind them. The factors governing a particular species' southern range boundary likely will dictate the extent of its northward retreat. To date, however, the specific factors shaping southern range limits remain unclear. This project will directly address this issue, thereby providing input for models forecasting the responses of tree species distributions to global climate change. The data gathered during the conduct of this project also will provide insight into the competitive interactions occurring within new species assemblages that may result from tree migrations. The project will help the scientific community improve its ability to generate credible forecasts concerning the impacts of global change for the structure and function of terrestrial ecosystems in North America and elsewhere. These predictions are important products used by policymakers at the state, regional, national and international levels, addressing global change impacts as well as potential mitigation strategies. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.