Using thermal imagery and changes to stem radius to assess water stress in two coniferous tree species

With a warming climate and greater evaporative demand, many forest ecosystems are increasingly affected by water limitation as prolonged water deficits reduce tree-level growth and survival. Monitoring water deficit traditionally requires daily measurements of sap flow or radial flux from automated sensors mounted on individual trees. As an alternative approach, we evaluated the use of airborne thermal imagery from unmanned aerial vehicles as a rapid, scalable tool for assessing tree-level water stress. Plant water stress leads to higher leaf temperatures when soil moisture is low and evaporative demand is high. To detect this response, we modelled the difference between leaf and air temperature (∆T) as a function of local soil moisture vapour pressure deficit, and wind speed for two tree species, lodgepole pine (Pinus contorta var. latifolia) and white spruce (Picea glauca). We used those same weather and soil conditions to model dendrometer-based measurements of daily changes in internal tree water deficit (∆TWD). While canopy leaf temperature and daily change in tree water deficit showed little direct correlation with one another, both variables responded to soil moisture, vapour pressure deficit, and wind speed in a manner that reflects a common response to water stress as soil became progressively drier over the summer months. The two species showed similar responses to environmental conditions, with some differences related to species-specific strategies for drought avoidance. The application of thermal imagery to detect water stress in natural ecosystems can improve understanding how trees experience water stress across species and environmental conditions.

随着气候变暖与蒸发需求加剧，持续的水分亏缺会抑制树木个体的生长，同时降低其存活率，许多森林生态系统正日益受到水分限制的困扰。传统的水分亏缺监测方法需依托安装于单棵树木的自动传感器，每日获取树液流量或径向通量的测量数据。作为替代方案，本研究评估了利用无人机（unmanned aerial vehicle, UAV）搭载的机载热成像技术，将其作为一种快速且可规模化的树木个体水分胁迫评估工具。当土壤湿度偏低且蒸发需求较高时，植物水分胁迫会导致叶片温度升高。为捕捉这一响应特征，本研究针对扭叶松（Pinus contorta var. latifolia）和白云杉（Picea glauca）两个树种，将叶-气温差（∆T）建模为局地土壤湿度、水汽压亏缺（vapour pressure deficit, VPD）以及风速的函数；同时利用相同的气象与土壤条件，对基于树木径向生长仪（dendrometer）测得的树木内部水分亏缺日变化量（∆TWD）进行建模。尽管冠层叶片温度与树木水分亏缺日变化量之间几乎不存在直接相关性，但二者均对土壤湿度、水汽压亏缺以及风速产生响应，其响应模式反映出：随着夏季土壤逐渐变干，两类变量均表现出对水分胁迫的共性响应。两个树种对环境条件的响应模式整体相似，但存在部分与各自特有避旱策略相关的差异。利用热成像技术开展自然生态系统水分胁迫监测，能够增进我们对不同树种、不同环境条件下树木水分胁迫响应机制的理解。