Thermal infrared (TIR) techniques to collect thermal imagery have been useful for recording quasi-continuous plant surface temperatures. In this study, we applied a thermal camera to measure canopy skin temperatures in a mature ponderosa pine forest in central Oregon over one growing season from May to September 2014. This study had the following objectives: (1) to examine spatial and temporal variations of canopy temperature; (2) to explore the effects of climate and soil conditions on canopy temperature; and (3) to quantify the relationships of canopy temperatures to forest-atmosphere heat and carbon fluxes. The temporal variation of 30min mean canopy temperature was large, and leaf temperatures ranged from-2.1 to 33.6 ºC during the study period. The temperature difference was small between the whole canopy and leaf regions, while tree stems had warmer temperatures than leaves, especially during the afternoon (12:00 to 19:59 hr). The canopy thermal regime was largely controlled by climatic conditions and related to the soil thermal states. Air temperature, relative humidity, longwave radiation, and soil temperature at 2-cm depth were tightly correlated with 30-min and daily/subdaily mean canopy leaf temperatures (r ≥ 0.6 or ≤-0.6, p < 0.01). The daily/sub-daily mean canopy temperatures contained stronger relationships with the climatic and soil variables than the 30-min mean temperatures. During the afternoon, the mean leaf temperature was more closely related to net ecosystem exchange (r 2 = 0.69) than air temperature, driven by the strong relationship between tissue temperature and photosynthesis and respiration. Our results show that canopy thermal conditions can be monitored almost continuously for extended time periods to better characterize how canopies respond to environmental conditions. Finally, thermal measurements show great promise for quantifying linkages to carbon exchange in forest ecosystems.

Quaking aspen (Populus tremuloides Michx.), a widespread and keystone tree species in North America, experienced heat and drought stress in the years 2002 and 2003 in the southwestern United States. This led to widespread aspen mortality that has altered the composition of forests, and is expected to occur again if climate change continues. Understanding interactions between aspen and its environments is essential to understanding future mortality risk in forests. Polyploidy, which is common in aspen, can modify plant structure and function and therefore plant-environment interactions, but the influence of polyploidy on aspen physiology is still not well understood. Furthermore, the ploidy types of aspen have different biogeographies, with triploids being most frequent at lower latitudes in generally warmer and drier climates, while the northerly populations are virtually 100% diploid. This suggests that ploidy-environment interactions differ, and could mean that the ploidy types have different vulnerabilities to environmental stress. In this study, to understand aspen ploidy-environment interactions, we measured 38 different traits important to carbon uptake, water loss and water-use efficiency in diploid and triploid aspen in Colorado. We found that triploid aspen had lower stand density, and greater leaf area, leaf mass, leaf mass per area, percent nitrogen content, chlorophyll content and stomatal size. These differences corresponded to greater potential net carbon assimilation (A, measured using A/Ci curves, and chlorophyll fluorescence) and stomatal conductance (gs) in triploids than diploids. While triploid aspen had higher intrinsic water-use efficiency (iWUE, calculated from measurements of δ13C in leaf tissue), they also had greater potential water loss from higher measured gs and lower stomatal sensitivity to increasing vapor pressure deficit. Therefore, despite greater iWUE, triploids may have lower resilience to climate-induced stress. We conclude that ploidy type strongly influences physiological traits and function, and mediates drought stress responses in quaking aspen.

1. Ploidy level in plants may influence ecological functioning, demography and response to climate change. However, measuring ploidy level typically requires intensive cell or molecular methods. 2. We map ploidy level variation in quaking aspen, a dominant North American tree species that can be diploid or triploid and that grows in spatially extensive clones.We identify the predictors and spatial scale of ploidy level variation using a combination of genetic and ground-based and airborne remote sensing methods.3. We show that ground-based leaf spectra and airborne canopy spectra can both classify aspen by ploidy level with a precision-recall harmonic mean of 0.75-0.95 and Cohen's kappa of c. 0.6-0.9. Ground-based bark spectra cannot classify ploidy level better than chance. We also found that diploids are more common on higher elevation and steeper sites in a network of forest plots in Colorado, and that ploidy level distribution varies at subkilometer spatial scales. 4. Synthesis. Our proof-of-concept study shows that remote sensing of ploidy level could become feasible in this tree species. Mapping ploidy level across landscapes could provide insights into the genetic basis of species' responses to climate change. K E Y W O R D S adaptation, ploidy level, polyploidy, quaking aspen, reflectance, remote sensing, spectrometry, UAS S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section at the end of the article. How to cite this article: Blonder B, Graae BJ, Greer B, et al. Remote sensing of ploidy level in quaking aspen (Populus tremuloides Michx.). J Ecol. 2020;108:175-188. https ://doi.

Quaking aspens (Populus tremuloides Michx.) are found in diverse habitats throughout North America. While the biogeography of aspens' distribution has been documented, the drivers of the phenotypic diversity of aspen are still being explored. In our study, we examined differences in climate between northern and southwestern populations of aspen, finding large‐scale differences between the populations. Our results suggest that northern and southwestern populations live in distinct climates and support the inclusion of genetic and phenotypic data with species distribution modeling for predicting aspens' distribution.