Spectral characteristics of pine needles at the limit of tree growth in subarctic Finland

Background: Forests in subarctic regions inhabit the transitional zone between the temperate region and the Arctic, where plant survival depends on the capability of individuals to adjust to changing environments. This transitional zone, generally falling between 50° N and 70° N, is becoming increasingly vulnerable under a changing climate, with treelines moving further north and also higher up slopes. Aims: The current study was carried out at 68° N, the most sensitive zone of the subarctic and northern limit of tree growth in Finnish Lapland. The physiology of trees in these conditions is still not fully understood, and an attempt is made here to contribute to a better understanding of the functionality of leaves by studying the spectral characteristics of needles of Pinus sylvestris along an altitudinal transect. Methods: A variety of spectral indices derived from high-resolution reflectance spectra were measured on freshly excised pine needles. The specific hypothesis is that spectral reflectance data can detect the stress that pine needles suffer at high elevations. We also wish to know which of the several possible indices is most indicative of stress. Results: All of the spectral indices, with the exception of NDVI, showed elevational as well as age-related trends consistent with the hypothesis that needles from the highest elevation suffer degradation of chlorophyll. Conclusions: We conclude that appropriate reflectance measurements provide useful information that may in future be used to detect stress effects on conifer foliage. The visual appearance of trees included in the study did also reflect in their spectral characteristics.