Linking repeat photography and remote sensing to assess treeline rise with climate warming: Mount of the Holy Cross, Colorado

Most ecological studies are by necessity cast on rather short timescales, such that documenting change in phenomena that occur slowly (e.g., over decades to centuries) is quite difficult. Here, we explore variation in and covariation of two sources of information to address rates of treeline change on one mountain face in the central Rocky Mountains: repeat photography and remote sensing. The repeat photography work involved assembling and quantitatively comparing four photographs that span 148 years. The remote sensing analyses covered a shorter period (35 years) but provided quantitative measurements and fuller landscape coverage. The repeat photography results demonstrated pronounced elevational advance of erect forest, at a rate <0.3 m/year during 1873–1979 but accelerating to >1.8 m/year during 2004–2020. The remote sensing analyses reflected similar processes but also reflected considerable infilling of tree cover near treeline; consequent increases in Normalized Difference Vegetation Index (NDVI) values in the upper forested areas may thus indicate treeline advance or changes in forest characteristics below treeline. Overall, these results document (1) acceleration in rise of treeline at this site in recent decades and (2) elevational compression of vegetation zones above treeline, with important implications for conservation of nonforested alpine ecosystems. Integrating multiple streams of evidence offers complementary views and insights in analyzing this phenomenon.