ATR-Fourier transform infrared spectroscopy was used to determine the carbon and nitrogen content in the leaves of herbaceous forest plant species and functional traits associated with the leaf economic spectrum (one of the two-dimensional global spectrum of plant form and function), and monitoring plant physiological status under elevated temperature conditions. The content of carbon and nitrogen determined by traditional methods validated the accuracy of ATR-FTIR method. It was also shown that in the case of forest herbs, the ATR-FTIR method is an efficient tool for determining functional traits (such as specific leaf area (SLA) and leaf dry matter content (LDMC)) related to the leaf economics spectrum, and to diagnose the photophysiological state of plants after changes of temperature (changes of day/night temperature from 21/13 °C to 25/17 °C). Measuring the areas of three absorption bands of the ATR-FTIR spectra related to amides I and II (between 1700 cm