Functional traits of Atlantic Forest plants, leaf optics and PROSPECT modeling

Plant functional traits are features strongly associated with plant colonization, survival, growth, and mortality. Those attributes can affect the ecosystem’s functions and the responses of the vegetation to environmental changes. Traditional methods to measure plant functional traits are time-consuming, and spectroscopy has been used as an alternative technology to assess leaf functional traits on a large scale because it is faster and non-destructive. The radiative transfer model PROSPECT is an open-source leaf optical properties model that in the forward mode simulates the leaf optical properties using biochemical constituents as input parameters and in the inverse mode uses as input parameters the leaf optical properties to simulate leaf biochemical constituents. This study aims to explore how accurate the leaf characteristics are estimated from leaf spectroscopy measurements performing the inversion of PROSPECT using different methods as input. Four different optic-derived tools were used to acquire leaf spectral data of 128 species from the Atlantic Forest in Brazil, and these are compared. The tools are (i) the integrating sphere, that measures leaf directional-hemispherical reflectance and transmittance, (ii) the contact probe, that measures bidirectional reflectance, and (iii) the SPAD and (iv) DUALEX leaf clips that measure, respectively, indices related to leaf chlorophyll content and chlorophyll, flavonols and anthocyanins. The measurements of the integrating sphere and the contact probe were used as input parameters to run the PROSPECT model in the inverse mode to estimate biochemical and structural leaf properties. Then the Spearman’s correlation test was used to assess the correlation of the estimated biochemical trait values with each other and with LMA and EWT measured in the laboratory to determine the consistency of the estimates using the PROSPECT model and confirm if the contact probe measurements can be used as an input parameter to invert PROSPECT. Our results showed that the contact probe reflectance measurements can be used as input to invert the PROSPECT model and estimate chlorophyll, carotenoids, anthocyanins, equivalent water thickness, and leaf mass per area but did not perform well in the inversions to estimate proteins and carbon-based compounds. The SPAD and DUALEX clips measurements correlated strongly with the CHL estimated by PROSPECT with the DUALEX clip slightly outperforming the SPAD. And although the integrating sphere measurements are used traditionally to invert PROSPECT, the LMA estimated by the inversion of PROSPECT with the contact probe correlated stronger with the laboratory measurements than the traditional inversion configuration. This study contributes to identifying the trade-offs between spectroscopy methods for estimating leaf traits and allowing for faster trait estimation on a larger scale compared to traditional trait measurement methods. It will likely support potential users in selecting the leaf traits measurement methods to apply for tropical forest tree species.