This study investigates the temporal behavior of the gravimetric vegetation water content (M_g) derived from SMOS (L-band) optical depth (τ) values. The analysis is done for the year 2010 over a coniferous forest site in the U.S. Resulting values of M_g are compared to values of leaf water potential obtained with the Soil-Plant-Atmosphere model and in situ data. A significant nonlinear correlation is found between the two (R=0.72, p<;0.001). It is found that the temporal behavior of M_g, based on plant water status, is different from that of existing optical-based vegetation indices, which are mainly based on chlorophyll/biomass changes. Therefore, τ-derived M_g could provide complementary information for future global-scale dynamic monitoring of vegetation water status.