Accuracy of Visible and Ultraviolet Light for Estimating Live Root Proportions with Minirhizotrons

The minirhizotron technique has been widely used to study roots in both natural and managed ecosystems (Box and Hammond 1990, Atkinson 1991, Murphy et al. 1994). Parameters estimated include root depth (Gregory 1979, Upchurch and Ritchie 1983, Parker et al. 1991, Hendrick and Pregitzer 1992a), root length density (Upchurch 1985, 1987, Bland and Dugas 1988, Box and Ramseur 1993, Volkmar 1993, Samson and Sinclair 1994), root morphology (Upchurch 1985, Hendrick and Pregitzer 1992b), root dynamics (Hansson and Andr6n 1987, Eissenstat and Caldwell 1988), and root turnover (Cheng et al. 1990, 1991, Hendrick and Pregitzer 1992b). For estimates of dynamics or turnover, the reliability of the minirhizotron method depends on the accuracy of assessing physiological status of roots, i.e., whether roots are live or dead. Root color is the most frequently used criterion for determining physiological status of roots in minirhizotrons (Cheng et al. 1990, Hendrick and Pregitzer 1992a, b). When roots become dark brown or black, they are counted as dead. However, root color may darken as a result of secondary growth or suberization, and this can complicate visible estimates of physiological status. Furthermore, under visible light, it is sometimes difficult to distinguish between roots and organic debris. To aid in separation of live and dead roots, some of the more recently manufactured minirhizotron systems