The taxa in this genus are sometimes recognized as varieties (e.g. var. distichum and var. imbricarium). Part of the taxonomic difficulty may be that the taxa often appear distinct, while sometimes they appear to intergrade (Tsumara et al. 1999; Lickey & Walker 2002; Lickey et al. 2002; Denny & Arnold 2007; Adams et al. 2012). Cypress knees are light and porous and have little bark near the apex. A study by Martin & Francke (2015) found that during inundation knees exposed to air provide significantly more oxygen to the roots below, but they note that the knees may have additional advantages such as increased tree stability and trapping of detritus and nutrients.