Evidence for xylem adaptations to drought in ancient cordaites of the Carboniferous

Background/Question/Methods

Ancient land plants faced the same challenges to growth and survival as modern land plants, including the need to resist xylem embolisms imposed by drought and freezing in order to maintain water supply to leaves. Cordaites, considered to be ancestors of the conifers, were some of the first trees on Earth and are often described as the most drought-resistant plants in the N. American landscape from the Late Mississipian (~320 Mya) to the early Permian (~250 Mya). Cordaites were common in both mires and dry uplands, however, suggesting considerable variation in drought tolerance, but neither the extent of this variation nor the particular xylem features associated with dryland habitats have been previously examined. We measured xylem anatomical traits, including conduit diameter (D), conduit wall thickness (t) and implosion strength (t/b), of cordaite roots and stems from 5 North American sites. We estimated vulnerability to drought embolism (P₅₀) and freeze-thaw embolism rate based on comparisons with modern plants. In addition, the model of Wilson et al. (2008) was employed to calculate the specific conductivity (K_sp), a measure of xylem water transport capacity.

Results/Conclusions

D and K_sp of cordaite stems were higher, while t/b was lower than that typical of many modern conifers. Consequently, cordaites were predicted to have greater stem water transport capacity and greater vulnerability to drought and freezing embolism compared to the most resistant modern conifers. However, among the samples K_sp varied by a factor of two, P₅₀ ranged from -4 to -9 MPa and freeze-thaw embolism rates ranged from 5–40%, depending on the site. We also found differences between stems and roots of the most drought-tolerant cordaites. Compared to stems, roots of drought-tolerant trees had larger D and higher K_sp, but lower t and t/b, resulting in lower P₅₀ (-2 MPa). Our data suggest that cordaite trees inhabited more mesic habitats compared to modern conifers, but our observation that their drought tolerance varied supports the idea that they were an ecologically diverse plant group. Furthermore, among more drought-tolerant forms, root embolisms occurring early during drought could have hydraulically isolated the plant from drying soil, potentially facilitating survival in drier upland habitats. Thus, we provide evidence from fossilized plants that associations between xylem features and habitat, as well as some modern drought adaptations, may be nearly as old as trees themselves.