Vessel wall reinforcement metrics as drought resistance indicators in angiosperm fossil wood assemblages

Background Plant ecologists have developed methods to measure xylem drought resistance but these cannot be used in fossil woods. There is, however, one anatomical trait highly correlated with cavitation resistance: the squared vessel-wall thickness-to-span ratio ((t/b)2h). This metric though, could be in many cases impractical to measure in fossil samples because they often are small and sample sizes are seldom reached. Questions are there alternative anatomical metrics that could be used instead of (t/b)2h to infer drought resistance of fossil wood assemblages? Study site and dates 279 species belonging to 14 extant communities from North and South America. Three fossil wood floras from the Oligocene and Miocene of Mexico. Methods We calculated three alternative wall reinforcement metrics to determine their relationship with (t/b)2h and drought resistance. These are based on vessel diameter and vessel wall thickness. Results We found that one of the alternative metrics ((t/b)2hydraulic mean) could potentially be used instead of (t/b)2h. The widely measured vessel wall-to-lumen ratio (VWLR), was the closest related to climate, and thus helpful in identifying broad precipitation differences among floras. VWLR and (t/b)2h might be describing slightly different ecological axes of ecological variation, with the latter associated with investment in support tissue, in addition to water availability alone. Conclusions Some of the alternative metrics we explored can be used, in combination with other functional traits, to better describe fossil forest functional strategies. ### Competing Interest Statement The authors have declared no competing interest.