Insular woody daisies (Argyranthemum , Asteraceae) are more resistant to drought-induced hydraulic failure than their herbaceous relatives

1. Insular woodiness refers to the evolutionary transition from herbaceousness towards derived woodiness on (sub)tropical islands, and leads to island floras that have a higher proportion of woody species compared to floras of nearby continents. 2. Several hypotheses have tried to explain insular woodiness since Darwin’s original observations, but experimental evidence why plants became woody on islands is scarce at best. 3. Here, we combine experimental measurements of hydraulic failure in stems (as a proxy for drought stress resistance) with stem anatomical observations in the daisy lineage (Asteraceae), including insular woody Argyranthemum species from the Canary Islands and their herbaceous continental relatives. 4. Our results show that stems of insular woody daisies are more resistant to drought-induced hydraulic failure than the stems of their herbaceous counterparts. The anatomical character that best predicts variation in embolism resistance is intervessel pit membrane thickne...