Wood anatomy of some non-Cape Protea and Faurea species (Proteeae, Proteaceae): structural responses of secondary xylem to climate and post-fire regeneration strategies

This study investigates patterns of wood structural diversity within the tribe Proteeae (Proteaceae), which includes the genera Protea and Faurea. We examined the wood structure of four Protea species (P. afra, P. gaguedi, P. roupelliae, P. welwitschii) and two Faurea species (F. saligna and F. rochetiana) distributed outside the Cape Floristic Region. Our data were combined with results from Stepanova et al. (2021) on Faurea and Cape species of Protea, creating a unified dataset for analyzing correlations between wood traits, climatic factors, and post-fire regeneration strategies (seeders vs resprouters). We found a positive correlation between the size of pit borders on fiber walls and a negative correlation between vessel grouping and the expression of Mediterranean-type climate. These findings suggest a shift in adaptive strategies within Proteeae in response to seasonal water stress: Cape species, influenced by winter rainfall and summer drought, typically exhibit solitary vessels and water-conducting ground tissue, while non-Cape species from regions with moist summers and dry winters tend to have larger vessel groups and non-conducting ground tissue. Additionally, the reduction in ray size and/or frequency associated with a Mediterranean-type climate aligning with broader patterns observed across southern African biomes. Within Proteeae, seeders generally have higher vessel frequency, larger intervessel pits, and narrower rays compared to resprouters. The higher vessel frequency in seeders is consistent with their greater cavitation resistance, which is critical for coping with water stress due to their shallow roots. Larger intervessel pits may also reflect adaptation to water stress, though their precise hydraulic role remains unclear. The wider rays in resprouters likely play a role in starch storage for post-fire regeneration and may serve as sites for adventitious epicormic bud formation, which is vital for resprouting.