Evidence from Cape Proteaceae that high relative bark thickness is correlated with high bark thickness growth rates

Abstract

Plants with thick bark at the time of fire are able to maintain some above-ground structure after fire, rather than having to recruit from seed. Despite this advantage, thick-barked plants are rare globally including in fire-prone Cape fynbos shrublands and they are globally restricted to only some fire habitats. Plants can attain thick bark by either having high relative bark thickness or by having high bark thickness growth rates. There is considerable information on relative bark thickness but almost none on bark thickness growth rates. Cape fynbos Proteaceae produce annual growth scars that allow the age of stems to be determined. Using data for three thick-barked and four co-occurring thin-barked species, I report the first information that indicates that relative bark thickness is positively correlated with bark thickness growth rates. Maximum bark thickness growth rates are close to 1 mm yr−1 which is almost double that of savanna plants. However, this growth rate appears to be insufficient to protect stems in fires because the few thick-barked species all grow in rocky relatively low-fire intensity environments. Lack of thick-barked species in the Cape is therefore due to bark thickness growth rates being insufficient to protect aerial stems in local fires.