Fire and the persistence, decline and extirpation ofProtea roupelliaesubsp.roupelliaetrees in a montane grassland

Abstract

We investigated long-term changes, from 1979 to 2015, in the abundance of Protea roupelliae subsp. roupelliae trees growing in a montane grassland of the uKhahlamba-Drakensberg mountains. The study involved counting the numbers of adults and juveniles (saplings and seedlings) in six groves on three occasions over 36 years, documenting the fire-return intervals and the seasons when fires occurred in each grove, and measuring the fuel loads of the grass swards in five of the groves. Only in one grove did adults persist in their original numbers, but not the juveniles. Adults and juveniles declined in three groves, and in two groves both adults and juveniles died. Neither mean fire-return interval nor its variation, or seasonal distributions of fire could account for the transition from tree persistence to extirpation among the six groves after 36 years. Instead, the only plausible explanation for the persistence of Protea trees in one grove was that its grass sward produced a significantly lower mean, but higher variation in fuel load (biomass), compared with groves where Protea tree abundances either declined or were extirpated. As such, the recruitment bottleneck imposed on seedling establishment by the grass sward is an obvious avenue for future research.