Failure of adult recruitment in Quercus buckleyi populations on the Eastern Edwards Plateau, Texas

Abstract

In many semiaridr egions woodlands, savannas and grasslands form an unstable landscape mosaic; the physiognomy of a particular patch in the mosaic changes over time. To explain such temporal and spatial variation in tree abundance in semiarid landscapes, greater understanding of woodland tree population dynamics and factors that affect population dynamics of trees in woodlands is required. We examined the apparent failure of adult recruitment in Quercus buckleyi (Spanish oak) populations in woodlands on the Edwards Plateau, Texas. To verify adult recruitment failure, we quantified age structures of adult stems (stems taller than 150 cm) in five Q. buckleyi stands. We found that adult recruitment has been low to absent for 35 to 60 y in four of our live sites. At the fifth site, some stems recruited above 150 cm (the browseline) in the past 30 y. Most recruitment of adult stems occurred between 1900 and 1935, a period of low deer abundance. At the one site with recent adult stem recruitment, relatively low deer densities have been maintained since 1970 by hunting and by a perimeter fence that prevents immigration. Based on coincidence of adult stem recruitment with low deer populations, we hypothesize that intense browsing pressure or the interaction between fire suppression and intense browsing pressure is limiting adult recruitment of Q. bucklwyi. If the documented patterns persist, many Q. buckleyi stands on the eastern Edwards Plateau are unlikely to replace themselves.