Warming promotes growth of seedlings of a woody encroacher in grassland dominated by C4 species

Abstract

Little is known of warming effects on growth and defence of woody seedlings that potentially invade grasslands. We predicted that elevated temperatures would increase growth and spine length of Vachellia sieberiana seedlings growing (1) with or without grass in the dormant season and early growing season, but (2) only when grass cover is absent in the mid-late growing season. To test these predictions, 120 seedlings were grown in 20 field plots in the dormant season, early growing season, middle of the growing and late growing season. Ten plots were warmed with open-top chambers that raised air temperature by 1.0 to 2.5 °C depending on season. Warming generally increased growth metrics regardless of grass cover across seasons and generally increased spine length only when grass cover was absent. The results imply that under a warmer climate, V. sieberiana will thrive irrespective of grass cover.