Effects of conspecifics and rocks on seedling recruitment of Brachystegia spiciformis Benth.in a dry miombo woodland in southern Africa

Abstract

Natural objects that buffer climate extremes and potentially function as regeneration niches are ubiquitous in dry miombo woodlands, yet few studies have explored their effects on early stages of plant recruitment. We used Brachystegia spiciformis to investigate the influence of conspecifics and rocks on early seedling recruitment in Cecil Kop Nature Reserve in Zimbabwe. B. spiciformis-dominated woodland was stratified into canopy and gap sites which were further divided into rock and rock-free microsites. In each microsite, we laid 3 blocks of 100 × 100 m and sow seeds of B. spiciformis in five 1 m2 plots close to rocks under canopies (CR), close to rocks in gaps (GR), rock-free in gaps (GF) and rock-free under canopies (CF), making a total of 60 plots. We laid additional 1 m2 plots to quantify the presence of naturally occurring seedlings at 1–12 m distances from boles of 15 trees in four cardinal directions. We found that photosynthetic photon flux density (PPFD), soil and air temperatures were significantly lower in CR, CF and GR than in GF while soil moisture displayed a reverse trend. Although seedling emergence was above 65% in all microsites, more than 60% of the seedlings died because of drought-related mortality which was increased in GF compared with other microsites. Survival probabilty of seedlings was highest in CF and CR, intermediate in GR and lowest in GF. Seedling biomass was increased in GR than under canopies despite higher concentrations of soil organic C, N and P in the latter. The density of naturally occurring seedlings was highest at the parent location than away from it, contradicting the Janzen-Connell recruitment pattern. We conclude that in dry miombo, conservation efforts may be well-served by focusing on the role of nurse objects in the plant recruitment process.