From bush encroachment to biome shift: the ecology of thicket pioneers
Susi Vetter Botany Department Rhodes University Parr et al. (2012) distinguished between savanna thickening and thicket expansion Savanna Thicket encroachment Closed-canopy thicket
Which spp? What is their effect on the grass layer? What are their traits?
e.g. Parr et al. (2012) in Hluhluwe: “new thickets” characterized by Diospyros simii, Berchemia zeyheri Effect of woody composition and structure on herbaceous cover and composition • 3 Sites • Fine scale data (5m diameter circular plots) • Path analysis (SEM)
Species associated with tall / dense woody cover Woody Woody structure composition (canopy area, (no. stems) height)
Effect of particular species Effect of tall and dense on grass canopy on grasses
Herbaceous basal cover and composition Data: Daisy Chiloane, Susi Vetter Eastern Cape Bhisho Thornveld; commercial cattle + game; MAP ~ 650 mm
Brachylaena Canopy elliptica Vachellia area Grewia karroo occidentalis Canopy height Scutia Woody myrtina Woody 0.66 structure Maytenus composition R2 = 0.44 heterophylla
-0.31 N = 235 -0.38
Karochloa Basal cover curva Herbaceous R2 = 0.40 Grass height Panicum maximum Digitaria eriantha Forb Sporobolus Themeda fimbriatus triandra Thicket pioneer spp. – what are their functional traits?
Ability to colonize Ability to transform • Dispersal • Accumulation of deep canopies • Bud protection, bark thickness, growth rate • High LAI • Resprouting strategies • Adaptations to escape browse trap
Bark thickness:
32 Savanna Canopy: Pioneer Recurved branches 10 Closed canopy Hedge-forming with browsing 3 Leaves: 1 Evergreen
Total bark thickness (mm)thicknessbark Total 0.3 Simple Data from Eastern Cape and Buffelskloof* Horizontal angle
1 10 100 (cf. Nondlazi and Archibald, unpubl.) Stem diameter (mm)
* Data: Julieta Rosell, Susi Vetter, Mark Olson, Michelle Greve Welverdiend: Lowveld granite catenas, communal livestock, MAP ~ 500 mm
Dichrostachys Canopy Vachellia cinerea area exuvialis Canopy Terminalia height sericea Woody Woody 0.70 structure composition R2 = 0.48
-0.15 N = 102 -0.54
Tricholaena Herbaceous monachne R2 = 0.44 Basal cover
Sporobolus nitens Aristida Grass height adscensionis Manyeleti: Lowveld granite catenas, game reserve, MAP ~ 500 mm
Combretum Senegalia Canopy apiculatum nigrescens area
Terminalia Canopy height sericea Woody Woody 0.81 structure Flueggea R2 = 0.65 virosa composition
-0.09 N = 99 -0.83
Herbaceous R2 = 0.58
Panicum Basal cover maximum Urochloa Forb mossambicensis Impact on grazing Trees Grass e.g. our data from the Eastern Cape: 0.61 R2 = 0.37 Trees reduce grazing via their effect on grass (not directly e.g. landscape of fear) -0.03 N = 235 0.71
% Bitten R2 = 0.53 Impact on fire spread Eastern Cape – 5-year drought – no fire = no data yet!
What can we generalize about the thicket encroachment process? From our data, not much yet! The species and their effects differ between vegetation types and land use.
More work on where savanna thickening vs thicket expansion occurs, and why (MAP, soils?)
vs.