Secondary Seed Dispersal and its Role in Landscape Organization

1
Secondary Seed Dispersal and its Role in
Landscape Organization Sally Thompson and Gabriel
Katul Duke University

• Secondary Dispersal Inhibits Band Migration
• An advective biomass flux can slow or reverse
  band migration directions. Anisotropic kernels
  constrain band evolution to the cross-slope
  direction.
• The ratio of the band velocity with secondary
  dispersal to that for solely primary dispersal
  for the advection-diffusion (A) and kernel-based
  (B) approaches.

Migration of Banded Vegetation a Long-standing
Debate Banded vegetation, or Tiger Bush,
consists of vegetation growing in regular bands
interspersed by bare ground. Vegetation
patterning has been linked with desertification.
Studying band dynamics might clarify this link.
Models of banded vegetation predict that bands
move upslope rapidly (10-100 m/year). Field
studies show much slower, or no movement (0 -1
m/year). Accounting for seed dispersal in models
may resolve this conflict.
Model Equations The starting point for the
research is the model of banded vegetation
developed by Rietkerk et al (2001).
To include secondary dispersal as
an advective flux, the biomass equation is
modified, introducing a biomass velocity Vp,
applying to the mobile proportion of the biomass
(?P), in addition to the rate of isotropic
biomass movement Dp Or an anisotropic
dispersal kernel can be used f represents the
dispersal kernel (Thompson and Katul 2008)
1.5 km
0.5 km
Sources Valentin et al 1999, Google Earth
Seed Dispersal in Banded Landscapes Primary seed
dispersal is isotropic. Secondary seed dispersal
in overland flow is oriented downslope, conveying
seeds to a vegetated band. The anisotropy is
reflected in the seed bank. Secondar
y seed dispersal can be incorporated into models
by an anisotropic dispersal kernel or as a new
advective process in a reaction-diffusion model.
Anisotropic Band Velocity Isotropic Band
Velocity
References Valentin, dHerbes and Poesen, 1999.
Soil and water components of banded vegetation
patterns. Catena 37 1-24 Rietkerk et al 2001.
Self-organization of vegetation in arid
ecosystems. American Naturalist 160
524-530. Thompson, Katul and MacMahon, 2008. The
role of biomass spread in vegetation pattern
formation within arid ecosystems. WRR, 44L
W10421. Contact us set8_at_duke.edu For related
manuscripts check http//www.nicholas.duke.edu/pe
ople/faculty/katul/pub2008.html
Acknowledgements S. Thompson thanks the General
Sir John Monash Foundation. G. Katul
acknowledges support from NSF-EAR-06-35787,
NSF-EAR-06-28432, NSF-ATM-0724088, BARD research
grant IS3861-06.
Downslope weighted dispersal kernel