Table 1.

Generalized additive mixed model results for testing the effect of shrub density and the dingo fence on various geomorphological metrics of dunes. Shrub density is a smooth term (penalized thin plate regression spline) and dingo fence is a standard categorical linear term with outside set to the contrast level. Random effects were fitted for individual dunes, and a Gaussian spatial correlation structure was imposed on model residuals. Figure 4 shows the effect of these terms on the metric predictions (easiest way to interpret smooth terms).

geomorphology metriceffect of dingo fenceeffect of shrub density
outside (treatment contrast)inside dingo fence
maximum height (m)lower outside
βoutside = −0.659
(F = 39.45, p < 0.001)
not significant
(F = 0.31, p = 0.58)
height deviation (m)less deviation outside
βoutside = −0.21
(F = 37.65, p < 0.001)
less deviation at higher densities
(F = 18.35, p ≤ 0.001)
slope root mean square (°)not significant
(F = 0.31, p = 0.581)
marginal, possibly range increases with density
(F = 3.92, p = 0.063)
slope coefficient of variation (%)not significant
(F = 0.08, p = 0.772)
decreases with shrub density
(F = 8.85, p = 0.003)
flatness (log ratio)not significant
(F = 0.12, p = 0.735)
marginal, possibly flatter at higher densities
(F = 2.46, p = 0.078)
rectangularity (ratio)more rectangular outside
βoutside = 0.025 (F = 18.16, p < 0.001)
more rectangular at higher densities
(F = 4.82, p = 0.001