A phytolith index as a proxy of tree cover density in tropical areas: calibration with Leaf Area Index along a forest?savanna transect in southeastern Cameroon

Methodological concerns for analysis of phytolith assemblages: Does count size matter?

“…To explore how the value of the "true" assemblage D/P and Iph affects confidence interval width, several different hypothetical phytolith assemblage compositions are examined. The results are discussed with reference to recent studies employing D/P and Iph and similar indices for reconstructing vegetation (Bremond et al, 2005a(Bremond et al, , 2005bStromberg, 2005). Specifically, I…”

“…For example, Bremond et al (2005aBremond et al ( , 2005b) combine all dumbbell-shaped grass silica short cells into one class, whereas other authors split this morphotype class into several subclasses (e.g., Fredlund and Tieszen, 1997;Pipemo and Pearsall, 1998;Stromberg, 2005). Different numbers of morphotype classes will undoubtedly result in different sampling curves and consequently arrive at different optimal count sizes, in particular because a lower number of (broader) morphological classes will also likely correspond to higher evenness.…”

“…Several indices have been devised to express both absolute and relative aspects of vegetation type. For example, the tree cover index, D/P, uses the ratio of certain forest indicator phytoliths to grass phytoliths to give a measure of vegetation structure in absolute terms (a D/P ratio >1.82=forest ;Bremond et al, 2005a) and in relative terms (a decrease in the D/P ratio signals an opening up of the landscape; e.g. Alexandre et al, 1997;Bremond et al, 2005a).…”

“…For example, the tree cover index, D/P, uses the ratio of certain forest indicator phytoliths to grass phytoliths to give a measure of vegetation structure in absolute terms (a D/P ratio >1.82=forest ;Bremond et al, 2005a) and in relative terms (a decrease in the D/P ratio signals an opening up of the landscape; e.g. Alexandre et al, 1997;Bremond et al, 2005a). Other commonly used indices include the aridity index (Iph=tall vs. short grass savannah), and the climatic index (Ic=C3 vs. C4 grasslands) (e.g., Alexandre et al, 1999;Bremond et al, 2005b;Diester-Haass et al, 1973;Twiss et al, 1987).…”

Decoupling the spread of grasslands from the evolution of grazer-type herbivores in South America

“…However, in general, soil phytolith studies show that the abundance of grass phytoliths faithfully reflects the relative abundance of grasses in many modern ecosystems 15,53 -although note that authors disagree on what phytolith morphotypes should indicate grass cover (compare, for example, refs 57 and 14). For example, applying the methods used herein to two large, modern phytolith data sets from West Africa 57,58 shows that grass-dominated vegetation (densely tree-covered savannah to open grasslands, including short grass steppe with shrubs) produces assemblages with, on average, 16.4% FI forms (of a sum of FI þ GSSC), with maximum values in assemblages from dense tree/shrub savannah (53-64%) and short grass steppe with shrubs (11-58%) (data not shown). This suggests that grass-dominated habitats will typically produce phytolith assemblages with at least B35-40% GSSC, with assemblages from more open savannahs usually 4450% GSSC.…”