Studying succession (continued)

Stable-isotope analysis

chemical evidence of vegetation change; useful for assessing changes in photosynthetic pathway of dominant plants

e.g., in most grasslands of the southern US, grasses have C₄ Ps pathway, woody plants have C₃ pathway --> useful for studying woody plant abundance in former grasslands or savannas

basis: C₃ and C₄ plants take up carbon-13 and carbon-12 (stable isotopes) at different rates from the atmosphere

decomposition rate is same for both guilds

thus, organic carbon in the soil leaves a signature which indicates the relative proportion of C₃ and C₄ plants which contributed the carbon

age is usu. inferred from soil depth; direct measure of age requires carbon-dating ($$)

Opal phytoliths (plant microfossils)

microscopic silica particles formed within plant cells and cell walls that can persist in soils for many millennia after carbon-based biomass has decomposed

different taxa leave characteristic microfossils --> technique is more specific to taxa than stable-isotope analysis

production and persistence of silica particles varies between plants

very expensive --> ltd. sample size

as w/ stable-isotope analysis, direct measure of age requires carbon-dating--in this case, C from within microfossils is used, and dating is done w/ accelerator mass spectrometry ($$)

Pollen-grain analysis

pollen in soil used to indicate relative proportions of plants

production and persistence of pollen grains varies between plants

Case-study: Woodland/grassland boundary change in southeastern Arizona (oak woodland, semi-desert grassland)

Hastings and Turner (1965, The Changing Mile)

Bahre (1991, A Legacy of Change)

McPherson et al. (1993 Oecologia 93:95-101)