Studying succession

Graphics

Permanent plots

most direct method

plot markers should:

persist at least 50 yr

be readily relocatable

written directions easy to follow, but names of trails/roads, etc. may change

use great care in documenting locations

also leave initial data in several locations

ideally, permanent plots mark areas w/ different manipulations

analysis involves repeated-measures ANOVA, which can be complicated; ordination also can be used

Population structure (dendrochronological techniques) (see caveats by E.A. Johnson and colleagues)

provides direct evidence

limited to woody plants that can be accurately aged

assumes mortality is unimportant or is consistent across age-classes and treatments

if used for inferring succession from community structure, assumes understory trees will replace overstory trees

Photographs (ground-based or aerial) (see caveats in Bahre 1991 Legacy of Change or McPherson 1997)

ground-based photos available since mid- to late-1800s

aerial photos available since 1930s in North America

useful for large-scale physiognomic changes; spp.-specific differences can rarely be detected

(-) field of view is oblique and narrow

(-) most historic photos are of disturbed sites

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)

Managing succession

More graphics

We usu. do not manage for late-successional ("climax") vegetation for several reasons:

1. we usu. do not know what it is (they are?), for any particular site

   historical records are poor

   photos and vegetation sampling started after large- scale disturbances

2. other communities may be desirable

3. late-successional communities are not necessarily more stable than mid- or early-successional ones

4. rate and path of succession depend on soil properties

   therefore, mgmt. activities which influence soil properties --> altered successional pathways

   return to late-successional veg. may not occur over time frames relevant to mgmt.

5. the concept of single-equilibrium communities that progress steadily toward "climax" as a function of disturbance does not apply in many communities

   alternative steady states exist

   stochastic events influence rate and path of succession

   discontinuous and irreversible transitions exist

We can find many situations in which the successional stage desired by land managers is different than that currently found --> attempts to manipulate ("manage" succession)

Forests: any properly-used silvicultural system represents an attempt to hasten or retard succession

simplest system (clearcut): returns forest to pioneer stage; early-successional spp. typically grow faster than late- successional ones

partial cuts: retain seed source on site --> accelerated establishment of early- successional trees

selection systems (uneven-aged stand mgmt): retain seed source and site protection (from existing trees)

pioneer hardwoods: weeds?

Tropical dry forest (Guanacaste National Park, Costa Rica):

multi-storied forest has been converted to savanna or grassland via timber harvest and conversion to pastures for livestock grazing

fire frequency has increased --> positive feedback of grassland --> grassland

"natural" rate of succession to tropical dry forest would be very slow because of frequent fires and long distance to seed source of woody plants

Janzen et al. have been trying to "speed up" succession by:

1. reducing fire frequency (active suppression program and cattle grazing --> fuel decrease)

2. encouraging dispersal and establishment of woody plants

   cattle grazing --> grass inteference decrease

   cattle fed woody plant seeds

   active enforcement of anti-poaching laws --> sustained populations of native seed dispersers

   planting seedlings in pasture centers