Managing succession

Lecture 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