February 4, 2004
Revegetation
1. Land degradation – overview
· Land degradation has many causes: deforestation by fire or timber harvest, mining, grazing, climate change or a combination of these.
· The result is often and increase in soil erosion, compaction and crusting, altered nutrient cycling, soil pollution, including salinization, loss of biodiversity and trophic complexity.
2. Revegetation as part of a restoration program
a. Some seed and seedling biology
· For revegetation to be successful, some seeds or transplants have to eventually grow into mature, fertile plants.
· There are many species differences in the “environmental cues” that break seed dormancy, in the water requirements for successful germination and establishment, and in the amount and cause of mortality between establishment and maturation.
· An important question for revegetation is: how does the degraded environment affect the species chances to get from seed to reproductive adult?
b. Working models for revegetation
· There are two competing models that affect restoration planning: the “Succession Model” and the “Alternative Stable State Model”.
· The Succession Model assumes that for every level of disturbance, there is a matching system state. – Remove the disturbance, and the ecosystem returns to its pre-disturbance state. If local seed sources are intact, revegetation may not be necessary.
· The Alternative Stable State Model assumes that degraded systems develop resilience to change through the establishment of new positive feedback cycles. – Remove the disturbance, and the ecosystem does not change or changes into another state different from the pre-disturbance state. Revegetation alone is unlikely to be successful.
· Sometimes the succession model works (prairie restoration by introducing historic fire regime, riparian restoration by introducing historic flooding regime).
· Often, the degraded ecosystem behaves more like it is in an Alternative Stable State. To restore this ecosystem, the stabilizing positive feedbacks need to be identified and broken (e.g. removal or addition of topsoil, removal of dominant vegetation, manipulation of soil nutrient cycles). Only then can revegetation efforts take hold.
c. Genetic considerations
· Local species extinctions, shifts in species dominance, and changes in ecosystem structure and function can lead to changes in the genetic makeup of populations.
· Revegetation can have negative effects on the “genetic health” of populations through “founder effects” (new plants are genetically too similar), “genetic swamping” (remnants of the local gene pool are over-run by a different ecotype), and “outbreeding depression” (hybrids of the remnant and introduced ecotype may produce hybrids that are inferior to both parental lines).
· Restoration has to be mindful of possible unintended genetic consequences of revegetation (e.g. grow hybrids in common garden, select seed sources carefully).
3. Case studies
a. Reintroduction of silverswords to Mauna Kea, Hawaii
· A revegetation attempt in the 1970’s unwittingly failed to introduce genetically diverse new population members. This is a problem, because the species does not self.
· In the late 1990’s, flowers were artificially pollinated to ensure high genetic diversity in the seeds produced by the remnant population, as well as in new outplants.
b. Mine restoration in South Africa
· An experiment demonstrated how land stripped of vegetation and topsoil can be restored to a diverse shrubland, through simultaneous application of topsoil and seed mix.
· However, long-term stability of the restored shrubland can only be ensured through removal of alien seedlings and re-establishment of the original fire regime.