disturbance and succession

disturbances

succession

the change in species composition and associated substrate over time

primary or secondary determines trajectory

measured by progression to an intact, undisturbed, reference or climax community but can often end up at different outcomes

long time interval, focus on natural processes, embedded in science

primary succession

disturbance is so severe that little or no biological legacy remains (plants, microbes, SOM)

ex post glacial, volcanoes (natural) [or post-mining, dam removal (human)]

secondary succession

substantial biological legacy remains post disturbance

ex low severity fire, insect caused tree mortality (natural) [or ploughing, invasives (human)]

succession vs restoration

disturbances are generally natural vs human caused, but both concerned with response to disturbance

both predict sequences of events, based on ecological function and community structure

no values vs well integrated with values, socioeconomic and political dimensions

major differences is who caused the disturbance and values

restoration

assisting in the recovery of damaged, degraded, or destroyed ecosystem

has a desired end state, integrated with values

phases of succession

from bare site → amelioration, establishment (colonization, biotic interactions), species assembly, ecosystem development to recovered ecosystem

amelioration influencing restoration

degraded substances may be infertile, dry, lack OM

restoration might add soil amendments, CWD, or increase heterogeneity

dispersal and colonization influencing restoration

most species are dispersal limited

restoration will add seeding or planting to eliminate dispersal limitation

establishment influencing restoration

safe sites are critical, facilitation can improve establishment → nurse plants, N fixers to facilitiate N, etc

and herbivory can reduce establishment → fences or enclosures

assembly or ecosystem development influencing restoration

species and community composition accumulates and adjusts over time

dense vegetation and non-target species may need to be removed

restoration may replant, or remove weeds, disturbances may need to be added as part of management

types of disturbances

biological, physical or chemical

can also be frequency (number events per time period), extent (area disturbed per time, size), shape, magnitude (intensity or severity), and timing

biological disturbance type

consumes or kills biomass

ex grazing, insects, disease

physical disturbance type

rearranges biomass

ex flooding, wind

chemical disturbance type

reforms biomass

ex fire, grazing, etc

causes of fire exclusion

banning of traditional burning, land conversion, livestock grazing, active suppression of human and lightning caused fires

protecting our infrastructure, farmers, ranchers, etc

impacts of fire exclusion

significantly less area is being burned compared to historical

fire severity is increasing due to fuel loads, weather and climate

how can fire stabilize forests

low severity fires

when there is an open stand structure, different aged trees, little ladder fuel

little overstory mortality and reduced understory competition

removes fuel loads so they dont build up and promote higher severity fire

allowed room for more plants and animals

negative feed back

how can fire destabilize forests

high severity fires, fire replacing stands

high fuel loads and connectivity

leads to over story mortality, recruitment failure and can lead to conversions of land to non woody vegetation, etc *******

native species and values

some scientists argue to exclude normative values from science, but restoration has values - how can this co-exist?

many people bring values into discussions on native species and invasive or introduced species - but science doesn’t always support the idea that native species are better for the ecosystem

transparency required, its okay to have values but be clear about it

traits of megafauna impacting plants

diet selectivity - bulk feeding increases native plant diversity

as muzzle size increases so does native plant diversity (muzzle width being a measure for selectivity)

global tree restoration potential methods

1) quantified tree cover with pictures

2) used known environmental variables to correlate with extent of tree cover

3) applied to the rest of the world

4) investigated current land uses which cannot be forested

5) assessed carbon storage potential of restored forests

6) future canopy cover with climate change predictions