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BCB 322:
Landscape Ecology
Lecture 4:
Emerging processes I
Disturbance and soil & nutrient
dynamics
Process & pattern
• Processes operating on the landscape level
affect the pattern of distributions
• Some are nearly indistinguishable (rivers act
as corridors: both process & pattern).
• Others are clearly disparate – fire as a
process can clearly drive distribution in a
savannah landscape
• Many processes are related to each other,
such as disturbance & fragmentation
• In the next two lectures we’ll look at some of
these processes
Disturbance
• A process which changes landscape structure,
population distributions & ecosystem function
over time
• Affects resource availability and physical
environment (White & Pickett, 1985)
• Often a driving factor in many other processes,
including organism migration, extinctions,
fragmentation, even soil nutrient potential
• All landscapes are affected by disturbance, and
are often even shaped by disturbance
processes (eg: latter-day sandplain fynbos
fragmentation is driven entirely by human
development)
Disturbance attributes
• Works at all scales, both
spatial & temporal
• Consequently, regimes
vary in size, duration &
intensity
• In order to understand
how a disturbance will
affect communities & a
landscape, one needs to
understand the spatial &
temporal structure of the
Fire disturbance (Baker, 1992)
disturbance
• Severe disturbance generally decreases biodiversity &
complexity, whilst milder disturbance may actually
enhance them
• (eg): burns at 13yr intervals in fynbos prevents
competitive exclusion of smaller, short lifecycle plants.
Disturbance types
• Two landscapes with similar abiotic components and
organismal populations may be significantly different
depending on disturbance regime.
• Eg: wildfire-disturbed forest in northern Ontario tends to
have smaller fragments and more regular edges than
clearcut forest (Gluck & Rempel, 1996)
• Abiotic disturbance drivers:
–
–
–
–
water
wind
landslides
solar input
• Biotic disturbance drivers:
– disease
– competition
– predation
Human disturbance
• Similar to nonanthropogenic sources in
many ways, but can differ
in intensity, frequency, and
duration
• Agriculture, forestry &
urban development tend to
have long term, high
intensity effects
• Can also differ in frequency (anthropogenic fires tend to
have similar effects to other fires, but near urban or
agricultural areas, can be as often as every season
• Area affected can be extremely large, and can cover
significantly different
• Generally, human disturbance overwhelms the
landscape’s ability to absorb it, and ecosystems are
reduced in complexity
Forest gaps
• These are small openings in the canopy
of a forest’s overstory, usually caused by
tree fall
• Inhabited by different species from the
forest understorey (maintains species
diversity)
• Plays a fundamental role in forest
ecosystems – allows growth to maturity
of surrounding trees
• Hurricanes and large disturbances in
forests can lead to a reduced diversity
between gap species & disturbed
understorey
• Hurricanes and large disturbances in forests can lead to a reduced
diversity between gap species & disturbed understorey
• Generally found throughout landscape (in mature forests, as much
as 50% of forest area contains gaps from deaths of several trees)
(Lertzman et al.,1996)
• Low-intensity, small scale gap disturbance allows turnover of tree
population in ~350 - 950 years in British Columbia (Canada
Savannah gaps
• Strictly speaking, savannah does not
have gaps, since it consists of a matrix of
grassland with patches of trees.
• However, in some respects trees
represent a disturbance in the grassland
matrix, since they are a departure from
the norm.
• Trees give the savannah different local
conditions:
– wet season: soil is dryer due to
shelter
– dry season: soil is damper due to
cooling & reduced evapotranspiration
– soil nutrient levels are boosted by tree roots
• Bush encroachment can be damaging to the life cycle of grazers if it
spreads too far
• Otherwise, the tree “gaps” boost biodiversity.
Fire disturbance
• Essential shaping force in dry landscapes
• Removes undecomposed leaf litter & biomass, and
enriches soils through ash nutrient deposition
• Assists certain species in germination & dispersion (eg:
fynbos proteas)
• Charcoal may reduce allelopathy by phenol-secreting
species by fixing phenols in the soil
• May also retain more water, enhancing soil moisture.
• Consequently may
rejuvenate ecosystems
• Has been used by man as
a management tool for
millennia for soil
enrichment
• Frequent fires destroy the
landscape
Animal disturbance
• Grazing is the primary form of
disturbance, although the actions of
burrowing animals (moles, worms,
rabbits) can aerate soil
• May enhance the growth of some
grass species through stress
hormones
• Urine & faeces can enhance the soil
• Trampling can destroy plants, and reduces seedling
growth rates
• Grazers tend to move from location to location, so highly
trampled areas may not correspond with excretaenhanced spots
• Produces highly varied local structure, and consequently
increases diversity
Physical landscape
• Dispersion in the landscape is not altogether random,
or related to distance
• Roughness (topography & vegetation cover) of
landscape shows patterns that can be modelled.
• Eg: propagule dispersion can be modelled given
sufficiently accurate data regarding prevailing wind &
topography. This affects distribution of fungi, grasses,
& insect species to name a few.
• Hence, physical character of a site affects patch
pattern & species selection pressure
• Soil formation is a complex process, involving
weathering, plant decomposition & movement through
the landscape
• Vital in determining plant growth, and consequently all
secondary species distributions, as well as surface
temperature & precipitation
Soil landscape
• Many descriptors of topography:
–
–
–
–
–
elevation
gradient
slope direction
catchment area
slope curvature
• Precipitation, runoff, evaporation &
seepage depend not only on slope & soil
depth, but soil character & use (meadows
have minimal runoff & high seepage
compared with woods) (Ripl, 1995)
• Furthermore, change in topography can
cause changes in soil character
• (eg) correlation between slope character
with (2)P, (3)pH, (4)organic C, (5)A
horizon thickness: 50% of variation
explained (Moore et al, 1993
Nutrient dynamics
• Nutrients such as C, N & P vary according to edaphic
conditions & topographical position.
• Both C & N can be carried in solution, and hence move
by leaching & in rivers.
• P is carried as particles, so soil accumulation processes
correspond to P-fixing locations (river bends, dunes, etc)
• Soil quality is fundamental in nutrient retention: clay
soils fix nutrients, sandy soils are leached
• Land use is also highly relevant – the discharge rates of
nutrients from 3 different watersheds are significantly
different Parameter Cropland Pasture Forest
Total N
13.80
5.95
2.74
NH4+
0.45
0.51
0.15
NO3-
6.35
3.20
0.36
Total P
4.16
0.68
0.63
All units in
kg/ha
Correll et al,
1992
Nutrients & rivers
• Rivers act as transport mediums for nutrients, and
overland flooding can lead to movement of nutrients far
downstream
• Riverine vegetation, however, often filters this nutrient
load.
• During floods, riverine forest can remove over 80% of N
& P from washout from bordering fields
• It also removes up to 85% of all nitrates from
groundwater runoff
• Similar riparian growth on differing soil types can have
different nutrient retention capacities
• On sandy soils, riparian vegetation acts as a source of
nutrients, whilst on clay soils, it acts as a sink
• In terms of landscapes, this means that riparian growth
can drive patch selection in neighbouring areas.
Summary
• Disturbance acts on all scales, both temporal & spatial
• Low intensity/frequency disturbance often enhances
biodiversity, whilst over a certain threshold, biodiversity
suffers.
• In savannah, woodland “gaps” play an important role in
maintaining diversity & soil character, as does fire
• Grazing can add to local patchiness and overall
landscape variation
• Soil character (and vegetation) are strongly affected by
minor topographical variation
• Soil nutrients differ in behaviour depending on land
cover & position
• Riparian vegetation plays an important role in filtering C,
N & P.