BCB 322: Landscape Ecology

1
BCB 322Landscape Ecology

• Lecture 4
• Emerging processes I
• Disturbance and soil nutrient dynamics

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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 well look at some of
  these processes

3
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)

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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 disturbance

Fire disturbance (Baker, 1992)

• 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.

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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

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Human disturbance

• Similar to non-anthropogenic 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
  landscapes ability to absorb it, and ecosystems
  are reduced in complexity

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Forest gaps

• These are small openings in the canopy of a
  forests 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

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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.

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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

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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
  excreta-enhanced spots
• Produces highly varied local structure, and
  consequently increases diversity

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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

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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

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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
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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.

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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.