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)

• 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 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)

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 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 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

• Consequently may rejuvenate ecosystems

• Has been used by man as a management tool for millennia for soil enrichment

• Frequent fires destroy the landscape

• 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.

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

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.