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Transcript
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.