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Soil erosion, as a landscape ecology phenomenon
Soil Erosion, as a Landscape Ecology Phenomenon Mark Westoby Ecological experiments give results that apply to experimental plots. For practical reasons these plots are usually small - often a square metre, rarely as much as a hectare. It is now becoming apparent that there may be problems in extrapolating experimental results to the wider spatial scales that managers have to deal with, from 1 up to 10 000 km*. A recent increase in activity in ‘landscape eco!ogy’1-4 reflects worry about the problem of scale. Judging from people I have talked to, however, most scientists in the population ecology tradition remain cynical about landscape ecology. It has introduced new ways of coding the heterogeneity of landscapes, but does not seem to have a clear program for going beyond description to prediction. The general problem is: when is ecoiogical functioning at one place affected by what is happening in neighbouring places? When effects across space are not important, landscape-level phenomena should be a simple sum of what is happening at each place. To scale up we need only to sample in a properly stratified way and compute a weighted average. This may be technically exacting, but no new concepts are needed. However, when an ecosystem is strongly affected by one or more of its neighbours, then the exact disposition in space of different landscape components matters. If neighbours matter, it must be because something of biological significance is moving sideways. Recent work by Pickup and otherssg at the Central Australian Laboratory of CSlRO (Commonweaith Scientific and Industrial Research Organization) illustrates how predictive power can flow from focusing on a real transport process.
Mark Westoby is at the School of Biological Sciences, Macquarie University, NSW 2109, Australia.
A prime management issue on rangelands is soil erosion. Erosion is viewed as one aspect of a general degradation of rangeland - the least reversible aspect, and therefore the most undesirable. Applied research organizations such as CSIRO are continually being urged to find out how much grazing causes soil erosion, and how much loss of longterm production results. But erosion is a transport process; erosion in one place means deposition somewhere else. Around the footslopes of the Central Ranges of Australia, the important transport process is the flow of water over land. The first step taken by Pickup and his group5s6 was to identify the components of landscape heterogeneity that were specifically related to this key transport process. They distinguished a ‘production’ zone of runoff and erosion, and a ‘sink’zone of runon and deposition. In between there is a ‘transfer’ zone where sediment may be deposited temporarily, depending on the size of individual runoff events. Because erosion is due to sheet flow of water, source zones have a relatively high percentage of bare ground and high reflectance. Sink zones receive water and fine soil in runon, which encourages growth of woody plants. (This being cattle country, they are known as ‘woody weeds’.) Because of this, production and sink zones can be
Images (128 x 128 Landsat pixels) of initial, observed and predicted landscapes: from Ref. 10 with permission, detailed explanation ,n Ref. 9. Top image shows conditions in ?972. bottom shows the same site in 1984, while mlddle shows forecast for 1984 derived from the 1972 image. The colours from blue through green and yellow to red indicate states ranging from eroded bare ground through fairly stable conditions with moderate vegetation cover to densely vegetated deposition areas. The site underwent good rainfall years during the period, producing a general increase in vegetation cover but also considerable eroston and deposition
TREE vol. 2, no. 1 I, November 1987 CROSS-SECTION
0 Productfon Zone a Transfer Zone m Sink Zone
PLAN y-1
Fis. 1.
Production, transfer and sink zones linked in erosion cells (outlined by dashed lines). After Ref. 7, with
p&mission.
mapped not only from air photograph interpretation but also from Landsat imagery5s6. Production zones, transfer zones and sink zones are linked in ‘erosion cells’7 (Fig. 1). These linkages express the scale of runoff events. Thus small erosion cells, associated with small rainfall events, can be nested inside larger cells. A landscape is a mosaic of interlocking and overlapping erosion cells. The effect of grazing is to reduce ground cover, especially in production zones. This increases the percentage of rain that runs off; runoff events tend to become larger in volume and so in spatial extent. At the landscape scale, grazing tends to
increase the size of erosion cells. The effect is not a simple landscape-wide increase in soil loss. Pickup and Chewings8,g have developed a method to predict the pattern of change in erosion and deposition, and in the vegetation. They reasoned that the frequency distribution of erosion cell sizes can be characterized by the degree of correlation among nearby Landsat pixels in whether they belong to production, sink or transfer zones. The increase in runoff associated with grazing shifts the frequency distribution in the direction of larger erosion cell sizes, within the framework of the basic pattern established by the topography. The increased cell size
could be simulated by increasing the degree of correlation among neighbouring pixels. They used a simultaneous autoregressive process to generate predicted landscape patterns given increased spatial autocorrelation. The amount of increase in spatial autocorrelation was estimated empirically, by examining what had happened in similar landscapes with known histories. In landscapes favourable to the model, predictions derived from Landsat scenes of the early 1970s are strikingly similar to the current situation (see Figure on previous page). How widely applicable might this approach be? The model applies for sheet erosion by water movement. It is worth noting that although the work is oriented to effects of grazing, the actual mechanism is that higher runoff from ground with less grass cover generates larger erosion cells. The approach is therefore also suited to predicting effects of climate changes due to greenhouse warming, which in this area will probably take the form of larger rainfall events. The model cannot be applied directly to other erosive processes such as wind erosion, or gully erosion. However, the underlying principle applies not only to other erosive processes but to landscape ecology in general. The principle is that the researchers began by focusing on the sideways movement of a real commodity - water, plus the soil, nutrients and seed carried in it that is critical to ecosystem behaviour in their region. The message for landscape ecology is that progress will come not by considering heterogeneity in the abstract, but by focusing on concrete lateral transport processes of important biological commodities.
References 1 Forman, R.T.T. and Godron, M. (1981) Bioscience 31 I 733-740 2 Naveh, Z. and Lieberman, AS. (1984) Landscape Ecology: Theory and Application, Springer-Verlag 3 Forman, R.T.T. and Godron, M. (1986) Landscape Ecology, John Wiley 4 Urban, D.L., O’Neil, R.V. and Shugart, H.H., Jr(1987) Bioscience37,119-127 5 Pickup, G. and Nelson, D.J. (1984) Remote Sensing Environ. 16,195-209 6 Pickup, G. and Chewings, V.H. (19861 Aust. Rangel. J. 8,57-62 7 Pickup, G. (1985) Aust. Range/. J. 7, 114-121 8 Pickup, G. and Chewings,
V.H. (1986)
Ecol. Model. 33,269-296 9 Pickup, G. and Chewings, V.H. ht. J. Remote Sensing (in press) 10 Bell, A. (1987) Ecos 51,14-17
322
Mark Westoby is at the School of Biological Sciences, Macquarie University, NSW 2109, Australia.
A prime management issue on rangelands is soil erosion. Erosion is viewed as one aspect of a general degradation of rangeland - the least reversible aspect, and therefore the most undesirable. Applied research organizations such as CSIRO are continually being urged to find out how much grazing causes soil erosion, and how much loss of longterm production results. But erosion is a transport process; erosion in one place means deposition somewhere else. Around the footslopes of the Central Ranges of Australia, the important transport process is the flow of water over land. The first step taken by Pickup and his group5s6 was to identify the components of landscape heterogeneity that were specifically related to this key transport process. They distinguished a ‘production’ zone of runoff and erosion, and a ‘sink’zone of runon and deposition. In between there is a ‘transfer’ zone where sediment may be deposited temporarily, depending on the size of individual runoff events. Because erosion is due to sheet flow of water, source zones have a relatively high percentage of bare ground and high reflectance. Sink zones receive water and fine soil in runon, which encourages growth of woody plants. (This being cattle country, they are known as ‘woody weeds’.) Because of this, production and sink zones can be
Images (128 x 128 Landsat pixels) of initial, observed and predicted landscapes: from Ref. 10 with permission, detailed explanation ,n Ref. 9. Top image shows conditions in ?972. bottom shows the same site in 1984, while mlddle shows forecast for 1984 derived from the 1972 image. The colours from blue through green and yellow to red indicate states ranging from eroded bare ground through fairly stable conditions with moderate vegetation cover to densely vegetated deposition areas. The site underwent good rainfall years during the period, producing a general increase in vegetation cover but also considerable eroston and deposition
TREE vol. 2, no. 1 I, November 1987 CROSS-SECTION
0 Productfon Zone a Transfer Zone m Sink Zone
PLAN y-1
Fis. 1.
Production, transfer and sink zones linked in erosion cells (outlined by dashed lines). After Ref. 7, with
p&mission.
mapped not only from air photograph interpretation but also from Landsat imagery5s6. Production zones, transfer zones and sink zones are linked in ‘erosion cells’7 (Fig. 1). These linkages express the scale of runoff events. Thus small erosion cells, associated with small rainfall events, can be nested inside larger cells. A landscape is a mosaic of interlocking and overlapping erosion cells. The effect of grazing is to reduce ground cover, especially in production zones. This increases the percentage of rain that runs off; runoff events tend to become larger in volume and so in spatial extent. At the landscape scale, grazing tends to
increase the size of erosion cells. The effect is not a simple landscape-wide increase in soil loss. Pickup and Chewings8,g have developed a method to predict the pattern of change in erosion and deposition, and in the vegetation. They reasoned that the frequency distribution of erosion cell sizes can be characterized by the degree of correlation among nearby Landsat pixels in whether they belong to production, sink or transfer zones. The increase in runoff associated with grazing shifts the frequency distribution in the direction of larger erosion cell sizes, within the framework of the basic pattern established by the topography. The increased cell size
could be simulated by increasing the degree of correlation among neighbouring pixels. They used a simultaneous autoregressive process to generate predicted landscape patterns given increased spatial autocorrelation. The amount of increase in spatial autocorrelation was estimated empirically, by examining what had happened in similar landscapes with known histories. In landscapes favourable to the model, predictions derived from Landsat scenes of the early 1970s are strikingly similar to the current situation (see Figure on previous page). How widely applicable might this approach be? The model applies for sheet erosion by water movement. It is worth noting that although the work is oriented to effects of grazing, the actual mechanism is that higher runoff from ground with less grass cover generates larger erosion cells. The approach is therefore also suited to predicting effects of climate changes due to greenhouse warming, which in this area will probably take the form of larger rainfall events. The model cannot be applied directly to other erosive processes such as wind erosion, or gully erosion. However, the underlying principle applies not only to other erosive processes but to landscape ecology in general. The principle is that the researchers began by focusing on the sideways movement of a real commodity - water, plus the soil, nutrients and seed carried in it that is critical to ecosystem behaviour in their region. The message for landscape ecology is that progress will come not by considering heterogeneity in the abstract, but by focusing on concrete lateral transport processes of important biological commodities.
References 1 Forman, R.T.T. and Godron, M. (1981) Bioscience 31 I 733-740 2 Naveh, Z. and Lieberman, AS. (1984) Landscape Ecology: Theory and Application, Springer-Verlag 3 Forman, R.T.T. and Godron, M. (1986) Landscape Ecology, John Wiley 4 Urban, D.L., O’Neil, R.V. and Shugart, H.H., Jr(1987) Bioscience37,119-127 5 Pickup, G. and Nelson, D.J. (1984) Remote Sensing Environ. 16,195-209 6 Pickup, G. and Chewings, V.H. (19861 Aust. Rangel. J. 8,57-62 7 Pickup, G. (1985) Aust. Range/. J. 7, 114-121 8 Pickup, G. and Chewings,
V.H. (1986)
Ecol. Model. 33,269-296 9 Pickup, G. and Chewings, V.H. ht. J. Remote Sensing (in press) 10 Bell, A. (1987) Ecos 51,14-17
322