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Moorland plant community recognition using landsat MSS data
REMOTE SENSING OF ENVIRONMENT 20:291-298 (1986)
291
Moorland Plant Community Recognition Using Landsat MSS Data
A. J. MORTON Department of Pure and Applied Biology, Imperial College, Silwood Park, Ascot, Berks. SL5 7PY, United Kingdom
Landsat MSS data are examined to determine to what extent moorland plant community types can be recognized in the Plynlimon area of Wales. A detailed comparison is made between spectral radiance data and community composition. Attention is drawn to the importance of a phenological understanding of the dominant plant species and in particular the propertion of living and standing dead plant parts as an important aid to distinguishing between species and communities.
Introduction Landsat MSS data have been available since 1972, but the data and derived imagery have been little used in the United Kingdom for ecological projects, Two important reasons for this have been the low availability of image analysis facilities and the relatively coarse resolution (79 m) of the data. Recent developments, however, have altered this situation; image analysis facilities are much more widely available and 30 m resolution data are now available (Landsat 5 Thematic Mapper). Much of the current and future research interest will, of course, focus on the new sources of higher resolution data, but Landsat MSS data have the advantage that they can at present be used for monitoring vegetation change, particularly in extensive areas of seminatural vegetation such as moorland. Fundamental to this is the question: To what extent can ecologically meaninghd plant community types be recognized using satellite-derived multispectral data? Much research effort has been directed towards biomass estimation (e.g., Curran, 1983) and relatively little towards classification of seminatural plant communities in Britain. Weaver (1984), however, has ap©Elsevier Science Publishing Co., Inc., 1986 52 Vanderbilt Ave., New York, NY 10017
praised Landsat data for studying heather moorland in relation to management, and Milner and Wyatt (1984) refer to the future possibility of using satellite imagcry for moorland mapping. The loss of moorland through afforestation and agricultural improvement is causing concern among conservationists and a recent study of the mid-Wales uplands (Parry and Sinclair, 1985) has shown that rough pasture occupied 55% of the area in 1983 compared with 78% in 1948. The aim of the work described in this paper is to relate, in a detailed way, Landsat MSS data to community tomposition data collected in the field and also available in the form of an existing vegetation map.
Methods The study area is 5 km northeast of Plynlimon, Wales in an area of moorland with continuous vegetation cover and very little bare rock or peat. The main vegetation types shown on the Nature Conservancy Council vegetation map (NCC 1981) are Nardusstricta grasslands(C2a), Juncus squarrosus grasslands (C3a, usually intermixed with C2a), Molinia 0034-4257/86/$3.50
292
A . j . MORTON
caerulea grasslands (C4a), CallunaEriophorum vaginatum blanket bog (G4a, often dominated by CaUuna), and Eriophorum vaginatum dominated mire (G4f). The classification is that of Ratcliffe and Birks (1980). A Landsat scene (220/23, 29 May 1982) was acquired, and a subscene which included the study area was extracted using an International Imaging Systems Model 70 Image Processor. A transect line was selected using both the vegetation map and the image processor to ensure that the major plant community types were sampled, Data, representing spectral radiance in the four MSS bands (MSS4; green, MSS5; red, MSS6; infrared, MSS7; irffrared) were extracted for an east-west transect of 52 contiguous pixels for subsequent analysis, The original scene had been deskewed but not contrast-stretched or resampled in any other way. The pixel data were ordinated using R-type Principal Components Analysis (Orloci, 1966)to examine
,
1Km
the spectral a~mties between pixels and the spatial dimensionality of the data. The data were also classified by a directed search polythetic divisive procedure. This method uses PCA to order the individuals to be classified, and then the ordered individuals are split into two groups using minimum combined within groups sums of squares as the division criterion (lambert et al., 1973). The derived classification is therefore totally unsupervised. The transect line was located as accurately as possible in the field and walked by compass bearing, noting major changes in the vegetation composition by visual observation. The changes in tommunity composition were found to be approximately at right angles to the transect line so that the exact location of the line was not critical. The Nardus stricta grassland was frequently found to be very intimately mixed with 1uncus squarrosus "grassland," so that the two were regarded as one community and
Grid North
,
FIGURE 1. Map of the study area showing the transect line annotated with the visually dominant species recorded in the field. The positions of the two sample areas for cover estimates are also indicated (e). Key to the visually dominant species: C ffi CaUuna tmlgaris, N ffi Nardus stricta, V ffi V a c c i n i u m rayrtillus, E ffi Eriophorum v a g i n a t u m , M ffi Molinia caerulea.
MSS CLASSIFICATION OF MOORLAND VEGETATION
referred to as Nardus grassland. The Nard~ grassland contained short, grazed Vaccinium myrtillus, and where the abundance of this was visually significant (more than about 20%), a record was made. The Eriophorum vaginatum mire contained significant amounts of Molinia caerulea, which was often visually dominant, horizontally. This was noted when recording the transect, and, in order to obtain a more accurate estimate of the composition of this community, a representative area was selected and recorded using 100 randomly placed cover pins. Percentage horizontal ground cover was recorded noting the species which made the uppermost contact ("top cover") in each case, and distinguishing between green and senescent plant parts. The concept of "top cover" may not be a particularly useful one for many ecological studies, but it was considered appropriate here, for comparison with remotely sensed data. The recording was carried out on 3 June 1985, to be comparable seasonally with the imagery, and was repeated for an area of Na~dus grassland. The positions of the transect and the two areas used for estimating top cover are shown in Fig. 1.
Results The ordination of the pixel data by Principal Components Analysis is shown in Fig. 2. The first component, which accounts for 88% of the total variance, is dominated by the two infrared bands (MSS6 and MSS7). The second component, which accounts for 10% of the total variance, is dominated by the red band (MSS5). The visually dominant plant species of the area corresponding to the
293
approximate position of the pixel on the ground are also shown in Fig. 2. Multiple lettering indicates codominance. The main features are as follows: 1) The Cal/una-dominated pixels are well separated from the remainder, being characterized by low infrared and low red radiance. 2) The remaining pixels form a continuum, but the Molinia caerulea-Eriophorum vaginatura pixels occupy an extreme position, characterized by high infrared and high red radiance. 3) The Nardus grassland pixels show a wide scatter along component 1 (infrared), but have low scores on component 2 indicating low red radiance relative to the MoliniaEriophorum pixels. 4)Pixels representing mixed communities of CaUuna and other species fall in intermediate positions. The polythetic divisive classification of the pixel data is shown in Fig. 3 which clearly demonstrates the separation of the CaUuna dominated pixels (group 1) from the other groups. All four groups are shown delimited on the ordination (Fig. 2) and it can be seen that the classific a t i o n s e p a r a t e s the MoliniaEriophorum community (group 4), but several of the Nardus pixels are classified in the same group as the mixed Calluna communities (group 2). Table 1 gives the composition of the Nardus grassland and the Molinia-Eriophorum mire in terms of top cover and also proportion of green matter for each species. The data demonstrate that, in early June, the Nardus community has a much higher proportion (66%) of green plant parts as top cover than the Molinia-Eriophomm community (31%). The Molinia has a particularly low proportion; the current year's green leaves only just beginning to overtop the previous season's leaf litter.
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E = Eriophorum vaginatum, M = Molinia caerulea.
FIGURE 2. Ordination by tMneit~l Components Analysis of 52 pixels of Landsat MSS data of moorland vegetation. Varhnee contributions to the vectors are: Vector 1, 43% MSS7; 41% MSS6; 8% MSS5; 7% MSS4. Vector 2, 66% MSS5; 18% MSS7; 15% MSS4; 1% MSS6. The groups derived from the classification (Fig. 3) are numbered on the ordination and are del~ited by broken lines. Key to the visually dominant species: C = Calluna vulgaris, N = Nardus s ~ c t a , V = Vaccinium myrtillus,
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2
Vector
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MSS CLASSIFICATION OF MOORLAND VEGETATION
295
200-
150"
100"
50'
m .--~
1
I
4
3
2
FIGURE 3. Polythetic divisive classification of 52 pixels of MSS data of moorland vegetation. TABLE I Composition (Top Cover %) of Two Plant Communities and Proportion of Green Matter, Sampled 3 June 1985 ToP COVF_.R(%) (Gr,zzs oB SENESCENT)
Gm~ PROPORTION (%)
Nardus Grassland Nardus stricta Vaccinium mttrtillus Festuca ov/na Carex pilulifera Carex binervis PotentiUa erecta All species
44 39 9 6 1 1 I00
59 69 89 60 0 100 66
Molinia- Eriophorum Mire Eriophorum vaginatum Molinia caerulea Eriophorum angustifolium Tricophorum cespitosum Nardus stricta Erica tetralix All species
56 32 6 3 2 1 100
32 16 83 0 100 100 31
296
Discussion
The results indicate that, within the area studied, CaUuna dominated vegetation is distinct, Molinia-Eriophorum mire is fairly distinct, and Nardus grassland is rather variable in terms of multispectral radiance properties at the time of year sampled. It is well known that reflectance properties of vegetation are related to a large number of factors, among which the properties of individual leaves and canopy structure are particularly important (Horler and Barber, 1981). Both of these vary with species and are also influenced by environmental factors; for example, grazing has a profound effect on the canopy structure of moorland dwarf shrub communities. The spectral distinctiveness of the Calluna-dominated vegetation in this study can be attributed to its evergreen and shrubby habit, giving the observed low red and low infrared radiance. This low radiance from areas of CaUuna is consistent with the findings of other workers (e.g., Weaver, 1984). The multispectral data do not, however, allow a distinction to be made between Calluna dominated blanket mire (G4a) and Calluna heaths (Bla) in the study area. The high radiance in both the red and infrared bands of the MoliniaEriophorum mire is attributable to the properties of the predominant senescent vegetation. This community is shown in Table 1 to have a much higher proportion of senescent and standing dead vegetation than the Nardus grassland and thus a much lower absorption of red light due to photosynthesis (Colwell, 1974). Molinia is particularly important in this respect. The near infrared radiance of senescent vegetation, however, is not significantly different from that of green
h.J. MORTON
vegetation (Curran, 1985), and, in this study, the infrared radiance of the largely senescent Molinia-Eriophorum mire is seen to be similar to that of the greener Nardus grassland. It is thus the red radiance which is important in distinguishing the Molinia-Eriophorum mire. Senescent vegetation is often considered to be a problem in studies which are concerned with estimating living biomass (Colwell, 1974; Curran, 1983), but in ecological studies where the recognition or classification of community types is an objecrive, the presence and quantity of senescent and dead plant tissue is a useful characteristic in distinguising between community types where there is consistent correlation. It is clear that phenological information about the more abundant plant species in moorland communities would be a useful tool in interpreting remotely sensed data, and in determining optimum times of year for distinguishing particular communities. Further irdormation of this kind may prove useful in distinguishing between species of similar structural morphology, such as Molinia and Eriophorum which are both tussock formers. Odenweller and Johnson (1984) have demonstrated the usefulness of temporal-spectral profiles for characterizing crop species, and it is suggested that the determination of such profiles for the main dominant species of moorlands would greatly facilitate interpretation and classification. The wide scatter of the Nardus pixels along the first component i~ probably related partly to natural variation in the community composition and partly to an aspect effect. The relationship between species composition and ordination position was not, however, apparent in the field, and the pixels with higher scores on
MSS CLASSIFICATION OF MOORLAND VEGETATION
component 1 were mostly located on an east-facing slope while the remainder were located on a west-facing slope. The time of overpass of the satellite is approximately 10.30 and at this time the eastfacing slopes would be more strongly irradiated. This aspect effect is most pronounced in the Nardus grasslands because, in the study area, they tend to occupy the sloping ground whereas the Molinia, Eriophorum, and Calluna dominated communities are on level or very gentle sloping terrain. The implication of this for classification using an image analyzer is that training areas for supervised classification must include both aspects, The polythetic division classification of the pixel data, which is an unsupervised classification, performs satisfactorally for the Calluna and Molinia-Eriophorum communities but not for the Nardus grassland. The reason for this is the wide variation in infrared radiance of the Nardus grassland and the fact that pixels from mixed communities have very similar infrared radiance to the Nardus tommunities on east-facing slopes. The problem of "mixed pixels" is well known; e.g., Weaver (1984) working with Landsat MSS data of the North Yorks Moors found that the spatial scale of the data was limiting when attempting to distinguish between management units in heather moorland. The use of higher resolution multispectral data might reduce the problem of "mixed pixels" to some extent in moorland areas, but natural small scale variation and continuity which exists in the composition of most seminatural communities will always be a problem if classification is an objective. If classification is to be successful, an understanding of this variation is essential in the selection
297
of training areas for supervised classification.
Thanks are due to George Greenshields for assistance with image processing and field work and to the Imperial College Centres for Environmental Technology and Remote Sensing for financial and other resources. Permission to carry out field work in the study area was kindly granted by the Bugeilyn Estate.
References Colwell, J. E. (1974), Vegetation canopy reflectance, Remote Sens. Environ. 3:175-183. Curran, P. J. (1983), Problems in the remote sensing of vegetation canopies for biomass estimation, in Ecological Mapping flrmn Ground, Air and Space, (R. M. Fuller, Ed.), Institute of Terrestrial Ecology, Cambridge, U.K., pp. 84-100. Curran, P. J. (1985), Pr/nc/p/es of Remote Sens/ng, Longman, London, p. 28. Horler, D. N. H., and Barber, J. (1981), Principles of remote sensing of plants, in Plants and the Daylight Spectrum, (H. Smith, Ed.), Academic, London, pp. 43-63. Lambert, J. M., Meacock, S. E., Barrs, J., and Smartt, P. F. M. (1973), AXOR and MONIT: two new polythetic divisive strategies for hierarchical classification, Taxon22:173-176. Milner, C., and Wyatt, B. K. (1984), Future techniques for monitoring vegetation change in upland areas: data collection and interpretation, in Moor and Heath: Its Conservationand Management, Snowdonia National Park Authority, U.K., pp. 48-59. Nature Conservancy Council (1981), Upland VegetationSurvey, Section 2: Site Reports, Number 4: Plitnlimon, Wales Field Unit, N.C.C., Bangor, U.K.
298 Odenweller, J. B., and Johnson, K. I. (1984), Crop identification using Landsat temporal-spectral profiles, Remote Sens. Env/ton. 14:39-54. Orloci, L. (1966), Geometric models in ecology. 1. The theory and application of some ordination methods, ]. Ecoi. 54:193-216. Parry, M., and Sinclair, G. (1985), Mid Wales Up/ands Study, Countryside Commission, Cheltenham, U.K. Ratcli~e, D. A., and Birks, H. J. B. (1980), A Classification of Up~and Vegetation Types
A.j. MORTON in Britain, Internal report, Nature conservancy Council, U.K. Weaver, R. E. (1984), Integration of remote sensing data for moorland mapping, in Satellite Remote Sensing--Review and Preview. Proceedings o f the Tenth Anniversand International Conference, Remote Sensing Society, Reading, U.K., pp. 191-200.
Received 5 November1985, revised171une 1986.
291
Moorland Plant Community Recognition Using Landsat MSS Data
A. J. MORTON Department of Pure and Applied Biology, Imperial College, Silwood Park, Ascot, Berks. SL5 7PY, United Kingdom
Landsat MSS data are examined to determine to what extent moorland plant community types can be recognized in the Plynlimon area of Wales. A detailed comparison is made between spectral radiance data and community composition. Attention is drawn to the importance of a phenological understanding of the dominant plant species and in particular the propertion of living and standing dead plant parts as an important aid to distinguishing between species and communities.
Introduction Landsat MSS data have been available since 1972, but the data and derived imagery have been little used in the United Kingdom for ecological projects, Two important reasons for this have been the low availability of image analysis facilities and the relatively coarse resolution (79 m) of the data. Recent developments, however, have altered this situation; image analysis facilities are much more widely available and 30 m resolution data are now available (Landsat 5 Thematic Mapper). Much of the current and future research interest will, of course, focus on the new sources of higher resolution data, but Landsat MSS data have the advantage that they can at present be used for monitoring vegetation change, particularly in extensive areas of seminatural vegetation such as moorland. Fundamental to this is the question: To what extent can ecologically meaninghd plant community types be recognized using satellite-derived multispectral data? Much research effort has been directed towards biomass estimation (e.g., Curran, 1983) and relatively little towards classification of seminatural plant communities in Britain. Weaver (1984), however, has ap©Elsevier Science Publishing Co., Inc., 1986 52 Vanderbilt Ave., New York, NY 10017
praised Landsat data for studying heather moorland in relation to management, and Milner and Wyatt (1984) refer to the future possibility of using satellite imagcry for moorland mapping. The loss of moorland through afforestation and agricultural improvement is causing concern among conservationists and a recent study of the mid-Wales uplands (Parry and Sinclair, 1985) has shown that rough pasture occupied 55% of the area in 1983 compared with 78% in 1948. The aim of the work described in this paper is to relate, in a detailed way, Landsat MSS data to community tomposition data collected in the field and also available in the form of an existing vegetation map.
Methods The study area is 5 km northeast of Plynlimon, Wales in an area of moorland with continuous vegetation cover and very little bare rock or peat. The main vegetation types shown on the Nature Conservancy Council vegetation map (NCC 1981) are Nardusstricta grasslands(C2a), Juncus squarrosus grasslands (C3a, usually intermixed with C2a), Molinia 0034-4257/86/$3.50
292
A . j . MORTON
caerulea grasslands (C4a), CallunaEriophorum vaginatum blanket bog (G4a, often dominated by CaUuna), and Eriophorum vaginatum dominated mire (G4f). The classification is that of Ratcliffe and Birks (1980). A Landsat scene (220/23, 29 May 1982) was acquired, and a subscene which included the study area was extracted using an International Imaging Systems Model 70 Image Processor. A transect line was selected using both the vegetation map and the image processor to ensure that the major plant community types were sampled, Data, representing spectral radiance in the four MSS bands (MSS4; green, MSS5; red, MSS6; infrared, MSS7; irffrared) were extracted for an east-west transect of 52 contiguous pixels for subsequent analysis, The original scene had been deskewed but not contrast-stretched or resampled in any other way. The pixel data were ordinated using R-type Principal Components Analysis (Orloci, 1966)to examine
,
1Km
the spectral a~mties between pixels and the spatial dimensionality of the data. The data were also classified by a directed search polythetic divisive procedure. This method uses PCA to order the individuals to be classified, and then the ordered individuals are split into two groups using minimum combined within groups sums of squares as the division criterion (lambert et al., 1973). The derived classification is therefore totally unsupervised. The transect line was located as accurately as possible in the field and walked by compass bearing, noting major changes in the vegetation composition by visual observation. The changes in tommunity composition were found to be approximately at right angles to the transect line so that the exact location of the line was not critical. The Nardus stricta grassland was frequently found to be very intimately mixed with 1uncus squarrosus "grassland," so that the two were regarded as one community and
Grid North
,
FIGURE 1. Map of the study area showing the transect line annotated with the visually dominant species recorded in the field. The positions of the two sample areas for cover estimates are also indicated (e). Key to the visually dominant species: C ffi CaUuna tmlgaris, N ffi Nardus stricta, V ffi V a c c i n i u m rayrtillus, E ffi Eriophorum v a g i n a t u m , M ffi Molinia caerulea.
MSS CLASSIFICATION OF MOORLAND VEGETATION
referred to as Nardus grassland. The Nard~ grassland contained short, grazed Vaccinium myrtillus, and where the abundance of this was visually significant (more than about 20%), a record was made. The Eriophorum vaginatum mire contained significant amounts of Molinia caerulea, which was often visually dominant, horizontally. This was noted when recording the transect, and, in order to obtain a more accurate estimate of the composition of this community, a representative area was selected and recorded using 100 randomly placed cover pins. Percentage horizontal ground cover was recorded noting the species which made the uppermost contact ("top cover") in each case, and distinguishing between green and senescent plant parts. The concept of "top cover" may not be a particularly useful one for many ecological studies, but it was considered appropriate here, for comparison with remotely sensed data. The recording was carried out on 3 June 1985, to be comparable seasonally with the imagery, and was repeated for an area of Na~dus grassland. The positions of the transect and the two areas used for estimating top cover are shown in Fig. 1.
Results The ordination of the pixel data by Principal Components Analysis is shown in Fig. 2. The first component, which accounts for 88% of the total variance, is dominated by the two infrared bands (MSS6 and MSS7). The second component, which accounts for 10% of the total variance, is dominated by the red band (MSS5). The visually dominant plant species of the area corresponding to the
293
approximate position of the pixel on the ground are also shown in Fig. 2. Multiple lettering indicates codominance. The main features are as follows: 1) The Cal/una-dominated pixels are well separated from the remainder, being characterized by low infrared and low red radiance. 2) The remaining pixels form a continuum, but the Molinia caerulea-Eriophorum vaginatura pixels occupy an extreme position, characterized by high infrared and high red radiance. 3) The Nardus grassland pixels show a wide scatter along component 1 (infrared), but have low scores on component 2 indicating low red radiance relative to the MoliniaEriophorum pixels. 4)Pixels representing mixed communities of CaUuna and other species fall in intermediate positions. The polythetic divisive classification of the pixel data is shown in Fig. 3 which clearly demonstrates the separation of the CaUuna dominated pixels (group 1) from the other groups. All four groups are shown delimited on the ordination (Fig. 2) and it can be seen that the classific a t i o n s e p a r a t e s the MoliniaEriophorum community (group 4), but several of the Nardus pixels are classified in the same group as the mixed Calluna communities (group 2). Table 1 gives the composition of the Nardus grassland and the Molinia-Eriophorum mire in terms of top cover and also proportion of green matter for each species. The data demonstrate that, in early June, the Nardus community has a much higher proportion (66%) of green plant parts as top cover than the Molinia-Eriophomm community (31%). The Molinia has a particularly low proportion; the current year's green leaves only just beginning to overtop the previous season's leaf litter.
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FIGURE 2. Ordination by tMneit~l Components Analysis of 52 pixels of Landsat MSS data of moorland vegetation. Varhnee contributions to the vectors are: Vector 1, 43% MSS7; 41% MSS6; 8% MSS5; 7% MSS4. Vector 2, 66% MSS5; 18% MSS7; 15% MSS4; 1% MSS6. The groups derived from the classification (Fig. 3) are numbered on the ordination and are del~ited by broken lines. Key to the visually dominant species: C = Calluna vulgaris, N = Nardus s ~ c t a , V = Vaccinium myrtillus,
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MSS CLASSIFICATION OF MOORLAND VEGETATION
295
200-
150"
100"
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FIGURE 3. Polythetic divisive classification of 52 pixels of MSS data of moorland vegetation. TABLE I Composition (Top Cover %) of Two Plant Communities and Proportion of Green Matter, Sampled 3 June 1985 ToP COVF_.R(%) (Gr,zzs oB SENESCENT)
Gm~ PROPORTION (%)
Nardus Grassland Nardus stricta Vaccinium mttrtillus Festuca ov/na Carex pilulifera Carex binervis PotentiUa erecta All species
44 39 9 6 1 1 I00
59 69 89 60 0 100 66
Molinia- Eriophorum Mire Eriophorum vaginatum Molinia caerulea Eriophorum angustifolium Tricophorum cespitosum Nardus stricta Erica tetralix All species
56 32 6 3 2 1 100
32 16 83 0 100 100 31
296
Discussion
The results indicate that, within the area studied, CaUuna dominated vegetation is distinct, Molinia-Eriophorum mire is fairly distinct, and Nardus grassland is rather variable in terms of multispectral radiance properties at the time of year sampled. It is well known that reflectance properties of vegetation are related to a large number of factors, among which the properties of individual leaves and canopy structure are particularly important (Horler and Barber, 1981). Both of these vary with species and are also influenced by environmental factors; for example, grazing has a profound effect on the canopy structure of moorland dwarf shrub communities. The spectral distinctiveness of the Calluna-dominated vegetation in this study can be attributed to its evergreen and shrubby habit, giving the observed low red and low infrared radiance. This low radiance from areas of CaUuna is consistent with the findings of other workers (e.g., Weaver, 1984). The multispectral data do not, however, allow a distinction to be made between Calluna dominated blanket mire (G4a) and Calluna heaths (Bla) in the study area. The high radiance in both the red and infrared bands of the MoliniaEriophorum mire is attributable to the properties of the predominant senescent vegetation. This community is shown in Table 1 to have a much higher proportion of senescent and standing dead vegetation than the Nardus grassland and thus a much lower absorption of red light due to photosynthesis (Colwell, 1974). Molinia is particularly important in this respect. The near infrared radiance of senescent vegetation, however, is not significantly different from that of green
h.J. MORTON
vegetation (Curran, 1985), and, in this study, the infrared radiance of the largely senescent Molinia-Eriophorum mire is seen to be similar to that of the greener Nardus grassland. It is thus the red radiance which is important in distinguishing the Molinia-Eriophorum mire. Senescent vegetation is often considered to be a problem in studies which are concerned with estimating living biomass (Colwell, 1974; Curran, 1983), but in ecological studies where the recognition or classification of community types is an objecrive, the presence and quantity of senescent and dead plant tissue is a useful characteristic in distinguising between community types where there is consistent correlation. It is clear that phenological information about the more abundant plant species in moorland communities would be a useful tool in interpreting remotely sensed data, and in determining optimum times of year for distinguishing particular communities. Further irdormation of this kind may prove useful in distinguishing between species of similar structural morphology, such as Molinia and Eriophorum which are both tussock formers. Odenweller and Johnson (1984) have demonstrated the usefulness of temporal-spectral profiles for characterizing crop species, and it is suggested that the determination of such profiles for the main dominant species of moorlands would greatly facilitate interpretation and classification. The wide scatter of the Nardus pixels along the first component i~ probably related partly to natural variation in the community composition and partly to an aspect effect. The relationship between species composition and ordination position was not, however, apparent in the field, and the pixels with higher scores on
MSS CLASSIFICATION OF MOORLAND VEGETATION
component 1 were mostly located on an east-facing slope while the remainder were located on a west-facing slope. The time of overpass of the satellite is approximately 10.30 and at this time the eastfacing slopes would be more strongly irradiated. This aspect effect is most pronounced in the Nardus grasslands because, in the study area, they tend to occupy the sloping ground whereas the Molinia, Eriophorum, and Calluna dominated communities are on level or very gentle sloping terrain. The implication of this for classification using an image analyzer is that training areas for supervised classification must include both aspects, The polythetic division classification of the pixel data, which is an unsupervised classification, performs satisfactorally for the Calluna and Molinia-Eriophorum communities but not for the Nardus grassland. The reason for this is the wide variation in infrared radiance of the Nardus grassland and the fact that pixels from mixed communities have very similar infrared radiance to the Nardus tommunities on east-facing slopes. The problem of "mixed pixels" is well known; e.g., Weaver (1984) working with Landsat MSS data of the North Yorks Moors found that the spatial scale of the data was limiting when attempting to distinguish between management units in heather moorland. The use of higher resolution multispectral data might reduce the problem of "mixed pixels" to some extent in moorland areas, but natural small scale variation and continuity which exists in the composition of most seminatural communities will always be a problem if classification is an objective. If classification is to be successful, an understanding of this variation is essential in the selection
297
of training areas for supervised classification.
Thanks are due to George Greenshields for assistance with image processing and field work and to the Imperial College Centres for Environmental Technology and Remote Sensing for financial and other resources. Permission to carry out field work in the study area was kindly granted by the Bugeilyn Estate.
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Received 5 November1985, revised171une 1986.