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Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century
SAJB-01311; No of Pages 8 South African Journal of Botany xxx (2015) xxx–xxx
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Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century Z.C. Poulsen ⁎, M.T. Hoffman Plant Conservation Unit, Department of Biological Sciences, University of Cape Town, Rondebosch 7701, South Africa
a r t i c l e Edited by A Potts Keywords: Aerial photography Environmental history Climate change Fire Land cover change Repeat photography Temporal dynamics
i n f o
a b s t r a c t Long-term changes in the distribution of Western Cape Afrotemperate Forest and Western Cape Milkwood Forest in Table Mountain National Park (TMNP) were examined using paired aerial and repeat groundbased photographs. All forest patches mapped on the aerial photographs taken in 1944 and 2008 were visited and boundaries between forest and adjacent vegetation types checked in the field. In addition, 50 historical ground-based photographs covering the period 1888 to 1980 were revisited between 2011 and 2012 and the change in forest cover was quantified using a 360 point sampling grid overlaid on each of the photograph pairs. Changes in fynbos, woody alien vegetation, alien grass, development and exposed rock and sand were also quantified. The analysis of aerial photographs showed that between 1944 and 2008 the number of forest patches on the Peninsula increased from 149 to 174 and total forest cover increased by 65.3% from 884.2 ha to 1461.5 ha. Only 13 of the forest patches decreased in cover after 1944 while 65 patches showed stasis and 96 increased in size. More than a third of the patches that decreased in size were of Western Cape Milkwood Forest located in proximity to expanding coastal development. The analysis of repeat ground-based photographs showed an increase in cover of Western Cape Afrotemperate Forest, alien grasses and urban development while fynbos vegetation, Western Cape Milkwood Forest, woody alien vegetation as well as exposed rock and sand decreased in cover. Historical land use practices such as firewood collection and a reduction in fire frequency relative to the fire regimes of the 19th century may also account for the changes observed in the historical photos. These results hold significance for the ecological management of TMNP in the face of changing climate and increased urbanisation. © 2015 SAAB. Published by Elsevier B.V. All rights reserved.
1. Introduction The world's forests are major reservoirs of terrestrial biodiversity and provide vital ecosystem services (Venter and Venter, 2009). In present debates on climate change, they are considered an integral part of climate mitigation owing to their carbon storage capacity. However, indigenous forests are under threat worldwide and the high level of global deforestation has increased the focus on effective forest conservation (Thompson et al., 2009), including in South Africa. Although indigenous forests cover only 0.56% of the land surface of South Africa (Low and Rebelo, 1996) they are among the most species-rich temperate forests worldwide (Lawes et al., 2004). How they respond to changes in climate and land use has been a matter of conservation concern for decades (Pillans, 1926; Wicht, 1945).
⁎ Corresponding author. Tel.: +27 73 341 2430. E-mail address: [email protected] (Z.C. Poulsen).
While not as rich in terms of plant species as neighbouring fynbos vegetation the forests of Table Mountain National Park (TMNP) are considered to be of high conservation importance (Alston and Richardson, 2006). They are home to several endemic species including two species of moss (Von Maltitz et al., 2003), numerous arthropods (Pauw and Johnson, 1999) and the Critically Endangered Table Mountain Ghost Frog (Pauw and Johnson, 1999). However, in terms of research and conservation planning, the Peninsula forests have been neglected and the current spatial extent of the Western Cape Afrotemperate Forests has not been comprehensively mapped (Von Maltitz et al., 2003). Euston-Brown et al. (2008) were commissioned by the South African National Parks (SANParks) to map the distribution of the Cape Peninsula forests. However, time and financial resources were not available to ground-truth all 212 of the forest patches that were recorded in this survey. A comprehensive assessment of the current spatial extent and distribution of the Cape Peninsula forests is, therefore, long overdue. This is crucial for effective monitoring and future conservation management of TMNP particularly in terms of the dynamics and distribution of important vegetation types such as Western Cape Afrotemperate Forest and the endangered Peninsula Granite Fynbos (Von Maltitz et al., 2003).
http://dx.doi.org/10.1016/j.sajb.2015.05.002 0254-6299/© 2015 SAAB. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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In the Cape Floristic Region (CFR) fire is an important determinant of the forest-fynbos ecotone (Manders, 1990). The frequency and intensity of fire determine whether forest patches expand or contract in size over time (Manders and Richardson, 1992; Luger and Moll, 1993). Frequent and intense fires will erode the forest edge while infrequent and cool fires or fire exclusion will enable the forest patch to expand and new forests to develop (Luger and Moll, 1993; Midgley et al., 2003). There is currently a paucity of data available on forest distribution dynamics in TMNP. It is not known whether the Peninsula forests are undergoing change or stasis. Recent research by Forsyth and Van Wilgen (2008) suggested that fire frequency in TMNP has increased since 1975. This is significant in the face of changing climate whereby fire frequencies in the CFR are projected to increase in response to future warming and increasing aridity (Midgley et al., 2001; Bomhard et al., 2005; Hannah et al., 2005). This has highlighted concern that the spatial extent of the Peninsula forests may decline in response to increasing fire frequencies. Such trends have been predicted in response to global change drivers in other Mediterranean climate forests in southern Europe (Morales et al., 2007). There is also the likelihood of negative impact on forest owing to predicted decreases in rainfall (Engelbrecht et al., 2008). In South Africa areas of potential forest distribution are strongly governed by moisture availability. In the winter rainfall zone, forests only occur in areas that receive more than 525 mm/yr precipitation (Von Maltitz et al., 2003). In contrast, several authors have recently expressed concern that Western Cape Afrotemperate Forest species are invading Peninsula Granite Fynbos and Peninsula Shale Fynbos on the Cape Peninsula in response to a long term absence of fire in some areas due to expanding urban development (Rebelo et al., 2006; Rebelo et al., 2010; Van Wilgen et al., 2012). Peninsula Granite Fynbos has a high diversity of plant species with many endemic to this community. It is classified as an endangered vegetation type and 56% has already been transformed by urbanisation, vineyards and pine plantations (Rebelo et al., 2006). Colonisation by afrotemperate forest taxa would further threaten this fynbos community (Rebelo et al., 2006). Luger and Moll (1993) used aerial photos to investigate changes in Western Cape Afrotemperate Forest distribution from 1933 to 1993 in Orange Kloof on Table Mountain. They discovered that forest had doubled in extent since 1933 and long term fire exclusion was given as the main reason for the observed changes. It remains unknown whether these trends are replicated elsewhere on the Peninsula or if this trend has continued. Here we provide further quantitative data on the distribution and temporal dynamics of the forests of the Cape Peninsula. This is vital to inform future forest and fynbos management in the TMNP. The main objectives of the study were to map the contemporary and historic distribution and spatial area of all indigenous forest on the Cape Peninsula and to examine the temporal dynamics of the forest-fynbos ecotone from 1880 to present.
2. Materials and methods 2.1. Study site The Table Mountain National Park (TMNP) is the focal area of this study and situated on the Cape Peninsula. The Cape Peninsula is a rugged and mountainous area of 470 km2 in size at the southwestern tip of the African continent (Cowling et al., 1996). The Peninsula Mountain Chain of TMNP lies within the bounds of the City of Cape Town which is one of South Africa's largest urban areas (Anderson and O'Farrell, 2012). The Cape Peninsula is an internationally renowned centre of exceptional plant species diversity and endemism (Helme and Trinder-Smith, 2006). It is part of South Africa's Cape Floristic Region (CFR) which is a recognised
biodiversity hotspot and is classified as a UNESCO World Heritage Site (Rebelo et al., 2010). The dominant vegetation of the Cape Peninsula is fynbos, which is a Mediterranean type shrubland that is both fire prone and fire dependent (Helme and Trinder-Smith, 2006). There are three main forest types which also occur on the Peninsula: Western Cape Afrotemperate Forest, Western Cape Talus Forest and Western Cape Milkwood Forest (Von Maltitz et al., 2003; Mucina and Geldenhuys, 2006). However, for the purposes of this research Western Cape Talus Forest is considered to be a subtype of Western Cape Afrotemperate Forest. The area experiences a Mediterranean climate with predominantly winter rainfall (Cowling et al., 1996). The Cape Peninsula has exceptionally steep gradients in precipitation driven by the high topographic heterogeneity of the area (Cowling et al., 1996) (Fig. 1). 2.2. Research approach Contact prints of aerial photographs of the Cape Peninsula from 1944 were scanned at 1000 dpi prior to georeferencing in ArcGIS 10 (ESRI, USA, Redlands). These were aligned with contemporary aerial images from 2008 to map forest change. The Euston-Brown et al. (2008) SANParks forest shapefile was used as a baseline for the study. Ground-truthing of all 212 forest patches in this shapefile was undertaken to verify the accuracy of forest/fynbos ecotonal boundaries delimited by Euston-Brown et al. (2008) and to determine whether the forest patches listed in the survey had been correctly classified. Ground-truthing was carried out from 2010 to 2012. For this analysis forest was defined as having a closed canopy greater than four metres in height and dominance of woody forest taxa (after Euston-Brown et al., 2008). Species and cover abundance of all woody taxa were recorded for each forest patch during ground-truthing. The 2008 Euston-Brown et al. shapefile was used as a base for digitising contemporary forest distribution. It was edited or redrawn as necessary with forest patches being reshaped, added or deleted depending on the outcome of ground-truthing carried out over the period 2010–2012. The contemporary forest distribution shapefile was then used as a baseline in conjunction with sets of historical aerial photographs to produce another shapefile delimiting forest distribution on the Peninsula in 1944. These were both then used to measure change in forest distribution and extent between 1944 and 2008 using ArcGIS 10. For the ground-based repeat photography study, 50 historical images showing Western Cape Afrotemperate Forest and Western Cape Milkwood Forest on the Peninsula were used in the analysis of forest cover change (Appendix I). The historical photographs covered the period 1882 to 1980. The location of each historic image was relocated in the field between 2010 and 2012 and a replicate image was taken. The repeat and original images were matched as closely as possible using Adobe Photoshop CS4. The complete ground-based repeat photo collection together with associated metadata (primarily photo number, site name, GPS coordinates, altitude, photographic information, geology, landforms, vegetation types, description of main changes and detailed species lists with estimated percentage cover values) was then used to investigate land cover change on the Cape Peninsula. In the analysis we focussed on the temporal change in the distribution of Western Cape Afrotemperate Forest and Western Cape Milkwood Forest. To do this, the ground-based photographs were subdivided according to forest type and date of the original photograph (historic images prior to 1970 and more recent photos) and then analysed separately. A 360 point grid was placed over each image in Adobe Photoshop CS4. At each gridline intersection the land cover type was recorded and assigned to a land cover class of either Western Cape Afrotemperate Forest, Western Cape Milkwood Forest, fynbos, woody alien vegetation, alien grass, development and exposed rock and sand. Sky and sea were
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
Z.C. Poulsen, M.T. Hoffman / South African Journal of Botany xxx (2015) xxx–xxx
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Fig. 1. Site 766 Blinkwater Ravine original image (upper) by Jurgens (1888). Repeat image (lower) by Timm Hoffman (7 September 2011).
recorded as null values. This dataset was then used to examine changes in land cover between the time steps of the original and repeat photographs.
3. Results
Table 1 Changes in forest cover of Western Cape Afrotemperate Forest and Western Cape Milkwood Forest on the Peninsula from 1944 and 2008 based on an analysis of aerial photographs.
The analysis of aerial photographs found that most patches of both Western Cape Afrotemperate Forest and Western Cape Milkwood Forest either experienced stasis or increased in extent from 1944 to 2008 (Table 1). There was an overall increase in cover of 65.3%. There was an increase in cover of Western Cape Afrotemperate Forest of 51.2% and the number of patches increased from 107 to 127 (Table 1). The rate of increase in Western Cape Milkwood Forest was higher than Western Cape Afrotemperate Forest at 81.4% and there have been a total of five new patches which had formed since 1944. The highest increases in cover of Western Cape Afrotemperate Forest were recorded in Orange Kloof, Disa Gorge and Blinkwater Ravine on Table Mountain. Overall the total number of forest patches increased since 1944 from a total of 149 to 174. Only 13 forest patches had
# forest patches in 1944 # forest patches in 2008 # forest patches with greater cover # forest patches with no change # forest patches with reduced cover Total area (ha) 1944 Total area (ha) 2008 % increase in forest cover
Western Cape Afrotemperate Forest
Western Cape Milkwood Forest
Total
107 127 68 51 8 471.9 713.7 51.2%
42 47 28 14 5 412.2 747.8 81.4%
149 174 96 65 13 884.2 1461.5 65.3%
3.1. Change in forest cover as determined from analysis of aerial photographs
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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Fig. 2. Site 981 Kommetjie original image (upper) (G246) from the Green Collection (Western Cape Archives and Records Service) (c. 1900). Repeat image (lower) by Zoë C Poulsen (27 October 2011).
reduced cover. Most of these were patches of Western Cape Milkwood Forest on the southern Peninsula in the Cape of Good Hope section of TMNP and around densely settled areas along the coast (e.g. at Kommetjie). 3.2. Ground-based photographic evidence of change in cover of Western Cape Afrotemperate Forest An increase in the cover of the Western Cape Afrotemperate Forest was recorded in 30 of the repeat ground-based photos although there was a high degree of variance in the extent of forest cover increase. This is unsurprising given the differences in the time period between photographs and the amount of area covered by each image. The highest increase in relative forest cover was recorded at Blinkwater Ravine on the western side of Table Mountain (Fig. 1). The original image taken in 1888 showed that the lower slopes of Blinkwater Ravine were comprised predominantly of fynbos taxa. The vegetation in the original photograph was relatively lowgrowing and comprised mainly shrubs with a smaller component of grass and geophytes. On the lower slopes of Porcupine Buttress (right distance) there was a small patch of scree forest. Blinkwater Ravine itself was forested in the upper reaches only along the drainage line and there was also a long and narrow patch of scree forest visible in the adjacent Fountain Ravine (left distance).
Since the original image was taken there has been a substantial increase in the extent of Western Cape Afrotemperate Forest in Blinkwater and other adjacent ravines. Relative forest cover in the image has increased by 52% at this site over the last 130 years. The present vegetation on the lower slopes is now dominated by forest precursor species, in particular Cassine peragua, Phylica buxifolia, Searsia tomentosa and S. lucida. These are typically forest margin species but here they now dominate the vegetation which is far taller with a significant increase in woody biomass. 3.3. Ground-based photographic evidence of change in cover of Western Cape Milkwood Forest The 12 repeat photographs of Western Cape Milkwood Forest demonstrated a general decrease in cover of this vegetation primarily due to coastal housing development. This is best illustrated by the repeat photograph dataset from Site 981 at Kommetjie. The original image taken by Green in c. 1900 (Fig. 2) was taken prior to the town of Kommetjie being built. This shows an extensive area of lowland fynbos with a few scattered houses and a coastline dominated by an uninterrupted belt of Western Cape Milkwood Forest. Since then almost the entire coastal plain has been covered by urban development. Only a few fragments of Western Cape Milkwood Forest remain at this site, which are dominated by Sideroxylon inerme.
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
Z.C. Poulsen, M.T. Hoffman / South African Journal of Botany xxx (2015) xxx–xxx
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Fig. 3. Box plot showing the change in land cover classes from 1888 to 2012 between 50 historical and repeat ground-based photographs of the Cape Peninsula.
3.4. Change in land cover classes on the Cape Peninsula using repeat ground-based photographs The average relative percentage of change in land cover classes between the original and repeat images was recorded for all 50 ground-based photographs. There was considerable variation in the change in fynbos cover. However, on average there was a decrease in fynbos cover of 5%. Western Cape Afrotemperate Forest increased in cover by almost 5% (Fig. 3). This is in contrast to a slight decrease in cover of 1% in Western Cape Milkwood Forest. An overall decrease in cover of more than 5% of visible rock and sand was recorded in the repeat ground-based photographs. There was a slight decrease in cover of alien trees and shrubs while there was a slight increase in the cover of alien grasses. There was a relatively high increase in cover of 7% from building development.
4. Discussion 4.1. Forest cover change on the Cape Peninsula Results from the aerial and repeat ground-based photograph datasets have shown that there has been a significant increase in the number of patches of forest as well as in forest cover on the Cape Peninsula since 1888 when the earliest repeat photos were taken. In most cases the increase in forest cover has been at the expense of adjacent fynbos vegetation. With the exception of the work of Luger and Moll (1993), this is the first time that such high rates of increase in the spatial extent of indigenous forest cover have been recorded in the Western Cape. Furthermore, the increase in forest cover reported here only takes into account the increase in closed canopy forest. There are other areas of the Peninsula where long-term absence of fire has led to the forest-fynbos ecotone becoming less well-defined and often dominated by thicket and forest precursor species such as Canthium inerme, Cassine peragua, Kiggelaria africana, Phylica buxifolia
and Gymnosporia buxifolia (Mucina and Geldenhuys, 2006). This is the case, for example, in Blinkwater Ravine, Orange Kloof and Kirstenbosch where these taxa have colonised extensive areas of Peninsula Granite Fynbos after long-term absence of fire (Van Wilgen et al., 2012). The development of urban areas on the lower slopes of Table Mountain has further enabled this process by creating a “fire shadow” within which forest species expand (Geldenhuys, 2000). In the past, strong south-easterly winds would have driven fires across the Cape Flats up to the lower margins of the forests. The expansion of urban environments, however, now buffers these areas and prevents fires from being driven on to these slopes by prevailing winds (Geldenhuys, 2000). Colonisation of adjacent biomes with woody vegetation after a long-term absence of fire has been widely documented in numerous ecosystems throughout the world. In the Bunya Mountains in eastern Australia, for example, woody vegetation has expanded into adjacent montane grasslands (Fairfax et al., 2009). This has been attributed to a decrease in anthropogenic burning in association with the cessation of traditional land management practices by Aboriginal communities. Regular burning used to take place to maintain habitats for yam beds, to provide habitat for native game and as a game management tool (Bowman et al., 2001). This trend has also been reported by numerous studies examining change at the forest-prairie ecotone in North America (Bragg and Hulbert, 1976; Mast et al., 1997; Briggs et al., 2002). For example, Coop and Givnish (2007) investigated invasion of upland coniferous forest across the ecotone into neighbouring montane grassland in the Valles Caldera in New Mexico. A significant increase in tree cover was reported and adjacent grasslands reduced by nearly 18% from 1935 to 1996. Decrease in fire frequency was cited as one of the most significant reasons for the change (Coop and Givnish, 2007).
4.2. Explanations for forest cover change on the Cape Peninsula The recent trend shown in this study of an increase in woody vegetation cover on the Cape Peninsula contrasts with earlier reports
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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of a decline in forest and shrub cover between the 17th and 19th centuries (Pillans, 1926). This was attributed to firewood harvesting and to high fire frequencies which were implemented to promote grass cover (Denslow, 1984). The early colonists farmed the Peninsula intensively and the area was heavily utilised by both cattle and horses (Adamson and Salter, 1950; Opie, 1967). In order to increase the grass component of the fynbos and in so doing to enhance the grazing potential of the veld it was burnt at regular intervals (Marloth, 1924; Adamson and Salter, 1950; Pooley, 2015). The effects of these practices were described at the 1924 National Symposium on Veld Fire burning. For example, Marloth (1924) described how, since the early colonists arrived, the fynbos in the mountains of the south-western Cape had become degraded and in some areas most of the shrubby components including many species within the Proteaceae, Ericaceae, Bruniaceae and others had been lost from the landscape as a result of annual veld burning. The only place where woody vegetation was present was in protected rocky areas (Marloth, 1924). At the same symposium Neville Pillans (1926) indicated that veld burning to improve forage production, took place in all seasons with the exception of the wettest winter months (May–August) and at intervals of 1–3 years, thus far higher than natural fire frequencies in fynbos. He also suggested that as a result of frequent, successive fires, the shrubby vegetation component had been gradually replaced by annuals, grasses and species within the Cyperaceae (Pillans, 1926). The vegetation described by Pillans and Marloth is similar in appearance to the vegetation observed in historic images of the Cape Peninsula in the 1880s and early 1900s and as exemplified in the view of Blinkwater Ravine (Fig. 1). Pillans (1926) suggested further that in the pre- and early-colonial period almost every ravine was clothed with forest and most mountain streams on the Peninsula had a thick riparian vegetation of forest taxa. Fires were so frequent, he argued, that eventually this was no longer the case and most stream banks were then lined with charred tree stumps. A few patches of forest remained in fire refugia on scree slopes but these were few and far between (Pillans, 1926). By the 1930s veld burning for agriculture was seen as such a destructive practice that widespread fire exclusion practices were introduced to protect the veld (Wicht, 1945; Pooley, 2012). More recently, Forsyth and Van Wilgen (2008) have shown that since 1975 mean fire return intervals for Table Mountain National Park (TMNP) have declined by 18.1 years, from 31.6 to 13.5 years. Pooley (2015) has reported a similar pattern for the region. Concerns were raised about the impact of these changes on fynbos vegetation of TMNP and particularly the likely impacts of decreasing fire intervals on obligate reseeding Proteaceae (Forsyth and Van Wilgen, 2008). These findings have been widely discussed in the global change literature (Abbot and Le Maitre, 2009; Syphard et al., 2009; Chown, 2010). However, a fire return interval of 13.5 years appears significantly less frequent than the fire return intervals which were in place on the Cape Peninsula in the 18th and 19th centuries and which were described by delegates to the 1924 veld fire symposium. What also needs to be clarified is the nature of the pre- and earlycolonial fire regime. A palaeoecological reconstruction of hunter– gatherer and herder burning practices is urgently needed to better understand the long-term dynamics of forest cover change in the region. Increases in woody vegetation cover have increasingly been attributed to increases in elevated atmospheric CO2 levels in response to changing climate. In savanna ecosystems where bush encroachment is a widespread problem, this potential driver of change has received considerable attention (Bond and Midgley, 2012). While elevated levels of CO2 may be a contributing factor in the increase of woody vegetation biomass on the Cape Peninsula, historical evidence (Marloth, 1924; Botha, 1926; Levyns, 1926; Pillans, 1926; Opie, 1967; Pooley, 2012) suggests that changes in land use and fire frequencies are probably the most important drivers of forest change. The aerial and repeat
ground-based photo datasets, which when used in conjunction, provide useful insights across different time scales. The evidence shows that expansion of forest on the Cape Peninsula is not a recent phenomenon of the last half century but started as soon as the frequent burning regimes of the 19th Century were halted. For example, the analysis of aerial photographs showed that forest taxa were already widespread at the Blinkwater Ravine site in 1944 long before the impact of CO2 would have had a significant effect. However, it is extremely difficult to differentiate the effects of CO2 on woody vegetation cover increase from other key drivers. Responses of woody taxa to elevated CO 2 vary greatly between species. Some taxa experience significantly enhanced growth rates whereas others show no response at all (Bond and Midgley, 2012). There has been no research on the effects of elevated CO2 on South African indigenous forest taxa. Further research is therefore needed to gain a better understanding of likely responses to better predict forest ecotonal dynamics in the context of changing climates.
5. Conclusion The Cape Peninsula's forests have high conservation importance and their long-term dynamics are of interest to managers and researchers alike. Results from an analysis of repeat aerial (1944–2008) and ground-based (1880–2012) photographs show that over the course of the 20th century there has been an increase in the number of patches as well as in the cover of Western Cape Afrotemperate Forest and Western Cape Milkwood Forest on the Cape Peninsula. Areas where this has not occurred are primarily situated along the coast where developments have expanded and replaced Cape Milkwood Forest. The increase in closed canopy forest has occurred largely at the expense of Peninsula Granite Fynbos. Although not mapped as forest in this study, the forestfynbos ecotone has also become increasing dominated by forest precursor species. The increase in forest cover is best explained by long-term fire exclusion which has occurred on the Cape Peninsula. Although fire return intervals have declined in the last 50 years, they remain less frequent than those in place in the 18th and 19th centuries. An important research question, however, is whether the current extent of forests is more or less than that observed by early colonists. A more detailed understanding of Khoisan burning practices is also needed. Finally, forest and thicket trees and shrubs will likely continue to expand into Peninsula Granite Fynbos. The intense, hot fires needed to remove forest taxa and keep them from encroaching into fynbos environments on the Cape Peninsula are difficult to achieve in practice.
Acknowledgements This study was funded by UCT's Plant Conservation Unit and the Department of Biological Sciences Cameron Fund. Special thanks go to Stuart Hall, Rick Rohde, Ellen Fedele, Eduard Smit, Darin Taitz, Charmaine Lacock and Jane and Sebastian Wyngaard for field assistance. Doug Euston-Brown, Zishaan Ibrahim, Tom Slingsby and Nick Lindenberg for aerial photographs and GIS data; Katya Mauff and especially Natalie Kunz for help with statistical analysis and The Mountain Club of South Africa, UCT Plant Conservation Unit and Western Cape Archives for use of historical images. The Mazda Wildlife Fund is also thanked for their use of a courtesy vehicle.
Appendix II. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.sajb.2015.05.002.
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
Z.C. Poulsen, M.T. Hoffman / South African Journal of Botany xxx (2015) xxx–xxx
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Appendix I Table 1 Relevant information for each of the 50 ground-based photographs analysed. Abbreviations: WCAF = Western Cape Afrotemperate Forest; WCMF = Western Cape Milkwood Forest. All modern photographs taken by Zoë C Poulsen; Photo pair numbers refer to the UCT Plant Conservation Unit's repeat photo collection. Photo pair
Forest type
Date of historical photo
Historical photographer
Date of modern photograph
603 Orange Kloof West 1 604 Orange Kloof West 2 605 Orange Kloof Valley 606 Orange Kloof East 1 607 Orange Kloof South 1 608 Orange Kloof East 2 969 Kirstenbosch 970 Kirstenbosch 971 Kirstenbosch 972 Kirstenbosch 973 Orange Kloof 974 Disa Gorge 975 Disa Gorge 976 Slangolie Ravine 977 Kirstenbosch 978 Kirstenbosch 979 Kirstenbosch 980 Scarborough 981 Kommetjie 982 Kommetjie 983 Olifantsbos 984 Buffelsbaai 985 Gifkommetjie 986 Tafelberg Road 987 Noordhoek Estate 988 Hangberg 989 Hout Bay East 990 Hout Bay Battery 991 Camps Bay Sports Field 992 Fountain Buttress 993 Groote Schuur Old Zoo 994 Hout Bay Drive 995 Llandudno 996 Camps Bay Sports Field 2 997 Camps Bay 998 Twelve Apostles 999 Theresa Drive 1000 Platteklip Gorge 1001 Woodhead Tunnel 1002 Wynberg Caves 1003 Klein Kogelbaai
WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCMF WCMF WCMF WCMF WCMF WCMF WCAF WCAF WCMF WCMF WCMF WCAF WCAF WCAF WCAF WCMF WCMF WCAF WCAF WCAF WCAF WCAF WCAF WCMF/WCAF
c. 1970 c.1970 c. 1970 c. 1970 c. 1970 c. 1970 c. 1980 c. 1900 c. 1900 c. 1900 c. 1900 c. 1900 c. 1900 1889 c. 1900 c. 1970 c. 1900 1972 c. 1900 1972 c. 1900 c. 1900 1968 c. 1900 c. 1900 c. 1900 c. 1900 c. 1900 c. 1980 c. 1900 c. 1900 c. 1900 c. 1900 c. 1980 c. 1900 c. 1900 c. 1980 c. 1900 c. 1900 c. 1972 c. 1900
Eugene Moll Eugene Moll Eugene Moll Eugene Moll Eugene Moll Eugene Moll Hall Elliot Collection Elliot Collection Elliot Collection Elliot Collection Whitworth Cameron Cairncross/Jurgens Elliot Collection Eugene Moll Elliot Collection CA Collection Green Collection CA Collection Green Collection Green Collection Hugh Taylor AG Collection Wood CA Collection Elliot Collection CA Collection Lambert Cobern Elliot Collection AG Collection Jurgens Lambert Elliot Collection Elliot Collection Hall Cobern Steer Collection Eugene Moll Steer Collection
2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2012 2012 2012 2012 2012 2012
Fig. 1. Mean monthly precipitation rates throughout TMNP (data sourced from the South African Weather Service).
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century Z.C. Poulsen ⁎, M.T. Hoffman Plant Conservation Unit, Department of Biological Sciences, University of Cape Town, Rondebosch 7701, South Africa
a r t i c l e Edited by A Potts Keywords: Aerial photography Environmental history Climate change Fire Land cover change Repeat photography Temporal dynamics
i n f o
a b s t r a c t Long-term changes in the distribution of Western Cape Afrotemperate Forest and Western Cape Milkwood Forest in Table Mountain National Park (TMNP) were examined using paired aerial and repeat groundbased photographs. All forest patches mapped on the aerial photographs taken in 1944 and 2008 were visited and boundaries between forest and adjacent vegetation types checked in the field. In addition, 50 historical ground-based photographs covering the period 1888 to 1980 were revisited between 2011 and 2012 and the change in forest cover was quantified using a 360 point sampling grid overlaid on each of the photograph pairs. Changes in fynbos, woody alien vegetation, alien grass, development and exposed rock and sand were also quantified. The analysis of aerial photographs showed that between 1944 and 2008 the number of forest patches on the Peninsula increased from 149 to 174 and total forest cover increased by 65.3% from 884.2 ha to 1461.5 ha. Only 13 of the forest patches decreased in cover after 1944 while 65 patches showed stasis and 96 increased in size. More than a third of the patches that decreased in size were of Western Cape Milkwood Forest located in proximity to expanding coastal development. The analysis of repeat ground-based photographs showed an increase in cover of Western Cape Afrotemperate Forest, alien grasses and urban development while fynbos vegetation, Western Cape Milkwood Forest, woody alien vegetation as well as exposed rock and sand decreased in cover. Historical land use practices such as firewood collection and a reduction in fire frequency relative to the fire regimes of the 19th century may also account for the changes observed in the historical photos. These results hold significance for the ecological management of TMNP in the face of changing climate and increased urbanisation. © 2015 SAAB. Published by Elsevier B.V. All rights reserved.
1. Introduction The world's forests are major reservoirs of terrestrial biodiversity and provide vital ecosystem services (Venter and Venter, 2009). In present debates on climate change, they are considered an integral part of climate mitigation owing to their carbon storage capacity. However, indigenous forests are under threat worldwide and the high level of global deforestation has increased the focus on effective forest conservation (Thompson et al., 2009), including in South Africa. Although indigenous forests cover only 0.56% of the land surface of South Africa (Low and Rebelo, 1996) they are among the most species-rich temperate forests worldwide (Lawes et al., 2004). How they respond to changes in climate and land use has been a matter of conservation concern for decades (Pillans, 1926; Wicht, 1945).
⁎ Corresponding author. Tel.: +27 73 341 2430. E-mail address: [email protected] (Z.C. Poulsen).
While not as rich in terms of plant species as neighbouring fynbos vegetation the forests of Table Mountain National Park (TMNP) are considered to be of high conservation importance (Alston and Richardson, 2006). They are home to several endemic species including two species of moss (Von Maltitz et al., 2003), numerous arthropods (Pauw and Johnson, 1999) and the Critically Endangered Table Mountain Ghost Frog (Pauw and Johnson, 1999). However, in terms of research and conservation planning, the Peninsula forests have been neglected and the current spatial extent of the Western Cape Afrotemperate Forests has not been comprehensively mapped (Von Maltitz et al., 2003). Euston-Brown et al. (2008) were commissioned by the South African National Parks (SANParks) to map the distribution of the Cape Peninsula forests. However, time and financial resources were not available to ground-truth all 212 of the forest patches that were recorded in this survey. A comprehensive assessment of the current spatial extent and distribution of the Cape Peninsula forests is, therefore, long overdue. This is crucial for effective monitoring and future conservation management of TMNP particularly in terms of the dynamics and distribution of important vegetation types such as Western Cape Afrotemperate Forest and the endangered Peninsula Granite Fynbos (Von Maltitz et al., 2003).
http://dx.doi.org/10.1016/j.sajb.2015.05.002 0254-6299/© 2015 SAAB. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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In the Cape Floristic Region (CFR) fire is an important determinant of the forest-fynbos ecotone (Manders, 1990). The frequency and intensity of fire determine whether forest patches expand or contract in size over time (Manders and Richardson, 1992; Luger and Moll, 1993). Frequent and intense fires will erode the forest edge while infrequent and cool fires or fire exclusion will enable the forest patch to expand and new forests to develop (Luger and Moll, 1993; Midgley et al., 2003). There is currently a paucity of data available on forest distribution dynamics in TMNP. It is not known whether the Peninsula forests are undergoing change or stasis. Recent research by Forsyth and Van Wilgen (2008) suggested that fire frequency in TMNP has increased since 1975. This is significant in the face of changing climate whereby fire frequencies in the CFR are projected to increase in response to future warming and increasing aridity (Midgley et al., 2001; Bomhard et al., 2005; Hannah et al., 2005). This has highlighted concern that the spatial extent of the Peninsula forests may decline in response to increasing fire frequencies. Such trends have been predicted in response to global change drivers in other Mediterranean climate forests in southern Europe (Morales et al., 2007). There is also the likelihood of negative impact on forest owing to predicted decreases in rainfall (Engelbrecht et al., 2008). In South Africa areas of potential forest distribution are strongly governed by moisture availability. In the winter rainfall zone, forests only occur in areas that receive more than 525 mm/yr precipitation (Von Maltitz et al., 2003). In contrast, several authors have recently expressed concern that Western Cape Afrotemperate Forest species are invading Peninsula Granite Fynbos and Peninsula Shale Fynbos on the Cape Peninsula in response to a long term absence of fire in some areas due to expanding urban development (Rebelo et al., 2006; Rebelo et al., 2010; Van Wilgen et al., 2012). Peninsula Granite Fynbos has a high diversity of plant species with many endemic to this community. It is classified as an endangered vegetation type and 56% has already been transformed by urbanisation, vineyards and pine plantations (Rebelo et al., 2006). Colonisation by afrotemperate forest taxa would further threaten this fynbos community (Rebelo et al., 2006). Luger and Moll (1993) used aerial photos to investigate changes in Western Cape Afrotemperate Forest distribution from 1933 to 1993 in Orange Kloof on Table Mountain. They discovered that forest had doubled in extent since 1933 and long term fire exclusion was given as the main reason for the observed changes. It remains unknown whether these trends are replicated elsewhere on the Peninsula or if this trend has continued. Here we provide further quantitative data on the distribution and temporal dynamics of the forests of the Cape Peninsula. This is vital to inform future forest and fynbos management in the TMNP. The main objectives of the study were to map the contemporary and historic distribution and spatial area of all indigenous forest on the Cape Peninsula and to examine the temporal dynamics of the forest-fynbos ecotone from 1880 to present.
2. Materials and methods 2.1. Study site The Table Mountain National Park (TMNP) is the focal area of this study and situated on the Cape Peninsula. The Cape Peninsula is a rugged and mountainous area of 470 km2 in size at the southwestern tip of the African continent (Cowling et al., 1996). The Peninsula Mountain Chain of TMNP lies within the bounds of the City of Cape Town which is one of South Africa's largest urban areas (Anderson and O'Farrell, 2012). The Cape Peninsula is an internationally renowned centre of exceptional plant species diversity and endemism (Helme and Trinder-Smith, 2006). It is part of South Africa's Cape Floristic Region (CFR) which is a recognised
biodiversity hotspot and is classified as a UNESCO World Heritage Site (Rebelo et al., 2010). The dominant vegetation of the Cape Peninsula is fynbos, which is a Mediterranean type shrubland that is both fire prone and fire dependent (Helme and Trinder-Smith, 2006). There are three main forest types which also occur on the Peninsula: Western Cape Afrotemperate Forest, Western Cape Talus Forest and Western Cape Milkwood Forest (Von Maltitz et al., 2003; Mucina and Geldenhuys, 2006). However, for the purposes of this research Western Cape Talus Forest is considered to be a subtype of Western Cape Afrotemperate Forest. The area experiences a Mediterranean climate with predominantly winter rainfall (Cowling et al., 1996). The Cape Peninsula has exceptionally steep gradients in precipitation driven by the high topographic heterogeneity of the area (Cowling et al., 1996) (Fig. 1). 2.2. Research approach Contact prints of aerial photographs of the Cape Peninsula from 1944 were scanned at 1000 dpi prior to georeferencing in ArcGIS 10 (ESRI, USA, Redlands). These were aligned with contemporary aerial images from 2008 to map forest change. The Euston-Brown et al. (2008) SANParks forest shapefile was used as a baseline for the study. Ground-truthing of all 212 forest patches in this shapefile was undertaken to verify the accuracy of forest/fynbos ecotonal boundaries delimited by Euston-Brown et al. (2008) and to determine whether the forest patches listed in the survey had been correctly classified. Ground-truthing was carried out from 2010 to 2012. For this analysis forest was defined as having a closed canopy greater than four metres in height and dominance of woody forest taxa (after Euston-Brown et al., 2008). Species and cover abundance of all woody taxa were recorded for each forest patch during ground-truthing. The 2008 Euston-Brown et al. shapefile was used as a base for digitising contemporary forest distribution. It was edited or redrawn as necessary with forest patches being reshaped, added or deleted depending on the outcome of ground-truthing carried out over the period 2010–2012. The contemporary forest distribution shapefile was then used as a baseline in conjunction with sets of historical aerial photographs to produce another shapefile delimiting forest distribution on the Peninsula in 1944. These were both then used to measure change in forest distribution and extent between 1944 and 2008 using ArcGIS 10. For the ground-based repeat photography study, 50 historical images showing Western Cape Afrotemperate Forest and Western Cape Milkwood Forest on the Peninsula were used in the analysis of forest cover change (Appendix I). The historical photographs covered the period 1882 to 1980. The location of each historic image was relocated in the field between 2010 and 2012 and a replicate image was taken. The repeat and original images were matched as closely as possible using Adobe Photoshop CS4. The complete ground-based repeat photo collection together with associated metadata (primarily photo number, site name, GPS coordinates, altitude, photographic information, geology, landforms, vegetation types, description of main changes and detailed species lists with estimated percentage cover values) was then used to investigate land cover change on the Cape Peninsula. In the analysis we focussed on the temporal change in the distribution of Western Cape Afrotemperate Forest and Western Cape Milkwood Forest. To do this, the ground-based photographs were subdivided according to forest type and date of the original photograph (historic images prior to 1970 and more recent photos) and then analysed separately. A 360 point grid was placed over each image in Adobe Photoshop CS4. At each gridline intersection the land cover type was recorded and assigned to a land cover class of either Western Cape Afrotemperate Forest, Western Cape Milkwood Forest, fynbos, woody alien vegetation, alien grass, development and exposed rock and sand. Sky and sea were
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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Fig. 1. Site 766 Blinkwater Ravine original image (upper) by Jurgens (1888). Repeat image (lower) by Timm Hoffman (7 September 2011).
recorded as null values. This dataset was then used to examine changes in land cover between the time steps of the original and repeat photographs.
3. Results
Table 1 Changes in forest cover of Western Cape Afrotemperate Forest and Western Cape Milkwood Forest on the Peninsula from 1944 and 2008 based on an analysis of aerial photographs.
The analysis of aerial photographs found that most patches of both Western Cape Afrotemperate Forest and Western Cape Milkwood Forest either experienced stasis or increased in extent from 1944 to 2008 (Table 1). There was an overall increase in cover of 65.3%. There was an increase in cover of Western Cape Afrotemperate Forest of 51.2% and the number of patches increased from 107 to 127 (Table 1). The rate of increase in Western Cape Milkwood Forest was higher than Western Cape Afrotemperate Forest at 81.4% and there have been a total of five new patches which had formed since 1944. The highest increases in cover of Western Cape Afrotemperate Forest were recorded in Orange Kloof, Disa Gorge and Blinkwater Ravine on Table Mountain. Overall the total number of forest patches increased since 1944 from a total of 149 to 174. Only 13 forest patches had
# forest patches in 1944 # forest patches in 2008 # forest patches with greater cover # forest patches with no change # forest patches with reduced cover Total area (ha) 1944 Total area (ha) 2008 % increase in forest cover
Western Cape Afrotemperate Forest
Western Cape Milkwood Forest
Total
107 127 68 51 8 471.9 713.7 51.2%
42 47 28 14 5 412.2 747.8 81.4%
149 174 96 65 13 884.2 1461.5 65.3%
3.1. Change in forest cover as determined from analysis of aerial photographs
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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Fig. 2. Site 981 Kommetjie original image (upper) (G246) from the Green Collection (Western Cape Archives and Records Service) (c. 1900). Repeat image (lower) by Zoë C Poulsen (27 October 2011).
reduced cover. Most of these were patches of Western Cape Milkwood Forest on the southern Peninsula in the Cape of Good Hope section of TMNP and around densely settled areas along the coast (e.g. at Kommetjie). 3.2. Ground-based photographic evidence of change in cover of Western Cape Afrotemperate Forest An increase in the cover of the Western Cape Afrotemperate Forest was recorded in 30 of the repeat ground-based photos although there was a high degree of variance in the extent of forest cover increase. This is unsurprising given the differences in the time period between photographs and the amount of area covered by each image. The highest increase in relative forest cover was recorded at Blinkwater Ravine on the western side of Table Mountain (Fig. 1). The original image taken in 1888 showed that the lower slopes of Blinkwater Ravine were comprised predominantly of fynbos taxa. The vegetation in the original photograph was relatively lowgrowing and comprised mainly shrubs with a smaller component of grass and geophytes. On the lower slopes of Porcupine Buttress (right distance) there was a small patch of scree forest. Blinkwater Ravine itself was forested in the upper reaches only along the drainage line and there was also a long and narrow patch of scree forest visible in the adjacent Fountain Ravine (left distance).
Since the original image was taken there has been a substantial increase in the extent of Western Cape Afrotemperate Forest in Blinkwater and other adjacent ravines. Relative forest cover in the image has increased by 52% at this site over the last 130 years. The present vegetation on the lower slopes is now dominated by forest precursor species, in particular Cassine peragua, Phylica buxifolia, Searsia tomentosa and S. lucida. These are typically forest margin species but here they now dominate the vegetation which is far taller with a significant increase in woody biomass. 3.3. Ground-based photographic evidence of change in cover of Western Cape Milkwood Forest The 12 repeat photographs of Western Cape Milkwood Forest demonstrated a general decrease in cover of this vegetation primarily due to coastal housing development. This is best illustrated by the repeat photograph dataset from Site 981 at Kommetjie. The original image taken by Green in c. 1900 (Fig. 2) was taken prior to the town of Kommetjie being built. This shows an extensive area of lowland fynbos with a few scattered houses and a coastline dominated by an uninterrupted belt of Western Cape Milkwood Forest. Since then almost the entire coastal plain has been covered by urban development. Only a few fragments of Western Cape Milkwood Forest remain at this site, which are dominated by Sideroxylon inerme.
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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Fig. 3. Box plot showing the change in land cover classes from 1888 to 2012 between 50 historical and repeat ground-based photographs of the Cape Peninsula.
3.4. Change in land cover classes on the Cape Peninsula using repeat ground-based photographs The average relative percentage of change in land cover classes between the original and repeat images was recorded for all 50 ground-based photographs. There was considerable variation in the change in fynbos cover. However, on average there was a decrease in fynbos cover of 5%. Western Cape Afrotemperate Forest increased in cover by almost 5% (Fig. 3). This is in contrast to a slight decrease in cover of 1% in Western Cape Milkwood Forest. An overall decrease in cover of more than 5% of visible rock and sand was recorded in the repeat ground-based photographs. There was a slight decrease in cover of alien trees and shrubs while there was a slight increase in the cover of alien grasses. There was a relatively high increase in cover of 7% from building development.
4. Discussion 4.1. Forest cover change on the Cape Peninsula Results from the aerial and repeat ground-based photograph datasets have shown that there has been a significant increase in the number of patches of forest as well as in forest cover on the Cape Peninsula since 1888 when the earliest repeat photos were taken. In most cases the increase in forest cover has been at the expense of adjacent fynbos vegetation. With the exception of the work of Luger and Moll (1993), this is the first time that such high rates of increase in the spatial extent of indigenous forest cover have been recorded in the Western Cape. Furthermore, the increase in forest cover reported here only takes into account the increase in closed canopy forest. There are other areas of the Peninsula where long-term absence of fire has led to the forest-fynbos ecotone becoming less well-defined and often dominated by thicket and forest precursor species such as Canthium inerme, Cassine peragua, Kiggelaria africana, Phylica buxifolia
and Gymnosporia buxifolia (Mucina and Geldenhuys, 2006). This is the case, for example, in Blinkwater Ravine, Orange Kloof and Kirstenbosch where these taxa have colonised extensive areas of Peninsula Granite Fynbos after long-term absence of fire (Van Wilgen et al., 2012). The development of urban areas on the lower slopes of Table Mountain has further enabled this process by creating a “fire shadow” within which forest species expand (Geldenhuys, 2000). In the past, strong south-easterly winds would have driven fires across the Cape Flats up to the lower margins of the forests. The expansion of urban environments, however, now buffers these areas and prevents fires from being driven on to these slopes by prevailing winds (Geldenhuys, 2000). Colonisation of adjacent biomes with woody vegetation after a long-term absence of fire has been widely documented in numerous ecosystems throughout the world. In the Bunya Mountains in eastern Australia, for example, woody vegetation has expanded into adjacent montane grasslands (Fairfax et al., 2009). This has been attributed to a decrease in anthropogenic burning in association with the cessation of traditional land management practices by Aboriginal communities. Regular burning used to take place to maintain habitats for yam beds, to provide habitat for native game and as a game management tool (Bowman et al., 2001). This trend has also been reported by numerous studies examining change at the forest-prairie ecotone in North America (Bragg and Hulbert, 1976; Mast et al., 1997; Briggs et al., 2002). For example, Coop and Givnish (2007) investigated invasion of upland coniferous forest across the ecotone into neighbouring montane grassland in the Valles Caldera in New Mexico. A significant increase in tree cover was reported and adjacent grasslands reduced by nearly 18% from 1935 to 1996. Decrease in fire frequency was cited as one of the most significant reasons for the change (Coop and Givnish, 2007).
4.2. Explanations for forest cover change on the Cape Peninsula The recent trend shown in this study of an increase in woody vegetation cover on the Cape Peninsula contrasts with earlier reports
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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of a decline in forest and shrub cover between the 17th and 19th centuries (Pillans, 1926). This was attributed to firewood harvesting and to high fire frequencies which were implemented to promote grass cover (Denslow, 1984). The early colonists farmed the Peninsula intensively and the area was heavily utilised by both cattle and horses (Adamson and Salter, 1950; Opie, 1967). In order to increase the grass component of the fynbos and in so doing to enhance the grazing potential of the veld it was burnt at regular intervals (Marloth, 1924; Adamson and Salter, 1950; Pooley, 2015). The effects of these practices were described at the 1924 National Symposium on Veld Fire burning. For example, Marloth (1924) described how, since the early colonists arrived, the fynbos in the mountains of the south-western Cape had become degraded and in some areas most of the shrubby components including many species within the Proteaceae, Ericaceae, Bruniaceae and others had been lost from the landscape as a result of annual veld burning. The only place where woody vegetation was present was in protected rocky areas (Marloth, 1924). At the same symposium Neville Pillans (1926) indicated that veld burning to improve forage production, took place in all seasons with the exception of the wettest winter months (May–August) and at intervals of 1–3 years, thus far higher than natural fire frequencies in fynbos. He also suggested that as a result of frequent, successive fires, the shrubby vegetation component had been gradually replaced by annuals, grasses and species within the Cyperaceae (Pillans, 1926). The vegetation described by Pillans and Marloth is similar in appearance to the vegetation observed in historic images of the Cape Peninsula in the 1880s and early 1900s and as exemplified in the view of Blinkwater Ravine (Fig. 1). Pillans (1926) suggested further that in the pre- and early-colonial period almost every ravine was clothed with forest and most mountain streams on the Peninsula had a thick riparian vegetation of forest taxa. Fires were so frequent, he argued, that eventually this was no longer the case and most stream banks were then lined with charred tree stumps. A few patches of forest remained in fire refugia on scree slopes but these were few and far between (Pillans, 1926). By the 1930s veld burning for agriculture was seen as such a destructive practice that widespread fire exclusion practices were introduced to protect the veld (Wicht, 1945; Pooley, 2012). More recently, Forsyth and Van Wilgen (2008) have shown that since 1975 mean fire return intervals for Table Mountain National Park (TMNP) have declined by 18.1 years, from 31.6 to 13.5 years. Pooley (2015) has reported a similar pattern for the region. Concerns were raised about the impact of these changes on fynbos vegetation of TMNP and particularly the likely impacts of decreasing fire intervals on obligate reseeding Proteaceae (Forsyth and Van Wilgen, 2008). These findings have been widely discussed in the global change literature (Abbot and Le Maitre, 2009; Syphard et al., 2009; Chown, 2010). However, a fire return interval of 13.5 years appears significantly less frequent than the fire return intervals which were in place on the Cape Peninsula in the 18th and 19th centuries and which were described by delegates to the 1924 veld fire symposium. What also needs to be clarified is the nature of the pre- and earlycolonial fire regime. A palaeoecological reconstruction of hunter– gatherer and herder burning practices is urgently needed to better understand the long-term dynamics of forest cover change in the region. Increases in woody vegetation cover have increasingly been attributed to increases in elevated atmospheric CO2 levels in response to changing climate. In savanna ecosystems where bush encroachment is a widespread problem, this potential driver of change has received considerable attention (Bond and Midgley, 2012). While elevated levels of CO2 may be a contributing factor in the increase of woody vegetation biomass on the Cape Peninsula, historical evidence (Marloth, 1924; Botha, 1926; Levyns, 1926; Pillans, 1926; Opie, 1967; Pooley, 2012) suggests that changes in land use and fire frequencies are probably the most important drivers of forest change. The aerial and repeat
ground-based photo datasets, which when used in conjunction, provide useful insights across different time scales. The evidence shows that expansion of forest on the Cape Peninsula is not a recent phenomenon of the last half century but started as soon as the frequent burning regimes of the 19th Century were halted. For example, the analysis of aerial photographs showed that forest taxa were already widespread at the Blinkwater Ravine site in 1944 long before the impact of CO2 would have had a significant effect. However, it is extremely difficult to differentiate the effects of CO2 on woody vegetation cover increase from other key drivers. Responses of woody taxa to elevated CO 2 vary greatly between species. Some taxa experience significantly enhanced growth rates whereas others show no response at all (Bond and Midgley, 2012). There has been no research on the effects of elevated CO2 on South African indigenous forest taxa. Further research is therefore needed to gain a better understanding of likely responses to better predict forest ecotonal dynamics in the context of changing climates.
5. Conclusion The Cape Peninsula's forests have high conservation importance and their long-term dynamics are of interest to managers and researchers alike. Results from an analysis of repeat aerial (1944–2008) and ground-based (1880–2012) photographs show that over the course of the 20th century there has been an increase in the number of patches as well as in the cover of Western Cape Afrotemperate Forest and Western Cape Milkwood Forest on the Cape Peninsula. Areas where this has not occurred are primarily situated along the coast where developments have expanded and replaced Cape Milkwood Forest. The increase in closed canopy forest has occurred largely at the expense of Peninsula Granite Fynbos. Although not mapped as forest in this study, the forestfynbos ecotone has also become increasing dominated by forest precursor species. The increase in forest cover is best explained by long-term fire exclusion which has occurred on the Cape Peninsula. Although fire return intervals have declined in the last 50 years, they remain less frequent than those in place in the 18th and 19th centuries. An important research question, however, is whether the current extent of forests is more or less than that observed by early colonists. A more detailed understanding of Khoisan burning practices is also needed. Finally, forest and thicket trees and shrubs will likely continue to expand into Peninsula Granite Fynbos. The intense, hot fires needed to remove forest taxa and keep them from encroaching into fynbos environments on the Cape Peninsula are difficult to achieve in practice.
Acknowledgements This study was funded by UCT's Plant Conservation Unit and the Department of Biological Sciences Cameron Fund. Special thanks go to Stuart Hall, Rick Rohde, Ellen Fedele, Eduard Smit, Darin Taitz, Charmaine Lacock and Jane and Sebastian Wyngaard for field assistance. Doug Euston-Brown, Zishaan Ibrahim, Tom Slingsby and Nick Lindenberg for aerial photographs and GIS data; Katya Mauff and especially Natalie Kunz for help with statistical analysis and The Mountain Club of South Africa, UCT Plant Conservation Unit and Western Cape Archives for use of historical images. The Mazda Wildlife Fund is also thanked for their use of a courtesy vehicle.
Appendix II. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.sajb.2015.05.002.
Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002
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Appendix I Table 1 Relevant information for each of the 50 ground-based photographs analysed. Abbreviations: WCAF = Western Cape Afrotemperate Forest; WCMF = Western Cape Milkwood Forest. All modern photographs taken by Zoë C Poulsen; Photo pair numbers refer to the UCT Plant Conservation Unit's repeat photo collection. Photo pair
Forest type
Date of historical photo
Historical photographer
Date of modern photograph
603 Orange Kloof West 1 604 Orange Kloof West 2 605 Orange Kloof Valley 606 Orange Kloof East 1 607 Orange Kloof South 1 608 Orange Kloof East 2 969 Kirstenbosch 970 Kirstenbosch 971 Kirstenbosch 972 Kirstenbosch 973 Orange Kloof 974 Disa Gorge 975 Disa Gorge 976 Slangolie Ravine 977 Kirstenbosch 978 Kirstenbosch 979 Kirstenbosch 980 Scarborough 981 Kommetjie 982 Kommetjie 983 Olifantsbos 984 Buffelsbaai 985 Gifkommetjie 986 Tafelberg Road 987 Noordhoek Estate 988 Hangberg 989 Hout Bay East 990 Hout Bay Battery 991 Camps Bay Sports Field 992 Fountain Buttress 993 Groote Schuur Old Zoo 994 Hout Bay Drive 995 Llandudno 996 Camps Bay Sports Field 2 997 Camps Bay 998 Twelve Apostles 999 Theresa Drive 1000 Platteklip Gorge 1001 Woodhead Tunnel 1002 Wynberg Caves 1003 Klein Kogelbaai
WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCAF WCMF WCMF WCMF WCMF WCMF WCMF WCAF WCAF WCMF WCMF WCMF WCAF WCAF WCAF WCAF WCMF WCMF WCAF WCAF WCAF WCAF WCAF WCAF WCMF/WCAF
c. 1970 c.1970 c. 1970 c. 1970 c. 1970 c. 1970 c. 1980 c. 1900 c. 1900 c. 1900 c. 1900 c. 1900 c. 1900 1889 c. 1900 c. 1970 c. 1900 1972 c. 1900 1972 c. 1900 c. 1900 1968 c. 1900 c. 1900 c. 1900 c. 1900 c. 1900 c. 1980 c. 1900 c. 1900 c. 1900 c. 1900 c. 1980 c. 1900 c. 1900 c. 1980 c. 1900 c. 1900 c. 1972 c. 1900
Eugene Moll Eugene Moll Eugene Moll Eugene Moll Eugene Moll Eugene Moll Hall Elliot Collection Elliot Collection Elliot Collection Elliot Collection Whitworth Cameron Cairncross/Jurgens Elliot Collection Eugene Moll Elliot Collection CA Collection Green Collection CA Collection Green Collection Green Collection Hugh Taylor AG Collection Wood CA Collection Elliot Collection CA Collection Lambert Cobern Elliot Collection AG Collection Jurgens Lambert Elliot Collection Elliot Collection Hall Cobern Steer Collection Eugene Moll Steer Collection
2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2012 2012 2012 2012 2012 2012
Fig. 1. Mean monthly precipitation rates throughout TMNP (data sourced from the South African Weather Service).
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Please cite this article as: Poulsen, Z.C., Hoffman, M.T., Changes in the distribution of indigenous forest in Table Mountain National Park during the 20th Century, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.05.002