13:45 〜 15:15
[HGG01-P09] ケニア中央部、半乾燥地帯における過去20年間の侵略的外来植物種の分布変化
キーワード：侵略的外来植物種、地形プロセス、人為的地表攪乱、ケニア、リモートセンシング
In recent years, invasive alien plant species (IAPS), such as Opuntia spp., Ipomoea spp. and Prosopis spp., have been growing widely in the arid and semi-arid regions of East Africa and other similar environments in the world. The distribution and expansion of Opuntia stricta in particular is remarkable. The expansion of IAPS is considered a serious environmental problem in numerous countries, including Kenya, due to the destruction of native vegetation in pastoral areas and direct stinging injuries to humans and livestock. The invasion of O. stricta in semi-arid areas of Laikipia County, central Kenya, has been discussed by Strum et al. (2015), and Muthoka et al. (2021) mapped O. stricta with high accuracy. We also attempted to understand the distribution of IAPS and estimate its expansion process through satellite image analysis (Otsuki et al., 2022, JpGU). The aim of this presentation is to provide a review of the process of improvement and updating of those results and the relationship with environmental conditions, such as geomorphic processes and anthropogenic ground disturbance.
The analysis area is located at 0.28–0.55°N, 37.00–37.20°E (approximately 30 km north-south and 23 km east-west) in the northeast of the Laikipia County. Several vegetation indices were compared, and based on these results and the consideration that O. stricta in particular was highly detectable (Ouko et al., 2020) the EVI was adopted as the main index in this study. To account for differences in seasonal plant activity of plant species, including IAPS, we compared EVI values in September and February, corresponding to the latter part of the biannual dry season, respectively, from year to year. Satellite images used were Sentinel-2 MSI (Level-1C) from 2016 onwards and Landsat 8/7 ETM (C02/T1_L2) from 2000 onwards. From the field surveys, we selected locatable O. stricta growth sites, and obtained EVI values on the both images in February and September 2021-2018. Antecedent rainfall is thought to have a significant influence on the variability of each index, including EVI, and we have reported that rainfall two months before the image date, especially rainfall one month before, has a high correlation with the vegetation index (Nirei & Otsuki, 2022, JpGU), and we focused on these points in our study. For precipitation, the IMERG estimated precipitation data (from June 2000) were used. However, since this estimated rainfall has a large deviation from the actual rainfall, we calculated the actual rainfall based on our 2019-2020 observed field rainfall.
Based on the trends in IAPS pixel (including O. stricta site) rates within the analysis area obtained from these procedures, the IAPS invasion process from around 2000 to present can be considered as follows. O. stricta appears to spread from areas of relatively high soil moisture in the vicinity of the river channel to interfluves. Although there is no perennial streamflow in the river channel in this area, it can be assumed that a relatively suitable growing environment for plants in general, including IAPS, will continue to exist close to the river channel because the shallow groundwater level in the river channel is observed in the field even during the dry season, except at the base rock exposure points. Seed dispersal of O. stricta also occurs during intermittent surface flow, and growth is more enhanced near the river channel. In contrast, the interfluves are gentle slopes corresponding to the pediment above the present gully, where sheet erosion is predominant. Although the groundwater table is at least several meters below the surface, the relatively deep root system of IAPS is advantageous even in drought years.
As previously reported (e.g. Otsuki et al., 2021, JpGU), sand extraction (sand harvesting) by non-local contractors has become more active in this area in response to the growing construction demand in urban areas in Kenya, and in response, local people have implemented resource management based on "group ranch". As a result, the amount of cash invested in this area has increased in recent years. The recent expansion of pioneering IAPS distribution is one of the important events to note in this area, in consideration of the anthropogenic conversion to bare land by sand extraction in addition to continued soil erosion. Furthermore, the above, combined with the decrease in vegetation and increase in bare land in the past decades near Il Polei (Nirei & Otsuki, 2022), may put more pressure on the livelihood of local pastoralists and even advance livelihood conversion. Further research is necessary, including the clarification of social structural changes in the area.
The analysis area is located at 0.28–0.55°N, 37.00–37.20°E (approximately 30 km north-south and 23 km east-west) in the northeast of the Laikipia County. Several vegetation indices were compared, and based on these results and the consideration that O. stricta in particular was highly detectable (Ouko et al., 2020) the EVI was adopted as the main index in this study. To account for differences in seasonal plant activity of plant species, including IAPS, we compared EVI values in September and February, corresponding to the latter part of the biannual dry season, respectively, from year to year. Satellite images used were Sentinel-2 MSI (Level-1C) from 2016 onwards and Landsat 8/7 ETM (C02/T1_L2) from 2000 onwards. From the field surveys, we selected locatable O. stricta growth sites, and obtained EVI values on the both images in February and September 2021-2018. Antecedent rainfall is thought to have a significant influence on the variability of each index, including EVI, and we have reported that rainfall two months before the image date, especially rainfall one month before, has a high correlation with the vegetation index (Nirei & Otsuki, 2022, JpGU), and we focused on these points in our study. For precipitation, the IMERG estimated precipitation data (from June 2000) were used. However, since this estimated rainfall has a large deviation from the actual rainfall, we calculated the actual rainfall based on our 2019-2020 observed field rainfall.
Based on the trends in IAPS pixel (including O. stricta site) rates within the analysis area obtained from these procedures, the IAPS invasion process from around 2000 to present can be considered as follows. O. stricta appears to spread from areas of relatively high soil moisture in the vicinity of the river channel to interfluves. Although there is no perennial streamflow in the river channel in this area, it can be assumed that a relatively suitable growing environment for plants in general, including IAPS, will continue to exist close to the river channel because the shallow groundwater level in the river channel is observed in the field even during the dry season, except at the base rock exposure points. Seed dispersal of O. stricta also occurs during intermittent surface flow, and growth is more enhanced near the river channel. In contrast, the interfluves are gentle slopes corresponding to the pediment above the present gully, where sheet erosion is predominant. Although the groundwater table is at least several meters below the surface, the relatively deep root system of IAPS is advantageous even in drought years.
As previously reported (e.g. Otsuki et al., 2021, JpGU), sand extraction (sand harvesting) by non-local contractors has become more active in this area in response to the growing construction demand in urban areas in Kenya, and in response, local people have implemented resource management based on "group ranch". As a result, the amount of cash invested in this area has increased in recent years. The recent expansion of pioneering IAPS distribution is one of the important events to note in this area, in consideration of the anthropogenic conversion to bare land by sand extraction in addition to continued soil erosion. Furthermore, the above, combined with the decrease in vegetation and increase in bare land in the past decades near Il Polei (Nirei & Otsuki, 2022), may put more pressure on the livelihood of local pastoralists and even advance livelihood conversion. Further research is necessary, including the clarification of social structural changes in the area.