Analysing the rate of land use and land-cover changes in Gambari Forest Reserve, Nigeria

Gambari Forest Reserve is located between latitude 7°10′51”N and longitude 3°52′34”E. Figure 1 shows the natural forest reserve plot at 6.5 km from the Cocoa Research Institute of Nigeria (CRIN), on the Idi-Ayunre-Ijebu-Ode Road, Oyo State.

Figure 1:

Gambari Forest Reserve has one of the most complex vegetation types with the highest number of plant species per unit area. This vegetation is dominated by woody trees to the exclusion of grasses; grasses are shade-intolerant and cease to grow under such tall vegetation, which has a heavy total canopy [19]. The topography of the study area is generally undulating with the presence of lianas, climbers, twiners, stranglers, scramblers, and epiphytes, which makes movement almost impossible. The valuable indigenous species are Terminalia spp K. Shum (Afara), Triplochiton scleroxylon K. Shum (Arere, Obeche), Irvingia garbonensis (Oro) and Treculia africana, among others. Exotic species such as Gmelina arborea (Gmelina) and Tectona grandis (teak) are found in the study area. The study area lies at an altitude between 90 m and 147 m above sea level [20]. The study area is bounded by two rivers (rivers Ona and Awon).

The major dry season occurs between December and March. Temperatures are high throughout the year, with a mean annual value of about 27°C and an annual range of 30°C [18,21].

Landsat images of the study area for three different years with the space span intervals of 20 and 16 years (for 1984, Landsat 5; for 2004, Landsat 7; and for 2020, Landsat 8 OLI) are used for this study, and the orthorectified images for 1984, 2004, and 2020 were downloaded from the United States Geological Survey (USGS), as shown below (Table 1). The software package ArcGIS 10.1 (2013) version was used to visually and digitally process and interpret the Landsat satellite images.

The RS image data of 1984, 2004, and 2020 were radially calibrated and atmospherically corrected. The relative geometric corrections of the three images were conducted to remove geometric distortion caused by the sensor or the Earth’s rotation. Considering the study area and the goal of this study, the land-use types are divided into three categories: dense forest (DF), light forest (LF) and non-forest (NF) (as shown in Table 2). With the help of the maximum likelihood method classifier, classification was carried out on these three images.

The Gambari Forest Reserve area in 1984 recorded more LF than NF. DF covered an area of 31.7 km² (23.4%), while LF covered 79.2 km² (65.4%), and NF covered 14.7 km² (11.2%), as shown below (Table 3). The LULC distribution for the study area in 1984, as derived from the map, showed that LF occupied the largest class, with 65.4% of the total classes, which implies that the forest reserve was still intact or minimally tampered with at the time under study. The Gambari Forest Reserve is a reserved area allocated to the Forestry Research Institute of Nigeria for experimental purposes, which restricted the Gambari Forest from being encroached (Figure 2).

Figure 2:

The static LULC change detection for the Gambari Forest Reserve area in 2004, as derived from the imagery, indicates that there is a significant increase in DF and NF areas, while the area of LF reduced over the 20 years (1984-2004). DF covered an area of 36.6 km² (31.2%), while LF covered 51.2 km² (43.7%), and NF covered 37.8 km² (25.1%), as shown in Table 4.

The area of DF increased from 23.4% in 1984 to 31.2% in 2004, which resulted from re-afforestation (amounts to 7.8%), while the area of LF decreased from 65.4% in 1984 to 43.7% in 2004, which implied a percentage size loss of -21.7% due to anthropogenic activities such as selective logging and legal felling, which led to the reduction of the volume of trees left in the area.

For the NF area, there was also a geometric increase from 11.2% in 1984 to 25.1% in 2004, which implies that a percentage size of 13.9% could be a result of the above stated anthropogenic activities such as major felling of trees without replanting (Figure 3).

Figure 3:

The Gambari Forest Reserve area in 2020 recorded more DF than LF. DF covered an area of 72.8 km² (54.4%), while LF covered 51.2 km²(41.5%), and NF covered 2.6 km² (4.1%).

An assessment of the classification data in 2020 imagery reveals that there has been a transition in the status of the forest reserve in the span of 36 years, which indicates that the DF area increased from 23.4% in 1984 to 54.4% in 2020, which implies a percentage change of 31.0%, due to favourable environmental factors such as climatic weather, decomposition, and conservation.

For the LF area, it was established that there was a decrease from 41.5% in 2020 to 65.4% in 1984, with a percentage variation of -23.9%, which is a decrease from what was classified in 1984. This could be as a result of poor management of the experimental plots, illegal fellers, and climate variability.

Furthermore, the NF status stands at 4.1% in 2020, which is also a decrease compared to 11.2% in 1984, with a percentage variation of −7.1% due to increase in sizes of both the DF and LF, respectively, which increased their LU over time.

The LULC distribution for the study area in 2020, as derived from the map, shows that DF occupied the largest class, with 54.4% of the total classes due to anthropogenic activities that occurred to the other classes (Table 5, Figure 4).

Figure 4:

The changes outlined in Table 6 occurred over the past 20 years in the Gambari Forest Reserve area from 1984 to 2004, where changes of the land covers are presented. LF declined in percentage size drastically by −21.7%, NF increased drastically by 13.9%, while DF also increased by 7.8%.

The changes mentioned below occurred over the past 16 years in the study area during 2004–2020. As shown in Table 7, representing 2004 to 2020 satellite images, changes of the land cover during this period show that DF changed by 23.20%, LF changed by – 2.20%, and NF changed by – 21.00%.

The overall trend of thr e36-year change that occurred between 1984 and 2020—as shown in Table 8—indicates that the DF area positively increased in percentage change by 31.00%, from 31.7 km² (23.4%) to 72.8 km² (54.4%), which could be as a result of environmental factors such as climatic weather, decomposition, and conservation. The LF area decreased in percentage change by −23.9% from 79.2 km² (65.4%) to 51.2 km² (41.5%). And finally, the NF area also decreased in percentage change by −7.1% from 14.7 km² (11.2%) to 2.6 km² (4.1%). These above declines in both the LF and NF areas were attributed to anthropogenic activities such as degradation, poor management of the experimental plots, and legal fellers.