Browsing by Subject "multitemporal"

Tropical montane forests are important environmental factors globally as they preserve biodiversity, carbon, and moisture. That is why it is necessary to have knowledge on the distribution and condition of these forests. Acknowledging what kind of changes happen and in what timescale, assist in forest management and planning. Remote sensing based change detection is one of the ways of investigating these changes. In this study change detection is conducted with multitemporal airborne laser scanning (ALS) data from the years 2014/2015 and 2022. ALS produces three-dimensional point data which can be further processed into different elevation models and canopy height model (CHM). This study focuses on observing changes in tropical montane forests in the Taita Hills, in Kenya. The study area included two peaks, Yale and Ngangao, which are part of the Eastern Arc Mountains, which is a bigger mountain chain stretching from Tanzania to Kenya. Both forest areas include native montane and exotic vegetation. Both positive and negative canopy height changes (i.e., tree height growth and tree loss) between these different forest segments were studied. The forest segmentation used in this study is based on earlier mapping in the area, but it was updated with the help of field observations, orthophotos acquired in January 2022 and CHM. In addition, the effects of point density on canopy height metrics were studied. The results indicate that overall trend in the forests has been positive height change. The canopy height changes show that tree growth in the forests differ between the forest segments. Areas with exotic tree species grow faster than the areas with native montane vegetation. When comparing the different tree species, eucalyptus seems to grow fastest, followed by pine and then cypress. Furthermore, some spatial differences were also noted, as similar forest types had grown more in the southern parts of Ngangao forest. Also, negative changes were observed in the data and treefall gaps were identified from the CHM. Results indicate that there is no distinctive pattern between the different forest segments and tree species in treefall. Falling of a tree can be a result of many things. It can be done purposely, or it may happen due to natural causes. Inspections of the effects of point density proved that the attributes do affect canopy height metrics derived from the data. Higher point density differences between the data resulted in larger difference values in the canopy height difference models. In accordance with other studies, it could then be concluded that high point densities create overestimations and lower point densities create underestimations of vegetation heights. Point density differences are one of the issues that should be considered when working with multitemporal data. In addition, variations in the data acquisition create uncertainties to accurate comparison between the data. This thesis provides valuable information about the changes happening in tropical montane forests in the Taita Hills. As the results demonstrate that different forest types have different growth speeds, the information can be further applied in practises that recognise forest type segments. This is crucial in determining montane forest segments. These results are suitable for further analysis and research. When considering the environmental effects tropical montane forests have and how their changes effect the local and global climate, it is useful to know how different species grow and survive in different environments. As the results show, eucalyptus seems to thrive in the area but the effects of exotic species to the biodiversity should be noted as well. In the case of eucalyptus, it uses a lot of water resources to grow, and the undergrowth might not be as rich as in native montane tree species areas.