Resilience of forest trees to debarking by elephants in Rabongo Forest, Murchison Falls National Park - Uganda.

Abstract

The main purpose of the study was to ascertain tree stem abundance, composition, tree species diversity, elephant damage, and the resilience mechanisms employed by the damaged trees over the last 20 years since 1992. Data were collected by re-measurement of the seven (7) one hectare Permanent Sample Plots established by Douglas Sheil in 1992 as baseline data. These plots were first re-measured 10 years later by Simon Nampindo in 2001. The present study has been conducted after another ten years bringing the total period under observation to 20 years (1992-2011). Enumeration of seedlings, saplings and poles and measurement of diameters at breast height (dbh) at 1.3 m for normal stems and diameters at point of measure (dPoM) for all stems ≥10 cm dbh was done from quadrats established at 20 m*20 m intervals. The study was carried out in Rabongo Forest located 1o 57' to 2o 35' N and 31o 22' to 32o 08' E in the Southeastern part of Murchison Falls National Park and Northwest of Budongo Central Forest Reserve. Rabongo Forest is an important habitat for most large herbivores such as elephants, buffalos and primates.

The results indicated an increase in density of tree stems ≥ 10 cm dbh from 2474 to 3206 (1992-2011) respectively. Recruitment rate (2.37%yr-1) was higher than mortality rate (1.53%yr-1) while regeneration potential was quite high at 35,715 seedlings ha-1. The forest was species diverse with H1 =2.65 in 2011and H1 =2.85 for 2001 (X2 = 6.082, df = 6, p = 0.414) indicating a significant difference between the two measurements and p = 0.09 for Budongo RP15. The data supported H01 that there is no difference in tree species abundance and diversity of 2001 and 2011 measurements in Rabongo forest and Budongo’s RP7. Number of damaged trees ≥ 10cm dbh reduced from 470 stems in 2001 to 132 in 2011 representing a 4% decline. Diospyros abyssinica, Holoptelea grandis, Trichilea prieuriana and Cynometra alexandri were the most damaged tree species. Damaged trees exhibited various resilience mechanisms such as scar healing, coppicing, other forest ecosystem resilience mechanism such as regeneration causing forest expansion from forest colonization. Therefore, elephant damaged trees were resilient enough and elephant activities did not alter stem abundance, community composition and tree species diversity of Rabongo forest. Tree species composition and structure of Rabongo forest is, therefore not likely to change due to elephant activities over time. However, there is need to continue monitoring the threats caused to the forest by elephants to see how this relates to the general forest ecosystem.