Recruitment and volume increment of selected indigenous tree species in Mabira Forest Reserve, Central Uganda

Abstract

An assessment of recruitment and volume increment of selected indigenous tree species was conducted in Mabira Forest Reserve. The aim was to guide determination of sustainable harvesting quota and harvesting cycles under a polycyclic harvesting system. The specific objectives were to assess recruitment levels of selected indigenous tree species; to determine their growth rates and to determine sustainable harvesting intensities. Data collection was based on individual tree counts and diameter (dbh) measurements carried out in 2001 and 2010 from 10 Permanent Sample Plots (PSPs). A utilizable volume model (V= - 4.4+28g) derived from curvilinear volume and basal area relationship of sample tree height and dbh measurements was used to estimate volume from dbh measurements of trees whose heights had not been measured. The results show a net annual species increment of 3.5%/yr attributed to a higher annual species recruitment level of 7.3%/yr compared to lower annual mortality level of -3.8% /yr. By far, Celtis mildbraedii has the highest annual recruitment levels of 1.7%/yr followed by Funtumia elastica and Trilepisium madagascariense with 1.2%/yr. The lowest is 0.01%/yr recorded for Monodora myristica, Lovoa trichiliodes and Entandrophragma cylindricum. Average growth rate of 0.8 cm/yr (in terms of dbh increment) has been determined from 196 trees representing 64 species. Fast growing species with average growth rate of 1.8cm/yr include Monodora myriastica, Lovoa trichiliodes, Ficus Mucuso, Ficus exasperata, Celtis zenkeri, Albizia zygia, Antiaris toxicaria and Trilepisium madagascariense. Slow growing species with average growth rates of 0.2 cm/yr include Phyllanthus discoideus, Funtumia africana, Lasiodiscus mildbraedii, and Celtis wightii. The Mean Annual Increment (MAI) was 14m3/ha/yr determined from 892 trees. Celtis mildbraedii had the highest annual volume increment (4.32m3/ha/yr) while the lowest (<0.001 m3/ha/yr) was recorded for Harrisonia occidentalis, Tabernaemontana holostii and Parkia filicoidea. Further analysis of volume increment in relation to felling intensity show that volume increment of 0.5-0.35 m3/ha/yr and felling intensities of 14-15 m3/ha are sustainable with an Annual Allowable Cut (AAC) of 9,800 m3/yr and 7,350 m3/yr respectively under polycyclic felling cycles between 30 and 40 years. From such volume increment and harvesting intensities, a harvesting intensity of 14-15 m3/ha and longer felling cycles of up to 75 and 90 years is proposed in order to ensure sustainable harvesting that can enhance restoration of Mabira forest ecosystem under polycyclic harvesting system.