Tree Stock, Structure and Use of Common Woody Species of a Town Neighboring Forest Reserve in Tanzania: Implication for Managing Carbon Accumulation

3.1. Floristic composition and population structure of the common woody species

A total of 102 woody species were recorded in the reserve and were represented by 30 families as presented in Table 1. The most dominant family was Fabaceae with 31 species, whereas several other families were represented by only one species. Of the 102 woody species recorded, 78 were trees while 24 were shrubs. The first 10 common woody species determined using the IVI classification procedure were Acacia nilotica, Brachystegia boehmii, Brachystegia microphylla Combretum molle, Dalbergia boehmii, Diplorhynchus condylocarpon, Julbernardia globiflora, Lannea welwitschii, Pteleopsis myrtifolia and Pterocarpus angolensis. The relative density of these common woody species for stems with dbh > 4 cm is presented in Figure 2, whereby, D. condylocarpon had the highest relative density of 50 stem/ha, while L. welwitschii had the least density that is,13 stem/ha.

The population structure of the common woody species in NFR was assessed based on dbh class size distribution, mean crown diameter and area, and density distribution of seedlings, saplings and mature trees. The dbhs of the common woody species recorded during the study are presented in Figure 3. All the 10 representative woody species had more individuals in their first classes with an abrupt drop beginning in the following class indicating the characteristic inversed J-shapes. However, except for C. molle of which its graph had a smooth inversed J-shape, the graphs for the rest of the species seemed interrupted. This is an indication of selective removal of individuals, especially those at pole size stages and the mature ones. The Morogoro region in which NFR is located has a history of human exploitation of tree species. For example, Wells and Wall [39], reported that, between 1970 and 1980 Morogoro Region was the source of much of the hardwood timber going to the Dar es Salaam market, but from early 1990s, the Morogoro region was exhausted of valued timber species such as P. angolensis and the production shifted to Tabora region, and even further west to Rukwa Region by late 1990s. Therefore, this denotes that, NFR being in Morogoro region, its quality timber trees has long been extracted, which is also evidenced by small dbhs in almost all the common tree species Figure 3. Moreover, the trend in dbh classification of the 10 common woody species (Figure 3) indicates that the graphs of some species such as B. microphylla are almost flat for individuals from the sapling stage. This is not a health state for the NFR since it is evident that trees are harvested in all stages from sapling to mature trees. Newton [40] pointed out that, when there is a very few trees recorded in lower dbh classes, extracting of large sized trees is detrimental to the future populations. Therefore, if unsustainable harvest continues in NFR, it is likely that the knock-off effect of the forest valuable trees will occur within a short span of time.

Figure 4 shows that of the common woody species B. microphylla had the biggest mean canopy diameter while that of C. molle was the smallest. Moreover, the mean canopy area was also the biggest for B. microphylla and the smallest for C. molle as shown in Table 2. The bigger crown diameter and area of B. microphylla as well as its higher density are indication that the species is potential especially in influencing the ecosystem functioning of the reserve. Thus, B. microphylla can be regarded as a foundation species for the NFR. By definition, a foundation species is “a single species that defines much of the structure of a community by creating locally stable conditions for other species and by modulating and stabilizing fundamental ecosystem processes” [41, 42]. These kinds of species with greater influence on ecosystem usually possess greater repercussions on ecosystem functioning in case they disappear [43]. Therefore, for NFR B. microphylla can be regarded as a candidate foundation species, and in case it disappears there could be far reaching consequences [43]. Probably the species we see today in NFR especially the shade tolerant ones are a masterpiece of B. microphylla. Thus, if B. microphylla vanishes from NFR the species composition is likely to change with cascading effects on carbon accumulation potential of the reserve.

The classification of stems into seedlings, saplings and mature trees showed the least number of seedlings/ha for the highly valued timber species P. angolensis Figure 5. The 24 seedlings/ha of P. angolensis were far less compared to that of C. mole and D. condylocarpon (88/ha) respectively. This could be an indication that there are difficulties in seedling germination in NFR for this species. Probably fires, damages from tree felling activities and rolling logs, or all of these combined are hampering the regeneration of the species. Other common woody species namely B. boehmii, B. microphylla, J. globiflora and P. myrtifolia had a bit higher number of seedlings but the saplings seemed to suffer a high mortality. This could be an indication that the saplings are removed to cater for different purposes such as house construction or instrument making [10].

3.2. Tree conservation priority

The tree conservation priority was assessed based on IVI classes and regeneration status (Table 3). Categorization of the common woody species into conservation priority classes based on IVI criteria placed L. welwitschii, D. boehmii, P. angolensis and A. nilotica in class 4. This means that these species have insufficient stock and are recommended for conservation priority, for example, Zegeye et al. [44], Kacholi [45]. The IVI criterion is used in determining the conservation priority of species whereby those with low IVI values are considered of main concern for conservation [44]. Therefore, the management of NFR should place special consideration in protecting A. nilotica, D. boehmii, L. welwitschii and P. angolensis against anthropogenic pressures for their persistence. On the other hand, the remainder of the common woody species recorded in NFR, that is, J. globiflora, B. boehmii, D. condylocarpon, C. molle, P. myrtifolia and B. microphylla are of less conservation concern since they fall under a higher IVI class value. However, the management of NFR is not advised to relax as protection of every single species in the reserve is paramount. As pointed out earlier, B. microphylla has been categorized by this study as a foundation species, thus, despite the fact that this species appears common, it still deserves special consideration for protection in respect of its ecological implication in case it disappears.

Moreover, classification of the 10 common woody species into regeneration status placed the highly valued timber species P. angolensis in class 2 denoting that the species had insufficient number of seedlings and saplings (Table 4). This suggests that either the species recruitment is insufficient or the seedlings and perhaps the saplings suffer high mortality before making it to maturity, the phenomena which are likely to jeopardize its long-term persistence in the reserve. As pointed out earlier, P. angolensis had the least number of seedlings observed among the 10 common woody species and this is attributed to a collective number of factors including disturbances brought about by tree felling activities, or fires. Omeja et al. [34] claims that human disturbance influence seed dispersal mechanisms, fruiting, germination and regeneration of tree species. Thus, disturbance by activities involved during charcoal burning (see Plate 1 a and b) perhaps contributes in killing young seedlings as logging was observed almost everywhere in the reserve especially on lower altitudes. Similarly, van Daalen [46], pointed out that very high intense fire reduces the viability of seeds of P. angolensis limiting their germination potential. Thus, the intense fires that the NFR faces (Personal Communication) could be killing more seeds of P. angolensis, allowing only a few to germinate.

3.3. Exploitation and spatial distribution of the common woody species

Harvesting rate of the common woody species was evaluated based on basal area/ha of the cut trees. During this study, some stumps could not be identified simply because most of them had already lost identification evidences such as barks and had no sprouts. Of the 10 common woody species, stumps were recorded for B. microphylla, C. molle, J. globiflora, P. angolensis and P. myrtifolia (Table 5). From this perspective, the total basal area for the harvested species seemed to be small (0.32 m/ha). Of the common woody species found harvested, J. globiflora seemed to be the highly preferred species accounting for 0.14 m²/ha followed by P. angolensis with 0.10m²/ha, while B. microphylla was the least harvested. The fact that this study has considered B. microphylla as a foundation species following its bigger canopy area, the removal of this species though at a smaller scale triggers alarm. As pointed out earlier, this species has a high chance of influencing other species especially in suppressing shade intolerant species. Therefore, if harvest continues for B. microphylla, there is a high possibility of seeds for other species that were being suppressed to start emerging and this could change the species composition and ultimately the ecosystem functioning and carbon accumulation potential of the area. Some previous studies pointed out that increasing ground illumination through felling bigger canopy trees has many consequences including allowing colonization of new plant species and development of new communities such as grasses [9, 20], thus impairing the ecological succession and carbon accumulation potential of the affected area [20]. Therefore, exploitation of trees in NFR especially among key species needs to be monitored by the responsible authorities for stability of ecosystem processes in the reserve.

One of the important questions under the current study concerned the distribution of valued timber species in NFR. This was so in order to elucidate the probable influence of the surrounding community on the reserve’s valued biological resources. To understand this, we plotted the spatial distribution of the highly valued timber species P. angolensis on the reserve’s map as shown in Figure 6. From the map, it was apparent that quadrats into which P. angolensis were recorded concentrated more in the higher altitudes of the reserve to the interior. We did not record P. angolensis from the northern, western or southern parts of the reserve. Moreover, the range at which P. angolensis occurred in NFR was between 600 to 1240 m a.s.l. The absence of P. angolensis especially in the northern, western and southern parts of the reserve perhaps was a result of selective logging, fires or edaphic factors. The eastern part of NFR consists of steep slopes and difficult terrain. This could be a reason why mature P. angolensis were recorded just close to the boundary of the eastern part of the reserve—perhaps tree loggers might be experiencing difficulties in accessing this area. According to Lovett et al. [26], the edges of NFR face fire incidences, deforestation for charcoal making and tree cutting for timber sawing. The villagers at Mkundi and Lukobe also disclosed that people prefer poaching trees for charcoal making and timber just a few hundreds of meters from the edges of the reserve in fear of the forest officers who usually conduct ambush patrols (Personal Communication). Therefore, tree cutting and frequent fires in the edge of NFR are probably the root of mortality and poor re-establishment of P. angolensis in the northern, western and southern parts of the reserve. Although P. angolensis is considered to be resistant to fires, heavy and/or frequent fires damage seedlings and prevent them passing to sapling stages [47].