Browsing by Author "Mgimba, Christopher"

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Ecological Dynamics Of Helichrysum (Asteraceae) Shrubs Cover And Their Effect On Orchids In Kitulo National Park, Tanzania
(NM-AIST, 2025-08) Mgimba, Christopher
Grassland ecosystems are undergoing major ecological transformations due to anthropogenic disturbances. In protected areas such as Kitulo National Park (KNP) in Tanzania, the spread of dwarf Helichrysum shrubs has potential impact on native herbaceous plant species like orchids. While Helichrysum species are known to thrive in degraded, low-nutrient soils, there is limited empirical evidence on the mechanisms driving their spread and the ecological consequences of their encroachment in tropical montane grasslands. This study aimed to assess the drivers of Helichrysum expansion and their ecological interaction with orchids. Fieldwork was conducted across multiple zones within KNP, categorized based on their historical land use as ‘grazed crop’, ‘grazed’, and ‘wild’. A total of 222 quadrats were established to assess the abundance and diversity of Helichrysum and orchid species. Soil samples were analyzed to determine nutrient composition, and controlled experiments were conducted to evaluate Helichrysum regeneration under different fire treatments (unburned, early burned, late burned) and disturbance regimes (cutting stems, uprooting). Additionally, germination trials were carried out in the laboratory to test the effect of fire-simulated temperatures on Helichrysum splendidum seed germination. The orchid species' responses were evaluated in terms of relative interaction intensity (RII), physiological vigor, and flowering metrics under varying Helichrysum cover levels. Results revealed that Helichrysum splendidum was the most dominant shrub species, with significantly higher abundance in historically disturbed zones; the 73% and 46% more in ‘grazed crop’ and ‘grazed’ areas, respectively, than in the ‘wild’ zone. Orchid richness and abundance significantly declined with increasing Helichrysum cover. Fire played a dual role while complete burning induced vigorous Helichrysum resprouting, early burns significantly suppressed seedling recruitment and reduced seed bank density. Temperature treatments confirmed that seeds exposed to 80°C and 110°C had significantly lower germination than unheated and those heated at 50°C (p < 0.001), suggesting fire-inhibits germination of Helichrysum seed. This study concludes that the spread of Helichrysum shrubs in Kitulo National Park is largely driven by legacy effects of past land use and soil nutrient depletion. High shrub cover negatively affects orchid diversity and species richness. Management strategies that include burning early in the season and physically removing shrubs can reduce the spread of Helichrysum and support the protection of native orchid species and the variety of plants in montane grasslands.
Effect of fire timing on the regeneration capacity of Helichrysum species in Tanzania
(Ecol Solut Evid., 2025) Mgimba, Christopher; Ngondya, Issakwisa; Treydte, Anna
Shrub encroachment poses a significant challenge in many grassland ecosystems, particularly where disturbances such as heavy livestock grazing and uncontrolled fires are prevalent. Fire is frequently employed as a management tool to mitigate shrub encroachment and preserve native vegetation, as it influences both resprouting and seed germination. However, few studies have examined the effects of fire timing on resprouting and seed germination both in the field and ex situ. We investigated the resprouting ability of Helichrysum shrubs after being completely, partially or not at all burned. In addition, we collected soil seed bank samples from each early burned, late burned and unburned area at depths of 0–2, 2–4 and 4–6 cm to assess plant species recruitment via seedlings. The data were analysed using a generalized linear regression model with a Gaussian link function. We found significant differences in the number of resprouts and seedling abundance across early burned, late burned and unburned sites. Resprouting of both partially burned and completely burned shrubs was more abundant compared to unburned shrubs. The surface soil in unburned sites contained a greater abundance of Helichrysum seedlings compared to that of burned sites, suggesting that fire negatively affects seed germination, particularly in the upper soil layers. Overall seedling recruitment was lower in early and late burned areas, whereas resprouting was higher in areas burned later in the season. Practical implication. The findings highlight the significance of fire timing in grassland management. We advocate for the implementation of planned early burning to mitigate shrub encroachment, promote biodiversity and save ecologically sensitive grasslands.
The effect of Helichrysum shrub encroachment on orchids in a tropical, montane grassland ecosystem, Tanzania
(CSIRO PUBLISHING, 2024-11-08) Mgimba, Christopher; Ngondya, Issakwisa; Treydte, Anna
Context Although shrub encroachment is a common phenomenon in grasslands, which often suppress co-existing herbaceous plants, little is known about how encroaching native shrubs affect endemic and threatened orchid species. Aims We assessed the effect of the native dwarf shrub Helichrysum species on orchid species in a protected mountainous grassland system in Tanzania. Methods We selected five Helichrysum shrub-dominated blocks and applied four treatments in each, i.e. no or low encroachment (<20% Helichrysum cover; ‘low cover’), high encroachment (>50% Helichrysum cover; ‘high cover’), cutting all stems of Helichrysum shrubs to ground level (‘stem cut’) and removing both stems and roots of all Helichrysum shrubs (‘uprooted’). We then compared orchid species diversity, abundance and functional traits by using a mixed linear model across treatments. Key results Orchid species diversity and abundance were significantly lower in ‘high cover’ plots than in other treatments. In ‘high cover’ plots, orchid species such as Disa robusta, Satyrium acutirostrum, and S. sphaeranthum had a significantly lower chlorophyll content than they did in ‘low cover’ plots. The ‘uprooting’ treatment showed significantly higher orchid species diversity in the second field season. Conclusion The expansion of Helichrysum shrubs adversely affected orchid abundance, diversity, and individual vigour, which in turn affected the regenerative ability of orchids. Implications We suggest that management should focus on shrub removal, because only ‘cutting’ had a beneficial effect on orchids. Shrub removal should be focused on areas of high shrub cover to promote further orchid species growth in this mountainous grassland of Tanzania.
The interplay between historical land-use and the distribution of Helichrysum shrubs in an African-protected grassland
(Wiley, 2024-05-01) Mgimba, Christopher; Smith, Stuart; Ngondya, Issakwisa; Treydte, Anna
Human land use can have lasting impacts on landscape characteristic, yet there remains a lack of information on how former land use affects plant communities in protected African grasslands. In this study, we investigated how land uses prior to the creation of Kitulo National Park, Tanzania, shaped the presence and abundance of the native shrub, Helichrysum species. We evaluated both plant species composition and soil properties across the park by dividing our sample into three different zones of historical land use based on participatory mapping. We divided the park into three former land uses: (1) livestock grazed and cultivated; (2) grazed only and (3) wild grazing with limited human impact. We observed that former grazed cultivated land use had five times higher Helichrysum abundance than former ‘wild’ land use. Soil pH, magnesium and phosphorus levels varied significantly across zones of historical land use but not between sites with and without Helichrysum species. Helichrysum splendidum was more abundant in soils with low soil phosphorus and magnesium concentrations. Our study demonstrates that historic grazing and cropping land uses through changes in soil nutrient properties can explain current Helichrysum species spread in protected areas. As such, conservation management plans would benefit from integrating mapping of former land uses to target interventions for problematic encroaching shrubs.