Extent, rate of invasion and conditions favouring occupancy and dominance of typha latifolia in selected wetlands of Harare, Zimbabwe.

Abstract

Typha latifolia is a global aggressive invader of wetlands. It is now one of the major threats to wetland ecosystems in Harare. Factors that promote its establishment and dominance are still poorly understood. The present study aimed at determining conditions that favour T. latifolia establishment and dominance in selected wetland sites of Harare. It also sought to establish the species present distribution and predict its future areal extent in the city’s wetland sites. Google Earth satellite imagery was used to map Harare wetland sites. Rate of aerial expansion of T. latifolia occupied sites was determined from aerial images acquired from Google Earth dating back some 10 years (2008-2018). MaxEnt software (version 3.3.3k) was used to predict future distribution of T. latifolia in Harare. A comparison of historical and current images showed that there has been significant change in T. Latifolia patch sizes within the largest three wetland sites, namely Prospect (176.3 ha), Monavale (159.1 ha) and Tynwald (655.5 ha). Results indicated a marked increase in the spread of T. latifolia within Monavale wetland from 2008 to 2018. The area covered by T. Latifolia increased from 0.12 ha to 1.83 ha during this period, indicating a 93% increase. Area occupied by the species in Prospect wetland expanded from 1.53 ha in 2011 to 5.40 ha in 2018, indicating a 72% increase in areal distribution. That of Tynwald wetland increased from 0.59 ha in 2008 to 4.49 ha in 2018, indicating an 87% increase. The MaxEnt species distribution model predicted a future spatial distribution of T. latifoliain areas with suitability greater than 0.38 in proximity to water course. Soils from the twelve wetlands differed with respect to pH, but had similar moisture content, phosphorus, nitrogen and carbon. To establish the relationship between plant morphological and edaphic characteristics, a multi-linear regression analysis was carried out. The results showed significant relationships between nitrogen and shoot height. Significant differences in mean nitrogen concentration within vegetative parts of T.latifolia collected from the twelve sites during the study period were observed. No significant differences in vegetative parts of T.latifolia in terms of phosphorus and carbon contents were observed from the twelve wetland sites.The study identified soil carbon and nitrogen to be significant environmental variables that explained nutrient concentrations in vegetative parts. It is concluded that T.latifolia is indeed invading most of the wetlands of Harare, and major contributing variables to T.latifolia invasion were found to be pH, soil moisture content, and soil nitrogen and carbon. Water availability is critical in facilitating T. latifolia encroachment. This implies that other that high nutrient levels, the expansion of Typha stands in Harare wetland areas will largely be governed by moisture availability. It can be concluded that the primary drivers of invasion are high soil nutrient status and moisture. Hence, management of this noxious species must focus on controlling nutrient loading and managing the hydrologic dynamics of the wetlands.