Effects of ungulate Herbivory and tree canopy on Ecosystem processes in Ruma National park, Kenya

Abstract

Herbivores and tree canopies may have significant influences on savanna ecosystem functioning that are not yet clearly understood. Positive influences of grazers and trees may be associated with fecal deposition and the ability to fix atmospheric nitrogen respectively. On the other hand, grazers could negatively influence savanna functioning through trampling and vegetation removal, while negative influence of trees could be through shading, which reduces the amount of photosynthetic photon flux density ~ (PPFD) reaching_ the understory. Field studies were carried out in a humid tropical savanna in Kuma National Park, Kenya to understand how grazing by domestic animals and the Acacia trees influence ecosystem processes in the herbaceous layer community. The objectives of the study were; to monitor microclimate and soil moisture during an annual growth period, to measure the amount of biomass and nitrogen concentration in the soil and herbaceous layer vegetation during an annual growth period, to measure net ecosystem CO2 exchange (NEE) and ecosystem respiration (Reco) during the vegetative period and to determine how trees and grazing influence soil moisture, N distribution, biomass development and ecosystem CO2 exchange. Soil moisture was measured through gravimetiic method, plant and soil nutrients were monitored through elementary analysis, biomass was measured using harvest method while CO2 exchange was determined through gas chamber method. Measurements were stratified so that locations under the trees (understory) and in the open, outside the tree canopies were monitored separately to understand the interactive role of trees. Significant (psO.OOI) differences in peak aboveground+ biomass at the different locations was recorded with highest peak aboveground biomass of 902.7±115~ g m-2 Jecorded in non grazed _plots. Trees had significant Influence on soil moisture content as well as biomass production in both - grazed and non-grazed plots. Plants in the non grazed plots exhibited significantly (psO.OOI) higher N content compared to those in the grazed plots and similar trends were also .observed in the soil N content. Soil moisture, soil N, biomass production and ecosystem CO2 fluxes exhibited seasonality. Highest N concentration both in the plant material and in the soil occurred during the rainy period and higher N concentrations were observed in the tree understory compared to the open locations outside tree canopies. Significant seasonal changes were observed for NEE, Reco and GPP with Net ecosystem CO2 exchange increasing during the rainy season up to l Sumol m-2 S-I. Highest Reco values occurred during the rainy season, with values of 2004 ±O.5 and 23.5 ±O.5 umol m-2 S-1 in grazed and non grazed plots respectively. The study showed that grazing affects soil moisture and soil nutrient content and may significantly influence the savanna ecosystem functioning. This influence is however, modified by the Acacia trees through improved soil N content and reduced PPFD in the understory. The observed trends create a unique production mosaic (understory and open locations) of ecosystem function and productivity in the humid savanna. Humid tropical savanna ecosystem can therefore be a significant source and sink of both Nand C with processes that control their emissions being complex and influenced by a variety of interrelated factors such as quality and rates of organic matter input to the soil, soil chemical status, soil moisture status and soil temperature with herbivory acting as modifiers. These findings indicate that plant dynamics in humid tropical savannas are not a simple function of rainfall patterns or herbivory, but regulated by interactive effects of rainfall, grazing pressure and soil nutrients.