Sensitivity of vegetation to annual rainfall variations over Nigeria

Abstract

Distribution and composition of vegetation are strongly controlled by climatic conditions. Of the climatic elements, precipitation has been found to be a major influence on the abundance, health and productivity of vegetation. This study investigated the spatial and temporal pattern in the sensitivity of vegetation to rainfall in Nigeria between 1980 and 2009. Rainfall data collected from 23 observation points were analysed and spatially correlated with monthly Normalized Difference Vegetation Index (NDVI) values for the same period to estimate the responsiveness of vegetation to annual variations in rainfall. The results revealed a non-uniform pattern in the sensitivity of vegetation to rainfall across the country. Areas in the forest and guinea savannah zones with higher mean annual rainfall exhibited stronger correlation, while the Sudan savannah zone exhibited weaker correlations. However, further northwards in the Sahel fringes, the correlations between rainfall and vegetation were found to be very negligible (mostly zero), except for the forested wetland around the Lake Chad where stronger positive coefficient of correlation were found. The findings have implications for ecological studies, agricultural planning and other socio-economic activities, which are directly concerned with vegetation and vegetal cover. This knowledge could be put to use to ensure better planning and decisions that concern the environment.

Introduction

One of the fundamental assumptions in plant ecology is that, on local to global scales, the distribution and composition of vegetation is strongly controlled by climatic conditions through the key processes of photosynthesis, respiration and transpiration (Gritti et al., 2010). This has been found to be true as various studies on the relationship between vegetation and climatic elements have revealed (Zeng et al., 1999, Zhang et al., 2007, Raynolds et al., 2008, Sun and Kafatos, 2007, Klein and Roehrig, 2006, Davenport and Nicholson, 1993). Vegetation over a geographical area is essentially a response to the climate in that area. There have always been a strong link between both and it is virtually impossible to separate climate from vegetation because even from the earlier times, climates were named after their characteristic vegetation (Oliver, 2005). This close link is also reflected even in the west-to-east demarcation of vegetation belts in Nigeria (Aweda and Adeyewa, 2011).

Factors that influence the vegetation composition of tropical forests are usually grouped as natural and anthropogenic. In Nigeria, anthropogenic factors include deforestation, bush burning, application of chemical fertilizers, fuel wood extraction, logging, urbanisation and climate change (Unanaonwi and Amonum, 2014). On the other hand, climatic factors such as, temperature, evaporation, and solar radiation play limiting roles, while precipitation is known to have a marked influence on vegetation. This is due to the roles of water in all the major processes carried out in plants: it is required in virtually all the activities needed to keep a plant alive and productive.

In many ecosystems, particularly grasslands and croplands, seasonal plant growth occurs in perfect harmony with the rainy season. The possibility of expressing the relationship between precipitation and vegetation has been greatly improved upon by the development of meteorological instruments and the collection of systematic climatic observations. Precipitation data have been made available for daily, monthly, seasonal, yearly, and decadal studies to reveal a lot of useful information, noticeable variations and to establish trends. When used together with remotely sensed data, numerous characteristics of vegetation are unveiled, and environmental dynamics are better understood. Assessing vegetation condition, cover, processes, and changes are major components of global change research programs which have gained considerable societal relevance. Remote sensing data come in handy in this area and have resulted in major breakthroughs in ecological and vegetation studies. Among the most widely used satellite data products for measuring the amount and quality of vegetation are spectral vegetation indices, which are seamless data products that are computed from the same mathematic formulae across all pixels in time and space, without prior assumptions of biome type, land cover condition, or soil type, and thus represent actual, long-term measurements of the land surface (Alfredo et al., 2006). They are useful in assessing parameters such as plant biomass, stress, health, production and water use.

Broadly speaking, there are two classes of vegetation indices: ratios and linear combination. While ratio vegetation indices are usually simple ratios of any two spectral bands, ratio of sums, differences or products of any number of bands, linear combinations are orthogonal sets of n linear equations calculated using data from n spectral bands. Among the numerous vegetation indices, the Normalized Difference Vegetation Index (NDVI) is preferred for global vegetation monitoring because it partially compensates for changing illumination conditions, surface slope, and viewing aspect (Yengoh et al., 2014). They are widely used as proxies in estimating canopy state variables (leaf area index, fraction of absorbed photosynthetically-active radiation, chlorophyll content, vegetation fraction) and canopy biophysical processes including photosynthesis, transpiration, and net primary production (Alfredo et al., 2006, Aweda and Adeyewa, 2011, Davenport and Nicholson, 1993, Gritti et al., 2010, Klein and Roehrig, 2006).

Indications of the sensitivity of vegetation to variations in precipitation highlight climate change as an important factor in vegetation ecosystem studies. An evidence of climate variation and change existed in precipitation records over the years in Nigeria (Fasona et al., 2011, Gritti et al., 2010, Odjugo, 2009). The general view was that the amount of rainfall in Nigeria decreases with increasing distance from the coast to the hinterlands. However, a number of authors have discovered that although this claim presents the overall pattern, it is not entirely true (Ishaku and Majid, 2010; Odjugo, 2010; Adefolalu, 1986; Fasona et al., 2011). They have shown that variations that do not follow the general pattern exist in certain areas within the country.

Therefore, there is the to take a closer look at the real situation, determine the exact spatial and temporal variations in precipitation and accurately estimate the sensitivity of vegetation to annual variations in precipitation. This is the crux of the present study. An understanding of vegetation responses to fluctuations in climate in Nigeria is critical to predictions of plant and ecosystem responses to climate change, and would provide useful insights into the dynamics of the Nigerian ecosystems and assist in monitoring environmental change process and prediction of biological productivity changes under different climatic scenarios.

Loss of vegetal cover is a serious issue that ecologists, environmentalist and other stakeholder are concerned about due to the pivotal role of vegetation in the physical and human environment. Vegetation is not only a major component of the natural environment; but an important factor that impacts biodiversity, soil, water and air. It provides a natural and effective cover and protection for the soil, preventing erosion and land degradation. The importance of vegetation in correcting soil erosion and land degradation is seen in the roles of plants in slowing down water runoff and increases infiltration and water holding capacity of soils. Vegetation slows down the flow of the water in wetlands and on the banks of rivers. It helps in slope stability by providing root strength and by modifying the saturated soil water regime (Ziemer, 2000). Vegetation also reduce urban heat island, enhance carbon sequestration, remove air pollution, and also improve urban water infrastructure and ambience (Tyubee and Anyadike, 2012, Balogun et al., 2011, Adinna et al., 2009, Adamu, 2009).

This study attempts to determine and quantify the effects of annual variations in precipitation on vegetation in Nigeria using satellite derived measures of surface greenness (NDVI). Specifically, the annual trends and anomalies in precipitation over Nigeria between 1981 and 2009 are analysed; the trend of the annual variations in vegetation are established, and the temporal as well as the spatial sensitivity of vegetation to the annual variations in precipitation are estimated.