Growth and Development of Cyperus papyrus in a Tropical Wetland

Alfonse Opio, Mike B. Jones, Frank Kansiime, Tom Otiti

Department of Biology, Faculty of Science, Gulu University, Gulu, Uganda.
Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland.
Department of Environmental Management, College of Agriculture and Environmental Sciences, Makerere University, Kampala, Uganda.
Department of Physics, College of Natural Sciences, Makerere University, Kampala, Uganda.

DOI: 10.4236/oje.2014.43013                

Abstract

Relative growth rate (RGR) is an important parameter in plant growth models. However, no field measurements of RGR have been made in Cyperus papyrus (papyrus) wetlands. In this study, the growth dynamics of aerial shoots were investigated in a tropical papyrus wetland. The aim was to estimate temporal changes in the growth rates, and also establish the effect of water level and temperature change on total culm number, culm recruitment and senescence. Dry weights of shoots during their life cycle were determined using a non-destructive method after establishment of relationship between culm diameter and dry weight. Measurements were made over period of seven months. During the course of the measurements, culm density significantly increased from 16.1 to 35.9 culms.m^-2. There were significant changes in both culm recruitment and senescence. Maximum RGR of developing culms was 1.04 g g^-1 d^-1. Using a culm density of 27 per m^-2, productivity range was 16.74 to 37.37 g m^-2 d^-1. There was an inverse and significant relationship between rate of change in leaf weight ratio (LWR) and RGR of the culms. In conclusion, RGR of the aerial shoot development was controlled by rate of change in LWR from 41 to 156 days of growth. There was a strong and negative influence of water level fluctuation on total culm density.

Keywords

Cyperus papyrus; Growth Analysis; Wetland

1. Introduction

Cyperus papyrus (papyrus) is a large herbaceous sedge commonly found in waterlogged environments in the African tropics. These wetland ecosystems provide ecological and socio-economic services related to the harvesting of aerial biomass, wastewater treatment, hydrological functions and climate modification [1] . It has been suggested that removal of large quantities of nutrients associated with papyrus harvesting results in reduced production rates in subsequent re-growth periods [2] . In addition, negative influences of frequent and indiscriminant harvesting on growth of aerial biomass have been reported [3] -[5] . Therefore, in order to achieve sustainable production, there should be a suitable management program to regulate harvesting [3] [5] . This could maximize wetlands potential as carbon sink [1] .

For sound management, there is need to understand the growth and development which form the basic components of plant population dynamics [6] . In papyrus wetlands, the aerial structure (shoot) consists of sheath, culm and umbel (Figure 1).

Figure 1. A Cyperus papyrus plant showing, on a single rhizome, the typical range of the culm units of increasing age from I to VI [7] .

During development, the vegetative shoot grows out from the rhizome with the umbel fully enclosed in a sheath. The sheath surrounds the base of the culm in later growth stages. The developing papyrus goes through different growth stages starting as young elongating culms with closed umbels, then elongating culms with umbels just opening, fully elongated culms and fully expanded umbels, senescent culms (≤ 40% achlorophyllous) and finally dead culms (≥60% achlorophyllous).

Tropical wetlands are often unstable environments and in particular they are subject to substantial water level fluctuation [8] -[10] . However, the loose structure of the floating papyrus rhizomes allows a free exchange of water and dissolved matter from underneath into the rhizome mat [11] . The effect of these fluctuations in water level on the natural regeneration of papyrus is unknown, although natural regenerative capacity is known to be influenced by water depth [12] .

The potential economic importance of papyrus wetlands has stimulated many studies on growth and development of the vegetation. These studies have shown that papyrus wetlands are characterized as having regular turnover of individual plant units [3] . However, estimates of growth rate of single shoots in terms of relative growth rate (RGR) [13] have not been made although measures of absolute growth rates of unit area have been a common comparison tool for performance in different wetlands [14] -[17] . Growth rates can be determined using culm-girth biomass relationships [3] [18] . An effort in understanding how RGR of the papyrus depends partly on the umbels and how actual value of RGR at an instant of time depends on the amount of foliage which presented at that time is an important aspect of their physiology.

RGR or growth efficiency index expresses growth in terms of a rate increase in size per unit of size and is an important parameter in plant growth models [19] [20] . This study is a growth analysis using biomass data of aerial shoot of C. papyrus. Mathematical equations in the form of fitted functions have been employed to analyze time series of dry weight [21] -[23] . The objective of this investigation was to estimate the temporal changes in growth and development in a papyrus wetlands. This involved determining the pattern of individual papyrus shoot relative growth rates. In addition, measurement of leafiness was made by determining the LWR over time. The effect of some environmental variables on culm recruitment and senescence, and total culm density was also investigated. It was hypothesized that growth rate of shoots and their total culm density, recruitment and senescence are dependent on water level and water temperature variation in the wetland.

2. Materials and Methods

2.1. Study Area

The study was conducted in Lubigi C. papyrus wetland in Kampala District, Uganda. The wetland is located at 7.5 km West of Kampala City in Kawempe Division 0˚17′ N to 0˚22′ N and 32˚30′ E to 32˚34′ E (Figure 2). The wetland has a tropical wet and dry climate with daily average temperature ranging from 17˚C to 27˚C during the year. Rainy seasons are from August-December and March-May of each year, however, delay in onset of rainfall has been observed in recent years.

2.2. Numbers of Total, Recruited and Senescencing Culms

Measurements were made along a 900 m transect in the wetland (Figure 2). The different culms categories in eight (8) replicates of 3 × 3 m² quadrats along the transect were counted and marked. The cut transect provided an access into the wetland using a ladder lying on the papyrus mat constructed from eucalyptus poles. New shoots, mature and senescence culms were identified at each sampling time. Recruits were all the papyrus culms that appeared and had not been marked at the previous sampling time. Senescent culms were those that were 40% achlorophyllous while mature culms were those between senescence and newly recruited. The changes were monitored for a period of seven (7) months from October, 2010 to April, 2011 at an interval of at most two weeks.

2.3. Assessment of Relationship between Aerial Shoot Biomass and Culm-Girth

Culm-girth was measured at the top of the sheathing leaves. One hundred and twenty culm units from along the transect were cut at rhizome level after which girth measurements was made. They were taken to laboratory at Makerere University, Department of Environmental Management for drying to constant weight at 80˚C. A relationship between culm-girth and dry weight measurements of aerial shoots and their umbels was determined by regression analysis.

Figure 2. Location of transect and productivity quadrats in Lubigi wetland in Kampala District, Uganda.

Conflicts of Interest

The authors declare no conflicts of interest.

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