Mangrove Ecosystems Rehabilitation in Cameroon: Effects of Two Abiotic Factors on the Growth of Rhizophora Seedlings

Din N, Boubakary S, Leopold EKG, Laurant NM, Alphonse KS, Flavien KME, Sarah MI and Francois MM

Published on: 2019-11-17

Abstract

Mangrove ecosystems are undergoing anthropogenic and natural pressures worldwide. Reforestation projects have been developed but very few are achieving the expected goal. Data on the parameters that influence the growth and development of seedlings in natural conditions appear as key factors. The objective of this work located in the Wouri estuary mangroves is to evaluate the impact of light and conservation period of propagules on seedling growth of Rhizophora spp. Four packages of 120 propagules each of the above species were harvested from tree shaking and were preserved 28 days, 14 days, 7 days and 0 day respectively. A fifth package was constituted with 120 propagules gathered from mangrove substrate. The propagules were distributed into three types of nurseries (lighted, semi-lighted and unlighted) and planted the same day. The development of radicles was found from the 14th day during the conservation period. No leaf appearance or perceptible color change of the seed coat were observed throughout this period. The best conservation period was 7 days associated with a semi-lighted nursery. The largest length of the seedlings in any nursery after eight weeks was 56.90 ± 6.74 cm and the average diameter is 1.32 ± 0.04 cm. The sproductions of seedlings in semi-lighted nursery appear to be the most effective approach.

Keywords

Conservation Period; Light; Nursery; Propagules; Reforestation; Wouri Estuary

Introduction

Ecosystems are worldwide subject to degradations of all kinds of amplitudes due to natural factors and anthropogenic pressures [1-3]. Few coasts in the world have been subject of a mangrove restoration experiment on their disaster sites. Development conditions are however optimal, and seeds are abundant and available in all seasons [4,5]. The intensification of human pressure is largely due to the upsurge of the regression process of mangrove ecosystems which is not specific to Africa. The average annual rate of the mangroves deforestation has been estimated between 0.2 - 0.7% from 2000 to 2012 [6]. In cameroon, 53.16% of mangroves around douala city have been lost in 35 years [7]. These losses of the vegetation cover are mainly due to the extension of the douala port, the exploitation of fuelwood and mangrove wood intended to overlay the needs of fence and ceiling of houses, but also for the processing of fresh fish into smoked fish, sampling sand and also building the huts following rampant urbanization [8].
Public and private agencies try to mitigate this trend by planting mangroves [5,9,10]. Some examples of mangrove reforestation projects implemented around the world in recent year’s show that a number of wetland functions and values can be restored [11]. In africa, mangrove ecosystem rehabilitation initiatives have been implemented in several countries notably benin, cameroon, côte d'ivoire, kenya, nigeria, senegal [12-14]. in cameroon, these mangrove restoration projects are continuing until now. Large mangrove reforestation projects implemented either fail completely or rarely achieve pre-established restoration objectives [5,15]. These failures are related to non-compliance with pre-planting, planting and post-planting procedures [16]. Most projects involve planting propagules in nurseries near or far from the target site. Few of these projects had detailed monitoring plans, and in most cases few documentation or recommendations for modifications to the original planting plan exist [17-18]. There is a lack of information on the biotic and abiotic parameters that influence the growth and development of seedlings in the specific mangrove ecosystem. Also, very little is known about the variation of structural and functional parameters of vegetation in the mangrove ecosystem [5,19,20]. in cameroon, numerous mangrove rehabilitation projects have been set up with the participation of local populations however, very few studies have taken into account the impact of biotic and abiotic parameters (light, temperature, humidity, chemical composition of water, atmospheric and hydrostatic pressure, physical and chemical structure of the substrate) in reforestation of mangrove vegetation. The purpose of this study is to determine, in natural conditions, the influence of light and conservation period of propagules on seedling growth of rhizophora spp. in the wouri estuary mangroves.

Materials And Methods

Study Sites
The work was carried out in the Wouri estuary in Douala Cameroon (4°00'26.2 '' N and 9°40'48.6'' E). The selected study sites are located in Wouri Bridge where the nursery was installed because seedling need daily to be flooded and this location were nearest with our laboratory to facilitate us monitoring of seedling in nursery. The other sites were in “Cité Berge” and “Mbanga Pongo” (Figure 1). The climate of the region belongs to the equatorial domain of a particular type or Cameroonian. It is characterized by a long rainy season (March - November) with abundant rains (4000 mm per year) and a short dry season (December - February), high and stable temperatures (26.7 °C), and high humidity throughout the year with a maximum always near 100% [21]. Tropical cyclones are unknown despite the strong influence of the tides.

Figure 1: Map of Cameroon estuary showing the study sites [22].

Harvest and Conservation of Propagules
Data collection began with the identification of the individuals of Rhizophora spp. which can be used as seed trees. The stem size was measured with a tape and propagules were harvested from seed trees with circumference ≥ 40 cm at the upper end of the strongest stilt roots. The propagules that fell before the harvest were removed under the selected seed trees. Harvesting consisted in shaking lightly the fruited branches with hands or by the help of a stringed instrument so that only mature propagules could fall (Figure 2a).
Four packages of 120 propagules each were harvested in different periods and stored as follow:
? a first package was harvested and stored for 28 days;
? a second package was harvested after two weeks and kept for 14 days;
? a third package was harvested after three weeks and kept for 7 days;
? the last one was harvested the day of planting.
In addition, another package of 120 propagules collected on the mud under seed trees or floating on the water were selected and transplanted directly into the nursery without conservation. For conservation, propagules was deposited on the mangrove mud in situ and surrounded by a mesh fencing with a very small mesh favoring on the one hand their stability against the tidal current and on the other hand the normal passage of light and tidal water (Figure 2b). The appearance of the plant and diaspores harvested was described with emphasis base on the coloring of the integuments. The time of appearance of the roots and leaves was noted, the propagules size was also measured. Every package of 120 propagules was divided into three groups of 40 individuals.
Nursery, Data Collection and Analysis
Three nurseries of 2m x 2m, varying according to the intensity of sunlight reaching the ground have been constructed in situ. The first nursery was totally opened (unprotected) favoring the arrival of all the light radiation incident on the seedlings (Figure 3a). The second nursery was established under a covered allowing the passage of soft light simulating a weak shading of shrub foliage in the natural environment (Figure 3b). This cover consists of few palms resting on a wooden frame. The third nursery consists of an opaque cover that completely prevents incident radiation from reaching the seedlings (Figure 3c). The nurseries have been arranged so that the propagules are flooded by the tides as in the natural environment. (Figure 2) Seedling: (a) Matures propagules harvested; (b) Conservation of propagules. The blocks of each conservation treatment were separated by a passage allowing the collection of data. Data collection consisted on the one hand to raise in each type of nursery and according to the block, the diameter of the seedlings with the help of a Vernier caliper and on the other hand their length with the help of a measuring tape. The variation in the number of leaves as a function of time was also noted. These parameters were recorded weekly and successively for eight weeks. Data analysis started with interpretation of results from observations.

Figure 2: Seedling:(a) Matures propagules harvested; (b) Conservation of propagules.

The ANOVA test was performed to compare the different lighting environment (lighted, semi-lighted, unlighted) according to the conservation time. The Excel 2010 and Graph pad Prism 5 software allowed to analyze the evolution of the different parameters studied through the graphical representation.

Results And Discussion

Biology of Propagules

The 600 propagules harvested have all a tapered shape and are of variable dimensions. By dividing all propagules collected into three groups (short = length ≤ 20 cm; intermediate = length [20; 50 cm] and long = length > 50 cm) as some authors used, 81% belongs to the medium group follow by the long with 13% and the short group represents 6%. The mean length was 49.8 ± 3.4 cm; the propagules of this area can be considered as long. These results get closer to those obtained in Gazi Bay (Kenya) where the dispersal of two of the most widespread eastern mangrove species (Ceriops tagal and Rhizophora mucronata) has provided a mean propagule size of 44.4 ± 4.3 cm [23].

Figure 3: Different types of nurseries: (a) Without cover; (b) Slightly
cover; (c) Opaque cover.

They are significantly different from the study carried out for the influence of propagules float longevity and the availability of light on the development of Rhizophora mangle and Bruguiera sexangula, two introduced mangrove species in Hawaii where the authors found a mean propagule size between 11.7 and 36.6 cm [24]. The diameter of propagules varies from 0.6 cm to 1.3 cm. These propagules have various colors: mostly brown, sometimes green and rarely yellow. When a propagule is mature, the cotyledon enveloping the plumule becomes red, and comes off easily. It secretes on its point of attachment, a sticky liquid, translucent and odorless. These observations are similar to those of [25] who observed germination of propagules of Rhizophora spp. until maturity.

Growth of Propagules

The coloring of the integuments did not vary and no leaf was observed throughout the conservation period. However, depending on the time, an appearance of the radicles is noted. Thus, 47.5% of individuals in the 14-days-old packages and 60% of individuals in the 28-days-old packages were found to have radicles. During the harvest of propagules directly transplanted in the nursery, it was found that several had been partially consumed by the benthic macro invertebrates. Another study has shown the impact of these benthic macro invertebrates on propagules in their natural environment and highlighted their influence on seedling distribution and the limitation of competition between mangroves’ predator species [26].

Growth In Heights

 

Effect of the Conservation

 Regarding the conservation period, distribution of the average heights of the seedlings is uneven (Figure 4). The situation is similar to that occurring naturally in mangroves. Mature propagules that have fallen from the seed tree can either immediately settle in the mud or begin to grow, or float on the water for days, weeks, or sometimes months before finding ideal conditions for growth. During flotation, the tide provides nutrients useful for the future growth and development of propagules. The growth observed is better in the semi-lighted environment for the packages of the propagules kept 7 days in situ before breeding in nursery. These results have confirmed those of [27] who observed the best development of seedlings after one week of flotation in full-strength seawater.

Figure 4: Variation of the average heights of the seedlings in the different environment according to the conservation period.

Figure 5: Variation of average diameter of seedlings in the different environment according to the conservation period.

Effect of Light

One week after nursery establishment, the green and yellow staining propagules contained in previous treatments turned completely brown. All propagules harvested and collected are healthy and a germination rate of 100% has been obtained. These results do not agree with those of [28,29] who revealed in the study of rehabilitation of mangroves between Fresco and Grand-Lahou in Côte d'Ivoire, important areas for fishing that, only the collected seedlings floating on the water have more interesting germination qualities. The appearance of the first leaves in the seedlings were observed in the second week in different environment: 7.5% of individuals in the lighted had leaves, 21% in the semi-lighted, 5% in the unlighted (Table 1). Overexposure to the sun can have adverse effects on plant life. When the plant is subjected to excessive light intensity the phenomenon of turgor (process of renewing the water reserves of the plant) stops, the vacuoles retract, the cytoplasmic membranes are detached from the wall, and the flexible parts of the plant soften. Without light, a plant cannot photosynthesize and therefore cannot support itself. If the plant does not have enough nutrient reserves produced by photosynthesis, its growth stops and it dies. [30,31].

TTable 1: Appearance of the first leaf in the seedling the second week.

Conservation periods (days)

Number of individuals with leaf after two weeks

Lighted

Semi-lighted

Unlighted

0*

0

0

0

0

0

0

0

7

8

17

2

14

1

21

2

28

6

4

6

Total

15

42

10

Table 2: Average height of seedling in each environment after 8 weeks according to conservation treatment.

Conservation periods (days) Mean height (cm) Lighted Semi-lighted Unlighted P0* 51.05 ± 6.94 51.50 ± 5.99 51.06 ± 6.11 P0 50.89 ± 6.81 48.99 ± 8.86 46.78 ± 5.80 P7 53.05 ± 7.18 56.90 ± 6.74 52.85 ± 5.47 P14 51.41 ± 6.08 47.33 ± 4.94 48.66 ± 5.24 P28 50.49 ± 6.22 46.02 ± 5.39 42.61 ± 4.08

Conservation periods (days)

Mean height (cm)

Lighted

Semi-lighted

Unlighted

P0*

51.05 ± 6.94

51.50 ± 5.99

51.06 ± 6.11

P0

50.89 ± 6.81

48.99 ± 8.86

46.78 ± 5.80

P7

53.05 ± 7.18

56.90 ± 6.74

52.85 ± 5.47

P14

51.41 ± 6.08

47.33 ± 4.94

48.66 ± 5.24

P28

50.49 ± 6.22

46.02 ± 5.39

42.61 ± 4.08

Table 3: Average seedlings diameter in each environment as a function of treatments.

Conservation periods (days)

Mean diameters (cm)

Lighted

Semi-lighted

Unlighted

P0*

1.23 ± 0.05

1.25 ± 0.03

1.19 ± 0.04

P0

1.27 ± 0.06

1.19 ± 0.03

1.19 ± 0.05

P7

1.26 ± 0.06

1.32 ± 0.04

1.25 ± 0.07

P14

1.19 ± 0.06

1.21 ± 0.03

1.18 ± 0.06

P28

1.22 ± 0.01

1.21 ± 0.03

1.13 ± 0.12

Mean seedling height distribution is unevenly distributed among the three environments: lighted, semi-lighted and unlighted (Table 2). Of the three-environment studied, the semi-lighted appears to be more favourable for the seedling height growth. The amount of light that arrives is sufficient for photosynthesis of mangrove seedlings. These results are similar to those of [32] who showed during the study photosynthetic responses and fluorescence of chlorophyll in seedlings of Rhizophora mucronata in the shade that semi-lighted treatments provide the best condition for a good carbon fixation capacity. These results also corroborate with those of [33] who observed during the study of the adaptation of red mangrove trees in the shade, the good development of seedlings in an area receiving sufficient solar radiation. However, inside the semi-lighted environment, there are differences between the various treatment packages. The highest mean height 56.90 ± 6.74 cm was obtained in the 7 days semi-lighted environment treatment batch. Moreover, in the unlighted environment, seedlings completely deprived of light grow very slowly compared to those of the semi-lighted. These results corroborate with those of [34], who showed the low growth of seedlings hermetically covered by plant canopies in the mangrove. These results are also consistent with those of [35] who observed the impact of irradiance on seedling mortality of Rhizophora spp. covered by canopies in their natural environment. High mortality of seedlings should be attributed to unfavorable lighting conditions starting from the poles structure. Seedling growing in full light died more rapidly than seedlings growing under vegetation cover. Canopy cover is known to offer protection against temperature extremes and excessive evapotranspiration, although drought might be increasingly harmful under shadier conditions because water absorption and light capture cannot be maximized simultaneously explain [36]. Mortality could be due to several interacting factors, mainly predation and desiccation under extremes temperatures [31].

 

 

 

Diametric Growth

Effect of the Conservation

The diameter varies from one batch of treatment to another. The greatest mean 1.32 ± 0.04 cm is obtained in the batch of propagules which have been stored for 7 days before being prick out in the nursery and the smaller mean diameter of 1.13 ± 0.12 cm in unlighted nursery in the batch of propagules store for 28 days (Figure 5). This result confirms that of [25] who came to a conclusion after observations that propagules should be kept for 6 to 7 days in their natural environment, sheltered from the sun before being transplanted to obtain vigorous plants. Generally, in the mangrove, the propagules which fall and settle immediately in the mud give less robust seedlings, on the other hand the propagules which float during several days before settling in the mud give robust seedlings [24].

Effect of Light

The distribution of diameters within nurseries shows significant differences (P <0.05). In fact, the most vigorous seedlings were observed in a semi-lighted nursery with the highest mean diameter of 1.32 ± 0.04 cm and smaller mean diameter of 1.13 ± 0.12 cm in unlighted nursery (Table 3). Diameter growth of Rhizophora spp. could result from the inputs of light. Generally in mangroves, Rhizophora seedlings establish and sprout most readily under the shady canopy of larger trees. [37] Demonstrated that the seedlings grow faster in the shade and show more efficient photosynthesis contrary to mangrove seedlings under a closed canopy. This result corroborates with those of the seedlings in a semi-lighted which also received a mean quantity of light. Shading can also have negative effects. For example, the mangrove seedlings under a closed canopy showed lower growth in south Florida [34]. This phenomenon is also observed in other ecosystems than mangroves [38]. The intensity of the incident light could be limiting factors for seedlings growing in vegetation contrary to those which are developing in exposed environment.

Conclusion

The present study has contributed to the improvement of the knowledge of some parameters that influence the growth and development of Rhizophora seedlings and has adjusted mangrove restoration methods at the local level. Nursery propagule rearing showed the development of root buds from the second week and the appearance of first leaves in lighted, semi-lighted and unlighted nursery in propagule package kept for 7 days, 14 days and 28 days in situ with high meaning semi-lighted nursery. Seedling development was important in the semi-lighted environment for propagules kept for 7 days. For the success of a mangrove reforestation project, it is important to first store the propagules in situ for one week, then to rear them in situ in semi-lighted environment for two months before planting. The results obtained in this study contributed to improve the science of mangrove management in general and Cameroon in particular.

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