We investigated seeds germination and seedlings growth in Khaya senegalensis and Parkia biglobosa, two local forest trees species highly exploited and threatened. The gained knowledge will be helpful for forestation and afforestation program in Côte d’Ivoire. These species showed short time to begin germination, generally less than 20 DAS. In addition, the germination was not spread out enough, less than 60 days. These results were similar to those previously found in Terminalia superba, Mansonia altissima and Pterygota macrocarpa (Akaffou et al. 2019) and in Pterocarpus erinaceus (Adji et al. 2021e). By contrast for some others trees species such as Sideroxylon contrerasii and Elaeocarpus prunifolus, the germination was long since 434 days and 146 days were required to reach maximum or mean germination respectively (Toledo-Aceves 2017 ; Iralu and Upadhaya, 2018).
Elsewhere the seeds germination dynamic and rate were significantly influenced by both seeds size and experimental site. For Khaya senegalensis, large and medium seeds showed fast germination and reached earlier maximum germination contrarily to small seeds. They also yield the highest germination rate. Conversely, for Parkia biglobosa, small and middle seeds this time exhibited earlier and fast germination and gave the best germination rate. Ours results did not support those of (Agboola, 1996 ; Mtambalika et al. 2014) who found no effect of seeds weight on germination rate in Terminalia superba, Terminalia ivorensis, Gmelina arborea and Afzelia quanzensis. In contrast our findings were consistent with those of (Burlem and Miller, 2001 ; Deb and Sundriyal, 2017) who reported fast and best germination in large seeds respectively in three and four tropical trees species. Such a result was also found by (Mao et al. 2019 ; Iralu and Upadhaya, 2018 ; Kuniya et al. 2013 ; Mandal et al. 2008) in Pinus thunbergii, Elaeocarpus prunifilius and Terminalia bellerica. In addition, the results obtained for Parkia biglobosa agreed with those published in Copaifera langsdorffii by (Souza and Fagundes, 2014) who reported that smaller seeds showed faster and greater germination than the larger ones. Hence, our results were not surprising. These contrast behaviour depending on species and well known in trees species were attributed to species strategies for their regeneration and early colonization of an area (Burlem and Miller, 2001).
The high germination rate obtained at Korhogo than at Daloa could be due to slightly warm temperature close to 30°C since this temperature is reported to be the optimal for seeds germination in trees species (Simao and Takaki, 2008 ; Vargas-Figneroa and Torres-Gonzales, 2018).
In final, the best germination rate found for large seeds in Khaya senegalensis and for small seeds in Parkia biglobosa were high, ranging between 71% and 86% on average. These rates were similar to that obtained in Mansonia altissima, Pterygota macrocarpa, Terminalia superba (Akaffou et al. 2019) and Pterocarpus erinaceus (Adji et al. 2021e). While they were superior than twice fold to those recorded in others trees species viz. Ceiba pentadra, Delonix regia, Sterculia urens, Albizia lebbeck and Leucaena leucocphala for which germination rate were very poor ≤ 30% (Nakar and Jadeja, 2014).
The growth analysis revealed that seedlings issued from large seeds, as well as in Khaya senegalensis as in Parkia biglobosa, showed best growth performance. This result was in conformity with the findings of (Ellis, 1992 ; Milberg and Lamont, 1997). These authors argued that best growth of seedlings issued from large seed should be due to efficient use of the more resources stored in these seeds.
Since in Parkia biglobosa, best growth performance was recorded for seedlings emerged from large seeds while they exhibited feeble germination rate (50%), research to increase their germination is required. Therefore, pre-soaking treatments in warm or hot water could be applied as done by (Fredrick et al. 2017).
The best growth performance scored at Daloa than that of Korhogo could be attributed to the favourable climatic conditions prevailing in the first experimental site, viz. more rainfall, 1300 mm vs 1000 mm and less hot temperature 26°C vs 30°C.
This study showed that seedling mortality is lower up to the age of 12 months in the target species. This fact clarifies, demonstrates and proves that large seeds can be recommended to farmers, agroforesters and forestry management structures as quality seeds for successful and optimised cultivation of target species. The results also showed that the individuals have statistically the same dimensions regardless of their origin at the age of 12 months. This was not the case at 4 months. Indeed, seedlings at the seedling stage are not yet independent. They therefore continue to use the seed's reserve (Milberg and Lamont, 1997). The larger the seed, the more starch reserves it contains. The seedling therefore draws on the reserve; it has more nutrient reserves in its procession to develop better than seedlings from small seeds (development and growth limited by the quantity of starch available in the seed).
Once the seedling regains its independence from photosynthetic activity, development and growth will depend on environmental conditions (soil fertility, availability of light and water etc.). Individuals therefore lose their vigour, which depends on the seed reserve. They acquire a growth capacity that depends on the environment. An individual resulting from the germination of small seeds under favourable conditions may have the same or greater vigour than an individual resulting from the germination of large seeds. In our study, the plants from the large seeds had slightly larger sizes than the other categories because of the identical environmental conditions. However, statistical analysis showed that these individuals were identical in size.
Even if the individuals tend to be morphologically identical later on, the most important thing is the selection of the right seed type for successful reforestation. In this sense, the mortality study is a priority and has shown that large seeds are the best candidates for any species.