Globally, planting trees is considered one of the most efficient strategies to combat climate change due to its ability to rapidly capture atmospheric carbon dioxide (CO2) and for restoration potential in any degraded areas (Bastin et al., 2019; Griscom et al., 2017). However, the selection of suitable species is critical in any plantation and the suitability of plantation type such as mono- or mixed-culture for different ecosystems is still under scrutiny (Lewis et al., 2019). While monoculture plantations dominate (Evans, 2009), mixed-species plantations are frequently being recommended due to their multiple functional benefits compared to timber production from monoculture (Lamb et al., 2005; Li et al., 2022; Paquette & Messier, 2010). Especially, in a country with low per capita land mass and high climate vulnerability, like Bangladesh, mixed species plantations are encouraged through various form of plantations activities such as social forestry and agroforestry (Iftekhar, 2006).
Despite multiple benefits, the success of mixed-species plantations is challenging as the intra- and inter-specific competition among species has a strong influence on individual fitness and the ways these species co-exist into communities (Antonovics & Levin, 1980). At the very early stage of plant growth, the outcomes of competition are driven and defined by resource capturing and dominating available spaces quickly (Aschehoug et al., 2016; Trinder et al., 2013). Broadly, plant competitive ability has two components: i) the ability to suppress a neighbour, or competitive effect, and ii) the ability to tolerate a neighbour’s competitive effect or competitive response (Goldberg & Barton, 1992). Trinder et al. (2013) stated resource competition as sensu stricto and considered competition for both direct and indirect resources including, light, water, soil moisture and nutrients. When two plant species interact, the ability to response of both competitors determine the outcome of the competition, which is usually measured as the overall performance of each species (Tilman et al., 2014). Though, plant competition studies predominantly focus on individual and direct negative effects of competition [e.g., community assembly theory (Gotzenberger et al., 2012; Weiher & Keddy, 2001)], or co-occurring plant species where both affect and response to one another and these interactions influence structure, dynamics and evolution within plant communities (Mathias & Chesson, 2013; Silvertown, 2004; Tilman, 2004). Despite its importance, there is still debate and scope to address further inter-specific competitive phenomena (Connolly et al., 2001; Silvertown, 2004). More specifically, by focusing on mutualistic interactions of species from the same family or taxa (Jones et al., 2012).
Competition not only takes place within and between species, but also varies by growth stage; especially when plants go through different demographic phases such as seed germination, seedling growth dynamics, and even initial development of root-shoot (Bergelson & Perry, 1989; Cameron et al., 2007; Navarro-Cerrillo et al., 2020; Soliveres et al., 2010). Nonetheless, competition between seeds or the continuation of competition from seed to seedling growth stage is rarely studied as most of the studies focused on the mature stages gain or dominancy (Silvertown, 2004; Trinder et al., 2013). These posit to study covering such continuum is necessary to understand and execute different plantation strategies, especially under current global climate change scenarios.
Albizia lebbeck (L. Benth.) and Albizia saman (Jacq.) Merr. are large deciduous trees native to tropical America, which have now become widespread throughout humid and subhumid tropics (NAS, 1979). They are from Fabaceae family and have multiple potentials other than producing wood; namely, pasture trees, shade trees in agroforestry and medicinal uses (Nair et al., 1984). Besides, due to their nitrogen fixing capabilities they often are recommended for afforestation and reforestation activities (Durr, 2001; Mishra et al., 2010). They are commonly found in roadside, agroforestry and social forestry plantations with different admixture with other species in Bangladesh (Azad et al., 2006). A. saman has been identified with an allelopathic effect on other species when it appeared in combination with other species, as all plant parts (i.e., seed, stem and leaf) contain and leach considerable amount of allelochemicals, including tannins, flavonoids, steroids, glycosides and terpenoids (Ghosh et al., 2013; Noor & Khan, 1994).
There is a myriad amount of research on individual germination and growth with different seed treatment strategies for these species (Khurana & Singh, 2004; Kumar et al., 2018), answering how higher germination success can be achieved? Nevertheless, there is no research on their inter-specific interaction, especially from the community organisational perspective when they shared a same space and time. Understanding the competitive and mutualistic interactions between these two important species at different developmental stages will be critical for evaluating their seed germination capacity and seedling production. Such knowledge will ultimately ensure their success in production forestry as well as their establishment in restoration program.
Therefore, we investigated the germination and growth performance of A. lebbeck and A. saman at seed germination and seedling growth stage and their inter-specific competition with a replacement series (RS) experiment whereby variable mixtures of these species were grown alongside monospecific treatment. Specifically, we addressed the following research questions: 1) do interactions between seeds of the two species influence the probabilities and timing of their emergence? 2) what is the degree of inter-specific competition between two confamilial species grown in a mixture of variable proportions and 3) how to the effects of inter-specific competition influence seedling height growth?