Agricultural Foam as a Propagation Substrate for Lowland Tropical Forest Tree Species

Lowland tropical forests are home to the greatest biodiversity on Earth and provide important ecosystem services such as climate regulation and the provisioning of timber, food, and fibers (Brandon 2014). However, these ecosystems have been heavily impacted by anthropogenic activities, having the highest rates of deforestation and land use change of the entire planet (Hansen et al. 2013, Sullivan et al. 2019). Consequently, their ecological restoration is a current priority in several Latin American countries (Alcazar-Caicedo et al. 2018, Brancalion et al. 2019). Despite this, restoration programs in lowland tropical ecosystems are modestly successful (Brancalion et al. 2019). Plant propagation is a particularly critical step of the restoration process that is usually limited due to the extensive use of soil substrates where pathogens and weeds proliferate (Bertacchi et al. 2015).

Seedling propagation in the greenhouse is a crucial step for the success of restoration (Haase and Davis 2017). The general recommendation is that seedlings reach at least one meter of height before being transplanted to the restored field site (Sanchún et al. 2016). This initial growth phase may take weeks or months, depending on seedling growth rate, which varies broadly across species (e.g., Bloor and Grubb 2003). Currently, soil is the most commonly used substrate in seed propagation; however, it may be problematic due to the logistics required such as filling bags and transporting them to the field, the space it occupies in the greenhouse, and the fact that it may be a vector of weeds and pathogens (K. Castaño and E. Murgueitio, CIPAV, unpub. data, Haase and Davis 2017).

Using sterile substrates such as horticultural foam (HF) can be an alternative to the use of soil. Owing to its aseptic nature, HF provides an inert medium where seeds and seedlings can develop without pathogens. This medium also ensures a transplanting free of weeds and pathogens into the restored area, which could enhance plant success (Tucker 2001). Furthermore, HF provides ideal conditions for optimal root development, and facilitates the transportation of seedlings especially when geographic conditions are adverse (K. Castaño and E. Murgueitio, CIPAV, unpub. data). The limited studies on this subject indicate that HF might be an ideal substrate for plant propagation because of its aseptic nature, its nutrient contents similar to soil, and because it reduces water consumption and allows for a high degree of air flux (Masiunas et al. 2003, Muñoz 2014a). Additionally, it has been demonstrated that it is similar to soil in terms of germination, growth, and root performance (K. Castaño and E. Murgueitio, CIPAV, unpub. data).

Figure 1.

A) Final germination proportion (n = 76) and B) final stem height (n = 994) of five fruit tree species as a response to two substrates (Foam and Soil). Seedling survival percentage is shown on the upper part of violin plots.

To assess if horticultural foam could be used as an alternative for plant propagation in restoration projects of lowland tropical forest trees, we compared the propagation performance of seeds from five forest native species seeds (Annona squamosa [Sweetsop], Annona cherimola [Cherimoya], Maclura tinctoria [Dyer's Mulberry], Psidium guajava [Guava] and Manilkara zapota [Sapodilla], on two substrates: HF and soil. We purchased seeds from the online seed provider "El semillero" (elsemillero.net). We selected these species because they are drought tolerant, grow rapidly, are broadly distributed in lowland forests and provide fruits for wildlife (Hall 2011, Ribeiro da Silva et al. 2015). However, because seed germination and seedling performance is dependent upon seed size (Doust et al. 2006), we planted a total of 3109 seeds: 1403 small seeds of M. tinctoria (n = 652) and P. guajava (n = 751), and 1706 medium-size seeds of A. squamosa (n = 662), A. cherimola (n = 520), and M. zapota (n = 524). Our prediction was that medium-seeded species, having larger seed reserves and seedlings better defended against pathogens, would grow equally well on both substrates, while small-seeded species, with smaller seed reserves and...