Soil underpins the health of natural ecosystems and people’s livelihoods. It is also the foundation that supports biodiversity and its ecosystem services (Wagg et al., 2014; Wall et al., 2015). Unfortunately, threats such as climate change, habitat loss, urban development, poor agricultural practices, and anthropogenic pollutants have impacted soil health severely (Tibbett et al., 2020). The deterioration of soil health would adversely affect the numerous taxa that lives in the soil, such as invertebrates, bacteria, archaea, protists, and fungi (Decaëns, 2010). Many with important ecosystem functions would disappear without ever being identified (Cameron et al., 2018). Fortunately, soil is receiving increasing attention because of its role in carbon sequestration, agricultural productivity, and ecosystem services (Bhattacharyya et al., 2022; Brevik et al., 2018). Many of these services are provided by the beneficial symbiotic fungi of plants, the ectomycorrhizae (ECM).
ECM contribute to the survival of its hosts and stabilise soil structures. They grow large networks of hyphae known as mycelia that transport nutrients to the host plant directly in exchange for photosynthates (Figueiredo et al., 2021). ECM protect plant hosts from toxic chemicals, pathogens, and predators (Lalaymia et al., 2014; Schausberger et al., 2012). ECM can make up more than a quarter of living biomass in soils, they lock carbon belowground and protect soil from erosion (Averill & Hawkes, 2016; Frey, 2019).
Despite the ubiquity of ECM, they face increasing threats that will undermine their abilities to provide ecosystem services. Habitat destruction is the ultimate challenge for the conservation of ECM(Nic Lughadha et al., 2020) and this includes habitat fragmentation which exposes sensitive fungal species to harsher environments (Boeraeve et al., 2016). When natural ecosystems are replaced by human developments, the original soil and belowground biodiversity are significantly altered (Rusterholz et al., 2020). Poor agricultural practices such as excessive use of fertilizers and pesticides change these networks (Vincent & Declerck, 2021). Chemical pollution from farms severely affects ECM (Arnolds, 1991). These threats will cascade through the food web, depriving arthropods and other fauna that depend on fungi as a food source (Fitter & Garbaye, 1994; Suz et al., 2021). With ECM playing critical roles in soil integrity, and the many-pronged anthropogenic threats known thus far, many studies exploring ECM emerged in the last few decades.
ECM research was mainly conducted in temperate regions. Research has shown complex relationships of ECM with multiple taxa, including its host. Not only do ECM channel mineral nutrition to their plant hosts, but other herbaceous parasitic plants such as temperate myco-heterotrophs also tap into the mycelial network (Cullings et al., 1996; Feild & Brodribb, 2005). The fungal network helps temperate trees to withstand droughts (Claire, 2019). Responses to environmental changes have been documented using isotopes to track the movement of carbon, nitrogen and phosphorous (Pellitier et al., 2021). New technologies are being used to visually capture the network, create global models, and measure the percentage colonisation of fungi on plant roots (Freschet et al., 2021; Wu et al., 2022). Unlike the temperate areas and its new technologies, there are much fewer studies in the tropics (Corrales et al., 2018).
Tropical ECM are associated with the Dipterocarpaceae, Fabaceae and many others (Brearley, 2011; Founoune et al., 2002). Like their temperate fungal counterparts, these associations are found in polyphyletic plant families. Although identifications of ectomycorrhiza, cultivation of ECM fungi with their tree hosts and their benefits have been reported with on-going research, physiological and ecological approaches are still lacking (Brearley, 2011; Brearley et al., 2016; Kaewgrajang et al., 2013; S. S. Lee et al., 2008). As there is greater diversity of plant taxa in the tropics, ECM may display different levels of host specificity (Peay et al., 2015a). One of the most studied ECM are those associated with Dipterocarps in Southeast Asia (Corrales et al, 2022).
Dipterocarps are the dominant tree family in Southeast Asian Forest, characterised by their winged fruits (Appanah et al., 1998; Ashton et al., 2021). Those nearest to the equator undergo masting, a synchronised flowering and fruiting period for many Dipterocarp species (Kurten et al., 2018). The trees are mainly pollinated by insects such as thrips and seeds can be dispersed by wind and water (Momose et al., 1998). Masting also provides food for many animals (Nakagawa et al., 2003). Local communities also used them as butter, as the seeds are rich in oil and fats (Seirbet, 1996). Seeds are recalcitrant and germinate quickly when environmental conditions are right (Umarani et al. 2015). Dipterocarps are one of the most threatened plant family due to deforestation, legal as well as illegal harvest of its valuable timber. In situ conservation is a major challenge, while ex situ conservation efforts are dampened by irregular mast fruiting and recalcitrant seeds- both leading to shortage of seedlings. Also, lack of information on reforestation methodologies, especially on ECM relationships, among others complicate matters further.
Dipterocarp ECM belong to the same families as those in the temperate forests of Amanitaceae, Boletaceae, Russulaceae, Thelophraceae and studies into their identities are still ongoing (Essene et al., 2017; Segnitz et al., 2020). The research in tropical Asia has shown mixed results in understanding ECM host specificity and benefits. Research shows that composition of soil affects what species of fungi grow on the trees (Essene et al., 2017). The mycorrhizal strategies used in dipterocarp forests also determine the community structure of other soil microbes (Cowan et al., 2023). In tree nurseries, Dipterocarps inoculated with ECM were tested under different light conditions with varying results for different species such as the Shorea spp. (Brearley et al., 2016; Ramadhani et al., 2018; Rimbun & Chaidir, 2016; Saner et al., 2011). Such studies should be conducted more frequently in tropical urban environments like Singapore.
Singapore is an island nation located near the equator in Southeast Asia. It is highly urbanised but has substantial areas of greenery in remaining forest fragments and parks. In Singapore, identities of fruiting bodies of ECM were compiled as part of the major fungal surveys (Corner & Bas, 1962; Lee, 2019; Tan et al., 2011; Tham & Watling, 2017). The fungal biodiversity list is incomplete as only fungi with visible fruiting bodies have been captured and DNA work was only done sporadically. For the DNA identification of ectomycorrhiza, the nuclear internal transcribed spacer (ITS) is generally sequenced. This region contains the linear order of 18S small subunit, 5.8S, and 28S large subunit with ITS1 and ITS2 sub-regions in between (Gardes & Bruns, 1993; Janowski et al., 2019; Joos et al., 2020; Manter & Vivanco, 2007).
There is much needed research into ECM in tropical urban city such as Singapore to better harness nature-based climate solutions and mitigate climate change. The first step to understanding any ecosystem is to identify the organisms. This is also true for urban parks. Urban parks are known to provide essential ecosystem services in cities and such services vary depending on the biodiversity richness (Mexia et al., 2018). Nurseries which provide the trees to urban parks need to be investigated as well. According to Trappe (2003), nurseries are sources of ECM for trees that grow in parks. The introduction of ECM into parks through its plant hosts can shape its belowground biodiversity such as rhizosphere bacteria communities (Cowan et al, 2023). After identification of these ECM, the ecological functions can be interpreted in future studies. It is also important to understand the factors that affect the ECM communities such as soil properties. With this understanding, we can inoculate specific ECM in urban parks to maximise its ecosystem services. Given the importance of Dipterocarps to the eco-region, this study aimed to answer two main questions: 1) What are the ECM found in Dipterocarps planted in parks, Hopea odorata in particular? 2) What are the ECM found in Dipterocarp seedlings grown in a nursery?