1 INTRODUCTION

Lianas contribute significantly to the dynamics of tropical and subtropical forests (Paul & Yavitt, 2010; Putz & Mooney, 1991; Schnitzer & Bongers, 2002), but our understanding of their contribution to food webs and biochemical cycling remains limited. In liana-rich forests, lianas provide substantial food resources and habitats for animals (Odell et al., 2019). To date, most studies have focused on ‘green’ food webs associated with living liana organs. By contrast, we know very little about the role of lianas in ‘brown’ food webs and associated biogeochemical nutrient turnover, including limited knowledge of how differences between microbial and invertebrate organisms impact the decomposition of liana organs.

The role of lianas in biogeochemical cycling may be understood from their contribution to total litter biomass and by the difference in functional traits of their organs compared to co-occurring trees (Powers, 2014). These trait differences may affect rates of decomposition and, in turn, nutrient turnover in forests (Cornwell et al., 2008). The contribution of lianas to total forest biomass is small, usually below 5% (van der Heijden et al., 2013; van der Heijden et al., 2014), but their contribution to specific components of the total forest litter pool may be much greater. For example, liana contribution to leaf litter fraction can be as high as 20–40% (Hegarty, 1991; Hora et al., 2008; Kusumoto & Enoki, 2008; Pragasan & Parthasarathy, 2005; Putz, 1983), and lianas have been found to contribute ~50% of seed and flower fall in seed traps in a tropical Dipterocarp forest (Wright et al., 2015). Global functional trait datasets suggest that liana leaves are richer in nutrients (higher nitrogen (N) and phosphorus (P) content per leaf mass) and have higher specific leaf area (SLA) than trees (Asner & Martin, 2012; Kazda, 2014). Comparisons to date include liana-host pairs (Cai & Bongers, 2007; Kazda et al., 2009; Kazda & Salzer, 2000), congeners (Cai et al., 2007), dominant species in forests (Cai & Bongers, 2007; Kusumoto & Enoki, 2008) or global datasets of leaf traits (Asner & Martin, 2012). Due to these structural and chemical characteristics, liana leaves as well as the litter lianas produce likely represent a more palatable food source for invertebrates and microbes compared to those of co-occurring trees in tropical and subtropical forests. Moreover, soil macro- and mesofauna, which in concert, can consume up to 50% of litter (Frouz, 2018), might benefit from enhanced litter quality provided by lianas. If that is the case, then a mixture of litter from lianas with litter from trees may enhance litter mixture decomposition rates and speed up biochemical cycling.

Liana litter in forest communities may enhance tree litter decomposition, as decomposers can access different resources from various litter sources and qualities; this is known as the complementarity effect or positive non-additive effect. Complementarity effects have been detected in several decomposition studies on tree litter (Gartner & Cardon, 2004; Guo et al., 2019; Hättenschwiler et al., 2005; Meier & Bowman, 2008; Vos et al., 2013, but see Pan et al., 2015). One mechanism supporting the complementarity effect is nitrogen transfer which can enhance decomposition rates (Bonanomi et al., 2014). The broad range of trait diversity across both lianas and co-occurring trees represents an opportunity to test complementarity effects. Yet, decomposition studies leveraging this opportunity remain rare. Only a handful of studies examined the decomposition of liana litter of single invasive species, finding accelerated decomposition rates compared to native woody species or communities of non-infested areas (Ashton et al., 2005; Leicht-Young et al., 2009; Scowcroft, 1997). Few studies have considered liana litter decomposition at the community scale and the results regarding decomposition rates differ. On the one hand, Cornelissen (1996) found that woody climbers s.l. had relatively fast decaying litter in a temperate flora and Jo et al. (2020) assigned half of the liana species in a temperate forest to the group of rapidly decaying species. On the other hand, Santiago (2009) found no difference in decomposition rate of litter between lianas and canopy trees in a tropical community.

To date, we have little understanding of how liana and tree litter decomposition change across forest types or environmental gradients. Forest communities which host lianas can differ in many abiotic and biotic aspects, which, in turn, may affect decomposition properties. First, plant communities consist of different plant species (e.g. Pasion et al., 2018). These representative species might differ in their functional traits, even within the same growth form (e.g. Roeder et al., 2019). And it is well established that functional traits drive decomposition rates (Cornwell et al., 2008; Dias et al., 2017). Second, different communities arise because of underlying variation in soil and climate properties (Liu & Slik, 2014), which also affect decomposition; high soil moisture, temperature and air humidity will increase the decomposition rates of plant material (Swift et al., 1979).

In this study, we investigated the decomposition of liana and tree litter at community level across three forest types. We tested the following hypotheses:

H1: Liana litter generally decomposes faster than tree litter because liana leaves have higher litter quality than trees (Asner & Martin, 2012; Mello et al., 2020), and mesofauna contribute significantly to this process.

H2: Litter of lianas and trees decomposes differently across different forest types (significant forest x growth form interaction) due to differences in litter traits, microclimate or soil fertility.

H3: As a consequence of H1, liana leaf litter mixed in tree leaf litter enhances total litter decomposition (complementarity or positive non-additive effect e.g. Vos et al., 2013).

H4: As a consequence of H1, lianas release nutrients faster than trees (Hobbie, 2015).

We tested these hypotheses using liana and tree species growing in three different tropical forest communities that are interspersed in tropical southwest China. These forest types differ in tree, liana and herb species composition (Pasion et al., 2018) and vary in several abiotic aspects such as water availability, elevation and soil fertility (Liu & Slik, 2014; Roeder et al., 2019). We collected community-level data of 40 species (20 lianas and 20 trees), using mixed species bags as well as single species bags, to get a comprehensive understanding of liana and tree litter decomposition.