Effect of lianas on forest‐level tree carbon accumulation does not differ between seasons: Results from a liana removal experiment in Panama

Abstract

Lianas are prevalent in Neotropical forests, where liana‐tree competition can be intense, resulting in reduced tree growth and survival. The ability of lianas to grow relative to trees during the dry season suggests that liana‐tree competition is also strongest in the dry season. If correct, the predicted intensification of the drying trend over large areas of the tropics in the future may therefore intensify liana‐tree competition resulting in a reduced carbon sink function of tropical forests. However, no study has established whether the liana effect on tree carbon accumulation is indeed stronger in the dry than in the wet season. Using 6 years of data from a large‐scale liana removal experiment in Panama, we provide the first experimental test of whether liana effects on tree carbon accumulation differ between seasons. We monitored tree and liana diameter increments at the beginning of the dry and wet season each year to assess seasonal differences in forest‐level carbon accumulation between removal and control plots. We found that median liana carbon accumulation was consistently higher in the dry (0.52 Mg C ha−1 year−1) than the wet season (0.36 Mg C ha−1 year−1) and significantly so in three of the years. Lianas reduced forest‐level median tree carbon accumulation more severely in the wet (1.45 Mg C ha−1 year−1) than the dry (1.05 Mg C ha−1 year−1) season in all years. However, the relative effect of lianas was similar between the seasons, with lianas reducing forest‐level tree carbon accumulation by 46.9% in the dry and 48.5% in the wet season. Synthesis. Our results provide the first experimental demonstration that lianas do not have a stronger competitive effect on tree carbon accumulation during the dry season. Instead, lianas compete significantly with trees during both seasons, indicating a large negative effect of lianas on forest‐level tree biomass increment regardless of seasonal water stress. Longer dry seasons are unlikely to impact liana‐tree competition directly; however, the greater liana biomass increment during dry seasons may lead to further proliferation of liana biomass in tropical forests, with consequences for their ability to store and sequester carbon.

Causes and consequences of liana infestation in southern Amazonia

“…Lianas strongly affected tree growth rate and the degree of liana infestation was the most important explanatory factor in determining individual tree growth. The negative effect of lianas on tree growth has also been observed in other neotropical forests (Clark & Clark, 1990;Fadrique & Homeier, 2016;Ingwell et al, 2010;van der Heijden & Phillips, 2009;van der Heijden, Powers, & Schnitzer, 2015). The fact that a negative effect of liana infestation on tree growth has now been observed in multiple environments across a large region suggests that this represents a general effect across most tropical forests.…”

“…The negative impact of lianas on tree growth and tree survival (Ingwell et al, 2010;Phillips et al, 2005) means that lianas significantly impact on the capacity of tropical forests to store and sequester carbon (e.g. Phillips et al, 2002;Tymen et al, 2016;van der Heijden et al, 2015).…”

“…We also showed that lianas may reduce the growth rate of the trees, especially for trees growing in well-lit conditions. Thus, in all, liana increases are capable of affecting the structure, stand-level growth, floristic composition and reproductive success of tropical forests and trees (García-León, Martínez Izquierdo, Mello, Powers, & Schnitzer, 2018;van der Heijden et al, 2015). Not only do lianas reduce the capacity of forests to sequester atmospheric carbon dioxide, but they also affect competitive interactions among their tree species, and hence are capable of altering their tree biodiversity.…”

“…Changing climatic conditions that include lengthening of dry seasons (e.g. DeWalt et al, 2010;van der Heijden, Powers, & Schnitzer, 2019) provide one leading potential explanation for this.…”

The negative effect of lianas on tree growth varies with tree species and season

“…Lianas appear to grow particularly well during seasonal drought (Schnitzer, 2005; Schnitzer & van der Heijden, 2019), which may lead to stronger competition for limited resources, particularly water. By contrast, trees grow more during the wet season (Schnitzer & van der Heijden, 2019), when competition from lianas may actually have a larger negative effect on tree growth (van der Heijden, Powers, & Schnitzer, 2019). However, the season in which lianas exert stronger negative effects on trees is poorly understood.…”