Abstract

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.

Highlights

  • Many Neotropical forests experience seasonality in rainfall with periods of abundance in precipitation being followed by pronounced periods of seasonal drought (Allen et al, 2017; Feng, Porporato, & Rodriguez-Iturbe, 2013)

  • Longer dry seasons are unlikely to impact liana-tree competition directly; 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

  • Forest-level median tree biomass increment was generally higher in the wet than in the dry season for both the control and removal plots

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Summary

Introduction

Many Neotropical forests experience seasonality in rainfall with periods of abundance in precipitation being followed by pronounced periods of seasonal drought (Allen et al, 2017; Feng, Porporato, & Rodriguez-Iturbe, 2013). Due to anthropogenic climate change, the length of the dry season has been increasing in many Neotropical forests (Fu et al., 2013), and tropical forests have been subjected to additional severe drought events, e.g. in 2005 and 2010 in the Amazon (Marengo, Tomasella, Alves, Soares, & Rodriguez, 2011; Zeng et al, 2008). These drying trends are projected to continue into the future Liana-tree competition is generally much more intense than tree-tree competition (Tobin, Wright, Mangan, & Schnitzer, 2012), resulting in reduced growth

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