The ecology of a liana community in a SW Chinese rainforest: functional traits, dynamics, and distribution

Summary

Tropical forests are global hotspots of biodiversity and important for the global carbon and water cycle. Lianas form a conspicuous element of tropical forests but have largely been ignored in species-level vegetation surveys. As a result, their ecology, in terms of functional traits, dynamics, and distribution are not fully understood. This study aims to improve our understanding of liana community ecology by analyzing species differences in functional traits, liana dynamics, and liana distributions in a tropical rainforest in SW China. I addressed three main questions in this thesis.

In chapter 2, I address the question how species distribution and abundance are associated with soil nutrients, canopy gaps, and topography. I evaluated in a 20-ha forest plot the distribution of the 50 most abundant liana species, comprising > 18,000 individuals. Soil pH and P were the strongest drivers of liana species distribution and also strongly correlated with each other. Most liana species occurred under significantly higher or lower soil nutrient conditions than expected at random. Lianas mainly separated along the P gradient, whereas for N and K most liana species tended to occupy locations with high soil nutrient concentrations. This study highlights the importance of soil nutrient status, and especially phosphorus for liana niche partitioning in wet tropical forests on highly weathered soils. Most liana species had high resource requirements for N, K, and light, which come along with their fast growth and acquisitive resource use strategy. Hence, belowground resource availability plays an important role in shaping the assembly of liana communities.

In chapter 3 I evaluate how functional trait associations shape abiotic niche dimensions. Eighteen functional traits related to light, water, or nutrient acquisition, storage and use were measured, and liana niche dimensions were quantified for 29 species, based on their distributions. I found two trait spectra; a tissue toughness spectrum ranging from soft to hard tissues, along which species also vary from acquisitive to conservative water use, and a resource acquisition spectrum ranging from low to high light and nutrient acquisition and use. Different traits were important for different niche dimensions. These studies indicate that plant functional traits and strategies can indeed explain species distributions. Lianas show a broad spectrum from conservative to acquisitive trait strategies which allows them to partition soil fertility gradients (in line with chapter 2).

In chapter 4, I ask how abiotic and biotic conditions explain liana dynamics and community changes over time. I monitored from 2013-2019 >20,000 lianas and analyzed how changes in liana communities within twenty 1-ha plots were driven by underlying vital rates (i.e., recruitment, growth and survival), abiotic factors (i.e., topography, light gaps, water, and soil phosphorus), biotic factors (tree and liana basal area), and the functional composition of liana communities based on 18 traits. I found that liana abundance decreased because of high mortality of small lianas, whereas liana basal area increased because of high survival and growth of large lianas. Liana communities showed a spectrum of slow to fast carbon and nutrient use, and a spectrum from slow to fast water use. Abiotic environmental factors (i.e., elevation and soil phosphorous and water) and liana functional composition influenced liana demographic processes, whereas competition by lianas and trees had little effect. Over the 5 years period, the liana community changed toward more, large, acquisitive lianas with rapid water use. This indicates that the liana community is undergoing succession, possibly driven by increased CO2 fertilization.

In this thesis, I show and discuss that abiotic resource conditions and biotic conditions (including forest structure and species traits) indeed determine the species performance (i.e., recruitment, growth, and mortality) and shape liana species distribution and niches. The observed decrease in liana abundance and increase in basal area in this SW Chinese rainforest partly contrasts with the widely documented liana increase across various Neotropical forests, which indicates that local drivers can overrule global change drivers.