Drivers of aboveground wood production in a lowland tropical forest of West Africa: teasing apart the roles of tree density, tree diversity, soil phosphorus, and historical logging.

Tommaso Jucker,Harriet D Allen,Jeremy A Lindsell,Aida Cuni Sanchez,Gabriel S Amable,David A Coomes

doi:10.1002/ece3.2175

Abstract

Tropical forests currently play a key role in regulating the terrestrial carbon cycle and abating climate change by storing carbon in wood. However, there remains considerable uncertainty as to whether tropical forests will continue to act as carbon sinks in the face of increased pressure from expanding human activities. Consequently, understanding what drives productivity in tropical forests is critical. We used permanent forest plot data from the Gola Rainforest National Park (Sierra Leone) – one of the largest tracts of intact tropical moist forest in West Africa – to explore how (1) stand basal area and tree diversity, (2) past disturbance associated with past logging, and (3) underlying soil nutrient gradients interact to determine rates of aboveground wood production (AWP). We started by statistically modeling the diameter growth of individual trees and used these models to estimate AWP for 142 permanent forest plots. We then used structural equation modeling to explore the direct and indirect pathways which shape rates of AWP. Across the plot network, stand basal area emerged as the strongest determinant of AWP, with densely packed stands exhibiting the fastest rates of AWP. In addition to stand packing density, both tree diversity and soil phosphorus content were also positively related to productivity. By contrast, historical logging activities negatively impacted AWP through the removal of large trees, which contributed disproportionately to productivity. Understanding what determines variation in wood production across tropical forest landscapes requires accounting for multiple interacting drivers – with stand structure, tree diversity, and soil nutrients all playing a key role. Importantly, our results also indicate that logging activities can have a long‐lasting impact on a forest's ability to sequester and store carbon, emphasizing the importance of safeguarding old‐growth tropical forests.

Highlights

By sequestering CO2 from the atmosphere and storing it in wood, tropical forests currently act as a net carbon sink and play a critical role in abating climate change (Pan et al 2011)
The SEM provided a good fit to the data (v2 = 4.7, df = 8, P = 0.79; root mean square error of approximation (RMSEA) = 0.001; comparative fit index (CFI) = 0.999; SRMR = 0.027), and as a whole explained 81% of the variation in aboveground wood production (AWP) among plots (Fig. 2)
In addition to basal area, both tree diversity and soil P contributed to promoting AWP (Fig. 3B–C), with the direct effect of the two drivers being comparable in magnitude (Fig. 2)

Summary

Introduction

By sequestering CO2 from the atmosphere and storing it in wood, tropical forests currently act as a net carbon sink and play a critical role in abating climate change (Pan et al 2011). Brienen et al 2015), especially as tropical forests continue to be threatened by human activities (Laurance 1999; Chazdon 2003; Asner et al 2009) Part of this uncertainty stems from the fact that while multiple drivers are known to influence rates of aboveground wood production (AWP) in tropical forests (e.g., climate, soils, forest structure, functional traits, human disturbance; Malhi et al 2004; Asner et al 2009; Banin et al 2014; Lasky et al 2014), few studies have considered how these drivers act together to shape AWP. We continue to lack a clear understanding of the multiple interacting factors which together control AWP, especially in the context of the African tropics which remain relatively understudied (Lewis et al 2009; Cleveland et al 2011; Banin et al 2014)

Methods

Results

Discussion

Conclusion