# Seasonal drought shapes the relationship between stem CO2 efflux and belowground respiration in an even-age rubber plantation on Hainan Island, China

## TL;DR

Seasonal drought affects how much CO2 is released from rubber tree stems, with different factors influencing this process in dry and wet seasons.

## Contribution

This study reveals seasonal dependencies in the relationship between stem CO2 efflux and belowground respiration in rubber plantations.

## Key findings

- Tapping activity increased stem CO2 efflux in rubber trees by 10.37% to 233.66% compared to non-tapped trees.
- Stem CO2 efflux showed a single-peak seasonal pattern, with lower values in the dry season.
- Root respiration and sap flow were key drivers in the dry season, while soil moisture and air temperature were dominant in the wet season.

## Abstract

Stem CO2 efflux (Ec
) reflects the amount of photo-assimilated carbon released back into the atmosphere and plays a critical role in the carbon balance of tree and forest ecosystems. Despite previous studies indicating that a portion of stem CO2 originates from root respiration (Rroot
), the seasonal dynamics of Ec
and its relationship with belowground respiration remain poorly understood, particularly in tropical regions characterized by distinct dry and wet seasons.

To address this gap, we investigated Ec
in tapping and non-tapped rubber trees, along with environmental factors and physiological traits (sap flow flux density, root respiration, and leaf area index) from 2018 to 2021.

Our results showed that tapping activity increased the Ec
of rubber trees compared to non-tapped trees, with increases ranging from 10.37% to 233.66%. However, the magnitude of this increase varied between the dry and wet seasons. Although tapping enhanced the Ec
, it did not alter the Ec
seasonal pattern. Consequently, Ec
in both tapped and non-tapped rubber trees displayed an overall single-peak pattern, with significantly lower values during the dry season compared to the wet season, suggesting growth phenology primarily regulates Ec
seasonal dynamics. Structural equation modeling revealed that root respiration (Rroot
), sap flow flux density (Fd
), and soil moisture at 50 cm depth as the primary drivers of the Ec
variations during the dry season. In contrast, soil moisture at 5 cm depth and air temperature (Ta
) were identified as dominant factors influencing Ec
in the wet season, with belowground respiration having a negligible influence.

These results suggest that the relationship between Ec
and belowground respiration is environmentally sensitive and exhibits seasonal dependency.