Micrometeorological drivers affecting the variability of CO2 uptake in the Himalayan Oak and Pine dominated ecosystems: An assessment of causal relationships

Abstract

Micrometeorological variability significantly impacts the structures, functions, and dynamics of ecosystems. However, the assessment of feedback and causal relationships among microclimatic drivers and various ecosystems in the Himalayan region is rarely evaluated. Here, we studied the micrometeorological drivers controlling the variability in the net ecosystem exchange (NEE) of Himalayan Oak (Banj-Oak/Quercus leucotrichophora) and Pine (Chir-Pine/Pinus roxburghii) dominated ecosystems, as NEE is an indicator of ecosystem functioning. We used half-hourly eddy covariance flux data of CO₂ fluxes from two sites established over Pine and Oak dominated ecosystems in Uttarakhand, India. We conducted the analysis with the information theory-based Temporal Information Partitioning Networks (TIPNets) approach to generate weekly process networks. TIPNets represent directed lag-structured causal graphs to identify the causal relationships and capture the temporal association among the variables. Our analysis aimed to capture fluctuations in variables with up to 6 h of memory. Based on the data availability, we generated the weekly networks at both the sites for the monsoon and post-monsoon seasons of 2016 and 2017. In both ecosystems, the sub-daily scale variations among the micrometeorological variables are responsible for the fluctuations in NEE. The Pine ecosystem is found to be more sensitive to changes in air temperature (TA) and uptakes more CO₂ as compared to the Oak ecosystem throughout the study period. The transfer entropy links show that the NEE of the Oak ecosystem is moisture-driven (precipitation and relative humidity), while the Pine ecosystem is heat-driven (TA and net solar radiation) in both seasons. The influence of precipitation is not observed within a short memory of 6 h in the Pine ecosystem. This is because lesser fine roots take time to show the precipitation signature on NEE through infiltration, soil moisture, and root water uptake, compared to Oak. However, the impacts of moisture stress are evident in the network structure of both ecosystems, with more causal links observed in the network during dry periods compared to wet periods.