Long-Term Patterns in Forest Soil CO2 Flux in a Pacific Northwest Temperate Rainforest

Abstract

Soil CO2 efflux (Fs) plays an important role in forest carbon cycling yet estimates of Fs can remain unconstrained in many systems due to the difficulty in measuring Fs over long time scales in natural systems. It is important to quantify seasonal patterns in Fs through long-term datasets because individual years may show patterns that are not reflective of long-term averages. Additionally, determining predictability of net patterns in soil carbon flux based on environmental factors, such as moisture and temperature, is critical for appropriately modeling forest carbon flux. Ecosystems in moderate climates may have strong CO2 efflux even during winter, and so continuous quantification of annual variability is especially important. Here, we used a 2008–2023 dataset in a lowland temperate forest ecosystem to address two main questions: (1) What are the seasonal patterns in Fs in a highly productive temperate rainforest? (2) How is average Fs across our study area predicted by average coincident temperature, soil moisture and precipitation totals? Data showed clear seasonality where Fs values are higher in summer. We also find Fs across our measurement network was predicted by variation in abiotic factors, but the interaction between precipitation/moisture and temperature resulted in greater complexity. Specifically, in spring a relatively strong relationship between air temperature and Fs was present, while in summer the relationship between temperature and Fs was flat. Winter and autumn seasons showed weak positive relationships. Meanwhile, a negative relationship between precipitation and Fs was present in only some seasons because most precipitation falls outside the normal growing season in our study system. Our data help constrain estimates of soil CO2 fluxes in a temperate rainforest ecosystem at ~14–20 kg C ha−1 day−1 in summer and autumn, and 6.5–10.5 kg C ha−1 day−1 in winter and spring seasons. Together, estimates suggest this highly productive temperate rainforest has annual soil-to-atmosphere fluxes of CO2 that amount to greater than 4.5 Mg C ha−1 year−1. Sensitivity of such fluxes to regional climate change will depend on the balance of Fs determined by autotrophic phenological responses versus heterotrophic temperature and moisture sensitivity. Relatively strong seasonal variation coupled with comparatively weak responses to abiotic variables suggest Fs may be driven largely by seasonal trends in autotrophic respiration. Accordingly, plant and tree responses to climate may have a stronger effect on Fs in the context of climate change than temperature or moisture changes alone.