Effects of temperature and precipitation anomalies on carbon dioxide and latent heat fluxes in wetland ecosystems

This study conducted a comprehensive assessment of the response of wetland ecosystems in temperate and polar latitudes, located on different continents, to extreme weather events. These events included temperature anomalies (unusually high/low temperatures) and precipitation anomalies (droughts/intense precipitation). The analysis of the response net ecosystem exchange (NEE) of CO₂ and latent heat (LE) fluxes to extreme temperature and precipitation events used ERA5 reanalysis data [Smith, 2011] and observations of CO₂ and LE fluxes from the global FLUXNET database [https://fluxnet.org/data/]. Fifteen greenhouse gas flux monitoring stations were selected for the study, representing the longest and most continuous time series of observations. These stations are located on different continents, with eight stations in temperate latitudes and seven in polar regions. It should be noted that this study focused exclusively on the warm season. The beginning and end of the warm season were defined as the sustained crossing of the daily mean air temperature above 0°C for at least seven consecutive days.

For each station, daily anomalies of CO₂ and LE fluxes were calculated as the deviation from the long-term mean values for the corresponding day of the year. Extremely high/low values of flux anomalies were identified as exceeding one standard deviation from the overall time series for each calendar month individually.

To identify periods with extreme air temperature values, ERA5 reanalysis data on two-meter air temperature every 3 hours with a spatial resolution of 0.25°×0.25° from 1991 to 2021 were used. To estimate extreme precipitation amounts, data from half-hourly station observations were used. Daily means were calculated from these data in a first step. Thresholds for defining extremely hot/cold periods were calculated as daily mean air temperature exceeding the 95th percentile (for anomalously hot periods) or not exceeding the 5th percentile (for anomalously cold periods) of a normal distribution with mean and standard deviation. The distribution was constructed for a specific month of the year and then averaged over the entire period considered. Two approaches were used to determine the extreme precipitation threshold. In the first approach, extreme precipitation days were defined as days with daily precipitation exceeding the 95th percentile of the probability density function (the Weibull distribution was used for precipitation). The second approach was based on the assessment of the Antecedent Precipitation Index (API), which determines the cumulative effect of precipitation on CO₂ fluxes.

For the quantitative assessment of the relationship between temperature and precipitation extremes and flux anomalies, the percentages of days on which both the NEE/LE anomaly exceeded the standard deviation and the temperature/precipitation exceeded the 95th percentile for the upper threshold or the temperature did not reach the 5th percentile for the lower threshold were calculated. The percentage was calculated based on the total number of days when one of the characteristics (air temperature, daily sum of precipitation) exceeded the threshold.

The analysis showed that temperate and polar wetland ecosystems can respond differently to temperature and precipitation anomalies. These differences can be attributed to the geographic location of the ecosystem, regional climatic conditions, plant species composition, and the intensity of temperature and precipitation extremes. During the warm half of the year, periods of extremely high temperatures in temperate latitudes were associated with a positive CO₂ flux anomaly, corresponding to an increased emission of CO₂ into the atmosphere. In contrast, polar latitudes showed an opposite response - an increase in CO₂ uptake by wetland ecosystems under anomalously high temperatures. This opposite response of CO₂ fluxes may be related to the different soil moisture regimes in polar wetland ecosystems and the different plant species composition. Extremely high temperatures were accompanied by positive LE anomalies due to the intensification of evaporation processes with rising temperatures, a trend observed in all wetland ecosystems analyzed.

The immediate response of wetland ecosystems to intense precipitation (above the 95th percentile) was manifested as an increase in CO₂ flux to the atmosphere at almost all stations analyzed. This observed response could be related to the "Birch effect" [Birch, 1964], which is characterized by an intensification of soil respiration due to a sudden increase in soil moisture and, consequently, an increase in the rate of decomposition and mineralization of organic matter during heavy precipitation and rising groundwater levels. LE flux decreases during intense precipitation, indicating suppression of evaporation due to high humidity and reduced incoming solar radiation.  The cumulative effect (API index) of extremely high precipitation is characterized by a predominance of extremely positive CO₂ flux anomalies over negative ones in wetland ecosystems at both temperate and polar latitudes. It should also be noted that the percentage of days with increased CO₂ uptake during the two weeks following intense precipitation is significantly higher than for the immediate response (10-25% of days in temperate latitudes and 5-20% of days in polar latitudes). The increase in CO₂ uptake after heavy precipitation may be related to enhanced photosynthetic rates of the vegetation cover under sunny weather and optimal soil moisture conditions. A prolonged absence of precipitation, represented by extremely low API values, is accompanied by negative CO₂ flux anomalies (enhanced uptake) at most of the studied wetland ecosystem stations, indicating a high adaptive potential of the studied wetland ecosystems to short-term (less than 14 days) dry periods. On the other hand, enhanced CO₂ uptake could be facilitated by clear weather conditions, which prevail during dry periods and are accompanied by an increase in direct solar radiation and corresponding acceleration of photosynthetic processes.

It is noteworthy that flux anomalies often did not coincide with temperature or precipitation extremes, indicating that the functioning of wetland ecosystems is strongly influenced by multiple abiotic and biotic factors, which vary among different plant communities.