When forests hold their breath: will increasing drought further disrupt carbon sequestration?

The 2020–2021 record drought in Taiwan halted carbon sequestration in its predominantly evergreen subtropical forests. The analysis uncovers a significant correlation between net ecosystem exchange, radiative factors, groundwater levels, and wildfires, indicating that the severity of droughts leads to a shift from carbon absorption to emission in these forests, thereby inviting a broader examination of the climate–carbon nexus in future scenarios.


Introduction
Subtropical forests act as significant carbon sinks, absorbing carbon dioxide (CO 2 ) from the atmosphere, thereby playing a crucial role in mitigating climate change.However, during the severe 2020-2021 drought experienced in Taiwan, an anomaly was observed.The typically lush forests were stressed, and surface observations in central Taiwan (figure 1 map) exhibited persistent positive net ecosystem exchange (NEE) values, at times approaching a value of 1 for several weeks, indicating a halt in the ecosystem's carbon sequestration function (figure 1(a)).This phenomenon metaphorically suggests the forests held their breath, i.e. their cessation of carbon sequestration for a length of time.
Long-term NEE observations in this subtropical forest (see supplemental text) have revealed a robust carbon sequestration capability, with an annual carbon absorption capacity of around 10 tons of CO 2 per year per hectare, thereby aiding in mitigating the effects of anthropogenic climate change [1].Fluctuations in NEE reflect the dynamics of vegetation growth in the forest, which are largely influenced by seasonality and environmental disturbances such as windthrow, monsoons, and drought events.Large NEE perturbations, as observed in 2013, 2017, and 2019, were triggered by events like cycloneinduced defoliation, continuous rainfall events in early 2016, and drought events in 2015 that lasted only briefly [2].However, the prolonged NEE perturbation observed in 2021 marks an unprecedented-and hence worrisome-intensity of such events.This perspective synthesizes compelling insights correlating prolonged NEE turnover with satelliteobserved shallow groundwater fluctuations, accentuating the invaluable role of hydrological assessments in understanding drought impacts.Additionally, the heightened incidence of wildfires, intensified by the drought, emerges as a plausible factor altering NEE dynamics.This scenario underscores the potential adversities climate change can levy on the carbon sink efficacy of subtropical forests, even in traditionally precipitation-rich regions like Taiwan, particularly through intensified droughts.By assimilating observations and broader analyses, this discourse endeavors to elucidate the seldom occasions where extreme climatic events perturb terrestrial carbon dynamics, hence enriching the ongoing narrative on climate-carbon cycle feedback amid evolving climate scenarios.

The carbon turnover
As spring transitioned into early summer in central Taiwan, a rise in NEE was noted at the Lianhuachi Hydrometeorological Observatory (location indicated in figure 1).Over a span of 3-5 months in 2021, a notable NEE turnover was recorded for several continuous weeks (figure 1(a)), indicating evolving ecosystem dynamics and a shift in carbon absorption capacity within this subtropical region.The phrase 'NEE turnover' marks this critical juncture where the ecosystem momentarily transitioned from functioning as a carbon sink to becoming a carbon source, visually captured in figure 1(a).During this period, NEE values approached 1 for a few weeks, signifying a near equilibrium between carbon uptake and release.The few weeks of NEE reaching 1 is an alarming indication of halted carbon sequestration, representing a temporary cessation of the forest's ability to act as a carbon sink, which is crucial for mitigating atmospheric CO 2 levels.Rainfall anomalies analyzed via automatic rain gauges across the island highlighted a widespread rainfall deficit during the drought (not shown), further underscoring the rough conditions of 2021.

Drought implications
During the same period, water budget analysis derived from the twin satellites of Gravity Recovery and Climate Experiment (GRACE [3]) revealed a significant reduction in the shallow groundwater component, reaching a nadir, thereby highlighting the severe drought conditions enveloping Taiwan from spring 2020 until May 2021 [4].In Taiwan, the reliance on groundwater metrics over meteorological drought indices becomes apparent given the latter's often misleading narrative due to the region's high humidity content.The depiction of depleted groundwater, as seen in figure 1(b), sheds light on the severity of the 2020-2021 drought, providing a satellitevalidated lens to the hydrological challenges faced in forested areas, especially where subsurface observations are scant.
To highlight the specificity of Taiwan's severe drought and NEE turnover in 2021, we not only focus on its duration and intensity but make a comparison with previous drought years of 2015 and 2018 (see figure 2).Despite 70% of climatological rainfall occurring from May to October, anomalous conditions led to over 15 months of groundwater levels below 25% by April 2021 (see figure 2(c)).Unlike year of 2015 and 2018, the continues drought in 2021 resulted in NEE turnover, making Taiwan's forest ecosystem into a carbon source through April, 2021.Moreover, the drought's intensity, with prolonged groundwater levels around 1% in 2020 and 2021, differed from 2015 and 2018, indirectly impacting carbon sequestration in 2021.This perspective invites a deeper understanding of the nuanced yet significant indicators that unveil the true extent of hydroclimatic adversities, enriching the discourse on how we interpret and respond to the evolving climate narrative in such ecologically rich yet vulnerable locales.
Further statistical analysis, depicted in figure 3, was conducted to examine the relationship between NEE, net radiation, GRACE-derived groundwater, and wildfire count data.The results in figure 3(a) revealed a notable association with a p-value below 0.01, suggesting that radiative factors significantly influence monthly NEE in this subtropical forest, although this relationship does not persist during other droughts.Notably, figure 3(b) highlights unusual conditions, such as water limitations (droughts in 2015, 2018, and 2021) or radiative constraints (excessive rainfall in 2016), which distinctly affect NEE, resulting in its reduction toward the boundaries/tails of the regression line.The analysis further reveals a time lag of 1-2 months, suggesting that the severity of drought conditions potentially leads to positive NEE values, supporting the implied transition from carbon absorption to emission in the stressed ecosystem.Additionally, figure 3(c) illustrates a significant statistical relationship between NEE and wildfire count, substantiating the more direct and interlinked dynamics of drought, wildfire occurrences, and ecosystem carbon exchange without much lead time.Furthermore, the green chlorophyll index data derived from Himawari-8 satellite imagery reveals an island-wide degradation in forest health from March 2021 through April 2021 (supplemental figure), coinciding with the onset of the Meiyu season that ended the drought.This decline in chlorophyll content, indicative of adverse environmental conditions, coincided with the fire outbreak in May, potentially undermining the forest's ability to absorb CO 2 , thereby impacting the carbon sequestration capacity of the region.This connection further underscores the complex interplay of hydroclimatic and wildfire disturbances with ecosystem carbon dynamics, shedding light on the broader environmental implications amid changing climate conditions [5][6][7] and calling for further examination.

Ways forward
The observation of NEE values turning positive, peaking at 1 during Taiwan's severe drought period, highlights a delicate balance between carbon emission and absorption in the ecosystem.This aligns with a broader scientific narrative showcasing forest carbon sinks' sensitivity to climate adversities like drought [8].The temporary halt in carbon sequestration of a subtropical forest during the recent drought signifies a notable deviation from typical carbon sink behavior, echoing global observations in various studies.This ecological shift, also marked by a rise in subcanopy tree species mortality, suggests a broader impact of various disturbance agents like wind-throw, droughts, fires, and insect outbreaks.The warming climate, along with recurrent severe droughts, may escalate the frequency and duration of such NEE turnovers, thereby increasing forest wildfires and possibly reducing the forests' carbon sequestration potential in the long run.As such, the anticipated increase in drought occurrences in Taiwan [9] suggests a potential decline in the region's carbon sinks' efficacy.This scenario underscores the need for a thorough understanding and deployment of adaptive and mitigation strategies to maintain carbon sink integrity amid evolving climate dynamics.The ongoing dialogue in the scientific community about climate adaptive mechanisms and sustainable forest management [10] could be crucial in bolstering resilience and sustaining carbon sink functionality amid escalating climate adversities.Recent studies emphasize the importance of understanding the interplay between forest management and climate change, significantly affecting forest productivity and carbon budgets [11].Moreover, adapting to climateinduced changes in forest dynamics, such as shifts in species distribution and community composition, is vital for maintaining forest carbon sinks and overall ecosystem resilience [12].The observed reduction in forest carbon uptake during severe drought should remind forest management and guide restoration strategies to ensure the resilience of forest carbon sinks facing the warming climate.

Figure 1 .
Figure 1.Historical wildfire distribution during 2012-2022 in Taiwan (map) and time series of NEE (a), groundwater (b), wildfire count (c), where contains monthly value (bars) and 4 month moving average (lines, centered).The Lien-Hua-Chi Hydrometeorological Observatory is located at the center of the 45 km circle that includes the wildfires in (c).

Figure 2 .
Figure 2. Historical drought period in Taiwan during 2015, 2018, 2021. 1 The red box represents periods when groundwater levels are below 25%.

Figure 3 .
Figure 3. Scatter diagrams between NEE and groundwater (a), net radiation (b), and wildfire count (c). 1 Red dots indicate the epicenter of major droughts (February-June 2015, February-June 2020, February-June 2021); green dots indicate the epicenter of NEE drops (February-June 2016); blue dots indicate the rest of the period. 2The trend line in figure (a) is calculated excluding the samples represented by red dots.