A Spatial and Temporal Risk Assessment of the Impacts of El Niño on the Tropical Forest Carbon Cycle: Theoretical Framework, Scenarios, and Implications

Abstract

Strong El Niño events alter tropical climates and may lead to a negative carbon balance in tropical forests and consequently a disruption to the global carbon cycle. The complexity of tropical forests and the lack of data from these regions hamper the assessment of the spatial distribution of El Niño impacts on these ecosystems. Typically, maps of climate anomaly are used to detect areas of greater risk, ignoring baseline climate conditions and forest cover. Here, we integrated climate anomalies from the 1982–1983, 1997–1998, and 2015–2016 El Niño events with baseline climate and forest edge extent, using a risk assessment approach to hypothetically assess the spatial and temporal distributions of El Niño risk over tropical forests under several risk scenarios. The drivers of risk varied temporally and spatially. Overall, the relative risk of El Niño has been increasing driven mainly by intensified forest fragmentation that has led to a greater chance of fire ignition and increased mean annual air temperatures. We identified areas of repeated high risk, where conservation efforts and fire control measures should be focused to avoid future forest degradation and negative impacts on the carbon cycle.

Effect of Various Types of ENSO Events on Moisture Conditions in the Humid and Subhumid Tropics

“…Considering the possible effects of the ENSO on the growth and functioning of tropical vegetation, it is also important to take into account the plasticity of tropical plant species and their ability for biological adaptation to current environmental conditions [93][94][95]. This can be extremely important given the projected increasing frequency of extremely strong ENSO events by the end of the 21st century [96] and the need to obtain adequate projections of the possible responses to such changes of the different plant communities [93].…”

Determining a Threshold to Delimit the Amazonian Forests from the Tree Canopy Cover 2000 GFC Data

“…For example, the lower thresholds evaluated—between 10% and 50%—overestimated forest cover, especially within savanna formations, where trees are sparser within the landscape. Thus, the choice of less conservative thresholds may inflate the quantification of forest disturbances, such as fires [27], deforestation [13], selective logging [23], forest fragmentation [24,25,26], and, consequently, carbon emissions accounting.…”

“…Any pixel that exceeded these thresholds was considered as forest. We selected these thresholds based on preliminary tests and the previous studies from the literature [23,24,26,27,28,29], representing a wide range of thresholds. Hereafter, we refer to these thresholds as 10%, 30%, and so on.…”

“…Several thresholds have been empirically chosen in the literature to define forest cover with the TreeCover2000 data. For example: Grecchi et al [23], Taubert et al [24], Brinck et al [25], and Esquivel-Muelbert et al [26] used 30%; Shimabukuro et al [27] used 50%; Wagner et al [28] used 80%; and Tyukavina et al [29] used 83%. However, as far as we know, no study has focused on testing the performance of these thresholds in delimiting the Amazonian rainforests in order to match with other countries’ definition of forest.…”

Effects of El Niño drought on seedling dynamics in a seasonally dry tropical forest in Northern Thailand

“…Although there is no consensus for a systematic change in the variability of El Niño over the 21st century, it is very likely that rainfall variability related to El Niño will be enhanced significantly by the latter half of the 21st century (Arias et al, 2021). This suggests that there will be stronger and more frequent droughts in the tropics owing to future climate change (Esquivel‐Muelbert et al, 2019; Lyon, 2004; Thirumalai et al, 2017); however, details of the impacts of drought on tropical forests remain to be elucidated (Feeley et al, 2011; Holmgren et al, 2001). Accumulating details of drought impacts is essential to more accurately predict the effects of global climate change on tropical forests and to understand the mechanisms involved.…”