Julia Isabella de Matos Rodrigues · Walmer Bruno Rocha Martins ·Victor Pereira de Oliveira · Myriam Suelen da Silva Wanzerley ·Hélio Brito dos Santos Júnior · Francisco de Assis Oliveira

Abstract Among the impacts of climate change,there is the intensification of phenomena such as the El Niño Southern Oscillation (ENSO) responsible for El Niño and La Niña.However,understanding their effects on the functional processes of forests is limited.Therefore,this study evaluated the effects of ENSO on litter stock and water holding capacity (WHC) in a successional forest in eastern Amazonia.Evaluations occurred in periods with the most rainfall in El Niño (2019) and least in La Niña (2021) years.Twelve permanent plots were used to sample litter.ENSO effects were evident for WHC,higher during El Niño.However,this influence was not clear for litter,as only in the rainy season effects were found.There was a positive correlation of WHC with precipitation and humidity,while litter stocks were negatively correlated with temperature and wind speed.Although the subject of this study requires long-term assessments,preliminary results suggests that,depending on the intensity of ENSO,forest functional processes can be strongly impacted and altered.The conclusion reinforces warnings by the scientific community about the impacts of climate change on the maintenance of litter stocks,decomposition and,consequently,the biogeochemical cycle and essential ecosystem services for the maintenance of Amazonia biodiversity.The need to develop long-term research to understand the effects of climatic change on litter stocks and water holding capacity is highlighted,especially in Amazonia.

Keywords Seasonality · Climate change · Succession ecosystem · Biogeochemical cycle · Amazonia biodiversity

Introduction

Climate change will intensify climate events such as theEl NiñoSouthern Oscillation (ENSO),which encompasses bothEl NiñoandLa Niña(Moura et al.2019).Increase in temperatures pose risks to survival (Roe et al.2019),food production (Leal Filho et al.2022),and social well-being,including increases in the incidence of fires and floods(Brando et al.2020).Therefore,considering the scenario of increasing deforestation,secondary forests are gaining importance,especially due to their relative increases in heights,diameters,and biomass,resulting in an efficient system for carbon sequestration (Barros et al.2020).

However,this shift to secondary forests can be disturbed by changes in climate patterns,especially in the Amazonia,where species are adapted to high temperatures and humidity(Garcia et al.2021).In 2019 for example,El Niño,combined with anthropogenic actions,had major impacts,including forest fires that burned over 19,617 km2of forest in the Amazon(Dong et al.2021;INPE 2021).However,in 2021,La Niñaresulted in historic Amazon floods,with soaring water levels leading to the declaration of a state of emergency in some regions (Espinoza et al.2022).

The consequences of increased temperatures include changes in life cycle events and consequently,alterations in food for wildlife (Antala et al.2022),leading to population reductions and community imbalance.Excessive temperature rise within the forest also results in the death of microorganisms responsible for litter decomposition,which significantly contribute to maintaining the vigor of the Amazonia forest through nutrient cycling (Bufacchi et al.2020).

In successional forest ecosystems,litter plays a vital role,including soil protection,mechanical control against erosion processes,shelter for edaphic entomofauna,and water holding capacity (Cuevas and Medina 1986;Kimmins 1987;Innangi et al.2018;Martins et al.2018;Caldeira et al.2019).The stock of litter is influenced by mechanisms of abscission and decomposition of dead material (Taiz et al.2017).Biotic factors such as soil quality,floristic composition,and forest structure have a significant impact on this process,which is regulated by climatic conditions (Moura et al.2017;Queiroz et al.2019).Increased temperatures,for example,induce the production of hormones responsible for leaf abscission,increasing litter production (Patharkar and Walker 2019).

While biotic factors such as composition and floristic diversity influence litter stock and water holding capacity (Rani et al.2016;Santos et al.2017),this study focuses on evaluating abiotic factors,particularly those related to climate elements,seeking to understand the effects of climate change on ecosystem functions.Additionally,monitoring how secondary vegetation responds to climate variations and events is essential for devising reforestation strategies for the Amazon.

The following question is highlighted: How does ENSO affect litter carbon stocks and water holding capacity (WHC)in a successional forest ecosystem in eastern Amazonia? Our hypothesis is that duringLa Niña,there will be a smaller litter stock but greater water holding capacity.Therefore,the objective of this study was to evaluate the effects of ENSO on litter stock and water holding capacity in a successional forest ecosystem in eastern Amazonia.

Material and methods

Study area

The study was carried out at the Escola de Castanhal Farm,belonging to the Federal Rural University of Amazonia in the municipality of Castanhal,eastern Amazonia,Brazil(1°19ʹ16ʺ S,47°57ʹ50ʺ W) (Fig.1).The topography is slightly undulating and the soils are Dystrophic Yellow Latosol of Rocky Phase I (Lateritic Concretionary),characterized by low fertility and high acidity due to the presence of toxic Fe and Al (Tenório et al.1999).According to Köppen’s classification system,the climate isAf3,with an average rainfall between 2,000 and 2,500 mm year-1(Alvares et al.2013).The rainiest period occurs from December to May,while the least rainy period is from June to November (INMET 2021).

Fig.1 Successional forest ecosystem evaluated during El Niño (2019) and La Niña (2021) for litter stock and WHC

The forest successional ecosystem under study was altered by shifting agriculture practices for approximately 47 years (1940–1987),and involved the cultivation of maize(Zea maysL.),cassava (Manihot esculentaCrantz.),and cowpea (Vigna unguiculataL.Walp).In 1987,the area was abandoned,allowing natural regeneration to cover the site.At present,34 years after abandonment,the predominant trees species includeLacistema pubescensMart.,Ocotea guianensisAubl.,Pourouma guianensisAubl.,andAnnona exsuccaDC.(Santos Junior et al.2021).

Experimental design and data collection

In early 2019,twelve 400 m2(20 × 20 m) permanent plots were demarcated.Samples were collected during the rainiest and least rainy months (March and September,respectively) of 2019 and 2021.For sampling,a metallic template 0.25 × 0.25 m was used,and five litter samples were randomly obtained within each plot,resulting in a total of 120 samples each year.All litter within the template was collected and placed and labeled in plastic bags.The collected material was sent to the Manejo de Ecossistemas e Bacias Hidrográficas Laboratory at the Federal Rural University of the Amazonia in the municipality of Belém,Pará,Brazil.

To evaluate water holding capacity (WHC),the methodology described by Blow (1955) was followed.The samples were arranged in plastic trays and submerged in water for 90 min,excess water removed by tilting the trays for 30 min.The wet mass (WM) of the samples was obtained on a precision scale with an accuracy of 0.01 g.To obtain dry mass (DM),the samples were dried in an air circulation oven at 65 ℃ for 48 h and dry mass determined on an analytical balance with a precision of 0.01 g.Using the equation,(WM -DM)/DM * 100,WHC was calculated.

Litter dry mass was converted into the international unit,mega grams per hectare (Mg ha-1),by dividing dry mass by area of the collector in hectares.Rainfall,average temperatures,relative humidity,and wind speed were collected as well as litter in the wettest and least rainy months of 2019 and 2021,which corresponded toLa NiñaandEl Niñoyears,respectively.The data were obtained from the meteorological station of the Instituto Nacional de Meteorologia (INMET 2021)located 1.8 km from the study area (Supplementary Table S1).

Data analyses

All variables were subjected to the Shapiro–Wilk normality test (p>0.05) and Bartlett’s homoscedasticity of variance(p>0.05).Under these assumptions,the data were tested by Student’sttest (p<0.05),where averages are compared between periods for the same year and between years for the same period.In addition,a multivariate principal component analysis (PCA) was carried out to assess the correlation between litter stock and WHC with rainfall,relative humidity,mean temperature,and wind speed.Analyses were conducted using the statistical software R (4.2.3),with the following:“factoextra”,“FactoMineR”,and “ggplot2”(R Development Core Team 2023).

Results

Seasonality influenced the litter stock both inEl NiñoandLa Niñayears.InEl Niño,the highest average(7.6 ± 1.5 Mg ha-1) was in the rainy season (t=2.75;p-value=0.01),while inLa Niña,the period of less rain had the highest litter stock (t=-8.63;p-value <0.01;Fig.2).Additionally,the effect of ENSO was only observed in the rainy season with higher litter stock duringEl Niño(t=11.43;p-value <0.01;Fig.2).

Litter WHC was higher in the wettest period of both years.In this same period,average WHC duringEl Niñowas significantly higher compared to theLa Niñayear (t=10.63;p-value <0.01;Fig.3).A difference was also observed for the least rainy period between the two years (t=10.54,p-value <0.01),with average values of 178.1 ± 16.3% and 309.4 ± 39.9% forLa NiñaandEl Niño,respectively.

In the PCA analysis,PC1 accounted for 61.6% of the total variance of the data related to climate elements.When combined with PC2,it explained 85.7% of the total variance (Fig.4).WHC showed a positive correlation with precipitation and humidity,while a negative correlation with temperature and wind speed.Litter stock on the other hand,showed a negative correlation with relative humidity(Fig.4).The PCA showed the dissimilarity between the two years in terms of climate elements,with high loadings along PC1 and low loadings along PC2,representing the difference betweenEl NiñoandLa Niña.In addition,the influence of temperature and wind speed on WHC duringEl Niñowas also observed.

Fig.4 Principal Component Analysis considering litter carbon stock,water holding capacity (WHC) and climate elements (precipitation,temperature,relative humidity,and wind speed) in a 34-year successional forest ecosystem,eastern Amazonia,Brazil

Discussion

The effects of ENSO on WHC were evident but its influence on litter stock was not clear.ENSO effects were observed during the rainy season,contradicting the initial hypothesis of this study.It was expected that the high rainfall characteristics ofLa Niñawould lead to increased decomposition and consequently,lower litter stocks.However,the negative correlation with temperature demonstrated the thermal impact on leaf degradation.Temperature is an important variable in this process and appeared to be the factor that most influenced decomposition and WHC (Bufacchi et al.2020;Simões et al.2022).

As of 2017,it was estimated that approximately 280 million hectares worldwide was under shifting cultivation(Heinimann et al.2017).Therefore,this suggests that human activities contribute to the intensification of forest fires in the Amazon region primarily due to higher temperatures within the forest and the subsequent drying natural flammable materials such as litter.

Water holding capacity (WHC) is instrumental in understanding the eco-hydrological aspects of the ecosystem,as it represents the litter’s ability to absorb water proportionally to its weight,potentially affecting water availability in the soil (Rosalem et al.2019).This mechanism is regulated by the processes of absorption and adsorption which depend on the morphological characteristics of the deposited material and the specific contact area (Xiao and McPherson 2016).In this context,the greater detritivore action of the edaphic fauna reduces the contact area of the litter,increases porosity,and consequently increases water retention capacity.This justifies the observation of a smaller litter stock while the WHC was significantly higher compared to the other period of the same year.

DuringLa Niña,when temperatures are generally mild and rainfall high,WHC minimizes problems with excessive waterlogging of the soil and consequently,it is one factor that slow surface runoff.In periods of high temperature such as duringEl Niño,WHC optimizes the use of rainwater and helps maintain soil moisture,ensuring the continued production of ecosystem goods and services for support and regulation.However,drastic and unexpected changes in climatological patterns can interfere with leaf renewal rates and the phenology of forest species in the Amazon (Lopes et al.2016),which may alter processes such as litter deposition and decomposition dynamics.

While ENSO is a natural phenomenon,the extreme and increasingly frequent changes in weather patterns will result in warming of the Equatorial Pacific Ocean,intensifying these phenomena and causing significant losses,especially of nutrients through leaching and/or volatilization (Zhang et al.2019).Additionally,even though litter production in successional ecosystems altered by human activities is a resilient process (Morais et al.2021),the quality of the deposited litter can be directly influenced as well as the biogeochemical cycle (Townsend et al.2008).

Therefore,changes in cycling patterns and forest structure consequently alter the availability of fuel material and,when combined with low precipitation,can lead to frequent and overwhelming forest fires (Brando et al.2020).These impacts may be even more pronounced in successional ecosystems due to low humidity and enhanced wind circulation within the forest (Moon et al.2019).Considering the expansion of secondary forests in the biome and the ongoing deforestation from prevailing productive models in Amazonia,this study reinforces warnings issued by the scientific community (Cox et al.2004;Dong et al.2021;Numata et al.2021) regarding the impacts of climate change on the maintenance of Amazonia biodiversity.It also highlights the negative impacts of climate change on the provision of ecosystem services,directly affecting local communities and the interactions between the different biotic and abiotic components of the forest.

Conclusion

DuringEl Niño,litter water holding capacity was intensified,but this study did not show an effect of ENSO on litter stock although it was clearly demonstrated seasonality.This indicates that,depending on the intensity of these climatic phenomena,a forest’s biogeochemical processes can be significantly impacted.Therefore,the need for long-term research in Amazonia is critical to understanding the effects these events have on forest ecosystems,and for maintaining the biogeochemical cycle and water holding.

AcknowledgementsThe Ecosystem and Watershed Management Laboratory of the Federal Rural University of the Amazonia processed data and samples.The Graduate Program in Forest Science is at the Federal Rural University of Amazonia.We are also grateful to three anonymous reviewers for their substantial contribution to improve the quality of this article.

Author contributionJIMR conceived the idea and designed the research;JIMR,WBRM,VPO and MSSW structured the manuscript and analyzed the data;JIMR and HBSJ collected the data;FAO assisted in the revision of the manuscript;all authors discussed the results and approved the latest version of the article.

Declarations

Conflict of interestThe corresponding author confirms on behalf of all authors that there have been no involvement that might raise the question of bias in the work reported or in the conclusions,implications,or opinions stated.