Wetter Wet Seasons Research Articles

Responses of plant water uptake sources to altered precipitation patterns in a tropical secondary forest

Gaining insights into the impacts of altered precipitation patterns on tree species' water uptake sources and water usage strategies is crucial for water resource management, but a comprehensive understanding is lacking. We explored water uptake patterns and water-related traits of four dominant tree species (Evergreen: Carallia brachiate, Symplocos poilanei, Schefflera heptaphylla; Deciduous: Tetradium glabrifolium) in two altered seasonal precipitation patterns (deferred wet season [DW] and wetter wet season [WW]) and a control (CK) in a secondary tropical forest. Using stable isotopes (δ2H and δ18O) and MixSIAR models, we determined water uptake sources of the target species from a 60 cm soil profile and groundwater. Our results showed that the DW treatment increased (2.3% to 11.5%) groundwater utilization by trees in June and November compared to the CK, and promoted greater water uptake by trees from 0 to 40 cm soil layer in March. However, the WW treatment had a mild impact on water utilization in plants, leading to a reduction in water uptake by 1% - 3.8% from the 0–40 cm soil layer for the four tree species in September. The four studied tree species primarily rely on soil water, with the deciduous T. glabrifolium exhibited more flexible water uptake strategies compared to the other three evergreen species. For plant traits, S. heptaphylla exhibited larger stomatal size under DW treatment, paralleling the elongated stomatal in C. brachiate under the DW treatment. Conversely, S. poilanei and T. glabrifolium exhibit greater flexibility in balancing midday water potential. Under altered precipitation patterns, these plants adopt different morphological and/or physiological strategies, along with shift their water uptake sources to adapt to fluctuating water conditions. Overall, this study improves our understanding of plant water use in secondary tropical forests and emphasizes the importance of considering species-specific responses in forest management under potential precipitation pattern changes.

Revisiting the phenology of El Triunfo cloud forest, Mexico, 30 years later

Abstract Climatic conditions have changed within the cloud forests of southern Mexico, but the effects of these changes on tree phenology are not well understood. Our study aimed to determine relationships between seasonal patterns of flowering and fruiting and the annual and long-term changes in temperature and rainfall in El Triunfo Biosphere Reserve, Mexico. Flowering and ripe fruiting intensity and fruit number were recorded for 158 trees of 21 zoochorous species during 17 months in 2019–2021. Circular statistics, synchronization calculations, and generalized linear mixed models were used to determine phenological patterns and relationships between phenophases and temperature, rainfall, and solar radiation. Flowering was most closely associated (negatively) with rainfall, whereas ripe fruiting was positively associated with solar radiation. We compared recent fruiting seasonality with phenological data collected in 1991–1993. The mean date of community fruiting in 2019–2020 was earlier (April 4) than in 1992 (May 1) and 1993 (May 13). Community fruit number in 2020–2021 decreased in comparison to 2019–2020. Our results suggest that decadal trends of increasing minimum daily temperature and an earlier and wetter wet season influenced the timing of community fruiting phenology, and fruit numbers varied greatly between years, possibly impacting resource availability for frugivores in El Triunfo.

Climate change-induced drought and implications on maize cultivation area in the upper Nan River Basin, Thailand

Abstract The escalating frequency of climate change-induced droughts poses a severe threat to rainfed maize cultivation in Thailand's upper Nan River Basin (NRB). Utilizing the standardized precipitation evapotranspiration index, this study comprehensively examines spatial and temporal drought patterns and their potential agricultural impact. Findings indicate a significant shift in precipitation patterns with wetter wet seasons, drier dry seasons and rising temperatures. The upper NRB experiences prolonged and severe droughts, while the lower region faces higher drought intensity, signalling an increased likelihood of extended and severe drought episodes in the upper region. Assessing maize cultivation suitability, factoring in environmental variables and drought impact under observed and climate change scenarios, reveals the current moderate suitability at 42.2%, projected to expand, and unsuitable regions expected to double. Different shared socioeconomic pathways (SSPs) show varied outcomes, with SSP5-8.5 indicating increased suitability in highly suitable areas and SSP2-4.5 demonstrating improvements in moderately suitable areas. The study underscores the need for tailored adaptation strategies in water management during droughts to enhance crop production, especially in dry seasons, in the upper NRB amid a changing climate.

Contrasting effects of intensified dry-season drought and extended dry-season length on soil greenhouse gas emissions in a subtropical forest

Over two-thirds of the Earth's land surface is subjected to seasonal precipitation changes along with climate warming, including the subtropical forests that represent one of the Earth's most important carbon sink and source. However, few experiments have been conducted to understand the response of soil greenhouse gas (GHGs) emissions from these forests to seasonal changes in precipitation. Herein, we conducted a field experiment in a subtropical forest of southern China including two precipitation seasonality treatments: an intensified dry-season (Oct–Mar) drought and wetter wet-season (Jun–Sep) treatment (ID) and an extended dry-season (Apr–May) length and wetter wet-season treatment (ED); for both ID and ED, the annual precipitation amount was kept the same as under ambient control (AC). Compared to AC, the decreased annual CO2 emissions for ID were mainly due to decreased WFPS in Oct–Mar of 2013–2014 and increased WFPS during Jun–Sep of 2013; the increased annual CH4 uptake for ID was predominantly attributed to decreased WFPS in Oct–Mar of 2013–2014; the decreased annual N2O emissions for ID were mainly due to decreased WFPS in Oct–Mar of 2013; the increased annual N2O emissions for ID in 2014 were mainly attributed to increased WFPS in Jun–Sep (p < 0.05). Relative to AC, the increased annual CO2 and N2O emissions from ED were predominantly attributed to decreased WFPS in Apr–May and increased WFPS in Jun–Sep during 2013–2014, respectively (p < 0.05). The average annual CO2-equivalent CH4 and N2O emissions increased under ED but decreased under ID compared to AC (p < 0.05). Although our two precipitation manipulation scenarios simulated seasonal drought impacts without changing annual precipitation amount, ED and ID had distinct impacts on soil GHGs emissions, which have important implications for modeling the subtropical forests GHG emissions and managing the forests to mitigate climate change.

Multi-model analysis of historical runoff changes in the Lancang-Mekong River Basin – Characteristics and uncertainties

Runoff changes are critical to the sustainable water resource in the Lancang-Mekong River Basin (LMRB). Changes in the LMRB’s runoff over the past decades are unclear because of inadequate streamflow observations. The advancement of global hydrological models (GHMs) has facilitated the understanding of runoff change worldwide. However, it is required to evaluate the performance of GHMs in simulating historical runoff change in the LMRB before assessing the runoff changes. This study aims to conduct a multi-model analysis of temporal-spatial changes in runoff in the LMRB for 1971–2010 using ten GHMs from the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) and evaluate the corresponding uncertainties among models. Results show that the model ensemble mean has the best performance than the individuals when compared with the reference data. Based on the model ensemble mean, large spatial heterogeneity of runoff is found in the LMRB, with an overall slightly positive trend (8.03%). Besides, the models perform better in estimating the trends of high flow than low flow. As to the trend of runoff in the wet and dry seasons, about 32% (70%) of the basin became drier (wetter) in the dry (wet) season. Meanwhile, 17% of the basin has experienced a trend of drier dry seasons and wetter wet seasons. Overall, our results highlight the uncertainty of the runoff changes in the LMRB in the low flow simulation, particularly requiring more attention in future model improvement. The complex change patterns of the runoff suggest the importance of accurate runoff observations and projections for better water management.

Response of leaf functional traits to precipitation change: A case study from tropical woody tree

Precipitation change is one of the most important forms of current global climate change, which will significantly influence the plant growth and development. Although numerous experiments have been conducted to investigate the effects of precipitation regulation on the plant growth and ecosystem process, the response of leaf functional traits of tropical woody species to the precipitation change is still poorly understood. In this study, three treatments, including delayed wet season (DW), wetter wet season (WW) and control check (CK), were established to examine the responses of leaf functional traits, including morphological traits, photosynthetic pigments, and non-structural carbohydrates (NSC) of the dominant woody tree, Symplocos chunii , to the simulated precipitation change in a secondary tropical forest of southern China. Our results showed that the specific leaf area (SLA), photosynthetic pigments, and NSC contents exhibited distinctly seasonal variations ( P < 0.05), but the simulated precipitation changes did not alter the leaf morphological traits (leaf length, width, or area) of S. chunii after six-year treatment. The leaf total chlorophyll contents increased observably by 83.9% in DW, and the chlorophyll b was increased by 181.5% compared to CK ( P < 0.05), while WW markedly reduced chlorophyll b content but remarkably increased carotenoid content ( P < 0.05). However, DW significantly reduced NSC contents in both dry and wet seasons. In contract, WW treatment significantly reduced leaf soluble sugar content only in the early stage of wet season, but after that, the WW did not alter the leaf soluble sugar and starch contents, even with the approach of dry season. These indicated that the precipitation change would alter the leaf photosynthetic rates thus influence the carbon assimilation, further regulate the allocation and transformation pattern of leaf NSC contents in tropical woody trees. The results suggest that the tropical woody trees may adapt to the frequent precipitation change via regulating the allocation and transformation of non-structural carbohydrates.

Future projection of seasonal drought characteristics using CMIP6 in the Lancang-Mekong River Basin

As the most widespread natural hazard, drought has significant impacts on the livelihood and ecosystems in the Lancang-Mekong River Basin (LMRB). However, few studies focus on the seasonal characteristics of drought in the LMRB, especially under future climate projections by the Sixth Phase of Coupled Model Inter-comparison Project (CMIP6). This study filled the knowledge gap using SPI and SPEI, based on eight GCMs of CMIP6 under three scenarios (i.e., SSP1-2.6, SSP2-4.5, SSP5-8.5). Our results show that the LMRB tends to experience wetter wet season and drier dry season with the rising temperature considered (based on SPEI), while the temporal trend of dry-season SPI is not significant. The future trends of SPEI are -0.006 per year and -0.011 per year under SSP2-4.5 and SSP5-8.5, respectively. The trend magnitudes demonstrate spatial heterogeneities. Our evaluation based on SPEI shows that the most notable increases of dry-season drought (in terms of duration and intensity) are distributed in the middle reaches of LMRB. The upper Lancang and middle Mekong basin will likely experience more wet-season droughts. The dry-season drought accounts for 60% of total drought events in the near future (i.e., 2021–2055) and more than 80% in the far future (i.e., 2061–2095) under SSP2-4.5. Effective strategies are needed to enhance food and drinking water security in the LMRB, especially for the dry seasons under a changing climate.

Delayed wet season increases soil net N mineralization in a seasonally dry tropical forest

Seasonal precipitation regime plays a vital role in regulating nutrient dynamics in seasonally dry tropical forests. Present evidence suggests that not only wet season precipitation is increasing in the tropics of South China, but also that the wet season is occurring later. However, it is unclear how nutrient dynamics will respond to the projected precipitation regime changes. We assessed the impacts of altered seasonal precipitation on soil net N mineralization in a secondary tropical forest. Since 2013, by reducing throughfall and/or irrigating experimental plots, we delayed the wet season by two months from April–September to June–November (DW treatment) or increased annual precipitation by 25% in July and August (WW treatment). We measured soil net N mineralization rates and assessed soil microbial communities in January, April, August and November in 2015 and 2017. We found that a wetter wet season did not significantly affect soil microbes or net N mineralization rates, even in the mid-wet season (August) when soil water content in the WW treatment increased significantly. By contrast, a delayed wet season enhanced soil microbial biomass and altered microbial community structure, resulting in a two-fold increase in net N mineralization rates relative to controls in the early dry season (November). Structural equation modeling showed that the changes in net N mineralization during the early dry season were associated with altered soil microbial communities, dissolved organic N, and litterfall, which were all affected by enhanced soil water content. Our findings suggest that a delayed wet season could have a greater impact on N dynamics than increased precipitation during the wet season. Changes in the seasonal timing of rainfall might therefore influence the functioning of seasonally dry tropical forests.

The mechanisms and potentially positive effects of seven years of delayed and wetter wet seasons on nitrous oxide fluxes in a tropical monsoon forest

Tropical forest soils contribute to global warming and ozone depletion due to large nitrous oxide (N2O) emissions. However, it is unknown whether the soil N2O fluxes will change under ongoing precipitation regime changes. In this study, two typical precipitation regimes were simulated in a tropical monsoon forest for seven years: delayed wet season (DW) and wetter wet season (WW). Although we did not find significant effects of the precipitation changes on soil N2O fluxes annually or seasonally, significant increases in soil N2O emission were observed in treatment months for DW and WW. Importantly, the underlying mechanisms for their effects were different. For the DW treatment, the increase in soil N2O emission was attributed to the increase in abundance of nitrifying bacteria (amoA), which provide nitrate for N2O production, and to the changed microbial structure of denitrifying bacteria (nosZ), which reduce N2O consumption. For the WW treatment, the stimulation of soil N2O emission was attributed to the relatively large increase in N2O production via an increased abundance of denitrifying bacteria (nirS) compared with the increased N2O consumption via increased abundance of N2O reducing bacteria (nosZ). If the projected precipitation regimes of delayed or wetter wet seasons last for more than two months, the N2O emitted from tropical forest soils has the potential to significantly increase due to specific microbial nitrogen transformation processes. This increase implies that the contribution of tropical forests to global warming and ozone depletion would increase, depending on precipitation regimes.

Effects of Climate Change on Weeds and Invasive Alien Plants in Sri Lankan Agro-Ecosystems: Policy and Management Implications

Changes in the climate have worsen the problems caused by weeds and invasive alien plants (IAPs) in agro-ecosystems at global scale resulting from their changes in the range and population densities. Over the past six decades, Sri Lanka has experienced a slow but steady increase in annual environmental temperature by 0.01–0.03°C. Increasing extreme events of rainfall, wetter wet seasons, and drier dry seasons are some of the characteristic features of the changes in the climate observed in Sri Lanka over the years. The Ministry of Environment (MOE) in Sri Lanka has established a National Invasive Species Specialist Group (NISSG) in 2012 and adopted the National Policy on Invasive Alien Species (IAS) in Sri Lanka, Strategies and Action Plan in 2016. Further, the MOE has developed and adopted protocols to assess the risk of IAS at pre- and post-entry level to the country while incorporating climate change concerns. Periodic risk assessments have being carried out to prioritize actions against IAS in Sri Lanka. The Ministry of Agriculture as adopted a National Weed Strategy (NWS) and has identified the Weeds of National Significance (WONS) under different priority crops. A study done in 2014 has clearly shown that weed control costs in agricultural lands in several district of Sri Lanka were nearly doubled during the years that experiencedEl NiñoSouthern Oscillation (ENSO). Further, studies have clearly indicated that IAPs also survive, expand and impact the continuously disturbed environments in agro-ecosystems.Panicum trichocladum, a species listed as a potential invasive based on the risk assessment done in 2016, has shown an increase in its population density and distribution in Sri Lanka during the last 2–3 years. However, weeds and IAPs in agro-ecosystems have drawn less attention of policy makers, scientists, and practitioners in relation to impact of climate change in island ecosystems. This paper focuses on the scientific evidence reported in agro-ecosystems in Sri Lanka on climate-related impacts on agriculturally important weeds and IAPs, and the efforts made to manage their introduction and spread across the country.

Seven decades of climate change across Mexico

Due to its geographical location, Mexico is one of the most vulnerable countries to climate change. However, we currently ignore the exact magnitude and particularities of past climate change in the Mexican territory and are missing a country-level spatially explicit analysis based on observed data. To fill this gap, I analyzed how temperature, precipitation and the water balance of Mexico changed over 1951-2017 at interannual and seasonal scales. My results show a clear national increment in temperature (+0.71 ºC) but no modification in annual mean precipitation. At the seasonal scale, the wet season (June-November) had higher rainfall (+31 mm) and no change was detectable on the dry season (December-May). However, when the full water balance was seasonally accounted for (precipitation minus potential evapotranspiration), the trend resulted in a wetter wet season and a much drier dry season across the country. Regionally, seasonal changes in water balance were larger in the area surrounding the Gulf of Mexico and positive in the Yucatan Peninsula and the central highlands. My results help explaining the recent increase in drought, storms and intense rainfall across Mexico and suggest even more extreme seasonal weather in the future if climate change exacerbates.

Seven years of wetter and delayed wet season enhanced soil methane uptake during the dry season in a tropical monsoon forest

Tropical monsoon forest soils have the potential to mitigate global warming by removing methane (CH4) from the atmosphere, yet whether this function would be altered under future precipitation regimes remains unclear. In this study, we conducted two rainfall manipulation treatments, including the delayed wet season (DW) and wetter wet season (WW), since 2012 in a tropical monsoon forest in southern China to investigate the impact of changing precipitation patterns on soil CH4 uptake over two measurement years between 2018 and 2019. We found that DW significantly increased the annual average soil CH4 uptake by 164%, mainly due to a significant increase during the dry season via enhancing soil moisture and soil enzyme activities. Although WW exerted a limited impact on the annual average soil CH4 uptake, it remarkably increased CH4 uptake during the dry season due to the alteration of soil temperature, nutrient conditions, and enzyme activities. Furthermore, we found that methanotrophic abundance increased and decreased with soil moisture during the dry and wet seasons, respectively. The optimum air-filled soil porosity for CH4 uptake was 0.45 m3 m−3. Our results suggested that the role of tropical monsoon forests as CH4 sinks would be enhanced under delayed wet season, which should not be overlooked for determining the global CH4 uptake capacity of forest soils in the future.

Identifying climate change impacts on surface water supply in the southern Central Valley, California

Mountain regions in arid and semi-arid climates, such as California, are considered particularly sensitive to climate change because global warming is expected to alter snowpack storage and related surface water supply. It is therefore important to accurately capture snowmelt processes in watershed models for climate change impact assessment. In this study we use the Soil and Water Assessment Tool (SWAT) to estimate projected changes in snowpack and streamflow in four alpine tributaries to the agriculturally important but less studied southern Central Valley, California. Watershed responses are evaluated for four CMIP5 climate models (HadGEM_ES, CNRM-CM5, CanESM2 and MIROC5) and two emission scenarios (RCP 4.5 and RCP 8.5) for 2020–2099. SWAT models are calibrated following a dual-objective, lumped calibration approach with an automatic calibration against observed streamflow (stage 1) and a manual calibration against reconstructed Parallel Energy Balance (ParBal) snow water equivalent (SWE) data (stage 2). Results indicate that under a warming climate, peak streamflow is expected to increase 0.5–4 times in magnitude in the coming decades and to arrive 2–4 months earlier in the year because of earlier snowmelt. In the foreseeable future, snow cover will reduce gradually in the lower elevations and diminish at higher rates at higher elevation towards the end of the 21st century. Surface water supply is predicted to increase in the southern Central Valley under the evaluated scenarios but increased temporal variability (wetter wet seasons and drier dry seasons) will create new challenges for managing supply. The study further highlights that the use of remote sensing based, reconstructed SWE data could fill the current gap of limited in-situ SWE observations to improve the snow calibration of SWAT to better predict climate change impacts in semi-arid, snow-dominated watersheds.

Species‐specific transpiration and water use patterns of two pioneer dominant tree species under manipulated rainfall in a low‐subtropical secondary evergreen forest

AbstractLike many other ecosystems, subtropical forests are suffering more intense and longer droughts with the ongoing climate change. This study aimed to explore the seasonal transpiration and physiological responses of two dominant tree species, Schima superba and Michelia macclurei, to manipulated precipitation patterns in a subtropical evergreen forest of South China, in which an ambient control treatment (BC), a drier dry and wetter wet season treatment (DD) and an extended dry and wetter wet season treatment (ED) were applied. Tree water use and associated ecophysiological characters, such as the daily whole‐tree transpiration (EL), intrinsic water use efficiency (WUEi), Huber values (As:Al) and utilization proportions from different water sources were determined during the period from October 2012 to September 2013. For both tree species, no significant difference in transpiration among the three treatments was observed in the wet season, and a relatively stronger decrease of transpiration occurred under DD and ED treatments during the later dry season. Moreover, the higher transpiration of M. macclurei and its advantage of utilizing the shallow water derived from light rainfall under dry condition suggested that it has more survival and growth advantages in this subtropical forest. Therefore, under the seasonal drought caused by uneven distribution of rainfall in the future, M. macclurei that inclines to use shallow soil water would adopt a drought‐avoidance strategy, whereas S. superba that could uptake deeper soil water would be drought tolerant. The different spatial and temporal patterns of water use, together with the contrasting water use strategies, could reduce competition of the two species and facilitate their coexistence under potential precipitation distribution changes.

Effects of seasonal precipitation change on soil respiration processes in a seasonally dry tropical forest.

Precipitation is projected to change intensity and seasonal regime under current global projections. However, little is known about how seasonal precipitation changes will affect soil respiration, especially in seasonally dry tropical forests. In a seasonally dry tropical forest in South China, we conducted a precipitation manipulation experiment to simulate a delayed wet season (DW) and a wetter wet season (WW) over a three‐year period. In DW, we reduced 60% throughfall in April and May to delay the onset of the wet season and irrigated the same amount water into the plots in October and November to extend the end of the wet season. In WW, we irrigated 25% annual precipitation into plots in July and August. A control treatment (CT) receiving ambient precipitation was also established. Compared with CT, DW significantly increased soil moisture by 54% during October to November, and by 30% during December to April. The treatment of WW did not significantly affect monthly measured soil moisture. In 2015, DW significantly increased leaf area index and soil microbial biomass but decreased fine root biomass. In contrast, WW significantly decreased fine root biomass and forest floor litter stocks. Soil respiration was not affected by DW, which could be attributed to the increased microbial biomass offsetting the decrease in fine root biomass. In contrast, WW significantly increased soil respiration from 3.40 to 3.90 μmol m−2 s−1 in the third year, mainly due to the increased litter decomposition and soil pH (from 4.48 to 4.68). The present study suggests that both a delayed wet season and a wetter wet season will have significant impacts on soil respiration‐associated ecosystem components. However, the ecosystem components can respond in different directions to the same change in precipitation, which ultimately affected soil respiration.

Changes in seasonal precipitation distribution but not annual amount affect litter decomposition in a secondary tropical forest.

In the tropics of South China, climate change induced more rainfall events in the wet season in the last decades. Moreover, there will be more frequently spring drought in the future. However, knowledge on how litter decomposition rate would respond to these seasonal precipitation changes is still limited. In the present study, we conducted a precipitation manipulation experiment in a tropical forest. First, we applied a 60% rainfall exclusion in April and May to defer the onset of wet season and added the same amount of water in October and November to mimic a deferred wet season (DW); second, we increased as much as 25% mean annual precipitation into plots in July and August to simulate a wetter wet season (WW). Five single‐species litters, with their carbon to nitrogen ratio ranged from 27 to 49, and a mixed litter were used to explore how the precipitation change treatments would affect litter decomposition rate. The interaction between precipitation changes and litter species was not significant. The DW treatment marginally accelerated litter decomposition across six litter types. Detailed analysis showed that DW increased litter decomposition rate in the periods of January to March and October to December, when soil moisture was increased by the water addition in the dry season. In contrast, WW did not significantly affect litter decomposition rate, which was consistent with the unchanged soil moisture pattern. In conclusion, the study indicated that regardless of litter types or litter quality, the projected deferred wet season would increase litter decomposition rate, whereas the wetter wet season would not affect litter decomposition rate in the tropical forests. This study improves our knowledge of how tropical forest carbon cycling in response to precipitation change.

A new normal for streamflow in California in a warming climate: Wetter wet seasons and drier dry seasons

In this study, we investigate changes in future streamflows in California using bias-corrected and routed streamflows derived from global climate model (GCM) simulations under two representative concentration pathways (RCPs): RCP4.5 and RCP8.5. Unlike previous studies that have focused mainly on the mean streamflow, annual maxima or seasonality, we focus on projected changes across the distribution of streamflow and the underlying causes. We report opposing trends in the two tails of the future streamflow simulations: lower low flows and higher high flows with no change in the overall mean of future flows relative to the historical baseline (statistically significant at 0.05 level). Furthermore, results show that streamflow is projected to increase during most of the rainy season (December to March) while it is expected to decrease in the rest of the year (i.e., wetter rainy seasons, and drier dry seasons). We argue that the projected changes to streamflow in California are driven by the expected changes to snow patterns and precipitation extremes in a warming climate. Changes to future low flows and extreme high flows can have significant implications for water resource planning, drought management, and infrastructure design and risk assessment.

Climate change, agricultural production and civil conflict: Evidence from the Philippines

Using unique data on conflict-related incidents in the Philippines, we exploit seasonal variation in the relationship between rainfall and agricultural production to learn about the mechanism through which rainfall affects civil conflict. We find that an increase in dry-season rainfall leads to an increase in agricultural production and dampens conflict intensity. By contrast, an increase in wet-season rainfall is harmful to crops and produces more conflict. Consistent with the hypothesis that rebel groups gain strength after a bad harvest, we find that negative rainfall shocks lead to an increase in conflict incidents initiated by insurgents but not by government forces. These results suggest that the predicted shift towards wetter wet seasons and drier dry seasons will lead to more civil conflict even if annual rainfall totals remain stable. We conclude that policies aimed at mitigating the effect of climate change on agriculture could have the added benefit of reducing civil conflict.

Responses of Tree Transpiration and Growth to Seasonal Rainfall Redistribution in a Subtropical Evergreen Broad-Leaved Forest

Precipitation changes such as more frequent drought and altered precipitation seasonality may impose substantial impacts on the structure and functioning of forest ecosystems. A better understanding of tree responses to precipitation changes can provide fundamental information for the conservation and management of forests under future climate regimes. We conducted a 2-year seasonal rainfall redistribution experiment to assess the responses of tree transpiration and growth to manipulated precipitation changes in a subtropical evergreen broad-leaved forest. Three precipitation treatments were administered including a drier dry season and wetter wet season treatment (DD), an extended dry season and wetter wet season treatment (ED), and an ambient control treatment, with the total amount of annual rainfall being kept the same among the three treatments. Our results showed that the DD and ED treatments reduced daily transpiration of Schima superba by 8–16 and 13–25%, respectively. The ED treatment also reduced the DBH increment of larger S. superba individuals. In contrast, neither treatment showed obvious effects on the transpiration and DBH increment of another dominant species Michelia macclurei. However, the transpiration of both species showed clear inter-annual differences between the 2 years with contrasting annual rainfall (2094 vs 1582 mm). S. superba had a lower transpiration-to-precipitation ratio (T/P) compared to M. macclurei and showed decreased sensitivities to total solar radiation and vapor pressure deficit under the DD and ED treatments. These results indicate the deep-rooted S. superba may be suppressed with a lower ability to obtain water and assimilate carbon compared to the shallow-rooted M. macclurei under the precipitation seasonality changes, which could potentially cause shifts in species dominance within the forest community.

Physiological homeostasis and morphological plasticity of two tree species subjected to precipitation seasonal distribution changes

General circulation models and empirical observations show that rainfall patterns will become increasingly erratic in the future as the paradigm “dry gets drier, wet gets wetter” becomes globally prevalent. Currently, few field studies have evaluated the long-term effects of extreme precipitation on forest ecosystems based on tree physiology. To provide a better mechanistic understanding of these effects, a two-year “Precipitation Seasonal Distribution Changes” experiment (PSDC) was conducted in lower subtropical China. The treatments included “Drier dry season and wetter wet season (DD)”, ‘Extended dry season and wetter wet season (ED)” and “Ambient control (AC)’. The throughfall of the DD treatment was excluded during the dry season (October-March of the following year) to simulate drought, whereas the throughfall of the ED treatment was excluded in the spring (April-May) to simulate spring drought without significant altering the total annual precipitation input. The sap flow, water use efficiency, leaf and wood nutrient content and morphological parameters of two co-occurring tree species, Michelia macclurei Dandy and Schima superba Gardner & Champ., were quantified to elucidate the physiological mechanisms that occur in response to rainfall pattern changes. Our results indicated that the whole-tree transpiration per unit sapwood area (Js) and intrinsic water use efficiency (WUEi) showed homeostatic responses. Tree transpiration and response parameters to environmental variables (vapor pressure deficit [VPD] and photosynthetically active radiation [PAR]) did not show significant treatment effects regardless of drought, spring drought or irrigation periods. However, the leaf nitrogen (Narea) and phosphorus (Parea) content showed fluctuations, particularly the leaf N:P ratio, which showed that rainfall pattern changes had adverse effects on nutrient acquisition in this dual nitrogen- and phosphorus- limited forest ecosystem. The wood stoichiometry was more conserved than the leaf stoichiometry. For a given leaf 13C discrimination (≈ci/ca), the wood 13C discrimination did not show treatment or species effects, suggesting that the carbon transfer of trees was not affected by rainfall patterns. The branch As:Al was significantly negatively correlated with the parameters representing the sensitivity of tree transpiration to VPD, indicating that the trees maintained homeostatic responses by combining stomatal control and morphological adjustments. In mesic environments, the two studied tree species showed high resilience to seasonal rainfall pattern changes and homeostatic responses to nutrient acquisition, light use and water uptake, which is consistent with the ‘ecohydrological equilibrium theory’. In conclusion, the shallow-rooted M. macclurei presented a growth advantage in mesic environments or under mild water stress, whereas the deeply rooted S. superba is more competitive under predicted prolonged drought scenarios, such as DD and ED rain patterns, because of its greater morphological plasticity and increased water uptake from groundwater. Both species are common in south and eastern China but present inherently different responses to seasonal precipitation changes, which implies that changes in the timing and magnitude of precipitation may have consequences for plant nutrient acquisition and water balance and the potential to alter community composition.