Shift in the migration trajectory of the green biomass loss barycenter in Central Asia.

Revealing the vegetation response law under drought stress has become a hot issue in global climate change research. Against the background of human beings actively responding to climate change, quantitatively revealing the change and migration laws of green biomass loss (GBL) caused by drought in historical and future periods is insufficient. In this regard, we innovatively constructed a joint kNDVI-SPEI (kernel normalized difference vegetation index and standardized precipitation evapotranspiration index) distribution based on copula theory to accurately capture GBL dynamic under various drought scenarios unlike previous studies conducted in a deterministic way. Taking the drought-sensitive and ecologically vulnerable Central Asia (CA) as a typical region, we verified that an average 94.4 % of region showed greater vegetation vulnerability in times of water shortage from May to October, which exhibited the greatest probability of GBL under different drought scenarios, mainly in Kazakhstan and Uzbekistan. Significantly intensified drought due to high emissions will cause an 18.16 percentage-point increase in GBL probability in the far future (FFP, 2061-2100) compared to the near future (NFP, 2019-2060), which is much higher than in the lower-emission (0.38 %) and moderate-emission scenarios (9.82 %). In the NFP, the GBL barycenter will shift from Kazakhstan to Xinjiang, China; in the FFP, it will shift back to Kazakhstan due to the measures taken by the Chinese government to conserve energy and reduce emissions. Results illustrate that against the background of worsening drought, active climate change coping strategies can reverse the migration trajectory of the GBL barycenter caused by drought, which provides a new idea for vegetation protection research in response to global climate change.

Environmental impact assessment of dam-break floods considering multiple influencing factors.

Dam-break floods cause substantial damage to the environment, and evaluating the negative impacts of dam-break floods on the environment (EI) is an important part of flood risk management. EI has been evaluated using various methods with different indices. However, the evaluation results of EI are typically one-sided or inaccurate because of diverse indices and complex influencing factors. A new method was proposed herein to calculate EI with an index system, including geomorphic changes (GC), water pollution (WP), plant biomass loss (PB), and biodiversity loss (BL). Eight factors that influence EI were sorted out, namely, erosion or deposition depth, non-point source (NPS) and point source (PS) pollution, plant biomass, species richness, plant height, and flood depth, velocity, and duration. After combining the proposed damage functions to calculate the influence of flood depth, velocity, and duration, and plant height on the environment, methods to calculate GC, WP, PB, BL, and EI were proposed. A dam-break flood scenario for Luhun Reservoir was used to verify the method. The results showed that (1) the trend in EI was similar to that in geomorphic changes in the inundated area and seriously affected by PS in local areas, (2) the average EI of woodland was the highest, while that of towns was the slowest, and (3) GC and WP contributed 93.7% of EI in the entire inundated area. This study summarized the complex impacts of dam-break floods on the environment from four aspects and proposed a method to quantify the overall impact of dam-break floods on the environment. The evaluation model could evaluate the impact of floods on the environment accurately, presenting the results on a flood inundation map. This provides a scientific basis for evaluating flood consequences and managing flood risk.

Lianas do not reduce tree biomass accumulation in young successional tropical dry forests.

Young successional tropical forests are crucial in the global carbon cycle because they can quickly sequester large quantities of atmospheric carbon. However, lianas (woody vines) can significantly decrease biomass accumulation in young regenerating forests. Lianas are abundant in tropical dry forests, and thus we hypothesized that lianas reduce biomass accretion in dry forests. Lianas may be particularly detrimental to the growth of young trees, which are vulnerable to competition from lianas. Alternatively, lianas may have a stronger negative effect on the largest trees because lianas seek the high-light environment at the top of the forest canopy. We tested these hypotheses using a liana-removal experiment in 13 dry forest stands that ranged from 1 to 70 years in southwestern Panama. We measured biomass accumulation annually for more than 10,000 stems from 2013 to 2017. Contrary to our expectations, liana removal had no effect on tree biomass accumulation across our successional forests and throughout our study period. Liana removal did not benefit smaller trees or larger trees. Lianas did not increase biomass accumulation on recruits, and did not increase biomass loss due to mortality. Surprisingly, removing lianas had a negative effect on three out of 41 tree species. Lianas had no effect on biomass accumulation and loss, possibly because: (1) trees allocated resources to roots instead of stems, (2) trees and lianas partitioned water, (3) higher irradiance after liana removal reduced soil moisture, or (4) low water availability might have been such a strong stressor that it reduced plant-plant competition.

Estimation of direct damage to maize seedlings by the corn leafhopper, Dalbulus maidis (Hemiptera: Cicadellidae), under different watering regimes.

The corn leafhopper Dalbulus maidis (Hemiptera: Cicadellidae), a specialist herbivore, is the cause of serious losses in maize yield for its capacity to transmit three important plant pathogens. They are also active phloem feeders, that insert stylets into the plant as they feed. Females place their eggs endophytically, totally inserted in the central midrib or the leaf blades, leaving conspicuous openings in the place where the ovipositor was inserted. In spite of the consequences that feeding and oviposition may have on the water status of the plant and the production of biomass, direct damage caused by the leafhopper has been only scarcely studied. In the present contribution, we measured biomass loss due to direct damage in maize plants under two watering regimes, with water supply ad libitum and with a watering restricted regime, emulating the most frequent field conditions. Moreover, we analyzed the effects of increasing densities of the vector on the biomass loss and plant mortality and the effects of females vs males. We observed that a density of 10 insects is sufficient to cause damage to 10-day-old seedlings, even in an ad libitum watering regime; however, in drought conditions, damage can be significantly greater, causing plant mortality. Also, females cause more damage than males, due to their oviposition habits.

Carbohydrate saving or biomass maintenance: which is the main determinant of the plant's long-term submergence tolerance?

It is hypothesized that plant submergence tolerance could be assessed from the decline of plant biomass due to submergence, as biomass integrates all eco-physiological processes leading to fitness. An alternative hypothesis stated that the consumption rate of carbohydrate is essential in differing tolerance to submergence. In the present study, the responses of biomass, biomass allocation, and carbohydrate content to simulated long-term winter submergence were assessed in four tolerant and four sensitive perennials. The four tolerant perennials occur in a newly established riparian ecosystem created by The Three Gorges Dam, China. They had 100% survival after 120 days' simulated submergence, and had full photosynthesis recovery after 30 days' re-aeration, and the photosynthetic rate was positively related to the growth during the recovery period. Tolerant perennials were characterized by higher carbohydrate levels, compared with the four sensitive perennials (0% survival) at the end of submergence. Additionally, by using a method which simulates posterior estimates, and bootstraps the confidence interval for the difference between strata means, it was found that the biomass response to post-hypoxia, rather than that to submergence, could be a reliable indicator to assess submergence tolerance. Interestingly, the differences of changes in carbohydrate content between tolerant and sensitive perennials during submergence were significant, which were distinct from the biomass response, supporting the hypothesis that tolerant perennials could sacrifice non-vital components of biomass to prioritize the saving of carbohydrates for later recovery. Our study provides some insight into the underlying mechanism(s) of perennials' tolerance to submergence in ecosystems such as temperate wetland and reservoir riparian.

Ecosystem heterogeneity and diversity mitigate Amazon forest resilience to frequent extreme droughts.

The impact of increases in drought frequency on the Amazon forest's composition, structure and functioning remain uncertain. We used a process- and individual-based ecosystem model (ED2) to quantify the forest's vulnerability to increased drought recurrence. We generated meteorologically realistic, drier-than-observed rainfall scenarios for two Amazon forest sites, Paracou (wetter) and Tapajós (drier), to evaluate the impacts of more frequent droughts on forest biomass, structure and composition. The wet site was insensitive to the tested scenarios, whereas at the dry site biomass declined when average rainfall reduction exceeded 15%, due to high mortality of large-sized evergreen trees. Biomass losses persisted when year-long drought recurrence was shorter than 2-7 yr, depending upon soil texture and leaf phenology. From the site-level scenario results, we developed regionally applicable metrics to quantify the Amazon forest's climatological proximity to rainfall regimes likely to cause biomass loss > 20% in 50 yr according to ED2 predictions. Nearly 25% (1.8 million km2 ) of the Amazon forests could experience frequent droughts and biomass loss if mean annual rainfall or interannual variability changed by 2σ. At least 10% of the high-emission climate projections (CMIP5/RCP8.5 models) predict critically dry regimes over 25% of the Amazon forest area by 2100.

High midday temperature stress has stronger effects on biomass than on photosynthesis: A mesocosm experiment on four tropical seagrass species.

The effect of repeated midday temperature stress on the photosynthetic performance and biomass production of seagrass was studied in a mesocosm setup with four common tropical species, including Thalassia hemprichii, Cymodocea serrulata, Enhalus acoroides, and Thalassodendron ciliatum. To mimic natural conditions during low tides, the plants were exposed to temperature spikes of different maximal temperatures, that is, ambient (29-33°C), 34, 36, 40, and 45°C, during three midday hours for seven consecutive days. At temperatures of up to 36°C, all species could maintain full photosynthetic rates (measured as the electron transport rate, ETR) throughout the experiment without displaying any obvious photosynthetic stress responses (measured as declining maximal quantum yield, Fv/Fm). All species except T. ciliatum could also withstand 40°C, and only at 45°C did all species display significantly lower photosynthetic rates and declining Fv/Fm. Biomass estimation, however, revealed a different pattern, where significant losses of both above- and belowground seagrass biomass occurred in all species at both 40 and 45°C (except for C. serrulata in the 40°C treatment). Biomass losses were clearly higher in the shoots than in the belowground root-rhizome complex. The findings indicate that, although tropical seagrasses presently can cope with high midday temperature stress, a few degrees increase in maximum daily temperature could cause significant losses in seagrass biomass and productivity.

Early physiological consequences of fire as an abiotic stressor in metabolic source and sink of young Brutian pine (Pinus brutia Ten.).

Climatic change causes gradual deforestation, partly through forest fires. However, fire has not been seen as an oxidative stressor on surviving forest trees. In addition, discrimination of stress-induced responses from acclimation steps cannot be examined under prolonged stress. Thus, four young Brutian pine (Pinus brutia Ten.) trees, a fire-related species, were subjected to a simulation of a crown-fire event to evaluate its impact on the availability of soluble carbon (C) and nitrogen (N) and the redox status near fire-afflicted tissue. Total soluble sugars, amino acids and non-structural (NS) proteins in needles and phloem, the antioxidant ascorbic acid (AsA) and reactive oxygen species (ROS) in needles were investigated together with the phloem transport velocity. To examine the temporal progress of these parameters, samples were obtained prior to fire (pre-fire), 2 h after fire, the following day (Day 1) and the following week (Week 1). Findings were categorized into shock reactions (2 h) and acclimation steps. Phloem transport accelerated 2 h postfire by almost 30% and correlated negatively to phloem sugars. At the same time the phloem ratio of sugars/amino acids correlated negatively to needle ROS. The trees' main response at 2 h and particularly on Day 1 was a massive increase in phloem NS proteins. The acclimation process involved also significant increases in needle NS proteins and AsA, as well as significant depletion of phloem amino acids by 65% by Week 1. The highest availability of soluble C and N was recorded on Day 1 in the phloem. Regression models explained significantly the variability of most soluble compounds postfire. Our findings suggest sink control over the source and an advanced role of phloem transport in defense processes.