Nitrogen dynamics in oak model ecosystems subjected to air warming and drought on two different soils

Global warming changes biomass and C:N:P stoichiometry of different components in terrestrial ecosystems

Global warming has significantly affected terrestrial ecosystems. Biomass and C:N:P stoichiometry of plants and soil is crucial for enhancing plant productivity, improving human nutrition, and regulating biogeochemical cycles. However, the effect of warming on the biomass and C:N:P stoichiometry of different components (plant, leaf, stem, root, litter, soil, and microbial biomass) in various terrestrial ecosystems remains uncertain. We conducted a comprehensive meta-analysis to investigate the global patterns of biomass and C:N:P stoichiometry responses to warming, as well as interaction relationships based on 1399 paired observations from 105 warming studies. Results indicated that warming had a significant impact on various aspects of plant growth, including an increase in plant biomass (+16.55%), plant C:N ratio (+4.15%), leaf biomass (+16.78%), stem biomass (+23.65%), root biomass (+22.00%), litter C:N ratio (+9.54%) and soil C:N ratio (+5.64%). However, it also decreased stem C:P ratio (-23.34%), root C:P ratio (-12.88%), soil N:P ratio (-14.43%) and soil C:P ratio (-16.33%). The magnitude of warming was the primary drivers of changes of biomass and C:N:P stoichiometry. By establishing the general response curves of changes in biomass and C:N:P ratios with increasing temperature, we demonstrated that warming effect on plant, root, and litter biomass shifted from negative to positive, whereas that on leaf and stem biomass changed from positive to negative as temperature increased. Additionally, the effect of warming on root C:N ratio, root biomass, and microbial biomass N:P ratios shifted from positive to negative, whereas the effects on plant N:P, leaf N:P, leaf C:P, root N:P ratios, and microbial biomass C:N ratio changed from negative to positive with increasing temperature. Our research can help assess plant productivity and optimize ecosystem stoichiometry precisely in the context of global warming.

Responses of native and invasive woody seedlings to combined competition and drought are species-specific

Woody species invasions are a major threat to native communities with intensified consequences during increased periods of summer drought as predicted for the future. Competition for growth-limiting nitrogen (N) between native and invasive tree species might represent a key mechanism underlying the invasion process, because soil water availability and N acquisition of plants are closely linked. To study whether the traits of invasive species provide an advantage over natives in Central Europe in the competition for N under drought, we conducted a greenhouse experiment. We analyzed the responses of three native (i.e., Fagus sylvatica L., Quercus robur L. and Pinus sylvestris L.) and two invasive woody species (i.e., Prunus serotina Ehrh. and Robinia pseudoacacia L.) to competition in terms of their organic and inorganic N acquisition, as well as allocation of N to N pools in the leaves and fine roots. In our study, competition resulted in reduced growth and changes in internal N pools in both native and invasive species mediated by the physiological characteristics of the target species, the competitor, as well as soil water supply. Nitrogen acquisition, however, was not affected by competition indicating that changes in growth and N pools were rather linked to the remobilization of stored N. Drought led to reduced N acquisition, growth and total soluble protein-N levels, while total soluble amino acid-N levels increased, most likely as osmoprotectants as an adaptation to the reduced water supply. Generally, the consequences of drought were enhanced with competition across all species. Comparing the invasive competitors, P. serotina was a greater threat to the native species than R. pseudoacacia. Furthermore, deciduous and coniferous native species affected the invasives differently, with the species-specific responses being mediated by soil water supply.

Effects of drought on leaf carbon source and growth of European beech are modulated by soil type

Drought potentially affects carbon balance and growth of trees, but little is known to what extent soil plays a role in the trade-off between carbon gain and growth investment. In the present study, we analyzed leaf non-structural carbohydrates (NSC) as an indicator of the balance of photosynthetic carbon gain and carbon use, as well as growth of European beech (Fagus sylvatica L.) saplings, which were grown on two different soil types (calcareous and acidic) in model ecosystems and subjected to a severe summer drought. Our results showed that drought led in general to increased total NSC concentrations and to decreased growth rate, and drought reduced shoot and stem growth of plants in acidic soil rather than in calcareous soil. This result indicated that soil type modulated the carbon trade-off between net leaf carbon gain and carbon investment to growth. In drought-stressed trees, leaf starch concentration and growth correlated negatively whereas soluble sugar:starch ratio and growth correlated positively, which may contribute to a better understanding of growth regulation under drought conditions. Our results emphasize the role of soil in determining the trade-off between the balance of carbon gain and carbon use on the leaf level and growth under stress (e.g. drought).

Experimental Air Warming of a Stylosanthes capitata, Vogel Dominated Tropical Pasture Affects Soil Respiration and Nitrogen Dynamics

Warming due to global climate change is predicted to reach 2°C in tropical latitudes. There is an alarming paucity of information regarding the effects of air temperature on tropical agroecosystems, including foraging pastures. Here, we investigated the effects of a 2°C increase in air temperature over ambient for 30 days on an established tropical pasture (Ribeirão Preto, São Paulo, Brazil) dominated by the legume Stylosanthes capitata Vogel, using a T-FACE (temperature free-air controlled enhancement) system. We tested the effects of air warming on soil properties [carbon (C), nitrogen (N), and their stable isotopic levels (δ13C and δ15N), as well as soil respiration and soil enzymatic activity] and aboveground characteristics (foliar C, N, δ13C, δ15N, leaf area index, and aboveground biomass) under field conditions. Results show that experimental air warming moderately increased soil respiration rates compared to ambient temperature. Soil respiration was positively correlated with soil temperature and moisture during mid-day (when soil respiration was at its highest) but not at dusk. Foliar δ13C were not different between control and elevated temperature treatments, indicating that plants grown in warmed plots did not show the obvious signs of water stress often seen in warming experiments. The 15N isotopic composition of leaves from plants grown at elevated temperature was lower than in ambient plants, suggesting perhaps a higher proportion of N-fixation contributing to tissue N in warmed plants when compared to ambient ones. Soil microbial enzymatic activity decreased in response to the air warming treatment, suggesting a slower decomposition of organic matter under elevated air temperature conditions. Decreased soil enzyme capacity and increases in soil respiration and plant biomass in plots exposed to high temperature suggest that increased root activity may have caused the increase seen in soil respiration in this tropical pasture. This response along with rapid changes in soil and plant 15N may differ from what has been shown in temperate grasslands.

Changes in the dynamics of foliar N metabolites in oak saplings by drought and air warming depend on species and soil type

Climate change poses direct or indirect influences on physiological mechanisms in plants. In particular, long living plants like trees have to cope with the predicted climate changes (i.e. drought and air warming) during their life span. The present study aimed to quantify the consequences of simulated climate change for foliar N metabolites over a drought-rewetting-drought course. Saplings of three Central European oak species (i.e. Quercus robur, Q. petraea, Q. pubescens) were tested on two different soil types (i.e. acidic and calcareous). Consecutive drought periods increased foliar amino acid-N and soluble protein-N concentrations at the expense of structural N in all three oak species. In addition, transient effects on foliar metabolite dynamics were observed over the drought-rewetting-drought course. The lowest levels of foliar soluble protein-N, amino acid-N and potassium cation with a minor response to drought and air warming were found in the oak species originating from the driest/warmest habitat (Q. pubescens) compared to Q. robur and Q. petraea. Higher foliar osmolyte-N and potassium under drought and air warming were observed in all oak species when grown on calcareous versus acidic soil. These results indicate that species-specific differences in physiological mechanisms to compensate drought and elevated temperature are modified by soil acidity.

Responses of sap flow, leaf gas exchange and growth of hybrid aspen to elevated atmospheric humidity under field conditions

An increase in average air temperature and frequency of rain events is predicted for higher latitudes by the end of the 21st century, accompanied by a probable rise in air humidity. We currently lack knowledge on how forest trees acclimate to rising air humidity in temperate climates. We analysed the leaf gas exchange, sap flow and growth characteristics of hybrid aspen (Populus tremula × P. tremuloides) trees growing at ambient and artificially elevated air humidity in an experimental forest plantation situated in the hemiboreal vegetation zone. Humidification manipulation did not affect the photosynthetic capacity of plants, but did affect stomatal responses: trees growing at elevated air humidity had higher stomatal conductance at saturating photosynthetically active radiation (gs sat) and lower intrinsic water-use efficiency (IWUE). Reduced stomatal limitation of photosynthesis in trees grown at elevated air humidity allowed slightly higher net photosynthesis and relative current-year height increments than in trees at ambient air humidity. Tree responses suggest a mitigating effect of higher air humidity on trees under mild water stress. At the same time, trees at higher air humidity demonstrated a reduced sensitivity of IWUE to factors inducing stomatal closure and a steeper decline in canopy conductance in response to water deficit, implying higher dehydration risk. Despite the mitigating impact of increased air humidity under moderate drought, a future rise in atmospheric humidity at high latitudes may be disadvantageous for trees during weather extremes and represents a potential threat in hemiboreal forest ecosystems.

A phenological timetable of oak growth under experimental drought and air warming

Climate change is expected to increase temperature and decrease summer precipitation in Central Europe. Little is known about how warming and drought will affect phenological patterns of oaks, which are considered to possess excellent adaptability to these climatic changes. Here, we investigated bud burst and intra-annual shoot growth of Quercus robur, Q. petraea and Q. pubescens grown on two different forest soils and exposed to air warming and drought. Phenological development was assessed over the course of three growing seasons. Warming advanced bud burst by 1-3 days °C⁻¹ and led to an earlier start of intra-annual shoot growth. Despite this phenological shift, total time span of annual growth and shoot biomass were not affected. Drought changed the frequency and intensity of intra-annual shoot growth and advanced bud burst in the subsequent spring of a severe summer drought by 1-2 days. After re-wetting, shoot growth recovered within a few days, demonstrating the superior drought tolerance of this tree genus. Our findings show that phenological patterns of oaks are modified by warming and drought but also suggest that ontogenetic factors and/or limitations of water and nutrients counteract warming effects on the biomass and the entire span of annual shoot growth.