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Nutrient dynamics along a precipitation gradient in European beech forests
No full textPrecipitation as a key determinant of forest productivity influences forest ecosystems also indirectly through alteration of the nutrient status of the soil, but this interaction is not well understood. Along a steep precipitation gradient, we studied the consequences of reduced precipitation for the soil and biomass nutrient pools and dynamics in 14 mature European beech (Fagus sylvatica L.) forests on Triassic sandstone. We tested the hypotheses that lowered summer precipitation (1) is associated with less acid soils and (2) a reduced accumulation of organic matter on the forest floor, and (3) reduces nutrient supply from the soil and leads to decreasing foliar and root nutrient concentrations. Soil acidity, the amount of forest floor organic matter, and the associated organic matter N and P pools decreased to about a half from wet to dry sites; the C/P and N/P ratios, but not the C/N ratio, of forest floor organic matter were reduced as well. Net N mineralization and P and K pools in the mineral soil did not change with decreasing precipitation. Foliar P and K concentrations (beech sun leaves) increased while N remained constant, resulting in decreasing foliar N/P and N/K ratios. Estimated N resorption efficiency increased toward the dry sites. We conclude that a reduction in summer rainfall significantly reduces the soil C, N and P pools but does not result in decreasing foliar N and P contents in beech. However, the decreasing foliar N/P ratios towards the dry stands indicate that the importance of P limitation for tree growth declines with decreasing precipitation
Belowground drought response of European beech: fine root biomass and carbon partitioning in 14 mature stands across a precipitation gradient
No full textHow tree root systems will respond to increased drought stress, as predicted for parts of Central Europe, is not well understood. According to the optimal partitioning theory, plants should enhance root growth relative to aboveground growth in order to reduce water limitations. We tested this prediction in a transect study with 14 mature forest stands of European beech (Fagus sylvatica L.) by analysing the response of the fine root system to a large decrease in annual precipitation (970-520 mm yr(-1)). In 3 years with contrasting precipitation regimes, we investigated leaf area and leaf biomass, fine root biomass and necromass (organic layer and mineral soil to 40 cm) and fine root productivity (ingrowth core approach), and analysed the dependence on precipitation, temperature, soil nutrient availability and stand structure. In contrast to the optimal partitioning theory, fine root biomass decreased by about a third from stands with > 950 mm yr(-1) to those with < 550 mm yr(-1), while leaf biomass remained constant, resulting in a significant decrease, and not an increase, in the fine root/leaf biomass ratio towards drier sites. Average fine root diameter decreased towards the drier stands, thereby partly compensating for the loss in root biomass and surface area. Both delta C-13-signature of fine root mass and the ingrowth core data indicated a higher fine root turnover in the drier stands. Principal components analyses (PCA) and regression analyses revealed a positive influence of precipitation on the profile total of fine root biomass in the 14 stands and a negative one of temperature and plant-available soil phosphorus. We hypothesize that summer droughts lead to increased fine root mortality, thereby reducing root biomass, but they also stimulate compensatory fine root production in the drier stands. We conclude that the optimal partitioning theory fails to explain the observed decrease in the fine root/leaf biomass ratio, but is supported by the data if carbon allocation to roots is considered, which would account for enhanced root turnover in drier environments
Nutrient dynamics along a precipitation gradient in European beech forests
No full textPrecipitation as a key determinant of forest productivity influences forest ecosystems also indirectly through alteration of the nutrient status of the soil, but this interaction is not well understood. Along a steep precipitation gradient (from 970 to 520 mm yr-1 over 150 km distance), we studied the consequences of reduced precipitation for the soil and biomass nutrient pools and dynamics in 14 mature European beech (Fagus sylvatica L.) forests on uniform geological substrate. We tested the hypotheses that lowered summer precipitation (1) is associated with less acid soils and a reduced accumulation of organic matter on the forest floor, and (2) reduces nutrient supply from the soil and leads to decreasing foliar and root nutrient concentrations. Soil acidity, the amount of forest floor organic matter, and the associated organic matter N and P pools decreased to about a half from wet to dry sites; the C/P and N/P ratios, but not the C/N ratio, of forest floor organic matter decreased. Net N mineralization (and nitrification) rate and the available P and K pools in the mineral soil did not change with decreasing precipitation. Foliar P and K concentrations (beech sun leaves) increased while N remained constant, resulting in decreasing foliar N/P and N/K ratios. N resorption efficiency increased toward the dry sites. We conclude that a reduction in summer rainfall significantly reduces the soil C, N and P pools but does not result in decreasing foliar N and P contents in beech. However, more effective tree-internal N cycling and the decreasing foliar N/P ratio towards the dry stands indicate that tree growth may increasingly be limited by N and not by P with decreasing precipitation.Open-Access-Publikationsfonds 201
Variation of soil and biomass carbon pools in beech forests across a precipitation gradient
No full textTemperate forests have recently been identified as being continuing sinks for carbon even in their mature and senescent stages. However, modeling exercises indicate that a warmer and drier climate as predicted for parts of Central Europe may substantially alter the source/sink function of these economically important ecosystems. In a transect study with 14 mature European beech (Fagus sylvatica L.) forests growing on uniform geological substrate, we analyzed the influence of a large reduction of annual precipitation (970-520 mm yr-1) on the carbon stocks in fast and slow pools, independent of the well-known aging effect. We investigated the C storage in the organic L, F, H layers, the mineral soil to 100 cm, and in the biomass (stem, leaves, fine roots), and analyzed the dependence of these pools on precipitation. Soil organic carbon decreased by about 25% from stands with > 900 mm yr-1 to those with < 600 mm yr-1; while the carbon storage in beech stems slightly increased. Reduced precipitation affected the biomass C pool in particular in the fine root fraction but much less in the leaf biomass and stem fractions. Fine root turnover increased with a precipitation reduction, even though stand fine root biomass and SOC in the organic L, F, and H layers decreased. According to regression analyses, the C storage in the organic layers was mainly controlled by the size of the fine root C pool suggesting an important role of fine root turnover for the C transfer from tree biomass to the SOC pool. We conclude that the long-term consequence of a substantial precipitation decrease would be a reduction of the mineral soil and organic layer SOC pools, mainly due to higher decomposition rates. This could turn temperate beech forests into significant carbon sources instead of sinks under global warming
Does reduced precipitation trigger physiological and morphological drought adaptations in European beech (Fagus sylvatica L.)? Comparing provenances across a precipitation gradient
No full textGlobal warming and associated decreases in summer rainfall may threaten tree vitality and forest productivity in many regions of the temperate zone in the future. One option for forestry to reduce the risk of failure is to plant genotypes which combine high productivity with drought tolerance. Growth experiments with provenances from different climates indicate that drought exposure can trigger adaptive drought responses in temperate trees, but it is not well known whether and to what extent regional precipitation reduction can increase the drought resistance of a species. We conducted a common garden growth experiment with five European beech (Fagus sylvatica L.) populations from a limited region with pronounced precipitation heterogeneity (816–544 mm year−1), where phylogenetically related provenances grew under small to large water deficits. We grew saplings of the five provenances at four soil moisture levels (dry to moist) and measured ∼30 morphological (leaf and root properties, root : shoot ratio), physiological (leaf water status parameters, leaf conductance) and growth-related traits (above- and belowground productivity) with the aim to examine provenance differences in the drought response of morphological and physiological traits and to relate the responsiveness to precipitation at origin. Physiological traits were more strongly influenced by provenance (one-third of the studied traits), while structural traits were primarily affected by water availability in the experiment (two-thirds of the traits). The modulus of leaf tissue elasticity ϵ reached much higher values late in summer in plants from moist origins resulting in more rapid turgor loss and a higher risk of hydraulic failure upon drought. While experimental water shortage affected the majority of morphological and productivity-related traits in the five provenances, most parameters related to leaf water status were insensitive to water shortage. Thus, plant morphology, and root growth in particular, did respond to reduced water availability with higher phenotypic plasticity than did physiology. We conclude that beech provenances exposed to different precipitation regimes have developed some genotypic differences with respect to leaf water status regulation, but these adaptations are associated with only minor adaptation in plant morphology and they do not affect the growth rate of the saplings
Nutrient return with leaf litter fall in Fagus sylvatica forests across a soil fertility gradient
No full textLeaf litter fall is an important nutrient flux in temperature deciduous forests which supplies a large part of the rapidly mineralisable nutrient fraction to the soil. This study investigates nutrient return with leaf litter fall in 36 old-growth forest stands of Fagus sylvatica across a broad gradient of soil fertility covering 9 mesozoic and kaenozoic parent material types (three limestones, two sandstones, two clay stones, one sand and one loess substrate). Study objectives were to analyse (i) the dependency of leaf litter nutrient concentrations on soil fertility, and (ii) the relationship between soil fertility and nutrient return with leaf litter at the stand level. Beech stands on the nine parent material types produced similar annual leaf litter masses irrespective of soil fertility or acidity. Leaf litter from the nine parent materials showed only minor variation with respect to N and K concentrations (factors of 1.5 and 1.4), moderate variation for Ca, Mg and P concentrations (factors of 2.2 to 2.9), and high variation for Al and Mn concentrations (factors of 6.7 and 10.5). Consequently, annual nutrient return with litter fall (leaf litter mass x litter nutrient concentration) was more similar among the parent materials for N (165 - 273 mmol m(-2) yr(-1)) and K (16 - 30 mm m(-2) yr(-1)) than for Ca, P, Mg, Mn and Al. A possible explanation is increased N deposition in recent time. According to a correlation analysis, return rates of N, P, K and Mg (but not Ca) were independent of the pool size of the respective nutrient in the soil. N return rate was neither influenced by the soil pools of N(t), plant-available P (P(a)) or exchangeable Ca, K and Mg, nor by soil acidity or the exchangeable Al pool. P return, in contrast, showed a negative relation to soil fertility. We hypothesize that nutrient fluxes with leaf litter fall do not necessarily reduce the fitness of tree populations as has been postulated from a tree-centred view. Rather, we suggest that nutrient fluxes with litter fall can increase, instead of decrease, plant fitness by improving nutrient availability in the densely rooted topsoil which reduces the roots' carbon and nutrient costs of nutrient acquisition
Relationship between species diversity, biomass and light transmittance in temperate semi-natural grasslands: is productivity enhanced by complementary light capture?
No full textA positive plant diversity–above-ground productivity relationship is often demonstrated in synthetic grassland stands established for functional biodiversity research, but this relationship is rarely found along diversity gradients in natural and semi-natural grasslands. One of the key mechanisms proposed to cause a positive species diversity–above-ground productivity relationship is increased complementarity in resource use. Using light transmittance to the ground as a measure of resource use intensity in semi-natural grasslands, we tested the hypothesis that peak above-ground biomass (as a proxy for productivity) increases and light transmittance decreases with increasing species richness, which would reflect higher complementarity in light capture. Semi-natural temperate grasslands in Lower Saxony, Germany. We investigated 31 grasslands with variable species richness on three different geological substrates (greywacke, limestone and sandstone) at two spatial scales (sub-regional and regional). Structural equation modelling (SEM) and generalized linear models (GLM) revealed that species richness (5–22 species · 0.09 m−2) was negatively related to above-ground biomass (AGB; 200–1350 g·m−2) and sward cover. The most influential determinant of AGB at the regional scale was temperature. Light transmittance was determined by sward cover, the cover of competitive species and AGB at the regional, and in part also at the sub-regional level. We found no evidence for increased light capture complementarity with higher species richness. This suggests that competitive exclusion, but not complementarity in above-ground resource use, mediates above-ground productivity in species-rich plant assemblages
Genotypic variation and phenotypic plasticity in the drought response of fine roots of European beech
No full textHow temperate trees respond to drier summers, as predicted by climate change models for parts of Europe and eastern North America, will depend on the drought susceptibility of the root systems. We investigated the importance of the genetic constitution for the belowground drought response of European beech (Fagus sylvatica L.), in four populations from regions differing in precipitation (520-970 mm year(-1)). Saplings were grown at ample (10 vol.%; well-watered) or reduced (5 vol.%; drought treatment) soil water content in the Gottingen Rhizolab Facility for two consecutive summers, and the responses of fine root biomass, root morphology, root depth distribution, and fine root production and turnover were investigated by a combined mini-rhizotron and harvest technique approach. In the drought treatment, total root mass per plant was reduced by 30-40% as a result of: (1) a reduction in median fine root lifespan by roughly 50% and hence an increase in fine root turnover; and (2) a 10-fold reduction in relative fine root growth rate (productivity per standing root biomass). The root: shoot ratio did not increase with drought. Although beech plants originating from drier climates tended to reduce their root biomass in response to drought less than those from wetter climates, analyses of variance revealed no significant influence of genotype on root mass, morphology, growth rate or turnover. However, most fine root traits showed marked differences between the well-watered and drought treatments. We conclude that beech saplings respond to summer drought primarily by shortening root lifespan, whereas root system structure and root: shoot carbon partitioning pattern are unaltered. Beech fine root growth and turnover exhibited high phenotypic plasticity, but genotypic variation was of minor importance. In contrast, genotype had a strong influence on leaf and shoot morphogenesis and growth
Leaf Size and Leaf Area Index in Fagus sylvatica Forests: Competing Effects of Precipitation, Temperature, and Nitrogen Availability
No full textPlants across diverse biomes tend to produce smaller leaves and a reduced total leaf area when exposed to drought. For mature trees of a single species, however, the leaf area-water supply relationship is not well understood. We tested the paradigm of leaf area reduction upon drought by a transect study with 14 mature Fagus sylvatica forests along a steep precipitation gradient (970-520 mm y(-1)) by applying two independent methods of leaf size determination. Contrary to expectation, average leaf size in dry stands (520-550 mm y(-1)) was about 40% larger and SLA was higher than in moist stands (910-970 mm y(-1)). As a result of increased leaf sizes, leaf area index significantly increased from the high- to the low-precipitation stands. Multiple regression analyses suggested that average leaf size was primarily controlled by temperature, whereas the influence of soil moisture and soil C/N ratio was low. Summer rainfall of the preceding year was the most significant predictor of total leaf number. We assume that leaf expansion of beech was independent of water supply, because it takes place in May with ample soil water reserves along the entire transect. In contrast, bud formation, which determines total leaf number, occurs in mid-summer, when droughts are severest. We conclude that leaf expansion and stand leaf area of beech along this precipitation gradient are not a simple function of water availability, but are controlled by several abiotic factors including spring temperature and possibly also nitrogen supply, which both tend to increase toward drier sites, thus overlaying any negative effect of water shortage on leaf development.Deutsche Bundesstiftung Umwel
Species-specific effects of temperate trees on greenhouse gas exchange of forest soil are diminished by drought
No full textTree species identity and root-associated microbes are assumed to play an important role in the global terrestrial fluxes of the key biogenic greenhouse gases (GHG; CO2, CH4, N2O), but the specific processes driving this influence and the importance against abiotic impacts are poorly understood. To what extent changes in the species composition of temperate forests and increases in the frequency and duration of summer droughts in the course of global climate change will alter GHG emissions remains unclear. We analyzed the effect of tree species identity and mycorrhizal association type vs. soil drought on GHG fluxes by conducting a greenhouse experiment with four important deciduous tree species which form either ectomycorrhizal or arbuscular mycorrhizal associations. We combined soil gas flux measurements with analyses of leaf gas fluxes, potential fine root respiration, fine root growth and turnover, and N turnover in soil microsites. Our experiment tests the hypotheses that (1) GHG emissions differ between tree species and mycorrhizal association type mainly due to differences in root activity and root-induced processes, and (2) soil drought decreases the amount of GHG exchange from different tree species to a different extent. We found a two times higher global warming potential (GWP) from soil gas exchange in European ash than in the other three tree species (1.9 vs. 0.8–1.0 g CO2-eq kg−1 h−1) mainly due to much higher root mass-specific CO2 emission rates (495 vs. 210–236 mg C kg−1 h−1). Apart from the influence of species differences in fine root productivity, we show a stronger increase in CO2 emission rates per portion of white roots in ash which may indicate a higher metabolic activity of unsuberized fine roots in this tree species. Ectomycorrhizal tree species differed from arbuscular mycorrhizal tree species by a two times greater increase in CO2 emissions per fine root production. The N2O emissions per root mass were up to five times higher in beech than in the other species, caused either by higher nitrate production in the rhizosphere or by lower nitrate consumption. Soil porosity drove the amount of methane uptake, while biotic influences were subordinate. Soil drought generally exerted an important control on GHG fluxes: low water-filled pore space decreased the GWP from soil emissions by only 9% in sycamore, but by 40% (European beech) to 68% (European ash) in the other tree species and largely diminished any tree species differences. This suggests that tree species identity may substantially alter the GWP of temperate forests through rhizosphere processes, but this influence on GHG exchange is diminished by soil drought
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