Growth and posture control strategies in Fagus sylvatica and Acer pseudoplatanus saplings in response to canopy disturbance

Background and Aims

Forest tree saplings that grow in the understorey undergo frequent changes in their light environment to which they must adapt to ensure their survival and growth. Crown architecture, which plays a critical role in light capture and mechanical stability, is a major component of sapling adaptation to canopy disturbance. Shade-adapted saplings typically have plagiotropic stems and branches. After canopy opening, they need to develop more erect shoots in order to exploit the new light conditions. The objective of this study was to test whether changes in sapling stem inclination occur after canopy opening, and to analyse the morphological changes associated with stem reorientation.

Methods

A 4-year canopy-opening field experiment with naturally regenerated Fagus sylvatica and Acer pseudoplatanus saplings was conducted. The appearance of new stem axes, stem basal diameter and inclination along the stem were recorded every year after canopy opening.

Key Results

Both species showed considerable stem reorientation resulting primarily from uprighting (more erect) shoot movements in Fagus, and from uprighting movements, shoot elongation and formation of relay shoots in Acer. In both species, the magnitude of shoot uprighting movements was primarily related to initial stem inclination. Both the basal part and the apical part of the stem contributed to uprighting movements. Stem movements did not appear to be limited by stem size or by stem growth.

Conclusions

Stem uprighting movements in shade-adapted Fagus and Acer saplings following canopy disturbance were considerable and rapid, suggesting that stem reorientation processes play a significant role in the growth strategy of the species.     

Frequency of inversions affects senescence phenology of Acer pseudoplatanus and Fagus sylvatica

In mountainous regions, inversion situations with cold-air pools in the valleys occur frequently, especially in fall and winter. With the accumulation of inversion days, trees in lower elevations experience lower temperature sums than those in middle elevations. In a two-year observational study, deciduous trees, such as Acer pseudoplatanus and Fagus sylvatica, on altitudinal transects responded in their fall leaf senescence phenology. Phenological phases were advanced and senescence duration was shortened by the cold temperatures in the valley. This effect was more distinct for late phases than for early phases since they experienced more inversion days. The higher the inversion frequency, the stronger the signal was. Acer pseudoplatanus proved to be more sensitive to cold temperatures compared to Fagus sylvatica. We conclude that cold-air pools have a considerable impact on the vegetation period of deciduous trees. Considering this effect, trees in the mid hillside slopes gain advantages compared to lower elevations. Our findings will help to improve knowledge about ecological drivers and responses in mountainous forest ecosystems.     

Competition for nitrogen sources between European beech (Fagus sylvatica) and sycamore maple (Acer pseudoplatanus) seedlings

To investigate the short‐term consequences of direct competition between beech and sycamore maple on root N uptake and N composition, mycorrhizal seedlings of both tree species were incubated for 4 days (i.e. beech only, sycamore maple only or both together) in an artificial nutrient solution with low N availability. On the fourth day, N uptake experiments were conducted to study the effects of competition on inorganic and organic N uptake. For this purpose, multiple N sources were applied with a single label. Furthermore, fine roots were sampled and analysed for total amino acids, soluble protein, total nitrogen, nitrate and ammonium content. Our results clearly show that both tree species were able to use inorganic and organic N sources. Uptake of inorganic and organic N by beech roots was negatively affected in the presence of the competing tree species. In contrast, the presence of beech stimulated inorganic N uptake by sycamore maple roots. Both the negative effect of sycamore maple on N uptake of beech and the positive effect of beech on N uptake of sycamore maple led to an increase in root soluble protein in beech, despite an overall decrease in total N concentration. Thus, beech compensated for the negative effects of the tree competitor on N uptake by incorporating less N into structural N components, but otherwise exhibited the same strategy as the competitor, namely, enhancing soluble protein levels in roots when grown under competition. It is speculated that enhanced enzyme activities of so far unknown nature are required in beech as a defence response to inter‐specific competition.     

Light and competition gradients fail to explain the coexistence of shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings

Background and Aims

The coexistence of forest tree species has often been linked to differences among species in terms of their response to light availability during the regeneration stage. From this perspective, species coexistence results from growth–growth or mortality–growth trade-offs along spatial light gradients. Experimental evidence of growth–growth trade-offs in natural conditions is sparse due to various confounding factors that potentially hinder the relationship. This study examined growth hierarchies along light gradients between two tree species with contrasting shade tolerance by controlling potential confounding factors such as seedling size, seedling status, seedling density and species composition.

Methods

Natural regenerated shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings were used, and growth rankings over a 4-year period were compared in 8- to 10-year-old tree seedlings.

Key results

No rank reversal occurs between the two species along the light gradient, or along the density, mixture or seedling size gradients. The shade-tolerant species was always the more competitive of the two. Pronounced effects of initial size on seedling growth were observed, whereas the effects of light and competition by neighbours were of secondary importance. The paramount effect of size, which results from the asymmetric nature of interseedling competition, gives a strong advantage to tall seedlings over the long term.

Conclusions

This study extends previous efforts to identify potential drivers of rank reversals in young tree mixtures. It does not support the classical assumption that spatial heterogeneity in canopy opening explains the coexistence of the two species studied. It suggests that spatial variation in local size hierarchies among seedlings that may be caused by seedling emergence time or seedling initial performance is the main driver of the dynamics of these mixed stands.     

Influence of root temperature on growth of Pinus sylvestris,Fagus sylvatica,Tilia cordata and Quercus robur

One-year-old tree seedlings were incubated in a greenhouse from April to July, under natural daylight conditions, with their root systems at constant temperatures of 5, 10, 15, 20, 25, 30 and 35 °C and with the above ground parts kept at a constant air temperature of 18–20 °C. The course of height growth, total mass increment, root, shoot and leaf weight as well as leaf areas were measured. The results indicate that clear differences exist in the optimal root zone temperatures for various growth parameters in different tree species. Pinus sylvestris had a maximal height increment at about 5–10 °C and maximal total mass increment at 15 °C root temperature. In contrast, the optimum for Quercus robur was at 25 °C. Tilia cordata and Fagus sylvatica had their optima for most growth parameters at 20 °C. The root temperature apparently indirectly influenced photosynthesis (dry weight accumulation) and respiration loss. From the observed symptoms and indications in the literature it seems probable that a change in hormone levels is involved as the main factor in the described effects. Variation of root temperature had only an insignificant effect on bud burst and the time at which the shoots sprouted. Apparently species of northern origin seem to have lower root temperature optima than those of more southern origin. This is to be verified by investigation of other tree species.     

Effects of toxic heavy metals (Cd,Pb) on growth and mineral nutrition of beech (Fagus sylvatica L.)

The beech (Fagus sylvatica L.) is the dominant tree in Middle Europe under many different ecological conditions. But like other tree species, it is suffering in the last ten years increasingly by air pollutants including heavy metals which have been deposited and accumulated for decades in many forest soils. Increasingly mobilized by acidification processes, these metals may have toxic effects on trees.In autecological studies (dose-response-experiments) effects of root-applied Pb and Cd on various growth parameters, on uptake of mineral nutrients and on transpiration of young beech trees were evaluated, and critical concentrations (threshold levels) could be established. Significant leaf area reduction was found with 6 ppm Pb (0.3 ppm Cd) in the leaves (DW), but biomass reduction only with 18 ppm Pb (3.6 ppm Cd). Root elongation rates of seedlings were significantly reduced with 44 ppm plant-available Pb in the soil by about 30%, but only with 24 ppm Pb when combined with 2 ppm Cd, exhibiting synergistic effects. After treatments with 20 ppm Pb and 1 ppm Cd in sand culture, a considerable decrease in the contents of K, Ca, Mg, Fe, Mn, and Zn in roots and leaves of saplings was coincident with high (roots) and moderate (leaves) accumulation of Pb and Cd. A 20% reduction of transpiration rates was measured in ten-year-old beech trees after three months of exposure to a forest soil containing 2.5 ppm plant-available Cd.The data indicate that present-day concentrations mainly of Pb, but not yet of Cd, in acidified European forest soils are sufficiently high to affect germination, growth and mineral nutrition of natural rejuvenation of beech.The data published here are part of a doctoral and of various diploma theses: Mrs. Christiane Bertels, Barbara Buschmann, Martina Hücker, Gritli Noack, Ute Röder, Swantje von Oy and Mr. Rüdiger Ahrend, Peter Rüther and Frank Weisser.     

Growth of adult Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) under free-air ozone fumigation

This study attempted to detect the impact of ozone on adult trees of Norway spruce ( Picea abies [L.] Karst.) and European beech ( Fagus sylvatica L.) in an experimental mixed stand in Southern Bavaria, Germany. The aim was to examine whether there is a decrease in growth when trees are exposed to higher than atmospheric concentrations of ozone. This exposure was put into effect using a free-air fumigation system at tree crown level. Growth analysis was carried out on a group of 47 spruce and 36 beech trees, where radial stem increment at breast height - a sensitive index for stress - was measured. The ozone monitoring system allowed values to be obtained for the accumulated ozone exposure (SUM00) of each individual tree, so that their radial increment over three years could be correlated with the corresponding ozone exposure for the same time period. Correlation and regression analysis were then carried out to test the influence of ozone on diameter increment. In both spruce and beech, the initial stem diameter was the most influential factor on radial increment in the following year. A linear model was applied, including the diameter of the preceding year and the ozone exposure of the current year as predicting factors. For spruce trees, a significant negative influence of ozone exposure was found. In contrast, no significant ozone effect on diameter increment of beech was detected. The effect of ozone stress on a large spruce tree can lead to a decrease in potential radial increment of 22 %. The results are discussed in relation to other stress factors such as drought and lack of light.     

Root growth inhibition effects of holorganic moder humus layer under spruce (Picea abies KARST.) and beech (Fagus sylvatica L.)

Field observations demonstrated inhibition of root growth by moder humus material of the Of₂ and Oh-layers under beech and spruce. Growth chamber experiments with spruce seedlings showed that root growth on a natural Of₂-substrate is ten times lower than on resin-quartz mixtures. Added phenolic acid (protocatechic a.) inhibited root growth only in sterilized substrate. Leachates of Of₂-material inhibited root growth in a mineral substrate devoid of absorption complex (pure quartz sand), but not in a resin-quartz mixture. It is concluded that inhibiting substances are probably ionized molecules that can withstand biodegradation.     

Past and future radial growth and water-use efficiency of Fagus sylvatica and Quercus robur in a long-term climate refugium

The low-latitudinal range margins of many temperate and boreal tree species consist of scattered populations that persist locally in climate refugia. Recent studies have shown that such populations can be remarkably resilient, yet their past resilience does not imply that they are immune to threats from future climate change. The functioning of refugial tree populations therefore needs to be better understood if we are to anticipate their prospects correctly. We performed a detailed study of tree radial growth and vigor in a long-term climate refugial population of beech (Fagus sylvatica), comparing the observed trends with those of co-occurring pedunculate oak (Quercus robur). Annual growth rates (basal area increment, BAI) for both species were similar to those observed in range-core populations, but natural lifespan was half that in the mountains. The master chronologies spanning 1870–2015 revealed 22% (Fagus) and 20% (Quercus) increases in BAI until the 1980s and a smaller decrease (−6% for Fagus, −9% for Quercus) since then. Stable carbon isotope measurements (δ¹³C) revealed no effect of cambial age and an increase in water-use efficiency (iWUE) from 1870–2015 of about 50% for Fagus and 20% for Quercus. The trend continued until 2015 in Fagus, whereas Quercus reached its maximum in the 1980s. A detailed analysis of the relationship between climate and annual growth based on a 118-year meteorological record revealed a major role of water availability in the current and previous year. We used the observed climatic relationships to model future growth trends until 2100 for the IPCC scenarios RCP4.5 and RCP8.5. Most projections revealed no change in current growth rates, suggesting that this climate refugium will be able to provide suitable conditions for the persistence of Fagus and Quercus over the coming decades even under warmer and drier regional climate conditions. Overall, our study provides valuable insight into the precise climatic and biological mechanisms enhancing the persistence of refugial tree populations under ongoing climate change.     

Temporal dynamics of exchangeable K, Ca and Mg in acidic bulk soil and rhizosphere under Norway spruce (Picea abies Karst.) and beech (Fagus sylvatica L.) stands

Anthropogenic nitrogen (N) deposition significantly affects forest soil microbial biomass and extracellular enzymatic activities (EEA). However, the influence of mixed N fertilizations on soil microbial biomass and EEA remains unclear. In this work, NH₄NO₃ was chosen as inorganic N, while urea and glycine were chosen as organic N. They were used to fertilize subtropical forest soil monthly for 1 year with different ratios (inorganic N : organic N?=?10 : 0, 7 : 3, 3 : 7 and 1 : 9 respectively.) and N inputs were equivalent to 7.2 g?N?m^?2?y^?1. Soil samples were harvested every 2 months. Subsequently, soil microbial biomass and enzymatic activities were assayed. Multiple regression analysis (MRA) and principle components analysis (PCA) were utilized to illustrate the relationship between soil microbial biomass and EEA. Results showed that soil EEA displayed different changes in response to various mixed N fertilizations. Invertase, cellulase, cellobiohydrolase, alkaline phosphatase, and catalase activities under mixed N fertilization were higher than those of single inorganic N (NH₄NO₃) fertilization. Polyphenol oxidase activities were depressed after inorganic N fertilization and accelerated after mixed N fertilization. Acid phosphatase activities were accelerated in all N fertilization plots, while the influence of various mixed N fertilizations were not significant. Soil microbial biomass was enhanced by mixed N fertilization, while no significant changes were observed after inorganic N fertilization. The result revealed that although N fertilization may alleviate soil N-limitation, single inorganic N fertilization may disturb the balance of inorganic N and organic N, and depress the increases of soil enzymatic activities and microbial biomass in the end. Soil enzymes activities and microbial biomass showed the highest activities after medium organic N fertilization (inorganic : organic N?=?3 : 7), which might be the most suitable N fertilizer for soil microbes. Meanwhile, PCA showed that the alleviation of N-limited reached a maximum after medium organic N fertilization. All results indicated that soil EEA, microbial biomass, and their relationship are all affected by N type and inorganic to organic N ratio.     

Morphological and physiological reactions of young deciduous trees (Quercus robur L., Q. petraea [Matt.] Liebl., Fagus sylvatica L.) to waterlogging

One-year-old seedlings of Quercus robur L., Q. petraea (Matt.) Liebl. and Fagus sylvatica L. were cultivated in lysimeters and subjected to waterlogging for 17 weeks, interrupted by a five-week drainage period during summer. The growth of Q. robur was less affected by waterlogging than that of Q. petraea and Fagus. Waterlogging resulted in the formation of adventitious roots in Q. robur and Q. petraea, but not in Fagus. In contrast to Fagus, Q. robur and, to a lesser extent, Q. petraea were able to generate roots even below the water table. The hydraulic conductance of the excised root systems, the stomatal conductance and, in Fagus, the leaf water potential and the leaf-mass related hydraulic conductance were decreased by waterlogging. The decrease in the hydraulic conductance was largest in Fagus, and smallest in Q. robur. The roots of Fagus responded to anaerobic conditions with an increase in ethanol concentration. The measurements of nitrate reductase activities in roots and leaves provided no indications of a persistent contribution of NO₃ ⁻ metabolism to the alleviation of waterlogging-induced stress. It is concluded that Q. robur and, to a lesser extent, Q. petraea can tolerate waterlogging periods better than Fagus due to a different pattern of root formation, and to a better adjustment of leaf biomass production to the hydraulic conductivity of the root system. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phenology and growth of Fagus sylvatica and Quercus robur seedlings in response to temperature variation in the parental versus offspring generation

• Plants are known to respond to warming temperatures. Few studies, however, have included the temperature experienced by the parent plant in the experimental design, in spite of the importance of this factor for population dynamics.
• We investigated the phenological and growth responses of seedlings of two key temperate tree species (Fagus sylvatica and Quercus robur) to spatiotemporal temperature variation during the reproductive period (parental generation) and experimental warming of the offspring. To this end, we sampled oak and beech seedlings of different ages (1–5 years) from isolated mother trees and planted the seedlings in a common garden.
• Warming of the seedlings advanced bud burst in both species. In oak seedlings, higher temperatures experienced by mother trees during the reproductive period delayed bud burst in control conditions, but advanced bud burst in heated seedlings. In beech seedlings, bud burst timing advanced both with increasing temperatures during the reproductive period of the parents and with experimental warming of the seedlings. Relative diameter growth was enhanced in control oak seedlings but decreased with warming when the mother plant experienced higher temperatures during the reproductive period.
• Overall, oak displayed more plastic responses to temperatures than beech. Our results emphasise that temperature during the reproductive period can be a potential determinant of tree responses to climate change.

     

Mixed Norway spruce (Picea abies [L.] Karst) and European beech (Fagus sylvatica [L.]) stands under drought: from reaction pattern to mechanism

Key message

We review causes of synergies in mixed-species stands, develop guiding hypotheses for revealing their mechanisms and present a rainfall exclusion experiment along with a transect (KROOF) for exploring drought effects.

Abstract

While monocultures have dominated forest research and practice in the past, in face of growing resource scarcity and climate change, mixed-species stands are on the advance. Long-term observations show that mixed-species stands frequently over-yield monocultures, and they further suggest that the over-yielding is often higher on poor than on fertile sites and in low-growth than in high-growth years. However, the underlying causes have not yet been clarified. We start with a review of, among others, hydraulic redistribution, complementary eco-physiological traits, and ectomycorrhizal networks as possible causes behind the observed productivity gains in mixed-species stands. Then, we develop guiding hypotheses for further exploration of synergies in mixed-species stands. Finally, we introduce into the concept and model approach of the rainfall exclusion experiment for examining the role of water supply in mixed versus pure forest stands of spruce and beech. At the Kranzberg site, six plots are equipped with water retention roofs, which only close during rain events. The remaining six plots serve as non-roofed control. Together with the rainfall exclusion experiment, an ecological gradient with five sites extending through SE-Germany forms the “Kranzberg Roof Experiment” (in short KROOF). Kranzberg Forest is a part of this gradient from moist to dry conditions, with each site providing mixed and pure associations each of spruce and beech. The ecophysiological approach will be complemented by tree ring analysis and modelling of productivity of the tree associations under intense drought.     

The effects of enhanced UV-B radiation on physiological activity and growth of Norway spruce planted outdoors over 5 years

The responses of Norway spruce [Picea abies (L.) Karst.] to enhanced UV-B radiation during the 5-year treatment performed outdoors have been subjected to ecophysiological and growth analysis. The plants were exposed to UV-B radiation, simulating 17% ozone depletion. Ecophysiological parameters were monitored three times a year on three needle age classes, while growth was analysed at the end of each growth season. Spruce exhibited great variability in the amounts of photosynthetic pigments and methanol-soluble UV-B absorbing compounds, light use efficiency, photosynthesis and respiratory potential. The needle, branch and plant biomass production was not significantly affected during the 5-year treatment. The repeated-measures procedure comparing growth parameters through subsequent seasons, revealed a decrease of branch diameter under enhanced UV-B, which could be interpreted as a cumulative UV-B effect. The effects of UV-B radiation depended on needle development stage, interaction with environmental conditions and stresses. A reduced negative effect of UV-B radiation was observed during the prolonged drought in 2003, which was hypothesised as an alleviating effect. The tolerance of Norway spruce to elevated UV-B was to a large extent due to the high content of methanol-soluble UV-B absorbing compounds that was related neither to environmental conditions, including UV-B dose, nor to the developmental stage of the needles. The current year needles exhibited a tendency to increased production of UV-B absorbing compounds under elevated UV-B radiation. The outdoor study performed under variable environmental conditions showed great complexity of spruce response to enhanced UV-B.     

Water relations and gas exchange in Fagus sylvatica L. and Quercus petraea (Mattuschka) Liebl. in a mixed stand at their southern limit of distribution in Europe

Water status and gas exchange of beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Mattuschka) Liebl.] were studied in a mixed stand in the Montejo de la Sierra forest (central Iberian Peninsula), one of the southernmost locations of both species in Europe. Gas exchange and water potential were measured in leaves at different canopy levels over several days in two growing seasons. The daily variation pattern was established with the measurements of three selected dates per year, representative of the soil moisture content situations in early, mid- and late summer. A similar daily time course of leaf water potential was found for the two species. Nevertheless, beech showed a most noticeable decrease of water potential at midmorning and maintained lower leaf water potential than oak in the early afternoon. In 1994 the sessile oak saplings showed higher values of predawn water potential (Ψ_pd) than beech at the end of summer, when soil moisture content was lowest (20 cm depth). Beech showed a significantly lower net assimilation rate (A) than sessile oak for leaves under the same PPFD. Maximum net photosynthesis values (A _max) for beech and sessile oak on sunny leaves were 10.1±0.4 μmol m^–2 s^–1 and 17.8±1.7 μmol m^–2 s^–1 respectively, and those for water vapour stomatal conductance (g _wv) were 265±31 mmol m^–2 s^–1 and 438±74 mmol m^–2 s^–1. Differences in A and g _wv between the two species were maintained throughout the day on all measurement dates. No clear relationship was found between water status of saplings and stomata performance; there was only a negative correlation between Ψ_pd and g _wvmid in beech. Nevertheless, a significant response to the air vapour pressure gradient between leaf and air was translated into stomata closure on an hourly basis, more intensively in beech. Received: 4 March 1999 / Accepted: 21 December 1999