Effect of ecological factors on intra-annual stem girth increment of holm oak

Key message

The intra-annual stem girth increment of Quercus ilex is mainly driven by water availability and secondly by temperature. Tree size and competition modulate the growth response to climate.

Abstract

Holm oak (Quercus ilex ssp. ballota [Desf.] Samp.) is the most widespread species in the Iberian peninsula, being one of the most representative trees in forests and open woodlands. The analysis of stem girth increment of holm oak may provide valuable information about how Mediterranean ecosystems will respond to the forecasted climate changes. However, due to the variability of the Mediterranean climate, the knowledge of intra-annual patterns of growth is needed for a better understanding of the influence of the climatic variables at this scale. To this end, we used band dendrometers to measure monthly stem girth increments of 96 holm oak trees from 2003 to 2010, located in open woodlands and dense Mediterranean forests in southwestern Spain. We assessed the effects of climate, competition, topography, and initial stem diameter on stem girth increment. The major stem increment periods were in spring and autumn whereas increment rates were very low or even negative in winter and summer. Spring was not every year the season with the higher stem increments, but autumn when spring was very dry. Higher precipitation, soil moisture, and relative humidity had significant positive effects on stem increment, whereas higher temperature, reference evapotranspiration, and solar radiation had significant negative effects. Initial tree diameter and competition from nearby trees partly explained significant differences in stem increment of individual trees. Therefore, the forecasted climatic changes, in which decreased rainfall in spring and increased summer drought are expected in the Mediterranean region, may be a significant threat to the Q. ilex ecosystems.     

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.     

Effects of soil warming and nitrogen foliar applications on bud burst of black spruce

Key message

In mature black spruce, bud burst process is anticipated by soil warming, while delayed by foliar applications of nitrogen; however, the effects depend on growth conditions at the site.

Abstract

The observation of phenological events can be used as biological indicator of environmental changes, especially from the perspective of climate change. In boreal forests, the onset of the bud burst is a key factor in the length of the growing season. With current climate change, the major factors limiting the growth of boreal trees (i.e., temperature and nitrogen availability) are changing and studies on mature trees are limited. The aim of this study was to investigate the effects of soil warming and increased nitrogen (N) deposition on bud burst of mature black spruce [Picea mariana (Mill.) BSP]. From 2008 onwards, an experimental manipulation of these environmental growth conditions was conducted in two stands (BER and SIM) at different altitudes in the boreal forest of Quebec, Canada. An increase in soil temperature (H treatment) and a canopy application of artificial rain enriched with nitrogen (N treatment) were performed. Observations of bud phenology were made during May–July 2012 and 2013. In BER, H treatment caused an anticipation (estimated as 1–3 days); while N treatment, a delay (estimated as 1–2 days but only in 2012) in bud burst. No treatments effect was significant in SIM. It has been demonstrated that soil temperature and N availability can play an important role in affecting bud burst in black spruce but the effects of these environmental factors on growth are closely linked with site conditions.

     

Variation of carbon age of fine roots in boreal forests determined from 14C measurements

Background and aims

The main objectives of this study were to determine how the carbon age of fine root cellulose varies between stands, tree species, root diameter and soil depth. In addition, we also compared the carbon age of fine roots from soil cores of this study with reported values from the roots of the same diameter classes of ingrowth cores on the same sites.

Methods

We used natural abundance of ¹⁴C to estimate root carbon age in four boreal Norway spruce and Scots pine stands in Finland and Estonia.

Results

Age of fine root carbon was older in 1.5–2 mm diameter fine roots than in fine roots with <0.5 mm diameter, and tended to be older in mineral soil than in organic soil. Fine root carbon was older in the less fertile Finnish spruce stands (11–12 years) than in the more fertile Estonian stand (3 and 8 years), implying that roots may live longer in less fertile soil. We further observed that on one of our sites carbon in live fine roots with the 1.5–2 mm diameter was of similar C age (7–12 years) than in the ingrowth core roots despite the reported root age in the ingrowth cores – being not older than 2 years.

Conclusions

From this result, we conclude that new live roots may in some cases use old carbon reserves for their cellulose formation. Future research should be oriented towards improving our understanding of possible internal redistribution and uptake of C in trees.     

How does the root system inhibit windthrow in thinned black spruce sites in the boreal forest?

Key message

The root shape and the angle between roots play an important role to prevent windthrow occurrence.

Abstract

Partial cutting is frequently applied to increase the volume growth of residual stems. However, the opening of the forest increases the wind speed within the site, and consequently, the risk of windthrow. In the case of black spruce, uprooted trees are normally characterized by a lifting of the root plate. This research was conducted to compare the root systems of standing and uprooted black spruces, after commercial thinning, by looking at root architecture, volume and radial growth. For this purpose, data from a pool of 18 standing and 18 uprooted trees from three study areas were analyzed. The distribution of roots around the stump was compared between both types of trees, standing and uprooted. The radial growth was measured at 30 cm in the stem, 10 cm and 60 cm in the roots. The shape (I and T-beam) and volume were recorded for each root system. The structure of the roots was also mapped to obtain a spatial overview of the angle between roots. The root shape (at 10 and 60 cm) and the angle between roots combined with the diameter of the stem at stump height seem to determine the vulnerability of black spruce to windthrow. Uprooted trees developed fewer roots, with a large sector around the stump without lateral roots which suggests its major implication in the resistance to windthrow.     

Can savannas become forests? A coupled analysis of nutrient stocks and fire thresholds in central Brazil

Aims

The effects of fire ensure that large areas of the seasonal tropics are maintained as savannas. The advance of forests into these areas depends on shifts in species composition and the presence of sufficient nutrients. Predicting such transitions, however, is difficult due to a poor understanding of the nutrient stocks required for different combinations of species to resist and suppress fires.

Methods

We compare the amounts of nutrients required by congeneric savanna and forest trees to reach two thresholds of establishment and maintenance: that of fire resistance, after which individual trees are large enough to survive fires, and that of fire suppression, after which the collective tree canopy is dense enough to minimize understory growth, thereby arresting the spread of fire. We further calculate the arboreal and soil nutrient stocks of savannas, to determine if these are sufficient to support the expansion of forests following initial establishment.

Results

Forest species require a larger nutrient supply to resist fires than savanna species, which are better able to reach a fire-resistant size under nutrient limitation. However, forest species require a lower nutrient supply to attain closed canopies and suppress fires; therefore, the ingression of forest trees into savannas facilitates the transition to forest. Savannas have sufficient N, K, and Mg, but require additional P and Ca to build high-biomass forests and allow full forest expansion following establishment.

Conclusions

Tradeoffs between nutrient requirements and adaptations to fire reinforce savanna and forest as alternate stable states, explaining the long-term persistence of vegetation mosaics in the seasonal tropics. Low-fertility limits the advance of forests into savannas, but the ingression of forest species favors the formation of non-flammable states, increasing fertility and promoting forest expansion.     

Linking transpiration reduction to rhizosphere salinity using a 3D coupled soil-plant model

Background and aims

In cold biomes, litter decomposition, which controls the nutrient availability for plants and the ecosystem carbon budget, is strongly influenced by climatic conditions. In this study, focused on the early litter decay within snowbed habitats, the magnitude of the short- and long-term influences of climate warming, the direction of the effects of warmer temperature and advanced snowmelt, and the control of microclimatic features and plant traits were compared.

Methods

Combining experimental warming and space-for-time substitution, mass loss and nutrient release of different plant functional types were estimated in different climatic treatments with the litter bag method.

Results

Plant functional types produced a larger variation in the early-decomposition compared to that produced by climatic treatments. Litter decay was not affected by warmer summer temperatures and reduced by advanced snowmelt. Structural-related plant traits exerted the major control over litter decomposition.

Conclusions

Long-term effects of climate warming, resulting from shifts in litter quality due to changes in the abundance of plant functional types, will likely have a stronger impact on plant litter decomposition than short-term variations in microclimatic features. This weaker response of litter decay to short-term climate changes may be partially due to the opposite influences of higher summer temperatures and advanced snowmelt time.     

Sap flow of the southern conifer, <Emphasis Type="Italic">Agathis australis</Emphasis> during wet and dry summers

Key message

Analysis of sap flux density during drought suggests that the large sapwood and rooting volumes of larger trees provide a buffer against drying soil.

Abstract

The southern conifer Agathis australis is amongst the largest and longest-lived trees in the world. We measured sap flux densities (F _d) in kauri trees with a DBH range of 20–176 cm to explore differences in responses of trees of different sizes to seasonal conditions and summer drought. F _d was consistently higher in larger trees than smaller trees. Peak F _d was 20 and 8 g m^?2 s^?1 for trees of diameters of 176 and 20 cm, respectively, during the wet summer. Multiple regression analysis revealed photosynthetically active radiation (PAR) and vapour pressure deficit (D) were the main drivers of F _d. During drought, larger trees were more responsive to D whilst smaller trees were more responsive to soil drying. Our largest tree had a sapwood area of 3,600 cm². Preliminary analysis suggests stem water storage provides a buffer against drying soil in larger trees. Furthermore, F _d of smaller trees had higher R ² values for soil moisture at 30 and 60 cm depth than soil moisture at 10 cm depth (R ² = 0.68–0.97 and 0.55–0.67, respectively) suggesting that deeper soil moisture is more important for these trees. Larger trees did not show a relationship between F _d and soil moisture, suggesting they were accessing soil water deeper than 60 cm. These results suggest that larger trees may be better prepared for increasing frequency and intensity of summer droughts due to deeper roots and/or larger stem water storage capacity.

     

Spatial Patterns in Hyphal Growth and Substrate Exploitation within Norway Spruce Stems Colonized by the Pathogenic White-Rot Fungus Heterobasidion parviporum

In Norway spruce, a fungistatic reaction zone with a high pH and enrichment of phenolics is formed in the sapwood facing heartwood colonized by the white-rot fungus Heterobasidion parviporum. Fungal penetration of the reaction zone eventually results in expansion of this xylem defense. To obtain information about mechanisms operating upon heartwood and reaction zone colonization by the pathogen, hyphal growth and wood degradation were investigated using real-time PCR, microscopy, and comparative wood density analysis of naturally colonized trees with extensive stem decay. The hyphae associated with delignified wood at stump level were devoid of any extracellular matrix, whereas incipient decay at the top of decay columns was characterized by a carbohydrate-rich hyphal sheath attaching hyphae to tracheid walls. The amount of pathogen DNA peaked in aniline wood, a narrow darkened tissue at the colony border apparently representing a compromised region of the reaction zone. Vigorous production of pathogen conidiophores occurred in this region. Colonization of aniline wood was characterized by hyphal growth within polyphenolic lumen deposits in tracheids and rays, and the hyphae were fully encased in a carbohydrate-rich extracellular matrix. Together, these data indicate that the interaction of the fungus with the reaction zone involves a local concentration of fungal biomass that forms an efficient translocation channel for nutrients. Finally, the enhanced production of the hyphal sheath may be instrumental in lateral expansion of the decay column beyond the reaction zone boundary.To grow to great heights, trees continually replace their water- and nutrient-conducting elements. Older elements, such as the heartwood that is formed in many trees, gradually become nonconductive. In contrast to the living sapwood, heartwood lacks active defense mechanisms against microbes. However, lignin, the polymer coating cell wall polysaccharides, is highly resistant to microbial degradation. In fact, white-rot fungi, besides having evolved the ability to tolerate or detoxify the secondary metabolites accumulating in heartwood, are the only organisms capable of efficiently degrading lignin. Following establishment in the heartwood of living trees, the colonies of pathogenic white-rot fungi expand and eventually also threaten the conductive sapwood.The white-rot fungus Heterobasidion annosum sensu lato, composed of three species with overlapping geographic distributions and host ranges in Europe (23), is the most important pathogen of Norway spruce (Picea abies L. Karst) in boreal forests. Primary infection of Norway spruce stands by H. annosum sensu lato takes place through fresh thinning stumps or wounds on roots and at the base of the stem. Basidiospores landing on these entrance points give rise to mycelia which colonize the root systems, and eventually the fungus spreads into the stem heartwood. At sites infested with Heterobasidion parviporum, a species primarily restricted to Norway spruce, roots of saplings can become infected by the fungus after around 10 years of growth (25). Stem colonization usually initiates only after the heartwood has started to develop, which in Norway spruce takes place in trees 25 to 40 years old (17). Due to relatively rapid axial spread within heartwood, the decay column caused by H. annosum sensu lato often is up to 10 m high in the stems of mature Norway spruce trees.In response to sapwood challenge by an expanding heartwood-based colony of H. annosum sensu lato, Norway spruce forms a so-called reaction zone (RZ) in the border area between healthy sapwood and colonized heartwood. This xylem defense is characterized by high pH due to increased carbonate content and enrichment of phenolic compounds, particularly lignans, some of which have shown antifungal properties in bioassays (14, 30, 31). Although several wood decay fungi are able to eventually penetrate the RZ regions formed in trees, the strategies employed by fungi to breach these unique defense barriers are poorly understood (24). The purpose of this study was to obtain information about the mechanisms operating in heartwood colonization and expansion of the decay column via penetration of the RZ. To do this, we examined spatial growth of H. parviporum and the associated substrate exploitation patterns within naturally colonized mature stems of Norway spruce.     

Interannual variations in primary and secondary growth of Nothofagus pumilio and their relationships with climate

Key message

The aim of this study is to evaluate the relationships between primary and secondary growth as well as the influences of climate variations on both types of growth.

Abstract

The relationships between apical (or primary) and radial (or secondary) growth, and climatic influences on both types of growth, were evaluated for Nothofagus pumilio (Nothofagaceae), the dominant subalpine tree in Patagonia. We measured the spacing and number of nodes of annual shoots developed in the period 2001–2010 in 40 N. pumilio trees growing near the upper treeline in the northern Patagonian Andes (41°S). Variations in ring width at the base of each trunk were also recorded. Interannual variations in primary and secondary growth were significantly related to each other, and to several climate variables. Mean temperatures in winter and early spring (June–October) prior to the period of shoot extension were positively associated with both primary and secondary growth. In addition, total summer precipitation (December–March) was positively related to shoot extension, whereas mean summer temperature during the previous growth season (January–March) was directly related to radial growth. These climatic influences on N. pumilio growth may play a major role in regulating the expressions of preformation and neoformation.     

Impact of understory mosses and dwarf shrubs on soil micro-arthropods in a boreal forest chronosequence

Aims

Plant species and functional groups are known to drive the community of belowground invertebrates but whether their effects are consistent across environmental gradients is less well understood. We aimed to determine if plant effects on belowground communities are consistent across a successional gradient in boreal forests of northern Sweden.

Methods

We performed two plant removal experiments across ten stands that form a 364-year post-fire boreal forest chronosequence. Through the removal of plant functional groups (mosses or dwarf shrubs) and of individual species of dwarf shrubs, we aimed to determine if the effects of functional groups and species on the soil micro-arthropod community composition varied across this chronosequence.

Results

Removal of mosses had a strong negative impact on the abundance and diversity of Collembola and Acari and this effect was consistent across the chronosequence. Only specific Oribatid families declined following dwarf-shrub species removals, with some of these responses being limited to old forest stands.

Conclusions

Our results show that the impacts of plants on micro-arthropods is consistent across sites that vary considerably in their stage of post-fire ecosystem development, despite these stages differing greatly in plant productivity, fertility, humus accumulation and moss development. In addition, mosses are a much stronger driver of the micro-arthropod community than vascular plants.     

Absorptive root area and stem resistivity in whole trees of contrasting structure and size – improvement of methods

Aims

The study was focused on comparing the results of the three instrumental methods applied simultaneously for root studies in several tree species representing contrasting situations: root systems of different structure and stems of a wide range of diameters (especially when considering their resistivity). We want to learn properties of the methods, make some improvements and test their validity, before they will be applied to a large series of trees at the stand level.

Material and methods

Douglas fir (Pseudotsuga menziessii (Mirbel) Franco) with very asymmetric root system and Blue spruce (Picea pungens Engelm.) with homogeneous root system growing in the Mendel University Training Forest Enterprise in K?tiny, were selected as the main sample trees. Three variants of stem impedance measurements needed for absorptive root area estimates were applied to an additional series of over 20 trees. In order to characterize vertical and circumferential (around stem) root distribution we applied (1) the sap flow radial patterns measured by the multi-point sensors based on the heat field deformation (HFD) method, and (2) a modified earth impedance (MEI) method from the group of thermodynamic and electric measuring methods and finally we (3) almost harmlessly excavated the whole root system by supersonic air stream. Three steps of absorptive root area measurements were improved: (a) Impact of stem impedance was almost eliminated, (b) Excessive variation of stem impedance values measured too close to stems (in a place with the most heterogeneous materials) was compensated by extrapolation of several close points, (c) Impact of high curvature of small stems was determined and eliminated by an equation.

Results

All the methods gave similar results when considering differences between individual trees as well as between stem sides. Sap flow density was interesting when expressed per measured absorptive root area and leaf area. Experimental data of main and additional sample trees confirmed validity of relationship, which can be applied to improve stem resistivity especially in small trees.

Conclusions

Results indicated, that all the instrumental methods are field applicable and suitable for quantitative measurements, when specific properties of the methods and stem macrostructure are taken into account. Soil electric parameters characterize the important properties related to presence of cracks, water content, and ion concentration, which are being analyzed now.     

Continuous VOC flux measurements on boreal forest floor

Background and Aims

Volatile organic compound (VOC) emissions from biogenic sources are important contributors to chemical reactions in the air. Soil/forest floor VOCs contribute significantly to the ecosystem scale emissions, however, these emissions and their temporal and spatial variations are poorly characterised. The below-canopy VOC emissions have been measured mainly in campaigns; continuous measurements over the whole growing season are rare.

Methods

VOCs were measured from boreal forest floor over the snow-free season 2010 in southern Finland with automated flow-through chambers connected to proton transfer reaction-mass spectrometer (PTR-MS). We measured 10 masses in total, of which five quantitatively (M33, M45, M59, M69, M137).

Results

All of the fluxes showed clear diurnal and seasonal variation, being at their highest in early summer. Spatial variation in the fluxes was great and the lowest rates were found in chambers with dense vegetation cover. Also, VOCs deposition was observed regularly. Monoterpene (M137) emissions were one magnitude higher (up to 264 ng?m^?2?s^?1) than other emissions. The VOC fluxes correlated positively with temperature and light, while relative humidity correlated negatively.

Conclusions

Results indicated that forest floor plays a substantial role in the boreal forest total VOC emissions. Understanding the processes controlling VOC emissions requires more detailed analysis and long-time measurements with sufficient time resolution and analytical accuracy.     

Greater accumulation of litter in spruce (Picea abies) compared to beech (Fagus sylvatica) stands is not a consequence of the inherent recalcitrance of needles

Background and aims

Replacement of beech by spruce is associated with changes in soil acidity, soil structure and humus form, which are commonly ascribed to the recalcitrance of spruce needles. It is of practical relevance to know how much beech must be admixed to pure spruce stands in order to increase litter decomposition and associated nutrient cycling. We addressed the impact of tree species mixture within forest stands and within litter on mass loss and nutritional release from litter.

Methods

Litter decomposition was measured in three adjacent stands of pure spruce (Picea abies), mixed beech-spruce and pure beech (Fagus sylvatica) on three nutrient-rich sites and three nutrient-poor sites over a three-year period using the litterbag method (single species and mixed species bags).

Results

Mass loss of beech litter was not higher than mass loss of spruce litter. Mass loss and nutrient release were not affected by litter mixing. Litter decay indicated non-additive patterns, since similar remaining masses under pure beech (47%) and mixed beech-spruce (48%) were significantly lower than under pure spruce stands (67%). Release of the main components of the organic substance (C_org, N_tot, P, S, lignin) and associated K were related to mass loss, while release of other nutrients was not related to mass loss.

Conclusions

In contradiction to the widely held assumption of slow decomposition of spruce needles, we conclude that accumulation of litter in spruce stands is not caused by recalcitrance of spruce needles to decay; rather adverse environmental conditions in spruce stands retard decomposition. Mixed beech-spruce stands appear to be as effective as pure beech stands in counteracting these adverse conditions.     

Mode of competition for light and water amongst juvenile beech and spruce trees under ambient and elevated levels of O3 and CO2

Key message

Our study provides evidence that neither elevated CO ₂ nor elevated O ₃ alters the positive asymmetric competition for light and the symmetric competition for water among beech and spruce individuals grown in monoculture. We conclude that the mechanism of competition (i.e. symmetric/asymmetric) above (e.g shading or overtopping effect) and belowground (e.g. non-preemption or foraging) rather than abiotic treatments such as elevated CO ₂ , O ₃ and CO ₂ /O ₃ regimes, plays a dominant role for ensuring competitive success among tree saplings.

Abstract

Despite numerous studies conducted on plant responses to increasing CO₂ and O₃ concentrations, there is still a gap in understanding on how these gasses would affect the mode of competition (e.g., the ability by which larger and smaller plants capture resources) at the individual level of intra-specific beech and spruce saplings. Using empirical data and simulations from the plant-growth model PLATHO, we analyzed underlying mechanisms of competition and extrapolated effects beyond the time span of the experiment. We hypothesized that among juvenile beech and spruce trees planted in monoculture, +CO₂ would diminish the positive asymmetric competition for light. Conversely, +O₃ would enhance this outcome. In addition, we hypothesized that the symmetric mode of competition belowground for water would remain unchanged, irrespective of +CO₂ and/or +O₃ treatments. Our results showed that +CO₂ and/or +O₃ treatments did not alter the mode of competition aboveground for light. Conversely, we accepted our hypothesis that the mode of competition for water would remain unchanged under both treatments. Overall, we conclude that neither +CO₂ nor +O₃ alters the positive asymmetric competition for light and the symmetric competition for water among beech and spruce individuals grown in monoculture. We further conclude that competitive mechanism above (e.g., shading or overtopping effect) and belowground (e.g., non-preemption or foraging) rather than abiotic treatments, such as elevated CO₂, O₃ and CO₂/O₃ regimes, plays a dominant role for ensuring competitive success among tree saplings.     

Model-assisted evaluation of crop load effects on stem diameter variations and fruit growth in peach

Key message

The paper identifies and quantifies how crop load influences plant physiological variables that determine stem diameter variations to better understand the effect of crop load on drought stress indicators.

Abstract

Stem diameter (D ^stem) variations have extensively been applied in optimisation strategies for plant-based irrigation scheduling in fruit trees. Two D ^stem derived water status indicators, maximum daily shrinkage (MDS) and daily growth rate (DGR), are however influenced by other factors such as crop load, making it difficult to unambiguously use these indicators in practical irrigation applications. Furthermore, crop load influences the growth of individual fruits, because of competition for assimilates. This paper aims to explain the effect of crop load on DGR, MDS and individual fruit growth in peach using a water and carbon transport model that includes simulation of stem diameter variations. This modelling approach enabled to relate differences in crop load to differences in xylem and phloem water potential components. As such, crop load effects on DGR were attributed to effects on the stem phloem turgor pressure. The effect of crop load on MDS could be explained by the plant water status, the phloem carbon concentration and the elasticity of the tissue. The influence on fruit growth could predominantly be explained by the effect on the early fruit growth stages.     

Mountain forest growth response to climate change in the Northern Limestone Alps

Key message

Growth response to climate differs between species and elevation. Fir is the most drought-tolerant species. The mountain forests are robust to the climatic changes until now.

Abstract

Alpine mountain forests provide a wide range of ecological and socio-economic services. Climate change is predicted to challenge these forests, but there are still considerable uncertainties how these ecosystems will be affected. Here, we present a multispecies tree-ring network of 500 trees from the Berchtesgaden Alps (Northern Limestone Alps, Southeast Germany) in order to assess the performance of native mountain forest species under climate change conditions. The dataset comprises 180 spruce, 90 fir, 110 larch and 120 beech trees from different elevations and slope exposures. We analyse the species with respect to: (1) the general growth/climate response; (2) the growth reaction (GR) during the hot summer in 2003 and (3) the growth change (GC) resulting from increasing temperatures since the 1990s. Spruce is identified as the most drought-sensitive species at the lower elevations. Fir shows a high drought tolerance and is well suited with regard to climate change. Larch shows no clear pattern, and beech remains unaffected at lower elevations. The unprecedented temperature increase of the last decades did not induce any distinct GC. The mountain forests of the Berchtesgaden Alps appear to be robust within the climatic changes until now.     

How important is woody tissue photosynthesis in poplar during drought stress?

Key message

Woody tissue photosynthesis might play a key role in maintaining plant carbon economy and hydraulic function under unfavourable conditions such as drought stress.

Abstract

Within trees, a portion of respired CO₂ is assimilated by bark and woody tissue photosynthesis, but its physiological role remains unclear, in particular under unfavour able conditions like drought stress. We hypothesised that woody tissue photosynthesis will contribute to overall tree carbon gain both under sufficient water supply and during drought, and plays a role in maintaining the hydraulic function. We subjected half of the trees to a stem and branch light-exclusion treatment to prevent bark and woody tissue photosynthesis. Then, we measured leaf gas exchange and stem growth in Populus deltoides x nigra ‘Monviso’ trees both under well-watered and dry conditions. We additionally measured cavitation using acoustic emission in detached control and light-excluded branches to illustrate the role of woody tissue photosynthesis in xylem embolism repair. Under well-watered conditions, light exclusion resulted in reduced stem growth relative to control trees by 30 %. In response to drought, stem shrinkage of light-excluded trees was more pronounced as compared to control trees. During drought stress also maximum photosynthesis and transpiration rate tended to decrease more rapidly in light-excluded trees compared to control trees. Leaf fall in light-excluded branches together with the larger number of acoustic emissions in control branches indicates that in the latter more xylem vessels were still hydraulically functional under drought. Therefore, our study highlights that photosynthesis at branch and stem level might be a key factor in the resilience of trees to drought stress by maintaining both the plant carbon economy and hydraulic function.

     

Decoupled responses of tree and shrub leaf and litter trait values to ecosystem retrogression across an island area gradient

Aims

In the long term absence of catastrophic disturbance ecosystem retrogression occurs, and this is characterized by reduced soil fertility, and impairment of plant biomass and productivity. The response of plant traits to retrogression remains little explored. We investigated how changes plant traits and litter decomposability shift during retrogression for dominant trees and understory shrubs.

Methods

We characterized changes in intraspecific, interspecific and community-averaged values of plant traits and litter decomposability, for six abundant species across thirty lake islands in boreal forest that undergo retrogression with increasing time since fire.

Results

For understory shrubs, trait values and litter decomposability often changed as soil fertility declined in a manner reflective of greater conservation (versus acquisition) of nutrients, particularly at the interspecific and whole community levels. Such responses were seldom observed for trees, meaning that trees and shrubs show a decoupled response to declining soil fertility during retrogression.

Conclusions

Our results only partially agree with previous studies on temperate and subtropical retrogressive chronosequences. Because traits of only shrubs were responsive, they also highlight that impairment of belowground ecosystem processes during retrogression is primarily driven by changes in the trait spectra of understory vegetation rather than that of the trees.