Tree water status and growth of saplings and mature Norway spruce (Picea abies) at a dry distribution limit

Acclimation of mature spruce and beech to five years of repeated summer drought - The role of stomatal conductance and leaf area adjustment for water use

Forests globally are experiencing severe droughts, leading to significant reductions in growth, crown dieback and even tree mortality. The ability of forest ecosystems to acclimate to prolonged and repeated droughts is critical for their survival with ongoing climate change. In a five-year throughfall exclusion experiment, we investigated the long-term physiological and morphological acclimation of mature Norway spruce (Picea abies [L.] KARST.) and European beech (Fagus sylvatica L.) to repeated summer drought at the leaf, shoot and whole tree level. Throughout the drought period, spruce reduced their total water use by 70 % to only 4-9 L per day and tree, while beech was less affected with about 30 % reduction of water use. During the first two summers, spruce achieved this by closing their stomata by up to 80 %. Additionally, from the second drought summer onwards, spruce produced shorter shoots and needles, resulting in a stepwise reduction of total leaf area of over 50 % by the end of the experiment. Surprisingly, no premature leaf loss was observed. This reduction in leaf area allowed a gradual increase in stomatal conductance. After the five-year drought experiment, water consumption per leaf area was the same as in the controls, while the total water consumption of spruce was still reduced. In contrast, beech showed no significant reduction in whole-tree leaf area, but nevertheless reduced water use by up to 50 % by stomatal closure. If the restriction of transpiration by stomatal closure is sufficient to ensure survival of Norway spruce during the first drought summers, then the slow but steady reduction in leaf area will ensure successful acclimation of water use, leading to reduced physiological drought stress and long-term survival. Neighboring beech appeared to benefit from the water-saving strategy of spruce by using the excess water.

Populus euphratica has stronger regrowth ability than Populus pruinosa under salinity stress

Pest infestation and soil salinization levels are increasing due to climate change. Comprehending plant regrowth after insect damage and salinity stress is crucial to understanding climate change's multifactorial impacts on forest ecosystems. This study examined Populus euphratica and P. pruinosa regrowth after different defoliation levels combined with salinity stress. Specifically, the biomass and regrowth ability, non-structural carbohydrate (NSC) and nitrogen (N) pools in different organs and the whole plant, and the leaf Cl- concentration of both poplars were analyzed. Our results showed that after 50% defoliation and no salt addition, the regrowth of both species recovered similarly to the control level, while their regrowth was about 70% after 90% defoliation. However, under salinity stress, the regrowth (% leaf biomass) of P. euphratica was significantly higher than P. pruinose at either the 50% or 90% defoliation levels. Additionally, P. euphratica had more soluble sugar, starch, NSC and N pools in leaf, stem, root and whole plant than P. pruinose under salinity stress. The regrowth based on leaf biomass increased linearly with soluble sugar, starch, NSC and N pools, and decreased linearly with leaf Cl- concentration across different salinity and defoliation levels. These results indicated that defoliation significantly decreased NSC and N pools, limiting the growth of both poplars, and salinity stress exacerbated the negative effect. Furthermore, when suffering from salinity stress, P. euphratica with higher NSC and N pools exhibited stronger regrowth ability than P. pruinose.

Drought responses and carbon allocation strategies of poplar with different leaf maturity

Leaf characteristics can reflect the adaptation of trees to drought stress. However, the effect of leaf maturity on drought stress has been neglected, leading to uncertainty in inferring individual tree responses to drought from leaves. The allocation strategy of photosynthetic carbon between leaf organs (fully expanded young and old leaves) under drought stress remains unclear. Poplar is a diverse and widespread tree species in arid and semi-arid regions. Here, three poplar genotypes (Populus cathayana, P. × euramericana 'Nanlin 895', and P. alba × P. tremula var. glandulosa) were selected and exposed to different watering regimes. The responses and carbon allocation strategies of leaves with different maturity to drought were investigated using a combination of leaf traits and 13 C pulse labelling technique. The results showed that (1) fully expanded young leaves had better osmotic regulation and antioxidant capacity than aged leaves under drought stress. (2) Aged leaves acted as a carbon source during water deficit, where their photosynthetic products were transferred and supplied to upper young leaves to promote stronger photosynthesis in young leaves to acquire resources for tree growth. This study highlights that the effect of leaf maturity should be considered in the future when investigating the effects of drought on woody plants, especially for continuously growing tree species. Therefore, our study not only demonstrates the existence of leaf-age-dependent responses to drought in poplar but also provides new insights into carbon allocation at the leaf level.

Dendrometers challenge the 'moon wood concept' by elucidating the absence of lunar cycles in tree stem radius oscillation

Wood is a sustainable natural resource and an important global commodity. According to the 'moon wood theory', the properties of wood, including its growth and water content, are believed to oscillate with the lunar cycle. Despite contradicting our current understanding of plant functioning, this theory is commonly exploited for marketing wooden products. To examine the moon wood theory, we applied a wavelet power transformation to series of 2,000,000 hourly stem radius records from dendrometers. We separated the influence of 74 consecutive lunar cycles and meteorological conditions on the stem variation of 62 trees and six species. We show that the dynamics of stem radius consist of overlapping oscillations with periods of 1 day, 6 months, and 1 year. These oscillations in stem dimensions were tightly coupled to oscillations in the series of air temperature and vapour pressure deficit. By contrast, we revealed no imprint of the lunar cycle on the stem radius variation of any species. We call for scepticism towards the moon wood theory, at least as far as the stem water content and radial growth are concerned. We foresee that similar studies employing robust scientific approaches will be increasingly needed in the future to cope with misleading concepts.

Sap flow and growth response of Norway spruce under long-term partial rainfall exclusion at low altitude

INTRODUCTION

Under ongoing climate change, more frequent and severe drought periods accompanied by heat waves are expected in the future. Under these conditions, the tree's survival is conditioned by fast recovery of functions after drought release. Therefore, in the presented study, we evaluated the effect of long-term water reduction in soil on tree water use and growth dynamics of Norway spruce.

METHODS

The experiment was conducted in two young Norway spruce plots located on suboptimal sites at a low altitude of 440 m a.s.l. In the first plot (PE), 25% of precipitation throughfall was excluded since 2007, and the second one represented the control treatment with ambient conditions (PC). Tree sap flow, stem radial increment, and tree water deficit were monitored in two consecutive growing seasons: 2015-2016, with contrasting hydro-climatic conditions.

RESULTS

Trees in both treatments showed relatively isohydric behavior reflected in a strong reduction of sap flow under the exceptional drought of 2015. Nevertheless, trees from PE treatment reduced sap flow faster than PC under decreasing soil water potential, exhibiting faster stomatal response. This led to a significantly lower sap flow of PE, compared to PC in 2015. The maximal sap flow rates were also lower for PE treatment, compared to PC. Both treatments experienced minimal radial growth during the 2015 drought and subsequent recovery of radial growth under the more the humid year of 2016. However, treatments did not differ significantly in stem radial increments within respective years.

DISCUSSION

Precipitation exclusion treatment, therefore, led to water loss adjustment, but did not affect growth response to intense drought and growth recovery in the year after drought.

Influence of Warmer and Drier Environmental Conditions on Species-Specific Stem Circumference Dynamics and Water Status of Conifers in Submontane Zone of Central Slovakia

The frequency and intensity of droughts and heatwaves in Europe with notable impact on forest growth are expected to increase due to climate change. Coniferous stands planted outside the natural habitats of species belong to the most threatened forests. In this study, we assess stem circumference response of coniferous species (Larix decidua and Abies alba) to environmental conditions during the years 2015–2019. The study was performed in Arboretum in Zvolen (ca. 300 m a.s.l., Central Slovakia) characterised by a warmer and drier climate when compared to their natural habitats (located above 900 m a.s.l.), where they originated from. Seasonal radial variation, tree water deficit (ΔW), and maximum daily shrinkage (MDS) were derived from the records obtained from band dendrometers installed on five mature trees per species. Monitored species exhibited remarkably different growth patterns under highly above normal temperatures and uneven precipitation distribution. The magnitudes of reversible circumference changes (ΔW, MDS) were species-specific and strongly correlated with environmental factors. The wavelet analysis identified species-specific vulnerability to drought indicated by pronounced diurnal stem variation periodicity in rainless periods. L. decidua exhibited more strained stem water status and higher sensitivity to environmental conditions than A. alba. Tree water deficit and maximum daily shrinkage were found appropriate characteristics to compare water status of different tree species.

Environmental Controls of Diurnal and Seasonal Variations in the Stem Radius of Platycladus orientalis in Northern China

Fine-resolution studies of stem radial variation over short timescales throughout the year can provide insight into intra-annual stem dynamics and improve our understanding of climate impacts on tree physiology and growth processes. Using data from high-resolution point dendrometers collected from Platycladus orientalis (Linn.) trees between September 2013 and December 2014, this study investigated the daily and seasonal patterns of stem radial variation in addition to the relationships between daily stem radial variation and environmental factors over the growing season. Two contrasting daily cycle patterns were observed for warm and cold seasons. A daily mean air temperature of 0 °C was a critical threshold that was related to seasonal shifts in stem diurnal cycle patterns, indicating that air temperature critically influences diurnal stem cycles. The annual variation in P. orientalis stem radius variation can be divided into four distinct periods including (1) spring rehydration, (2) the summer growing season, (3) autumn stagnation, and (4) winter contraction. These periods reflect seasonal changes in tree water status that are especially pronounced in spring and winter. During the growing season, the maximum daily shrinkage (MDS) of P. orientalis was positively correlated with air temperature (Ta) and negatively correlated with soil water content (SWC) and precipitation (P). The vapor pressure deficit (VPD) also exhibited a threshold-based control on MDS at values below or above 0.8 kPa. Daily radial changes (DRC) were negatively correlated with Ta and VPD but positively correlated with relative air humidity (RH) and P. These results suggest that the above environmental factors are associated with tree water status via their influence on moisture availability to trees, which in turn affects the metrics of daily stem variation including MDS and DRC.

Night and day: Shrinking and swelling of stems of diverse mangrove species growing along environmental gradients

Tree stems swell and shrink daily, which is thought to reflect changes in the volume of water within stem tissues. We observed these daily patterns using automatic dendrometer bands in a diverse group of mangrove species over five mangrove forests across Australia and New Caledonia. We found that mangrove stems swelled during the day and shrank at night. Maximum swelling was highly correlated with daily maxima in air temperature. Variation in soil salinity and levels of tidal inundation did not influence the timing of stem swelling over all species. Medium-term increases in stem circumference were highly sensitive to rainfall. We defoliated trees to assess the role of foliar transpiration in stem swelling and shrinking. Defoliated trees showed maintenance of the pattern of daytime swelling, indicating that processes other than canopy transpiration influence the temporary stem diameter increments, which could include thermal swelling of stems. More research is required to understand the processes contributing to stem shrinking and swelling. Automatic Dendrometer Bands could provide a useful tool for monitoring the response of mangroves to extreme climatic events as they provide high-frequency, long-term, and large-scale information on tree water status.

Environmental Modelling of Forest Vegetation Zones as A Support Tool for Sustainable Management of Central European Spruce Forests

The impact of climate change on forest ecosystems may manifest itself by a shift in forest vegetation zones in the landscape northward and into higher elevations. Studies of climate change-induced vegetation zone shifts in forest ecosystems have been relatively rare in the context of European temperate zone (apart from Alpine regions). The presented paper outlines the results of a biogeographic model of climatic conditions in forest vegetation zones applied in the Central European landscape. The objective of the study is a prediction of future silvicultural conditions for the Norway spruce (Picea abies L. Karst.), which is one of the principal tree species within European forests. The model is based on a general environmental dependence of forest vegetation zones on the long-term effect of altitudinal and exposure climates defined by the mean and extreme air temperatures and the amount and distribution of atmospheric precipitation. The climatological data for the model were provided by a validated regional climate database for 2010 – 2090 according to the SRES A1B scenario, bound to specific geo-referenced points in the landscape. The geobiocoenological data in the model were provided by the Biogeography Register database which contains ecological data on the landscape bound to individual cadastres of the entire Czech Republic. The biogeographic model applies special programs (the FORTRAN programming language) in the environment of geographic information systems. The model outputs can be clearly graphically visualized as scenarios of predicted future climatic conditions of landscape vegetation zones. Modelling of the regional scenario of changes in the climatic conditions of forest vegetation zones reveals that in the prediction period of 2070 and beyond, good and very good climatic conditions for the cultivation of forests with dominant Norway spruce will be found only in some parts of its today’s native range in forest vegetation zones 5 – 8. Based on the results provided by the regional scenario, the authors of this paper recommend fundamental reassessment of the national strategy of sustainable forest management in the Czech Republic, stipulating that the current practice of spruce cultivation be reduced only to areas specifically defined by the biogeographic model. The paper shows that biogeographic models based on the concept of vegetation zoning can be applied not only in regional scenarios of climate change in the landscape but also as support tools for the creation of strategies of sustainable forest management.

Distinct growth phenology but similar daily stem dynamics in three co-occurring broadleaved tree species

Dendrometers offer a useful tool for long-term, high-resolution monitoring of tree responses to environmental fluctuations and climate change. Here, we analyze a 4-year dendrometer dataset (2014-17) on European beech (Fagus sylvatica L.), common hornbeam (Carpinus betulus L.) and pedunculate oak (Quercus robur L.), co-occuring in a mixed broadleaved forest in northeastern Germany. In our analyses, we focus both on seasonal growth dynamics as well as on the environmental forcing of daily stem-size variations. Over the study period with contrasting weather conditions, we observed species- and year-specific differences in growth phenology (i.e., growth onset, cessation and duration). Oak was characterized by early growth onset and long growth duration in all years as compared with beech and hornbeam. The analysis on the environmental forcing of daily stem dynamics revealed, however, highly similar responses for the studied species, with current-day vapor pressure deficit and sunshine duration negatively, and relative humidity and precipitation positively affecting stem size. When considering lagged effects, environmental conditions often oppositely affected stem-size changes. No consistent seasonality in environmental responses was detected, though specific weather conditions were found to affect temporal patterns in individual years. We suggest that the high similarity in environmental forcing observed between tree species can be explained by daily stem-size changes mainly reflecting tree water status rather than tree growth. Our results stress that correcting dendrometer series for reversible stem hydrological changes is of utmost importance to better quantify tree growth from dendrometers in future.

Water stress limits transpiration and growth of European larch up to the lower subalpine belt in an inner-alpine dry valley

Climate change will further constrain water availability in dry inner-alpine environments and affect water relations and growth conditions in mountain forests, including the widespread larch forests. To estimate the effects of climate conditions on water balance and growth, variation in sap flow and stem radius of European larch was measured for 3 yr along an elevation transect from 1070 to 2250 m above sea level (asl) in an inner-alpine dry valley in South Tyrol/Italy. Additionally, long-term climate-growth relations were derived from tree cores. Sap flow and radial growth were reduced in dry periods up to an elevation of 1715 m, leading to maximum annual growth at 2000 m. In a wet year no growth difference between elevations was observed. Long-term tree ring data showed a positive growth response to precipitation up to 1715 m and to temperature only above 2000 m. Our results demonstrate that reduced water availability and higher atmospheric water demand limit larch at low elevation within dry Alpine regions. This indicates a general upward shift of this species' elevational amplitude upon climate change, and respective negative effects on future silvicultural use and ecosystem services at lower elevations in the European Alps.

Cambial response of Norway spruce to modified carbon availability by phloem girdling

We tested the hypothesis that increase in carbon (C) availability in Norway spruce saplings (Picea abies (L.) Karst.) intensifies cambial cell division and increases cell lumen diameter (CLD) and cell wall thickness (CWT) when water availability is adequate. To accomplish this, we experimentally subjected 6-year-old P. abies saplings (n = 80 trees) to two levels of soil humidity (watered versus drought conditions) and manipulated tree C status by physically blocking phloem transport at three girdling dates (GDs). Stem girdling occurred in mid-March (day of the year (doy) 77) and in mid-May (GD doy 138), i.e., ~4 weeks before the onset of bud break and during vigorous stem growth, respectively, and in early July (GD doy 190), i.e., 6 and 4 weeks after cessation of radial growth in drought-stressed trees and shoot growth in both soil humidity (SH) treatments, respectively. In response to phloem blockage a striking increase in the number of xylem cells at all GDs and reactivation of cambial activity in drought-stressed trees was detected above the girdling zone, while below girdling xylem formation stopped in both SH-treatments. Although girdling differently affected wood anatomical parameters (CLD, CWT and CLD:CWT ratio) during earlywood and latewood formation, GD had a minor effect on cambial cell division and xylem cell differentiation. Results also revealed that phloem girdling outweighed drought effects imposed on cambial activity. We explain our findings by accumulation of carbohydrates, osmotically active sugars and/or C based signaling compound(s) in response to girdling. Altogether, we conclude that wood formation in P. abies saplings is limited by C availability, which is most likely caused by high C demand belowground especially under drought.

Linking wood traits to vital rates in tropical rainforest trees: Insights from comparing sapling and adult wood

PREMISE OF THE STUDY

Wood density is the top predictor of growth and mortality rates (vital rates) but with modest explanatory power at best. Stronger links to vital rates are expected if wood density is decomposed into its anatomical properties at sapling and adult stages, since saplings and adults differ in wood traits and vital rates. We examined whether anatomical determinants of wood density and strength of the relationship between wood traits and vital rates shift between saplings and adults.

METHODS

Using wood segments from near pith (sapling) and near bark (adult) for 20 tree species (three adults each) from Barro Colorado Island, Panama, we quantified wood traits. Vital rates for saplings and adults were obtained from an earlier study.

KEY RESULTS

Anatomical predictors of wood density were similar for sapling and adult wood, with wood density variation largely explained by fiber lumen area and fiber wall fraction. In sapling wood only, growth rates decreased with fiber wall fraction and increased with fiber lumen area, while mortality rates increased with vessel area but decreased with fiber wall fraction and vessel density.

CONCLUSIONS

Wood traits of sapling trees provide functional insight into the growth-mortality tradeoff. Sapling wood with relatively large fiber lumen area and wide vessels, enabling faster hydraulic transport but less mechanical strength, is associated with fast growth and high mortality. Sapling wood with relatively more fiber wall and many narrow vessels, enabling greater mechanical strength but slower hydraulic transport, is associated with slow growth and low mortality.

Soil water availability and evaporative demand affect seasonal growth dynamics and use of stored water in co-occurring saplings and mature conifers under drought

High-resolution time series of stem radius variations (SRVs) record fluctuations in tree water status and temporal dynamics of radial growth. The focus of this study was to evaluate the influence of tree size (i.e., saplings vs. mature trees) and soil water availability on SRVs. Dendrometers were installed on Pinus sylvestris at an open xeric site and on Picea abies at a dry-mesic site, and the SRVs of co-occurring saplings and mature trees were analyzed during two consecutive years. The results revealed that irrespective of tree size, radial growth in P. sylvestris occurred in April-May, whereas the main growing period of P. abies was April-June (saplings) and May-June (mature trees). Linear relationships between growth-detrended SRVs (SSRVs) of mature trees vs. saplings and climate-SSRV relationships revealed greater use of water reserves by mature P. abies compared with saplings. This suggests that the strikingly depressed growth of saplings compared with mature P. abies was caused by source limitation, i.e., restricted photosynthesis beneath the dense canopy. In contrast, a tree size effect on the annual increment, SSRV, and climate-SSRV relationships was less obvious in P. sylvestris, indicating comparable water status in mature trees and saplings under an open canopy. The results of this study provided evidence that water availability and a canopy atmosphere can explain differences in temporal dynamics of radial growth and use of stem water reserves among mature trees and saplings.