Temperature thresholds for the onset of xylogenesis in alpine shrubs on the Tibetan Plateau

Key message

The threshold minimum air temperature driving xylem growth of alpine  Rhododendron aganniphum is lower than that commonly observed at the treeline of conifers.

Abstract

Understanding how alpine shrubs grow and which environmental factors drive their biomass gain could help to functionally differentiate trees and shrubs. The cambium is the main meristem responsible for wood formation in trees and shrubs. Thus, a better knowledge of cambium growth dynamics in alpine shrubs would allow explaining why shrubs displace trees above the treeline. Here, we aim to investigate the timings and dynamics of xylogenesis and to identify the thermal thresholds controlling the onset of xylem growth of Rhododendron aganniphum, a tall shrub growing above the alpine treeline on the Tibetan Plateau. Timings of xylogenesis and radial growth rates were assessed from anatomical observations of the developing xylem during three growing seasons (2011, 2012, and 2013). The threshold temperature at which xylogenesis had a 0.5 probability of being active was calculated with logistic regressions. The onset of xylogenesis was observed between mid and late June, whereas the end of xylogenesis lasted from mid to late September. Overall, the duration of xylem growth lasted 88–101 days, and 94 % of the ring was formed from June to August. The threshold for the onset of xylem growth was observed at 2.0 ± 0.6 °C for the minimum air temperature, lower than that commonly observed for treeline conifers (ca. 6 °C). This low thermal threshold allows alpine shrubs to have a growing season long enough to complete xylem production and maturation during the warmest summer months. Our results suggest that the time required to complete xylogenesis is critical to understand why shrubs displace trees above the treeline.

     

Stem growth rhythms in trees of a tropical rainforest in Southern Brazil

Key message

We demonstrate that tropical trees growing in wet climates can have a marked seasonality in cambium activity and stem growth associated with high temperature and day length of summer.

Abstract

Monitoring the rhythm of tree growth associated with the growth rings can contribute substantially to understanding forest dynamics and the management of tropical forests. In this study, we monitored the girth increment rhythm and described the wood characteristics (anatomy of growth rings, wood specific gravity) in 10 tropical tree species (103 individuals) naturally occurring in a wet and weakly seasonal region of Atlantic Forest in southern Brazil. We aimed to verify whether tree growth dynamics are associated with climate and woody anatomy in tropical trees with contrasting ecological characteristics. We installed permanent dendrometer bands and monthly assessed the girth increment for 22 months. We collected wood samples (non-destructive method), measured wood specific gravity and prepared permanent slides to characterize the growth ring markers. We found growth rings in all species (distinct in six species); deciduous species produced more distinguishable tree rings compared with semi-deciduous and evergreen tree species. Species varied in their accumulated girth growth (in average, from 1.83 to 62.64 mm), growth rates (1–15 %), and annual radial increment (0.16–5.44 mm). Girth increment was positively related to temperature and day length in five out of ten tree species, indicating the possible effects of these climatic variables in triggering cambial activity in these species. The growth pattern varied among species and was marginally associated to the tree deciduousness. We concluded that even in wet and less seasonal climates, there can be an association in the cambium activity and stem growth with the hotter and longer days of summer months.

     

Stem growth and phenology of two tropical trees in contrasting soil conditions

Background and aims

Phenological variations in tropical forests are usually explained by climate. Nevertheless, considering that soil water availability and nutrient content also influence plant water status and metabolism, soil conditions may also be important in the regulation of plant reproductive and vegetative activities over time. We investigated whether phenological patterns and stem growth differ in trees growing in two types of soil that display contrasting water and nutrient availability, namely, Gleysol (moist and nutrient-poor) and Cambisol (drier and nutrient-rich).

Methods

Phenological observations (flushing, leaf fall, flowering and fruiting) and stem diameter growth were recorded for 120 trees fitted with fixed dendrometer bands, at 15 days intervals, for 1 year. Two species of contrasting deciduousness were investigated: Senna multijuga (semi-deciduous) and Citharexylum myrianthum (deciduous).

Results

Both species were seasonal in all phenophases, regardless of soil type. However, frequency, mean date and intensity of phenophases varied according to soil type. Girth increment of C. myrianthum was four times greater in Cambisol than in Gleysol, whereas the type of soil had no significant effect on that of S. multijuga.

Conclusions

These results show that soil characteristics also play an important role in determining phenological patterns and growth and must be considered when analysing phenological patterns in tropical forests.     

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.     

A novel insight into the structure of amphivasal secondary bundles on the example of Dracaena draco L. stem

Key message

Pattern of tracheids found along the bundles extends understanding of their cross - sectional anatomy and sheds a new light on the issue of radial transport in monocotyledons with secondary growth.

Abstract

Secondary growth of Dracaena draco L. stem is connected with the formation of amphivasal vascular bundles in which a centrally located phloem is surrounded by a ring of xylem cells (tracheids). However, as visible in a single transverse section, there is a tendency towards variation among the secondary bundles from such with a xylem ring to ones in which the tracheids do not completely surround the phloem, i.e., are separated by vascular parenchyma cells. We aimed to elucidate the cross-sectional anatomy of amphivasal secondary bundles using the method of serial sectioning (with sections 3 μm thick), which allowed us to follow very precisely the bundle structure along its length. The analysis revealed that the xylem arrangement in these bundles depends on the position of a section in the bundle path. Each amphivasal bundle is composed of sectors where tracheids form a ring, as well as of such where tracheids are separated by vascular parenchyma cells. We hypothesize that this structure of amphivasal vascular bundles facilitates radial transport of assimilates to the sink tissues. The result of the anatomical analysis is discussed in a physiological context.     

A life cycle carbon dioxide inventory of the Million Trees Los Angeles program

Purpose

This study seeks to answer the question, “Will the Million Trees LA (Million Trees Los Angeles, MTLA) program be a carbon dioxide (CO₂) sink or source?” Because there has never been a full accounting of CO₂ emissions, it is unclear if urban tree planting initiatives (TPIs) are likely to be effective means for reaching local reduction targets.

Methods

Using surveys, interviews, field sampling, and computer simulation of tree growth and survival over a 40-year time period, we developed the first process-based life cycle inventory of CO₂ for a large TPI. CO₂ emissions and reductions from storage and avoided emissions from energy savings were simulated for 91,786 trees planted from 2006 to 2010, of which only 30,813 (33.6 %) were estimated to survive.

Results and discussion

The MTLA program was estimated to release 17,048 and 66,360 t of fossil and biogenic CO₂ over the 40-year period, respectively. The total amount emitted (83,408 t) was slightly more than the ?77,942 t CO₂ that trees were projected to store in their biomass. The MTLA program will be a CO₂ sink if projected 40-year-avoided fossil fuel CO₂ emissions from energy savings (?101,679 t) and biopower (?1,939 t) are realized. The largest sources of CO₂ emissions were mulch decomposition (65.1 %), wood combustion (14.5 %), and irrigation water (9.7 %).

Conclusions

Although trees planted by the MTLA program are likely to be a net CO₂ sink, there is ample opportunity to reduce emissions. Examples of these opportunities include selecting drought-tolerant trees and utilizing wood residue to generate electricity rather than producing mulch.     

Precision and accuracy of tree-ring-based death dates of mountain pines in the Swiss National Park

Key message

Mountain pines in the Swiss National Park show evidence of partial cambial mortality, which affects the precision of tree-ring-based death dates, followed by lagged crown mortality.

Abstract

The time of tree death is commonly reconstructed by dating the outermost ring of tree-ring series. However, due to the occurrence of partial cambial mortality, the date of the outermost tree ring may vary between different locations on the tree stem. Furthermore, a tree may continue to live following the formation of the most recent tree ring. In this study, we quantified precision and accuracy of tree-ring-based death dates from 229 dead mountain pines (Pinus montana) from a 28 km² study area in the Swiss National Park. For almost two-thirds of the trees, a maximum difference of just 0–4 years between the dates of cambial mortality from three increment cores was observed, however, for a few trees the difference reached 30–65 years. Higher maximum differences between the dates of cambial mortality are expected for trees on steep slopes, for old trees or for trees that died a long time ago. For 84 % of dead mountain pines, which were sampled in a permanent sample plot with 2-year remeasurement intervals, the difference between the date of observed crown mortality and the death date determined from three cores was 0–5 years. Sampling two or just one core per tree decreases the accuracy of tree-ring-based death dates. Based on the findings of our study, we recommend a prior assessment of the precision and accuracy of tree-ring-based death dates for any dendroecological study dealing with the reconstruction of tree mortality.     

Xylem and phloem phenology in co-occurring conifers exposed to drought

Key message

Variability in xylem and phloem phenology among years and species is caused by contrasting temperatures prevailing at the start of the growing season and species-specific sensitivity to drought.

Abstract

The focus of this study was to determine temporal dynamics of xylem and phloem formation in co-occurring deciduous and evergreen coniferous species in a dry inner Alpine environment (750 m a.s.l., Tyrol, Austria). By repeated micro-sampling of the stem, timing of key phenological dates of xylem and phloem formation was compared among mature Pinus sylvestris, Larix decidua and Picea abies during two consecutive years. Xylem formation in P. sylvestris started in mid and late April 2011 and 2012, respectively, and in both years about 2 week later in P. abies and L. decidua. Phloem formation preceded xylem formation on average by 3 week in P. sylvestris, and c. 5 week in P. abies and L. decidua. Based on modeled cell number increase, tracheid production peaked between early through late May 2011 and late May through mid-June 2012. Phloem formation culminated between late April and mid-May in 2011 and in late May 2012. Production of xylem and phloem cells continued for about 4 and 5–6 months, respectively. High variability in xylem increment among years and species is related to exogenous control by climatic factors and species-specific sensitivity to drought, respectively. On the other hand, production of phloem cells was quite homogenous and showed asymptotic decrease with respect to xylem cells indicating endogenous control. Results indicate that onset and culmination of xylem and phloem formation are controlled by early spring temperature, whereby strikingly advanced production of phloem compared to xylem cells suggests lower temperature requirement for initiation of the former.     

Partitioning of belowground C in young sugar maple forest

Background and aims

Trees allocate a high proportion of assimilated carbon belowground, but the partitioning of that C among ecosystem components is poorly understood thereby limiting our ability to predict responses of forest C dynamics to global change drivers.

Methods

We labeled sugar maple saplings in natural forest with a pulse of photosynthetic ¹³C in late summer and traced the pulse over the following 3 years. We quantified the fate of belowground carbon by measuring ¹³C enrichment of roots, rhizosphere soil, soil respiration, soil aggregates and microbial biomass.

Results

The pulse of ¹³C contributed strongly to root and rhizosphere respiration for over a year, and respiration comprised about 75 % of total belowground C allocation (TBCA) in the first year. We estimate that rhizosphere carbon flux (RCF) during the dormant season comprises at least 6 % of TBCA. After 3 years, 3.8 % of the C allocated belowground was recovered in soil organic matter, mostly in water-stable aggregates.

Conclusions

A pulse of carbon allocated belowground in temperate forest supplies root respiration, root growth and RCF throughout the following year and a small proportion becomes stabilized in soil aggregates.     

Fine root biomass and turnover of two fast-growing poplar genotypes in a short-rotation coppice culture

Background and aims

The quantification of root dynamics remains a major challenge in ecological research because root sampling is laborious and prone to error due to unavoidable disturbance of the delicate soil-root interface. The objective of the present study was to quantify the distribution of the biomass and turnover of roots of poplars (Populus) and associated understory vegetation during the second growing season of a high-density short rotation coppice culture.

Methods

Roots were manually picked from soil samples collected with a soil core from narrow (75 cm apart) and wide rows (150 cm apart) of the double-row planting system from two genetically contrasting poplar genotypes. Several methods of estimating root production and turnover were compared.

Results

Poplar fine root biomass was higher in the narrow rows than in the wide rows. In spite of genetic differences in above-ground biomass, annual fine root productivity was similar for both genotypes (ca. 44 g DM m^?2 year^?1). Weed root biomass was equally distributed over the ground surface, and root productivity was more than two times higher compared to poplar fine roots (ca. 109 g DM m^?2 year^?1).

Conclusions

Early in SRC plantation development, weeds result in significant root competition to the crop tree poplars, but may confer certain ecosystem services such as carbon input to soil and retention of available soil N until the trees fully occupy the site.     

Formation mechanisms of the alpine Erman’s birch (Betula ermanii) treeline on Changbai Mountain in Northeast China

Key message

The treeline on Changbai Mountain controlled by low temperature and water stress, has not reached the position most commonly expected.

Abstract

Treeline pattern is an important consideration in exploring the general mechanisms controlling the response of treelines to climatic change. However, most of the present conclusions were derived from evergreen and/or conifer treeline, it is still not clear about the deciduous treeline. This study analyzed concentrations of non-structural carbohydrates (NSC) and their components (total soluble sugars and starch) in tree tissues of the deciduous species Erman’s birch (Betula ermanii) at four points along an elevational gradient ranging from 1,908–2,058 m a.s.l at the end of the growing season on Changbai Mountain in Northeast China. The mean 10-cm soil temperature of 8.2 °C under trees across the 129-day growing season at the treeline in this region was higher than that of the average threshold temperature found at treeline positions in the global and China’s climate studies. However, altitudinal trends of NSC concentrations increased significantly in all tissue types along the altitudinal gradients, revealing no depletion of carbon reserves at the treeline on Changbai Mountain. At the same time, the pronounced variation of δ¹³C in leaves and aged branches suggested that low temperature and water stress may simultaneously be operating at high altitudes to restrict the growth and NSC accumulation in trees above the treeline. In light of the above, we conclude that treeline formation on Changbai Mountain is no carbon depletion at the end of growing season, and most likely the result of sink limitation reflecting the combined effects of low temperature and water stress that determined the actual position of the treeline.     

Shoot hydraulic characteristics, plant water status and stomatal response in olive trees under different soil water conditions

Aims

To evaluate the impact of the amount and distribution of soil water on xylem anatomy and xylem hydraulics of current-year shoots, plant water status and stomatal conductance of mature ‘Manzanilla’ olive trees.

Methods

Measurements of water potential, stomatal conductance, hydraulic conductivity, vulnerability to embolism, vessel diameter distribution and vessel density were made in trees under full irrigation with non-limiting soil water conditions, localized irrigation, and rain-fed conditions.

Results

All trees showed lower stomatal conductance values in the afternoon than in the morning. The irrigated trees showed water potential values around ?1.4 and ?1.6 MPa whereas the rain-fed trees reached lower values. All trees showed similar specific hydraulic conductivity (K _s) and loss of conductivity values during the morning. In the afternoon, K _s of rain-fed trees tended to be lower than of irrigated trees. No differences in vulnerability to embolism, vessel-diameter distribution and vessel density were observed between treatments.

Conclusions

A tight control of stomatal conductance was observed in olive which allowed irrigated trees to avoid critical water potential values and keep them in a safe range to avoid embolism. The applied water treatments did not influence the xylem anatomy and vulnerability to embolism of current-year shoots of mature olive trees.     

The role of harvest residues to sustain tree growth and soil nitrogen stocks in a tropical Eucalyptus plantation

Aims

Tropical plantations are likely to supply a growing share of the increasing world demand for forest products. We aimed to gain insight into the role of the nitrogen (N) contained in harvest residues (HR) for tree growth and soil N stocks.

Methods

We used 15N-labeled harvest residues to (1) study the dynamic of N release throughout decomposition, (2) determine the vertical transport pathways of N from the forest floor to the upper soil layers, and (3) quantifying the contributions of HR to soil N stocks and the supply of N to young Eucalyptus trees.

Results

Almost all of the 15N initially contained in the HR was recovered 27 months after deposition, with 21 % remaining in HR, 38 % being transferred to the underlying O layer, 21 % being transferred to the 0–15 cm soil layer, and approximately 15 % accumulating in the tree biomass. Our results supported the presence of two pathways of N transfers from the O layer to the mineral soil: (1) the leaching of dissolved 15N from fresh litter during the first year after planting which actively contributed to Eucalyptus N nutrition and (2) the transport of particulate organic matter in percolating water which contributed to maintain N stocks in the first 15 cm of the soil. Approximately 40 % of the N content in 2-year-old Eucalyptus trees was derived from the labeled HR.

Conclusions

The sustainability of fast-growing Eucalyptus trees established on N-poor sandy tropical soils largely relies on organic residues, as an early source of mineral N for tree and as a source of organic N in the top soil.     

Leaf litter of a dominant cushion plant shifts nitrogen mineralization to immobilization at high but not low temperature in an alpine meadow

Aims

We evaluated the effects of temperature and addition of leaf litter of Androsace tapete MaximWe–a dominant cushion plant species of alpine meadows on the Tibetan Plateau–on carbon (C) and nitrogen (N) mineralization, microbial biomass C (MBC) and N (MBN).

Methods

A laboratory incubation experiment with and without cushion plant litter addition was conducted for 112 days at three temperature regimes (?1, 5 and 11 °C). C and net N mineralization were simultaneously measured during the incubation period.

Results

C and N mineralization were affected by interactions between litter addition and temperature. Litter addition increased C mineralization and MBN but shifted N mineralization to immobilization at higher temperature. The positive relationship between net N mineralization and MBC and MBN was shifted to a negative one through cushion plant litter addition. Cushion plant litter also changed the relationship between C mineralization and net N mineralization from insignificantly positive to significantly negative.

Conclusions

These findings indicate that low temperature in winter could be important for alpine plants because low temperature can increase net N mineralization and supply plants with available N for their growth in the early growing season. During the growing season, climate warming–either directly through a temperature effect or indirectly through triggering increased cushion plant litter production–might lead to stronger competition for N between alpine plants and microorganisms.     

Foliar application of Sili-K® increases chestnut (Castanea spp.) growth and photosynthesis, simultaneously increasing susceptibility to water deficit

Background and aims

The beneficial effects of Si have mainly been observed in herbaceous plants, while little is known about its role in deciduous trees. The aim of this work was to evaluate the effect of foliar application of Si on chestnut leaf growth, photosynthesis and water relations in the presence of short, but intense water deficit.

Methods

Sili-K® solution (containing 0.12 % Si and 0.15 % K) was repeatedly (× 3) sprayed onto leaves of potted chestnut plantlets and irrigation was suspended 7 weeks later, for 8 days. Leaf growth, anatomy, as well as physiological and biochemical traits of the plantlets were studied.

Results

Si application enhanced chestnut growth, due to increased photosynthetic traits, including higher chlorophyll content and chlorophyll a to b ratio, photochemical efficiency of PSII, gas exchange (stomatal conductance, transpiration rate, net CO₂ assimilation) and oxygen evolution rate. Meanwhile, Si yielded larger and thinner leaves, higher xylem, specific leaf area and transpiration rate, thus being beneficial to the tree in absorbing sunlight energy for photosynthesis and in alleviating heat stress. However, Si also lowered leaf sap osmotic pressure, causing the plant to lose water more quickly, thus being more susceptible to water stress.

Conclusions

Si improved chestnut photosynthesis, growth, and heat stress tolerance, but it also increased the susceptibility to drought.     

Boron delays dehydration and stimulates root growth in Eucalyptus urophylla (Blake,S.T.) under osmotic stress

Background

Water and nutritional restrictions are limiting factors for the growth of Eucalyptus trees in tropical climates. In the dry season, boron (B) uptake is severely affected.

Aims

The objectives of this study were to evaluate the phloem mobility of B and whether its deficiency can increase plant sensitivity to osmotic stress. It was also tested to what extent foliar application of B could mitigate the negative effects of drought under low B supply.

Methods

Seedlings of a drought tolerant Eucalyptus urophylla (Blake, S. T.) clone were grown in nutrient solution, subjected to low availability of B for 25 days, and then submitted to a progressive osmotic stress. After imposition of osmotic stress, B was applied to young or mature leaves.

Results

B applications, mainly to mature leaf, stimulated root growth and delayed dehydration under osmotic stress and led to an increased B translocation and carbon isotopic composition. The expression of B transporters and pectin metabolism genes were also increased in water-stressed plants supplied with B by foliar application.

Conclusions

B deficiency led to increased plant dehydration and decreased root growth under osmotic stress. The application of B to mature leaf of water-stressed plants proved effective in mitigating the negative effects of water deficit in root growth.     

Climate variability,tree increment patterns and ENSO-related carbon sequestration reduction of the tropical dry forest species <Emphasis Type="Italic">Loxopterygium huasango</Emphasis> of Southern Ecuador

Key message

Striking hydro-climatic differences of 2 years (wet; dry) dramatically control the increment pattern of L. huasango in varying extent, even causing a “growth collapse” during the La Niña drought 2010/2011.

Abstract

We present the first multi-year long time series of local climate data in the seasonally dry tropical forest in Southern Ecuador and related growth dynamics of Loxopterygium huasango, a deciduous tree species. Local climate was investigated by installing an automatically weather station in 2007 and the daily tree growth variability was measured with high-resolution point dendrometers. The climatic impact on growth behaviour was evaluated. Hydro-climatic variables, like precipitation and relative humidity, were the most important factors for controlling tree growth. Changes in rainwater input affected radial increment rates and daily amplitudes of stem diameter variations within the study period from 2009 to 2013. El Niño Southern Oscillation (ENSO) related variations of tropical Pacific Ocean sea surface temperatures influenced the trees’ increment rates. Average radial increments showed high inter-annual (up to 7.89 mm) and inter-individual (up to 3.88 mm) variations. Daily amplitudes of stem diameter variations differed strongly between the two extreme years 2009 (wet) and 2011 (dry). Contrary to 2009, the La Niña drought in 2011 caused a rapid reduction of the daily amplitudes, indicating a total cessation (‘growth collapse’) of stem increment under ENSO-related drought conditions and demonstrating the high impact of climatic extreme events on carbon sequestration of the dry tropical forest ecosystem.

     

The influence of stem guying on radial growth,stem form and internal resin features in radiata pine

Key message

Stem guying to prevent wind-induced swaying of radiata pine trees resulted in significant changes in radial growth, but did not affect the frequency of compression wood or resin features.

Abstract

Mechanical stress resulting from wind forces acting on trees can cause a number of direct and indirect effects ranging from microscopic changes in cambial activity through to stem breakage and uprooting. To better understand these effects on radial stem growth and wood properties, an experiment was established in a 13-year-old radiata pine (Pinus radiata D Don) stand in which 20 trees were guyed to prevent them from swaying. Radial growth was monitored in these trees and 20 matched controls at monthly intervals for 5 years. The trees were then felled and radial growth, resin features and compression wood were assessed on cross-sectional discs taken at fixed locations up the stem. There was a significant reduction in radial growth at breast height (1.4 m above the ground) in the guyed trees, but an increase in growth immediately above the guying point. A total of 277 resin features were observed in the growth rings formed following guying. The overall frequency of such features was related to height within the stem and annual ring number. No effect of stem guying was found on the incidence of compression wood. Interestingly, the distribution of resin features also did not differ between guyed and un-guyed trees. There was no evidence of a link between stem restraint as a result of guying and the incidence of resin features, suggesting that other factors, such as soil moisture may be more influential.     

Linking above- and belowground phenology of hybrid walnut growing along a climatic gradient in temperate agroforestry systems

Background and aims

Plant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France.

Methods

Eight rhizotrons were installed at each site for 21 months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1 mm, 1–2 mm and 2–5 mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year.

Results

Root growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1 mm and 1–2 mm diameter classes lived for significantly shorter periods compared to those in the 2–5 mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1 mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter.

Conclusions

We conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined.