Tree-ring wood anatomy and stable isotopes show structural and functional adjustments in olive trees under different water availability

Background and Aims

Olive tree (Olea europaea L.) is a drought-tolerant tree species cultivated in Mediterranean-type environments. Although it is tolerant to drought, dry conditions decrease its productivity. A thorough analysis of the hydraulic architecture and wood anatomical plasticity, as well as of their physiological significance, is needed to understand how olive trees will adapt to the predicted increase in frequency and severity of drought in the Mediterranean region.

Methods

Dendrochronological, stable isotopic (δ¹³C, δ¹⁸O) and wood anatomical analyses were applied to understand how different water availability can affect wood stem structure and function, in rainfed and irrigated at 100 % of crop evapotranspiration (ETc) olive trees in an experimental orchard close to Benevento (Italy) from 1992 to 2009.

Results

Dendrochronological data indicate that cross-dating and synchronization of ring-width time series in olive tree is possible. After the start of irrigation, significantly more negative δ¹³C and lower δ¹⁸O values were recorded in irrigated trees indicating higher stomatal conductance and transpiration rates. Increased water balance induced the formation of a higher number of vessels with higher diameter.

Conclusions

Water balance variations affected wood anatomy and isotopic composition. Anatomical analyses detected structural and functional adjustments in rainfed trees that produced more vessels with lower diameter to prevent cavitation. Isotopic analyses confirmed that irrigated trees continuously showed enhanced transpiration rates.     

Carbon isotope discrimination and growth response of old Pinus ponderosa trees to stand density reductions

Stand density reductions have been proposed as a method by which old‐growth ponderosa pine (Pinus ponderosa) forests of North America can be converted back to pre‐1900 conditions, thereby reducing the danger of catastrophic forest fires and insect attacks while increasing the productivity of the remaining old‐growth individuals. However, the duration of productivity response of individual trees and the physiological mechanisms underlying such a response remain speculative issues, particularly in old trees. Tree‐ring measurements of carbon isotope ratios (δ¹³C) and basal area increment (BAI) were used to assess the response of intrinsic water‐use efficiency (the ratio of photosynthesis, A to stomatal conductance, g) and growth of individual> 250‐year‐old‐ponderosa pine trees to stand density reductions. It was hypothesized that reductions in stand density would increase soil moisture availability, thus decreasing canopy A/g and increasing carbon isotope discrimination (Δ). Cellulose‐δ¹³C of annual tree rings, soil water availability (estimated from pre‐dawn leaf water potential), photosynthetic capacity, stem basal growth and xylem anatomy were measured in individual trees within three pairs of thinned and un‐thinned stands. The thinned stands were treated 7 to 15 years prior to measurement. The values of δ¹³C and BAI were assessed for 20 consecutive years overlapping the date of thinning in a single intensively studied stand, and was measured for 3 years on either side of the date of thinning for the two other stands to assess the generality of the response. After thinning, Δ increased by 0.89‰ (± 0.15‰). The trees in the un‐thinned stands showed no change in Δ (0.00‰ ± 0.04‰). In the intensively studied trees, significant differences were expressed in the first growing season after the thinning took place but it took 6 years before the full 0.89‰ difference was observed. BAI doubled or tripled after disturbance, depending on the stand, and the increased BAI lasted up to 15 years after thinning. In the intensively studied trees, the BAI response did not begin until 3 years after the Δ response, peaked 1 year after the Δ peak, and then BAI and Δ oscillated in unison. The lag between BAI and Δ was not due to slow changes in anatomical properties of the sapwood, because tracheid dimensions and sapwood‐specific conductivity remained unchanged after disturbance. The Δ response of thinned trees indicated that A/g decreased after thinning. Photosynthetic capacity, as indexed by foliar nitrogen ([N]) and by the relationship between photosynthesis and internal CO₂ (A–C_i curves), was unchanged by thinning, confirming our suspicion that the decline in A/g was due to a relatively greater increase in g in comparison with A. Model estimates agreed with this conclusion, predicting that g increased by nearly 25% after thinning relative to a 15% increase in A. Pre‐dawn leaf water potential averaged 0.11 MPa (± 0.03 MPa) less negative for the thinned compared with the un‐thinned trees in all stands, and was strongly correlated with Δ post‐thinning (R² = 0.91). There was a strong relationship between BAI and modelled A, suggesting that changes in water availability and g have a significant effect on carbon assimilation and growth of these old trees. These results confirm that stand density reductions result in increased growth of individual trees via increased stomatal conductance. Furthermore, they show that a physiological response to stand density reductions can last for up to 15 years in old ponderosa pines if stand leaf area is not fully re‐established.     

Disbudding treatments on pistachio trees cv. mateur: dry matter accumulation and distribution within fruiting and non-fruiting branches under dry climate

Key message

After applying disbudding treatments, removal of fifty percent of flower bud each year improves dry matter accumulation in fruiting and non-fruiting branches of pistachio trees, which could minimize alternate bearing.

Abstract

Dry matter accumulation and distribution within branches of pistachio trees were investigated during 2005-2008 to determine the effects of fruiting on shoot growth under rain-fed conditions in arid climate. Four treatments were applied: T ₀ normal alternation cycle, T ₁ trees disbudded for 1 year, T ₂ trees disbudded for two successive years, and T ₃ removal of 50 % of all floral buds for each year. Consecutive disbudded treatment (T ₂) allowed a higher growth potential of pistachio trees with reference to normal biennial cycle (T ₀). Individual current shoot of T ₂ accumulated 44 % as much dry matter cm^?1 as those of ‘On’ trees, which have the highest yield. Removal of 50 % of floral buds (T ₃) significantly increased the dry matter accumulation in fruiting branch to reach 57 g in postharvest compared to 42.6 g for the control T ₀. Trees disbudded for 2 years (T ₂) had increased dry matter accumulation in the non-fruiting branch from 3.3 to 16.3 g. Leaves, current shoot, 1-year-old wood and inflorescence buds represented, respectively, 87, 5.3, 5 and 2.7 % of the total dry matter of individual branch of T ₂. In fruiting branches, nuts consisted of 83–87 %, leaves 7–10 %, rachises 4 %, 1-year-old wood 1.6–2 % and current shoot 0.8–1.3 % of the total dry matter. One-year-old wood played a major role as sources and sinks for developing current year shoot, leaves, inflorescence buds and nuts. Removal of 50 % of flower bud (T ₃) improves the dry matter accumulation in fruiting and non-fruiting branches. Thus, under rain-fed conditions, annual pruning could be used to minimize alternate bearing of pistachio.     

Jack pine foliar δ15N indicates shifts in plant nitrogen acquisition after severe wildfire and through forest stand development

Background and aims

Natural abundance of the stable nitrogen (N) isotope ¹⁵N can elucidate shifts in plant N acquisition and ecosystem N cycling following disturbance events. This study examined the potential relationship between foliar δ¹⁵N and depth of plant N acquisition (surface organic vs. mineral soil) and nitrification as conifer stands develop following stand-replacing wildfire.

Methods

We measured foliar δ¹⁵N along an 18-site chronosequence of jack pine (Pinus banksiana) stands, 1 to 72 years in age post-wildfire. Foliar δ¹⁵N was compared to total δ¹⁵N of the organic (Oe + Oa) and mineral (0–15 cm) soil horizons, and organic horizon N mineralization and nitrification as functions of total mineralization.

Results

Foliar δ¹⁵N declined with stand age, yet wildfire effects were heterogeneous. Jack pine seedlings on burned, mineral soil patches in the youngest stand were significantly more enriched than those on unburned, organic patches (P?=?0.007). High foliar values in the youngest stands relative to mineral-horizon δ¹⁵N indicate that nitrification also likely contributed to seedling enrichment.

Conclusions

Our results suggest jack pine seedlings on burned patches obtain N from the mineral soil with potentially high nitrification rates, whereas seedlings on unburned patches and increasingly N-limited, mature jack pine acquire relatively more N from organic horizons.     

Interpreting species‐specific variation in tree‐ring oxygen isotope ratios among three temperate forest trees

Although considerable variation has been documented in tree‐ring cellulose oxygen isotope ratios (δ¹⁸O_cell) among co‐occurring species, the underlying causes are unknown. Here, we used a combination of field measurements and modelling to investigate the mechanisms behind variations in late‐wood δ¹⁸O_cell (δ¹⁸O_lc) among three co‐occurring species (chestnut oak, black oak and pitch pine) in a temperate forest. For two growing seasons, we quantified among‐species variation in δ¹⁸O_lc, as well as several variables that could potentially cause the δ¹⁸O_lc variation. Data analysis based on the δ¹⁸O_cell model rules out leaf water enrichment (Δ¹⁸O_lw) and tree‐ring formation period (Δt), but highlights source water δ¹⁸O (δ¹⁸O_sw) as an important driver for the measured difference in δ¹⁸O_lc between black and chestnut oak. However, the enriched δ¹⁸O_lc in pitch pine relative to the oaks could not be sufficiently explained by consideration of the above three variables only, but rather, we show that differences in the proportion of oxygen exchange during cellulose synthesis (p_ex) is most likely a key mechanism. Our demonstration of the relevance of some species‐specific features (or lack thereof) to δ¹⁸O_cell has important implications for isotope based ecophysiological/paleoclimate studies.     

Leaf δ18O of remaining trees is affected by thinning intensity in a semiarid pine forest

Silvicultural thinning usually improves the water status of remaining trees in water‐limited forests. We evaluated the usefulness of a dual stable isotope approach (δ¹³C, δ¹⁸O) for comparing the physiological performance of remaining trees between forest stands subjected to two different thinning intensities (moderate versus heavy) in a 60‐year‐old Pinus halepensis Mill. plantation in semiarid southeastern Spain. We measured bulk leaf δ¹³C and δ¹⁸O, foliar elemental concentrations, stem water content, stem water δ¹⁸O (δ¹⁸O_{stem water}), tree ring widths and leaf gas exchange rates to assess the influence of forest stand density on tree performance. Remaining trees in low‐density stands (heavily thinned) showed lower leaf δ¹⁸O, and higher stomatal conductance (g_s), photosynthetic rate and radial growth than those in moderate‐density stands (moderately thinned). By contrast, leaf δ¹³C, intrinsic water‐use efficiency, foliar elemental concentrations and δ¹⁸O_{stem water} were unaffected by stand density. Lower foliar δ¹⁸O in heavily thinned stands reflected higher g_s of remaining trees due to decreased inter‐tree competition for water, whereas higher photosynthetic rate was largely attributable to reduced stomatal limitation to CO₂ uptake. The dual isotope approach provided insight into the early (12 months) effects of stand density manipulation on the physiological performance of remaining trees.     

Isotopic and anatomical signals for interpreting fire-related responses in Pinus halepensis

Key message

Our study aims to define isotopic and anatomical responses to fires of P. halepensis . Main results: decrease in tree growth and relative conductivity and increase in water use efficiency.

Abstract

We investigated the ecophysiological responses of a Pinus halepensis Mill. stand surviving two wildfires in southern France. Basal area, isotope composition and anatomical traits were analysed before and after fires, using tree rings to assess the ecological responses of trees to heat-related damage. The years were determined based on the presence of fire scars. Stable isotopes (δ¹³C and δ¹⁸O) were measured in tree rings before and after the “fire years”. Anatomical observations allowed qualitative analysis of the scar region and quantification of tracheid size in tree rings before and after the fire years. Relative and percentage conductivity of earlywood and latewood far from the woundwood were estimated. Results showed a decrease in tree growth after the fire events accompanied by an increase in ¹³C-derived water use efficiency (WUE_i) and a decrease in relative conductivity. The positive relationship between δ¹³C and δ¹⁸O suggested that both isotopic variations are mostly driven by changes in stomatal conductance following fire events. P. halepensis proved to be a strong isohydric species, able to survive frequent fires with temporary ecophysiological modifications and anatomical adaptations. Our findings afford new insights into post-fire survival strategies of this species in an environment where fires are predicted to increase in frequency during the twenty-first century.     

Tree ring δ<Superscript>15</Superscript>N as validation of space-for-time substitution in disturbance studies of forest nitrogen status

Forest ecosystem nitrogen (N) response to disturbance has often been examined by space-for-time substitution, but there are few objective tests of the possible variation in disturbance type and intensity across chronosequence sites. We hypothesized that tree ring δ¹⁵N, as a record of ecosystem N status, could validate chronosequence assumptions and provide isotopic evidence to corroborate N trends. To test this we measured soil N availability, soil δ¹⁵N, and foliar N attributes of overstory Douglas-fir (Pseudotsuga menziesii) and understory western hemlock (Tsuga heterophylla) across three old-growth stands and nine second-growth plantations on southeast Vancouver Island, British Columbia (Canada). Increment cores for wood δ¹⁵N were retrieved from three co-dominant Douglas-fir per plot. Bulk soil δ¹⁵N was well aligned with both foliar and recent wood δ¹⁵N, demonstrating the utility of wood δ¹⁵N in monitoring ecosystem N status. Strongly contrasting trends in tree ring δ¹⁵N were evident among second-growth stands, with most trees from plantations older than 50 years exhibiting steep declines (3–4‰) in δ¹⁵N but with no temporal trends detected for younger plantations. The discrepancy in tree ring δ¹⁵N suggests disturbance history varied considerably among second-growth sites, likely because of greater slash loads and hotter broadcast burns on older cutblocks. As a consequence, the pattern of increased soil N availability and foliar N concentration with time since disturbance derived from the chronosequence could not be validated. Tree ring δ¹⁵N may provide insights into disturbance intensity, especially fire, and correlations with foliar N concentration could inform the extent of changes in stand nutrition.     

Foliar chemistry and tree ring δ13C of Pinus densiflora in relation to tree growth along a soil pH gradient

Background and aims

Soil acidification is known to be one of the constraints of tree growth; however, it is unclear how it affects tree growth at photosynthesis level (i.e., through affecting stomatal conductance vs. carboxylation rate) during the growth of trees. This paper studied the effects of soil acidification on Pinus densiflora foliar chemistry and tree ring C isotope ratio (¹³C/¹²C, expressed as δ¹³C) and their relationship with tree growth.

Methods

Tree growth (diameter, annual growth ring area, and root biomass), soil chemistry (pH, mineral N, and exchangeable Ca and Al), foliage chemistry (N, Ca/Al, and δ¹³C), and tree ring δ¹³C in P. densiflora stands along a soil pH gradient (from 4.38 to 4.83, n?=?9) in southern Korea were investigated.

Results

Overall, trees with relatively poor growth under more acidic soil conditions (low pH and Ca/Al) had lower values of foliar N concentration and δ¹³C and tree ring δ¹³C, suggesting that restricted N uptake under more acidic soil conditions caused N limitation for photosynthesis, leading to poor tree growth. In addition, relationships between mean annual area increment and carbon isotope discrimination of tree rings at five-yr intervals from 1968 to 2007 revealed that the impact of soil acidification on tree growth became severer during the last 15 yrs as negative correlations between them became significant after 1993.

Conclusions

Reduced N uptake under acidic soil conditions resulted in lower radial growth of P. densiflora via non-stomatal limitation of photosynthesis.     

Introducing Acacia mangium trees in Eucalyptus grandis plantations: consequences for soil organic matter stocks and nitrogen mineralization

Background and aims

Eucalyptus plantations cover 20 million hectares on highly weathered soils. Large amounts of nitrogen (N) exported during harvesting lead to concerns about their sustainability. Our goal was to assess the potential of introducing A. mangium trees in highly productive Eucalyptus plantations to enhance soil organic matter stocks and N availability.

Methods

A randomized block design was set up in a Brazilian Ferralsol soil to assess the effects of mono-specific Eucalyptus grandis (100E) and Acacia mangium (100A) stands and mixed plantations (50A:50E) on soil organic matter stocks and net N mineralization.

Results

A 6-year rotation of mono-specific A. mangium plantations led to carbon (C) and N stocks in the forest floor that were 44% lower and 86% higher than in pure E. grandis stands, respectively. Carbon and N stocks were not significantly different between the three treatments in the 0–15?cm soil layer. Field incubations conducted every 4?weeks for the two last years of the rotation estimated net soil N mineralization in 100A and 100E at 124 and 64?kg?ha^?1?yr^?1, respectively. Nitrogen inputs to soil with litterfall were of the same order as net N mineralization.

Conclusions

Acacia mangium trees largely increased the turnover rate of N in the topsoil. Introducing A. mangium trees might improve mineral N availability in soils where commercial Eucalyptus plantations have been managed for a long time.     

Functional convergence in water use of trees from different geographical regions: a meta-analysis

Functional convergence in water use of trees across species from diverse geographic locations was examined using data on tree water use parameters, with the intention of gaining an understanding on the capacity for water transport for trees with varying structural and functional traits. Wood density (ρ_w), which is reported to have a negative exponential relation with sap flow density (SFD), showed a bell-shaped curve when the daily SFD data from 101 tree species belonging to 35 angiosperm and gymnosperm families were plotted. The species came from 23 different geographical locations representing all continents. Trees were most efficient in water transport when the ρ_w was between 0.51 and 0.65 g cm^?3. When the ρ_w increased or decreased from this range, there was a gradual fall in their water transport rate as indicated by lower daily SFD. The unexpected reduction in SFD with decreasing ρ_w is explained in terms of reduced conductance in the transport pathway, which is a precaution taken by the tree for avoiding cavitation or implosion in larger conducting tubes, which is characteristic of low density wood. The development of severe leaf water potential variations, which is frequently reported in such trees, supports this notion. The SFD versus ρ_w relation has a potentially wide applicability in predicting water use by forest stands with varying ρ_w. In addition, the occurrence of a high number of tree species with ρ_w values in the range of 0.51–0.65 g cm^?3 across all continents examined points towards the importance of ρ_w in the evolutionary process as related to efficient functioning of the water transport mechanism.     

Postfire carbon pools and fluxes in semiarid ponderosa pine in Central Oregon

Forest fire dramatically affects the carbon storage and underlying mechanisms that control the carbon balance of recovering ecosystems. In western North America where fire extent has increased in recent years, we measured carbon pools and fluxes in moderately and severely burned forest stands 2 years after a fire to determine the controls on net ecosystem productivity (NEP) and make comparisons with unburned stands in the same region. Total ecosystem carbon in soil and live and dead pools in the burned stands was on average 66% that of unburned stands (11.0 and 16.5 kg C m⁻², respectively, P<0.01). Soil carbon accounted for 56% and 43% of the carbon pools in burned and unburned stands. NEP was significantly lower in severely burned compared with unburned stands (P<0.01) with an increasing trend from −125±44 g C m⁻² yr⁻¹ (±1 SD) in severely burned stands (stand replacing fire), to −38±96 and +50±47 g C m⁻² yr⁻¹ in moderately burned and unburned stands, respectively. Fire of moderate severity killed 82% of trees <20 cm in diameter (diameter at 1.3 m height, DBH); however, this size class only contributed 22% of prefire estimates of bole wood production. Larger trees (> 20 cm DBH) suffered only 34% mortality under moderate severity fire and contributed to 91% of postfire bole wood production. Growth rates of trees that survived the fire were comparable with their prefire rates. Net primary production NPP (g C m⁻² yr⁻¹, ±1 SD) of severely burned stands was 47% of unburned stands (167±76, 346±148, respectively, P<0.05), with forb and grass aboveground NPP accounting for 74% and 4% of total aboveground NPP, respectively. Based on continuous seasonal measurements of soil respiration in a severely burned stand, in areas kept free of ground vegetation, soil heterotrophic respiration accounted for 56% of total soil CO₂ efflux, comparable with the values of 54% and 49% previously reported for two of the unburned forest stands. Estimates of total ecosystem heterotrophic respiration (R_h) were not significantly different between stand types 2 years after fire. The ratio NPP/R_h averaged 0.55, 0.85 and 1.21 in the severely burned, moderately burned and unburned stands, respectively. Annual soil CO₂ efflux was linearly related to aboveground net primary productivity (ANPP) with an increase in soil CO₂ efflux of 1.48 g C yr⁻¹ for every 1 g increase in ANPP (P<0.01, r²= 0.76). There was no significant difference in this relationship between the recently burned and unburned stands. Contrary to expectations that the magnitude of NEP 2 years postfire would be principally driven by the sudden increase in detrital pools and increased rates of R_h, the data suggest NPP was more important in determining postfire NEP.     

Within-canopy variation in photosynthetic capacity,SLA and foliar N in temperate broad-leaved trees with contrasting shade tolerance

Key message

The relative shade tolerance of T. cordata , F. sylvatica , and C. betulus in mature stands is based on different species-specific carbon and nitrogen allocation patterns.

Abstract

The leaf morphology and photosynthetic capacity of trees are remarkably plastic in response to intra-canopy light gradients. While most studies examined seedlings, it is not well understood how plasticity differs in mature trees among species with contrasting shade tolerance. We studied light-saturated net photosynthesis (A _max), maximum carboxylation rate (V _cmax), electron transport capacity (J _max) and leaf dark respiration (R _d) along natural light gradients in the canopies of 26 adult trees of five broad-leaved tree species in a mixed temperate old-growth forest (Fraxinus excelsior, Acer pseudoplatanus, Carpinus betulus, Tilia cordata and Fagus sylvatica), representing a sequence from moderately light-demanding to highly shade-tolerant species. We searched for species differences in the dependence of photosynthetic capacity on relative irradiance (RI), specific leaf area (SLA) and nitrogen per leaf area (N _a ). The three shade-tolerant species (C. betulus, T. cordata, F. sylvatica) differed from the two more light-demanding species by the formation of shade leaves with particularly high SLA but relatively low N _a and consequently lower area-based A _max, and a generally higher leaf morphological and functional plasticity across the canopy. Sun leaf morphology and physiology were more similar among the two groups. The three shade-tolerant species differed in their shade acclimation strategies which are primarily determined by the species’ plasticity in SLA. Under low light, T. cordata and F. sylvatica increased SLA, mass-based foliar N and leaf size, while C. betulus increased solely SLA exhibiting only low intra-crown plasticity in leaf morphology and N allocation patterns. This study with mature trees adds to our understanding of tree species differences in shade acclimation strategies under the natural conditions of a mixed old-growth forest.     

Elemental and stable isotope composition of Pinus halepensis foliage along a metal(loid) polluted gradient: implications for phytomanagement of mine tailings in semiarid areas

Background and aims

Aleppo pine (Pinus halepensis Mill.) is a widely used species for restoring degraded semiarid areas, but its use for the revegetation of metal(loid) polluted soils has not been thoroughly investigated. The main goal of this research was to study the ecophysiological status and elemental composition of spontaneous populations of Pinus halepensis growing on mine tailings to assess their use in phytomanagement of mine spoils in semiarid climates.

Methods

Edaphic characteristics and the physiological (by stable isotopes) and nutritional status of pine trees were determined on mine tailings, in the metalloid-polluted surroundings and a non-polluted control area.

Results

Low soil phosphorus availability at the tailings was found to be a more important limiting factor for pine physiological performance than high soil metal(lloid)s concentrations. Foliar phosphorus concentrations showed a strong negative correlation with foliar sulphur concentrations along the studied transect. The carbon and oxygen isotopic composition (δ¹³C and δ¹⁸O) of pine needles indicated that trees at the tailings were less water stressed than those in surroundings or control areas. The low foliar δ¹⁵N of pines growing at the tailings was due to low soil fertility and/or a heavy reliance on symbiotic ectomycorrhizal fungi for nitrogen uptake.

Conclusions

The results of this study indicate that Pinus halepensis is a suitable tree species for the phytostablisation of neutral or slightly-alkaline mining wastes in semiarid environments, thanks to its drought hardiness and good adaptation to low soil fertility and salinity.     

Foliar and litter needle carbon and oxygen isotope compositions relate to tree growth of an exotic pine plantation under different residue management regimes in subtropical Australia

Background and aim

Significant differences in tree growth were observed in an exotic pine plantation under different harvest residue management regimes at ages 2–10 years. However, the variations in tree growth between residue management treatments could not be explained by soil and foliar nutrient analyses, except by potassium (K) concentration. Therefore, this study determined the carbon isotope composition (δ¹³C) and oxygen isotope composition (δ¹⁸O) of current and archived foliar samples from the exotic pine plantation to establish relationships with foliar K concentration and tree growth indices as a means to determine changes in stomatal conductance (g_s) and photosynthetic rate (A_max) or water use efficiency (WUE), and therefore understand the variations in tree growth across treatments.

Methods

The harvest residue treatments were: (1) residue removal, RR₀; (2) single level residue retention, RR₁; and (3) double level residue retention, RR₂. Foliar δ¹³C and δ¹⁸O were determined for samples at ages 2, 4, 6 and 10 years, and the atmospheric ¹³C discrimination (Δ¹³C), intercellular CO₂ concentration (C_i) and WUE were determined from the δ¹³C data. Litter needle δ¹³C and δ¹⁸O were also determined over 15 months between ages 9 and 10 years. These parameters or variables where correlated to each other as well as to the periodic mean annual increment of basal area (PAIB) and the periodic mean annual increment of tree diameter at breast height (PAID) across the treatments and over time. Foliar δ¹³C and δ¹⁸O were also related to published data of foliar K concentrations of the same trees.

Results

Significant variations of foliar δ¹³C, and therefore WUE and Δ¹³C, across treatments were only observed at ages 4 and 10 years old, and foliar δ¹⁸O at age 4 years old only. The results showed increasing foliar δ¹³C, δ¹⁸O and WUE, and decreasing Δ¹³C and Ci, from RR₀ to RR₂ treatments. However, while the WUE was positively related to the PAID and PAIB at age 4 years, it was negatively related to PAID and PAIB at age 10 years old. Litter needle δ¹³C, indicative of WUE, was also negatively related to the PAID at age 10 years old. . At age 4 years, foliar δ¹³C and δ¹⁸O were positively related with a steep slope of 7.70 ‰ across treatments, and that both isotopes were positively related to foliar K concentrations. Similarly, δ¹⁸O was negatively related to the Δ¹³C. No significant relationship can be determined between foliar δ¹³C, or Δ¹³C, and δ¹⁸O at age 10 years old. In addition, WUE was increasing (p?<?0.001) and Δ¹³C and C_i decreasing (p?<?0.001) with decreasing PAID over time.

Conclusions

The variations at age 4 years in foliar δ¹³C or Δ¹³C and δ¹⁸O and increasing WUE with increasing growth rate suggest growth induced water-stress with increasing residue-loading rate as a result of the nutritional effect of the harvest residues on tree growth. At age 10 years, the negative relationships between WUE and PAID indicate nutrient limitation has an over-riding effect on δ¹³C variations rather than g_s. This was due to the lack of a significant relationship between foliar Δ¹³C and δ¹⁸O at this age, as well as over time.     

Comparison of measured and modeled variations in piñon pine leaf water isotopic enrichment across a summer moisture gradient

Stable hydrogen and oxygen isotopic composition of bulk leaf water (δD_lw and δ¹⁸O_lw) in piñon pine (Pinus edulis and P. monophylla) and gas exchange parameters were measured under field conditions to examine the effects of seasonal moisture stress on leaf water isotopic enrichment. Study sites were located near the lower elevation limit for piñon in the southwestern USA. Leaf-level transpiration measurements were made four times daily in spring, summer and early autumn; simultaneously, leaf samples were collected for water extraction and stable isotope analysis. Diurnal variations in δD_lw and δ¹⁸O_lw values were small, especially when leaf water residence times (molar leaf water content divided by transpiration rate) were high. Stomatal conductance explained most of the variance (60%) in leaf water enrichment across the dataset. Observed leaf water enrichment was compared with predictions of steady-state and nonsteady-state models. Nonsteady-state predictions fit observations the best, although D enrichment was often lower than predicted by any model. Hydrogen isotope ratios of leaf water and cellulose nitrate were strongly correlated, demonstrating preservation of a leaf water signal in wood and leaf cellulose.     

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.     

Combined effects of soil properties and Phytophthora cinnamomi infections on Quercus ilex decline

Aims

The importance of soil properties as determinants of tree vitality and Phytophthora cinnamomi root infections was analysed.

Methods

The study comprised 96 declining stands in western Spain, where declining and non-declining holm oak (Quercus ilex L.) trees were sampled. Soil properties (soil depth, Ah horizon thickness, texture, pH, redox potential, soil bulk density and N-NH₄ ⁺ and N-NO₃ ^? concentrations) and P. cinnamomi infections were assessed.

Results

Tree mortality rates increased with low soil bulk densities, which were also associated with more P. cinnamomi-infected trees. Occurrence of infected trees was higher in fine textured soils and in thick Ah horizons. Fine textured soils favoured trees, but with the presence of P. cinnamomi their health status deteriorated. Soil under declining trees had higher N-NO₃ ^?/N-NH₄ ⁺ ratio values than under non-declining trees. Additional soil properties changes associated to grazing were not related to decline and P. cinnamomi infections.

Conclusions

The implications of P. cinnamomi in holm oak decline and the influence of soil properties as contributors to pathogen activity were demonstrated. Fine soil textures and thick Ah horizons, usually favourable for vigour and vitality of trees growing in the Mediterranean climate, were shown to be disadvantageous soil properties if P. cinnamomi was present. Fine soil textures and thick Ah horizons are frequently related with higher levels of soil moisture, which increase the inoculum of the pathogen and favours root infection. Grazing does not seem to be directly linked to Q. ilex health status or P. cinnamomi root rot.     

How stand productivity results from size- and competition-dependent growth and mortality

Background

A better understanding of the relationship between stand structure and productivity is required for the development of: a) scalable models that can accurately predict growth and yield dynamics for the world''s forests; and b) stand management regimes that maximize wood and/or timber yield, while maintaining structural and species diversity.

Methods

We develop a cohort-based canopy competition model (“CAIN”), parameterized with inventory data from Ontario, Canada, to examine the relationship between stand structure and productivity. Tree growth, mortality and recruitment are quantified as functions of diameter and asymmetric competition, using a competition index (CAI_h) defined as the total projected area of tree crowns at a given tree''s mid-crown height. Stand growth, mortality, and yield are simulated for inventoried stands, and also for hypothetical stands differing in total volume and tree size distribution.

Results

For a given diameter, tree growth decreases as CAI_h increases, whereas the probability of mortality increases. For a given CAI_h, diameter growth exhibits a humped pattern with respect to diameter, whereas mortality exhibits a U-shaped pattern reflecting senescence of large trees. For a fixed size distribution, stand growth increases asymptotically with total density, whereas mortality increases monotonically. Thus, net productivity peaks at an intermediate volume of 100–150 m³/ha, and approaches zero at 250 m³/ha. However, for a fixed stand volume, mortality due to senescence decreases if the proportion of large trees decreases as overall density increases. This size-related reduction in mortality offsets the density-related increase in mortality, resulting in a 40% increase in yield.

Conclusions

Size-related variation in growth and mortality exerts a profound influence on the relationship between stand structure and productivity. Dense stands dominated by small trees yield more wood than stands dominated by fewer large trees, because the relative growth rate of small trees is higher, and because they are less likely to die.     

Tree age,site and climate controls on tree ring cellulose δ18O: A case study on oak trees from south-western France

A main concern of dendroclimatic reconstruction is to distinguish in the tree ring proxy the influence of the climate variables of interest from other controlling factors. In order to investigate age, site and climate controls on tree ring width and cellulose δ¹⁸O, measurements have been performed in nearby groups of young (145 years old) and older (310–405 years old) oak trees in south-western France, covering the period 1860–2010.Within a given site, inter-tree deviations are small, pointing to a common climatic signal. Despite a similar inter-annual variability, the average level of cellulose δ¹⁸O in the young tree group is ∼0.8‰ higher than in the old trees. Such offsets might be caused by different soil properties and differences in the fraction of the source water used by trees from different depths. The δ¹⁸O of water in the top soil layer is directly related to the current growing season precipitation, while deeper water can have a lower and more constant δ¹⁸O. Local cave drip waters at 10 m depth indeed show a constant isotopic composition, which corresponds to pluri-annual mean precipitation.A 2‰ increasing trend is observed in cellulose δ¹⁸O of young trees in the first 30 years of growth, during a period when no trend is visible in older trees. This increase can be quantitatively explained by humidity gradients under the forest canopy, and a changing microclimate around the crown as trees grow higher.While relationships between tree ring width and climate appear complex, the isotopic composition of cellulose is strongly correlated with summer maximum temperature, relative humidity and evapotranspiration (r ≈ 0.70). Weaker correlations (r ≈ 0.40) are identified with precipitation δ¹⁸O from a 15-year long local record and from the REMOiso model output. These results imply that leaf water enrichment has a stronger control on the inter-annual variability of cellulose δ¹⁸O than the δ¹⁸O of precipitation.This study demonstrates the suitability of oak tree ring cellulose δ¹⁸O for reconstructing past summer climate variability in south-western France, provided that the sampling and pooling strategy accounts for the fact that trees from different sites and of different age can introduce non-climatic signals.