The number of days on which increment occurs is the primary determinant of annual ring width in Callitris intratropica

Key message

The number of days on which a measureable increment occurred, and the average rate of stem growth, rather than the overall duration of the wet season, were the main determinants of ring width in young Callitris intratropica trees. These effects were amplified by competition.

Abstract

Dendroclimatology of tropical tree species is an important tool for understanding past climatic variability at low latitudes where long-term weather records are often absent. Despite the growing number of published tropical tree-ring chronologies, however, still little is known of the factors that control annual ring formation in tropical tree species. In this paper we used an endemic Australian conifer, Callitris intratropica, to study the intra-annual dynamics of seasonal growth and xylem formation, and the effects of environmental conditions and competition, on growth ring formation. We combined high-resolution growth and climate monitoring (every 15 min for 2 years) with less frequent cambial sampling. Trees exhibited marked reductions in growth during certain periods within the rainy season when rainfall was not as regular and VPD was high. Overall, we found that ring width was most influenced by the number of days when increment occurred; regardless of how early the growing season began or ended, and by the rates of tracheid production. The effect of competition was also important. Trees growing in dense groves had narrower annual rings (4.6 mm) than trees that were growing in the open (6.7 mm), due to less active cambia, slower rates of xylem production and expansion and more increment days, although the overall growing season duration was also shorter in grove trees.     

Cambial dormancy induced growth rings in <Emphasis Type="Italic">Heritiera fomes</Emphasis> Buch.- Ham.: a proxy for exploring the dynamics of Sundarbans,Bangladesh

Key message

Cambial marking experiment and cambial activity analysis offer strong evidence on existence of annual growth rings in Heritiera fomes and revealing the potential of dendrochronological applications in Bangladesh mangroves.

Abstract

Despite enormous significance in coastal protection, biodiversity conservation and livelihood support to the local communities, mangrove ecosystems have been continuously degrading mainly due to anthropogenic disturbances and climate change. Time series based on dated tree ring is an option to identify the causes of forest dilapidation. In this study, we investigated the structure and periodicity of the growth ring in Heritiera fomes, the flagship tree species of the Bangladesh Sundarbans, combining cambial marking experiment and cambial activity analysis. Distinct growth rings were found which are delineated by a band of marginal parenchyma, predominantly one cell wide but up to three and occasionally interrupted with fiber. Of the 13 trees with cambium marking experiment, one growth ring was found in each tree during a year. The dormant cambium was characterized by the abrupt boundary between xylem and cambial zone, absence of enlarging or differentiating cambial derivatives, lower number of cambial cells and thicker radial walls in cambial cells. Growth ring anomalies, i.e., wedging and partially missing rings were also found. In most of the cases, the lower part of the eccentric discs had low radial increment (<0.75 mm) and therefore the growth ring in that area merged with previous one and produced wedging or partially missing ring. However, the existence of annual rings suggests its great potential for future dendrochronological applications to reveal the dynamics of vegetation and climate in Sundarbans.

     

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.     

Hydraulic limitation on maximum height of Pinus strobus trees in northern Minnesota,USA

Key message

Using comparisons within and between trees, the authors show evidence for hydraulic limitation of tree height in a humid-climate species that is far from the global maximum tree height.

Abstract

We measured water status and two indicators of drought stress as a function of height within the canopies of four tall (32–35 m) eastern white pines (Pinus strobus) at an old-growth site in northern Minnesota, USA. Pre-dawn and midday xylem pressure potential measured on terminal shoots (Ψ _shoot), needle length, and foliar carbon isotope discrimination (δ ¹³C) all showed within-canopy gradients consistent with increasing drought stress with height. Midday Ψ _shoot near tree tops was ?1.8 MPa, close to values associated with stomatal closure for other temperate conifers. Pre-dawn Ψ _shoot decreased with height at >2× the gradient in gravitational potential. δ ¹³C was strongly correlated with height and weakly correlated with light. Needles were 15–25 % shorter at canopy top compared to the bottom of the canopy. Midday Ψ _shoot and needle length showed significant differences in regression model coefficients from tree to tree. The patterns are consistent with hydraulic constraints on height growth of white pine at this site.     

Tracking of fluorescence nanoparticles with nanometre resolution in a biological system: assessing local viscosity and microrheology

The aim of the study was to establish a user-friendly approach for single fluorescence particle 3D localization and tracking with nanometre precision in a standard fluorescence microscope using a point spread function (PSF) approach, and to evaluate validity and precision for different analysis methods and optical conditions with particular application to microcirculatory flow dynamics and cell biology. Images of fluorescent particles were obtained with a standard fluorescence microscope equipped with a piezo positioner for the objective. Whole pattern (WP) comparison with a PSF recorded for the specific set-up and measurement of the outermost ring radius (ORR) were used for analysis. Images of fluorescent particles were recorded over a large range (about $7\,\upmu \text{ m }$ ) of vertical positions, with and without distortion by overlapping particles as well as in the presence of cultured endothelial cells. For a vertical range of $6.5\,\upmu \text{ m }$ , the standard deviation (SD) from the predicted value, indicating validity, was 9.3/8.7 nm (WP/ORR) in the vertical and 8.2/11.7 nm in the horizontal direction. The precision, determined by repeated measurements, was 5.1/3.8 nm in the vertical and 2.9/3.7 nm in the horizontal direction. WP was more robust with respect to underexposure or overlapping images. On the surface of cultured endothelial cells, a layer with 2.5 times increased viscosity and a thickness of about $0.8\,\upmu \text{ m }$ was detected. With a validity in the range of 10 nm and a precision down to about 3–5 nm obtained by standard fluorescent microscopy, the PSF approach offers a valuable tool for a variety of experimental investigations of particle localizations, including the assessment of endothelial cell microenvironment.     

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.

     

Airborne LiDAR derived canopy height model reveals a significant difference in radiata pine (Pinus radiata D. Don) heights based on slope and aspect of sites

Key message

Significant relationship between tree height and ALS-derived topography was shown. Taller trees were found on slopes <10° and southerly aspects. Potential value of ALS in forest management applications was defined.

Abstract

Accurate information on tree height distribution can provide a better understanding of forest productivity and biomass estimation. Airborne light detection and ranging remote sensing, also known as airborne laser scanning (ALS), has proven to be an effective tool for deriving tree height information in forests. While tree height has been reported to vary in response to many environmental factors, few researchers have demonstrated the effect of topography on tree height variation using ALS data. This study investigated the relationship between tree height variation and ALS-derived topographic aspect and slope factors within two even-aged radiata pine (Pinus radiata D. Don) plantation sites in Nundle, New South Wales, Australia. A total of 447 trees was sampled from 77 plots in two plantation age classes: 193 trees from a 34-year-old site and 254 trees from a 9-year-old site. ALS height estimates were highly correlated with field heights (R ² = 0.90 and RMSE = 0.66 for 2002 and R ² = 0.87 and RMSE = 1.49 for 1977 sites). ALS-derived slope and aspect metrics were shown to have a significant relationship with height variation across the stands. The slope (P < 0.01) and aspect (P < 0.001) were significant in the mixed linear models. Overall taller trees were found on slopes below 10° and on southerly aspects, while shorter trees dominated steeper slopes (>20°) and on northerly aspects. However, aspect gradient appeared to have more significant effect on tree heights than slope classes. These results were further verified using an additional 2,000 randomly located trees sampled across the plantations. The study demonstrates a significant relationship between tree height variation and ALS-derived ground aspect and slope categories which may have potential benefits for improving current wood resource inventories and future productivity models.     

Soil organic carbon and root distribution in a temperate arable agroforestry system

Aim

To determine, for arable land in a temperate area, the effect of tree establishment and intercropping treatments, on the distribution of roots and soil organic carbon to a depth of 1.5 m.

Methods

A poplar (Populus sp.) silvoarable agroforestry experiment including arable controls was established on arable land in lowland England in 1992. The trees were intercropped with an arable rotation or bare fallow for the first 11 years, thereafter grass was allowed to establish. Coarse and fine root distributions (to depths of up to 1.5 m and up to 5 m from the trees) were measured in 1996, 2003, and 2011. The amount and type of soil carbon to 1.5 m depth was also measured in 2011.

Results

The trees, initially surrounded by arable crops rather than fallow, had a deeper coarse root distribution with less lateral expansion. In 2011, the combined length of tree and understorey vegetation roots was greater in the agroforestry treatments than the control, at depths below 0.9 m. Between 0 and 1.5 m depth, the fine root carbon in the agroforestry treatment (2.56 t ha^-1) was 79% greater than that in the control (1.43 t ha^?1). Although the soil organic carbon in the top 0.6 m under the trees (161 t C ha^?1) was greater than in the control (142 t C ha^?1), a tendency for smaller soil carbon levels beneath the trees at lower depths, meant that there was no overall tree effect when a 1.5 m soil depth was considered. From a limited sample, there was no tree effect on the proportion of recalcitrant soil organic carbon.

Conclusions

The observed decline in soil carbon beneath the trees at soil depths greater than 60 cm, if observed elsewhere, has important implication for assessments of the role of afforestation and agroforestry in sequestering carbon.     

Water relations of ponderosa pines in Patagonia Argentina: implications for local water resources and individual growth

Replacement of grasslands by forests may result in increased water consumption, and the magnitude of this will depend on stand density. To test this hypotheses and evaluate the impact of pine plantations on hydric resources in Patagonia Argentina, we measured over two seasons (1999–2000 and 2000–2001) and at two densities of ponderosa pine plantations (350 and 500 trees ha^?1) the following variables: soil water content, leaf water potential (ψ), individual tree growth, individual sap flow, and response of sap flow density (u) to vapor pressure deficit (VPD). Stand transpiration (T) and whole-plant liquid-phase hydraulic conductance (L) were also estimated. Pre-dawn ψ varied from about –0.5 to –1.0 MPa. No differences were found in midday maximum u (1100–1800 hours) on clear days between the 2 measurement years, throughout each season, or between different densities of plantation. Sapflow density was also not correlated with soil water storage up to 1.4 m soil depth. Sapflow increased until VPD of about 2.3 kPa, and decreased at VPD >4 kPa, describing a hysteresis in the afternoon. Values of L in Patagonian trees were similar to those recalculated from published data for pines of the same height in the USA. Average stand transpiration increased with increased density (2.07 and 3.08 mm day^-1 for 350 and 500 pines ha^-1, respectively) and size of the trees. We conclude that ponderosa pines in Patagonia Argentina use more water, in a magnitude depending on the density of the trees, than native grasslands. Accordingly, ponderosa pines use deep water to maintain high water potential and transpiration rates even during the dry season.     

Long-term individual-level variation of reproductive features in Sorbus aucuparia,a fleshy-fruited tree

Key Message

Synchrony and fluctuation in reproductive output was not associated in individual trees.

Abstract

In a study conducted at the central Cantabrian Range, northern Iberia, we analyzed the fruiting pattern of 54 rowans (Sorbus aucuparia), a fleshy-fruited tree, over 16 consecutive years. Our objectives were: (a) to assess the covariation between several variables related to the reproductive performance of individual trees; (b) to measure the degree of synchrony shown by individuals; and (c) to address whether the reproductive behavior of individuals changed over the period of study. The fruiting performance of individuals was assessed in terms of the individual coefficient of variation in fruit output (CV _i ), synchrony (as the correlation between fruiting patterns), and the frequency of heavy crop years. Mean synchrony (0.52 ± 0.18) and CV _i values were large (1.92 ± 0.33) and correlated negatively. The average tree was synchronized with 60 ± 22 % of its conspecifics and about 36 ± 14 % of its fruiting years were heavy crop years. The study population included a distinct and small set of asynchronous trees and the synchrony between individual fruiting patterns was markedly reduced during the second half of the study period.     

Declining hydraulic performances and low carbon investments in tree rings predate Scots pine drought-induced mortality

Key message

The retrospective analysis of wood anatomical features evidences how a long-term deterioration of hydraulic performance and carbon use portend drought-induced mortality in Scots pine.

Abstract

Widespread episodes of drought-induced tree mortality are predicted to become more frequent as climate becomes warmer and drier. Nevertheless, growth trends and their links to changes in wood anatomy before tree dies are still poorly understood. Wood anatomical features provide valuable information that can be extracted to infer the mechanisms leading to tree death. In this study, we characterize drought-induced mortality affecting two Scots pine (Pinus sylvestris) sites (Prades and Arcalís) located in the North Eastern Iberian Peninsula. Co-occurring now-dead and living Scots pine trees were sampled and their wood anatomical features were measured and compared. We aimed to detect differences in anatomical features between living and dead trees, and to infer past physiological performances that might have determined their subsequent death or survival. Now-dead trees showed lower tracheid and resin duct production, and smaller radial lumen diameters than co-occurring living trees. At the more xeric Prades site, these anatomical differences were larger and chronic, i.e. were observed over the three studied decades, whilst they were less pronounced at the other, more mesic Arcalís site, where tree mortality episodes were more recent. This indicates that dead trees’ hydraulic conductivity was severely affected and that carbon investment in xylem formation and resin duct production was constrained prior to tree death. Our findings show that both hydraulic deterioration and low carbon allocation to xylem formation were associated to drought-induced mortality in Scots pine. Nevertheless, the temporal dynamics of these processes differed between populations as a function of site climatic conditions.     

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.     

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.     

Sap flow-based quantitative indication of progression of Dutch elm disease after inoculation with Ophiostoma novo-ulmi

Key message

The rate of progression of Dutch elm disease can be continuously and quantitatively estimated from sap flow measurements.

Abstract

Response of sap flow to inoculation with Ophiostoma novo-ulmi, a causal agent which causes vascular mycosis called Dutch elm disease, was studied in a field experiment comprised of 4-year-old wych elm trees (Ulmus glabra). Sap flow was measured on inoculated trees using the trunk heat balance method with external heating (EMS 62, Czech Republic) throughout the experiment. The first detectable symptoms of reduction in sap flow occurred 6 days after inoculation and all inoculated trees died within 16 days. Our experiment confirmed the ability of O. novo-ulmi to quickly kill young elm trees. The disease progressed faster than in previous experiments utilizing O. ulmi. To the best of our knowledge, this is the first experiment using sap flow measurements on trees inoculated by O. novo-ulmi. The trunk heat balance sap flow method is an effective non-invasive tool for continuous quantitative monitoring of the progression of vascular tree diseases, and show increased potential for field and greenhouse studies on changes in xylem hydraulic conductivity in a wide range of broadleaved and coniferous tree species.     

Spatial distribution of the soil organic carbon pool in a Holm oak dehesa in Spain

Aims

Dehesas are agroforestry systems characterized by scattered trees among pastures, crops and/or fallows. A study at a Spanish dehesa has been carried out to estimate the spatial distribution of the soil organic carbon stock and to assess the influence of the tree cover.

Methods

The soil organic carbon stock was estimated from the five uppermost cm of the mineral soil with high spatial resolution at two plots with different grazing intensities. The Universal Kriging technique was used to assess the spatial distribution of the soil organic carbon stocks, using tree coverage within a buffering area as an auxiliary variable.

Results

A significant positive correlation between tree presence and soil organic carbon stocks up to distances of around 8 m from the trees was found. The tree crown cover within a buffer up to a distance similar to the crown radius around the point absorbed 30 % of the variance in the model for both grazing intensities, but residual variance showed stronger spatial autocorrelation under regular grazing conditions.

Conclusions

Tree cover increases soil organic carbon stocks, and can be satisfactorily estimated by means of crown parameters. However, other factors are involved in the spatial pattern of the soil organic carbon distribution. Livestock plays an interactive role together with tree presence in soil organic carbon distribution.     

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.     

Forest decline after a 15-year “perfect storm” of invasion by hemlock woolly adelgid,drought, and hurricanes

Invasions by introduced pests can interact with other disturbances to alter forests and their functions, particularly when a dominant tree species declines. To identify changes after invasion by the insect hemlock woolly adelgid (Adelges tsugae; HWA), coinciding with severe droughts and hurricanes, this study compared tree species composition of eastern hemlock (Tsuga canadensis) forests on 11 plots before (2001) and 15 years after (2016) invasion in the southern Appalachian Mountains, USA. Losses of hemlock trees after HWA invasion were among the highest reported, with a 90% decline in density, 86% decline in basal area, and 100% mortality for individuals ≥ 60 cm in diameter. In contrast to predictions of theoretical models, deciduous tree density declined after HWA invasion, while basal area changed little, at least during the initial 15 years after invasion. Overall, forest density declined by 58%, basal area by 25%, and tree species richness by 8%. Factors additional to HWA likely exacerbating forest decline included: droughts before (1999–2001) and after HWA invasion (2006–2008); tree uprooting from hurricane-stimulated winds in 2004; pest-related declines of deciduous tree species otherwise likely benefitting from hemlock’s demise; death of deciduous trees when large hemlocks fell; and competition from aggressive understory plants including doghobble (Leucothoe fontanesiana), rosebay rhododendron (Rhododendron maximum), and Rubus spp. Models of forest change and ecosystem function should not assume that deciduous trees always increase during the first decades after HWA invasion.     

Pinyon pine (Pinus edulis) mortality and response to water addition across a three million year substrate age gradient in northern Arizona, USA

Background and aims

Pinyon pine (Pinus edulis Engelm.) is an important tree species in the western United States that has experienced large-scale mortality during recent severe drought. The influence of soil conditions on pinyon pine response to water availability is poorly understood. We investigated patterns of tree mortality and response of tree water relations and growth to experimental water addition at four sites across a three million year soil-substrate age gradient.

Methods

We measured recent pinyon mortality at four sites, and tree predawn water potential, leaf carbon isotope signature, and branch, leaf, and stem radial growth on 12 watered and unwatered trees at each site. Watered trees recieved fifty percent more than growing season precipitation for 6 years.

Results

Substrate age generally had a greater effect on tree water stress and growth than water additions. Pinyon mortality was higher on intermediate-aged substrates (50–55%) than on young (15%) and old (17%) substrates, and mortality was positively correlated with pinyon abundance prior to drought.

Conclusions

These results suggest high soil resource availability and consequent high stand densities at intermediate-age substrates predisposes trees to drought-induced mortality in semi-arid regions. The response of tree water relations to water addition was consistent with the inverse texture hypothesis; watering reduced tree water stress most in young, coarsely textured soil, likely because water rapidly penetrated deep in the soil profile where it was protected from evapotranspiration.     

Natural recovery from Verticillium wilt in olive: can it be exploited in a control strategy?

Aims

Our objective was to evaluate if natural recovery may be exploited in disease control of Verticillium wilt in olive. Therefore, we evaluated the following: the incidence of natural recovery; the Verticillium dahliae viability within olive tissues over time and the effectiveness of soil solarization, calcium cyanamide and pollarding of trees at soil level in promoting natural recovery.

Methods

Three different experiments (A, B and C) were performed in commercial olive orchards planted with the highly susceptible cv. ‘Bella di Cerignola’ and infested with the non-defoliating V. dahliae pathotype.

Results

In experiment A, in the period 2010–2012, natural recovery occurred on 35 of 138 diseased trees (25 %); however, this recovery was transient and lasted between 3 months for 11 trees (8 %) and 21 months for one tree (0.7 %). V. dahliae tended to be inactivated in twigs within 1 or 2 years after symptom onset (experiment A). However, it was evident that V. dahliae was more abundant in larger (trunk and first- or second-order branches) versus thinner woody parts of olive trees (roots; experiment B). In the attempt to explore whether natural recovery could be further stimulated artificially, it was observed that soil solarization and soil application of calcium cyanamide were ineffective in promoting its occurrence. Tree pollarding at soil level induced a transient recovery, which lasted only 1 year (experiment C).

Conclusions

Based on our observations, natural recovery of susceptible olive from Verticillium wilt has a low impact on the disease epidemiology in the short-term only and cannot be effectively stimulated in practice by soil solarization, calcium cyanamide or tree pollarding.