Tree Resilience Research Articles

Winter's bite: beech trees survive complete defoliation due to spring late-frost damage by mobilizing old C reserves.

Late frost can destroy the photosynthetic apparatus of trees. We hypothesized that this can alter the normal cyclic dynamics of C-reserves in the wood. We measured soluble sugar concentrations and radiocarbon signatures (Δ14 C) of soluble nonstructural carbon (NSC) in woody tissues sampled from a Mediterranean beech forest that was completely defoliated by an exceptional late frost in 2016. We used the bomb radiocarbon approach to estimate the time elapsed since fixation of mobilized soluble sugars. During the leafless period after the frost event, soluble sugar concentrations declined sharply while Δ14 C of NSC increased. This can be explained by the lack of fresh assimilate supply and a mobilization of C from reserve pools. Soluble NSC became increasingly older during the leafless period, with a maximum average age of 5yr from samples collected 27d before canopy recovery. Following leaf re-growth, soluble sugar concentrations increased and Δ14 C of soluble NSC decreased, indicating the allocation of new assimilates to the stem soluble sugars pool. These data highlight that beech trees rapidly mobilize reserve C to survive strong source-sink imbalances, for example due to late frost, and show that NSC is a key trait for tree resilience under global change.

Forest ecological heterogeneity determines contrasting relationships between crown defoliation and tree diversity

Tree diversity is found to enhance ecosystem functions in forests and increase the resistance and resilience of trees subjected to environmental stress, including climate change. This effect, however, can be different depending on tree species assemblages and ecological contexts. The pan-European programme for monitoring forest health (ICP Forests) considers crown defoliation as an indicator of tree vitality. Only a few studies have analysed the role of tree diversity in crown defoliation, with contrasting results. This paper analyses the relationships between defoliation and forest diversity in a country (Italy) characterised by heterogeneous bioclimatic and edaphic characteristics (from Mediterranean to Alpine) and with a wide range of forest tree species and functional groups. National-scale results show that defoliation increased in more diverse forest stands. This result can be explained by the fact that the tree mixtures reflect varying ecological conditions in heterogeneous environments. The most relevant factors that explain tree defoliation at the national level, investigated through random forest analysis, were the geographic position, tree species identity and forest stand age and structure. Cluster analysis was applied to identify homogeneous groups of plots based on their ecological and vegetation features. Four clusters were selected. Within each homogeneous cluster, the crown defoliation of the tree species showed different and contrasting patterns, changing their relationships with diversity over time. A beneficial role of diversity on defoliation (i.e. decrease of defoliation) has been observed in Quercus ilex, growing in Mediterranean conditions, and Picea abies in Alpine areas after dry periods. In the latter species, drought led to a greater increase in defoliation in monospecific than in mixed stands, suggesting the stabilising role of tree diversity.

Interactions between southern Ips bark beetle outbreaks, prescribed fire, and loblolly pine (Pinus taeda L.) mortality

The southeastern U.S. is considered the “wood-basket” of the world where loblolly pine (Pinus taeda L.) plantations provide tremendous ecological and economic benefits to the region. These plantations are susceptible to various natural and anthropogenic abiotic and biotic stressors and disturbances. The southern pine engravers, Ips avulsus (Eichhoff), I. calligraphus (Germar), and I. grandicollis (Eichhoff), are considered secondary colonizers of stressed, damaged, and dying loblolly pines. However, they may undergo outbreaks and colonize live pine hosts if environmental conditions cause physiological stress to trees. In 2016, >230 concurrent Ips infestations >2 ha in size were documented in Georgia, U.S., reportedly due to severe drought. In these forests, prescribed burning is often conducted every 2–3 years to reduce fuel-loads, improve wildlife habitat, and manage understory vegetation. However, the effects of low-severity prescribed fire on active southern Ips infestations are unknown; fire may exacerbate or alleviate beetle outbreaks. Our objectives were to: (1) compare Ips infestations between burned and unburned sites to determine the short-term effects of prescribed fire on loblolly pine mortality; and (2) determine which site-level and tree-level variables were the best predictors of short-term levels of tree mortality. We monitored 838 pines on ten sites for eight months following prescribed fire in spring 2017. Overall, 69 (8%) trees died with 3.6 times higher tree mortality on unburned sites, and a higher probability of survival on burned sites. At the site-level, binomial logistic regression models including treatment (unburned versus burned) and time since burn were the best predictors of loblolly pine mortality. At the tree-level, model selection showed that treatment, crown mortality level (1–5), Ips activity level (none, low, medium, and high), and tree diameter provided the best predictions of mortality. Prescribed burning may thus help alleviate pest pressure and increase tree resilience in loblolly pine forests in the southeastern U.S.

Modeling δ18O as an early indicator of regime shift arising from salinity stress in coastal vegetation

In many important coastal habitats, a combination of increasing soil salinization due to sea level rise, reduced precipitation and storm surges may induce regime shift from salinity-intolerant glycophytic vegetation to salinity-tolerant halophytic species. Early detection of regime shift due to salinity stress in vegetation may facilitate conservation efforts. It has been shown that the 18O value of water in the xylem of trees can be used as a surrogate for salinity in the rooting zone of plants. Coupling measured δ18O values in the tree xylem with simulated δ18O values in trees and salinity in the vadose zone can be used to investigate competitive responses of glycophytic versus halophytic trees. MANTRA-O18 simulations suggest that the impacts of salinization on diminishing the resilience of salinity-intolerant trees can be detected up to 25 years before the glycophytic trees are threatened with regime shift to halophytic species. This early detection provides critical lead time and valuable information and insights useful for planning adaptation strategy to mitigate against the adverse impacts of sea level rise and climate change.

Tree resilience to drought increases in the Tibetan Plateau.

Forests in the Tibetan Plateau are thought to be vulnerable to climate extremes, yet they also tend to exhibit resilience contributing to the maintenance of ecosystem services in and beyond the plateau. So far the spatiotemporal pattern in tree resilience in the Tibetan Plateau remains largely unquantified and the influence of specific factors on the resilience is poorly understood. Here, we study ring‐width data from 849 trees at 28 sites in the Tibetan Plateau with the aim to quantify tree resilience and determine their diving forces. Three extreme drought events in years 1969, 1979, and 1995 are detected from metrological records. Regional tree resistance to the three extreme droughts shows a decreasing trend with the proportion of trees having high resistance ranging from 71.9%, 55.2%, to 39.7%. Regional tree recovery is increasing with the proportion of trees having high recovery ranging from 28.3%, 52.2%, to 64.2%. The area with high resistance is contracting and that of high recovery is expanding. The spatiotemporal resistance and recovery are associated with moisture availability and diurnal temperature range, respectively. In addition, they are both associated with forest internal factor represented by growth consistence among trees. We conclude that juniper trees in the Tibetan Plateau have increased resilience to extreme droughts in the study period. We highlight pervasive resilience in juniper trees. The results have implications for predicting tree resilience and identifying areas vulnerable to future climate extremes.

Soil fertilization transiently increases radial growth in sessile oaks but does not change their resilience to severe soil water deficit

The future climate is projected to become increasingly problematic for European forest ecosystems, likely leading to the decline of many tree species. Forest management must adapt to cope with the negative effects of these changes. Despite the close interactions between nutrients and water in plant physiology, very few studies have investigated the link between soil fertility and tree response to soil water availability and vapour pressure deficit. The objective of this study was to test whether fertilization would modify forest tree resilience to soil water deficit.The potential impact of fertilization on trees during soil water deficit was assessed in a fertilization experiment carried out on sessile oaks. Three fertilization treatments (NCa, NPKCaMg and an unfertilized control) were applied to 40-year-old oaks that were cored almost 40 years later. The responses to drought in the fertilized and control trees were compared by analysing the radial growth chronologies following two complementary approaches. Firstly, the relationships between radial growth and inter-annual climate variations were investigated through dendroclimatic modelling including water deficit indices computed with the daily soil water balance model Biljou©. Secondly, resistance, recovery and resilience to drought-induced crises were evaluated from tree-ring width and compared among treatments. In addition, the carbon isotope composition (δ13C) of the tree rings was measured to evaluate potential differential gas-exchange processes between fertilized and control trees.Fertilization had an immediate, strong positive effect on oak growth which disappeared after nine years. NCa and NPKCaMg fertilizations had similar effects, suggesting that adding P, K and Mg did not increase radial growth. Whatever the treatment, ring width variations appeared to be identically constrained by climate, especially by the summer water deficits of the current and previous years. Tree resistance to soil water deficit was negatively affected by the severity of the drought event but did not significantly differ between control and fertilized trees. Tree resilience was not affected by fertilization and remained stable across crises. The tree-ring δ13C clearly increased during the severe 1976 water deficit but did not vary among treatments. We discuss the possible mechanisms involved in changes in growth and the lack of any significant effect of fertilization on resilience to drought and conclude that, under the relatively dry conditions and moderately poor soil fertility of our study site, fertilization could temporarily stimulate aboveground growth without inducing imbalances that would modify resilience and resistance to severe drought episodes.

Better tree performance and water use efficiency through resilient root systems: rapid screening for drought and salinity stress tolerance using relative growth rate

Abiotic stresses, such as soil water deficit and high soil solution salts, can lead to negative impacts on crop growth, productivity and quality, impacting industry profitability. The first contact of the tree with these stresses is the root system, providing the possibility of improving tree resilience by rootstock choice. Identifying almond rootstock genotypes with high stress tolerance is, consequently, of high importance in enabling the Australian almond industry to cope with these environmental challenges. This study developed and tested a potential tool to screen genotypes for tolerance to abiotic stresses, using the response of relative growth rate (RGR) to water deficit or salinity stress. Seedling plants of six almond rootstock cultivars were randomly allocated into three treatments (control, drought or salinity stress) for two months in a glasshouse. Plants in the well-watered treatment were irrigated to the point of run-off daily. The drought treatment was imposed by adding 33% of evapotranspiration of well-watered plants daily. Plants in the salinity treatment were supplied with mixed chloride solution (3.3 dS m-1) to the point of run-off daily. Plants were harvested at three time points to calculate RGR. The percentage of RGR reduction in stressed plants, relative to the non-stressed plants, was calculated and used as an index of stress tolerance. Considerable variation for stress tolerance was found among the six almond rootstock genotypes, with some genotypes being able to maintain growth rates - whereas others stopped growing - under drought or salinity stress. The impacts of the drought treatment used were generally larger than those of salinity stress. The screening approach introduced in this study provides a relatively simple and robust method to compare rootstock genotypes, contributing to horticultural management decisions, whilst also being applicable to the generation of fundamental knowledge in this area.

Tree carbon allocation explains forest drought-kill and recovery patterns.

The mechanisms governing tree drought mortality and recovery remain a subject of inquiry and active debate given their role in the terrestrial carbon cycle and their concomitant impact on climate change. Counter-intuitively, many trees do not die during the drought itself. Indeed, observations globally have documented that trees often grow for several years after drought before mortality. A combination of meta-analysis and tree physiological models demonstrate that optimal carbon allocation after drought explains observed patterns of delayed tree mortality and provides a predictive recovery framework. Specifically, post-drought, trees attempt to repair water transport tissue and achieve positive carbon balance through regrowing drought-damaged xylem. Furthermore, the number of years of xylem regrowth required to recover function increases with tree size, explaining why drought mortality increases with size. These results indicate that tree resilience to drought-kill may increase in the future, provided that CO2 fertilisation facilitates more rapid xylem regrowth.

Dynamic responses of tree-ring growth to multiple dimensions of drought.

Droughts, which are characterized by multiple dimensions including frequency, duration, severity, and onset timing, can impact tree stem radial growth profoundly. Different dimensions of drought influence tree stem radial growth independently or jointly, which makes the development of accurate predictions a formidable challenge. Measurement-based tree-ring data have obvious advantages for studying the drought responses of trees. Here, we explored the use of abundant tree-ring records for quantifying regional response patterns to key dimensions of drought. Specifically, we designed a series of regional-scaled "natural experiments," based on 357 tree-ring chronologies from Southwest USA and location-matched monthly water balance anomalies, to reveal how tree-ring responds to each dimension of drought. Our results showed that tree-ring was affected significantly more by the water balance condition in the current hydrological year than that in the prior hydrological year. Within the current hydrological year, increased drought frequency (number of dry months) and duration (maximum number of consecutive dry months) resulted in "cumulative effects" which amplified the impacts of drought on trees and reduced the drought resistance of trees. Drought events that occurred in the pregrowing seasons strongly affected subsequent tree stem radial growth. Both the onset timing and severity of drought increased "legacy effects" on tree stem radial growth, which reduced the drought resilience of trees. These results indicated that the drought impact on trees is a dynamic process: even when the total water deficits are the same, differences among the drought processes could lead to considerably different responses from trees. This study thus provides a conceptual framework and probabilistic patterns of tree-ring growth response to multiple dimensions of drought regimes, which in turn may have a wide range of implications for predictions, uncertainty assessment, and forest management.

Tall Amazonian forests are less sensitive to precipitation variability

Climate change is altering the dynamics, structure and function of the Amazon, a biome deeply connected to the Earth’s carbon cycle. Climate factors that control the spatial and temporal variations in forest photosynthesis have been well studied, but the influence of forest height and age on this controlling effect has rarely been considered. Here, we present remote sensing observations of solar-induced fluorescence (a proxy for photosynthesis), precipitation, vapour-pressure deficit and canopy height, together with estimates of forest age and aboveground biomass. We show that photosynthesis in tall Amazonian forests, that is, forests above 30 m, is three times less sensitive to precipitation variability than in shorter (less than 20 m) forests. Taller Amazonian forests are also found to be older, have more biomass and deeper rooting systems1, which enable them to access deeper soil moisture and make them more resilient to drought. We suggest that forest height and age are an important control of photosynthesis in response to interannual precipitation fluctuations. Although older and taller trees show less sensitivity to precipitation variations, they are more susceptible to fluctuations in vapour-pressure deficit. Our findings illuminate the response of Amazonian forests to water stress, droughts and climate change. Tall trees are less sensitive to variation in precipitation than short trees, according to analyses of photosynthetic sensitivity to drought in tall and short Amazon forests. The results demonstrate higher resilience of tall trees to drought.

An intra-stand approach to identify intra-annual growth responses to climate in Pinus nigra subsp. laricio Poiret trees from southern Italy

The growth of Pinus nigra tree stands is known to be limited by spring-summer precipitation (P). We explored the intra-annual growth dynamics (early-wood EW and late-wood LW of tree-rings) and their responses to climate (in monthly, seasonal and annual scale) in Pinus nigra subsp. laricio at the intra-stand level in Calabria, at the pines' mesic to xeric distribution range. We used a variety of age detrending methods to assess how the adaptive potential to climate change of each tree varies within the even-aged forest stand. In years of wet climate, when precipitation (P) could infiltrate deeper below ground, higher growth rates occurred in 83% of trees, best explained by P accumulated over several previous years. The variability of EW increment was best explained by 3–5 previous year P (including the growth year) in 61% of trees, while LW increment was best explained by 1–3 year P in 78% of trees. This would suggest that in wet years most trees utilized not only surface but also deeper moisture pools using their taproot to produce both EW and LW. In contrast, during dry years, for 39% of trees the most significant predictor for EW was June rainfall. August P explained LW variability in 35% of the trees, while the influence of 1–3 year P on LW was reduced to 48%. Thus, under a drier climate ca. 1/3 of the trees within the stand significantly reduced their capacity to utilize deeper ground moisture, indicating higher vulnerability to drought stress. Multiple-year P appeared as the main climatic driver for growth in most trees, but only became evident through age detrending methods retaining low frequency growth variability. Our findings are the first to provide such insight into the wide spectrum of climatic factors that may drive P. laricio’s inter-stand and inter-annual productivity. They also assist to identify the most vulnerable trees to drought stress within a forest stand. Such information could prove very useful in the application of silvicultural treatments (e.g., selective thinning) aiming to increase the resilience of tree stands to future drought intensification.

Estimating the contribution of arbuscular mycorrhizal fungi to drought tolerance of potted olive trees (Olea europaea)

The aim of the present study is to investigate the contribution of mycorrhization to the resilience of olive trees to drought. One-year-old olive plants were inoculated (Myc+) or not (Myc−) with arbuscular mycorrhizal fungi (AMF), and subjected to a 40-day-drought period. At regular intervals of the watering-off period and after rehydration period, water relations and gas exchanges parameters were measured. Similarly, the total soluble sugars, proline, and mineral nutrients concentrations were determined. The results revealed that Myc+ plants were less affected by drought than Myc− plants proving the involvement of the AMF in the alleviation of drought impact on olive tree. In fact, the turgor potential (Ψp) in Myc+ plants exhibited positive values during the whole treatment period, while Ψp in Myc− plants was negative mainly under severe stress intensity. Moreover, the stomatal function of Myc+ plants was less affected by drought compared to Myc− plants. The maximum of mycorrhizas relative drought alleviation rate (RDAR) was estimated to be 40% for Ψpd and RWC, 36% for the osmotic potential (ΨS), 86% for Ψp, 16% for gs, and 27% for E. The osmotic adjustment by proline was earlier in Myc+ plants than in Myc− ones. The inoculation with AMF also improved mineral uptake (K, N, Zn, and Fe). After 40 days of drought, Myc+ plants survive but not Myc− ones. In addition, the restoration of the irrigation permitted the Myc+ plants to recuperate from severe drought stress. To sum up, inoculation of young olive trees with the AMF improved their resilience to drought.

PREDICTING WOOD STRENGTH UPON COMPRESSION ALONG FIBERS

According to the basic principles of bionics, internal forces are formed in the tree stem during its growth. Appearance and development of such forces are mainly caused by external effects, where the main external effect is a wind load. Internal forces develop in response to the powerful effect of the wind load on tree stems. Such forces ensure the resilience of tree stems. They prevent the tree stem from breaking and falling together with the roots. The established system of internal forces does not disappear upon log sawing. All sawn-timber pieces obtained after log sawing are stressed to various extents. This affects the natural curvature and wood strength upon compression along fibers and static bending. The established mathematical relationship between the size of the core zone and the value of internal forces makes it possible to analyze changes in the wood strength upon compression along fibers along the board. The wood strength depends on the log taper, relative size of the core zone, and distribution of internal forces over the tree stem volume. According to the obtained data, the ratio of wood ultimate strengths upon compression along fibers and static bending depends on the nature of internal forces' distribution over the tree stem section. The paper presents the results of theoretical studies on the wood strength upon compression along fibers under various laws of internal forces' variation along the length of the log.

Foliar nitrogen metabolism of adult Douglas-fir trees is affected by soil water availability and varies little among provenances.

The coniferous forest tree Douglas-fir (Pseudotsuga menziesii) is native to the pacific North America, and is increasingly planted in temperate regions worldwide. Nitrogen (N) metabolism is of great importance for growth, resistance and resilience of trees. In the present study, foliar N metabolism of adult trees of three coastal and one interior provenance of Douglas-fir grown at two common gardens in southwestern Germany (Wiesloch, W; Schluchsee, S) were characterized in two subsequent years. Both the native North American habitats of the seed sources and the common garden sites in Germany differ in climate conditions. Total and mineral soil N as well as soil water content were higher in S compared to W. We hypothesized that i) provenances differ constitutively in N pool sizes and composition, ii) N pools are affected by environmental conditions, and iii) that effects of environmental factors on N pools differ among interior and coastal provenances. Soil water content strongly affected the concentrations of total N, soluble protein, total amino acids (TAA), arginine and glutamate. Foliar concentrations of total N, soluble protein, structural N and TAA of trees grown at W were much higher than in trees at S. Provenance effects were small but significant for total N and soluble protein content (interior provenance showed lowest concentrations), as well as arginine, asparagine and glutamate. Our data suggest that needle N status of adult Douglas-fir is independent from soil N availability and that low soil water availability induces a re-allocation of N from structural N to metabolic N pools. Small provenance effects on N pools suggest that local adaptation of Douglas-fir is not dominated by N conditions at the native habitats.

Growth recovery of mature Norway spruce and European beech from chronic O3 stress

Elevated O3 levels can strongly impair the health and vitality of forest ecosystems. Free-air exposure systems reveal that forest tree and stand growth can be reduced strongly under chronic O3 stress. Detailed knowledge of the effect of O3 exposure on photosynthesis, carbon sequestration, allometry and growth during chronic stress is available. However, knowledge of growth response after O3 reduction is scarce. Here, we analyse the growth of mature Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.) in the free-air O3 fumigation experiment at Kranzberg Forest. We compare tree growth over a 9-year period (2008–2016) after exposure to O3 (2000–2007). During 2 × O3 exposure, the annual basal area growth of Norway spruce and European beech decreased by 24 and 32%, respectively. After cessation of 2 × O3 exposure, the annual basal area growth of Norway spruce and European beech not only recovered but exceeded the growth of the trees in the control condition by 14 and 24%, respectively. The growth resilience and resistance of trees previously exposed to 2 × O3 towards drought stress and late frost was hardly lower than that of the trees in the control condition. The capacity for growth recovery even after long-term chronic O3 stress emphasizes the strong beneficial effect of air pollution control on the health of forest ecosystems and on the global land carbon sink.

Spatial and temporal variability of fragmentation effects in a long term, eucalypt forest fragmentation experiment

Although forest fragmentation is generally thought to impact tree growth and mortality negatively, recent work suggests some forests are resilient. Experimental forests provide an opportunity to examine the timing and extent of forest tree resilience to disturbance from fragmentation. We used the Wog Wog Habitat Fragmentation Experiment in southeastern Australia to test Eucalyptus growth and survivorship responses to forest fragmentation over a 26 year period. We measured 2418 tree diameters and used spline-regression techniques to examine non-monotonic fragmentation effect over two time periods. Over the first 4 years after fragmentation, individual eucalypt tree growth was greater than in continuous forest for large trees and mortality rates were higher only within 10 m of edges. Over the following 22 years only the effects on tree growth remained and on average all fragments rebounded so that their biomass and mortality rates were equivalent to continuous forest. Importantly non-monotonic patterns were observed in growth and mortality with respect to area and distance from edge in both study periods, demonstrating that fragmentation impacts on trees can be strong in localized areas (greatest in 3 ha fragments and 0–30 m edges) and over short time periods. Dry-sclerophyll eucalypt forests join the set of forest types that display resilient growth dynamics post fragmentation. Moreover, persistent non-monotonic impacts on tree growth with respect to tree size, fragment area, and fragment distance from edge, highlighting landscape fragmentation as a driver of habitat heterogeneity within remnant forest fragments.

Minor European broadleaved tree species are more drought-tolerant than Fagus sylvatica but not more tolerant than Quercus petraea

Abstract More frequent and intensive droughts are projected to affect the vitality of major European tree species. Therefore, it is important to search for alternative, more drought-tolerant species to ensure ecological stability, biodiversity, and productivity of forests in future. Based on their relative abundance at very dry and warm sites, the otherwise rare and little-known, minor broadleaved species like Sorbus torminalis, S. domestica, Acer campestre, and A. platanoides are thought to be drought-tolerant. However, there is so far only limited scientific evidence to support that notion. Here we quantified the effect of extreme droughts (1947, 1976, and 2003) on radial growth of mature trees of the minor broadleaves in relation to the common companion species Quercus petraea and Fagus sylvatica in southwest Germany. Based on tree-ring analyses with a commensurate sample size for such rare species, the ability to resist and recover from drought, as well as the medium-term resilience of mature trees was studied. Likewise, the sensitivity of the species to temperature and precipitation was assessed. Radial growth was positively related to spring and summer precipitation for all species, whereas high summer temperatures led to decreased growth rates for A. platanoides, S. domestica, and F. sylvatica. Whereas S. torminalis appeared to be resistant and resilient in relation to drought, S. domestica and A. platanoides were also resilient but less resistant to drought stress. A synoptic ranking of the drought tolerance of all tree species suggests that the minor broadleaved tree species are not more drought-tolerant than Q. petraea, except for A. campestre. We conclude that cultivation of these minor species as well as Q. petraea should be expanded on xerix sites, where F. sylvatica reaches its growth limits.

Porous pavement effects on rooting depth and growth of newly planted trees in sidewalk cutouts

Resin-bound gravel, a type of porous pavement, is a popular surface treatment around city trees, primarily because it creates a continuous level walking surface. These installations often take place where no mulch has been used, and thus may significantly alter water relations between trees and soils. Although presumed to increase stormwater capture without any detriment to trees, their effect on root distribution, tree growth, soil water distribution, and drought resilience are unknown. In November 2014 12 Platanus ×acerifolia 'Bloodgood' whips were planted in simulated urban pervious pavement profiles, six constructed with pervious resin-bound gravel pavement (PRBGP) and six bare soil (BS), in a clay-loam soil in Blacksburg, Virginia, USA. Root growth was monitored with minirhizotrons, stem diameter was measured throughout the growing season, and soil moisture was recorded at 10, 30 and 60 cm depths. After one growing season, trees in PRBGP were larger (59% greater stem diameter) than those in BS and developed a shallower root system, with 26% of minirhizotron frames having roots in the first 10 cm while 6% had roots below 37 cm, compared to trees in BS that had 12% of frames with roots in the first 10 cm and 21% with roots below 37 cm. These effects are congruent with the increased moisture (23% greater at 10 cm depth) observed under PRBGP during dry periods. A lag phase in root development was also observed in BS compared to trees in PRBGP. It was concluded that the installation of PRBGP increased growth of newly planted trees compared to BS. However, roots appeared to be shallower under PRBGP profiles, which may affect drought resilience of urban trees and potentially increase root/pavement conflicts.

Hydrodynamics of mangrove-type root models: the effect of porosity, spacing ratio and flexibility

Mangrove trees play a prominent role in coastal tropic and subtropical regions, providing habitats for many organisms and protecting shorelines against high energy flows. In particular, the species Rhizophora mangle (red mangrove) exhibits complex cluster roots interacting with different hydrological flow conditions. To better understand the resilience of mangrove trees, we modeled the roots as a collection of cylinders with a circular pattern subject to unidirectional flow. We investigated the effect of porosity and spacing ratio between roots by varying both the diameter of the patch, D, and inset cylinders, d. In addition, we modeled hanging roots of red mangroves as cantilevered rigid cylinders on a hinge. Force and velocity measurements were performed in a water tunnel (Reynolds numbers from 2200 to 11 000). Concurrently, we performed 2D flow visualization using a flowing soap film. We found that the frequency of the vortex shedding increases as the diameter of the small cylinders decreases while the patch diameter is constant, therefore increasing the Strouhal number, . By comparing the change of Strouhal numbers with a single solid cylinder, we introduced a new length scale, the effective diameter. The effective diameter of the patch decreases as the porosity increases. In addition, we found that patch drag scales linearly with the patch diameter but decreases linearly as the spacing ratio increases. After a spacing ratio of (), the force scales linearly with the free stream velocity, and the mean velocity behind the patch is independent of the Reynolds number and the patch effect disappears. For flexible cylinders, we found that a decrease in stiffness increases both patch drag and the wake deficit behind the patch in a similar fashion as increasing the blockage of the patch. This information has the potential to help in the development of methods to design resilient bio-inspired coastline structures.

Quantifying the synergistic effects of impervious surface and drought on radial tree growth

The global trend of increasing urbanization is generating isolated patches of vegetation that could be strongly influenced by the surrounding landscape. Simultaneously, many of these urban forests will be exposed to the increasing temperatures and higher risk of drought conditions brought up by climate change. The heat island effect associated with an increase in impervious surface cover and its influence on urban vegetation has been document mostly for large urban centers. However, how the effects of urbanization may interact with tree resilience to drought have been much less studied, especially in middle size cities like the majority of those populating eastern North America. In this study, we analyzed 30 years of tree radial growth data collected from individuals of three common native species in remnant forest patches distributed along a gradient of urbanization in the Midwestern USA. Acer saccharum and Quercus rubra show reduced growth rates in years with lower water availability. Our results also indicate that the detrimental effects of impervious surface cover eliminated any of the beneficial effects of growing in a wet year for A. saccharum and Q. rubra, and these effects of increase impervious surface cover on growth were more detrimental than those of droughty years. Carya ovata, the third species, did not show differential growth along the urbanization gradient or among years with varying water availability. This study illustrates how the combination of drought events and increasing impervious surface cover could have a differential impact on coexisting species in urban forests, which could further alter the species composition and functioning of these ecosystems.