Potted Saplings Research Articles

Repeated Mild Summer Drought in Crataegus monogyna Jacq. Provokes Compensation Growth in the Following Year

Water limitations will pose significant challenges to forest ecosystems across Europe. To gain a deeper understanding of the potential impacts, we investigated the response of the common shrub Crataegus monogyna to two summer droughts, each followed by rewatering. The experimental design consisted of a common garden with potted saplings from a local Belgian (n = 48), a Swedish (n = 47), and a Spanish-Pyrenean provenance (n = 48). We quantified the effects on growth and leaf phenology, focusing on the legacies in the year following the droughts. Responses were influenced by the severity of the drought and by its timing. Most strikingly, height increment was enhanced by 24% (p = 0.046) in comparison to the controls in the year following the droughts in the group of plants that endured the two drought treatments, each time without developing visible stress symptoms. Only one such mild drought, whether early or late summer, did not lead to this response, suggesting stress memory acting as a growth promoter. A late summer drought that resulted in visible drought symptoms led to a reduced diameter increment in the year following the droughts, independent of the preceding treatment (severe, mild, or no drought), whereas this was not the case for a similar drought in early summer. Minor leaf phenological responses were detected in the year following the droughts. Finally, the non-local provenances did not respond in a deviating way to the droughts compared to the local provenance. Our findings contribute to the prediction of carbon sequestration in forests and other woody vegetations in the temperate regions of Europe.

The effects of mixes of peat and olive pomace at various ratios on the vegetative growth of potted grapevine saplings

Numerous studies have been conducted in order to utilize the olive pomace, the solid waste left over from the oil processing of olives, which is widely cultivated, especially in Mediterranean countries, and to consider it a raw material that has added value instead of being considered waste. Some of these research are focused on establishing the re-utilization of olive pomace in agricultural production. Although some studies have reported that olive pomace can be utilized as fertilizer, soil improvement regulator, solid media culture, and even mulching material, this study was carried out due to the lack of sufficient scientific data on whether or not this material can be used as a growth medium in the cultivation of potted grapevine saplings. The study was carried out by growing ungrafted grapevine saplings of Vitis vinifera L. cv. Hatun Parmağı on media with peat and olive pomace at different ratios for six months, and then some vegetative growth parameters were examined. Due to the elevated olive pomace ratios in the growth medium, shoot and root growth of the grapevine saplings were restricted. The chlorophyll index and root fresh weight decreased dramatically when more than 25% (v/v) and 20% (v/v) crude pomace were available in the growth medium, respectively. Moreover, the availability of olive pomace in the medium significantly reduced shoot length, number of leaves, shoot weight, and leaf weight of grapevine saplings, regardless of the amount of olive pomace. However, it was found that 15% (v/v) or less of crude olive pomace could be utilized in growth medium mixtures when growing potted grapevine saplings; however, higher ratios would prevent the saplings from attaining marketable quality. While designing new studies, examining different grape varieties, fruit species, and lower olive pomace ratios would contribute to new and more comprehensive findings on the utilization of olive pomace in growing potted saplings.

Elevated CO2 concentration enhance carbon and nitrogen metabolism and biomass accumulation of Ormosiahosiei

Elevated CO2 concentrations may inhibit photosynthesis due to nitrogen deficiency, but legumes may be able to overcome this limitation and continue to grow. Our study confirms this conjecture well. First, we placed the two-year-old potted saplings of Ormosia hosiei (O. hosiei) (a leguminous tree species) in the open-top chamber (OTC) with three CO2 concentrations of 400 (CK), 600 (E1), and 800 μmol·mol−1 (E2) to simulate the elevated CO2 concentration environment. After 146 days, the light saturation point (LSP), light compensation point (LCP), apparent quantum efficiency (AQE), and dark respiration rate (Rd) of O. hosiei were increased under increasing CO2 concentration and obtain the maximum ribulose diphosphate (RuBP) carboxylation rate (Vc max) and RuBP regenerated photosynthetic electron transfer rate (Jmax) were also significantly increased under E2 treatment (P < 0.05). This results in a significant increase of the maximum assimilation rate (Amax) under elevated CO2 concentrations. Sucrose phosphate synthase (SPS) activity in sucrose metabolism increased in the leaves, more soluble sugars, starches, and sucrose was produced, but sucrose content only in leaves increased at E2, and more carbon flows to the roots. The activity of the NH4+ assimilating enzymes glutamine synthetase (GS), glutamate synthetase (GOGAT), and glutamate dehydrogenase (GDH) in the leaves of O. hosiei increases under elevated CO2 concentrations to promote nitrogen synthesis that reduces the content of ammonium nitrogen and increases the content of nitrate nitrogen. In addition, under E1 conditions, sucrose synthase (SS), direction of synthesis activity was highest and sucrose invertase (INV) activity was lowest, this means that the balance of C and N metabolism is maintained. While under E2 conditions SS activity decreased and INV activity increased, this increased C/N and nitrogen use efficiency. So, the elevated CO2 concentration promotes the accumulation of O. hosiei biomass, especially in the aboveground part, but did not have a significant effect on the accumulation of root biomass. This means that O. hosiei is able to cope under the elevated CO2 concentration without showing photosynthetic adaptation during the experimental period.

Timing of autumnal leaf senescence in a common shrub species depends on the level of preceding summer drought symptoms

More severe water limitations will challenge forest ecosystems in Europe. To gain more insight into the putative consequences for forests, we studied how the common shrub Cornus sanguinea L. responds to a period of water withholding in summer, followed by rewatering. We grew potted saplings of a local Belgian and a Pyrenean provenance in a common garden experiment. We quantified the impact on growth and several phenological traits, with a specific focus on autumnal leaf senescence. We showed that the timing of autumnal leaf senescence was earlier in plants that displayed no to little leaf desiccation due to the experienced drought, compared to the controls. On the contrary, plants that suffered heavy to total leaf desiccation resprouted after the drought period and displayed delayed leaf senescence. No shift in the timing of autumnal leaf senescence was observed for plants with intermediate leaf desiccation. Our results suggest two different post-drought recovery strategies acting in the same experiment. Which strategy was activated depended on the severity of the drought stress, and the two strategies cancelled each other for the group of plants with intermediate drought symptoms. The earlier leaf senescence is likely an acclimation response whereas the delayed leaf senescence enables plants to repair damaged tissues. Our results also suggest provenance-dependent responses as the earlier leaf senescence was activated earlier in the Pyrenean provenance (no difference in timing for the delayed leaf senescence).

The Impact of Nitrogen Application on Leaf and Root Functional Traits of Davidia involucrata Saplings

Understanding the effects of nitrogen (N) deposition on plant functional traits can provide insights into their adaptation strategies. We conducted an N application experiment (0, 5, 10, 15 g N m−2) with potted saplings of the endangered species Davidia involucrata and examined 24 functional traits of both leaves and roots. We found that N application increased the leaf morphological traits, except for a significant decrease (by 19.2%–27.0%) in specific leaf area (SLA). Compared to the control treatment, N application significantly increased the specific root surface area (SRA), specific root length (SRL), and root tissue density (RTD) by 9.2%–20.1%, 20.2%–47.9%, and 30.8%–46.4%, respectively, while root diameter was conservative and insensitive to N application. Additionally, N application had contrasting effects between leaf and root carbon, N, and phosphorus contents and their stoichiometry. SRL, SRA, and RTD were positively correlated with most leaf photosynthetic traits, but negatively correlated with SLA. Moreover, root biomass and root chemical traits were also tightly correlated with leaf photosynthetic traits and chemical traits. These results suggest that N application may trigger a resource-conservative strategy for leaves but a nutrient-acquisitive strategy for roots. Future N deposition combined with other practices, such as simultaneous P fertilizer application, can be effective for the scientific conservation of D. involucrata populations in their natural habitats.

Comparative transcriptomic responses of European and Japanese larches to infection by Phytophthora ramorum

Background and objectivesPhytophthora ramorum severely affects both European larch (EL) and Japanese larch (JL) trees as indicated by high levels of mortality particularly in the UK. Field observations suggested that EL is less severely affected and so may be less susceptible to P. ramorum than JL; however, controlled inoculations have produced inconsistent or non-statistically significant differences. The present study aimed to compare RNA transcript accumulation profiles in EL and JL in response to inoculation with P. ramorum to improve our understanding of their defence responses.MethodologyRNA-sequencing was carried out on bark tissues following the inoculation with P. ramorum of potted saplings in both EL and JL carried out under controlled environment conditions, with sampling at 1, 3, 10, and 25 days post inoculation in infected and control plants.ResultsAll of the inoculated trees rapidly developed lesions but no statistically significant differences were found in lesion lengths between EL and JL. RNA-Sequencing comparing control and inoculate saplings identified key differences in differentially expressed genes (DEGs) between the two larch species. European larch had rapid induction of defence genes within 24 hours of infection followed by sustained expression until 25 days after inoculation. Results in JL were more varied; upregulation was stronger but more transient and represented fewer defence pathways. Gene enrichment analyses highlighted differences in jasmonate signalling and regulation including NPR1 upregulation in EL only, and specific aspects of secondary metabolism. Some DEGs were represented by multiple responsive copies including lipoxygenase, chalcone synthase and nucleotide-binding, leucine-rich-repeat genes.ConclusionThe variations between EL and JL in responsive DEGs of interest as potentially related to differences seen in the field and should be considered in the selection of trees for planting and future breeding.

Narrow vessels cavitate first during a simulated drought in Eucalyptus camaldulensis.

Establishing drying-limits for mortality of different tree species and understanding the anatomical and physiological traits involved is crucial to predict forests' responses to climate change. The xylem of Eucalyptus camaldulensis presents a complex of solitary vessels surrounded by different imperforate tracheary elements and parenchyma that influence, in a poorly known way, its hydraulic functioning. We aimed at describing the dynamics of embolism propagation in this type of xylem, seeking any vessel-size pattern, and unraveling the threshold of xylem embolism leading to nonrecovery after drought in E. camaldulensis. We assigned potted saplings to a protracted water-stress for 70 days. We relied on colorimetric and hydraulic methods to test for links between xylem anatomy and embolism propagation in the main stem. On average, the occurrence of embolism was randomly distributed in the stem xylem, but the probability of embolized vessels was higher than predicted by chance in the narrowest vessels of individuals that experienced low to moderate water-stress. The saplings could recover from severe water-stress if their percentage loss of conductance (PLC) was <77%, but not when the PLC was ˃ 85%. We concluded that, contrary to results reported for most species, the narrowest vessels are the most vulnerable to cavitation in E. camaldulensis, suggesting a lack of tradeoff between xylem efficiency and safety (in response to drought) at the tissue level. These results challenge the well-established paradigm of the effect of vessel size on cavitation, which states that the widest conduits are the most vulnerable to both freeze-thaw and drought-induced cavitation.

Carbon allocation dynamics in conifers and broadleaved tree species revealed by pulse labeling and mass balance

Fixed carbon (C) is a central resource, whose dynamic allocation by the tree determines its ability to establish, grow and compete with other species. Traditionally, tree C allocation has been estimated by isotopic or mass balance approaches. Here we developed a setup of 13CO2 pulse-labeling, with a flux-coupled, three-phase detection, allowing us to follow C quantitatively. We examined C allocation patterns in 2-year-old potted saplings of five evergreen forest tree species that co-occur in a typical mixed forest in the Mediterranean region: two early succession conifers (Pinus, Cupressus) and three late succession broadleaf species (Quercus, Ceratonia, Pistacia). Across the five species, C moved from leaves to stem and fine roots following an exponential decay, with parameters that are consistent with the ecological role of each species. Eight days post-labeling, conifers allocated ~30% of the C belowground, and broadleaves <10%. In contrast, leaf C residence time was ~4 days in conifers and only ~1 day in broadleaves. Root exudation was quantified and shown to be small. We integrated our results into a compartmental model of 13C distribution in the tree, enhancing our understanding of divergent C allocation strategies among tree species within the same ecosystem.

Comparison of water-use characteristics of tropical tree saplings with implications for forest restoration

Tropical forests are experiencing reduced productivity and will need restoration with suitable species. Knowledge of species-specific responses to changing environments during early stage can help identify the appropriate species for sustainable planting. Hence, we investigated the variability in whole-tree canopy conductance and transpiration (Gt and EL) in potted saplings of common urban species in Thailand, viz., Pterocarpus indicus, Lagerstroemia speciosa, and Swietenia macrophylla, across wet and dry seasons in 2017–2018. Using a Bayesian modeling framework, Gt and EL were estimated from sap flux density, informed by the soil, atmospheric and tree measurements. Subsequently, we evaluated their variations with changing vapor pressure deficit (VPD) and soil moisture across timescales and season. We found that Gt and EL were higher and highly variable in L. speciosa across seasons than S. macrophylla and P. indicus. Our results implied that water-use in these species was sensitive to seasonal VPD. L. speciosa may be suitable under future climate variability, given its higher Gt and EL across atmospheric and soil moisture conditions. With their lower Gt and EL, P. indicus and S. macrophylla may photosynthesize throughout the year, maintaining their stomatal opening even under high VPD. These findings benefit reforestation and reclamation programs of degraded lands.

Drought promotes early leaf abscission regardless of leaf habit but increases litter phosphorus losses only in evergreens

Drought can alter leaf senescence and nutrient resorption, but whether these alterations are consistent between trees of contrasting leaf habits remains unclear. We conducted an experiment for 2 years with potted saplings to compare leaf habit and drought effects on leaf abscission dynamics and nutrient resorption proficiency in two evergreen and two deciduous Nothofagus (Nothofagaceae) species from southern temperate forests. A global meta-analyses has found similar nitrogen (N) proficiency between leaf habits, and greater phosphorus (P) proficiency for evergreens; however, it is unclear whether these differences are attributable to phylogeny or leaf habit. Leaf habit had no effect on either N or P resorption proficiency, but drought significantly decreased P resorption proficiency, particularly in evergreens. Additionally, drought reduced spring relative growth and promoted significantly earlier leaf abscission in both leaf habits. In evergreens, reduced P proficiency along with a non-significant trend of higher litter mass under drought led to increased litter P content. Thus, leaf P resorption proficiency of evergreens appeared to be hindered by drought, whereas drought-induced senescence was efficient in reducing P losses in deciduous saplings. Our findings suggest that nutrient cycling and species composition in strongly P-limited environments could change under sustained, moderate drought conditions.

Variation in nocturnal stomatal conductance and development of predawn disequilibrium between soil and leaf water potentials in nine temperate deciduous tree species.

It is widely acknowledged that many plant species can keep stomata open during night. We examined how nocturnal stomatal conductance differs among potted saplings of nine temperate tree species from diverse native habitats in wet and dry soil conditions, and how it affects plant predawn water status. Nocturnal stomatal conductance in dry soil conditions was low in all the species (with a maximum value of 14.6 mmol m-2 s-1); in wet conditions, it was the highest in Populus tremula L., a fast-growing and anisohydric pioneer species, and the lowest in Quercus robur L., a late-successional and isohydric species. Relatively high nocturnal stomatal conductance in wet conditions in P. tremula compared with the other species resulted in the highest difference in water potential values between the leaves and soil at predawn. As drought progressed, different species tended to keep stomata almost closed at night, and the observed differences between anisohydric and isohydric species disappeared. At an ample soil water supply, nocturnal stomatal behaviour was species dependent and varied according to both the water-use and the life strategies of the species. Keeping that in mind, one should therefore be careful when using predawn leaf water potential as a proxy for soil water potential, sampling different species.

Bark water vapour conductance is associated with drought performance in tropical trees

Bark water vapour conductance (gbark) is a rarely considered functional trait. However, for the few tree species measured to date, it appears high enough to create stem water deficits associated with mortality during droughts, when access to water is limited. I tested whether gbark correlates with stem water deficit during drought conditions in two datasets of tropical trees: one of saplings in forest understories during an annual dry season and one of potted saplings in a shadehouse during extreme drought conditions. Among all 14 populations of eight species measured, gbark varied more than 10-fold (0.86-12.98 mmol m-2 s-1). In the forest understories, gbark was highly correlated with stem water deficit among four deciduous species, but not among evergreen species that likely maintained access to soil water. In the shadehouse, gbark was positively correlated with stem water deficit and mortality among all six species. Overall, tree species with higher gbark suffer higher stem water deficit when soil water is unavailable. Incorporating gbark into soil-plant-atmosphere hydrodynamic models may improve projections of plant mortality under drought conditions.

An explanation for the isotopic offset between soil and stem water in a temperate tree species.

A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica. We measured δ2 H and δ18 O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes. Regardless of substrate, soil and stem water δ2 H were similar only near permanent wilting point. Under moister conditions, stem water δ2 H was 11±3‰ more negative than soil water δ2 H, coherent with field studies. Under drier conditions, stem water δ2 H became progressively more enriched than soil water δ2 H. Although stem water δ18 O broadly reflected that of soil water, soil-stem δ2 H and δ18 O differences were correlated (r=0.76) and increased with transpiration rates indicated by proxies. Soil-stem isotopic offsets are more likely to be caused by water isotope heterogeneities within the soil pore and stem tissues, which would be masked under drier conditions as a result of evaporative enrichment, than by fractionation under root water uptake. Our results challenge our current understanding of isotopic signals in the soil-plant continuum.

Asma Fidanı Üreten İşletmelerin Mevcut Durumu ve Sorunları

Bu araştırmanın amacı, asma fidanı üreten işletmelerin yapısal özelliklerini belirlemek ve fidancılık sektöründe karşılaştıkları sorunları ortaya koymaktır. Araştırmanın ana materyalini anket çalışmasından elde edilen veriler oluşturmaktadır. Araştırma kapsamında, Fidan Üreticileri Alt Birliği (FÜAB) üyesi olan 12 işletme ile yüz yüze görüşme gerçekleştirilmiş ve bu şekilde anket çalışması verileri derlenmiştir. Araştırma döneminde FÜAB‘a üye 33 işletme bulunmaktadır, ancak bu işletmelerin çoğunluğu küçük kapasiteli ve üretim yapmadan fidan alım satımı yapan işletmelerdir. Araştırmada, büyük kapasiteli hem üretim hem fidan satışı yapan işletmelerin tamamı ile görüşülmeye çalışılmış, bu şekilde 33 işletmenin 12‘si ile görüşülmüştür. Görüşme yapılan işletmeler asma fidan pazarının büyük bir kısmını ellerinde tutmaktadır. Veriler yüzde dağılım tabloları, ortalama, maksimum ve minimum hesaplamalar yoluyla değerlendirilmiş ve yorumlanmıştır. Araştırma sonuçlarına göre, işletmeler ticari düzeyde ortalama olarak 12 yıldır asma fidanı üretmektedir. Görüşülen işletmelerin tamamı açık köklü fidan üretimi yapmaktadır. Bunun yanı sıra tüplü (%58,33) ve Amerikan asma (%25) fidanı da üretilmektedir. İşletmelerin aşılı fidan randıman oranları, ortalama olarak açık köklü fidanlarda %45, tüplü fidanlarda %72 olarak belirlenmiştir. Üreticilerin%67'si fidan pazarlamada sorun yaşadığını belirtmişlerdir. İşletmeciler kayıt dışı üretim konusunda yetkililerin önlem almasını talep etmişlerdir.

Differential ecophysiological responses and resilience to heat wave events in four co-occurring temperate tree species

Extreme summer heat waves are known to induce foliar and stem mortality in temperate forest ecosystems, yet our mechanistic knowledge of physiological thresholds for damage is lacking. Current spatiotemporal simulations of forest growth responses to climate change fail to explain the variability between co-occurring tree species to climate extremes, indicating a need for new model frameworks that include mechanistic understanding of trait-specific responses. In this context, using manipulative heat wave (hw) experiments we investigated ecophysiological responses and physiological recovery in four co-occurring temperate tree species of the southeastern United States including three deciduous angiosperms: southern red oak (Quercus falcata Michx.), shumard oak (Q. shumardii Buckl.) and, tulip-poplar (Liriodendron tulipifera L.) and one evergreen conifer: eastern white pine (Pinus strobus L.). The objectives were to investigate inter-specific differences in ecophysiological responses to hw events to understand mechanistic differences in resilience that may be useful for future model development. Two-year-old, well-irrigated potted saplings were exposed to progressively increasing extreme hw diurnal cycles followed by a recovery cycle, with peak midday air temperature increasing from 37 °C to a maximum of 51 °C on the third day of the hw. Plants were assessed for various photosynthetic and water use responses, chlorophyll fluorescence and photosystem-II (PSII) activity, leaf temperature and foliar pigments. Intense heat caused progressive down-regulation in net photosynthesis, but the stomata remained operational, which helped cool leaves through loss of latent heat. Even though whole plant transpiration increased for all species, the rate plateaued at higher hw events that allowed leaf temperature to exceed 45 °C, well beyond the optimal range. A significant increase in non-photochemical quenching over the hw cycles was evident in all species though indications of both transient and chronic PSII damage were evident in the most heat sensitive species, pine and tulip poplar. The oaks, especially Q. falcata, showed greater thermotolerance than other species with a higher threshold for photodamage to PSII, rapid overnight recovery of photoinhibition and minimal heat-induced canopy necrosis. We conclude that these co-occurring tree species exhibit large variability in thermotolerance and in their capability to repair both transient and chronic photodamage. Our results indicate that extreme heat induced damage to PSII within the leaf chloroplasts may be a mechanistic trait that can be used to project how different species respond to extreme weather events.

Coarse roots prevent declines in whole-tree non-structural carbohydrate pools during drought in an isohydric and an anisohydric species.

Predicted increases in the frequency and severity of droughts have led to a renewed focus on how plants physiologically adjust to low water availability. A popular framework for understanding plant responses to drought characterizes species along a spectrum from isohydry to anisohydry based on their regulation of gas exchange and leaf water potential under drying conditions. One prediction that arises from this theory is that plant drought responses may hinge, in part, on their usage of non-structural carbohydrate (NSC) pools. For example, trees that respond to drought by closing stomates (i.e., isohydric) are predicted to deplete NSC reserves to maintain metabolism, whereas plants that keep stomata open during water stress (i.e., anisohydric), may show little change or even increases in NSC concentration. However, empirical tests of this theory largely rely on aboveground measurements of NSC, ignoring the potentially conflicting responses of root NSC pools. We sought to test these predictions by subjecting potted saplings of Quercus alba L. (an anisohydric species) and Liriodendron tulipifera L. (an isohydric species) to a 6 week experimental drought. We found that stem NSC concentrations were depleted in the isohydric L. tulipifera but maintained in the anisohydric Q. alba-as predicted. However, when scaled to whole-plant NSC content, the drought-induced decreases in stem NSCs in L. tulipifera were offset by increases in root NSCs (especially soluble sugars), resulting in no net change to whole-plant NSC content. Similarly, root sugars increased in Q. alba in response to drought. This increase was concurrent with declines in growth, suggesting a potential trade-off between allocation of photoassimilates to root sugars vs biomass during drought. Collectively, our results suggest that the responses of NSC in coarse roots can differ from stems, and indicate a prominent role of coarse roots in mitigating drought-induced declines in whole-tree NSC pools.

Effects of soil nutrient conditions on water transport properties and recovery from severe drought stress in Pinus densiflora saplings

ABSTRACTTo evaluate the acclimation of water transport properties to soil nutrient conditions, morphological and hydraulic traits were measured in 4-year-old potted saplings of Japanese red pine that were grown under differing nutrient conditions for 3 years. To compare recovery from severe drought stress under differing nutrient conditions, the well-watered saplings were subjected to drought treatments of almost Ψ50 by stopping irrigation; this is equivalent to the xylem water potential at which 50% loss of hydraulic conductivity occurs. After resuming daily watering, the xylem water potential and gas exchange rate were measured. Whole-plant transpirational demand from large total-leaf area tended to be high for plants under high-nutrient conditions. However, no differences were observed in water transport properties, such as the conducting area, hydraulic efficiency, and cavitation resistance, suggesting that acclimation of the hydraulic architecture did not occur when plants were grown under sufficient soil moisture conditions. The root dry weight tended to be large and the belowground dry mass to root dry mass (T/R ratio) tended to be low for plants under high-nutrient conditions. Plants grown under high-nutrient conditions recovered their gas exchange rate and leaf-specific hydraulic conductance slightly during the month following severe drought stress by irrigation. The recovery of Japanese red pine appears to vary under different levels of nutrient availability probably because of the influence of root water absorption capacity.

Retention of stored water enables tropical tree saplings to survive extreme drought conditions.

Trees generally maintain a small safety margin between the stem water potential (Ψstem) reached during seasonal droughts and the Ψstem associated with their mortality. This pattern may indicate that species face similar mortality risk during extreme droughts. However, if tree species vary in their ability to regulate Ψstem, then safety margins would poorly predict drought mortality. To explore variation among species in Ψstem regulation, I subjected potted saplings of six tropical tree species to extreme drought and compared their responses with well-watered plants and pretreatment reference plants. In the drought treatment, soil water potential reached <-10 MPa, yet three species, Bursera simaruba (L.) Sarg., Cavanillesia platanifolia (Bonpl.) Kunth and Cedrela odorata L. had 100% survival and maintained Ψstem near -1 MPa (i.e., desiccation-avoiding species). Three other species, Cojoba rufescens (Benth.) Britton and Rose, Genipa americana L. and Hymenaea courbaril L. had 50%, 0% and 25% survival, respectively, and survivors had Ψstem <-6 MPa (i.e., desiccation-susceptible species). The desiccation-avoiding species had lower relative water content (RWC) in all organs and tissues (root, stem, bark and xylem) in the drought treatment than in the reference plants (means 72.0-90.4% vs 86.9-97.9%), but the survivors of the desiccation-susceptible C. rufescens had much lower RWC in the drought treatment (44.5-72.1%). Among the reference plants, the desiccation-avoiding species had lower tissue density, leaf-mass fraction and lateral-root surface area (LRA) than the desiccation-susceptible species. Additionally, C. platanifolia and C. odorata had reduced LRA in the drought treatment, which may slow water loss into dry soil. Together, these results suggest that the ability to regulate Ψstem during extreme drought is associated with functional traits that favor retention of stored water and that safety margins during seasonal drought poorly predict survival during extreme drought.

Structures of Bordered Pits Potentially Contributing to Isolation of a Refilled Vessel from Negative Xylem Pressure in Stems of Morus australis Poir.: Testing of the Pit Membrane Osmosis and Pit Valve Hypotheses.

Two hypotheses have been proposed to explain the mechanism preventing the refilling vessel water from being drained to the neighboring functional vessels under negative pressure. The pit membrane osmosis hypothesis proposes that the xylem parenchyma cells release polysaccharides that are impermeable to the intervessel pit membranes into the refilling vessel; this osmotically counteracts the negative pressure, thereby allowing the vessel to refill. The pit valve hypothesis proposes that gas trapped within intervessel bordered pits isolates the refilling vessel water from the surrounding functional vessels. Here, using the single-vessel method, we assessed these hypotheses in shoots of mulberry (Morus australis Poir.). First, we confirmed the occurrence of xylem refilling under negative pressure in the potted mulberry saplings. To examine the pit membrane osmosis hypothesis, we estimated the semi-permeability of pit membranes for molecules of various sizes and found that the pit membranes were not semi-permeable to polyethylene glycol of molecular mass <20,000. For the pit valve hypothesis, we formed pit valves in the intervessel pits in the short stem segments and measured the maximum liquid pressure up to which gases in bordered pits were retained. The threshold pressure ranged from 0.025 to 0.10 MPa. These values matched the theoretical value calculated from the geometry of the pit chamber (0.0692-0.101 MPa). Our results suggest that gas in the pits is retained by surface tension, even under substantial positive pressure to resolve gases in the refilling vessel, whereas the molecule size required for the pit membrane osmosis mechanism in mulberry would be unrealistically large.

Effects of aphid infestation on the biogeochemistry of the water routed through European beech (Fagus sylvatica L.) saplings

Mass outbreaks of herbivore insects perturb the functional properties of forests, by substantially altering water and nutrient cycling of infested trees. Less clear is how low to moderate herbivory affects the vertical flow of nutrients from trees to the soil. We report on the effects of low to moderate infestation levels of the woolly beech aphid (Phyllaphis fagi L.) on the nutrient dynamics and hydrology of European beech (Fagus sylvatica L.). We traced the vertical dynamics of macro and micronutrients via throughfall (TF), stemflow (SF) and Oa litter leachates from potted saplings underneath infested and uninfested control trees. The low infestation levels were insufficient to significantly alter TF and litter leachate composition, although, on average, the release of K and DOC and the immobilization of N (NH4–N) in TF solution were more pronounced under aphid infestation. SF chemistry was significantly altered by aphid activity exhibiting intensified fluxes of K (+159 %), Mg (+82 %), Mn (+93 %), S (+86 %), SO4–S (+62 %), DOS (+137 %), DOC (+51 %) and DON (+62 %), compared to control trees. SF is likely enriched by low and mid-level aphid activity due to its increased residence time in the canopy. It has interaction with large areas of aboveground vegetative surface area representing a cumulative flux of solutes and particulates from both foliar and woody surfaces from above-lying portions of the canopy. Accordingly, SF might be a useful indicator to evaluate the impact of sap- and leaf-feeding herbivores on forest ecosystems.