Broadleaf Tree Species Research Articles

Transient Post-Fire Growth Recovery of Two Mediterranean Broadleaf Tree Species

Fires affect forest dynamics in seasonally dry regions such as the Mediterranean Basin. There, fire impacts on tree growth have been widely characterized in conifers, particularly pine species, but we lack information on broadleaf tree species that sprout after fires. We investigated post-fire radial growth responses in two coexisting Mediterranean hardwood species (the evergreen Quercus ilex, the deciduous Celtis australis) using tree-ring width data. We compared growth data from burnt and unburnt stands of each species subjected to similar climatic, soil and management conditions. We also calculated climate–growth relationships to assess if burnt stands were also negatively impacted by water shortage, which could hinder growth recovery. Tree-ring data of both species allowed us to quantify post-fire growth enhancements of +39.5% and +48.9% in Q. ilex and C. australis, respectively, one year after the fire. Dry spring climate conditions reduced growth, regardless of the fire impact, but high precipitation in the previous winter enhanced growth. High June radiation was negatively related to the growth of unburnt Q. ilex and burnt C. australis stands, respectively. Post-fire growth enhancement lasted for five years after the fire and it was a transitory effect because the growth rates of burnt and unburnt stands were similar afterwards.

Pollarding May Relieve Drought Stress in Black Poplars

Pollarding has historically been used in broadleaf tree species across European woodlands. However, despite pollarding enhances vigor growth in the short term, it is still unclear how long this effect lasts and whether it can alleviate drought stress in seasonally dry regions. We compared the radial growth and wood δ13C (13C/12C), a proxy of intrinsic water-use efficiency (iWUE), of trees pollarded 10 and 20 years ago in two black poplar (Populus nigra L.) riparian stands located in North Eastern Spain and subjected to different ecohydrological conditions. We also assessed if pollarded trees showed different leaf phenology as compared with uncut trees of coexisting white poplar (Populus alba L.) trees. The relationships between growth, climate variables, drought severity and river flow were quantified. Pollarded and uncut trees showed a similar leaf phenology with a trend towards earlier leaf unfolding as springs become warmer. Pollarding increased growth rates by 54% (ratio between trees pollarded 10 and 20 years ago, respectively), but this enhancement was transitory and lasted ca. 10 years, whereas wood δ13C decreased −5%. The growth of black poplar increased in response to high precipitation in the previous winter, cool wet conditions, and a higher river flow in summer. Pollarding improves growth and relieves drought stress.

Modification of xylan in secondary walls alters cell wall biosynthesis and wood formation programs and improves saccharification.

Wood of broad-leaf tree species is a valued source of renewable biomass for biorefinery and a target for genetic improvement efforts to reduce its recalcitrance. Glucuronoxylan (GX) plays a key role in recalcitrance through its interactions with cellulose and lignin. To reduce recalcitrance, we modified wood GX by expressing GH10 and GH11 endoxylanases from Aspergillus nidulans in hybrid aspen (Populus tremula L. × tremuloides Michx.) and targeting the enzymes to cell wall. The xylanases reduced tree height, modified cambial activity by increasing phloem and reducing xylem production, and reduced secondary wall deposition. Xylan molecular weight was decreased, and the spacing between acetyl and MeGlcA side chains was reduced in transgenic lines. The transgenic trees produced hypolignified xylem having thin secondary walls and deformed vessels. Glucose yields of enzymatic saccharification without pretreatment almost doubled indicating decreased recalcitrance. The transcriptomics, hormonomics and metabolomics data provided evidence for activation of cytokinin and ethylene signalling pathways, decrease in ABA levels, transcriptional suppression of lignification and a subset of secondary wall biosynthetic program, including xylan glucuronidation and acetylation machinery. Several candidate genes for perception of impairment in xylan integrity were detected. These candidates could provide a new target for uncoupling negative growth effects from reduced recalcitrance. In conclusion, our study supports the hypothesis that xylan modification generates intrinsic signals and evokes novel pathways regulating tree growth and secondary wall biosynthesis.

Leaf stomatal configuration and photosynthetic traits jointly affect leaf water use efficiency in forests along climate gradients.

Water use efficiency (WUE) represents the trade-off between carbon assimilation and water loss in plants. It remains unclear how leaf stomatal and photosynthetic traits regulate the spatial variation of leaf WUE in different natural forest ecosystems. We investigated 43 broad-leaf tree species spanning from cold-temperate to tropical forests in China. We quantified leaf WUE using leaf δ13C and measured stomatal traits, photosynthetic traits as well as maximum stomatal conductance ( ) and maximum carboxylation capacity ( ). We found that leaves in cold-temperate forests displayed 'fast' carbon economics, characterized by higher leaf nitrogen, Chl, specific leaf area, and , as an adaptation to the shorter growing season. However, these leaves exhibited 'slow' hydraulic traits, with larger but fewer stomata and similar , resulting in higher leaf WUE. By contrast, leaves in tropical forests had smaller and denser stomata, enabling swift response to heterogeneous light conditions. However, this stomatal configuration increased potential water loss, and coupled with their low photosynthetic capacity, led to lower WUE. Our findings contribute to understanding how plant photosynthetic and stomatal traits regulate carbon-water trade-offs across climatic gradients, advancing our ability to predict the impacts of climate changes on forest carbon and water cycles.

Assisted migration outcomes for oak species and seed sources in southern Ontario, Canada

IntroductionForest assisted migration has been proposed as a means to align tree populations with shifting climate habitats under climate change. Here we report on the growth and survival of oak species and seed sources at five assisted migration trials in southern Ontario – an important transition zone between boreal and temperate ecosystems.MethodsEach trial featured one or more oak species – including red oak (Quercus rubra), white oak (Q. alba), burr oak (Q. macrocarpa), and swamp white oak (Q. bicolor) – and seed sources from Ontario, Pennsylvania, and/or Tennessee. The trials were measured for survival and height at between 7 and 13 years after planting.ResultsFor several trials and species, southern seed sources performed nearly as well as local sources. However, southern seed sources of burr oak performed significantly worse than local sources at 2 trials in eastern Ontario. These outcomes may have been influenced by reduced quality of southern seed source planting stock at these trials.DiscussionOur findings generally support previous work that suggests northward movements of seed sources of several hundred kilometers may be safe for assisted migration efforts involving broadleaf tree species. Notably, the trial sites were located at the northern range limits of two oak species in this study (Q. alba and Q. bicolor), suggesting the potential for modest range expansions in this boreal-temperate transition zone. These findings help forest managers to better understand potential assisted migration outcomes under climate change.

Introduction of broadleaf tree species can promote the resource use efficiency and gross primary productivity of pure forests.

Long-term pure forest (PF) management and successive planting has result resulted in "low-efficiency artificial forests" in large areas. However, controversy persists over the promoting effect of introduction of broadleaf tree species on production efficiency of PF. This study hypothesised that introduced broadleaf tree species can significantly promote both water-nutrient use efficiency and gross primary productivity (GPP)of PF. Tree ring chronologies, water source, water use efficiency and GPP were analysed in coniferous Cunninghamia lanceolata and broadleaved Phoebe zhennan growing over the past three decades. The introduction of P. zhennan into C. lanceolata plantations resulted in inter-specific competition for water, probably because of the similarity of the main water source of these two tree species. However, C. lanceolata absorbed more water with a higher nutrient level from the 40-60-cm soil layer in mixed forests (MF). Although the co-existing tree species limited the basal area increment and growth rates of C. lanceolata in MF plots, the acquisition of dissolved nutrients from the fertile topsoil layer were enhanced; this increased the water use efficiency and GPP of MF plots. To achieve better ecological benefits and GPP, MFs should be constructed in southern China.

Do leaf lignin content or leaf mass-to-area bias the estimation of intrinsic water use efficiency from leaf bulk δ13C? A test with seedlings from five oak species

Key MessageLeaves of seedlings from five oak species (Quercus robur L.; Q. pubescens L.; Q. suber L.; Q. afares Pomel; Q. ilex L.) displayed large, mainly inter-specific, differences in leaf mass-to-area ratio (LMA) and lignin content, as well as in the 13C composition of bulk leaf biomass. The variation in leaf lignin content and LMA did not impact the offset between the 13C composition measured in bulk leaf material versus soluble sugars. This observation, as well as the similar correlations between intrinsic leaf water use efficiency and the 13C compositions of bulk material or soluble sugars extracted from leaves, confirms their reliable use as a proxy for the former even when there is a large variation in LMA or lignin among samples.ContextCarbon isotope composition (δ13C) of bulk leaf biomass is frequently used as a proxy for intrinsic water use efficiency (iWUE) in large-scale intra- and inter-specific comparisons. However, post-photosynthetic 13C discrimination during the synthesis of lignin combined with differences in leaf mass-to-area ratio (LMA) may bias the relationship between δ13C of bulk leaf matter and iWUE and thus its use as a proxy of iWUE.AimsTo quantify the impact of differences in lignin content and LMA on the relationship between δ13C of bulk leaf biomass and iWUE over a large gradient of lignin contents across five oak species (deciduous: Quercus robur, Q. pubescens, Q. afares and evergreen: Q. ilex and Q. suber).MethodsWe measured lignin content, LMA, and δ13C of bulk leaf biomass and of soluble sugars extracted from the leaves, as well as intrinsic water use efficiency (derived from leaf gas exchange) in seedlings of the five oak species grown under common conditions in a greenhouse and measured in a climate chamber.ResultsThere was a large range (mainly across species) in lignin content (4 to 33%) and LMA (60–180 g m−2). δ13C of bulk leaf biomass and soluble sugars were tightly correlated, showing a significant mean offset of − 0.4‰. This offset was stable across species and not correlated to the lignin content of the leaves. A very loose correlation was found between the offset and LMA, mainly due to one species.ConclusionOur results are a demonstration that potential variations in leaf lignin content or LMA have no or only a little effect on the δ13C of bulk leaf biomass. They are unlikely to cause a bias when using bulk leaf δ13C as a proxy for variations in intrinsic water use efficiency among Mediterranean and temperate broad-leaf forest tree species.

Detecting Vegetation Stress in Mixed Forest Ecosystems Through the Joint Use of Tree‐Water Monitoring and Land Surface Modeling

AbstractRecent European heatwaves have significantly impacted forest ecosystems, leading to increased plant water stress. Advances in land surface models aim to improve the representation of vegetation drought responses by incorporating plant hydraulics into the plant functional type (PFT) classification system. However, reliance on PFTs may inadequately capture the diverse plant hydraulic traits (PHTs), potentially biasing transpiration and vegetation water stress representations. The detection of vegetation drought stress is further complicated by the mixing of different tree species and forest patches. This study uses the Community Land Model version 5.0 to simulate an experimental mixed‐forest catchment with configurations representing standalone, patched mixed, and fully‐mixed forests. Biome‐generic, PFT‐specific, or species‐specific PHTs are employed. Results emphasize the crucial role of accurately representing mixed forests in reproducing observed vegetation water stress and transpiration fluxes for both broadleaf and needleleaf tree species. The dominant vegetation fraction is a key determinant, influencing aggregated vegetation response patterns. Segregation level in PHT parameterizations shapes differences between observed and simulated transpiration fluxes. Simulated root water potential emerges as a potential metric for detecting vegetation stress periods. However, the model's plant hydraulic system has limitations in reproducing the long‐term effects of extreme weather events on needleleaf tree species. These findings highlight the complexity of modeling mixed forests and underscore the need for improved representation of plant diversity in land surface models to enhance the understanding of vegetation water stress under changing climate conditions.

Soil fungal composition under decomposing deadwood is largely affected by tree bark density rather than soil properties

Deadwood is an important structural and biological component of forest ecosystems habitating for a variety of soil fungal communities. However, it is unknown how decomposition of deadwood impacts fungal communities in the soil underneath functionally different tree species in forests. This study compared the effects of the barks of seven tree species, differing in physicochemical characteristics, on soil fungal communities during the early stages of deadwood decomposition in a subtropical forest. A field experiment consisting of 3 evergreen broadleaf, 3 deciduous broadleaf and 1 coniferous tree species, was conducted for 18 months to examine the impact of deadwood decomposition on soil fungal community using real-time PCR and high-throughput sequencing. Our results demonstrated that wood decomposition led to significant declines in soil pH. Functionally distinct tree species significantly shaped the distinct soil fungal composition and abundances underneath the deadwood of both broadleaf and coniferous trees after 18 months of decomposition. The fungal ecological guilds underneath the deadwood were altered, with symbiotrophs being more prominent, and a higher proportion of pathogens were observed under certain broadleaf tree species. Out of the soil fungal ASVs detected, only 51 (3 %) were generalists, while 1053 ASVs (52 %) exhibited traits indicating potential fungal specialists. There were stronger coniferous tree species preferences in soil fungal composition, compared to broadleaf trees. The initial bark density from broadleaf and coniferous species illustrated the most obvious difference. Interestingly, tree bark density, rather than soil chemistry, was the key driver causing the fluctuations in fungal composition during deadwood decomposition. Tree species traits notably affected soil fungal community during early wood decomposition. These findings highlight the importance of considering the bark traits in understanding the dynamics of soil fungal communities and its implications for forest health and ecosystem management.

Changing Dynamic of Tree Species Composition and Diversity: A Case Study of Secondary Forests in Northern China in Response to Climate Change

Climate warming is believed to have irreversible effects on biodiversity and ecosystem functions. Secondary forests are non-negligible ecosystems in northern China that have attracted much attention because of their instability and sensitivity to global change. However, there is no consensus on the impact of warming on secondary forest succession. In this study, we explored the response of tree species diversity to climate warming in northern secondary forests in China using a series of field surveys combined with annual meteorological data from 2015 to 2021. Our results indicate that the temperature in the study area increased in spring and autumn, while the precipitation increased in spring, summer, and autumn from 2015 to 2021. Changes in species composition in the study area and climate warming were significant in the northern region of China. The importance values of many broadleaf tree species increased, whereas those of local coniferous and broadleaf tree species decreased. The Shannon–Wiener, Simpson, and Margalef indices for the pure forest were significantly lower than those for the broadleaf mixed forest and the conifer–broadleaf mixed forest (p < 0.05) in 2015 and 2021. The highest value for the Pielou index was in the conifer–broadleaf mixed forest (p < 0.05), whereas it was not significantly different between the pure forest and broadleaf mixed forest in 2021. Surprisingly, the secondary broadleaf mixed forest in northern China showed an unfavorable degradation trend under the influence of climate change, just the same as the secondary pure forest. Our work provides an experimental data source for research on secondary forests under various climate change scenarios and is an important reference for predicting and dealing with the impact of global climate change on the adaptive management and protection of secondary ecosystems.

Non-additive effects on biodegradation of moso bamboo litter- and broadleaf tree litter-leached dissolved organic matter mixtures in a subtropical forest of southern China

Phyllostachys pubescens (moso bamboo) has extensively expanded to subtropical broadleaf forests. However, how moso bamboo expansion influences litter-leached dissolved organic matter (DOM) biodegradation is unclear. In this study, we collected fresh leaf litter of moso bamboo and 10 broadleaf tree species from a subtropical forest in southern China and extracted litter-leached dissolved organic carbon (DOC), dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP). Then, using a 42-day incubation experiment, we measured litter-leached DOM biodegradation of the selected 11 species and assessed the relative mixing effects on biodegradation of bamboo litter- and broadleaf tree litter-leached DOM mixtures with volume mixing ratios of 1:3, 1:1, and 3:1. In the litter leachates, bamboo had lower DOC:DTN ratio, DOC:DTP ratio, and DOM aromaticity (i.e., lower SUVA254 and SUVA350 values) than most broadleaf tree species. Litter-leached DOM biodegradation did not differ among bamboo, Liquidambar formosana, Vernicia fordii, and Cyclobalanopsis glauca, but was greater for bamboo than for the other seven broadleaf tree species. Leaf litter-leached DOM biodegradation correlated negatively with DOC:DTN and DOC:DTP ratios, but exhibited no significant relationship with DOM aromaticity. Regardless of volume mixing ratios, antagonistic effects were observed when bamboo litter-leached DOM was mixed with broadleaf tree litter-leached DOM with comparable biodegradation, whereas synergistic effects occurred when bamboo litter-leached DOM was mixed with broadleaf tree litter-leached DOM with lower biodegradation. The relative mixing effects on DOM biodegradation increased linearly with elevated interspecific difference in litter-leached DOM biodegradation between bamboo and broadleaf tree species across the incubation periods. These findings indicate that moso bamboo expansion will substantially alter litter-leached DOM biodegradation by improving substrate quality and changing species interactions, and the magnitudes of such changing trends are dependent on the native tree litter-leached DOM biodegradation in subtropical broadleaf forests.

Uptake and physiological impacts of nanoplastics in trees with divergent water use strategies.

Anthropogenic contaminants can place significant stress on vegetation, especially when they are taken up into plants. Plastic pollution, including nanoplastics (NPs), could be detrimental to tree functioning, by causing, for example, oxidative stress or reducing photosynthesis. While a number of studies have explored the capacity of plants to take up NPs, few have simultaneously assessed the functional damage due to particulate matter uptake. To quantify NPs uptake by tree roots and to determine whether this resulted in subsequent physiological damage, we exposed the roots of two tree species with different water use strategies in hydroponic cultures to two concentrations (10 mg L-1 and 30 mg L-1) of model metal-doped polystyrene NPs. This approach allowed us to accurately quantify low concentrations of NPs in tissues using standard approaches for metal analysis. The two contrasting tree species included Norway spruce (Picea abies [L.] Karst), a water conservative tree, and wild service tree (Sorbus torminalis [L.] Crantz), an early successional tree with a rather water spending strategy. At both exposure concentrations and at each of the experimental time points (two and four weeks), NPs were highly associated and/or concentrated inside the tree roots. In both species, maximum concentrations were observed after 2 weeks in the roots of the high concentration (HC) treatment (spruce: 2512 ± 304 μg NPs per g DW (dry weight), wild service tree: 1190 ± 823 μg NPs per g DW). In the aboveground organs (stems and leaves or needles), concentrations were one to two orders of magnitude lower than in the roots. Despite relatively similar NPs concentrations in the tree aboveground organs across treatments, there were different temporal impacts on tree physiology of the given species. Photosynthetic efficiency was reduced faster (after 2 weeks of NPs exposure) and more intensively (by 28% in the HC treatment) in wild service trees compared to Norway spruce (ca. 10% reduction only after 4 weeks). Our study shows that both, evergreen coniferous as well as deciduous broadleaf tree species are negatively affected in their photosynthesis by NPs uptake and transport to aboveground organs. Given the likelihood of trees facing multiple, concurrent stressors from anthropogenic pollution and climate change, including the impact of NPs, it is crucial to consider the cumulative effects on vegetation in future.

Evaluation of Wood Anatomical Properties from 18 Tree Species in the Subtropical Region of China

The subtropical region of China possesses abundant broad-leaf tree species resources; however, the anatomical properties and microstructure of the wood are still unclear, which restricts the processing and utilization of wood. In this study, 14 broad-leaf trees and four coniferous trees were selected. Wood anatomical indices and wood microanatomy were used to evaluate the wood properties using a comprehensive index method. The results have shown that Dalbergia assamica exhibited the highest wood basic density among the 14 broad-leaved tree species, accompanied by a significant fiber proportion and vessel lumen diameter but a small vessel proportion and a high number of wood rays. Conversely, Parakmeria lotungensis and Michelia chapensis had relatively low wood basic densities, rendering them less suitable as valuable broad-leaved wood sources. Altingia chinensis, Castanopsis kawakamii, and the remaining 11 tree species exhibited medium-level wood basic densities. The 14 broad-leaved tree species had medium-length fibers. Phoebe bournei, Dalbergia assamica, and Castanopsis kawakamii demonstrated relatively high fiber proportion. Altingia chinensis, Dalbergia assamica, and Castanopsis kawakamii exhibited a large number of wood rays, making their wood more susceptible to cracking, whereas other broad-leaved tree species possessed fewer wood rays. The findings have provided a scientific basis for the exploration of precious broad-leaved tree resources and wood use.

Representation of in situ forest genetic resources of broadleaf tree species according to forest types in the Right-Bank Forest-Steppe of Ukraine

The article analyzes the formation and localization of in situ forest genetic resource conservation facilities in the conditions of the Right Bank Forest-Steppe of Ukraine in terms of the main forest types. Peculiarities of the state and selection structure of the forest genetic reserves (FGR) and plus stands (PS) were studied. The established that the largest area of forest genetic reserves and plus stands of broadleaves forests within the Right Bank Forest-Steppe is concentrated in the Vinnytsia region, based on the results of the research. The largest number and largest areas of these objects are concentrated in this region. It was established that the largest area of FGR located in fresh forest type conditions — 3475.9 ha, according to the distribution of forest genetic reserves and plus stands. The largest number of objects — 92 (78.0%) — is concentrated in this type of forest. The largest area of FGR is concentrated in fresh hornbeam beech — 842.3 ha in the Roztoch-Opil district of the forest typological region. There are 44 genetic reserves located in the conditions of the Podilsk sector of the Polysk-Prikarpattia district of wet hornbeam forests of the forest typological region of the wet cluster 3d. The largest area of forest stands is fresh beech forest — 234.0 ha. There are 40 FGR with a total area of 2,001 hectares of fresh hornbeam forests in the Podilskyi and Pravoberezhny sectors of the Dnipro tipologycal district. Forest genetic reserves are mainly concentrated in fresh hornbeam forest — 1584.6 hectares (29 units). The representation of the objects of in situ conservation of the gene pool of forest tree species in terms of forest types is insufficient and reflects only 16% of the entire diversity of forest ecosystems within the boundaries of the Roztoch-Opil district and 21–24% of the Dnipro districts of Polisko-Prikarpatskiy. This requires the introduction of measures for the additional creation of forest genetic reserves in other types of forest with the aim of the widest possible representation of the ecosystem diversity of the forest landscapes of the region.

The effect of mixed forest identity on soil carbon stocks in Pinus massoniana mixed forests

Increased productivity generally promotes the accumulation of soil organic carbon (SOC) stocks. The productivity of mixed forests is mainly influenced by plant species richness (PSR), mixed forest age (MFA), and mixed species proportion (MSP). However, the influence of PSR, MFA, and MSP on SOC stocks along the soil profiles in Pinus massoniana mixed forests remains to be determined. We conducted a meta-analysis employing paired observations of SOC stocks from 1010 paired mixed and pure stands of P. massoniana from 110 publications. The findings revealed that SOC stocks were highly dependent on MFA and increased with increasing MFA in various soil layers, rather than the expected influence of PSR. MFA contributed 48.97 %, 83.20 %, and 38.41 % to the increased SOC stocks in the topsoil, midsoil, and subsoil, respectively. Furthermore, MSP also significantly affected the increase in SOC stock in the topsoil and midsoil when 40 % < MSP ≤ 60 %. Over the next 60 years, subsoil SOC accumulation will be limited by increased PSR and MSP in mixed forests. Mixing between P. massoniana and broadleaf tree species (especially Schima superba and Lespedeza bicolor) significantly enhanced SOC stocks along the soil profiles. SOC stocks along the soil profiles decreased with increasing dominant mixed tree species richness (e.g., broadleaf, deciduous broadleaf, arbuscular mycorrhizal, and the sum of conifer and broadleaf trees). Incorporating lower PSR (e.g., 2 ≤ N ≤ 10) and dominant mixed tree species richness (e.g., N = 2) practices may be optimization options for increasing SOC stocks. Overall, based on the expected goals, including optimizing productivity, enhancing carbon storage, mitigating climate change, and promoting biodiversity conservation, we emphasize the importance of incorporating MFA, MSP, tree species identity, and subsoil into forest management.

Drivers and spatiotemporal patterns of post-drought growth resilience of four temperate broad-leaved trees

With the intensification of climate warming, more frequent and hotter droughts are a significant risk to forest ecosystems around the globe, including temperate biomes. To accurately predict the response of forests to drought, it is necessary to provide comprehensive information on the resilience of tree growth, its spatiotemporal changes and drivers. This information is often lacking for broadleaf species in temperate regions. To fulfill this aim, we analyzed 3177 ring-width series from 1981 trees covering 40 sites for four dominant broad-leaf tree species (Fraxinus mandshurica, Phellodendron amurense, Juglans mandshurica, and Quercus mongolica) in temperate forests in northeastern China. We quantified the resilience (resistance and recovery) of tree growth to extreme droughts. Minimum temperatures in most seasons and warm conditions and drought stress in early summer were the main factors limiting growth. Among the four tree species, Q. mongolica had the weakest growth resistance to drought and the fastest growth recovery from drought. Juglans mandshurica had a significantly (p < 0.05) higher drought resilience than F. mandshurica and P. amurense. Tree resilience presented a clear spatial pattern driven by differences in site condition and local climate. The resistance (recovery) to drought decreased (increased) with increasing latitude and longitude, and increased (decreased) with elevation. Q. mongolica exhibited a decrease in resistance and increase in recovery through time, whereas the other three tree species showed the opposite pattern. Fraxinus mandshurica, J. mandshurica, and Q. mongolica had high resistance (recovery) in warmer-wetter (colder-drier) regions. In contrast, P. amurense had reduced recovery in cold sites and low resistance in warm sites. The resistance and recovery indices of these sympatric tree species confirmed that different tree species have distinct strategies to cope with drought that is influenced by tree condition and local environment. The influence of biogeographical factors is often more important than other factors and should be considered when studying forest resilience to drought.

Aquatic hyphomycete assemblages of Abies sachalinensis leaf litter immersed in a stream in Hokkaido, Japan

ABSTRACT Aquatic hyphomycetes are key decomposers of plant-litter in freshwater environments. Previous studies have shown that conifer needles are colonized less frequently by aquatic hyphomycetes than broad-leaf species. Sakhalin fir (Abies sachalinensis) is the most common coniferous tree species planted for timber production in Hokkaido, Japan. Herein, we investigated aquatic hyphomycete assemblages involving Sakhalin fir needle litter and compared them with those of broad-leaf tree species, including Japanese oak (Quercus crispula) and Japanese linden (Tilia japonica), during immersion in a stream. Seventy percent of the total fungal species observed in fir needles were also observed in broad-leaf litter. However, the structure of the aquatic hyphomycete assemblages differed between the fir and two broad-leaf species, especially in the middle to late stages of the immersion period. The increase in species number and conidia number in fir needles was slower than that in oak leaves in the early stages of the immersion period. However, they continued to increase rapidly relative to the broad-leaf tree litter during the middle stages of the immersion period. These results suggest that fir needle litter was not difficult for aquatic hyphomycetes to colonize in the middle to late stages of the immersion period.

Decoupling between stomatal conductance and photosynthesis occurs under extreme heat in broadleaf tree species regardless of water access.

High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests worldwide. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (Anet ) and stomatal conductance (gs ). However, evidence for decoupling of gs from Anet under extreme heat has been found. Such a response both enhances survival of leaves during heat events but also quickly depletes available water. To understand the prevalence and significance of this decoupling, we measured leaf gas exchange in 26 tree and shrub species growing in the glasshouse or at an urban site in Sydney, Australia on hot days (maximum Tair > 40°C). We hypothesized that on hot days plants with ample water access would exhibit reduced Anet and use transpirational cooling leading to stomatal decoupling, whereas plants with limited water access would rely on other mechanisms to avoid lethal temperatures. Instead, evidence for stomatal decoupling was found regardless of plant water access. Transpiration of well-watered plants was 23% higher than model predictions during heatwaves, which effectively cooled leaves below air temperature. For hotter, droughted plants, the increase in transpiration during heatwaves was even more pronounced-gs was 77% higher than model predictions. Stomatal decoupling was found for most broadleaf evergreen and broadleaf deciduous species at the urban site, including some wilted trees with limited water access. Decoupling may simply be a passive consequence of the physical effects of high temperature on plant leaves through increased cuticular conductance of water vapor, or stomatal decoupling may be an adaptive response that is actively regulated by stomatal opening under high temperatures. This temperature response is not yet included in any land surface model, suggesting that model predictions of evapotranspiration may be underpredicted at high temperature and high VPD.

Species-specific growth responses to local and regional climate variability indicate the presence of a diversity portfolio effect in mediterranean tree assemblages

Understanding the role of tree biodiversity in stabilizing ecosystem processes is of paramount importance in forest ecology, especially because of increasing temperatures and droughts. The portfolio effect is one of the main hypotheses explaining the biodiversity-stability relationship that has been defined as a variance reduction effect of biodiversity in an ecosystem process, so that fluctuations over time in highly diverse communities become significantly smaller in low-diversity communities. In this work, two main objectives were established: to analyze a potential portfolio effect in a tree assemblage through functional mechanisms of compensation, and to identify the local climatic factors and teleconnections responsible for the interannual variations in the growth of tree species. The study was carried out in a region of the Iberian Peninsula with transitional climatic characteristics that allow the coexistence of needleleaf and broadleaf tree species with a wide variety of strategies and functional traits allowing them to respond differentially to climatic conditions. A dendrochronological approach was followed to analyze tree growth in relation to climate factors. We found that the magnitude and sensitivity of the response to both local climatic elements and teleconnections were species-dependent and that during climatically extreme years, the species exhibited marked differences in tree growth and compensatory responses that contributed to an increase in temporal stability. These results point to the existence of a portfolio effect in Mediterranean woodlands that supports tree diversity as an effective driver of ecosystem stability. Although the combination of dry and hot years always has a negative effect on tree growth, the association of coniferous and broadleaved (evergreen and marcescent) species at our study site could have a significantly positive influence by increasing temporal stability at landscape level through complementary functional responses.

Physiological and proteomic changes of Castanopsis fissa in response to drought stress

Castanopsis fissa is a native, broadleaf tree species in Guangdong with characteristics of barrenness and fast growth and is often used as a pioneer species for vegetation restoration with excellent ecological benefits. To explore the response of C.fissa to drought, this study investigated the drought tolerance mechanism of C.fissa using physiological and proteomic assessments. Using a potted continuous drought experimental method with normal water supply as a control, we measured photosynthetic parameters, antioxidant enzyme activities, and osmoregulatory substances of C. fissa in response to drought stress for 1 to 4 weeks, respectively. In addition, we used TMT quantitative proteomics to identify differentially expressed proteins (DEPs) between the drought-stress-treated C. fissa leaves and the control leaves. With the extension of drought stress time, the photosynthetic indexes and peroxidase (POD) activity of C. fissa leaves showed a decreasing trend. The malondialdehyde (MDA) content; superoxide Dismutase (SOD) and catalase (CAT) activities; and proline (Pro), soluble sugar (SS) and soluble protein (SP) contents showed an overall increasing trend, all of which reached significant differences at 4 w of stress. We identified 177 and 529 DEPs in the 2 and 4 weeks drought-stress leaves, respectively, in reference to the control leaves. These DEPs were closely related to physiological metabolic processes such as photosynthesis, energy and carbohydrate metabolism, stress response and defense, transcriptional regulation, and signal ion transduction. Drought stress mainly affects photosynthesis, carbohydrate metabolism, and protein synthesis and degradation in C. fissa leaves. At 2 weeks of stress, the expression of carbon metabolism, pyruvate metabolism and ribosome-related proteins was significantly changed, however, and at 4 weeks of stress, protein processing in the endoplasmic reticulum and spliceosome-related proteins were significantly increased in plant leaves. To alleviate the effect of water unavailability, the drought-stressed C.fissa leaves increased its oxidative protective enzyme system to eliminate excess reactive oxygen species (ROS) and also increased its Pro and SP contents to maintain the intracellular osmotic potential balance.