Mixing oak and pine trees in Mediterranean forests increases aboveground hydraulic dysfunctions.

<p>Mixed species forest stands have been advocated over monoculture stands for afforestation around the globe as they can be more resilient to pests and diseases. However, in arid and semi-arid regions, whether such forests are suitable for future climate conditions remains to be addressed. The overall objective of this study is to analyze ecohydrological processes of indigenous, drought-tolerant tree species in a Mediterranean mixed plantation forest. The specific objectives are i) to quantify transpiration of pine (<em>Pinus brutia</em>) and cypress (<em>Cupressus sempervirens</em>) trees and ii) to analyze the effects of environmental variables (meteorology and soil moisture) on transpiration. The study site is located in Athalassa Forest Park, in Cyprus. The site has a surface area of 10 ha with an average slope of 4%. Average annual rainfall is 315 mm with a mean daily minimum temperature of 5° C during winter and a mean daily maximum temperature of 37° C during summer. The site was converted in 2011 from rainfed agriculture to a mixed forest by planting seedlings of different tree and shrub species. The study site is located on two sedimentary formations: Athalassa (calcarenites interlayered with sandy marls) and Nicosia (siltstones and layers of calcarenites). Soil depths up to 1 m can be found on top of the impervious and semi-pervious strata. </p><p>The research field was stratified in two spatial geological units (strata). In each stratum, two <em>P. brutia</em> and two <em>C. sempervirens</em> trees were randomly selected (total eight trees) for sap flow monitoring with sensors (heat ratio method) attached to the tree trunks. In addition to the random trees, two representative (one per species) neighboring trees were selected where sap-flow sensors were installed and mid-day leaf water potential (pressure chamber) and stomatal conductance (porometer) were measured. Forty-five soil moisture sensors were installed between the representative trees at depths of 10 cm, 30 cm and 50 cm.</p><p>Data from November 2020 to January 2021 indicated that mean sap flow rate per tree (cm<sup>3</sup> h<sup>-1</sup>) is higher for <em>C. sempervirens</em> (min: 161, max: 503) than <em>P. brutia </em>(min: 68, max: 266). Total rainfall during these months was 88 mm, most of which fell in three main rainfall events (between 20 and 30 mm per event). Mean soil moisture before rain (15-day average) was 5% for all soil depths. After the rain, soil moisture was 12% for 10 and 30 cm depths and 8% for 50 cm. The increase in soil moisture resulted in 1.6 times higher transpiration for <em>C. sempervirens</em> and 1.4 times higher transpiration for <em>P. brutia</em>. The leaf water potential of <em>C. sempervirens</em> increased from -2.6 MPa before the rain to -0.8 MPa after the rain, whereas it remained near -0.5 MPa for <em>P. brutia</em>. This research of the different plant water-use strategies can contribute to an improved selection of species for afforestation in arid and semi-arid regions.</p><p>This research has received support from the Water JPI (Joint Call 2018) FLUXMED Project, funded through the Cyprus Research and Innovation Foundation.</p>

This study aimed to (i) investigate the congruence among the species composition and diversity of bryophytes and vascular plants in forests; (ii) test if site prioritization for conservation aims by the maximization of the pooled number of vascular plant species is effective to maximize the pooled number of bryophyte species. The study was performed in six forests in Tuscany, Italy. Four-hundred and twenty vascular plant species (61 of which were woody) and 128 bryophyte species were recorded in 109 plots. Despite the good predictive value of the compositional patterns of both woody plants and total vascular with respect to the compositional pattern of bryophytes, the species richness of the latter was only marginally related to the species richness of the former two. Bryophyte rare species were not spatially related to rare plant species and neither coincided with the sites of highest plant species richness. The species accumulation curves of bryophytes behaved differently with respect to those of woody plants or total vascular plants. Reserve selection analysis based on the maximization of the pooled species richness of either woody plants or total vascular plants were not effective in maximizing the pooled species richness of bryophytes. This study indicates that species diversity of vascular plants is not likely to be a good indicator of the bryophyte species diversity in Mediterranean forests.

In resprouting species, fire-induced topkill causes a reduction in height and leaf area without a comparable reduction in the size of the root system, which should lead to an increase in the efficiency of water transport after fire. However, large plants undergo a greater relative reduction in size, compared with small plants, so we hypothesized that this enhancement in hydraulic efficiency would be greatest among large growth forms. In the ecotone between long-leaf pine (Pinus palustris Mill.) savannas and wetlands, we measured stomatal conductance (gs), mid-day leaf water potential (Ψleaf), leaf-specific whole-plant hydraulic conductance (KL.p), leaf area and height of 10 species covering a range of growth forms in burned and unburned sites. As predicted, KL.p was higher in post-fire resprouts than in unburned plants, and the post-fire increase in KL.p was positively related to plant size. Specifically, large-statured species tended to undergo the greatest relative reductions in leaf area and height, and correspondingly experienced the greatest increases in KL.p. The post-fire increase in KL.p was smaller than expected, however, due to a decrease in absolute root hydraulic conductance (i.e., not scaled to leaf area). The higher KL.p in burned sites was manifested as an increase in gs rather than an increase in Ψleaf. Post-fire increases in gs should promote high rates of photosynthesis for recovery of carbohydrate reserves and aboveground biomass, which is particularly important for large-statured species that require more time to recover their pre-fire size.

in adult oaks (Quercus petraea and Q robur) during a period of water shortage in a simplified lysimeter. Sap flux densities and stomatal conductance were reduced by = 70% at maximal drought intensity. Predawn leaf water potential then ranged from -1.7 to -2.0 MPa. The slightly lower transpiration observed in pedunculate oaks could be ascribed to their smaller crown development. Nevertheless, no significant difference in stomatal conductance could be observed between the two species. They also had the same percent loss of conductivity (= 80%) in petioles at maximal drought intensity when midday leaf water potential had dropped to = -3.0 MPa. Finally, good agreement was found between observed losses of hydraulic conductivity during in situ dehydration and the vulnerability curves obtained under laboratory conditions. The shifts in maximal conductivity observed in some droughted trees probably accentuated discrepancies between field and laboratory data. However, a correction procedure helped overcome these artifacts.

Using a combination of remote sensing data, Pinus pinaster Ait. and Pinus pinea L. were distinguished at individual tree level in mixed Mediterranean stands with over 95% accuracy. This approach is easily applicable over large areas, enhancing the economic value of non-wood forest products, stone pine nuts, and resin, and aiding forest managers to accurately predict this production. The discrimination of tree species at individual level in mixed Mediterranean forest based on remote sensing is a field which has gained greater importance. In these stands, the capacity to predict the quality and quantity of non-wood forest products is particularly important due to the very different goods the two species produce. To assess the potential of using low-density airborne LiDAR data combined with high-resolution Pleiades images to discriminate two different pine species in mixed Mediterranean forest (Pinus pinea L. and Pinus pinaster Ait.) at individual tree level. A Random Forest model was trained using plots from the pure stand dataset, determining which LiDAR and satellite variables allow us to obtain better discrimination between groups. The model constructed was then validated by classifying individuals in an independent set of pure and mixed stands. The model combining LiDAR and Pleiades data provided greater accuracy (83.3% and 63% in pure and mixed validation stands, respectively) than the models which only use one type of covariables. The automatic crown delineation tool developed allows two very similar species in mixed Mediterranean conifer forest to be discriminated using continuous spatial information at the surface: Pleiades images and open source LiDAR data. This approach is easily applicable over large areas, enhancing the economic value of non-wood forest products and aiding forest managers to accurately predict production.

Genotypic variation in water relations and gas exchange of urban trees in Detroit, Michigan, USA

Aim of study: We aimed at identifying differences in the response to water shortage between Eucalyptus globulus clones. Area of study: Field trials were established in SW Spain. Material and methods: Potted plants from six clones were grown in a greenhouse for 35 days under two watering regimes. Two clones were F0 genotypes and the other four were F1 (hybrid) genotypes, including one inbred clone. Differences in stomatal conductance, hydraulic traits, growth and specific leaf area (SLA) were analyzed. Main results: Water shortage decreased SLA, growth in height and leaf area and leaf-specific hydraulic conductivity (KLmax). We measured the highest growth in F1 genotypes and the lowest in the clone in which SLA was lowest. The inbred clone showed the highest growth reduction under water shortage. There was substantial hysteresis between leaf water potential (Y) and native embolism, most probably a result of combined cavitation and refilling. High losses of hydraulic conductance were compatible with high stomatal conductances. Maximum values of stomatal conductance decreased with the soil water content estimated from predawn Y and were lowest in the inbred clone, showing less plasticity and a diminished ability to cope with high temperatures, which could explain its poor development under field conditions. Research highlights: Soil water content and predawn Y appeared as critical factors controlling stomata closure, while stomatal conductance and SLA could be useful to predict differences in growth and survival from early trials.

Limited information is available on the variation of plant leaf hydraulic traits in relation to soil rock fragment content (RFC), particularly for xerophytes native to rocky mountain areas. In this study, we conducted a field experiment with four gradients of RFC (0, 25, 50 and 75% ν ν-1) on three different xerophytic species (Sophora davidii, Cotinus szechuanensis and Bauhinia brachycarpa). We measured predawn and midday leaf water potential (Ψleaf), leaf hydraulic conductance (Kleaf), Ψleaf induced 50% loss of Kleaf (P50), pressure-volume curve traits and leaf structure. A consistent response of hydraulic traits to increased RFC was observed in three species. Kleaf showed a decrease, whereas P50 and turgor loss point (Ψtlp) became increasingly negative with increasing RFC. Thus, a clear trade-off between hydraulic efficiency and safety was observed in the xerophytic species. In all three species, the reduction in Kleaf was associated with an increase in leaf mass per area. In S. davidii, alterations in Kleaf and P50 were driven by leaf vein density (VLA) and Ψtlp. In C. szechuanensis, Ψtlp and VLA drove the changes in Kleaf and P50, respectively. In B. brachycarpa, changes in P50 were driven by VLA, whereas changes in both Kleaf and P50 were simultaneously influenced by Ψtlp. Our findings suggest that adaptation to increased rockiness necessarily implies a trade-off between leaf hydraulic efficiency and safety in xerophytic species. Additionally, the trade-off between leaf hydraulic efficiency and safety among xerophytic species is likely to result from processes occurring in the xylem and the outside-xylem hydraulic pathways. These findings contribute to a better understanding of the survival strategies and mechanisms of xerophytes in rocky soils, and provide a theoretical basis for the persistence of xerophytic species in areas with stony substrates.

Non‐structural carbohydrates (NSC) are essential for the osmotic adjustment and maintenance of the hydraulic functioning of trees, but knowledge about the relationship between NSC dynamics and hydraulics during drought stress is still limited, especially in dioecious plants. We investigated photosynthetic carbon assimilation, xylem hydraulics and related functional traits and explored whether hydraulics are linked to NSC dynamics in the leaves and branches of Populus euphratica females and males along an aridity gradient in the Xinjiang Province, China. Both sexes of P. euphratica had increased intrinsic water use efficiency (WUE), percent loss of conductivity (PLC) and xylem pressure inducing 50% loss of hydraulic conductivity (P50), but decreased net photosynthetic rate (Pn), sapwood‐specific hydraulic conductivity (Ks) and hydraulic safety margin (leaf mid‐day water potential‐P50, HSM50) associated with the reduction in the soil water content. Furthermore, females and males have different hydraulic strategies related to NSC dynamics under low soil water conditions. Males had higher Ks, wood density (WD), HSM50, Pn, WUE, leaf dry mass per area (LMA) and leaf soluble sugar levels and lower branch soluble sugar levels, PLC and P50 values than females under extreme drought conditions, indicating that males had a more resistant xylem and can maintain water flow and leaf turgor probably due to the greater availability of soluble sugars to be used for osmotic adjustments. In addition, females had a lower Ks, WD and LMA and higher branch soluble sugar levels and PLC, implying that females were more vulnerable to cavitation and required higher branch soluble sugar levels for embolism repair under extreme drought conditions. Synthesis. Due to the spatial sexual segregation across resource gradients, dioecious plants are more vulnerable to rapid climate change. The different hydraulic strategies linked to NSC dynamics between females and males may result in a situation that one sex is more prone to an increasingly long and intense drought than the other one. This study improves our predictions for future climate change impacts on dioecious P. euphratica and provides theoretical knowledge for restoration and afforestation in P. euphratica forests. Read the free Plain Language Summary for this article on the Journal blog.

The impacts of climate change, such as increased soil dryness and nutrient deficiency, highlight the need for environmentally sustainable restoration of forests and groundwater resources. However, it is important to consider that extensive afforestation efforts may lead to a depletion of groundwater supply due to higher evapotranspiration rates, exacerbating water scarcity issues. Consequently, we conducted a study to examine how the fast-growing tree species Populus sibirica (Horth ex Tausch) and Ulmus pumila (L.) respond morpho-physiologically to varying watering regimes and types of fertilizers, aiming to better understand their specific water and nutrient requirements. We used two-year-old nursery-growth seedlings (N=512) of P.sibirica and U. pumila with initial root collar diameter (RCD) and the height of 0.51±0.02mm and 68±2.94 cm and 0.33±0.01mm and 51±1.14 cm, respectively. The leaf area (LA), specific leaf area (SLA), chlorophyll concentration, stomatal conductance (gs), chlorophyll fluorescence, and predawn and midday leaf water potential were measured across treatments. Four different irrigation regimes and two different fertilizer types were applied: no irrigation (control, 0 L h-1), 2 L h-1 = 0.25 mm m-2, 4L h-1 = 0.5mm m-2, 8 L h-1 = 1.0 mm m-2 and 120 g and 500 g tree-1 of NPK and compost (COMP). Twelve plots (600 m2) were established in the study site for each species and treatments. During the first growing season (2021), the LA of P. sibirica was larger in the 4-8 L h-1 without fertilizer, but it was smaller in the 4 L h-1+ COMP during the second growing season (2022). The 2 L h-1 without fertilizer and 2 L h-1 + NPK had larger LA compared with the control (CONT) for the first and second growing seasons, respectively, for U. pumila. P. sibirica seedlings at 4 L h-1 without fertilizer had the highest SLA for 2021 and at 2 L h-1 + NPK for 2022, whereas CONT and 4 L h-1 had the highest SLA than the other treatments for 2021 and 2022 growing seasons, respectively, for U. pumila. The chlorophyll concentration of P. sibirica seedlings in the first year was generally higher in CONT, while the 2 L h-1 without any fertilizer yielded a significantly higher chlorophyll concentration of U. pumila. Chlorophyll fluorescence parameters (PIABS and Fm) were generally lower in CONT with/without NPK or COMP for both species. The CONT with NPK/COMP generally had a higher gs compared with the other treatments in both experimental periods for U. pumila, whereas CONT and 2 L h-1+ NPK-treated P. sibirica seedlings had a significantly greater gs during the first year and second year, respectively. The predawn and midday leaf water potentials of both species were generally the lowest in CONT, followed by 2 L h-1+ NPK/COMP during the first growing season, but a different pattern was observed during the second growing season. Overall, the morpho-physiological traits of the two species were affected by watering and fertilizer treatments, and the magnitude of the effects varied depending on growing season, amount of irrigation, and fertilizer type, and their interactions.

Leaf morphological and physiological traits control the carbon and water relations of mature trees and are determinants of drought tolerance, but it is not well understood how they are modified in response to water deficits. We analysed five sun-canopy leaf traits (mean leaf size (LS), specific leaf area (SLA), Huber value (HV), water potential at turgor loss point (Ψtlp ) and foliar carbon isotope signature (δ13 C)) in European beech (Fagus sylvatica L.) across three precipitation gradients sampled in moist (2010), dry (2019) and very dry (2018) summers, and tested their response to short-term water deficits (climatic water balance (CWB) preceding sample collection) and long-term water availability (mean annual precipitation (MAP), plant-available soil water capacity (AWC) and neighbourhood competition). Across the 34 sites, LS varied seven-fold (3.9-27.0 cm2 ), SLA four-fold (77.1-306.9 cm²·g-1 ) and HV six-fold (1.0-6.65 cm2 ·m-2 ). In the 2018 dataset, LS showed a negative and HV a positive relationship to MAP, which contradicts relations found in multi-species samples. Average Ψtlp ranged from -1.90 to -2.62 MPa and decreased across the sites with decreasing CWB in the month prior to measurement, as well as with decreasing MAP and AWC in 2019. Studied leaf traits varied considerably between years, suggesting that mast fruiting and the severe 2018 drought caused the formation of smaller leaves. We conclude that sun-canopy leaf traits of European beech exhibit considerable plasticity in response to climatic and edaphic aridity, and that osmotic adjustment may be an important element in the drought response strategy of this anisohydric tree species.

Several mistletoe species are able to grow and reproduce on both deciduous and evergreen hosts, suggesting a degree of plasticity in their ability to cope with differences in intrinsic host functions. The aim of this study was to investigate the influence of host phenology on mistletoe water relations and leaf gas exchange. Mistletoe Passovia ovata parasitizing evergreen (Miconia albicans) hosts and P. ovata parasitizing deciduous (Byrsonima verbascifolia) hosts were sampled in a Neotropical savanna. Photosynthetic parameters, diurnal cycles of stomatal conductance, pre-dawn and midday leaf water potential, and stomatal anatomical traits were measured during the peak of the dry and wet seasons, respectively. P. ovata showed distinct water-use strategies that were dependent on host phenology. For P. ovata parasitizing the deciduous host, water use efficiency (WUE; ratio of photosynthetic rate to transpirational water loss) was 2-fold lower in the dry season than in the wet season; in contrast, WUE was maintained at the same level during the wet and dry seasons in P. ovata parasitizing the evergreen host. Generally, mistletoe and host diurnal cycles of stomatal conductance were linked, although there were clear differences in leaf water potential, with mistletoe showing anisohydric behaviour and the host showing isohydric behaviour. Compared to mistletoes attached to evergreen hosts, those parasitizing deciduous hosts had a 1.4-fold lower stomatal density and 1.2-fold wider stomata on both leaf surfaces, suggesting that the latter suffered less intense drought stress. This is the first study to show morphophysiological differences in the same mistletoe species parasitizing hosts of different phenological groups. Our results provide evidence that phenotypical plasticity (anatomical and physiological) might be essential to favour the use of a greater range of hosts.

We investigated functional coordination between branch hydraulic properties and leaf functional traits among nine miombo woodlands canopy tree species differing in habitat preference and phenology. Specifically, we were seeking to answer the question: are branch hydraulic properties coordinated with leaf functional traits linked to plant drought tolerance in seasonally dry tropical forests and what are the implications for species habitat preference? The hydraulic properties investigated in this study were stem area specific hydraulic conductivity (KS), Huber value (Hv), and xylem cavitation vulnerability (Ψ50). The leaf functional traits measured were specific leaf area (SLA), leaf dry matter content (LDMC), and mean leaf area (MLA). Generalists displayed significantly (P < 0.05) higher cavitation resistance (Ψ50) and SLA, but lower sapwood specific hydraulic conductivity (KS), leaf specific conductivity (KL), MLA, and LDMC than mesic specialists. Although MLA was uncorrelated with Ψ50, we found significant (P < 0.05) positive and negative correlations between plant hydraulic properties and leaf functional traits linked to plant drought tolerance ability, indicating that the interactions between branch hydraulics and leaf functional traits related to plant drought tolerance ability may influence tree species habitat preference in water-limited ecosystems.

In alpine rocky desertification areas, environmental stress poses challenges to vegetation restoration and protection. Merely observing the changes in specific leaf area driven by environmental factors may overlook the risk of non-tree vegetation degradation. The leaf resource allocation strategies of non-tree plants need to be focused on. In the alpine rocky desertification areas of the Jinsha River Basin, three vegetation types were investigated. The leaf traits, vegetation coverage, species diversity of non-tree plants, and soil total nitrogen, rock bareness degree were measurement. An increase in altitude led to a decrease in vegetation coverage and an increase in species diversity. In grasslands with exceeded 35% rock bareness degree, the increase in species diversity intensified competition pressure, resulting in a decrease in the specific leaf area. In forests with less than 20% rock bareness degree, the species of shrubs have become homogeneous, resulting in a decrease in vegetation coverage but an increase in the specific leaf area. But due to environmental stress, the leaf resource allocation of different species may have favored leaf dry weight (allometric index < 1.0). An increase in soil total nitrogen alleviated environmental stress, causing leaf resources to be allocated to both leaf dry weight and leaf area (allometric index ≁1.0). However, it enhanced the above-ground competitiveness of few dominant species, squeezing out the living space of auxiliary species, and vegetation degradation risk increased. Species with similar specific leaf areas can have different leaf resource allocation strategies. By combining the changes in specific leaf area with leaf resource allocation strategies, the development of vegetation under environmental stress can be accurately revealed.

Question: Do coexisting plant life forms differ in overall phenology, leaf traits and patterns of leaf litterfall?Location: Patagonian Monte, Chubut Province, Argentina.Methods: We assessed phenology, traits of green and senesced leaves and the pattern of leaf litterfall in 12 species of coexisting life forms (perennial grasses, deciduous shrubs, evergreen shrubs).Results: We did not identify differences in phenology, leaf traits and patterns of leaf litterfall among life forms but these attributes contrasted among species. Independent of the life form, the maintenance of green leaves or vegetative growth during the dry season was mostly associated with leaves with high leaf mass per area (LMA) and high concentration of secondary compounds. Low LMA species produced low litterfall mass with low concentration of secondary compounds, and high N concentration. High LMA species produced the largest mass of leaf litterfall. Accordingly, species were distributed along two main dimensions of ecological variation, the dimension secondary compounds in leaves ‐ length and timing of the vegetative growth period (SC ‐ VGP) and the dimension leaf mass per area ‐ leaf litterfall mass (LMA ‐ LLM).Conclusions: Phenology, leaf traits and leaf litterfall varied among species and overlapped among life forms. The two dimensions of ecological variation among species (SC ‐ VGP, LMA ‐ LLM) represent distinct combinations of plant traits or strategies related to resource acquisition and drought tolerance which are reflected in the patterns of leaf litterfall.