Water-use strategies in two co-occurring Mediterranean evergreen oaks: surviving the summer drought

In the Mediterranean evergreen oak woodlands of southern Portugal, the main tree species are Quercus ilex ssp. rotundifolia Lam. (holm oak) and Quercus suber L. (cork oak). We studied a savannah-type woodland where these species coexist, with the aim of better understanding the mechanisms of tree adaptation to seasonal drought. In both species, seasonal variations in transpiration and predawn leaf water potential showed a maximum in spring followed by a decline through the rainless summer and a recovery with autumn rainfall. Although the observed decrease in predawn leaf water potential in summer indicates soil water depletion, trees maintained transpiration rates above 0.7 mm day(-1) during the summer drought. By that time, more than 70% of the transpired water was being taken from groundwater sources. The daily fluctuations in soil water content suggest that some root uptake of groundwater was mediated through the upper soil layers by hydraulic lift. During the dry season, Q. ilex maintained higher predawn leaf water potentials, canopy conductances and transpiration rates than Q. suber. The higher water status of Q. ilex was likely associated with their deeper root systems compared with Q. suber. Whole-tree hydraulic conductance and minimum midday leaf water potential were lower in Q. ilex, indicating that Q. ilex was more tolerant to drought than Q. suber. Overall, Q. ilex seemed to have more effective drought avoidance and drought tolerance mechanisms than Q. suber.     

Branch growth and biomass allocation in Abies amabilis saplings in contrasting light environments

Aboveground biomass allocation, and height and branch growth were studied in saplings of the shade-tolerant conifer, Abies amabilis Dougl. ex Forbes growing in large openings and in the understory of an old-growth forest in western Oregon. The presence of annual overwintering budscale scars was used to infer extension growth histories; annual growth rings in branches and stems were used in combination with extension histories to compute partitioning of new biomass among leaves, branches and stems. Saplings growing in large gaps had conical crowns, whereas understory saplings had umbrella shaped crowns as a result of much greater rates of branch extension than stem extension. Understory saplings grew slowly in height because of low rates of biomass production and low allocation of biomass to stem extension. About 40% of new biomass was allocated to foliage in both groups, but understory saplings allocated more of the remaining growth increment to branches and less to stem than did saplings growing in large gaps. These results differ from the patterns observed in shade-tolerant saplings of tropical forests, where allocation to foliage increases with shading and branch allocation is much lower than observed here. This difference in allocation may reflect mechanical constraints imposed by snow loads on the evergreen A. amabilis crowns, particularly on flat-crowned understory saplings.     

Differential light responses of Mediterranean tree saplings: linking ecophysiology with regeneration niche in four co-occurring species

The ecophysiological mechanisms underlying plant-plant interactions and forest regeneration processes in Mediterranean ecosystems are poorly understood, and the experimental evidence for the role of light availability in these processes is particularly scant. We analyzed the effects of high and low irradiances on 31 ecological, morphological and physiological variables in saplings of four late-successional Mediterranean trees, two deciduous (Acer opalus subsp. granatense (Boiss.) Font Quer & Rothm. and Quercus pyrenaica Willd.) and two evergreen (Pinus nigra Arnold subsp. salzmannii (Dunal) Franco and Quercus ilex L.), which coexist in mature montane forests. Species differed in both their capacity to withstand high radiation and in their shade tolerance. The two deciduous species were the least tolerant to high radiation, exhibiting both dynamic and chronic photoinhibition in full sunlight, with severe implications for gas exchange and photosynthetic performance. Excess light severely limited the survival of A. opalus subsp. granatense, even minor reductions of excessive radiation (from full sunlight to 80% sunlight) being crucial for sapling survival. Among species, P. nigra was the most tolerant of high irradiances but showed the poorest shade acclimation. Interspecific differences in the mechanisms of response to light provide a partial explanation of the differential regeneration patterns previously reported for these species, with shade-tolerant plants (i.e., deciduous broadleaf species) benefiting the most from associations with nurse plants. We conclude that light availability is an important environmental factor defining the regeneration niche of Mediterranean woody species.     

Ontogenetic changes in stomatal and biochemical limitations to photosynthesis of two co-occurring Mediterranean oaks differing in leaf life span

A quantitative analysis was applied to the stomatal and biochemical limitations to light-saturated net photosynthesis under optimal field conditions in mature trees and seedlings of the co-occurring evergreen oak, Quercus ilex L., and the deciduous oak, Q. faginea Lam. Stomatal limitation to photosynthesis, maximal Rubisco activity and electron transport rate were determined from assimilation versus intercellular leaf carbon dioxide concentration response curves of leaves that were subsequently analyzed for nitrogen (N) concentration, mass per unit area, thickness and percent internal air space. In both species, seedlings had a lower leaf mass per unit area, thickness and leaf N concentration than mature trees. The root system of seedlings during their third year after planting was dominated by a taproot. A lower leaf N concentration of seedlings was associated with lower maximal Rubisco activity and electron transport rate and with assimilation rates similar to or lower than those of mature trees, despite the higher stomatal conductances and potential photosynthetic nitrogen-use efficiencies of seedlings. Consequently, stomatal limitation to photosynthesis increased with tree age in both species. In both seedlings and mature trees, a lower assimilation rate in Q. ilex than in Q. faginea was associated with lower stomatal conductance, N allocation to photosynthetic functions, maximal Rubisco activity and electron transport rate, and potential photosynthetic nitrogen-use efficiency but greater leaf thickness and leaf mass per unit area. Tree-age-related changes differed quantitatively between species, and the characteristics of the two species were more similar in seedlings than in mature trees. Despite higher stomatal conductances, seedlings are more N limited than adult trees, which contributes to lower biochemical efficiency.     

Growth,biomass allocation,and water use efficiency of 31 apple cultivars grown under two water regimes

Plant growth, biomass allocation, carbon isotope composition (δ¹³C), and water use efficiency (WUE) of 31 cultivars of apple (Malus domestica Borkh.) grown under two water regimes were measured. Drought-stressed plants showed significant declines in tree height, trunk diameter, biomass production, and total leaf area, the extent to which depended upon cultivar. Also, gas exchange rates, instantaneous and long-term efficiencies (WUE_I and WUE_L, respectively), and values for δ¹³C differed among cultivars and watering regimes. Variations in WUE_I were mainly due to changes in stomatal conductance (g _s) under drought condition. ‘Qinguan’ and ‘Golden Delicious’ had greater trunk diameter, tree height, and had higher biomass production and WUE_L under drought stress, implying that they are more suitable for arid and semi-arid regions. Moreover, WUE_L was significantly and positively correlated with δ¹³C under two watering regimes, which suggests a potential for evaluating water use efficiency of Malus by measuring carbon isotope composition.     

Dry mass production,allocation patterns and water use efficiency of two conifers with different water use strategies under elevated [CO<Subscript>2</Subscript>], warming and drought conditions

Knowledge regarding the interactive effects of elevated [CO₂], warming and drought on dry mass production, allocation and water use efficiency (WUE) of tree seedlings is limited, particularly in trees exhibiting different stomatal regulation strategies. Seedlings of Callitris rhomboidea (relatively anisohydric) and Pinus radiata (relatively isohydric) were grown in two [CO₂] (C_a (400 μmol mol^?1) and C_e (640 μmol mol^?1)) and two temperature (T_a (ambient) and T_e (ambient?+?4 °C)) treatments in a sun-lit glasshouse under well-watered conditions prior to imposition of the drought. C_e increased mass production in C. rhomboidea (but not in P. radiata), while drought limited mass production in both species. Mass production was greatest in the combination of C_e, T_e and well-watered conditions. Pinus radiata allocated relatively more dry mass into roots and had higher plant WUE than C. rhomboidea. Noticeably, mass allocation patterns in C. rhomboidea varied as a function of the treatments, but those of P. radiata were constant. C_e enhanced leaf WUE of both species, but to a greater degree under drought stress than well-watered conditions. Moderate drought stress increased both leaf and plant WUE compared to well-watered conditions. C. rhomboidea exhibited plasticity to variable climate conditions through morphological adjustments, while P. radiata exhibited a highly conservative strategy. Collectively, these findings indicate that the two species have different strategies in resource acquisition and utilisation under changing environmental conditions. Future studies on tree response to climate change need to fully consider the integration of species traits, including stomatal behaviour and hydraulic strategies.     

Thigmomorphogenesis versus light in biomechanical growth strategies of saplings of two tropical rain forest tree species

? In the dense tropical rainforest understorey, saplings exhibit different growth strategies aiming at reaching light levels better fitting their ecology. Investing mainly in height growth, at the expense of their width, a lot are close to mechanical instability. Tachigali melinonii, a long living heliophilic tree species, is frequently observed to be extremely slender and supported by neighbours. Such observations suggest an active growth control through the perception of mechanical environment.

? Mechanical environment or light availability, which one is the most influent on growth and slenderness (H/D)? To test this question, we recorded growth of control and staked saplings of two species with contrasting habits and ecology: T. melinonii, and Dicorynia guianensis, along a natural light gradient.

? Dicorynia, the more stable, responded more clearly to the staking treatment, showing slenderness increase when light is available, whereas for Tachigali, only light availability governed growth.

? For Tachigali, growth allocation is mainly governed by light availability and ontogeny, whereas Dicorynia is probably similar to the average tree strategy, using the thigmomorphogenetic physiological process to control its stability.

     

Leaf traits and tree rings suggest different water-use and carbon assimilation strategies by two co-occurring Quercus species in a Mediterranean mixed-forest stand in Tuscany, Italy

We compared the water-use characteristics of co-occurring mature Quercus cerris L. and Quercus pubescens Willd. trees growing in resource-limited (mainly water) hilly habitats in Tuscany, Italy. The species differed in their distribution along soil water gradients and in their access to, and use of, water, even though the study year was wetter than average, though with a summer drought. Compared with Q. cerris, Q. pubescens had greater access to soil water (less negative predawn water potentials) and a more conservative water-use strategy based on its relatively low stomatal conductance, high instantaneous water-use efficiency, less negative midday water potential and high soil-to-leaf hydraulic conductance. Quercus cerris had less conservative water-use characteristics than Q. pubescens, exhibiting relatively high stomatal conductance, low instantaneous water-use efficiency, more negative midday water potentials and low soil-to-leaf hydraulic conductance; however, Q. cerris had higher photosynthetic rates than Q. pubescens. Photosynthesis and stomatal conductance were positively correlated in both species. Although a strong correlation between ring widths and precipitation patterns was not found, some dry periods influenced ring-width growth. Quercus pubescens has always grown faster than Q. cerris, probably because of more efficient water use, although stand dynamics (driven by exogenous disturbance factors, including coppicing, browsing and competition) cannot be excluded. Ring-width variability, as well as tree-ring growth in dry years, which should be unaffected by stand dynamics, were higher in Q. pubescens than in Q. cerris. Moreover, Q. pubescens recovered completely after the drought in the seventies, even showing higher tree-ring growth than in the recent past, whereas Q. cerris showed a minor growth decline followed by a recovery to values comparable with those observed before the 1970s drought. Beginning in the early eighties, tree-ring growth decreased in both species, though Q. pubescens showed consistently higher values than Q. cerris. These differences can be explained by differences in water-use efficiency. Despite differences between the species in water use and water status, the results are consistent with the interpretation that both are drought tolerant, but that Q. pubescens is at an advantage on xeric ridges because of its greater ability to access soil water and use it more conservatively compared with Q. cerris.     

Divergence among species and populations of Mediterranean pines in biomass allocation of seedlings grown under two watering regimes

Seedlings of four populations each of Pinus pinaster, P. halepensis, P. canariensis and P. pinea were grown in controlled conditions to evaluate both inter- and intra-specific differences in response to watering. We submitted half of the plants to a moderate water stress and after 22 weeks, we recorded height, stem diameter and root, stem and leaves dry weight. Patterns and amounts of phenotypic changes, including changes in biomass allocation, were analysed. We found a scant response in P. canariensis, P. pinaster and P. halepensis presented high population divergence for phenotypic changes, and P. pinea showed marked allocational shifts and no population divergence. The phenotypic changes observed within species are interpreted as a plastic response. The variation encountered within P. halepensis and P. pinaster may be indicative of specialisation to either resource-rich or resource-poor habitats, being populations from favourable sites more plastic. P. pinea exhibited a very uniform plastic response, indicating generalist behaviour.     

Phenology and growth dynamics in Mediterranean evergreen oaks: Effects of environmental conditions and water relations

Budburst date and shoot elongation were measured in two mature Mediterranean evergreen oaks (Quercus suber and Quercus ilex) and their relationships with meteorological and tree water status (predawn leaf water potential) data were analysed. Experimental work took place at two sites: Mitra 2 - Southern Portugal (2002-2003) and Lezirias - Central Portugal (2007-2010). Quercus suber phenology was studied at both sites whereas Q. ilex was only studied at Mitra 2. Quercus suber budburst date occurred at a photoperiod around 13.8 h (± 0.26) - late April/early May - and was highly related to the average daily temperature in the period 25 March - budburst date (ca. 1.5 months prior to budburst), irrespective of site location. In that period, budburst date was much more dependent on average maximum than average minimum daily temperature. Base temperature and thermal time for Q. suber were estimated as 6.2 °C (within the reported literature values) and 323 degree-days, respectively. Q. ilex budburst occurred about 6 weeks earlier than in Q. suber (photoperiod: 12.3 h (±0.3)). Relationships of Q. ilex budburst date and temperature were not studied since only 2 years of data were available for this species. Q. suber shoot elongation underlying mechanisms were quite different in the two sites. At Mitra 2 (Q. suber and Q. ilex), there was a considerable tree water stress during the dry season which restricted shoot elongation. Shoot growth was resumed later in the wet autumn when tree water status recovered again. At the Lezirias site Q. suber water status was not restrictive. Therefore, shoot elongation was mainly dependent on nutrient availability in top soil, as suggested by the strong and positive relationships between annual shoot growth and long-term cumulative rainfall (2-4 months) and short-term average temperature (1 month) prior to budburst. Annual shoot elongation at this well-watered site was higher than in Mitra 2, and variability of growth between trees was enhanced after warm, wet springs when shoot elongation was higher. Results obtained are relevant to the carbon balance, productivity and management of evergreen Mediterranean oak woodlands, particularly under the foreseen climate change scenarios.     

Shoot growth patterns in saplings of Cleyera japonica in relation to light and architectural position

To gain further insight into crown development, the influences of shoot architectural position (branch order) and light environment on patterns of shoot growth of Cleyera japonica Thunberg (Theaceae) were investigated. Annual shoot length and light environment were positively correlated within same-order branches. Shoot length differed significantly among branch orders: shoot length was greater for the lower-order branches when light environments were comparable. Lower-order branches lengthened to a certain extent even if light availability was relatively low, whereas higher-order branches did not grow vigorously even when light availability was relatively high. Within same-order branches, branching was independent of the light environment of the shoot. Sylleptic shoot production differed significantly among branch orders, with most sylleptic shoots being produced on second-order branches. It is concluded that both light condition and architectural position of shoots must be considered when examining the mechanisms underlying crown development.     

Interacting influence of light and size on aboveground biomass distribution in sub-boreal conifer saplings with contrasting shade tolerance

Plant size often influences shade tolerance but relatively few studies have considered the functional response of taller plants to contrasting light environments. Several boreal and sub-boreal Abies, Picea and Pinus species were studied along a light (0-90% full sunlight) and size (30-400-cm high) gradient to examine the interactive influence of tree size and light availability on aboveground biomass distribution. Sampling was conducted in two regions of Canada: (A) British Columbia, for Abies lasiocarpa (Hook.) Nutt., the Picea glauca (Moench.) Voss x P. engelmannii Parry ex. Engelm. complex and Pinus contorta Dougl. var. latifolia Engelm.; and (B) Quebec, for Abies balsamea (L.) Mill., Picea glauca (Moench. Voss) and Pinus banksiana (Lamb.). All biomass distribution traits investigated varied with size, and most showed a significant interaction with both size and light, which resulted in increasing divergences among light classes as size increased. For example, the proportion of needle mass decreased as size increased but the rate of decrease was much greater in saplings growing at below 10% full sunlight. Needle area ratio (total needle area:aboveground mass) followed a similar pattern, but decreased more rapidly with increasing tree size for small trees up to 1 m tall. The proportion of needle biomass (needle mass ratio) was always lower in taller trees (i.e., > 1 m tall) than in small trees (< 1 m tall) and increasingly so at the lowest solar irradiances (0-10% full sunlight). Thus, extrapolating the functional response to light from small seedling to taller individuals is not always appropriate.     

Photosynthetic and stomatal responses to high temperature and light in two oaks at the western limit of their range

Bur oak (Quercus macrocarpa Michx.) and chinquapin oak (Q. muehlenbergii Engl.) leaves were exposed to high temperatures at various photosynthetic photon flux densities under laboratory conditions to determine if species-specific responses to these factors were consistent with the distribution of these oaks in gallery forests in the tallgrass prairies of northeastern Kansas, USA. Measurements of the ratio of chlorophyll fluorescence decrease, R(fd), indicated that chinquapin oak maintained greater photosynthetic capacity than bur oak across all tested combinations of irradiance (100, 400, 700 and 1000 micro mol m(-2) s(-1)) and temperature (40, 42, 44, 46 and 48 degrees C). In both oak species, manipulation of leaf temperature to about 47 degrees C for 45 min in the field led to a 45% decrease in carbon assimilation up to one week after the heat treatment, and to sharp reductions in stomatal conductance. Photosynthetic recovery patterns indicated that bur oak took longer to recover from heat stress than chinquapin oak, suggesting that heat stress may be important in determining distribution patterns of these oak species. Based on a comparison of the results with data from other forest species, we conclude that the photosynthetic temperature tolerances of bur oak and chinquapin oaks facilitate their dominance at the western limit of the eastern deciduous forest.     

Gender-specific patterns of aboveground allocation, canopy conductance and water use in a dominant riparian tree species: Acer negundo

Acer negundo Sarg. (box elder) is a dioecious tree species that dominates riparian systems at mid elevations throughout the southwest and Intermountain West of the United States. Previous studies have shown that female A. negundo trees occur at higher frequencies along stream margins, whereas males occur at higher frequencies in drier microsites. To better understand the adaptive significance of sex ratio biases and their impact on the ecohydrology of riparian ecosystems, we examined whole-plant water relations and hydraulic properties of mature male and female A. negundo trees occurring within 1 m of a perennial stream channel. We hypothesized that (1) females would have significantly greater canopy water fluxes than males (particularly during periods of seed production: May-June), and (2) xylem in females is more hydraulically efficient but more vulnerable to cavitation than xylem in males. Mean sap flux density (J(s)) during the early growing season (May and June) was 43% higher in female trees than in male trees (n = 6 and 7 trees respectively, P < 0.0001). Mean J(s) in July and August remained 17% higher in females than in males (P = 0.0009). Mean canopy stomatal conductance per unit leaf area (g(s,leaf)) in May and June was on average 140% higher in females than in males (P < 0.0001). Mean g(s,leaf) in July and August remained 69% higher in female trees than in male trees (P < 0.0001). Canopy stomatal conductance scaled to basal area was 90 and 31% higher in females relative to males during May-June and July-August, respectively (P < 0.0001 during both periods). Conversely, there were no apparent differences in either branch hydraulic conductance or branch xylem cavitation vulnerability between genders. These results improve our capacity to describe the adaptive forces that shape the spatial distribution of male and female trees in dioecious species, and their consequences for ecohydrological processes in riparian ecosystems.     

Effects of a nursery CO2 enriched atmosphere on the germination and seedling morphology of two Mediterranean oaks with contrasting leaf habit

The use of an enriched CO₂ atmosphere in tree nurseries has been envisaged as a promising technique to increase productivity and to obtain seedlings with a higher root/shoot ratio, an essential trait to respond to water stress in Mediterranean-type ecosystems. In that framework, we have analyzed the effects of three levels of atmospheric CO₂ concentration (350, 500 and 700ppm) on the germination rate, growth and morphology of seedlings of two Mediterranean oaks used in reforestation programs: the evergreen Quercus ilex L. and the deciduous Quercus cerrioides Wilk. et Costa. CO₂ enrichment increased the germination rate of Q. cerrioides (from 70±7 to 81±3%) while it decreased that of Q. ilex (from 71±10 to 41±12%). Seedlings of both species increased approximately 60% their total biomass in response to CO₂ enrichment but at two different CO₂ concentrations: 500ppm for Q. cerrioides and 700ppm for Q. ilex. This increase in seedlings biomass was entirely due to an augmentation of root biomass. Considering germination and biomass partitioning, an enriched CO₂ atmosphere might not be appropriate for growing Mediterranean evergreen oaks, such as Q. ilex, since it reduces acorn germination and the only gains in root biomass occur at a high concentration (700ppm). On the other hand, a moderate CO₂ enrichment (500ppm) appears as a promising nursery technique to stimulate the germination, growth and root/shoot ratio of deciduous oaks, such as Q. cerrioides.     

Environmental and plant-based controls of water use in a Mediterranean oak stand

In this work, new information is reported on water relations of the Mediterranean oak species Quercus pyrenaica based on environmental and physiological measurements carried out during the growing seasons of 2006 and 2007. The interest in this species has increased due to its use in reforestation programs and its impacts on the water resources due to the extensive spontaneous afforestation after the abandonment of forest (firewood, charcoal, livestock, etc.) and agricultural activities, in general in some areas in the Mediterranean region and in particular, in the studied area (Sistema Central range, Spain). The objectives were to evaluate the long-term water use of this stand and its limit and to analyse the specific traits to cope with summer drought, especially the use of stem water storage and deep soil water reserves. Tree water stress associated with depletion of soil water reserves was not observed since the oak trees appeared to avoid a marked water stress using water reserves from deeper soil layers as summer drought progresses. The contribution of mean daily stem water storage to transpiration was low (4%), although it could be greater under dryer conditions. Only at the end of summer of 2006, the transpiration and canopy conductance were reduced due to soil drought. Despite the absence of marked water stress an upper limit was found in transpiration (slightly higher than 3 mm day⁻¹). The heavy use of soil water resources by this species (75% of available soil water in this study) should be considered when evaluating the impact of spontaneous afforestation and reforestation programs on water resources.     

Dendrometric and dasometric analysis of the bushy biomass in Mediterranean forests

The aim of this study was to define the dendrometric and dasometric parameters of five Mediterranean bushy plant species, to make their environmental assessment and to determine correlations with LIDAR data. The species studied were: Quercus coccifera, Rosmarinus officinalis, Ulex parviflorus, Cistus albidus, and Erica multiflora. We calculated the global form factor for each species so as to be able to determine the volume of the plants by measuring their base diameter and length. The model volume closest to the real volume was the cylinder, which had form factor values of between 0.8 and 2.33. We also studied the evolution of the moisture content of the vegetation after cutting. The initial values were from 20 to 30%, with a dry density of 1 t/m³. We defined an occupation factor to determine the relationship between the real volume occupied by the plants in an area and a model volume (hemisphere, paraboloid, cone, and cylinder). The occupation factor was similar for the five species studied in dm³ of material per m³ of (model) apparent volume: 3.5 dm³/m³ for the cylinder; 12.5 dm³/m³ for the cone; 8.5 dm³/m³ for the paraboloid; and 14.5 dm³/m³ for the hemisphere. These factors enabled us to calculate the biomass contained in an apparent volume of bushes (including materials and hollows) by density. The prediction models tested in this study will thus serve to determine the biomass of a bushy forest area when the Canopy Height Model (CHM) is calculated from LIDAR data.     

Biomass allocation and assimilation efficiency of naturalTilia amurensis samplings in response to different light regimes

Biomass allocation and assimilation efficiency of natural Amour linden (Tilia amurensis) samplings in different light regimes were analyzed in the paper. The results showed that shoot increment of samplings in gap was the highest and that of samplings under canopy was the least. Samplings in gap expressed apical dominance strongly but samplings in full sun and under canopy behaved intensive branching. Lateral competition or moderate shading was favored to bole construction. The patters of biomass allocation of samplings in different light environment were rather similar. The biomass translocated to stem was more than that to other organs, and about one half of photosynthate was used to support leaf turn over. On the contrary, photosynthates of samplings in full sun were mostly consumed in leaves bearing and energy balancing. The carbon assimilation for leaves of samplings in gap was the most efficient, and more carbons were fixed and translocated to non-photosynthetic organs, especially to stemwood. Responsible editor: Zhu Hong