Seasonal variability of photosynthetic characteristics influences growth of eight tropical tree species at two sites with contrasting precipitation in Panama

Relative to closed-canopy tropical forests, tree seedlings planted in open grown areas are exposed to higher light intensity, air temperatures, vapor pressure deficit, and greater seasonal fluxes of plant available water than mature tropical forests. The species-specific adaptive capacity to respond to variable precipitation and seasonality in open grown conditions, therefore, is likely to affect species performance in large-scale reforestation efforts. In the present study, we compared the photosynthetic characteristics of eight tropical tree species within and between seasons at two study sites with contrasting dry season intensities. All species except Pseudosamanea guachapele reduced leaf physiological function between the wet and dry seasons. The contrasting severity of seasonal drought stress at the study sites constrained growth rates and photosynthetic characteristics differently. Variation of photosynthetic characteristics at the species level was high, particularly in the dry season. Faster growing species at the less seasonal site, Terminalia amazonia, Inga punctata, Colubrina glandulosa, and Acacia mangium, exhibited a greater adaptive capacity than the other species to down-regulate leaf photosynthesis between seasons. As the dry season was more severe at the more seasonal site, most species strongly reduced physiological function regardless of relative growth rates, except two species (Tectona grandis and P. guachapele) with widespread distributions and relatively high drought tolerance. Our results underscore the need to consider seasonal drought tolerance when selecting tree species for specific reforestation sites.     

Survival, growth and physiological status of Acacia disparrima and Eucalyptus crebra seedlings with respect to site management practices in Central Queensland, Australia

An improved understanding of important ecophysiological mechanisms underpinning tree water and nutrient use is necessary for developing and testing effective revegetation and restoration techniques in disturbed landscapes. A field trial was established in Central Queensland, Australia, to evaluate tree ecophysiological response to site management methods, including site preparation, herbicide application versus top soil removal (scalping) and fertilisation versus non-fertilisation. The influence of site management practices on plant survival, growth and foliar ecophysiological traits of Acacia disparrima (M. W. McDonald and Maslin) and Eucalyptus crebra (F. Muell.) seedlings was investigated within 22 months following tree planting. There was no difference in the survival of A. disparrima and E. crebra in response to the site preparation. However, there was a significant difference in growth response with both species showing greater mean periodic height gain in the herbicide areas compared to the scalped areas. Plant growth and survival of both species were unaffected by fertilisation, regardless of site preparation treatment. We suggest that the effects of fertiliser may have been masked by drought conditions experienced by seedlings in the first 6 months after planting. Neither site preparation nor fertilisation affected the leaf-level ecophysiological traits of seedlings, including foliar total N, photosynthetic capacity, instantaneous water-use efficiency, carbon isotope composition and stomatal conductance, irrespective of species. Scalping was more effective than herbicide application to suppress weeds and reduced the costs of site preparation and maintenance. Surprisingly, scalping had no impact on plant survival and foliar ecophysiological traits. However, it should be noted that the scalping may not be a sustainable practice in plantation establishment with short rotations where organic matter levels may not have a chance to recover between disturbances.     

Ecosystem production in Douglas-fir plantations: Interaction of red alder and site fertility

Pure stands of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and mixed stands of Douglas-fir and naturally established red alder (Alnus rubra Bong.) were examined on two different sites for soil properties, tree growth and nutrition, and aboveground ecosystem biomass and net primary production. One site (Mt. Benson, Nanaimo, B.C.) was nitrogen (N)-deficient and had a low site index (expected Douglas-fir height of 24 m at 50 years). The other site (Skykomish, western Washington) was N-rich and had a site index of 45 m at 50 years. Soil N accretion on the red alder units was estimated at 65 (Mt. Benson) and 42 (Skykomish) kg ha^?1 year^?1 for 23 years to a soil depth of 50 cm. At the current stage of plantation development, presence of red alder at the infertile Mt. Benson site increased average Douglas-fir diameter but did not affect its basal area and basal area growth rate; including alder stem biomass increased total stand basal area and basal area growth 2.5 fold. Presence of red alder at the fertile Skykomish site decreased average diameter, basal area, and basal area growth of Douglas-fir; including alder biomass left total stand basal area and basal area growth unchanged. Douglas-fir foliar N concentrations on Mt. Benson increased from 0.93 without alder to 1.41% on the red alder unit but were 1.55% for both units at Skykomish. Although alder did not affect Douglas-fir aboveground biomass and net primary production on Mt. Benson, total ecosystem biomass doubled and production tripled when alder biomass was included. Conversely, at Skykomish, Douglas-fir biomass and production decreased, and total ecosystem values were essentially unchanged. Mixing red alder and Douglas-fir seems to have great potential for increasing Douglas-fir growth and ecosystem production on infertile, N-deficient sites but probably has limited value on fertile, N-rich sites.     

Changes in foliar carbon isotope composition and seasonal stomatal conductance reveal adaptive traits in Mediterranean coppices affected by drought

We estimated water-use efficiency and potential photosynthetic assimilation of Holm oak(Quercus ilex L.) on slopes of NW and SW aspects in a replicated field test examining the effects of intensifying drought in two Mediterranean coppice forests. We used standard techniques for quantifying gas exchange and carbon isotopes in leaves and analyzed total chlorophyll, carotenoids and nitrogen in leaves collected from Mediterranean forests managed under the coppice system. We postulated that responses to drought of coppiced trees would lead to differential responses in physiological traits and that these traits could be used by foresters to adapt to predicted warming and drying in the Mediterranean area. We observed physiological responses of the coppiced trees that suggested acclimation in photosynthetic potential and water-use efficiency:(1) a significant reduction in stomatal conductance(p0.01) wasrecorded as the drought increased at the SW site;(2) foliar δ13C increased as drought increased at the SW site(p0.01);(3) variations in levels of carotenoids and foliar nitrogen, and differences in foliar morphology were recorded, and were tentatively attributed to variation in photosynthetic assimilation between sites. These findings increase knowledge of the capacity for acclimation of managed forests in the Mediterranean region of Europe.     

Soil CO2 efflux in an Afromontane forest of Ethiopia as driven by seasonality and tree species

Variability of soil CO₂ efflux strongly depends on soil temperature, soil moisture and plant phenology. Separating the effects of these factors is critical to understand the belowground carbon dynamics of forest ecosystem. In Ethiopia with its unreliable seasonal rainfall, variability of soil CO₂ efflux may be particularly associated with seasonal variation. In this study, soil respiration was measured in nine plots under the canopies of three indigenous trees (Croton macrostachys, Podocarpus falcatus and Prunus africana) growing in an Afromontane forest of south-eastern Ethiopia. Our objectives were to investigate seasonal and diurnal variation in soil CO₂ flux rate as a function of soil temperature and soil moisture, and to investigate the impact of tree species composition on soil respiration. Results showed that soil respiration displayed strong seasonal patterns, being lower during dry periods and higher during wet periods. The dependence of soil respiration on soil moisture under the three tree species explained about 50% of the seasonal variability. The relation followed a Gaussian function, and indicated a decrease in soil respiration at soil volumetric water contents exceeding a threshold of about 30%. Under more moist conditions soil respiration is tentatively limited by low oxygen supply. On a diurnal basis temperature dependency was observed, but not during dry periods when plant and soil microbial activities were restrained by moisture deficiency. Tree species influenced soil respiration, and there was a significant interaction effect of tree species and soil moisture on soil CO₂ efflux variability. During wet (and cloudy) period, when shade tolerant late successional P. falcatus is having a physiological advantage, soil respiration under this tree species exceeded that under the other two species. In contrast, soil CO₂ efflux rates under light demanding pioneer C. macrostachys appeared to be least sensitive to dry (but sunny) conditions. This is probably related to the relatively higher carbon assimilation rates and associated root respiration. We conclude that besides the anticipated changes in precipitation pattern in Ethiopia any anthropogenic disturbance fostering the pioneer species may alter the future ecosystem carbon balance by its impact on soil respiration.     

Foliar carbon isotope discrimination and related traits along light gradients in two different functional-type tree species

To understand how different plant functional types respond to light intensities, foliar carbon isotope discrimination (Δ¹³C) and related traits, i.e., specific leaf area (SLA), mass- and area-based nitrogen concentrations (N_mass and N_area), leaf dry mass content (LDMC) of two evergreen coniferous and three deciduous broad-leaved species, were measured under four light intensities. Foliar Δ¹³C and SLA increased significantly from full- to low-light conditions for all species. These indicate that species studied could increase their light capture capacity under low-light conditions, leading to lower water-use efficiency (higher ¹³C discrimination). There were significant differences in the responses of foliar N_mass or N_area to light variations in the two functional types, indicating that different functional-type tree species may have different N-use strategies to adapt to the light variations. It was found that there were large functional-type-dependent differences with regard to the relationships between foliar Δ¹³C and other leaf traits. Our findings suggest that all tree species could change foliar morphology to increase their light-harvesting ability under low-light conditions at the expense of decreasing their water-use efficiency. However, large differences in N-use strategy may exist between deciduous and evergreen species, which may be vital for the survival of these two functional-type tree species in a shaded understory. More important, our findings reveal that changes in Δ¹³C are not directly related to foliar N if N investment does not proportionally increase the photosynthetic capacity; this should be considered when exploring the relationships of nitrogen concentrations with Δ¹³C.     

Impacts of repeated fuel reduction burning on tree growth,mortality and recruitment in mixed species eucalypt forests of southeast Queensland,Australia

The long-term effects of repeated prescribed burning on diameter growth of trees in mixed species dry and wet sclerophyll forest sites in southeast Queensland, Australia were assessed. In addition, fire effects on tree mortality and recruitment in the wet sclerophyll site were evaluated. The results show that growth responses of species to fire were variable. Nevertheless, for most species, recurrent burning had no deleterious effect on tree growth. At the dry sclerophyll site, annual burning since 1952 did not affect growth rates of Eucalyptus drepanophylla and E. acmenoides. On the other hand, E. tereticornis responded positively to annual burning. Smaller Corymbia variegata (formerly Eucalyptus maculata) trees appeared to respond positively to annual burning, while larger trees appeared to respond negatively but this response lessened over time. Periodic burning (every 2–3 years) since 1973 did not significantly affect the growth rates of any tree species. At the wet sclerophyll site, biennial burning since 1972 has enhanced the diameter growth of Lophostemon confertus but depressed that of Syncarpia glomulifera. Quadrennial burning did not affect the diameter growth of L. confertus but depressed that of S. glomulifera. The diameter growth of E. pilularis, C. intermedia, E. microcorys and E. resinifera was not affected by either burning treatment. Basal area growth of most eucalypts at this site was unaffected by burning. However, basal area growth of both S. glomulifera and L. confertus was adversely affected by burning due to tree mortality. For most species, tree mortality was both diameter-dependent and fire-related, that is, smaller trees have a lower chance of survival than larger trees and frequent burning further reduces this probability. Without fire, recruitment was dominated by S. glomulifera and to a lesser extent by L. confertus. Recruitment of these species was adversely affected by burning. This result and the greater mortality of smaller trees with frequent burning suggest that if these trends continue, future stand growth and hence productivity of these species could be jeopardized because of the reduction of the regenerative capacity of the forest. Recruitment was negligible for other tree species in this forest regardless of fire treatment.     

Plant limiting nutrients in Andean-Patagonian woody species: Effects of interannual rainfall variation,soil fertility and mycorrhizal infection

Andean-Patagonian forests are especially interesting for the study of N and P limitation because they receive minimal atmospheric pollution, have little influence of vascular N-fixing species, and grow on volcanic soils that retain P. In a previous study of 10 woody species (four broad-leaved deciduous species, three broad-leaved evergreens and three conifers) conducted during an exceptionally dry year in NW Patagonia, and on the basis of nutrient resorption efficiency and proficiency, we suggested that N was the most limiting nutrient except for the broad-leaved evergreen Lomatia hirsuta. In the present work, we compared patterns of nutrient limitation during a dry and a wet year, quantified the percentage of mycorrhizal infection, and related mycorrhizal behavior and nutrient limitation to soil fertility. We used N and P concentrations in green leaves as indicators of nutrient requirements, and N and P concentrations in senescent leaves (resorption proficiency) and the N/P ratio in green leaves as indicators of nutrient limitation. We also determined leaf mass area (LMA) and lignin concentration as indicators of structural and chemical defences. From previous works, the following soil fertility indicators were included: pH, organic C, total N, exchangeable cations, Olsen-P, potential N mineralization (pNmin) and N retained in microbial biomass (N-MB). Nitrogen, P and lignin concentrations in green and senescent leaves did not differ significantly between the dry and the wet year either by species or by functional groups. Most species behaved as N-proficient and P-non-proficient; this together with values of foliar N/P ratios lower than 14–16 confirmed N limitation in these forests. The only species limited by P but not by N was L. hirsuta (1.0–1.1% N in senescent leaves, N/P ratio = 21–23), a non-mycorrhizal species with cluster roots. The lack of P limitation in the other species was probably related to the high percentages of infection with arbuscular mycorrhizae (80–90% in Maytenus boaria and the conifers Araucaria araucana, Austrocedrus chilensis and Fitzroya cupressoides), and ectomycorrhizae (73–79% in five Nothofagus species). Nitrogen and P requirements were positively correlated among themselves and negatively with lignin and LMA. Soil fertility was positively correlated with nutrient requirements and negatively with lignin and LMA. Conifers had lower N and P requirements, higher LMA, lower foliar N/P ratio and grew on soils of lower soil N dynamics (lower pNmin and N-MB) than ectomycorrhizae-associated species.     

Corresponding patterns of site quality,decline and tree growth in a sessile oak stand

This study focuses on two rarely studied aspects of oak decline: relations with site characteristics and effects on tree growth. The study was carried out in a 5.5 ha stand in Hungary which is strongly affected by oak decline. The nearly pure sessile oak (Quercus petraea) stand of mostly coppice origin was 90 years old at the beginning of the study. Within-stand site heterogeneity was described by the herbaceous vegetation. Four ecological site types were distinguished by the species composition of herbs, and characterized by the ecological indicator values of the species. Tree growth between 1987 and 1993 was measured, and tree vigour was estimated from visual characteristics five times between 1987 and 1993. Potential volume increment of declining trees was estimated with the growth rates of healthy trees of the same size. Volume increment loss caused by oak decline was also assessed. Significant positive relationships were found between tree vigour and tree size and between tree vigour and tree growth. The growth of seriously declining trees dropped to almost one-half of that of healthy ones. Growth reduction of living trees at the stand level amounted to 5.4%, whereas growth reduction of all trees, including those that died during the observation period, amounted to 19.9% of the potential growth. Tree size and growth were greater on better sites. A strong relationship was also found between tree vigour and site type, but sessile oak was more susceptible to decline at better sites.     

Tree age and soil phosphorus conditions influence N2-fixation rates and soil N dynamics in natural populations of Acacia senegal

Acacia senegal, an important leguminous tree in arid and semi-arid environments, has shown promise as a multipurpose species, including gum production and soil fertility improvement, linked with N₂-fixation capabilities. Of particular interest are ontogenetic and edaphic effects on A. senegal performance in natural populations. Our research objectives were to investigate the effect of tree age and site phosphorus conditions on (1) tree N₂-fixation and (2) soil N and C dynamics in natural stands of A. senegal var. senegal, Baringo District, in the Rift Valley, Kenya. Sites consisted of A. senegal saplings (9 months) and mature A. senegal trees (7 years) along an edaphic gradient of soil P availability. A single-tree neighborhood approach was employed using a two by two factorial design: site conditions [high and low soil P contents] and tree age class [juvenile and mature]. Soil (N and C pools and fluxes) and plant metrics were quantified. A soil transfer experiment was also employed to confirm age and site effects on soil N mineralization. On the high soil P site, A. senegal had significantly lower foliar (¹⁵N levels than neighboring non-leguminous species (Balanites aegyptiaca), while foliar δ¹⁵N values in A. senegal on the low P site exhibited no significant difference with our reference plant, B. aegyptiaca. Across P sites, B. aegyptiaca had similar foliar δ¹⁵N values. These results indicate that the rate of N₂-fixation of A. senegal trees, as determined with foliar ¹⁵N natural abundance methodology, increased with increasing soil P availability in these natural populations. However, N₂-fixation rates declined with age. Although soil texture and soil CO₂ efflux did not differ between sites or across ages, soils under mature A. senegal at the high P site exhibited significantly greater total N content and total C content in comparison to soils at the low P site and under juvenile plants. Furthermore, under mature A. senegal trees, soil N mineralization rates were significantly greater as compared to under saplings. Soil transplants confirmed that soil microbial activity may be stimulated under mature trees as N mineralization rates were 2-3 fold greater compared to under A. senegal saplings. Our findings suggest that tree age and soil P availability are important factors in the nitrogen budget of natural populations of A. senegal, determining N₂-fixation rates, and potentially influencing soil total N and C pools and soil mineral N. This study provides information regarding the adaptation of A. senegal under differing edaphic conditions thus increasing accuracy of management support for A. senegal populations as productive agroforests.     

Comparison of the growth traits of a commercial pioneer tree species,paper mulberry (Broussonetia papyrifera L. Vent.), with those of shade-tolerant tree species: investigation of the ecophysiological mechanisms underlying shade-intolerance

In rural areas of northern Laos, a commercially valuable pioneer tree species, paper mulberry (Broussonetia papyrifera L. Vent.) has been recommended for intercropping under plantations of commercial trees. However, less is understood about the growth of this pioneer tree species in the understorey and the mechanism underlying the shade intolerance. We measured growth characteristics for seedlings of paper mulberry under four light intensities. We compared the relative growth rates in aboveground biomass and standing leaf area (RGR_mass and RGR_leaf), light-capture efficiency, and seeding-level mass-based daily photosynthetic rates (A _mass) with those of field-grown seedlings of eight shade-tolerant species to identify factors potentially responsible for shade-intolerance. Most growth traits of the paper mulberry seedlings did not differ consistently from those of the shade tolerant species. The ecophysiological–architectural model software showed higher A _mass and RGR_mass capacity in paper mulberry than in shade-tolerant species. Despite their higher RGR_mass, paper mulberry seedlings had negative RGR_leaf under shaded conditions due to short leaf lifespan. The linear RGR_mass–RGR_leaf relationship for paper mulberry had a high RGR_mass intercept, indicating that a high RGR_mass was required to provide positive RGR_leaf. Progressive decreases in standing leaf area with time, and possibly photosynthesis, appear to be responsible for the shade-intolerance of paper mulberry. Although intercropping of paper mulberry has been suggested in the species’ native region, understorey cultivation of paper mulberry would only be possible with relatively open canopies.     

Understory abundance,species diversity and functional attribute response to thinning in coniferous stands

Alternative strategies for stand density management in even-aged coniferous forests may increase plant species and functional diversity. We examined the effects of fixed and variable density thinning on tree seedling regeneration as well as on abundance (indexed by cover) and richness of understory vascular plants 11 years after harvesting 45- to 66-year old forests dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) or western hemlock (Tsuga heterophylla (Raf.) Sarg.) at three sites in western Oregon. Each site contained an unthinned control (CON), and thinning treatments selected to enhance overstory structural diversity and spatial variability within stands (HD, high density treatment at 300 trees ha⁻¹; MD, moderate density treatment at 200 trees ha⁻¹; VD300, VD200 and VD100, variable density treatments at 300, 200 and 100 trees ha⁻¹). Leave islands are included in HD and the other thinning treatments contain both leave islands and gap openings. Tree seedling regeneration was highly variable and generally increased with thinning. Cover of all understory species was greater in VD100 than in the control whereas richness was greater in HD and MD. Cover and richness of early seral species were greater in most thinning treatments than in the control. Understory plant communities were overwhelmingly dominated by native species. In general, vegetation dynamics was accelerated by thinning, especially in variable density treatments. Cover of N-fixing understory species was greater in VD200 than in the other treatments, and in MD and VD300 than in the control, whereas richness of understory N-fixing species increased in all thinning treatments. Cover of understory species with intermediate soil water requirements was greater in MD, VD200 and VD100 than in the control, whereas richness of these species increased in VD200 compared to the control, HD and VD300. Thinning promoted higher diversity of understory conditions without reducing density and species richness of crop tree regeneration, and seemed to increase functional effect and response diversity.     

Implications of multipurpose tree leaf application on wheat productivity in dry tropics

Leaves of multipurpose tree species (those providing more than one function or product of human use) can serve as sources of fer-tilizer for nutrient supply, especially nitrogen (N). In this study chop...     

Abundance, niche breadth, and niche occupation of Central European tree species in the centre and at the margin of their distribution range

How abundance, niche breadth and niche occupation of species change within the distribution range remains unresolved for tree floras. We compared populations of three common and three rare European tree species in the distribution centre and at the eastern margin with respect to abundance, niche breadth, and preferential occurrence along several soil-related niche axes. We hypothesised: (i) that abundance is higher, and niche breadth greater, in the distribution centre than at the margin, (ii) that the occupied niche differs between central and marginal populations with species being restricted to more favourable soils at the margin, and (iii) that common and rare species show different patterns.We characterised the occupied niche and niche breadth in a semi-quantitative way and discuss the role of human impact on tree abundance in the light of evidence from paleoecology and forest dynamics research.Five of the six species are more abundant in the centre than at the margin, at least in the natural vegetation prior to human intervention. In contrast to soil niche breadth, the preference for soil chemical and hydrological properties differed between centre and margin. Contrary to expectation, a shift to less fertile and partly also drier sites at the margin was observed, although bedrocks which give rise to the development of fertile soils were more frequent in Slovakia (ca. 49% of the total area compared to 20% in Central Germany) and bedrock types which produce infertile soils were more abundant in Central Germany (about 23% vs. 2% in Slovakia).While the abundant-centre-distribution hypothesis is supported by our data, we found no conclusive evidence of competitive displacement of weaker competitors and of a niche shift of superior competitors to more favourable sites at the range margin. Factors other than competition must be responsible for the observed niche shifts. Higher drought stress does not seem to be a limiting factor at the eastern margin, where dry sites were occupied more frequently compared to the centre.     

Low light acclimation in five temperate broad-leaved tree species of different successional status: the significance of a shade canopy

?Context

Tree species differ largely in their capability to produce characteristic shade leaves with effective morphological and physiological acclimation to low light.

?Aims

By examining the sun/shade leaf differentiation in leaf morphology, foliar nitrogen and photosynthetic capacity in five temperate tree species of different successional status, we aimed at identifying those leaf traits that determine the development of a typical shade crown with low light-acclimated leaves.

?Methods

Leaf morphology, foliar N content, photosynthetic capacity (V _cmax, J _max and A _max) and leaf dark respiration (R _d) were measured in the canopies of 26 adult trees of Fraxinus, Acer, Carpinus, Tilia and Fagus species.

?Results

Six traits (the sun/shade leaf differentiation in specific leaf area, leaf size, A _max per leaf area or per mass, photosynthetic N use efficiency and R _d) were found to characterise best the degree of low light acclimation in shade leaves. All five species exhibited certain modifications in leaf morphology and/or physiology in response to low light; Fagus sylvatica showed the highest and Fraxinus excelsior the lowest shade leaf acclimation.

?Conclusions

Our results indicate that the five early/mid- to late-successional species have developed species-specific low light acclimation strategies in their shade crowns which differ in terms of the relative importance of leaf morphological and physiological acclimation.     

Genetic variation in foliar nutrient concentration in relation to foliar carbon isotope composition and tree growth with clones of the F1 hybrid between slash pine and Caribbean pine

The objectives of this study were: (1) to quantify the genetic variation in foliar nutrient concentration in relation to foliar carbon isotope composition (δ¹³C) and tree growth of 122 clones of ca. 4-year-old F₁ hybrids between slash pine (Pinus elliottii Engelm var. elliottii) and Caribbean pine (P. caribaea var. hondurensis Barr. et Golf.) grown at two experimental sites with different water and nutrient availability in southeast Queensland, Australia and (2) to examine the potential of using foliar nutrient concentration of the 4-year-old tree canopies for selecting elite F₁ hybrid pine clones with improved nutrient-use efficiency (NUE) and water-use efficiency (WUE), and ultimately enhanced tree growth under ambient growing conditions. There were significant differences in foliar nutrient concentrations between two canopy positions (upper outer and lower outer canopy) sampled, between summer and winter, and between the two sites. This highlights that foliar nutrient concentrations are influenced by sampling and environment. Significant genetic variations in foliar nutrient concentrations were detected between the clones, between the female parents, and between the male parents of the clones in both sampling seasons at both sites. Depending on the nutrient concerned, canopy position, season, and site sampled, the clones accounted for 4.7–33.9% of the total variation in foliar nutrient concentrations, the clone female parents for 0–25.1% and the clone male parents for 0–28.6%. The site-by-clone interactions were statistically significant for foliar N, P, Mg, Cu, Zn, Mn, Fe and mineral concentrations at the upper outer canopy in summer, and for foliar N concentration in winter. There were significant, positive correlations between clone means of foliar δ¹³C and N concentration at the upper outer canopy in summer for the wet site, while clone foliar δ¹³C was also positively related to clone foliar N concentration at both canopy positions in summer for the dry site. This suggests that clone WUE as reflected in foliar δ¹³C may be improved by selecting elite clones with higher foliar N concentration and increased photosynthesis, leading to enhanced tree growth when both water and N are the major growth-limiting factors. This is supported by the positive correlation detected between clone tree height and foliar N concentration at the upper outer canopy for both sites. Thus, foliar nutrient (particularly N) concentration, together with foliar δ¹³C, may be useful for assisting in selection of exotic pine clones with improved NUE and WUE, and enhanced tree growth under the nutrient- and water-limiting environments.     

Effects of enhanced nitrogen deposition on foliar chemistry and physiological processes of forest trees at the Bear Brook Watershed in Maine

Effects of enhanced nitrogen deposition on nutrient foliar concentrations and net photosynthesis of sugar maple (Acer saccharum Marsh.), American beech (Fagus grandifolia Ehrh) and red spruce (Picea rubens Sarg.) were evaluated at the Bear Brook Watershed in Maine (BBWM). The BBWM is a paired-watershed forest ecosystem study with one watershed treated since 1989 with bimonthly dry ammonium sulfate ((NH₄)₂SO₄) additions at a rate of 25.2 kg N ha⁻¹ year⁻¹, while the other watershed serves as a reference. The (NH₄)₂SO₄ treatment resulted in significant increases in foliar N concentrations for all three species and significant reductions in foliar Ca, Mg and Zn concentrations for American beech and red spruce. Treatment effects on foliar concentrations of other nutrients were not significant in any species. Despite higher N concentrations in all species, only treated sugar maple showed significantly higher photosynthetic rates. The non-response in net photosynthesis to higher foliar N in American beech and red spruce might be attributed to their low foliar Ca and/or Mg concentrations. Higher net photosynthetic rates in sugar maple might be explained by the higher foliar N and by the ability of this species to maintain an adequate Ca and Mg supply. Results suggested that nutrient imbalances due to inadequate supply of Ca and Mg might have counteracted a potential increase in net photosynthesis induced by higher N concentrations in American beech and red spruce.     

Ontogenetic changes in water-use efficiency (δC) and leaf traits differ among tree species growing in a semiarid region of the Loess Plateau,China

On the Loess Plateau, China, several planted tree species such as Populus hopeinsis and Robinia pseudoacacia suffer occasional diebacks in the top shoots, reducing growth rates after maturation. However, this does not usually occur in other species, e.g., Ulmus pumila and Zizyphus jujuba. We compared stable carbon isotope ratios (δ¹³C) as indicators of leaf water-use efficiency (WUE), leaf mass per unit area (LMA), and leaf area- and mass-based nitrogen content (N_area and N_mass) by tree height (1, 5, and 10 m) in P. hopeinsis, R. pseudoacacia, U. pumila, and Z. jujuba. In P. hopeinsis and R. pseudoacacia, leaf δ¹³C and N_area were significantly lower in 10-m trees than in 1-m saplings, indicating that leaf WUE and photosynthetic rates of both species decreased with tree height. In contrast, δ¹³C in Z. jujuba varied little with tree height. The δ¹³C of 10-m-tall U. pumila trees was significantly higher than that of 1-m plants, demonstrating an increase in WUE with tree height. Decreasing WUE, leaf N_area, and N_mass with height increases in P. hopeinsis and R. pseudoacacia may be related to water and nutrient limitations for these species in semiarid regions. In contrast, stable or increasing δ¹³C with tree height in U. pumila and Z. jujuba may account (to some extent) for successful production of these species under identical environmental conditions. Diameter growth rate also decreased with maturity in P. hopeinsis and R. pseudoacacia, but increased or was stable for U. pumila and Z. jujuba. The differences in leaf WUE and LMA among species with tree maturity may be related to species’ growth patterns and susceptibility to drought stress, and are likely to be important new criteria for plantation species selection on the Loess Plateau.     

Diversity of northern plantations peaks at intermediate management intensity

Tree diversity is an important component of biodiversity. Management intensification is hypothesized to affect tree diversity. However, evidence to support the relationship between management intensity and tree diversity in northern forests is lacking. This study examined the effects of fertilization, site preparation, and brush control on tree species diversity, shade tolerance diversity and size diversity of jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana [Mill.] B.S.P.), white pine (Pinus strobus L.) and white spruce (Picea glauca [Moench] Voss) plantations, 15 years after planting in Ontario, Canada. Species diversity and shade tolerance diversity were highly correlated, so were diameter size diversity and height size diversity. Fertilization did not affect the tree diversity indices of any plantations. Species diversity and shade tolerance diversity was interactively influenced by site preparation and brush control in the black spruce, white pine, and white spruce plantations, showing that the highest diversity occurred on sites with intensive site preparation without brush control, whereas on sites with brush control, diversity was higher with least intensity of site preparation. However, in the jack pine plantation, neither species diversity nor shade tolerance diversity differed with management intensification, and is attributed to the fast capture of site resources by the planted crop trees of jack pine which minimized establishment of non-crop species. Tree size diversity increased with site preparation intensity in the jack pine and black spruce plantations, while it decreased with brush control in the white pine and white spruce plantations. We concluded that (1) the effects of management intensification on diversity of northern plantations differ with growth habit of planted crop tree species and (2) species diversity and tree size diversity tend to be highest at intermediate levels of silvicultural intensification during the stand establishment phase, supporting the intermediate disturbance hypothesis.     

Short-term effects of selection and clear cutting on the shrub and herb layer vegetation during the conversion of even-aged Norway spruce stands into mixed stands

The conversion of even-aged Norway spruce (Picea abies (L.) H. Karst.) stands into more site adapted mixed stands is the main silvicultural goal in Germany and many other European countries. The conversion will primarily be achieved with the help of selection cutting, removing single target diameter trees and creating small gaps. At sites highly susceptible to windthrow, however, small scale clear cutting (up to 1 ha) might be an appropriate alternative. Both selection and clear cutting will affect the understorey vegetation, an important component of forest biodiversity and ecosystem functioning. Thus, with the help of a BACI-experiment (before–after/control–impact), the short-term effects of selection and small scale clear cutting on the herb and shrub layer vegetation were studied at two sites in the Solling Hills (Lower Saxony, Germany). Both treatments led to an increase in herb and shrub layer species numbers, due to persistent and invading (mainly pioneer tree species and indicators of disturbance) species and to a shift in plant traits. Most responsible for the changes in vegetation was a higher light availability, and to a lesser degree soil disturbance. The number of annuals and ruderals, however, increased only immediately after clear cutting, whereas on selection cutting plots the numbers remained high. Gaining in number and coverage were competitive perennials (Epilobium angustifolium, Juncus effusus, Rubus idaeus) especially on the clear cuttings, fulfilling a nutrient retention function. The competitives did not deter tree species from regenerating in the clear cut plots, whereas on selection cutting plots the large expansion of R. idaeus appeared to be hindering tree colonisation. Suffering under clear cut conditions were predominantly shade-tolerant, closed-forest species, but no loss of these species could be detected. This indicates the potential for a fast recovery of these forest species, as soon as, pioneer trees expand.