Mechanism of water‐stress induced cavitation in conifers: bordered pit structure and function support the hypothesis of seal capillary‐seeding

Resistance to water‐stress induced cavitation is an important indicator of drought tolerance in woody species and is known to be intimately linked to the anatomy of the xylem. However, the actual mechanical properties of the pit membrane are not well known and the exact mode of air‐seeding by which cavitation occurs is still uncertain. We examined the relationship between cavitation resistance and bordered pit structure and function in 40 coniferous species. Xylem pressure inducing 50% loss of hydraulic conductance (P₅₀, a proxy for cavitation resistance) varied widely among species, from ?2.9 to ?11.3 MPa. The valve effect of the pit membrane, measured as a function of margo flexibility and torus overlap, explained more variation in cavitation‐resistance than simple anatomical traits such as pit membrane, pit aperture or torus size. Highly cavitation resistant species exhibited both a high flexibility of the margo and a large overlap between the torus and the pit aperture, allowing the torus to tightly seal the pit aperture. Our results support the hypothesis of seal capillary‐seeding as the most likely mode of air‐seeding, and suggest that the adhesion of the torus to the pit border may be the main determinant of cavitation resistance in conifers.     

Cavitation fatigue in conifers: a study on eight European species

After drought-induced embolism and repair, tree xylem may be weakened against future drought events (cavitation fatigue). As there are few data on cavitation fatigue in conifers available, we quantified vulnerability curves (VCs) after embolism/repair cycles on eight European conifer species. We induced 50% and 100% loss of conductivity (LC) with a cavitron, and analyzed VCs. Embolism repair was obtained by vacuum infiltration. All species demonstrated complete embolism repair and a lack of any cavitation fatigue after 50% LC . After 100% LC, European larch (Larix decidua), stone pine (Pinus cembra), Norway spruce (Picea abies), and silver fir (Abies alba) remained unaffected, while mountain pine (Pinus mugo), yew (Taxus baccata), and common juniper (Juniperus communis) exhibited 0.4–0.9 MPa higher vulnerability to embolism. A small cavitation fatigue observed in Scots pine (Pinus sylvestris) was probably biased by incomplete embolism repair, as indicated by a correlation of vulnerability shifts and conductivity restoration. Our data demonstrate that cavitation fatigue in conifers is species-specific and depends on the intensity of preceding LC. The lack of fatigue effects after moderate LC, and relevant effects in only three species after high LC, indicate that conifers are relatively resistant against cavitation fatigue. This is remarkable considering the complex and delicate conifer pit architecture and may be important considering climate change projections.     

Sapling survival and growth of coniferous and broad‐leaved trees in successional highland habitats in Mexico

Abstract. We assessed survival and growth of transplanted saplings of understory broad‐leaved trees (Oreopanax xalapensis, Rapaneajuergensenii, Rhamnus sharpii and Tern‐stroemia lineata) and canopy conifers (Abies guatemalensis, Pinus ayacahuite and P. pseudostrobus) into Grassland (GR), Mid‐Successional Oak Forest (MS), and Old‐Growth Oak Forest (OF) stands in the central highlands of Chiapas, Mexico. A total of 727 plants were monitored over eight years. The results suggest habitat preferences of the studied species that should be considered for their conservation oriented management. Conifers had highest survival and growth in GR, while broad‐leaved trees survived better under sparse canopy in MS, but grew tallest in full light. Saplings of all species in dense canopy sites in OF were smaller, and the survival of conifers was lower. An integrated response index (IRI) calculated for each species with survival and relative growth rates showed similar trends. Re‐introduction of the endangered A. guatemalensis is possible in open habitats if fire, trampling and grazing are suppressed. Open habitats in current man‐made landscapes in the highlands of Chiapas may limit the establishment of understory trees requiring partial shade. Inclusion of such species in restoration programs has not been usual so far; yet, local and global interest in native species for such programs is increasing. Information on the response of these kind of species in contrasting habitats may help to incorporate them in the management of high‐diversity forests that may follow pine plantations.     

Computational fluid dynamics models of conifer bordered pits show how pit structure affects flow

? The flow of xylem sap through conifer bordered pits, particularly through the pores in the pit membrane, is not well understood, but is critical for an understanding of water transport through trees. ? Models solving the Navier-Stokes equation governing fluid flow were based on the geometry of bordered pits in black spruce (Picea mariana) and scanning electron microscopy images showing details of the pores in the margo of the pit membrane. ? Solutions showed that the pit canals contributed a relatively small fraction of resistance to flow, whereas the torus and margo pores formed a large fraction, which depended on the structure of the individual pit. The flow through individual pores in the margo was strongly dependent on pore area, but also on the radial location of the pore with respect to the edge of the torus. ? Model results suggest that only a few per cent of the pores in the margo account for nearly half of the flow and these pores tend to be located in the inner region of the margo where their contribution will be maximized. A high density of strands in outer portions of the margo (hence narrower pores) may be more significant for mechanical support of the torus.     

A new method for vulnerability analysis of small xylem areas reveals that compression wood of Norway spruce has lower hydraulic safety than opposite wood

Compression wood is formed at the underside of conifer twigs to keep branches at their equilibrium position. It differs from opposite wood anatomically and subsequently in its mechanical and hydraulic properties. The specific hydraulic conductivity (k_s) and vulnerability to drought‐induced embolism (loss of conductivity versus water potential ψ) in twigs of Norway spruce [Picea abies (L.) Karst.] were studied via cryo‐scanning electron microscope observations, dye experiments and a newly developed ‘Micro‐Sperry’ apparatus. This new technique enabled conductivity measurements in small xylem areas by insertion of syringe cannulas into twig samples. The hydraulic properties were related to anatomical parameters (tracheid diameter, wall thickness). Compression wood exhibited 79% lower k_s than opposite wood corresponding to smaller tracheid diameters. Vulnerability was higher in compression wood despite its narrower tracheids and thicker cell walls. The P₅₀ (ψ at 50% loss of conductivity) was ?3.6 MPa in opposite but only ?3.2 MPa in compression wood. Low hydraulic efficiency and low hydraulic safety indicate that compression wood has primarily a mechanical function.     

Chilling and forcing temperatures interact to predict the onset of wood formation in Northern Hemisphere conifers

The phenology of wood formation is a critical process to consider for predicting how trees from the temperate and boreal zones may react to climate change. Compared to leaf phenology, however, the determinism of wood phenology is still poorly known. Here, we compared for the first time three alternative ecophysiological model classes (threshold models, heat‐sum models and chilling‐influenced heat‐sum models) and an empirical model in their ability to predict the starting date of xylem cell enlargement in spring, for four major Northern Hemisphere conifers (Larix decidua, Pinus sylvestris, Picea abies and Picea mariana). We fitted models with Bayesian inference to wood phenological data collected for 220 site‐years over Europe and Canada. The chilling‐influenced heat‐sum model received most support for all the four studied species, predicting validation data with a 7.7‐day error, which is within one day of the observed data resolution. We conclude that both chilling and forcing temperatures determine the onset of wood formation in Northern Hemisphere conifers. Importantly, the chilling‐influenced heat‐sum model showed virtually no spatial bias whichever the species, despite the large environmental gradients considered. This suggests that the spring onset of wood formation is far less affected by local adaptation than by environmentally driven plasticity. In a context of climate change, we therefore expect rising winter–spring temperature to exert ambivalent effects on the spring onset of wood formation, tending to hasten it through the accumulation of forcing temperature, but imposing a higher forcing temperature requirement through the lower accumulation of chilling.     

Vulnerability to embolism differs in roots and shoots and among three Mediterranean conifers: consequences for stomatal regulation of water loss?

We investigated the potential links between stomatal control of transpiration and the risk of embolism in root and shoot xylem of seedlings of three Mediterranean conifers (Cupressus sempervirens, Pinus halepensis and P. nigra) grown in a greenhouse under semi-controlled conditions. We measured the intrinsic vulnerability to embolism in roots and current year shoots by the air injection method. Root and shoot segments were subjected to increasing pressures, and the induced loss of hydraulic conductivity recorded. The three species displayed very different vulnerabilities in shoots, with P. nigra being much more vulnerable than P. halepensis and C. sempervirens. Roots were distinctly more vulnerable than shoots in C. sempervirens and P. halepensis (50% loss of conductivity induced at 3.0 MPa and 1.7 MPa higher xylem water potential in roots vs shoots). In P. nigra, no significant difference of vulnerability between shoots and roots was found. Seedlings were subjected to soil drought, and stomatal conductance, twig hydraulic conductivity and needle water potential were measured. The water potential resulting in almost complete stomatal closure (90%) was very close to the threshold water potential inducing loss of conductivity (10%) in twigs in P nigra, resulting in a very narrow safety margin between stomatal closure and embolism induction. The safety margin was larger in P. halepensis and greatest in C. sempervirens. Unexpectedly, this water potential threshold produced a 30–50% loss of conductivity in 3–5 mm diameter roots, depending on the species. The implications of this finding are discussed.     

A 40,000-year woodrat-midden record of vegetational and biogeographical dynamics in north-eastern Utah, USA

Aim A conspicuous climatic and biogeographical transition occurs at 40–45° N in western North America. This pivot point marks a north–south opposition of wet and dry conditions at interannual and decadal time‐scales, as well as the northern and southern limits of many dominant western plant species. Palaeoecologists have yet to focus on past climatic and biotic shifts along this transition, in part because it requires comparisons across dissimilar records [i.e. pollen from lacustrine sediments to the north and plant macrofossils from woodrat (Neotoma) middens to the south]. To overcome these limitations, we are extending the woodrat‐midden record northward into the lowlands of the central Rocky Mountains. Location Woodrat middens were collected from crevices and rock shelters on south‐facing slopes of Dutch John Mountain (2000–2200 m, 40°57′ N, 109°25′ W), situated on the eastern flanks of the Uinta Mountains in north‐eastern Utah. The site is near the regional limits for Pinus ponderosa, P. edulis, P. contorta, Cercocarpus ledifolius var. intricatus, Abies concolor, Ephedra viridis and other important western species. Methods We analysed pollen and plant macrofossils from the 40,000‐year midden sequence. The middens represent brief, depositional episodes (mostly years to decades). Four middens represent the early to full‐glacial period (40,000–18,000 cal‐yr bp ), eight middens are from the late‐glacial/early Holocene transition (13,500–9000 cal yr bp ), and 33 middens span the mid‐to‐late Holocene (last 7500 years). Temporal density of our Holocene middens (one every c. 210 years) is comparable with typical Holocene pollen sequences from lake sediments. Results Early to full‐glacial assemblages are characterized by low diversity and occurrence of montane conifers (Picea pungens, Pseudotsuga menziesii, P. flexilis, Juniperus communis) absent from the site today. Diversity increases in the late‐glacial samples with the addition of J. scopulorum, J. horizontalis, C. montanus, C. ledifolius var. intricatus and mesic understory species. The coniferous trees and J. communis declined and J. osteosperma appeared during the late‐glacial/Holocene transition. Juniperus osteosperma populations have occupied the site throughout the Holocene. Pinus ponderosa was established by 7500 cal‐yr bp , and has occurred at least locally ever since. Montane conifers and J. horizontalis persisted until c. 5500 cal‐yr bp . The signature events of the late Holocene were the invasions of P. edulis and Ephedra viridis and establishment of pinyon–juniper woodland in the last 800 years. Main conclusions The Dutch John Mountain midden record adds to an emerging picture in which mid‐elevation conifers (P. flexilis, Pseudotsuga menziesii, Picea pungens, J. scopulorum, J. communis) dominated vegetation over a wide area of the Colorado Plateau and adjacent Rocky Mountains. Rather than being fragmented, as often assumed in phylogeographical studies, these species had broader and more‐connected distributions than they do in the region today. Paradoxically, subalpine conifers (Picea engelmannii, A. lasiocarpa) occurred at higher elevations to the south, possibly representing declining precipitation from south to north owing to southward displacement of the polar jet stream. The Dutch John Mountain record displays a series of extinction and invasion events. Most of the extinctions were local in scale; nearly all constituents of fossil midden assemblages occur within a few kilometres of Dutch John Mountain, and some occur at least locally on its slopes. The sole exception is J. horizontalis, which is regionally extinct. In contrast to extinctions, Holocene invasions were regional in scale; J. osteosperma, P. ponderosa, P. edulis and Ephedra viridis immigrated from glacial‐age source populations far to the south.     

Contrasting acclimation abilities of two dominant boreal conifers to elevated CO2 and temperature

High latitude forests will experience large changes in temperature and CO₂ concentrations this century. We evaluated the effects of future climate conditions on 2 dominant boreal tree species, Pinus sylvestris L. and Picea abies (L.) H. Karst, exposing seedlings to 3 seasons of ambient (430 ppm) or elevated CO₂ (750 ppm) and ambient temperatures, a + 4 °C warming or a + 8 °C warming. Pinus sylvestris responded positively to warming: seedlings developed a larger canopy, maintained high net CO₂ assimilation rates (A_net), and acclimated dark respiration (R_dark). In contrast, carbon fluxes in Picea abies were negatively impacted by warming: maximum rates of A_net decreased, electron transport was redirected to alternative electron acceptors, and thermal acclimation of R_dark was weak. Elevated CO₂ tended to exacerbate these effects in warm‐grown Picea abies, and by the end of the experiment Picea abies from the +8 °C, high CO₂ treatment produced fewer buds than they had 3 years earlier. Treatments had little effect on leaf and wood anatomy. Our results highlight that species within the same plant functional type may show opposite responses to warming and imply that Picea abies may be particularly vulnerable to warming due to low plasticity in photosynthetic and respiratory metabolism.     

Primary attraction and kairomonal host discrimination in three species of Dendroctonus (Coleoptera: Scolytidae)

Abstract 1 Synthetic blends of bole and foliage volatiles of four sympatric species of conifers were released from pheromone‐baited multiple‐funnel traps to determine if three species of tree‐killing bark beetles (Coleoptera: Scolytidae): (i) exhibited primary attraction to volatiles of their hosts and (ii) discriminated among volatiles of four sympatric species of host and nonhost conifers. 2 Bole and foliage volatiles from Douglas‐fir, Pseudotsuga menziesii (Mirb.) Franco, increased the attraction of coastal and interior Douglas‐fir beetles, Dendroctonus pseudotsugae Hopkins, to pheromone‐baited traps. Primary attraction to bole volatiles was observed in interior D. pseudotsugae. Beetles were significantly less attracted to the pheromone bait when it was combined with volatiles of lodgepole pine, Pinus contorta var. latifolia Engelm. or interior fir, Abies lasiocarpa × bifolia. 3 The monoterpene myrcene synergized attraction of mountain pine beetles, Dendroctonus ponderosae Hopkins, to their aggregation pheromones, but there was no evidence of primary attraction to host volatiles or discrimination among volatiles from the four conifers. 4 There was significant primary attraction of the spruce beetle, Dendroctonus rufipennis Kirby, to bole and foliage volatiles of interior spruce, Picea engelmannii × glauca, but beetles did not discriminate among volatiles of four sympatric conifers when they were combined with pheromone baits. 5 Our results indicate that host volatiles act as kairomones to aid pioneer Douglas‐fir beetles and spruce beetles in host location by primary attraction, and that their role as synergists to aggregation pheromones is significant. For the mountain pine beetle, we conclude that random landing and close range acceptance or rejection of potential hosts would occur in the absence of aggregation pheromones emanating from a tree under attack.     

Leaf morphology of 40 evergreen and deciduous broadleaved subtropical tree species and relationships to functional ecophysiological traits

We explored potential of morphological and anatomical leaf traits for predicting ecophysiological key functions in subtropical trees. We asked whether the ecophysiological parameters stomatal conductance and xylem cavitation vulnerability could be predicted from microscopy leaf traits. We investigated 21 deciduous and 19 evergreen subtropical tree species, using individuals of the same age and from the same environment in the Biodiversity‐Ecosystem Functioning experiment at Jiangxi (BEF‐China). Information‐theoretic linear model selection was used to identify the best combination of morphological and anatomical predictors for ecophysiological functions. Leaf anatomy and morphology strongly depended on leaf habit. Evergreen species tended to have thicker leaves, thicker spongy and palisade mesophyll, more palisade mesophyll layers and a thicker subepidermis. Over 50% of all evergreen species had leaves with multi‐layered palisade parenchyma, while only one deciduous species (Koelreuteria bipinnata) had this. Interactions with leaf habit were also included in best multi‐predictor models for stomatal conductance (g_s) and xylem cavitation vulnerability. In addition, maximum g_s was positively related to log ratio of palisade to spongy mesophyll thickness. Vapour pressure deficit (vpd) for maximum g_s increased with the log ratio of palisade to spongy mesophyll thickness in species having leaves with papillae. In contrast, maximum specific hydraulic conductivity and xylem pressure at which 50% loss of maximum specific xylem hydraulic conductivity occurred (Ψ₅₀) were best predicted by leaf habit and density of spongy parenchyma. Evergreen species had lower Ψ₅₀ values and lower maximum xylem hydraulic conductivities. As hydraulic leaf and wood characteristics were reflected in structural leaf traits, there is high potential for identifying further linkages between morphological and anatomical leaf traits and ecophysiological responses.     

Stand development and regeneration during a 33-year period in a seral Picea glehnii forest, northern Japan

Stand development and regeneration were studied during a 33-year period (1965-1998) in a 1-ha plot in a seral Picea glehnii forest in northern Japan. P. glehnii was mono-dominant in the upper canopy layer, but its understory trees were rarely found in 1965. Other species were scarcely observed in 1965. Many recruited saplings of Abies sachalinensis which had grown to > 5 cm diameter at breast height (DBH) by 1998 had become dominant in the understory layer. Mortality of P. glehnii canopy trees was low. Therefore, the stand basal area increased during the census period due to the growth of surviving canopy trees. Stand development brought about intense competition among trees by increasing local crowding for each tree, and promoted dominance of larger trees and suppression of smaller trees. Although growth rates of understory trees of the two conifers decreased with the increase in local crowding, the growth rate of A. sachalinensis was consistently higher than that of P. glehnii at all extents of local crowding. The recruitment rate (growing to 5 cm DBH) of the two conifers was less affected by local crowding. However, the number of recruits of P. glehnii was only about a quarter of that of A. sachalinensis during the census period because the regeneration of P. glehnii was largely restricted to fallen logs and within 1 m of the base of any live tree > 20 cm DBH. Therefore, our long-term study suggests that A. sachalinensis will dominate over P. glehnii in the seral forest because of higher recruitment and growth rates of the former than the latter in the understory.An erratum to this article can be found at     

Seedling survival in a northern temperate forest understory is increased by elevated atmospheric carbon dioxide and atmospheric nitrogen deposition

We tested the main and interactive effects of elevated carbon dioxide concentration ([CO₂]), nitrogen (N), and light availability on leaf photosynthesis, and plant growth and survival in understory seedlings grown in an N‐limited northern hardwood forest. For two growing seasons, we exposed six species of tree seedlings (Betula papyrifera, Populus tremuloides, Acer saccharum, Fagus grandifolia, Pinus strobus, and Prunus serotina) to a factorial combination of atmospheric CO₂ (ambient, and elevated CO₂ at 658 μmol CO₂ mol⁻¹) and N deposition (ambient and ambient +30 kg N ha⁻¹ yr⁻¹) in open‐top chambers placed in an understory light gradient. Elevated CO₂ exposure significantly increased apparent quantum efficiency of electron transport by 41% (P<0.0001), light‐limited photosynthesis by 47% (P<0.0001), and light‐saturated photosynthesis by 60% (P<0.003) compared with seedlings grown in ambient [CO₂]. Experimental N deposition significantly increased light‐limited photosynthesis as light availability increased (P<0.037). Species differed in the magnitude of light‐saturated photosynthetic response to elevated N and light treatments (P<0.016). Elevated CO₂ exposure and high N availability did not affect seedling growth; however, growth increased slightly with light availability (R²=0.26, P<0.0001). Experimental N deposition significantly increased average survival of all species by 48% (P<0.012). However, seedling survival was greatest (85%) under conditions of both high [CO₂] and N deposition (P<0.009). Path analysis determined that the greatest predictor for seedling survival in the understory was total biomass (R²=0.39, P<0.001), and that carboxylation capacity (V_cmax) was a better predictor for seedling growth and survival than maximum photosynthetic rate (A_max). Our results suggest that increasing [CO₂] and N deposition from fossil fuel combustion could alter understory tree species recruitment dynamics through changes in seedling survival, and this has the potential to alter future forest species composition.     

Seasonal and age-related variation in the needle quality of five conifer species

Summary Age changes of foliage resource quality (water, nitrogen, fibre, phenolics and toughness) were studied in five species of conifer (Pinus sylvestris L.), Picea abies (L.) Karsten, Tsuga heterophylla (Rafinesque) Sargent (all Pinaceae), Chamaecyparis lawsonian (Murray) Parlatore and Thuja plicata D. Don (both Cupressaceae) over a 2-year period in an English forest.Mature foliage of Pinus sylvestris was characterized by higher levels of nitrogen, fibre and toughness, and lower phenolics, and that of Tsuga heterophylla by higher levels of phenolics, and lower fibre and toughness levels, than the mature needles of the other species studied. Immature needles had higher levels of water and nitrogen, and lower levels of fibre and toughness, than older needles. Immature needles of Picea abies and Tsuga heterophylla had a high concentration of phenolics, which decreased with needle maturity. By mid-August, the levels of most of the foliar constituents in current-year needles had stabilized at levels maintained for the next year. Sampling revealed a fall in the concentration of phenolics, fibre and water in mature needles between March and June. Possible reasons for this seasonal trend are discussed. The levels of conifer foliar constituents were compared with levels recorded in broadleaf trees. Conifers had greater concentrations of all measured foliar constituents, but, with the exception of the six fold greater toughness of conifer needles, the differences between broadleaves and conifers were less than those between the immature and mature conifer needles. Previous studies have related the abundance of Lepidoptera on conifers to hostplant taxonomic relationships. However, the foliar constituents measured in this study were poor predictors of taxonomic relationships between the conifers. It is suggested that the abundance of Lepidoptera on conifers is not determined by levels of general foliar constituents and the role of other hostplant factors in shaping lepidopteran utilization of conifers is discussed.     

Seasonal variation of growth and development of the roots of five second year conifer species in the nursery

Every other week over their second growing season, stem height, collar diameter, shoot and root dry masses, number of lateral roots and length of the tap root were measured on nursery grown seedlings ofAbies balsamea L. Mill.,Pinus banksiana Lamb.,Pinus resinosa Ait.,Picea mariana Mill. BSP andPicea glauca Moench Voss. Root elongation, branching and mycorrhizal development were also recorded.Given species showed distinct seasonal growth patterns. The rate and timing of maximum root growth (mg/dry weight/week) differed markedly between species.Except for the increase in height ofPinus banksiana, root and shoot growth were not negatively correlated.The results are discussed in relation to the performance of tree seedlings in the nursery.     

Airborne conifer pollen grains are rarely deposited on stigmas of coflowering insect‐pollinated angiosperms

Although plant species with either animal or wind pollination modes are widespread and usually sympatric in nature, the degree of pollen interference from wind‐pollinated species on animal‐pollinated species remains little known. Conifer trees generally release a huge number of pollen grains into the air, floating into our noses and sometimes causing an allergic response. Here we document airborne pollen from two conifers (Pinus densata Mast. and Picea likiangensis (Franch.) E. Pritz.) deposited on the stigmas of eight coflowering insect‐pollinated angiosperms over 2 years in a mountainous forest community, in Shangri‐La, southwest China. Pollen density in the air as well as conifer pollen deposited onto stigmas at short and long distances from the airborne pollen source were quantified. Our results showed that conifer pollen as a proportion of total stigmatic pollen loads in the insect‐pollinated plants varied from 0.16% to 8.67% (3.16% ± 0.41%, n = 735) in 2016 and 0.66% to 5.38% (2.87% ± 0.86%, n = 180), and pollen quantity per unit area was closely related to that of airborne pollen in the air. Conifer pollen deposition on stigmas of insect‐pollinated species decreased greatly with increased distance from the pollen source. In the 10 plant species flowering in summer after conifer pollen release had finished, heterospecific pollen deposited on these stigmas came mainly from other insect‐pollinated flowers, with little contribution from airborne conifer pollen. The results indicate that there might be little interference with coflowering angiosperms by airborne pollen from dominant conifers in natural communities.     

Growth and nutrient uptake of coniferous seedlings: Comparison among 10 species at various seedbed densities

Summary The evaluation of biomass production and uptake of N, P, K, Ca, and Mg for various plant components (roots, stems, and foliage) and totals by 10 species of 2-0 coniferous seedlings grown at a controlled range of densities in a highly productive forest nursery documents considerable differences among species and seedling parameters. The species are ranked by biomass and nutrient-element relationships on a unit area of seedbed basis, quantifying the magnitude of the differences among the species at the various density levels. The 10 species include Abies balsamea, Larix leptolepis, Picea abies, Picea glauca, Picea mariana, Picea pungens, Pinus resinosa, Pinus strobus, Pinus sylvestris, and Pseudotsuga menziesii.Contribution of the Forest Resources Council, School of Environmental and Resource Management, State University of New York College of Environmental Science and Forestry, Syracuse, New York 13210.Contribution of the Forest Resources Council, School of Environmental and Resource Management, State University of New York College of Environmental Science and Forestry, Syracuse, New York 13210.     

A latitudinal gradient in tree growth response to climate warming in the Siberian taiga

We investigated the climate response of three Siberian taiga species, Larix cajanderi, Picea obovata, and Pinus sylvestris, across a latitudinal gradient in central Siberia. We hypothesized that warming is more frequently associated with increased growth for evergreen conifers (P. obovata and P. sylvestris) than for L. cajanderi, and for northern than for southern sites; we also hypothesized that increased growth is associated with a positive trend in normalized difference vegetation index (NDVI). In mixed stands, growth of L. cajanderi and P. obovata increased over time, but the larger growth increases in P. obovata may presage a shift in competitive balance between these species. Climate response varied among and within populations of all species, and positive responses to temperature prevailed at northern sites, where trees grew faster in years with warm early summers. Negative responses to warming declined along the south to north latitudinal gradient. We observed considerable variability in climate response within populations which even exceeded that among species or sites. Tree response to climate was correlated with NDVI trends, indicating that patterns of tree‐growth response to climate were indicative of a coherent landscape‐scale response to warming. Our findings suggest that increased productivity with warming is likely only in the northern reaches of the Siberian taiga. An increased prevalence of evergreen conifers in areas currently dominated by deciduous Larix species also seems likely.     

Direct observation and modelling of embolism spread between xylem conduits: a case study in Scots pine

Xylem embolism is one of the main processes involved in drought‐related plant mortality. Although its consequences for plant physiology are already well described, embolism formation and spread are poorly evaluated and modelled, especially for tracheid‐based species. The aim of this study was to assess the embolism formation and spread in Pinus sylvestris as a case study using X‐ray microtomography and hydraulics methods. We also evaluated the potential effects of cavitation fatigue on vulnerability to embolism and the micro‐morphology of the bordered pits using scanning electron microscopy (SEM) to test for possible links between xylem anatomy and embolism spread. Finally, a novel model was developed to simulate the spread of embolism in a 2D anisotropic cellular structure. Results showed a large variability in the formation and spread of embolism within a ring despite no differences being observed in intertracheid pit membrane anatomical traits. Simulations from the model showed a highly anisotropic tracheid‐to‐tracheid embolism spreading pattern, which confirms the major role of tracheid‐to‐tracheid air seeding to explain how embolism spreads in Scots pine. The results also showed that prior embolism removal from the samples reduced the resistance to embolism of the xylem and could result in overestimates of vulnerability to embolism.     

SURFACE WAX DEPOSITS ON FOLIAGE OF PICEA PUNGENS AND OTHER CONIFERS

Epicuticular waxes of five cultivated varieties of Picea pungens and six other species of conifers were examined directly with a scanning electron microprobe. The quality, quantity, and distribution patterns of surface wax deposits varied between species, within species, within plants, and within individual leaves. Two basic types of wax were observed, structural and amorphous. Structural wax is associated with glaucousness and the blue foliage color in Picea pungens and other species. The degree of occlusion of the stomatal pore by surface waxes also varied from heavily occluded to completely free of wax. Wax in the epistomatal chamber may be another resistance barrier to gas exchange by conifer needles.