Postglacial recolonization routes for Picea abies K. in Italy as suggested by the analysis of sequence-characterized amplified region (SCAR) markers

The routes through which Norway spruce recolonized the Alps after the last ice age were investigated at the genetic level. Seven populations along the Alpine range plus one Apennine population were characterized for seven sequence-characterized amplified region (SCAR) loci, detecting an overall FST = 0.118. This rather high value for forest species reflects an uneven distribution of genetic variability, and was analysed through different statistical methods. Alternative hypotheses were tested under the isolation-by-distance model and using the analysis of molecular variance (AMOVA) frame. We conclude that the hypothesis of the existence of a glacial refugium in the Apennines should be rejected, while a putative relict population is identified in the Maritime Alps. The Alpine range of Norway spruce appears to be split in two parts across a north-south line. The results are discussed in comparison with data based on morphological markers, isozymes, chloroplast microsatellites and mitochondrial markers.     

Nitrate leaching from three afforestation chronosequences on former arable land in Denmark

In regions dominated by agricultural activities, nitrogen (N) is recognized as a major pollutant in aquatic environments. In north‐western Europe, afforestation of agricultural land is part of a strategy to improve water quality. In Denmark, former arable land has been afforested during the past 40–50 years. This study evaluated the effect of afforestation of former arable land on nitrate leaching, based on three afforestation chronosequences. Precipitation, canopy throughfall and soil water were collected and soil moisture was monitored at two Danish locations, Vestskoven (nutrient‐rich, medium deposition) and Gejlvang (nutrient‐poor, high deposition). Afforestation was performed using Norway spruce [Picea abies (Karst.) L.] and common oak (Quercus robur L.) at Vestskoven and Norway spruce at Gejlvang. The results suggest that afforestation of former arable land initially leads to lower nitrate leaching than that occurring under the former agricultural land use, and largely below the standard of 50 mg NO⁻₃ L⁻¹ for groundwater to be utilized as drinking water. Nitrate concentrations became almost negligible in forest stands of 5–20 years of age. However, after canopy closure (>20 years) nitrate concentrations below the root zone and nitrate leaching tended to increase. This was attributed to increased N deposition with increasing canopy development and decreased N demand once the most N‐rich biomass compartments had been built up. Nitrate leaching started to increase at a throughfall deposition level of about 10 kg N ha⁻¹ yr⁻¹. Compared with nutrient‐poor sandy soils, nutrient‐rich clayey soils appeared more vulnerable to disturbance of the N cycle and to increased N deposition, leading to N saturation and enhanced nitrate leaching. In approximately the first 35 years after afforestation, nitrate leaching below the root zone was generally higher below oak than below Norway spruce.     

Seasonal changes in the distribution of green spruce aphid Elatobium abietinum (Walker) (Homoptera: Aphididae) in the canopy of Sitka spruce

1 Seasonal changes in the distribution of green spruce aphid Elatobium abietinum (Walker) within the canopy of 20–25‐year‐old Sitka spruce are described based on data from two low‐altitude sites (310–420 m above sea‐level), two mid‐altitude sites (500–550 m a.s.l) and one high‐altitude site (610 m a.s.l). 2 Aphids were counted throughout the canopy on shoots representative of all needle age‐classes present at each whorl of branches. Counts were made during the middle week of each month from September to July for 4 years (1999–2003), and mean E. abietinum densities at each canopy position were calculated separately for each month and for the low‐, mid‐ and high‐altitude sites. 3 During September to November, the highest densities of E. abietinum occurred on 3–4‐year‐old needles on branches low in the canopy. Over the winter and spring, the centre of the aphid’s distribution shifted outward and upward, so that by June of the next year the highest aphid densities occurred on current and 1‐year‐old needles on branches near the top of the tree. 4 The aphid distribution was re‐set each year during July, at the time when the nutrient quality of the host was in decline and E. abietinum populations were decreasing. Aphid densities decreased less on 3–4‐year‐old needles than on current and 1‐year‐old needles, suggesting that older needles were a superior food resource at this time of year and in the autumn. However, other factors, such as higher temperatures in the upper canopy during the summer or differential mortality caused by natural enemies, could also have contributed to the change in distribution. 5 The outward and upward shift in the aphid distribution over the winter period provided no evidence that aphids at positions lower and deeper in the canopy were better insulated from freezing temperatures and had higher over‐winter survival rates. Mean air temperatures at the top and bottom of the canopy during the winter were also found to differ by only 0.1–0.2 °C. 6 The percentage of the total aphids per tree that occurred on current or 1‐year‐old needles varied widely between seasons and between sites. Consequently, sampling programmes designed to estimate total population numbers of E. abietinum have little option but to sample needles throughout the canopy, and at regular intervals during the period when the aphid is abundant.     

Non‐serotinous woody plants behave as aerial seed bank species when a late‐summer wildfire coincides with a mast year

Abstract

1. Trees which lack obvious fire-adaptive traits such as serotinous seed-bearing structures or vegetative resprouting are assumed to be at a dramatic disadvantage in recolonization via sexual recruitment after fire, because seed dispersal is invariably quite constrained. We propose an alternative strategy in masting tree species with woody cones or cone-like structures: that the large clusters of woody tissue in a mast year will sufficiently impede heat transfer that a small fraction of seeds can survive the flaming front passage; in a mast year, this small fraction would be a very large absolute number.
2. In Kootenay National Park in British Columbia, we examined regeneration by Engelmann spruce (Picea engelmannii), a non-serotinous conifer, after two fires, both of which coincided with mast years. Coupling models of seed survivorship within cones and seed maturation schedule to a spatially realistic recruitment model, we show that (1) the spatial pattern of seedlings on a 630 m transect from the forest edge into the burn was best explained if there was in situ seed dissemination by burnt trees; (2) in areas several hundred meters from any living trees, recruitment density was well correlated with local prefire cone density; and (3) spruce was responding exactly like its serotinous codominant, lodgepole pine (Pinus contorta).
3. We conclude that non-serotinous species can indeed behave like aerial seed bank species in mast years if the fire takes place late in the seed maturation period. Using the example of the circumpolar boreal forest, while the joint probability of a mast year and a late-season fire will make this type of event rare (we estimate P = 0.1), nonetheless, it would permit a species lacking obvious fire-adapted traits to occasionally establish a widespread and abundant cohort on a large part of the landscape.

     

Induced tree response to herbivory influences the foraging preference and performance of a conifer‐defoliating sawfly

Herbivory often alters the growth and development of woody plants and can thereby render hosts less susceptible to subsequent herbivores. We carried out field surveys and experiments to investigate how previous herbivory influences adult egg lay, larval feeding preference, and associated survival of the yellowheaded spruce sawfly, Pikonema alaskensis (Rohwer) (Hymenoptera: Tenthredinidae), within crowns of black spruce, Picea mariana (Mill.) Britton et al. (Pinaceae). Pikonema alaskensis females laid nearly twice as many eggs, and late‐instar larvae consumed nearly twice as much foliar biomass, on undefoliated vs. defoliated branches. The major factor driving the lower incidence of egg lay on defoliated branches was a significant reduction in the availability of preferred size classes of shoots. In general, adult females preferred medium‐sized shoots, whereas late instars preferred large shoots; previous defoliation generally decreased the availability of medium and large shoots. In a field bioassay, late‐instar survival generally declined with increasing defoliation, which again corresponded to significant reductions in shoot length on defoliated branches. The tendency of P. alaskensis to avoid shoots on severely defoliated branches is likely to increase the time lag between herbivory and the feedback effects of induction and could thus contribute to fluctuations in sawfly population density during outbreak. Our results differ from numerous past studies of closely related diprionid sawflies in conifers, most of which have reported weak or positive effects of previous herbivory on host susceptibility; we attribute the observed responses of spruce to the unusual sawfly feeding preference of P. alaskensis for developing conifer foliage.     

From glacial refugia to modern populations: new assemblages of organelle genomes generated by differential cytoplasmic gene flow in transcontinental black spruce

Assessing species' range-wide cytoplasmic diversity provides valuable insights regarding their dispersal and adaptive potential in a changing environment. Transcontinental chloroplast (cpDNA) and mitochondrial DNA (mtDNA) population structures were compared to identify putative ancestral and new cytoplasmic genome assemblages in black spruce (Picea mariana), a North American boreal conifer. Mean within-population diversity and allelic richness for cpSSR markers were 0.80 and 4.21, respectively, and diminished westward. Population differentiation based on G(ST) was lower for cpDNA than for mtDNA (G(ST) =0.104 and 0.645, respectively) but appeared comparable when estimated using Jost differentiation index (D=0.459 and 0.537, respectively). Further analyses resulted in the delineation of at least three genetically distinct cpDNA lineages partially congruent with those inferred from mtDNA data, which roughly corresponded to western, central and eastern Canada. Additionally, the patterns of variation in Alaska for both cpDNA and mtDNA markers suggested that black spruce survived the last glacial maximum in this northern region. The range-wide comparison of the geographic extent of cytoplasmic DNA lineages revealed that extensive pollen gene flow between ancestral lineages occurred preferentially from west to east during the postglacial expansion of the species, while seed-mediated gene flow remained geographically restricted. This differential gene flow promoted intraspecific cytoplasmic capture that generated new assemblages of cpDNA and mtDNA genomes during the Holocene. Hence, black spruce postglacial colonization unexpectedly resulted in an increase in genetic diversity with possible adaptive consequences.     

Changes in fine root biomass of Picea abies forests: predicting the potential impacts of climate change

Aims The impact of global warming on belowground processes, especially on fine root production, is poorly understood in comparison with its aboveground counterpart.Methods Here, we compiled 227 measurements to assess the influence of temperature and precipitation on fine root biomass of Norway spruce (Picea abies [L.] Karst) forest ecosystems in the Eurasia boreal region.Important findings We found that fine root biomass decreased significantly with latitudes. There was a biomass increase of 0.63 Mg ha^-1 and 0.32 Mg ha^-1 for fine roots <2 and <1 mm in diameter, respectively, with 1°C increase of mean annual temperature. There was an increase of 0.5 and 0.1 Mg ha^-1 per 100 mm year^-1 precipitation for the two size classes of fine roots. If the adaption of root production can match the pace of global warming and water is not a limiting factor for plant growth, fine root biomass would be expected to increase by 40–140% in response to the predicted increase in temperature (3–10°C) over the next century. Our analyses highlighted the strongly positive influences of temperature and precipitation on belowground function, suggesting that predicted future climate change could substantially enhance belowground biomass in the boreal region where the greatest warming is anticipated. This potential increase of belowground biomass, coupled with aboveground biomass, may provide a better understanding of climate–ecosystem feedbacks.