Effects of Soil Fertility and Atmospheric CO_2 Enrichment on Leaf, Stem and Root Dark Respiration of Populus tremuloides

An open-top chamber experiment was conducted at the University of Michigan Biological Station near Pellston, Michigan, USA, to study the effects of soil fertility and CO2 on leaf, stem and root dark respiration (Rd) of Populus tremuloides. Overall, area-based day-time leaf Rd (Rda) was significantly greater at elevated than at ambient CO2 in high-fertility soil, but not in low-fertility soil. Mass-based leaf Rd (Rdm) was overall greater for high- than for low-fertility soil grown trees at elected, but not at ambient CO2. Nighttime leaf Rd. and Rdm were unthected by soil fertility or CO2, nor was stem Rda, which ranged from 1.0 to 1.4 μmol m-2 s-1 in the spring and 3.5 to 4.5 μmol m-2 s-1 in the summer. Root Rda. was significantly higher in high- than in low-fertility soil, but was unaffected by CO2. Since biomass production of P. tremuloides will be significantly greater at elevated CO2 while specific Rd will either increase or remain unchanged, we predict that carbon loss to the atmosphere through respiration from this ecologically important species would increase at higher CO2. Soil fertility would also interact with elevated CO2 in affecting the carbon flow in the plant-soil-air system.

Observations on bud burst phenology in a field trial established with Poplar (Populus spp.)

Bud flushing is very important for the survival and growth of trees, a phenomenon matched each year with the annual course of temperature and the timing of bud flushing in the spring. Essentially it represents a serious ecological and evolutionary tradeoff between survival and growth. The most suitable timing of bud burst permits trees to begin growth sufficiently early to take advantage of favorable spring conditions, but late enough to decrease the risks of tissue damage from late frost. In the present study bud burst spring phenology of poplar （Populus tremula and P. tremuloides） from eight different provenances, originating from Eu- rope and the USA, was observed during March and April, 2009. The experimental plot was located at Solling, Germany （51~44＇0＂ N, 9036＇0＇＇ E）. A six stage subjective scoring system of bud burst phenology was used to identify the phenological stages of the seed- lings, where each plant was observed twice a week. The aim of the study was to predict phenotypic variation in poplar, originating from regions between 42~ and 60~ N latitude, growing in similar environments. Timing of bud flushing of poplar was recorded. It was found that seedlings of provenance 3, which originated from 42.35~ N latitude, started and completed flushing significantly earlier than those of other provenances, while seedlings of provenance 5, originating from 54.29~ N latitude, started flushing very late and only a few plants reached top scoring at the end of the experimental period. Analysis of variance showed statistically highly signifi- cant differences （p 〈 0.05） in bud flushing among the provenances. The correlation between scoring and flushing periods was very strong within provenances although the flushing pattern differed among provenances （origin of the planted seedlings）. Bud flushing showed a negative correlation with the origin of the planted seedlings. Given the field experience gained with this experiment, it is recommended that seedlings from provenances 5 and 8 could be used for future plantations where late frost may be a problem for the young shoots of seedlings.

Production of hybrid aspen on agricultural land during one rotation in southern Sweden

High potential productivity together with short rotation periods have made hybrid aspen an interesting option for wood production on former arable land in Nordic countries.In this study,some of the oldest active experimental plots with hybrid aspen in Sweden were remeasured at 23–30 years of age.A main aim was to assess age and productivity at the time of maximum mean annual volume increment.In addition,the infl uence of commercial thinning on stand development and diff erences in genetic gain among clones were investigated.Data from fi ve experiments in southern Sweden were used,including three genetic trials,one demonstration stand with a clone mixture and one stand regenerated from root suckers.The three genetic trials were treated as single plot experiments,subject to a standard thinning program.In the remaining experiments,diff erent thinning strategies were tested in a balanced block design.Volume growth had culminated or was close to maximum at age 25–30 years.Mean annual stem volume increment at culmination was 20–22 m 3 ha^−1 a^−1.Dominant height reached 30–35 m at 28–30 years of age.Mean diameter at breast height was 27–29 cm after 29–30 years in the genetic trials.Clonal ranking based on diameter at age 7–9 years was positively correlated with the ranking at the fi nal measurement in the genetic trials,20 years later.This indicates that clones can be selected for superior growth based on results from young trials.More intense thinning programs increased the mean diameter compared to light thinning.The study indicates that one or two early and relatively heavy thinnings can promote the development of crop trees,without jeopardizing total volume production during a rotation of 25–30 years.

Growth and chemical responses of trembling aspen to simulated browsing and ungulate saliva

Aims Woody plant-browser systems represent an understudied facet of herbivory.We subjected four genotypes of trembling aspen to artificial browsing,similar to that of a large mammalian herbivore,and applied deer saliva to clipped and unclipped trees to assess:(i)the effects of artificial browsing on aspen growth and phytochemistry of leaves and stems,(ii)genotypic variation in responses and(iii)potential alterations of responses by mammalian saliva.Methods Potted aspen trees were grown outdoors on the University of Wisconsin-Madison campus.The experiment consisted of a fullycrossed,2×2×4 randomized complete block design,with two levels of artificial browsing(unclipped and clipped),two levels of saliva application(no saliva and saliva)and four aspen genotypes.To simulate ungulate browsing damage,we removed the upper 50%of the stem of half of the trees by pinching the stem with needle-nosed pliers and then separating it by tearing.For half of the damaged trees,we immediately swabbed the wound with deer saliva.Trees in the unclipped plus saliva treatment were swabbed with saliva at the 50%height mark.To assess the effects of clipping and saliva application,we harvested all trees after 2 months and measured various growth and chemical properties.Growth measurements included height,vertical growth,mass of leaves,stems and roots,leaf number and area and bud set.Chemical parameters included defensive,nutritional and structural components of both foliage and stems.Important Findings Clipping affected most of the growth parameters measured,decreasing tree height,leaf,stem,root and total tree mass and leaf area.Clipped trees had greater vertical growth,more leaves and higher specific leaf area(SLA)than unclipped trees.Deer saliva had little to no effect on plant growth response to the clipping treatment.Terminal budset was delayed by clipping and varied among genotypes but not in response to saliva application.Clipping also affected most of the phytochemical variables measured,reducing defensive compounds(phenolic glycosides and condensed tannins(CTs))and nutrients(N),but increasing structural components(cellulose and lignin)in both leaves and stems.Saliva had very little effect on tree chemistry,causing only a slight decrease in the amount of CTs in leaves.In general,leaves contained more defensive compounds and nutrients,but much less cellulose,compared with stems.Genotypes differed for all physical and chemical indices,and in tolerance to damage as measured by vertical growth.In addition,for most of the physical and chemical variables measured,genotype interacted with the clipping treatment,suggesting that in natural stands some genotypes will resist or tolerate browsing better than others,affecting forest genetic composition and ultimately forest dynamics.