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.

Inventory of aspen trees in spruce dominated stands in conserva-tion area

Background： The occurrence of aspen trees increases the conservation value of mature conifer dominated forests. Aspens typically occur as scattered individuals among major tree species, and therefore the inventory of aspens is challenging. Methods： We characterized aspen populations in a boreal nature reserve using diameter distribution, spatial pattern, and forest attributes： volume, number of aspens, number of large aspen stems and basal area median diameter. The data were collected from three separate forest stands in Koli National Park, eastern Finland. At each site, we measured breast height diameter and coordinates of each aspen. The comparison of inventory methods of aspens within the three stands was based on simulations with mapped field data. We mimicked stand level inventory by locating varying numbers of fixed area circular plots both systematically and randomly within the stands. Additionally, we also tested if the use of airborne laser scanning （ALS） data as auxiliary information would improve the accuracy of the stand level inventory by applying the probability proportional to size sampling to assist the selection of field plot locations. Results： The results showed that aspens were always clustered, and the diameter distributions indicated different stand structures in the three investigated forest stands. The reliability of the volume and number of large aspen trees varied from relative root mean square error figures above 50% with fewer sample plots （5-10） to values of 25%-50% with ]0 or more sample plots. Stand level inventory estimates were also able to detect spatial pattern and the shape of the diameter distribution. In addition, ALS-based auxiliary information could be useful in guiding the inventories, but caution should be used when applying the ALS-supported inventory technique. Conclusions： This study characterized European aspen populations for the purposes of monitoring and management of boreal conservation areas. Our results suggest that if the number of sample plots is adequate, i.e. 10 or more stand level inventory will provide accurate enough forest attributes estimates in conservation areas （minimum accuracy requirement of RMSE% is 20%-50%）. Even for the more ecologically valuable attributes, such as diameter distribution, spatial pattern and large aspens, the estimates are acceptable for conservation purposes

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.