Stand structure and height-diameter relationship of a degraded Populus euphratica forest in the lower reaches of the Tarim River, Northwest China

Understanding stand structure and height-diameter relationship of trees provides very useful information to establish appropriate countermeasures for sustainable management of endangered forests. Populus euphratica, a dominant tree species along the Tarim River watershed, plays an irreplaceable role in the sustainable development of regional ecology, economy and society. However, as the result of climate changes and human activities, the natural riparian ecosystems within the whole river basin were degraded enormously, particularly in the lower reaches of the river where about 320 km of the riparian forests were either highly degraded or dead. In this study, we presented one of the main criteria for the assessment of vitality of P. euphrafica forests by estimating the defoliation level, and analyzed forest structure and determined the height-diameter （height means the height of a tree and diameter means the diameter at breast height （DBH） of a tree） relationship of trees in different vitality classes （i.e. healthy, good, medium, senesced, dying, dead and fallen）. Trees classified as healthy and good ac- counted for approximately 40% of all sample trees, while slightly and highly degraded trees took up nearly 60% of total sample trees. The values of TH （tree height） and DBH ranged from 0-19 m and 0-125 cm, respectively. Trees more than 15 m in TH and 60 cm in DBH appeared sporadically. Trees in different vitality classes had different distribution patterns. Healthy trees were mainly composed more of relatively younger trees than of degraded tress. The height-diameter relationships differed greatly among tress in different vitality classes, with the coefficients ranging from 0.1653 to 0.6942. Correlation coefficients of TH and DBH in healthy and good trees were higher than those in trees of other vitality classes. The correlation between TH and DBH decreased with the decline of tree vitality. Our results suggested that it might be able to differentiate degraded P. euphratica trees from healthy trees by determining the height-diameter correlation coefficient, and the coefficient would be a new parameter for detecting degradation and assessing sustainable management of floodplain forests in arid regions. In addition, tree vitality should be taken into account to make an accurate height-diameter model for tree height prediction.

Linnebjer—A South Swedish Oak Forest and Meadow Area—Revisited after Half a Century

An oak forest and three wet meadows/fens were reinvestigated after 50 years concerning tree vitality, biomass and productivity, and soil chemistry. Sulphur and nitrogen deposition has changed dramatically during these years, and the aim was to analyse the differences in both the oak forest and the open field ecosystems. Trees were re-measured and soil profiles were resampled. Important visible changes in the oak forest were stated concerning the vitality of oaks. Aboveground there was a decrease in tree biomass, production and litter fall, but a huge increase in standing dead logs. During the years, the deposition of sulphur had decreased drastically, but nitrogen deposition was still high. Soil acidification in the forest had decreased, reflected in an increased base saturation in the forest, in spite of slightly lowered pH-values. Strongly increased amounts of exchangeable Ca and Mg now appeared in the forest soil, and a substantial transport of calcium and magnesium had obviously taken place from the forest soil to the meadow and fens during the years. However, the most important soil change was the accumulation of organic matter. The increased accumulation of organic matter in turn meant increased amounts of colloid particles and microsites for ion exchange in the soil. This favoured 2-valence base cations, and especially Ca and Mg that increased very much in all the studied ecosystems. Carbon as well as nitrogen had strongly increased in the forest, meadow and fen soils. This was interpreted as a natural result of increased vegetation growth due to high nitrogen deposition, increased global annual temperature and increased carbon dioxide concentration in air. It was concluded that the decreased deposition of sulphur had had a positive effect on soil chemistry, and that the deposition of nitrogen probably had stimulated vegetation growth in general, and contributed to increased amount of organic matter in the soils. However, in this studied oak forest, the decreased vitality and many killed trees were also suspected to be a result of high nitrogen deposition. Obviously increased tree growth was counteracted by decreased stress resistance, and increased appearance of pathogens in the oak trees.