Age Structure and Growth of Degraded Populus euphratica Floodplain Forests in North-west China and Perspectives for Their Recovery (original) (raw)
Related papers
Landscape and Ecological Engineering, 2008
Populus euphratica Oliv. is a main tree species that forms natural riparian forests in arid and semi-arid areas from Morocco to the Ordos Plateau. This study is designed to clarify the forest structure and dynamics of P. euphratica and to elucidate the ecological mechanisms sustaining riparian forests under unreliable environmental conditions. This study was conducted in a P. euphratica forest of the Ejina Oasis in Inner Mongolia, China, which is a hyperarid area. According to their tree size distribution, P. euphratica forests can be grouped into juvenile, mature, and overmatured stages. Almost all large P. euphratica showed dieback. The regeneration density on the forest floor shows a relation with the degree of height decrease due to dieback damage, as evaluated using the ratio of actual height to the maximum height estimated from the D-H relation. Therefore, after the mature stage, individual trees continue to grow while controlling their canopy size to adjust to changing environmental conditions in the overmatured stage. Our results suggest that P. euphratica growing under large fluctuations in groundwater levels exhibit a sophisticated regeneration system with canopy degradation.
Landscape and Ecological Engineering, 2016
To explore the regeneration properties of a Populus euphratica Oliv. forest under unstable environmental conditions, we performed replicate censuses to determine the sapling dynamics of a P. euphratica population in the vicinity of the Ejina Oasis (Inner Mongolia). Even when the stands were established on riverside flatland, we detected slight variations in ground levels and salt concentrations. Due to leaching by flood water, the salinity of topsoil was lower in the riverbed than on the riverbank. Newly recruited saplings grew on riverbanks with highsalinity soil. Saplings were distributed over a wide area via fluctuating water levels, and those growing where salt levels were relatively low were able to grow more easily and become canopy trees. Small individuals comprise a sapling bank on the forest floor that is relatively stable. The dieback of larger saplings results in few reaching a height of more than 2 m. The growth of saplings (including new recruits) is balanced by the death of smaller saplings and the dieback of larger saplings. Individual saplings persist for about 4 years on the forest floor. In summary, hydrological events coupled with soil conditions may drive vegetation distribution patterns in riparian areas in arid regions. P. euphratica forests regenerate via a sapling bank rather than a seed bank. Yearly fluctuations in water flow facilitate the spread of the sapling bank, which guarantees regeneration of the forest. Dieback through partial defoliation is a mechanism used by saplings to escape adverse conditions, thereby maintaining a stable state in arid regions.
Forestry, 2012
In the Central Asian deserts, the floodplains along the large river systems like the Amu Darya, Syr Darya and Tarim harbour the highest biodiversity and provide a wide range of ecosystem services for men in those regions. Against the background of the potential use of wood biomass and carbon sequestration, we investigate the biomass of floodplain forest (Tugai) ecosystems along the Tarim River, northwest China, and the Amu Darya, Turkmenistan. These forests are dominated by Populus euphratica Oliv. Under very good water supply conditions, the stand biomass, comprising aboveand below-ground components of P. euphratica, was 58.4 t ha 21 , corresponding with the highest tree densities. The stand biomass of P. euphratica under non-optimal conditions, i.e. prolonged flooding or periods of drought with sinking groundwater levels, ranged from 16 to 28 t ha 21. The observed stand biomass values for P. euphratica are in the range of values found by others in Xinjiang and Inner Mongolia, China, but lower than the biomass of temperate forests. However, among the desert, semi-desert and steppe ecosystems, the riparian P. euphratica forest is the ecosystem with the highest above-ground biomass. Thus, the riparian forests in Central Asia contain a regionally remarkable amount of biomass.
Journal of Plant Ecology, 2016
Aims Along central-Asian rivers in arid regions, lowering of the groundwater level constitutes a major threat to the riparian forests, whose tree layers are dominated by Euphrates poplar (Populus euphratica). Some of these fragile ecosystems are supplied with additional ('ecological') water for protection and conservation. We investigated interrelationships among groundwater distance, stand structure and above-ground wood production (at the tree and the stand level) in P. euphratica stands along a groundwater gradient (distances of 2.0-12.0 m) that also included a plot supplied with 'ecological water'. Methods We measured the height, diameter at breast height and basal area of poplar trees and determined the annual above-ground wood production for the three most recent years on the basis of tree-ring analyses and allometric regressions at three sites at the upper, middle and lower reaches of the tarim River, Xinjiang, NW china. Important Findings the distribution of age classes differed significantly among the plots: in accordance with their life histories, stands growing at a larger distance to the groundwater harboured a larger fraction of old trees (>80 years) and, in most cases, a smaller fraction of young trees (≤20 years). the annual above-ground wood production per ha was highest on a plot with a short groundwater distance (2.0 m; maximum of the 3-year investigation period: 3.0 t ha −1 a −1) and lowest, at a plot with a large distance to the water table (6.6 m; minimum: 0.23 t ha −1 a −1). However, the plot located close to the groundwater (and to the river) at the middle reaches exhibited a relatively large interannual variation in above-ground wood production, which can be attributed to interannual variations in the river discharge. At the middle and the lower reaches, these interannual variations on plots with the most favourable water supply were even more obvious at the tree level. For the fraction of mature trees (60-99 years old), no significant differences in above-ground wood production were found between the plots with the most and the least favourable water supply. Overall, the productivity at the stand level was most closely correlated with the stand density (number of trees per ha, tree cover percentage). Productivity was negatively related to tree age, whereas groundwater distance alone was not a good predictor of above-ground wood production. In conclusion, vigorous growth of P. euphratica is possible at sites with groundwater distances of up to 12 m. Supply with 'ecological water' may be beneficial to trees growing at groundwater distances not larger than ~6 m. However, allocation of water should focus on stands with a short distance to the groundwater because only under these conditions, natural generative reproduction of the poplars is possible.
Forestry, 2008
Tugai forests are the riparian forests along the rivers in the continental desert regions of Central Asia, i.e. the Tarim River, Amu Darya and Syr Darya. They mainly consist of Populus euphratica Oliv., Populus pruinosa Schrenk. and Elaeagnus oxycarpa Schltdl. As a consequence of land opening campaigns, large areas of Tugai forests were destroyed after the 1950s. Due to excessive use of water for irrigation, the remaining Tugai forests are under severe threat. Near natural Tugai forests still exist along the Tarim middle reaches in the Tarim Huyanglin Nature Reserve, Xinjiang, NW China. There is a gap in understanding, how the seedlings of P. euphratica establish as trees which continuously connect to the groundwater. Therefore, the set of conditions which must be met for germination and successful establishment, i.e. formation of Tugai forests, was investigated along a representative transect still under natural conditions. P. euphratica seedlings germinate in belts during the retreat of the summer fl ood on freshly deposited sites bare of other vegetation. Such germination sites are formed by river dynamics. While germination takes place regularly in the study area, successful establishment is restricted to few germination events. Seedlings face dropping groundwater levels during spring and early summer of the second year after germination. Therefore, for successful establishment, it is essential that the fl ood of the second year starts in time and is high enough, in order to replenish the groundwater. Furthermore, clayey soil layers in the subsoil may play a role for successful establishment, too, as they store water better than sandy soil layers.
River Research and Applications
The riparian forests in the Tarim Basin (NW China) are mainly composed of poplar species (Populus euphratica, Populus pruinosa). In the hyper-arid climate of this region, the trees are phreatophytes, which rely on access to groundwater. Essential ecosystem services (ESS) of these forests comprise provisioning, regulating, and cultural services. The ESS of the forests are threatened by overuse and a continuous decrease in the groundwater level due to excessive use of water for agriculture, which resulted in a severe reduction of the poplar forest area during the past decades.
2010
Naturally, the floodplains of Central Asian rivers harbour riparian, so-called ‘Tugai’ forests, reeds with Phragmites australis, and shrub communities which form a mosaic depending on the variety of available ground water. In recent decades, these natural ecosystems have been strongly altered anthropogenically or even completely destroyed. In order to restore those ecosystems, knowledge on vegetation, ecosystem dynamics, and natural regeneration processes is essential. In our study, we present results of ecological investigations at the Tarim River. We gathered comprehensive data on soil, vegetation, forest stand age, tree vitality, river course dynamics, and land use and brought it to the landscape level. Thus, recommendations are derived for the maintenance of these floodplain ecosystems, in particular with regard to their biological diversity.
Journal of Arid Environments, 2017
Populus euphratica is a constitutive tree species of the mostly monospecific forests along Central-Asian rivers. In the hyper-arid climate along the Tarim River, Xinjiang, NW China, these trees are phreatophytes with continuous access to the groundwater. Those poplar stands are often moderately to completely defoliated by Lepidopteran larvae in spring. In a P. euphratica stand at the middle reaches of the Tarim River, which grew close to the river and to the water table, we tested whether severe defoliation significantly reduces the trees' radial stem increment and their production of above-ground wood biomass even at ample water supply by the river. Tree-ring analyses and allometric regressions revealed a drastic decrease in radial stem increment and annual above-ground wood production upon complete defoliation even in a period of ample water supply, which otherwise would have allowed maximum productivity. The loss of above-ground production by herbivory amounted to approximately 44% relative to the high production rate of the year before complete defoliation. During the recent decade, however, only the amount of river discharge e but not the estimated intensity of defoliation e was (marginally) significantly correlated with the stem increment. We conclude that in P. euphratica forests growing close to the river and to the groundwater, tree growth is mainly related to the amount of river discharge, but can be significantly reduced in years with complete defoliation. Therefore, defoliation events should be considered in assessing the productivity of these riparian forests.
Species-Specific Effects of Groundwater Level Alteration on Climate Sensitivity of Floodplain Trees
Forests
European floodplain forest is facing increasingly frequent and severe drought events related with ongoing climate change. Moreover, this ecosystem type was frequently affected by river regulation, leading to groundwater table lowering; however, river revitalization has, in some locations, achieved some restoration of groundwater levels. In this study, we investigated the growth–climate sensitivity and growth modulation after groundwater-level manipulation for Fraxinus angustifolia Vahl. and Quercus robur L. in one of the most important floodplain forest complexes in Central Europe. We constructed three different types of tree ring chronologies to reflect the high frequency variability, medium-low frequency variability, and basal area increment. We found F. angustifolia to be more sensitive than Q. robur to both drought and groundwater level fluctuations. Moreover, F. angustifolia showed more pronounced short-term and long-term growth decreases after artificial ground water level alt...
Drought-induced shift in tree response to climate in floodplain forests of Southeastern Europe
Scientific Reports
Floodplain forests are the most rapidly disappearing ecosystem in the world, especially in temperate regions of Europe where anthropogenic influence has been pronounced throughout history. Research on primeval forests is crucial to further our understanding of their natural dynamics and interaction with climate but is limited by the lack of such preserved forests. The aim of this study was to investigate how a primeval floodplain forest in Southeastern Europe has responded to climate variability during the last 250 years through comparison of tree growth and climate, canopy disturbance and recruitment dynamic of two dominant tree species with different tolerances to flooding/drought. Our analysis revealed induced stress caused by several consecutive severe drought events in the 1940s, which led to a significant increase in sensitivity to increasing temperatures and decreasing river water levels. This trend is particularly pronounced in pedunculate oak. Age structure analysis revealed one larger episode of oak regeneration culminating after periods of intense growth release. Such period co-occurs with summer drought, which is part of a complex system of natural disturbances and a significant natural driver of the cyclical regeneration of primeval oak ecosystems. Global forest decline caused by drought has been recorded worldwide and has significantly increased since 1970 1,2. Recent changes in climate are associated with increased temperatures and changes in precipitation patterns, with more frequent, prolonged and intense episodes of drought as a consequence. Such events result in long-lasting changes in ecosystem function, community composition and structure, especially in water sensitive ecosystems such as floodplain forests 3. Lowland floodplain forest ecosystems are characterized by high productivity, diverse microhabitat conditions and considerable biodiversity 4. They are widespread in all biogeographic regions of the world on alluvial deposits of large rivers with which they have a constant hydrologic interaction 5. According to a study by Tockner et al. in 2002 6 , the world's remaining floodplain forests cover an area of approximately 2.24 × 10 6 km 2. The continuous expansion of settlements and infrastructure, as well as exploitation of natural resources, has ultimately resulted in the widespread disappearance of primeval lowland floodplain ecosystems 7,8. In Europe, natural lowland floodplain forests have all but vanished, and with them, a very important research reference point for forestry and ecology. In the last century deforestation due to agriculture has wiped out 90% of Europe's floodplain forests 9,10. The remnants of relatively natural forest occur mostly in Eastern and Southeastern Europe 3,11. Apart from deforestation, floodplain forests have been impacted by numerous activities, particularly river regulation (construction of dams, dykes, drainage systems, etc.). These interventions have disrupted the sensitive flood patterns and assisted the progression of mesohydric species 12-14. Regional episode of pedunculate oak (Quercus robur L.) decline were recorded during the 20th century in floodplain forests in almost all of Europe 15. As oak and other species die out, another problem in lowland floodplain ecosystems is the spread of mesohydric species, such as hornbeam (Carpinus betulus L.), which are becoming increasingly dominant, especially in drier, oak dominated habitats. In the last 20 years significant decline in narrow-leaved ash (Fraxinus angustifolia Vahl) was observed through the whole landscape. The greatest threat to the stability of forest ecosystems of narrow-leaved ash is currently posed by the phytopathogen Hymenoscyphus fraxineus T. Kowalski 16 but also constant increase of temperature and environment dryness. The continuation of oak and ash decline could have long-lasting consequences