Xiaomei Peng

Human activity impacts on the stem radial growth of Populus euphratica riparian forests in China’s Ejina Oasis, using tree-ring analysis

Key messageThe heterogeneous stem radial growth ofPopuluseuphraticaand its correlation with hydrology and climate factors were investigated by a tree-ring study in Ejina Oasis in Northwest China.AbstractOver the last several decades, riparian forests in Northwest China have been extremely vulnerable to the increasing pressures from human activity and climate change. However, it is unclear how forests correlate with these influences over time and on a wide scale. In the present study, we developed 28 tree-ring chronologies of Populus euphratica in the Ejina Oasis, in the lower reaches of Northwestern China’s Heihe River, to evaluate the oasis-scale riparian forest growth variation and its correlation with the runoff regulation. We identified three hydrologic periods using regime-shift analysis: a natural runoff period from 1954 to 1989, when the oasis was sustained by natural river flows; a degradation period from 1990 to 2002, when excessive upstream withdrawals of runoff decreased flows to the oasis; and a recovery period, from 2003 to 2010, when water allocation projects restored flows. In general, stem radial growth of poplar trees in the oasis showed decreasing and increasing trends in the degradation period and recovery period, respectively, with high spatial heterogeneity in each period. Our analysis of the relationship between the chronologies and corresponding hydrologic and climatic data revealed that the runoff-recharged hyporheic groundwater depth was the major factor that limited tree stem radial growth. Thus, it is important to allocate water between the middle and lower reaches, and inside the lower reaches, to mitigate or prevent the adverse effects of low runoff. Our study also demonstrates that researchers must carefully consider the high spatial heterogeneity during dendrochronological research in an arid riparian forest.

Intra-annual stem diameter growth of Tamarix ramosissima and association with hydroclimatic factors in the lower reaches of China’s Heihe River

High-resolution observations of cambial phenology and intra-annual growth dynamics are useful approaches for understanding the response of tree growth to climate and environmental change. During the past two decades, rapid socioeconomic development has increased the demand for water resources in the oases of the middle reaches of the Heihe River in northwestern China, and the lower reaches of the Heihe River have changed from a perennial river to an ephemeral stream with a decreased and degraded riparian zone. Tamarisk (Tamarix ramosissima) is the dominant shrub species of the desert riparian forest. In this study, the daily and seasonal patterns of tamarisk stem diameter growth, including the main period of tree ring formation, were examined. Observations concerning the driving forces of growth changes, along with implications for the ecology of the dendrohydrological area and management of desert riparian forests in similar arid regions, are also presented. The diurnal-seasonal activity of stem diameter and the dynamics of growth ring formation were studied using a point dendrometer and micro-coring methods during the 2012 growing season in shrub tamarisk in a desert riparian forest stand in the lower reaches of the Heihe River in Ejin Banner, Inner Mongolia of northwestern China. Generally, the variation in diurnal diameter of tamarisk was characterized by an unstable multi-peak pattern, with the cumulative stem diameter growth over the growing season following an S-shaped curve. The period from late May to early August was the main period of stem diameter growth and growth-ring formation. Among all of the hydroclimatic factors considered in this study, only groundwater depth was significantly correlated with stem diameter increment during this period. Therefore, for the dendrochronological study, the annual rings of the tamarisk can be used to reconstruct processes that determine the regional water regime, such as river runoff and fluctuations in groundwater depth. For the management of desert riparian forests, suitable groundwater depths must be maintained in the spring and summer to sustain tree health and a suitable stand structure.

Microcoring and dendrometer-detected intra-annual wood formation of Populus euphratica in the Ejina Oasis, northwestern China

Seasonal stem radial growth and wood formation of trees have become research hotspots because of their significance for dendroclimatological and dendroecological studies. However, until recently, these studies concentrated on coniferous tree species in high-altitude and high-latitude regions, while detailed information on arid-zone riparian forests is scarce. The main focus of this study is to monitor the intra-annual dynamics of radial growth and tree ring formation in a deciduous species, Populus euphratica . In 2013, we combined the dendrometer and microcoring methods to study this species in the riparian forest of the Ejina Oasis, in arid northwestern China. Vessel enlargement began in early May, and the maximum rate of cell production occurred in early June. The cell division then ceased from early to mid-July. The dendrometer method failed to reliably detect the date of growth initiation and cessation, but succeeded to detect the time of maximum growth rate just like the microcoring method did. We found that weekly stem radial increment data described xylem growth more accurately than daily datasets. Based on correlation analysis among climatic and hydrologic variables, and weekly stem radial increment, weekly ring width increase dataset, the depth to groundwater was the main factor that limited tree ring growth. From a practical perspective, such studies of intra-annual wood formation can provide empirical guidance for seasonal water allocations within a river basin.