Haloxylon ammodendron is a typical tree species in arid region for windbreak and sand fixation. Understanding how stand structure (i.e., afforestation density, forest type, and stand age) of H. ammodendron impacts ecological benefits and tree growth status has great implications for desertification control in arid area. We obtained a dataset of 446 observations from 79 studies related to H. ammodendron plantations for meta-analysis. Stand age was the key factor affecting performances of windbreak, tree canopy, and soil properties while afforestation density was more important in determining tree survival rate. Wind speed in H. ammodendron plantations was reduced by 52% in general compared to that at the site without any plantations and decreased as the increasing density. With the increase of stand age, soil carbon, nitrogen, and phosphorus accumulated continuously, but soil moisture in 0-60 cm depth decrease when trees grew up to nearly 20 years. Within 4000 plants ha(-1), responses of survival rate, tree height, ground diameter, and canopy decreased with the increase of afforestation density. Compared with mono-species plantation, mixed afforestation was more beneficial to windbreak and survival, while it damaged tree growth. Combining the ecological functions and tree growth of H. ammodendron plantations, the afforestation density and forest age should be controlled to 450-900 plants ha(-1) and 20 years, respectively, as the upper limit. Therefore, besides reasonable afforestation density, management measures for regulations of forest age and existing stand density were important to maintain higher ecological benefits and better tree growth of H. ammodendron plantations.

Understanding the impact of management upon post-drought tree growth recovery and drought legacy effects is among the fundamental challenges hindering the improvement of forest conservation strategies in the face of increasingly frequent, longer, and intensified extreme droughts under ongoing climate change. Yet surprisingly little is known to date about how management practices can influence drought legacy effects; and previous studies of management impacts on forest resilience to drought have reached inconsistent and contentious conclusions. This study sought to tackle these pressing questions and gain insight by analyzing tree-ring datasets from non-managed and managed Qinghai spruce forests in northwestern China. The results show improved growth resilience to drought of those trees under management practices. Moreover, Qinghai spruce radial growth in non-managed forest exhibited significant legacy effects of extreme drought, whereas such legacy effects were mitigated in managed forest. Nevertheless, both the resilience augmentation and the mitigation of drought legacy effects by management were much weaker in the face of a three-year persistent drought than a single-year event. Hence, we may conclude that current management practices are advantageous and necessary for forest conservation under exacerbated drought conditions, for which strategies and measures should be better thought out and tailored to specific situations, rather than being one-size-fits-all, to better serve the goals of forest managers and conservationists.

Rising atmospheric carbon dioxide (CO2) may enhance tree growth and mitigate drought impacts through CO2 fertilization. However, multiple studies globally have found that rising CO2 has not translated into greater tree growth despite increases in intrinsic water-use efficiency (iWUE). The underlying mechanism discriminating between these two general responses to CO2 fertilization remains unclear. We used two species with contrasting stomatal regulation, the relatively anisohydric Qilian juniper (Sabina przewalskii) and the relatively isohydric Qinghai spruce (Picea crassifolia), to investigate the long-term tree growth and iWUE responses to climate change and elevated CO2 using tree ring widths and the associated cellulose stable carbon isotope ratios (delta C-13). We observed a contrasting growth trend of juniper and spruce with juniper growth increasing while the spruce growth declined. The iWUE of both species increased significantly and with similar amplitude throughout the trees' lifespan, though the relatively anisohydric juniper had higher iWUE than the relatively isohydric spruce throughout the period. Additionally, with rising CO2, the anisohydric juniper became less sensitive to drought, while the relatively isohydric spruce became more sensitive to drought. We hypothesized that rising CO2 benefits relatively anisohydric species more than relatively isohydric species due to greater opportunity to acquire carbon through photosynthesis despite warming and droughts. Our findings suggest the CO2 fertilization effect depends on the isohydric degree, which could be considered in future terrestrial ecosystem models.

Based on radial tree growth measurements in nine plots of area 625 m(2) (369 trees in total) and climate data, we explored the possibly changing effects of climate on regional tree growth in the temperate continental semi-arid mountain forests in the Tianshan Mountains in northwest China during 1933-2005. Tree growth in our study region is generally limited by the soil water content of pre-and early growing season (February-July). Remarkably, moving correlation functions identified a clear temporal change in the relationship between tree growth and mean April temperature. Tree growth showed a significant (p < 0 : 05) and negative relationship to mean April temperature since approximately the beginning of the 1970s, which indicated that the semi-arid mountain forests are suffering a prolonged growth limitation in recent years accompanying spring warming. This prolonged limitation of tree growth was attributed to the effects of soil water limitation in early spring (March-April) caused by the rapid spring warming. Warming-induced prolonged drought stress contributes, to a large part, to the marked reduction of regional basal area increment (BAI) in recent years and a much slower growth rate in young trees. Our results highlight that the increasing water limitation induced by spring warming on tree growth most likely aggravated the marked reduction in tree growth. This work provides a better understanding of the effects of spring warming on tree growth in temperate continental semi-arid forests.