Fading regulation of diurnal temperature ranges on drought-induced growth loss for drought-tolerant tree species

Different drought recovery strategy between Larix spp. and Quercus mongolica in temperate forests

Forests are experiencing increasingly severe drought stress worldwide. Although most studies have quantified how tree growth was affected by extreme droughts, how trees recover from different drought intensities are still poorly understood for different species. We used a network of tree-ring data comprising 731 Quercus mongolica trees across 29 sites, 312 Larix olgensis Henry trees from 13 sites, and 818 Larix principis-rupprechtii trees from 34 sites, covering most of their distribution range in northern China, to compare the influences of drought intensity on post-drought recovery. The results showed that summer droughts had strong negative influences on tree growth. Post-drought growth varied with drought intensity for the three species. Larix species exhibited strong legacy effects after severe droughts, which is related to the lack of compensatory growth. In contrast, the compensatory growth of Q. mongolica reduced drought legacy effect. However, the compensatory growth of Q. mongolica gradually weaken with increasing drought intensity and disappeared during severe drought. Our findings indicated that influence of drought on Q. mongolica growth mainly shown in drought years, but Larix species suffered from long-term drought legacy effects, implying Q. mongolica rapidly recovered from droughts but Larix species need several years to recover from droughts, thus the two genera have different recovery strategy.

Dissimilarity in radial growth and response to drought of Korshinsk peashrub (Caragana korshinskii Kom.) under different management practices in the western Loess Plateau

Quantitative assessment of tree responses to the local environment can help provide scientific guidance for planted forest management. However, research on the climate-growth relationship of Korshinsk peashrub (Caragana korshinskii Kom.) under different land preparation and post-management (irrigation) conditions is still insufficient. In this study, we collected 223 tree-ring samples from Korshinsk peashrubs using dendroecological methods and systematically quantified the relationships between shrub growth and climatic factors under different management practices in the western Loess Plateau of China. Our findings demonstrated that drought stress caused by scarce precipitation from April to August was the primary factor limiting the growth of Korshinsk peashrubs in the northern and southern mountains of Lanzhou. The "climwin" climate model results showed a weak correlation between natural Korshinsk peashrub growth and drought stress, whereas planted Korshinsk peashrub under rain-fed conditions in the southern mountain was significantly (p<0.05) limited by drought stress from April to August. Moreover, planted Korshinsk peashrub growth under irrigated conditions in the northern mountain was limited only by drought stress in January. Drought model explained 28.9%, 38.3%, and 9.80% of the radial growth variation in Xiguoyuan (XGY), Shuibaozhan (SBZ), and Zhichagou (ZCG) sites, respectively. Artificial supplementary irrigation alleviated the limitation of drought on planted forest growth, which may be implemented for Korshinsk peashrubs planted on sunny slopes, while planted Korshinsk peashrubs under natural rain-fed conditions can be planted on shady slopes through rainwater harvesting and conservation measures such as horizontal ditches and planting holes.

Higher latewood to earlywood ratio increases resistance of radial growth to severe droughts in larch

As drought has caused great losses of tree growth across the world, the mechanism of how trees adapt to drought has been extensively investigated. However, how trees change their late- to earlywood ratio (LER) to adapt to severe drought events remains poorly understood. We used a network of Larix principis-rupprechtii earlywood and latewood width data from 1979 to 2018, covering most of the distribution of planted larch across North China, to investigate how latewood proportion affected trees' resistance to drought. The interactions among LER, minimum temperature, vapor pressure deficit (VPD), growing season length, and their contributions to drought resistant (Rt) were estimated using structural equation models. The results show a significant increase in LER of the juvenile wood throughout the first 15 growth rings after which it stabilizes. The LER decreased significantly with elevation for the juvenile wood. March-May temperature and VPD were the main determinant in the LER of mature wood. The sensitivity of radial growth to droughts was positively changed with LER when LER was below 0.50, but negatively changed with LER when LER is above 0.50. We confirmed that high LER increases resistance of tree growth to severe droughts in L. principis-rupprechtii. Our results highlight that a higher proportion of latewood is formed in dry years, and this high drought sensitivity of LER in turn led to an increased resistance to drought. This combination of reduced radial growth during dry years, while the latewood proportion remains increases maybe an adaptive strategy of larch trees to cope with severe droughts.