The warming climate system has long been known as one of the main limiting factors for the establishment, growth, and survival of trees and perpetuation of stable forest ecosystems. The forest decline and tree mortality in Northern China (NC) have also been widely reported where could be confirmed as the sensitive area of climatic change. These mortality events can lead to irreversible changes in ecosystem properties with cascading effects on many physiological and metabolic processes relevant for water, carbon, and nutrient cycles. Nonetheless, the physiological mechanisms of drought-induced forest decline remain poorly understood with the concurrent or alternative role of carbon starvation and hydraulic failure. Plants can alleviate environmental stress through the regulation of physiological processes and the plastic response of functional traits. So far, there still be little focus on the issues of climate change challenges plant capacity to cope with intensified water stress in the temperate forest (including the planted forest) in NC.
In the study, we collected the tree-ring samples at Chinese Loess Plateau and Beijing Dongling Mountain in NC. Combining quantitative wood anatomy with the dendrochronological method. Multi-proxies (radial growth, mean vessel area, number of vessel and theoretical hydraulic conductivity, etc.) were obtained to analyze the interactive relationship between climatological data, radial growth, and wood anatomical features: i) along the precipitation and altitudinal gradient; ii) between different species. Finally, the adjustment of earlywood vessel structure and the hydraulic transport strategy of trees to climate change were explained. The main results are summarized as follows:
In the horizontal gradient, two patterns of climate-trees growth relationship corresponding with two functional groups regarding the different capacity of hydraulic strategies in xylem features responding to climate variables along a rainfall gradient: (i)increasing trend of both radial growth and hydraulic efficiency in the south and central sites of Loess Plateau; (ii) decreasing trend in radial growth and trade-off between hydraulic efficiency and safety at the north site of Loess Plateau. At the southern Loess Plateau, trees would suffer more risks of hydraulic failure to maintaining large vessels.
Continued climate change will trigger crown dieback and tree mortality. As for the semi-arid zone at northern Loess Plateau, increasing consequences of water shortage causes of hydraulic dysfunction and restricted radial growth so that forest productivity is strictly limited. In summary, xylem plastic response of climate stress shows the
negative effect on forest growth along the north.south transect; (iii) in the Yangjuangou catchment, Robinia pseudoacacia was strongly suffered by water stress which growth unsustainable in future while native trees were affected by drought weakly which can adapt to the trend of climate change in Chinese Loess Plateau in the semi-arid area. In the typical semi-arid area of Loess Plateau, (iv) the native species of Armeniaca sibirica at the Yangjuangou watershed has maintained a weak water-conducting capacity for a long period. Trees can adapt to the water stress environment in the growing season, and the introduced species of black locust was mainly promoted by the water-conducting efficiency priority strategy which may cause hydraulic failure; (v) in the Renjiatai watershed, the native tree species Quercus wutaishanica and the introduced species black locust respond to climate stress by generating large and small number of vessels,so hydraulic failures can occur at both species in future climate scenarios.
In the vertical gradient, (i) compared with high elevation, low and mid elevations could be described as drought-prone sites where trees suffer temperature. induced drought significantly and shape a different but complementary manner of xylem features to respond to harsh environment by reducing radial growth and abundance of vessels but increasing vessel diameter. Safety versus efficiency trade.off is not strong,and hydraulic efficiency is more crucial for xylem water transport under prolonged and server water stress environment; (ii) at high elevation, all tree-ring and xylem chronologies, increasing ring width with larger and more plentiful vessels, show a positive relationship with rising temperature suggesting a potential climate. warming advantage for trees xylem counter.balance of water transport and carbon allocation. In the middle elevation of Beijing Dongling Mountain, the under the comparison of different tree species responding to climate change, the results showed that: (iii) the rank of tree suffer from water stress by the order: Quercus wutaishanica> Juglans mandshurica> Phellodendron amurense> Ulmus macrocarpa; (iv) The number of vessels of Quercus wutashanicas, Juglans mandshurica and Ulmus macrocarpa decreased with the increase of the size of vessel, and the water transport efficiency was prioritized. While there was trade-off between efficiency and safety of Phellodendron amurense with the balance along with the number and the size of conduit. According to the characters of climate response and relationship between radial growth and anatomical traits of annual ring, Ulmus macrocarpa had strong drought tolerance characteristics, and Phellodendron amurense had a very flexible water transport strategy; while both Quercus wutaishanica and Juglans mandshurica were obviously affected by water stress with the decrease of carbon accumulation and the risk of hydraulic failure.
Long-term series of Robinia pseudoacacia and Quercus wutaishanica maintain similar anatomical characteristics, but the structure of vessel of Robinia pseudoacacia reflected the high elasticity of climate adjustment ability; Whether in the semi-arid region of the Loess Plateau or in the middle and low-altitude areas of Beijing Dongling Mountain, Quercus wutaishanica still maintains high water-conducting efficiency for a long time, showing strong resistance and accelerating forest decline and tree mortality under warming climate.
In this study, we innovatively made a preliminary discussion on the water transport mechanism of tree radial growth and wood anatomy in the climate-sensitive area of NC. These new knowledge of vessel anatomical features linked between tree acclimation and the physiological performance could provide the implication for ecological restoration and forest management in Loess Plateau of China and the forest management in Beijing Dongling Mountain in future.