Morphological trait as a determining factor for Populus simonii Carr. to survive from drought in semi-arid region
Introduction
Forests are one of the most important terrestrial ecosystems on Earth, and play a fundamentally important role in regulating water cycles, carbon sequestration (Cui et al., 2020), biodiversity protection (Newbold et al., 2015), desalinisation (Minhas et al., 2020), soil and water conservation (Hou et al., 2020) and as a shelter for agricultural land (Sun et al., 2018b, Deng et al., 2021). However, as global climate change proceeds, 10 million ha of forest have been lost for each year since 2015 (https://www.fao.org/forestry/FRA2020), which has drawn considerable attention. Studies on tree die-offs mainly focus on tree physiological (McDowell et al., 2011a) or morphological traits (Rais et al., 2014) in the literature. Hydraulic failure, carbon starvation, and biotic attack are widely considered as the three main causes of tree mortality, but all these causes originate from water stress (McDowell, 2011b, Choat et al., 2018). Trees exposed to low water availability may form cavitation in xylem tissues, blocking the water path from the root to the canopy (Nardini et al., 2013). Trees might lower the transpiration rate and canopy conductance by closing the leaf stomata to reduce water loss, while lowering the photosynthesis rate as well (Jia et al., 2017). This could lead to unavoidable damage to trees due to carbon starvation behaviour in a prolonged drought period, causing trees to be more vulnerable to pests (Ji et al., 2020).
Although drought seems fatal for trees, not all trees die in a certain drought event (McDowell et al., 2008). Intra- or inter-species morphological differences may explain this pattern. It is widely recognised that the vulnerability of trees to drought increases with the tree size (Bennett et al., 2015, Koch et al., 2004, Stephenson and Das, 2020). Height growth results in larger hydraulic resistance because higher trees consistently have wider conduit diameters, reducing the friction against the conduit wall for water molecules (Petit and Anfodillo, 2009). Taller trees need to overcome more gravity when transporting water to the canopy (McDowell and Allen, 2015). Meanwhile, trees with larger canopies usually demand more transpiration, but this could lead to issues when the water supply is in shortage (Wullschleger et al., 1998). In addition, larger trees have to face a more challenging hydraulic environment, such as, higher solar radiation or greater vapour pressure deficit. However, a study carried out in the Mediterranean region of southern Italy showed that local Hungarian oak (Quercus frainetto Ten.)trees with a larger size survived from a drought-induced mortality event in the 2000s, but the reason remained unexplained (Sánchez-Salguero et al., 2020). Subedi et al. (2020) found that relatively small trees in east Texas were more susceptible to drought, which was attributed to higher stand density and lower stem water storage. Yang et al. (2020) showed a similar result from the Daxing’an Mountains of Northeast China. The limitation of radiation resource was considered as the main factor; thus, smaller trees exposed to less sunlight had lower growth rates. In addition, trees in the dry or alpine regions generally grew to a lower tree height and smaller canopy, resulting in a much lower water usage and were more adapted to the local environment (Colangelo et al., 2017, Pandey et al., 2021).
Globally, plantation comprise a considerable percentage of forests. For example, planted forests in China account for over a third of forests nationwide. Planted forests provide as many ecological services as other forests (Trivino et al., 2017), while also experiencing mortality or degradation. Plantations are artificially designed which suggests that there could be a mismatch between the vegetation resource demand and local natural resource supply (Song et al., 2021). On the other hand, they are directly related to the livelihood of human beings by protecting farmland and residents from sandstorms or other kinds of natural disasters (Zheng et al., 2012). Therefore, it is of great importance to understand how would this type of vegetation responds to drought.
The Three North Shelter Forest Program(TNSF) is an important man-made ecological barrier for northern China, covering an area of 406.9 × 104 km2. Shelter forests of Zhangbei County (Heibei Province) were a typical part of the TNSF and formed an ecological barrier protecting Beijing and its surrounding area from sandstorms. It was also the ecotone between woodland and grassland, which makes it particularly vulnerable to climate change. The plantation programme in Zhangbei was started in year 1977, with the selection of Populus simonii Carr. as the pioneer tree species. Wide-spread mortality of the poplars has been observed since 2000, with a sharp decline in the regional fractional vegetation coverage (FVC) (Wang et al., 2020). Approximately 80% of the poplars in Zhangbei County suffered from degradation, while one-third of them died-off or near died-off (Sun et al., 2018b). Researchers are still debating the main causes of tree death. One reason for the degradation could be the decline in physiological functions as a natural result of ageing (Ji et al., 2020). Another reason could be drought according to the tree-ring study (Lu et al., 2020, Sun et al., 2018b) or the water use pattern studies (Liu et al., 2020, Sun et al., 2018a). However, the reason why some of the poplars died while others survived the drought in this particular region remains unclear.
In this study, we conducted a field survey measuring tree height, diameter at breast height (DBH), and canopy width of the poplars experiencing different degrees of degradation. Tree-ring data was obtained from the survey. The water use pattern of the poplars was also studied to reveal the water use strategies of the poplars. The aims of this study were as follows:1) to reveal the relationship between tree morphological traits and degradation degree in this region, 2) to predict the rooting depth of the poplars by revealing their water use patterns, and 3) to explain why some trees survived while others succumbed to drought from a morphological perspective (tree size and root depth). Our goal was to provide scientific information for the future development of plantation practices and vegetation management in the study area.
Section snippets
Study area
The study was conducted across Zhangbei County (Fig. 1, 41°34′N–41°57′N,114°10′E–115°27′E), with an average elevation of 1300 m. The area is characterised by a temperate continental monsoon climate with an average annual rainfall of 400 mm, of which 64–69% is concentrated between June and September. The annual average temperature was approximately 3–4 °C, while the maximum and minimum daily average temperature were 33.4 °C and − 34.8 °C, respectively. The soil texture is mainly chestnut soil,
Climate characteristics of the recent 50 years
The annual precipitation varied significantly (CV = 0.17) during the 1970–2019 period with a mean value of 385.8 mm, while the minimum value was 249 mm in the year 1997 (Fig. 2). The annual maximum, average, and minimum temperature increased by 0.46, 0.41, and 0.39 °C every ten years, respectively, showing a significant trend of climate warming (except for annual minimum temperature) (Fig. 3). Precipitation in 2019 was 420.5 mm and was mainly concentrated in the period from May to October. The
The effect of tree ageing and stand density on the growth drawback of poplars
The tree height time series (Fig. 6) of the poplars exhibited a decrease in growth rate after the year 2000, which was the transition period of the poplars from half-maturity to maturity. This is a natural phenomenon in which the growth rate of trees will decreases because of the larger tree size and higher hydraulic resistance (Linares et al., 2013). According to the scPDSI values (Fig. 5), the drought that occurred in 1999–2001 was not the most intensive in the entire growing period of the
Conclusions
Plantations are a crucial forest resource for the human society. In Zhangbei County, China, Populus simonii Carr. plantation was the shelterbelt for local agriculture and residents. However, extreme drought caused heterogeneous dieback in the poplars. To reveal the mechanism of their mortality, a field survey including tree morphological data, tree-ring analysis, and water use pattern study using stable water isotopes was conducted in 2017 and 2019. We found that tree growth began to slow down
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the Fundamental Research Funds for the Central Universities (2019ZY35) and the National Natural Science Foundation of China (41877152 & 41977149).
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