Abstract
Understanding the response of tree growth and drought vulnerability to climate and competition is critical for managing plantation forests. We analyzed the growth of Mongolian pines in six forests planted by the Three-North Shelter Forest Program with tree-ring data and stand structures. A retroactive reconstruction method was used to depict the growth-competition relationships of Mongolian pines during the growth period and their climatic responses under different competition levels. Drought vulnerability was analyzed by measuring the basal area increment (BAI) of different competition indices (CIs). In young trees, differences in BAIs in stands with different CIs were not statistically significant. After 15–20 years, medium- and high-CI stands had significantly lower tree-ring widths (TWs) and BAIs than the low-CI stands (p < 0.05). The standardized precipitation evapotranspiration index (SPEI), precipitation, relative humidity, and vapor pressure deficit were major factors affecting tree growth. On a regional scale, climate outweighed competition in determining radial growth. The relative contribution of climatic factors increased with the gap in SPEI between plantation sites and the native range, while the reverse pattern of the competition-growth relationship was observed. Drought reduced TWs and BAIs at all sites. Stands of different CIs exhibited similar resistance, but, compared with low-CI stands, high- and medium-CI stands had significantly lower recovery, resilience, and relative resilience, indicating they were more susceptible to drought stresses. Modeled CI was significantly negatively related to resistance, resilience, and relative resilience, indicating a density-dependence of tree response to drought. After exposure to multiple sequential drought events, the relative resilience of high-CI stands decreased to almost zero; this failure to fully recover to pre-drought growth rates suggests increased mortality in the future. In contrast, low-CI stands are more likely to survive in hotter, more arid climates. These results provide a better understanding of the roles of competition and climate on the growth of Mongolian pines and offer a new perspective for investigating the density-dependent recovery and resilience of these forests.
Highlights
Satellite data acquired by the National Aeronautics and Space Administration (NASA) of the USA indicate that the global forest area increased by 5.18 × 108 ha in the past two decades, with major contributions from China and India
This study found that, for all six sites, the tree-ring widths (TWs) indices were positively correlated with precipitation, RH, and standardized precipitation evapotranspiration index (SPEI), and negatively correlated with vapor pressure deficit (VPD) and peak temperature
The carbon assimilated by photosynthesis of natural forest in high latitude area was not depleted in the current year and would be used to build the new tissues in the year, while the wood formation of Mongolian pine in low latitude area used more carbohydrates of the current year (Zeng et al, 2019)
Summary
Satellite data acquired by the National Aeronautics and Space Administration (NASA) of the USA indicate that the global forest area increased by 5.18 × 108 ha in the past two decades, with major contributions from China and India. For China, 42% of this increase is attributed to afforestation programs (Chen et al, 2019). The Three-North Shelter Forest Program (TNSFP) is the largest afforestation project launched worldwide. Since its initiation in 1978, the program has planted 4.61 × 107 ha of forests in North China. Mongolian pines have been introduced to TNSFP shelterbelts in other regions because they are adaptable and are effective for windbreaks and sand fixation. They currently cover >3.0 × 106 ha. In recent years, an extensive decline in Mongolian pines has been observed in TNSFP shelterbelts (Zheng et al, 2012). Climate studies predict that the eastern part of the TNSFP area is likely to experience continuing warming and aridification in the future, suggesting there will be further extensive tree mortality
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