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Investigating the distribution of internal physiological indicators and the cold resistance of woody plants in the alpine treeline ecotone is of great ecological importance to explain the mechanism of alpine treeline formation. Less research has been conducted on the cold resistance mechanisms of alpine treeline woody plants than on commercial crops. In this paper, five different tree species in the alpine treeline ecotone of the Sygera Mountains were used as the research objects and the leaves, branches, and roots of 19 woody plants were collected in the non-growing season (November) of 2019. Their non-structural carbohydrate content (soluble sugar and starch), malondialdehyde, hydrogen peroxide (H2O2), proline, superoxide dismutase, and peroxidase levels were measured. The contents of C, N, P, and K elements were analyzed, along with the distribution characteristics of physiological indices and organs of various woody plants and their relationship to plant nutrients. Results showed that the MDA (5.46 ± 1.95 μg·g−1) and H2O2 (4.11 ± 0.76 mmol·g−1) of tree root organs and the MDA (3.03 ± 2.05 μg·g−1) and H2O2 (4.25 ± 1.03 mmol·g−1) of shrub leaf organs were higher than those of other organs, indicating that under the stress of low temperatures, the root organ of arbor species and the leaf organ of shrub species experienced the most damage. Osmotic substances, particularly soluble sugars, play a crucial role in the response of the woody plants in Sygera Mountains to low-temperature stress. Plant nutrients could enhance plant stress resistance by further activating the activity of the antioxidant system and increasing the synthesis of osmotic substances. This study hypothesized that the stress on the root organs of the arbor species in the treeline ecotone may not be repaired in time, which may be a key mechanism for the formation of the alpine treeline in the Sygera Mountains.

In order to elucidate the adaptation mechanisms of the stoichiometric characteristics of Abies georgei var. smithii in southeastern Tibet to different habitats, the spatial and temporal dynamics of the nutrient contents and stoichiometric ratios among the leaf, branch, stem, and root organs were analyzed on Sejila Mountain in southeastern Tibet at different elevations (3500 m, 3900 m, and 4300 m). The results show that (1) the C and K contents of the fir organs did not change significantly with increasing elevation in the same season, while the N and P contents showed an overall increasing trend with increasing elevation, with the C and N contents being the highest in the leaves. The distribution order was leaves > branches > roots > stem; the P and K content order in each organ was branches > leaves > roots > stem. (2) At the same elevation and in different seasons, the Abies georgei var. smithii organs showed a similar convergence in terms of nutrient storage and utilization strategies, and more nutrients were optimally allocated between the assimilated and stored organs in the alpine habitats, which represents a “trade-off” strategy. (3) Compared to the findings of the global-scale studies, this study area has low N, P, and K contents, and its growth is limited by both N and P. Due to physiological and nutrient balance constraints, the content of the N-limited elements in the plants is relatively stable, which is in line with the “limiting element stability hypothesis”. (4) Principal component analysis showed that the influence of environmental factors on the stoichiometric characteristics of the different organs of Abies georgei var. smithii had a spatial scale effect, and that Abies georgei var. smithii demonstrated increased accumulation of N and P contents when subjected to environmental stress, which promoted the domestication and adaptation of the plant, enabling it to show good nutrient accumulation capacity and good adaptation strategies even at high elevation; thus, it has become a pioneer tree species at high elevations. This research work shows that the resilient adaptation of Abies georgei var. smithiir to environmental change has led to differences in the nutrient uptake and use efficiency and the adaptation patterns of the organs at different altitudinal gradients, with each organ adapting to habitat changes by adjusting its nutrient storage strategy between habitats.