4. DISCUSSION
In this study, we found that Q. chungii increased in its overall abundance, frequency and tree size in the last decade, suggesting active regeneration and fast growth rate ofQ. chungii . These results indicate the effectiveness of conservation efforts from establishing the nature reserve. However, the in-depth analyses of population structures warn potential risks of the future population development of Q. chungii .
The age structure of Q. chungii population in 2023 shows a pyramidal shape, with a sharp decline in the numbers of individuals from germinated seeds to seedlings and from seedlings to saplings. Saplings, a key stage for determining the future population, had a low number of individuals. These lead to the high age-specific death probabilities at the early developmental stages. These results indicate potential risks of population decline in the future. The observed decline in sapling proportion from 2013 to 2023 may signify a population decline trend within the species. This decrease could be attributed to either accelerated ontogenetic progression of saplings into larger size classes or elevated mortality rates during early developmental stages and within the sapling cohort. If mortality factors predominate, it underscores a potential population decline over the past decade.
The seed-to-seedling stage may be the most important period limiting the regeneration of Q. chungii population, because the highest mortality probabilities occurred from germinated seeds to seedlings. This is also consistent with our observation in the field. We found a very low density of seedlings with DBH \(<\)1cm in 2023. Only one of them may be produced from seed, while the others were evidently sprouted from roots of established trees. The scarcity of seedlings produced from seeds were already spotted in more than a decade ago (Wang et al. 2011), which may be one of the reasons driving the decadal decline of sapling proportion observed in this study. The dependence of clonal reproduction can be one of the most important reasons driving the highly aggregated spatial distribution ofQ. chungii in natural forests (Wang et al. 2011). The aggregated distribution may increase the risk from pests and diseases. The reduced genetic diversity due to clonal reproduction may further increase these risks (Gilbert and Webb 2007, Bagchi et al. 2014, Liu et al. 2015, Liang et al. 2016), thus threatening the health and development of Q. chungii population in the future.
We estimated the number of germinated seeds from historical data of seeds. We assumed that trees with larger basal area will produce more germinated seeds, because larger trees can obtain more resources and produce more seeds (Visser et al. 2016). We acknowledge that the production and germination of seeds are complex processes determined by multiple factors, such as climate, soil, animal predation, plant physiology and human disturbance (Donohue et al. 2010, Baskin and Baskin 2014). The accuracy of our estimation approach thus relies on how these biotic and abiotic factors change in the past decade within the nature reserve. We thought that these changes should be slow due to the relative short temporal duration and the prevention from human activities by administrative office of the nature reserve. The findings of a high number of germinated seeds were also confirmed from the recent field observation by the officers of the nature reserve. But most of the seeds were found to be eaten by boars and rodents.
Despite the high predation risks from mammals, the structural and physiological traits of Q. chungii may also drive the severe decline in population numbers from seeds to seedlings. The germination pore of Q. chungii seed locates at a key place for water transportation, which may make it vulnerable to rapid water deficits (Sun et al. 2021). Q. chungii has a substantially delayed shoot emergence following a fast root emergence. The breakage of this shoot dormancy often needs 3–5 months with warm temperature (Sun et al. 2021). These seed traits can makeQ. chungii very sensitive to precipitation and temperature abnormality, thus reducing the probability of transition from seeds to seedlings.
The age-specific death probability of Q. chungiifirst decreased then increased with age classes, resulting in a U-shape pattern. Large and tall trees are more vulnerable to disease, wind, and other causes of death (Lu et al. 2021, Fernández‐de‐Uña et al. 2023), which may drive the increasing mortality probability from medium to large trees.