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.