4 | Discussion
To the best of our knowledge, this is the first attempt to overlap the
geographical distribution of all large-seeded pines and their primary
seed dispersers to explore the distribution relationship of these two
different trophic groups. The geological factors regarding the formation
of the pine trees distribution pattern in China and the effect of
reciprocal interaction on the distribution of the large-seeded pines,
seed dispersers, and the new reciprocal combinations was investigated.
We found that the distribution of large-seeded pines in China was
interlaced from the northeast to the southwest as dots, which consisted
of P. fenzeliana , P. gerardiana , and P.
dabeshanensis . The Pinus plant origins include Western Europe
and Eastern North America, where the early Pinus spread from
Europe to East Asia through the ancient Mediterranean coast. North of
the ancient Mediterranean region, the plants would spread freely and
ultimately formed a distribution pattern that is found in China (Tao,
2012). The eight species of the large-seeded pines make one-third of
China’s pines that are mainly distributed along the mountainous areas at
altitudes of 1000-2000m. They display adaptability to alpine and barren
habitats, which is consistent with previous findings (Richardson, 1998;
Tomback & Linhart, 1990).
Large seeds are considered to be a tradeoff in terms of promoting seed
germination and seedling growth as well as seed dispersal by animals.
The evolutionary process of bird dependency caused pines to grow at high
elevation or in barren environments that are sympatric with one or more
corvids with seed-storing behavior and increased seed size (Tomback &
Linhart, 1990). In addition, rodents with scatter-hoarding behavior
brought new changes to such interactions, even though their home range
is much smaller than those of a nutcracker (Vander, W., Stephen B. &
Beck, 2011; Vander, W., Stephen B. et al., 2006).
In this study, we found that the distribution of nutcrackers runs
through the distribution areas of only seven species of the large-seeded
pines, while the distribution area of the rodent was smaller and their
distribution with the pines had a large overlap. This difference between
birds and rodents could be related to movement and ecological traits.S. vulgaris showed a preference for pine seeds by storing large
amounts of seeds in the mature season to cope with food shortages in the
winter and early spring. Thus, they can only be distributed in areas
with larger pine seeds (Siepielski & Benkman, 2008; Yu et al., 2014).
As T. sibiricus are omnivores, they eat young buds, leaves, and
flowers, and they hibernate during the season when pine seeds are
scarce, hence, they can be distributed more widely. These rodents,
either scatter-hoarding or larder-hoarding, retrieve food by olfaction.
However, when seeds are buried too deep or for too long, then the seeds
are more difficult to find, thus improving the survival rate of seeds
(Fletcher et al., 2010; Zhang & Zhang, 2007). Moreover, N.
caryocatactes is a typical coniferous forest bird that disperses and
stores a large number of seeds for winter in the seed maturity season
and has a strong spatial memory for finding the seeds (Balda, 1980).
Furthermore, our results were based on the large overlap of the spatial
scale distribution, which could add uncertainty to widely distributed
species, such as P. armandii , but not evolutionary clues
regarding the pines due to the lack of seed dispersal research, which we
believe is crucial. For the four primary seed disperser of pines,S. vulgariss is mainly distributed across northern China, whileT. sibiricus and N. caryocatactes are distributed in the
same range as all the eight pine species. S. davidianus cover the
mountains of central China, which partially or a completely overlaps
with the range of P. fenzeliana P. dabeshanensis, P. bungeana ,
and P. armandii . This highlighted the central spreading pattern
of Pinus plants in central China. The results of the overlapping
distribution confirmed that the potential interaction not previously
discovered between large-seeded pines and animals, where N.
caryocatactes , S. davidianus , and T. sibiricus were found
in the distribution area of P. fenzeliana and N.
caryocatactes , while P. gerardiana , N. caryocatactes ,S. vulgaris , T. sibiricus , and S. davidianus were
found in the distribution area of P. bungeana , hence, these
animals may be potential seed dispersers of these pines. In addition, we
found T. sibiricus in the distribution area of P. pumila ,S. vulgaris , and T. sibiricus , which was in the
distribution area of P. sibirica , while N. caryocatacteswas in the distribution area of P. dabeshanensis , where these
animals were not identified in previous studies. Similarly, they may
have certain foraging and dispersal effects on the pine trees. We
identified new combinations of pines and seed dispersers that were
previously described but not found as well as combinations that have not
been studied. Sympatry studies are valuable to identify the
relationships between animals and plants, where Piazzon et al. predicted
that the interspecific variation in seed rain was based on plant
distribution and lizard movement patterns (Piazzon et al., 2012), while
Barrere et al. studied feeding behaviors of sympatric red and roe deer
relative to the oak mast seedling (Barrere et al., 2020).
More detailed studies should be performed regarding the process and
mechanism of the seed dispersal of large-seeded pines. The effects of
animal dispersal on Pinus species are often changed by the seed
disperser species, their foraging behavior, habitat type, seed-bearing
year (old or young), and other factors (Peters et al. , 2017). In
view of the reported large-seeded pines, long-term, large-scale, and
multi-factorial observations and tracking should be conducted to
understand the dynamic changes in the species, abundance, behavior, and
population dynamics of the seed disperser community in different
habitats. The large-seeded pines that have not yet been studied should
be investigated relative to the ones in China. Based on these data, an
evolutionary model of the large-seeded pine was established based on the
reciprocal relationships found between animals and plants.