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.