Conspecific negative density dependence (CNDD) is widely regarded as a key mechanism maintaining plant diversity and is classically attributed to host-specific pathogens or herbivores. However, whether animal-mediated seed dispersal may contribute to CNDD-like spatial patterns remains poorly understood. Such animal-mediated seed dispersal behaviors are widespread in forests but are difficult to study because of their stochastic nature. Here, we investigated the distance-dependent spatial distribution of seedlings of two Cyclobalanopsis species around parent trees (at seven distances from 0 to 30 m) in a tropical forest, comparing directions with and without observed nut-storage activity by two squirrel species that actively carve and store nuts away from parent trees. Results showed that seedling numbers exhibited a clear nonlinear distance-dependent pattern along directions with squirrel nut-storage activity, with significantly higher numbers at 20 m than in both same-tree and different-tree controls. Pathogen infection in sapling fine roots did not show a significant distance-dependent pattern. These findings provide a new perspective on CNDD-like pattern formation by highlighting animal-mediated seed redistribution as an additional process that may be associated with seedling spatial patterns.

Squirrels of temperate zones commonly store nuts or seeds under leaf litter, in hollow logs, or even in holes in the ground; however, in the humid rainforests of Jianfengling in Hainan, South China, we show that some flying squirrels cache elliptical or oblate nuts by hanging them securely in vegetation. These small flying squirrels were identified as Hylopetes phayrei electilis (G. M. Allen, 1925) and Hylopetes alboniger (Hodgson, 1870), in video clips captured of their behavior around focal nuts. Squirrels chewed grooves encircling ellipsoid nuts or distributed on the bottoms of oblate nuts, and then used these grooves to fix nuts tightly between small twigs 0.1–0.6 cm in diameter that were connected at angles of 25–40°. The grooves carved on the nuts (concave structure) connected with Y-shaped twigs (convex structure) and thus firmly affixed the nuts to the plant in a way similar to a mortise-tenon joint used in architecture and carpentry. Cache sites were on small plants located 10–25 m away from the closest potentially nut-producing tree, a behavior that likely reduces the discovery and consumption of the nuts by other animals. The adaptive squirrel behavior that shapes and fits nuts between twigs seems to be directed at providing more secure storage that increases food supply during dry periods in a humid tropical rainforest. In addition to providing such benefits for the squirrels, we suggest that this behavior also impacts the distribution of tree species in the forest.

Full text Figures and data Side by side Abstract Editor's evaluation eLife digest Introduction Results and discussion Materials and methods Data availability References Decision letter Author response Article and author information Metrics Abstract Squirrels of temperate zones commonly store nuts or seeds under leaf litter, in hollow logs, or even in holes in the ground; however, in the humid rainforests of Jianfengling in Hainan, South China, we show that some flying squirrels cache elliptical or oblate nuts by hanging them securely in vegetation. These small flying squirrels were identified as Hylopetes phayrei electilis (G. M. Allen, 1925) and Hylopetes alboniger (Hodgson, 1870), in video clips captured of their behavior around focal nuts. Squirrels chewed grooves encircling ellipsoid nuts or distributed on the bottoms of oblate nuts, and then used these grooves to fix nuts tightly between small twigs 0.1–0.6 cm in diameter that were connected at angles of 25–40°. The grooves carved on the nuts (concave structure) connected with Y-shaped twigs (convex structure) and thus firmly affixed the nuts to the plant in a way similar to a mortise-tenon joint used in architecture and carpentry. Cache sites were on small plants located 10–25 m away from the closest potentially nut-producing tree, a behavior that likely reduces the discovery and consumption of the nuts by other animals. The adaptive squirrel behavior that shapes and fits nuts between twigs seems to be directed at providing more secure storage that increases food supply during dry periods in a humid tropical rainforest. In addition to providing such benefits for the squirrels, we suggest that this behavior also impacts the distribution of tree species in the forest. Editor's evaluation This report of nut modification and storage in flying squirrels provides new insights into food caching behaviour in wild animals. Although further direct evidence is needed to corroborate some of the findings, the current study provides valuable documentation of an interesting behaviour that should motivate further observational and experimental research. https://doi.org/10.7554/eLife.84967.sa0 Decision letter Reviews on Sciety eLife's review process eLife digest The rainy forests of South China are home to Cyclobalanopsis trees whose smooth, elliptical nuts are favoured by many animal species. While doing fieldwork in the Jianfengling nature reserve in the southern province of Hainan, China, researchers came across an unusual sight: many of these nuts had been wedged into the Y-shaped forks between diverging twigs. A closer inspection revealed that a carefully crafted groove on the surface of the nuts helped them to stay wedged and secured between the branches. Which creature was responsible for such a feat? To investigate, Xu et al. set up motion-triggered, infra-red cameras near some of the hoarding sites. They discovered that the culprits were Hylopetes phayrei electilis and Hylopetes alboniger, two small species of flying squirrel that tend to store Cyclobalanopsis nuts to prepare for the dry, cool season. The footage showed that the squirrels first chewed the nuts before inserting them tightly between the branches. In fact, this process appeared to require much care – and, potentially, cognitive involvement – with the squirrels testing and adjusting their grooves many times until a perfect fit was achieved. Caching sites were usually found 10 to 25 meters away from the nearest Cyclobalanopsis tree, which probably helps to protect the hoards from other animals on the hunt for nuts. Squirrels from temperate regions typically prepare for winter by hiding food in the ground, between logs or inside hollow trees; in humid, tropical forests, however, such caching sites may promote mould, decomposition or germination. In these conditions, securely hanging nuts between branches may prove to be a more suitable strategy. By choosing caching sites that are away from the mother tree, squirrels may also inadvertently help Cyclobalanopsis to expand their range, with forgotten nuts becoming dislodged and sprouting in new locations across the reserve. Overall, these findings shed new light on animal adaptation and cognition, as well as on the forces that help to shape forest ecology. Introduction Storing food to buffer against periods of low resource supply is a common species-specific behavior used by squirrels and other rodents (Andersson and Krebs, 1978; Steele et al., 2006). Nuts, in particular, are harvested from trees and cached in various places. For example, many temperate-zone squirrels hoard nuts under leaf litter, in holes in trees or logs or in the ground (Cheng et al., 2005; Hadj-chikh et al., 1996). In subtropical zones, however, some species store nuts or mushrooms by hanging them on tree branches, a behavior thought to minimize fungal infection in humid environments (Lichti et al., 2017; Xiao et al., 2013) or decrease the risk of loss through decomposition or germination under warmer temperatures in the cache (Sechley et al., 2015). The present work was prompted by our inadvertent discovery of Cyclobalanopsis nuts with strange surface grooves, and that were suspended in Y-shaped crotches of twigs on understory plants on Hainan Island, South China. Cyclobalanopsis trees are dominant fagaceous trees in these tropical forests; however, their fruits are elliptical or oblate single nuts with smooth surfaces, features that make them difficult to hang on vegetation. Firmly suspending such nuts in the vegetation presents an ecological challenge to squirrels in such environments. We asked whether some of the nine squirrel species identified from Hainan forests used special behaviors to prepare these nuts and fix them securely on vegetation. In this paper, we show that the Indochinese Flying Squirrel, Hylopetes phayrei electilis (G. M. Allen, 1925), and the Particolored Flying Squirrel, H. alboniger (Hodgson, 1870), which co-occur in Hainan Island, cache these nuts individually between the twigs of small plants. Both of these small-bodied flying squirrels are widespread in the tropical forests in Southeast Asia, from Myanmar, south to northwestern Vietnam, and east into southern China (Duckworth et al., 2016; Duckworth et al., 2016). In China, H. phayrei electilis can be found in the mountainous areas of Hainan, Fujian, Guangxi, and Guizhou Provinces. H. alboniger is mainly found in the provinces of Hainan, Yunnan, Guizhou, Guangxi and, more rarely, in Zhejiang. Although these squirrels are reasonably common, there is little published information about their habits, and there are no studies from Hainan Province in China (Li et al., 2012). In particular, nut storage behavior hasn't been reported from elsewhere in the ranges of either of these two squirrels. Thus, the preparation of nuts to connect them firmly to twigs is a new finding, although other squirrel species are known to handle nuts prior to suspending them to improve the success rate of storage (Fox, 1982; Steele and Yi, 2020; Xiao et al., 2010). Here, we document in some detail the squirrel behaviors associated with this phenomenon in Hainan. Results and discussion We used images from infrared cameras to determine that the nocturnal flying squirrels, H. phayrei electilis and H. alboniger, two of the nine species of squirrels known from tropical forests of Hainan (Table 1), stored Cyclobalanopsis nuts by suspending them on vegetation in the Jianfengling forest (Videos 1–5). The videos further showed that the squirrels chewed grooves in the surfaces of the nuts before fixing between the twigs, and that they sometimes altered the previously carved grooves by further chewing, apparently to adjust the fit and suspend the nut more firmly (Videos 6–9). In footage from 32 field infrared cameras, we captured 48 film sequences that included chewing (Videos 6–7), fixing (Video 9, partial evidence) and removing nuts (Video 2, Video 3 and Video 5), or visiting a storage site (Video 1, Video 4 and Video 8). This direct evidence, together with the findings below, shows that this mode of nut storage is a reasonably common activity of these two squirrel species in the Jianfengling forest. Table 1 The nine squirrel species known from Jianfengling, Hainan Island, China. Species and subspecies nameBody length/mmTamiops maritimus (Bonhote,1900) (Liu et al., 2020; Pan et al., 2007)105~134Dremomys pyrrhomerus (Thomas, 1895) (Xu and Chen, 1989)194~215Hylopetes alboniger (Hodgson, 1870) (Liu et al., 2020; Andrew, 2008; Pan et al., 2007)180~203Hylopetes phayrei electilis (Allen, 1925) (Liu et al., 2020; Pan et al., 2007)123~173Dremomys rufigenis (Blanford, 1878) (Zheng et al., 2008)170~250Callosciurus erythraeus (Pallas, 1779) (Huang, 1995; Zheng et al., 2008)198~252Belomys pearsonii (Gray, 1842) (Pan et al., 2007; Huang, 1995)180~260Petaurista albiventer (Gray, 1834) (Jing et al., 2007)420~520Ratufa bicolor (Sparrmann, 1778) (Li et al., 2008)350~505 Note: The data in this table are referenced from the below literature. Video 1 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Squirrel 1 of Hylopetes alboniger was checking and re-fixing nuts at the storage sites with footage from infrared cameras. Video 2 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Squirrel 2 of Hylopetes alboniger was removing nuts from storage sites with footage from infrared cameras. Video 3 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Squirrel 3 of Hylopetes alboniger was removing nuts from storage sites with footage from infrared cameras. Video 4 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Squirrel 4 of Hylopetes phayrei electili was checking and re-fixing nuts at storage sites with footage from infrared cameras. Video 5 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Squirrel 5 of Hylopetes phayrei electili was removing nuts from storage sites with footage from infrared cameras. Video 6 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg One squirrel was cracking the nuts on the ground with footage from infrared cameras. Video 7 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg One squirrel was rotating and cracking the nuts on the trees with footage from infrared cameras. Video 8 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg One squirrel was re-fixing the nuts by cracking behavior with footage from infrared cameras. Video 9 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg One squirrel was fixing the nuts between the twigs with footage from infrared cameras. We merged several photos and a video successively taken by an infrared camera in 30 s. A total of 151 grooved and cached nuts were found suspended on more than 55 tree or shrub species distributed across 28 plant families during our censuses of approximately 5.5 ha of forest (Figure 1, Supplementary file 1). All suspended nuts found had surface grooves of the form carved by squirrels, as documented above. Examples of storage locations and carved nuts are shown in Figures 2–3. Most discovered nuts were fixed between plant twigs connected at angles of 25–40° on a variety of small saplings and shrubs (Figure 4). This range of angles accommodates the nut sizes of Cyclobalanopsis edithiae and C. patelliformis (2.4 cm (width) × 4.6 cm (length) and 2.4 cm (width) × 2.0 cm (height), respectively), which accounted for 96.7% of the nuts that we found cached (C. edithiae (40.4%), C. patelliformis (56.3%)). A few nuts of Lithocarpus fenzelianus A. Camus (n=4) and C. fleuryi (Hickel & A. Camus) Chun ex Q. F. Zheng (n=2) were also found similarly suspended on plants. Figure 1 Download asset Open asset Spatial distribution of the 151 suspended nuts observed in Jianfengling Nature Reserve, Hainan, China. Figure 2 Download asset Open asset Nuts are stored after surface preparation by flying squirrels. (A) Nut of C. edithiae (Skan) Schottky, with chewed grooves outlined in red. Nuts of C. edithiae fixed on trees, with (B–D) one groove, (E) two non-connected grooves, or (F) spiral carved grooves encircling the nuts. (G) Nut of C. patelliformis (Chun) Y. C. Hsu et H. W. Jen, with chewed grooves outlined in red. (H–I). Nuts of C. patelliformis fixed on trees, with carved grooves on the bottom fixed on (J) bamboos, (K–L) lianas, between the big petioles of (M) trees and (N) palms. Figure 3 Download asset Open asset Variation in carved grooves depends on the storage situation. The carved surface grooves on nuts of C. edithiae mostly encircle the middle of the nut, with (A) one groove, (B) one spiral groove, or (C) two separated grooves. The grooves on nuts of C. patelliformis are distributed on the bottom of the nuts, with (D) 2, (E) 4, (F) 6, (G) 8, (H) 10 symmetrically, or (I) randomly distributed grooves. Figure 4 Download asset Open asset Nuts were fixed tightly between twigs generally meeting at angles of 25–40°. (A) C. edithiae nuts. (B) C. patelliformis nuts. Nuts of the two predominant tree species were disproportionately stored on small plants with diameters at breast height (DBH) of 0.4–1.6 cm (Figure 5) and twig diameters of 0.10–0.60 cm (Figure 6a and b). For nuts of C. edithiae, plant twig diameter was significantly correlated with groove width on the nut, and generally varied from 0.20–0.60 cm (p<0.001, Figure 7). The widths of grooves carved in the nuts matched the typical width of the paired incisor tips of these squirrels (i.e. less than 5 mm). Most of the nuts were found stored on the first to third branches of a plant 1.50–2.50 m above the ground (45.9% of C. edithiae and 43.5% of C. patelliformis storage sites) (Figure 8). Figure 5 Download asset Open asset Most nuts were stored on small plants with the diameter at breast height (DBH) ranging from 0.4–1.6 cm. (A) C. edithiae nuts. (B) C. patelliformis nuts. Notes: The value on each bar is the actual number of stored nuts. Figure 6 Download asset Open asset Nuts were stored mainly on small plants between twigs with diameters of 0.10 - 0.60 cm. (A) Histogram of diameters of twigs used to store nuts of C. edithiae. (B) Histogram of diameters of twigs used to store nuts of C. patelliformis. Notes: The value on each bar is the actual twigs with the number of stored nuts. Figure 7 Download asset Open asset Grooves carved by squirrels on most C. edithiae nuts were 0.2 - 0.6 cm in width. Figure 8 Download asset Open asset Nuts were generally stored on the first to third branches at 1.5–2.5 m aboveground. (A) C. edithiae nuts. (B) C. patelliformis nuts. Notes: The value on each bar is the actual number of stored nuts. Squirrels of both species carved spiral zigzagged grooves that encircled the midsection surface of the ellipsoid nuts of C. edithiae (Figure 2a–f) with one, or occasionally, two grooves (Figure 3a–c). Two non-connected or spiral grooves appeared to be useful for adjusting the position of stored nuts to the specific orientation of the twigs. In contrast, up to 20 surface grooves were carved on the bottoms of the oblate nuts of C. patelliformis (Figure 2g–i). These grooves on oblate nuts varied considerably in pattern from symmetric (Figure 3d–i) to scattered. Symmetrical grooves on the bottom of nuts likely facilitate firm positioning as squirrels rotated nuts, apparently to optimize the nut's position for the most secure attachment. Interestingly, oblate nuts stored on living trees and shrubs had significantly more carved shallow scattered grooves than those stored on dead trees and lianas (5.1 ± 5.0 vs 2.8 ± 4.0, t=2.1591, df=46.402, p=0.036). Because the bark of dead trees and lianas is coarser than that of living trees, fewer grooves may be required to hold the nuts securely in place. We also note that the grooves on the ellipsoid nuts of C. edithiae were deeper (more than 0.5 mm) than those on the oblate nuts of C. patelliformis (less than 0.45 mm, p<0.05, Figure 9). Nonetheless, none of the chewed grooves that we observed were deep enough to damage the endosperm of the nut, and thus the squirrels seemed to minimize the potential impacts of fungi during storage. Figure 9 Download asset Open asset Number of grooves carved on the oblate nuts of C. patelliformis. (A) Most nuts had fewer than eight grooves. (B) The depth of most grooves was shallow to medium. Notes: The value on each bar is the actual number of stored nuts. The surface grooves allowed the squirrels to 'pressure fit' the nuts between the two plant twigs in a way functionally similar to a mortise-tenon joint (Qiao et al., 2021; Figure 3). Squirrels used the twigs as a convex 'tenon' to fit into the convex 'mortise' provided by the grooves on the nuts. Thus, carved nuts were inlayed between plant twigs (0.10–0.60 cm in diameter) intersecting at specific angles (25–40°) on various understory plants (Figure 4). We found nuts on small trees and shrubs, but also on lianas, bamboos, or dead trees, and even occasionally on large petioles of palms or trees (Figure 2j–n, Table 2). Once fixed in this manner, nuts were resistant to being blown off by strong wind or even by shaking that we administered experimentally (Videos 10–15). Table 2 The types of plants used for nut storage. Plant typeNumber of individualsPercentage of all individuals (%)Alive tree10871.5Dead tree1711.3Alive liana1912.6Dead liana21.3Bamboo53.3Total151100 Video 10 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Footage of shaking a liana does not dislodge nuts of Cyclobalanopsis edithiae stored by squirrels. Video 11 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Footage of shaking a sapling does not dislodge nuts of Cyclobalanopsis edithiae stored by squirrels. Video 12 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Footage of shaking a sapling does not dislodge nuts of Cyclobalanopsis edithiae stored by squirrels. Video 13 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Footage of shaking a liana does not dislodge nuts of Cyclobalanopsis patelliformis stored by squirrels. Video 14 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Footage of shaking a sapling does not dislodge nuts of Cyclobalanopsis patelliformis stored by squirrels. Video 15 Download asset This video cannot be played in place because your browser does support HTML5 video. You may still download the video for offline viewing. Download as MPEG-4 Download as WebM Download as Ogg Footage of shaking a sapling does not dislodge nuts of Cyclobalanopsis patelliformis stored by squirrels. The distance between the closest Cyclobalanopsis trees producing nuts and storage sites on smaller understory plants varied from 10–25 m (Figure 10), distances greater than the average canopy width of large trees in the Jianfengling forest (estimated to be 10 ± 5 m). This sort of distancing likely reduces discovery by other squirrels, mice, or other animals potentially searching for aboveground nuts below the parent trees (Cao et al., 2011), although recordings from our cameras show that some nuts were still found and eaten by mice. Figure 10 Download asset Open asset Distance from storage sites to potential parent trees for the nuts varied from 10–25 m. (A) C. edithiae nuts. (B) C. patelliformis nuts. Notes: The value on each bar is the actual number of stored nuts. Because of this spacing, most seedlings that result from dropped, fallen or forgotten nuts (Figure 11) will germinate at some distance from their parents. Thus, seed dispersal by these squirrels may decrease competition between seedlings and parent trees. This should increase seedling survival rates and could, in turn, truly decrease the negative density dependence of conspecific trees (see Detto et al., 2019). Unfortunately, we presently do not have sufficient data to estimate what proportion of the disappearance that we observed is the result of use by squirrels, although our video footage establishes that they do remove some nuts (Videos 2–5). Nonetheless, some proportion of nuts likely falls from storage sites and germinates nearby, as is common for seeds and nuts cached by squirrels, especially in hardwood forests (Steele and Yi, 2020). Figure 11 Download asset Open asset After long (e.g. >ca. 365 days) storage, nuts become not fresh. (A) Dried nuts, (B) Germinated, or (C–E) Destroyed by insects. Only 63.6% of nuts that we discovered on understory plants were fresh at the time of the survey. Under natural conditions, these nuts on the ground would likely germinate in ca. two to three months after they had fallen to the ground (Zhou, 2001). These stored nuts did not germinate during our 3.5 month investigation interval, which means that nuts can persist in these storage sites for longer periods than do nuts on the ground. Over the 44 days between the first and second surveys, 19.7% of the stored nuts had disappeared, and 15.0% of the nuts discovered during the second survey were new. Over the 61 days between the second and third surveys, 43.7% of the stored nuts had disappeared, and 20.6% of the nuts discovered were new. Thus, the numbers and composition of stored nuts in this forest are dynamic variables. In general, the number of nuts being stored decreased gradually from January to May after the fruiting season, i.e., it decreased gradually through the dry season toward the rainy season. The high precipitation and humidity of the Jianfengling forest environment (Xu et al., 2015) likely favors the storage of nuts above the ground by reducing fungal infection or germination and may influence the timing of removal of stored nuts. In more temperate forests with lower annual precipitation nuts can be safely stored under dry leaf litter or in the ground without special processing (Hadj-chikh et al., 1996). Hence, the suspended storage that we observed appears to be an effective adaptation for safe storage, mainly during drier seasons to improve the food supply for the squirrels during the colder months at Jianfengling. However, field comparative data about the fates of seeds stored on or above are needed to properly evaluate this hypothesis. In summary, we have demonstrated that individuals of the flying squirrel species H. phayrei electilis and H. alboniger collect and cache nuts from or beneath two species of Cyclobalanopsis trees in the Jianfengling forest. Nuts were carried 10–25 m away from parental fruiting trees and processed for storage by chewing grooves into their surfaces before they were suspended on shrubs or small trees (Videos 6–7). The pattern and depth of these grooves varied with the shape of nuts from these two Cyclobalanopsis trees so that they could be effectively fixed in the crotches of two twigs. Squirrels appear to check the strength of fixation, and sometimes iteratively modify the grooves to improve the attachment (Video 1, Video 4 and Video 8) before a nut is finally removed from a storage site (Video 2, Video 3 and Video 5). Clearly, individuals of these two squirrels store nuts of different shapes and sizes securely on a variety of plant twigs at some distance from the plants that produced the nuts. The behavior of these two squirrel species has evolved to prepare nuts for such storage by chewing surficial grooves on nuts to enable a 'mortise-tenon' connection between nuts and understory plant twigs. The significant efforts that we observed of squirrels testing and adjusting the fixation of nuts suggest that they employ active cognitive processes in storing these nuts. Taken together, our observations suggest that effective food storage behavior is a significant aspect of the adaptation of these two flying squirrel species to life in the humid tropical rainforest. Nut caching behavior helps to secure food for the coolest month in these rainforests. We have not yet compared the fates of suspended nuts with those of the same species buried in the ground or leaf litter, but predict that suspended storage will be far superior. The caching behavior may further affect the dispersal of nuts (Chang and Zhang, 2014; Xiao et al., 2004) in a way that alters the spatial and temporal distribution of the local plant community in the long run. Thus, this behavior could have a significant impact on the larger forest community. Although the importance of large DBH trees has been emphasized for the maintenance of forest ecosystem productivity (Lutz et al., 2018); however, from a broader perspective that includes the understanding of squirrel caching behavior, small understory plants may help sustain the diversity and complexity of forest structure. The possibility that such plant-animal interactions affect tree populations and distributions deserves more attention in the future (Goheen and Swihart, 2003; Rong et al., 2013). Materials and methods Study site Request a detailed protocol This study was conducted in the Jianfengling region of the Hainan Tropical Rainforest National Park in Hainan Province, China (108°46'–109°45'E). The area has a seasonal tropical monsoon climate with a rainy season from June to October and a dry season from November through May of the next year. The mean annual temperature in this forest is 19.7 °C and the annual average precipitation is 2461 ± 619 mm. Jianfengling is the second rainiest area on Hainan Island, with an average annual relative humidity of >88% (Jiang and Lu, 1991). The Jianfengling forest includes 992 free-standing tree and shrub species, and is dominated by trees of Fagaceae, Lauraceae, and Moraceae (Xu et al., 2012). Castanopsis, Lithocarpus, and Cyclobalanopsis are the three main genera of Fagaceae, which reproduce through nuts that are used as food by various mammals. The cupules of Castanopsis are solitary units produced on a rachis, completely or partially enclosing the nut, while cupules of Lithocarpus are grouped together in cymes on the rachis, completely or partly enclosing the nut. In contrast, cupules of Cyclobalanopsis are solitary, and do not enclose the nuts. Because enclosed nuts are difficult for squirrels to deal with, Cyclobalanopsis nuts are highly preferred as food by squirrels and other animals, although the smooth nut surfaces make them challenging to suspend on vegetation. Cyclobalanopsis edithiae (Skan) Schottky and Cyclobalanopsis patelliformis (Chun) Y. C. Hsu et H. W. Jen are the two most abundant species with naked nuts in the mountain forests of Jianfengling (Xu et al., 2015; Table 3). Both are in fruit from October to December, just before the coolest month (January) in Hainan. Table 3 Main Fagaceae species found in a 60 ha plot in the Jianfengling forest. SpeciesAbundanceCastanopsis carlesii (Hemsley) Hayata3269Castanopsis fissa (Champion ex Bentham) Rehder & E. H. Wilson2803Castanopsis jianfenglingensis Duanmu2297Castanopsis tonkinensis Seemen953Castanopsis ledongensis C. C. Huang & Y. T. Chang335Castanopsis fabri Hance113Castanopsis hystrix J. D. Hooker & Thomson ex A. de Candolle35Cyclobalanopsis edithiae (Skan) Schottky1645Cyclobalanopsis patelliformis (Chun) Y. C. Hsu & H. W. Jen1207Cyclobalanopsis phanera (Chun) Y. C. Hsu & H. W. Jen886Cyclobalanopsis fleuryi (Hickel & A. Camus) Chun ex Q. F. Zheng568Cyclobalanopsis neglecta Schottky392Cyclobalanopsis blakei (Skan) S

[Objective] Hopea hainanensis is not only a Wild Plant with Extremely Small Populations but also a national grade first-level key protected plant and a representative species of tropical rainforest in Hainan, China. Here, the population structure, dynamic change characteristics, and future development trend of Hopea hainanensis were explored for providing a scientific basis for protection and management.[Methods] In this study, the population structure and dynamic characteristics of Hopea hainanensis distributed in Jianfengling, Hainan was analyzed using population statistics, including static life table, survival curve, and time series prediction model. [Results]（1） The population structure of wild Hopea hainanensis in Jianfengling showed an inverted J shape, and the number of seedlings（age class I） accounted for95.24% of the total individuals.（2） The slop populations in the primary forest and secondary forest in the Jianfengling area tended to grow, but there was a limitation in the growth of young trees during the transition from seedlings to saplings, resulting in a very small number of adults.（3） The regeneration ability of Hopea hainanensis population in primeval forests was lower than that in secondary forests, but the recovery process in secondary forests was more susceptible to external disturbance.（4） The Hopea hainanensis population in primeval forests and secondary forests would recover steadily after 2-5 age classes if the existing Hopea hainanensis population and their habitats in Jianfengling were well protected.[Conclusion] There are still a certain number of wild saplings in Jianfengling, Hainan. It is urgent to strengthen the protection of the existing wild Hopea hainanensis populations and promote the transformation of the seedlings to saplings in the natural state.

为研究红厚壳(Calophyllum inophyllum)硬枝扦插育苗技术,以多年生硬枝为扦插材料,探讨不同浓度的萘乙酸(NAA)、吲哚丁酸(IBA)及ABT1#号生根粉对插穗抽叶特性和生根特性的影响,并运用隶属函数法,综合评价各处理组合扦插效果.结果 表明:不同激素处理对硬枝插穗的生根率、偏根率、不定根数量、不定根体积、侧根数量等生根特性指标有显著影响,但对抽叶率、新叶数、新叶面积等抽叶特性指标影响不显著.3种激素对促进红厚壳扦插育苗的综合效果排序为:NAA>IBA>ABT1#.红厚壳硬枝扦插的最佳激素处理组合为1200mg/L的NAA,生根率为75.0％、偏根率为0.21.