Chinese Torreya (Torreya grandis cv. Merrillii) is an important economic plant species and the main source of income for local farmers. This species is distributed in mountainous areas in southeastern China, particularly in Zhejiang, Anhui, and Fujian Provinces (Chen and Jin, 2019a; Li and Dai, 2007; Wu, 2013). The origin of the Chinese Torreya forest can be traced back to the late Tang Dynasty, which is more than a thousand years ago. It was developed from a quality tree species through grafting from ≈1500 years ago (Li and Dai, 2007). The seeds of this tree can be processed to produce nuts, which have been served as snack foods with some health effects (Chen and Chen, 2019; Chen and Jin, 2019a). The seeds have a high nutritional content, including proteins, fatty acids, carbohydrates, calcium, phosphorus, and iron (Li et al., 2005). The seeds also possess antioxidative and acute anti-inflammatory properties (Chen et al., 2006). Currently, the nuts cost ≈$50 to $70 per kg. Usually, one mature Chinese Torreya tree can bring an income of ≈$5000 from seed crops each year. Zhuji City (or County) of Zhejiang Province, which is within the central production area of Chinese Torreya, produces more than 1000 tons of nuts every year (Chen and Jin, 2019b). During the last years, more and more farmers have planted this tree species and produced nuts (People’s Government of Shaoxing City, 2013). With the increasing demand for seeds, scientific studies on the different perspectives for the growth of Chinese Torreya trees and seeds, including photosynthesis, drought tolerance, and nut quality, have been conducted (Hu et al., 2018; Lin et al., 2019; Zhang et al., 2017). However, there are high variations in the seed production of Chinese Torreya with time. So far, few studies have been conducted to analyze the dynamics of seed production for this important tree species at a community level.
The reproduction of Chinese Torreya is a lengthy process. The development of male and female gametophyte starts in April and May, pollination occurs in May of the second year, but seeds become mature during September to November in the third year (Li and Dai, 2007). Then, it can be hypothesized that 3-year cycles should exist in the dynamics of seed production for Chinese Torreya. But because of the lengthy processes in reproduction, environmental factors, such as weather conditions, may affect the final seed production. The crop production of Chinese Torreya can vary ≈20-fold. The production of many seed crops is typically correlated with the weather. It is necessary to study how annual weather can affect seed production in Chinese Torreya (Chen and Chen, 2019). No studies were conducted for the effects of climate on seed production. For the variable tree seed production, there are several hypotheses (e.g., Kelly and Sork, 2002). The resource-matching hypothesis indicates that better environmental conditions during the growing season will later result in a more significant production of seeds. But the resource accumulation hypothesis suggests that trees need to accumulate resources for several years until a threshold is reached, and then a large production occurs, which means a large number of seeds will be produced with a certain biological periodicity despite the weather effect. Pearse et al. (2017) found that the cv in plant seed production increased over time for many masting and long-lived plants because these species with most variable seed production responded strongly to interannual differences in weather or resources. Because Chinese Torreya can live more than a thousand years, it is expected that its cv of seed production may increase with time. Entropy, usually related to disorder and predictability in information, is used to characterize spatial and temporal complexity in ecology (e.g., Chen et al., 2005). Chen et al. (2016) found that the cone production of longleaf pine at different sites followed a similar pattern in complexity with the increase of time scales. It is necessary to know whether the seed production of Chinese Torreya has the same phenomenon. Comparisons of complexity, such as entropy of seed production at local sites through time, may provide insight about seed production from a new perspective. Using or developing useful methods at local areas to characterize the emergent behavior of seed production (e.g., regime shift) for Chinese Torreya needs to be investigated.
Various life forms may obey a host of simple and systematic empirical scaling laws because all biological systems must transform energy and materials to support multiple life functions and structures (West, 1999). Power laws are considered as ubiquitous in nature and are broadly found in the distribution of rain droplet size, earthquakes, drainage area for rivers, and populations of cities (Bak, 1996; Gutenberg and Richter, 1956; Zipf, 1949). In statistics, a power law describes a relationship between two variables, in which a relative change in one variable results in a relative proportional change in the other variable: one varies as the power of another. Although there were fluctuations in seed production, trees were found to have reproductive consistency (Herrera et al., 1998; Koenig and Knops, 1998). Chen et al. (2017) found invariant scaling properties in cone production for longleaf pine in the southeastern United States. Thus, we may derive a hypothesis that there existed possible power laws in the seed production of Chinese Torreya.
The primary goal of this study was to analyze the possible patterns in the dynamics of seed production for Chinese Torreya. The specific objectives were to determine 1) whether there were 3-year cycles in the dynamics of seed production; whether climate correlated with seed production; 2) whether the cv in seed production increased with time; 3) how the complexity of seed production changed with time scales; whether power laws existed in seed production. The results of this study can provide a new understanding of seed production dynamics for Chinese Torreya.
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