影响森林多样性和未来恢复的再生限制和种子大小-数量权衡
题目：Limits to reproduction and seed size-number trade-offs that shape forest dominance and future recovery (发表于Nature Communications,PNAS)
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报告人简介
Dr. Tong Qiu received his Ph.D. from the University of North Carolina at Chapel Hill in 2020. Since then, he works as a postdoc at Nicholas School of the Environment at Duke University. His major fields are remote sensing of ecology. His work concentrates on synthesizing geospatial big data from satellite and airborne remote sensing with ecological big data from monitoring network within a Bayesian Hierarchical Models to quantify global change impacts on forest ecosystems. Dr. Qiu’s works are the first to establish global quantifications in tree seed production and how the patterns are associated with tree size and nutrient requirement. Dr. Qiu’s personal website can be found at: www.ecotongqiu.com
邱桐博士于2020 年从北卡罗来纳大学教堂山分校获得博士学位，之后在杜克大学Nicholas School of the Environment做博士后。其主要研究领域为生态遥感，使用贝叶斯统计模型综合来自卫星和航空遥感的地理空间大数据与来自监测网络的生态大数据，用来量化全球变化对森林生态系统的影响。最近的工作是建立树木种子产量的全球量化，以及种子产量与树木大小和养分需求之间的关联。个人网址：www.ecotongqiu.com
报告摘要
The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacit to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential.
树木种子产量对于了解森林物种的进化及其从干旱、火灾和砍伐造成的损失中恢复的能力至关重要。基于全球 714 个物种的种子产量数据，我们验证了对量化树木繁殖至关重要的假设。这些假设是评估森林树木适应性的基础。这篇文章主要有四个主要发现。首先，种子生产不受种子大小和数量之间严格权衡的限制。相反，种子数量变化超过十个数量级，种子较大的物种产生的种子数量比 1:1 权衡所预期的要多。其次，裸子植物的种子产量低于被子植物，这可能是由于它们对木质锥的额外投资。第三，叶片磷浓度含量较高物种，它们的种子产量低。最后，不同物种对土壤肥力的敏感性差异很大，限制了种群种子生产对肥力梯度的反应。结合起来，这些发现对于提高森林的管理、保护和恢复具有重要意义。
研究背景
The emergence of extreme disturbance as a global change phenomenon highlights the need to understand how tree fecundity influences forest regeneration. Drought-induced forest diebacks are now observed in regions where they were rare in the past. Forest stands shaped bynon-destructive surface fires are experiencing catastrophic crown fires, withpost-burn seed production limited to survivors in unburned fragments. Species capable of vegetative regrowth will contribute to reforestation, but colonization and seedling success in many landscapes will depend on numbers and sizes of seeds from surviving trees. Large seeds that are well-provisioned for seedling survival are expensive to produce, apparent to seed predators, and dependent on animals for dispersal. If the costs of producing larger seeds are offset by producing fewer of them, then reproduction could be capped by asize-numbers trade-off. If this cap depends on resources, then landscape fertility gradients and differing nutrient requirements of species play important roles. While species differences in seed size are readily observed and the subject of a large literature, seed numbers are not. At least since Darwin pondered the rapid ascendance of angiosperms in the Cretaceous, seed production has been recognized as a fundamental component of fitness, that lacks systematic quantification for trees, the planet’s dominant life form.
The reproductive potential of trees as they grow and age is of special concern to ecologists because, despite being relatively rare, large trees can contribute disproportionately to forest biomass due to the allometric scaling that amplifies linear growth in diameter to a volume increase that is more closely related to biomass. Understanding the role of large trees can also benefit management in recovering forests. If allometric scaling applies to fecundity, then these large individuals might determine the species and genetic composition of seeds that compete for dominance in future forests. Unfortunately, under representation of big trees in forests frustrates efforts to infer how fecundity changes with size. Despite the increase with tree size assumed by ecologists, evidence for declining reproduction in large trees has continued to accumulate from horticultural practice and at least some ecological and forestry literature. However, we are unaware of studies that evaluate changes in fecundity that include substantial numbers of large trees.
Studies of nutrient effects are limited geographically and taxonomically, and results are equivocal. Through the assumption that reproductive allocation scales with net primary production (NPP), current models assume that resources stimulate fecundity, without consideration of responses that might come from variation within species versus between species. Nutrient additions in orchard practice use ratios intended to balance demands for nitrogen (N), phosphorus (P), and potassium (K), the latter being especially important for fruit yield in agriculture, but less frequently reported in ecological studies. Agricultural experience makes it clear that high-N fertilization can decrease reproductive yield due to allocation to vegetative growth. Variation between species couldcome from their differing nutrient demands, where in species with high averagefoliar nutrients tend to occupy fertile sites and, thus, might be expected toproduce more seed. Alternatively, species with high nutrient demands might preferentially allocate to growth and defense.
研究发现
A. 种子产量和树木直径之间的关系
Here we present a wall-to-wall identification of non-forest trees (defined as woody plants with a crown size over 3 m²) in the West African Sahara, Sahel and sub-humid zone, covering a rainfall gradient from hyper-arid (rainfall of 0–150 mm yr−1), arid (rainfall of 150–300 mm yr−1), semi-arid (rainfall of 300–600 mm yr−1) to sub-humid (rainfall of 600–1,000 mm yr−1) areas. We split the crown sizes into shrubs (3–15 m²), small trees (15–50 m²) and large trees (50–200 m²), as well as very large trees and clumped canopies that form thickets or forests (over 200 m²).
Sixty-three percent of the species in this study (Qiu et al., 2021) show eventual declines in fecundity with increasing diameter (type A in Materials and Methods) (Fig.1). Seventeen percent showed diminishing rates of increase (type B), indicating that fecundity might eventually plateau or even decrease. Only 20% show the continuous increase in fecundity (type C) that would be consistent with traditional allometric scaling.
Fig 1. The relationship between fecundity and diameter for species in temperate (A–C) and tropical (D–F) regions, where diameter and fecundity are scaled as 𝐷/𝐷𝑜𝑝𝑡 and 𝑓(𝐷)/𝑓(𝐷𝑜𝑝𝑡), respectively. A and D exhibit type A species(fecundity eventually declines); B and E show type B species (sigmoid increasein fecundity); C and F represent type C species (continuous increase in fecundity). Line transparency is proportional to the 90% credible interval width across the diameter ranges, such that confident predictions are opaque, and vice versa. 𝐷 is the diameter when maximum fecundity occurs.
图 1. 和传统生态学家们的假设不同，异速生长无法描述种子的产量和直径的关系
B. 每个物种的种子产量在全球尺度上的量化
Rather than being constrained to constant values by a size-numbers trade-off, species seed production perbasal area (SSP) spans ten orders of magnitude and increases with seed size(β = 0.546 ± 0.042, Fig. 2a). For a small-statured species this amounts to 0.62 kg at a diameter of 20 cm. For a large species, this is 15 kg for adiameter of 100 cm. For a small-seeded species of, say, 0.001 g (e.g., Betula), this amounts to annual averages of > 50, 000 seeds at 40 cm and > 300,000 seeds at 100 cm. If SSP is regulated by a proportionate sacrifice in seed size as the cost of producing more of them, then SSP would not vary with seed size(dashed line in Fig. 2a). Instead, seed numbers decline with seed size at lessthan half the proportionate rate for a slope of β − 1 = − 0.454 ± 0.042. This departure from expectation comes from variation in SSP that is related to species traits and partly captured by phylogeny.
Figure 2. A. Species seed production (SSP, g seed per m² tree basal area) is not constrained by the strict size-number trade-off (dashed line with a slope of zero). Instead, itvaries over ten orders of magnitude and has a positive correlation with seed mass across 714 tree species (log10 SSP =4.29+0.546⋅log10𝑚, R² = 0.189, p < 10−15, n = 714). B. SSP exhibits phylogenetic coherence for 482 species having phylogeny data (68% of species).Brown and green text highlight species that produce coniferous cones and fleshy fruits, respectively. The phylogenetic signal is estimated using Pagel's λ = 0.60 (p < 10−9, n = 482).
图 2. 全球尺度上种子的产量并不被种子数量-大小的关系所限制，与之相反，种子的产量可以有至少10个数量级的差异。
C. 种子产量和其对土壤肥力的响应
Phenotypic plasticity in individual seed production (ISP) to soil fertility varies widely between species, but with phylogenetic coherence (Fig. 3). In the well-represented Fagales, fertility responses in Castanea and the white and cerris oaks (sections Quercus and Cerris) are negative, while red oaks(section Lobatae) are positive. This divergence coincides with the tendency for many red oaks (Lobatae) to occupy more fertile soils. Increasing CEC reduces ISP in the angiosperm groups Fabales, Magnoliids, Acer, and Fraxinus, while stimulating ISP in Ericales/Cornales, and having mixed effects in Rosales and Betulaceae. Gymnosperm responses are primarily decreasing for Tsuga, Picea, Cedrus, and soft pines(section Haploxylon), and mixed for hard pines (section Diploxylon)and Abies. Despite species variation, the reconstructed ancestral lineage for gymnosperms is negative (purple ancestral branches marked by red dashed line in Fig. 3). By contrast, there is no angiosperm-wide tendency to produce fewer seeds on high-fertility sites.
Figure 3. Sensitivity of individual standardized production (ISP) to soil fertility based on within-species response to cation exchange capacity. Text color follows Fig. 2. Red dashed line indicates the ancestry of gymnosperms. Percentages of species that respond negatively to CEC are labeled for species groups. The phylogenetic signal, estimated for 129 species (91% of species) having phylogeny data, is highly significant (Pagel's λ = 0.87, p < 0.001, n = 129).
图 3. 种间差异对于土壤肥力的响应
D. 种子产量和其对土壤肥力的响应
By contrast with recruitment, the fecundity response little resemblance to both adults and recruits; fecundity response to climate and soil diverges from the life stages on either side of it. This divergence takes the form of a braided alluvial diagram in Figure 4, where community reorganization occurs in 57% of species from adults to fecundity and in 64% of species from fecundity to recruits. The adult to fecundity divergence is followed by a partial re-coalescence, with 69% of species returning to the adult community at the recruitment stage.
Figure 4. Map of community assemblages (A) and reorganization as an alluvial diagram (B). Community clustering is based on adult tree response to climate and soil fertility. (B) Shifts in assemblages across the three demographic stages (adult, fecundity, and conditional recruits) in eighty-seven species. Flows are color-coded by adult membership in communities mapped in(A).
图 4. 树木的不同生长阶段（成年树，种子，和幼苗）对于土壤和气候的响应不同，导致了种群在不同阶段的移动
结论
这一系列的文章得到了下面的结论：
1. 树木被认为是可以一直保持“年轻”和持续生长，但是我们发现当它们达到一定年龄之后，其繁殖能力（种子数量）会开始下降。掌握树木繁殖能力下降的时间和程度对于管理，保护，和恢复森林具有重要的作用。
2. 长期以来，生态学家认为种子的数量和种子的大小之间存在一个权衡，也就是种子的大小越大，那么种子数量越少。但是我们发现对于树种来说，这种权衡并不一定是1:1的。对于全球尺度上的树种，种子的产量有10个数量级的差异。而且被子植物的种子产量比裸子植物要高。这些发现对于研究森林多样性和森林的再生有重要的意义。
3. 大部分的生态学模型认为土壤肥力越高，种子的产量应该越高。但是我们发现不同树种对于土壤肥力的响应是不同的，有些树种的种子产量会提高但是有些不会。裸子植物的祖先更喜欢肥力较低的土壤。这项发现可以改进已有的生态学模型。
4. 最后，我们发现成年树和幼苗对于土壤和气候的响应非常类似，但是种子的响应和前两者完全不同。这些原因也就造成了树木的种群会在不同的生长阶段发生偏移。这项发现可以更好的管理森林来应对气候变化。
References:
Qiu,Tong,et al. "Limits to reproduction and seed size-number trade-offs that shape forest dominance and future recovery. Nature Communications 13.1 (2022): 1-12. DOI: https://doi.org/10.1038/s41467-022-30037-9
Qiu,Tong,et al. "Is there tree senescence? The fecundity evidence." Proceedings of the National Acedemy of Science 118.34(2021).DOI:https://doi.org/10.1073/pnas.2106130118
Qiu,Tong,et al. "Niche Shifts From Trees to Fecundity to Recruitment That Determine Species Response to Climate Change. "Frontiers in Ecology and Evolution 9(2021).DOI:https://doi.org/10.3389/fevo.2021.719141
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