Long inter-fire intervals do not guarantee a large seed bank in a serotinous shrub (Banksia spinulosa Sm.)

1. It is often assumed that long-lived woody perennials with canopy-stored seed banks steadily accumulate seeds over time since fire. Trends in flowering and fruiting have usually been inferred from synchronic studies of sites of different post-fire ages or counting stored seeds in apparent age classes, and mostly in obligate-seeder species. Long-term longitudinal studies on a broader range of species are needed to fully understand the dynamics of flower and fruit production and accumulation of viable seeds. Key questions are: What is the annual trend in flowering and cone production? Is this matched by accumulation of cones and seeds? How is seed germinability related to cone age at time of a fire?

2. We counted inflorescences and tagged cones produced annually by the resprouting shrub Banksia spinulosa in 315 plants over 13 years at one site and in 46 plants over 20 years at another. At the end of the study, we harvested all accumulated cones, burned them and assessed seed viability using germination trials.

3. We detected enormous inter-plant variation in reproductive effort and output. 50% of inflorescences were produced by only 10-15% of plants. There was potential for accumulation of massive seed banks. However, (i) only 8-10% of inflorescences became cones; (ii) only 44-50 % of these were retained until harvest; (iii) many retained cones suffered seed predation; and (iv) viability of retained seeds declined with cone age. The result of these processes meant that the accumulated seed bank was only 2 to 4 seeds per plant, 82% to 94% of the viable seeds had been produced in 6 years prior to harvest, and only 12% to 26% of plants contributed to this viable seed bank.

4. Synthesis: Cone and seed losses and declining seed viability in Banksia spinulosa mean that almost all viable seeds come from the past few years of flowering even though apparently intact cones may be retained for decades. If this is typical for resprouting serotinous shrubs, it is important to understand trends in flowering and fruit set over time because the magnitude of recruitment will depend on fecundity in the few years prior to a fire.

Methods
We established a long-term study using two populations of Banksia spinulosa var. spinulosa in south eastern Australia: Barren Grounds Nature Reserve (34° 40’ S, 150° 43’ E) and Kioloa State Forest (35° 34’ S, 150° 19’ E). At Barren Grounds (BG), a random sample of 49 mature plants was selected from a population of about 1,000. The Kioloa State Forest population (KSF) was added nearly 10 years later and comprised a sample of 349 mature plants from within a similarly sized population. All study plants were tagged and mapped. Towards the end of the flowering season each year we counted the numbers of new inflorescences and the numbers of cones resulting from the previous year’s flowering. Inflorescences that failed to produce any follicles were removed to simplify future censuses and each cone was tagged with a stainless-steel tag identifying the flowering year. We harvested all accumulated cones on the BG plants in December 2005 and on the KSF plants in mid 2006.

1. 学界通常认为，拥有冠层储种库（canopy-stored seed banks）的长寿木本多年生植物，会在火灾发生后的时间跨度内持续累积种子。此前针对开花与结实趋势的研究，大多基于对不同火后年龄样地的同步调查，或是通过统计不同表观年龄层级的储种数量开展，且研究对象多为专性火后播种物种（obligate-seeder species）。若要全面阐明结实与存活种子累积的动态变化，亟需针对更广范围物种开展长期纵向研究。核心问题包括：开花与球果产量的年度趋势如何？该趋势是否与球果及种子的累积相匹配？火灾发生时的球果年龄与种子萌发能力存在何种关联？
2. 本研究针对两个样地的旋卷班克木（Banksia spinulosa，萌蘖型灌木（resprouting shrub））开展长期调查：其一在13年间统计了315株个体的年度花序数量与球果产量，其二则在20年间追踪了46株个体。研究末期，我们收集所有累积的球果并进行灼烧处理，随后通过发芽试验（germination trials）评估种子活力。
3. 研究发现，个体间的繁殖投入与产出存在极大差异：仅10%-15%的植株贡献了50%的花序。理论上存在形成大规模冠层储种库的潜力，但实际存在多重限制：(1) 仅8%-10%的花序可发育为球果；(2) 其中仅44%-50%的球果能留存至收获期；(3) 大量留存的球果会遭遇种子捕食；(4) 留存种子的活力会随球果年龄增长而下降。上述过程共同导致最终累积的储种库仅为每株2-4粒种子；其中82%-94%的存活种子产自收获前6年，且仅12%-26%的植株参与了该存活种子库的构建。
4. 研究总结：尽管旋卷班克木的球果与种子会出现流失，且留存种子的活力随球果年龄下降，但几乎所有存活种子均产自近年的开花事件——即便外观完整的球果可留存数十年。若该特征在萌蘖型闭萌灌木（resprouting serotinous shrubs）中具有普遍性，那么明确随时间变化的开花与结实趋势便至关重要，因为种群补充规模将取决于火灾前数年的繁殖能力。
方法
本研究在澳大利亚东南部选取两个旋卷班克木原变种（Banksia spinulosa var. spinulosa）种群开展长期调查：分别为贫瘠地自然保护区（Barren Grounds Nature Reserve，南纬34°40′，东经150°43′）与基奥洛州立森林（Kioloa State Forest，南纬35°34′，东经150°19′）。在贫瘠地自然保护区（BG），我们从约1000株成熟个体中随机选取49株作为研究对象；基奥洛州立森林种群（KSF）则在近10年后加入研究，从规模相近的种群中选取349株成熟个体。所有研究植株均被标记并绘制位置图。
每年开花季末期，我们统计当年新生花序数量与上一年开花所结的球果数量。为简化后续普查，我们会移除未产生蓇葖果（follicles）的花序，并为每个球果安装不锈钢标签以标注其开花年份。研究末期，我们于2005年12月收集BG种群所有累积的球果，并于2006年中期收集KSF种群的全部球果。