Trade-offs in above and belowground biomass allocation influencing seedling growth in a tropical forest

1. Plants allocate biomass to different organs in response to resource variation for maximizing performance, yet we lack a framework that adequately integrates plant responses to the simultaneous variation in above and belowground resources. Although traditionally, the optimal partition theory (OPT) has explained patterns of biomass allocation in response to a single limiting resource, it is well known that in natural communities multiple resources limit growth. We study trade-offs involved in plant biomass allocation patterns and their effects on plant growth under variable below and aboveground resources –light, soil N, and P– for seedling communities. 2. We collected information on leaf, stem, and root mass fractions for more than 1,900 seedlings of 97 species paired with growth data and local-scale variation in abiotic resources from a tropical forest in China. 3. We identified two trade-off axes that define the mass allocation strategies for seedlings – allocation to photosynthetic vs. non-photosynthetic tissues and allocation to roots over stems – that responded to the variation in soil P and N and light. Yet, the allocation patterns did not always follow predictions of OPT in which plants should allocate biomass to the organ that acquires the most limiting resource. Limited soil N resulted in high allocation to leaves at expense of non-photosynthetic tissues, while the opposite trend was found in response to limited soil P. Also, co-limitation in above and belowground resources (light and soil P) led to mass allocation to stems at expense of roots. Finally, we found that growth increased under high light availability and soil P for seedlings that either invested more in photosynthetic over non-photosynthetic tissues or/and that allocated mass to roots at expense of stem. 4. Synthesis: Biomass allocation patterns to above and belowground tissues are described by two independent trade-offs that allow plants to have divergent allocation strategies (e.g., high root allocation at expense of stem or high leaf allocation at expense of allocation to non-photosynthetic tissues) and enhance growth under variable resources. Identifying the trade-offs driving biomass allocation is important to disentangle plant responses to the simultaneous variation in resources in diverse forest communities.

1. 植物会根据资源变异将生物量分配至不同器官以最大化自身性能，但目前仍缺乏能够充分整合植物对地上与地下资源同步变异响应的研究框架。传统上，最优分配理论（Optimal Partition Theory, OPT）曾用于解释单一限制性资源下的生物量分配模式，但学界早已明确，自然群落中往往有多种资源共同限制植物生长。本研究聚焦幼苗群落，探讨植物在地下与地上资源（光、土壤氮（N）、土壤磷（P））变异条件下的生物量分配权衡策略，及其对植物生长的影响。
2. 我们从中国境内一处热带森林中采集了97个物种共计1900余株幼苗的叶、茎、根生物量分数数据，并匹配了幼苗生长数据与当地非生物资源的局地尺度变异信息。
3. 本研究明确了两类定义幼苗生物量分配策略的权衡轴：一是光合组织与非光合组织的分配权衡，二是根相对于茎的分配权衡，这两类权衡均会响应土壤氮、磷与光照的变异。不过，生物量分配模式并不总是符合OPT的预测——该理论认为植物应将生物量分配给获取最限制性资源的器官。当土壤氮受限，植物会以非光合组织为代价增加叶片生物量分配；而在土壤磷受限的情况下则呈现相反趋势。此外，地上与地下资源（光与土壤磷）共同限制时，植物会以根系为代价增加茎部生物量分配。最后我们发现，对于那些更多将资源投入光合组织而非非光合组织，或/且以茎部为代价将生物量分配给根系的幼苗而言，高光照与高土壤磷条件下其生长速率会显著提升。
4. 研究总结：植物向地上与地下组织的生物量分配模式可通过两类独立的权衡策略加以阐释，这两类权衡允许植物演化出分化的分配策略（例如以茎部为代价的高根系分配，或以非光合组织为代价的高叶片分配），并帮助植物在资源变异环境中提升生长性能。厘清驱动生物量分配的权衡机制，对于阐明多样森林群落中植物对资源同步变异的响应规律具有重要意义。