Trade-offs in above- and below-ground 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 below-ground 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 above-ground 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 versus 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 the expense of non-photosynthetic tissues, while the opposite trend was found in response to limited soil P. Also, co-limitation in above- and below-ground resources (light and soil P) led to mass allocation to stems at the expense of roots. Finally, we found that growth increased under high-light availability and soil P for seedlings that invested more in photosynthetic over non-photosynthetic tissues or/and that allocated mass to roots at the expense of stem.

4. Synthesis. Biomass allocation patterns to above- and below-ground tissues are described by two independent trade-offs that allow plants to have divergent allocation strategies (e.g. high root allocation at the expense of stem or high leaf allocation at the expense of allocation to non-photosynthetic tissues) and enhance growth under different limiting resources. Identifying the trade-offs driving biomass allocation is important to disentangle plant responses to the simultaneous variation in resources in diverse forest communities.

Umaña, M.N., Cao, M., Lin, L.X., Swenson, N.G., & Zhang, C.C.
Journal of Ecology