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.