Data from: Effects of increased N and P availability on biomass allocation and root carbohydrate reserves differ between N‐fixing and non‐N‐fixing savanna tree seedlings

In mixed tree‐grass ecosystems, tree recruitment is limited by demographic bottlenecks to seedling establishment arising from inter‐ and intra‐life‐form competition, and disturbances such as fire. Enhanced nutrient availability resulting from anthropogenic nitrogen (N) and phosphorus (P) deposition can alter the nature of these bottlenecks by changing seedling growth and biomass allocation patterns, and lead to longer‐term shifts in tree community composition if different plant functional groups respond differently to increased nutrient availability. However, the extent to which tree functional types characteristic of savannas differ in their responses to increased N and P availability remains unclear. We quantified differences in above‐ and belowground biomass, and root carbohydrate contents in seedlings of multiple N‐fixing and non‐N‐fixing tree species characteristic of Indian savanna and dry forest ecosystems in response to experimental N and P additions. These parameters are known to influence the ability of plants to compete, as well as survive and recover from fires. N‐fixers in our study were co‐limited by N and P availability, while non‐N‐fixers were N limited. Although both functional groups increased biomass production following fertilization, non‐N‐fixers were more responsive and showed greater relative increases in biomass with fertilization than N‐fixers. N‐fixers had greater baseline investment in belowground resources and root carbohydrate stocks, and while fertilization reduced root:shoot ratios in both functional groups, root carbohydrate content only reduced with fertilization in non‐N‐fixers. Our results indicate that, even within a given system, plants belonging to different functional groups can be limited by, and respond differentially to, different nutrients, suggesting that long‐term consequences of nutrient deposition are likely to vary across savannas contingent on the relative amounts of N and P being deposited in sites.

乔草混合生态系统中，树木更新（tree recruitment）受限于幼苗建成的种群瓶颈（demographic bottlenecks），该瓶颈源于生活型间与生活型内竞争，以及火烧等干扰因子。人为氮（N）与磷（P）沉降引发的养分有效性提升，可通过改变幼苗生长与生物量分配（biomass allocation）格局，进而改变上述瓶颈的性质；若不同植物功能群（plant functional groups）对养分有效性提升的响应存在差异，则可能引发树木群落组成的长期转变。然而，以稀树草原（savanna）为特征的树木功能群对N、P有效性提升的响应差异程度仍不明确。本研究针对印度稀树草原与干旱森林生态系统的特征树种，选取多种固氮（N-fixing）与非固氮（non-N-fixing）树种的幼苗，通过模拟N、P添加实验，量化其地上、地下生物量以及根系碳水化合物含量的差异。上述参数已知会影响植物的竞争能力，以及火灾后的存活与恢复能力。本研究中的固氮树种受N与P有效性共同限制，而非固氮树种仅受N限制。尽管两类功能群在施肥后生物量产量均有所提升，但非固氮树种的响应更为显著，其生物量相对增幅高于固氮树种。固氮树种在地下资源分配与根系碳水化合物储备上的基础投入更高；尽管施肥均降低了两类功能群的根冠比（root:shoot ratio），但仅非固氮树种的根系碳水化合物含量在施肥后出现下降。本研究结果表明，即便在同一生态系统内，不同功能群植物可能受不同养分限制，且对养分的响应存在差异，这意味着养分沉降的长期效应可能因稀树草原生态系统而异，具体取决于各站点的N、P相对沉降量。