Explaining productivity variation in response to nitrogen addition and warming requires intraspecific variability

1. Recent studies have shown that intraspecific trait variability is an important source of total trait variation. However, the contribution of intraspecific variability to ecosystem functions in the face of global change remains unknown.

2. We measured plant height, leaf area, specific leaf area, leaf dry matter content and leaf thickness of 26 species at individual or leaf level and aboveground net primary productivity in 48 subplots subjected to 8 years of nitrogen addition and warming in a Tibetan alpine meadow. We split community weighted mean (CWM) and Rao’s quadratic entropy (Rao) of individuals’ traits within a given community into “fixed value” (only caused by interspecific difference of traits) and intraspecific variability (only caused by intraspecific difference), respectively, using a variance partitioning method.

3. We found that productivity showed a humped back response to nitrogen addition: it was highest at intermediate levels of nitrogen fertilization. The response trend was mediated by the changes of plant functional structure. Productivity was positive with fixed Rao of plant height and intraspecific variability of leaf area, i.e. community having higher fixed variability of plant height and individuals producing bigger leaf area can increase productivity via niche complementary and dominance effect. Warming reduced productivity directly and marginally decreased individuals’ leaf area which suppresses productivity indirectly.

4. Our research suggests the non-negligible role of plant intraspecific trait variability in maintaining ecosystem functions, especially in the face of global change.

1. 已有研究表明，种内性状变异（intraspecific trait variability）是总性状变异的重要来源。然而，在全球变化背景下，种内性状变异对生态系统功能的贡献仍不明确。
2. 本研究在青藏高原高寒草甸的48个小样方中，对26个物种的个体或叶片水平的株高、叶面积、比叶面积（specific leaf area）、叶干物质含量（leaf dry matter content）和叶厚度进行了测定，同时测定了经过8年氮添加和增温处理样地的地上净初级生产力（aboveground net primary productivity）。本研究采用方差分解法，将特定群落内物种个体性状的群落加权均值（community weighted mean，缩写CWM）与Rao二次熵（Rao’s quadratic entropy，缩写Rao）分别拆分为"固定值"（仅由性状的种间差异导致）与种内性状变异（仅由种内差异导致）两部分。
3. 研究发现，生产力对氮添加呈现驼峰型响应：在中等氮施肥水平下生产力达到峰值。该响应趋势由植物功能结构的变化所介导。生产力与株高的固定Rao二次熵以及叶面积的种内性状变异呈正相关，即群落中株高的固定变异程度更高、个体叶面积更大时，可通过生态位互补与优势效应提升生产力。增温会直接降低生产力，并小幅降低个体叶面积，进而间接抑制生产力。
4. 本研究表明，植物种内性状变异在维持生态系统功能中具有不可忽视的作用，在全球变化背景下尤为如此。