University of California, Berkeley; Lawrence Berkeley National Lab, United States
Abstract: The allocation of net primary productivity (NPP) to reproduction in trees (flowers, fruits, seeds, etc.) influences forest function through a tradeoff with growth and as a necessary driver of recruitment. Despite empirical evidence to the contrary, common parameterizations for RA assume it is a constant fraction of NPP across plant functional types and size, as well as across geographic, climatic, and edaphic gradients, potentially biasing predictions.
We use a novel metric for RA constructed from litterfall records to estimate differences in RA across forest sites globally and ask 1) How does ecosystem-level RA vary within and across biomes? and 2) To what extent is variation in RA explained by climate (MAP, MAT) and stand-level (soil, successional state) characteristics? Recent evidence suggests that RA may increase with temperature and be greater on fertile soils in tropical forests, and may differ across successional states, but these hypotheses have not been evaluated using field data on a global scale.
We collected 1520 observation-years of forest leaf and reproductive litterfall fluxes from 550 sites that span the forested continents. We use R/R+L (where R= reproductive and L = leaf annual litter flux) as a proxy for RA, which our previous work shows is strongly correlated with the more complete field measure R/NPP (R2 = 0.87). This allows us to compare reproductive allocation across a wide range of forest sites where biometric NPP estimates are not available. We used a Kruskal Wallis nonparametric test for differences across biomes and general linear mixed effects models on transformed data to quantify continuous relationships with climate and site characteristics.
Mean RA ranged from 0.11±0.012 in boreal forests to 0.14±0.005 in tropical forests (0.13±0.005 in temperate forests), representing a 27% difference. Differences in RA across tropical and boreal forests were statistically significant (Mann-Whitley, p < 0.001). Globally, we find evidence for a non-linear, concave down relationship between RA and temperature, with a peak at ~20°C. We also find a significant negative relationship between RA and precipitation in the tropics (p < 0.001), but no significant relationship with precipitation in other biomes. These results suggest weaker relationships between reproductive allocation and temperature and precipitation gradients than previously reported, and suggest additional drivers should be examined to explain variation in reproductive allocation.
