Climate mediates the effects of forest gaps on tree crown allometry

Allometric relationships between crown width (CW) and stem diameter at breast height (DBH) contribute in understanding forest dynamics and estimating forest biomass and carbon stocks. Nevertheless, the response of tree crown allometry to gap management and climate interactions remain unclear. We used 934 paired CW and DBH of Robinia pseudoacacia trees from 38 man-made gap forest sites (GPFs) of different sizes and 40 unmanaged forest sites (UMFs) in three counties with different climatic conditions in the Loess Plateau to (1) evaluate potential deviations in CW estimation from existing crown allometry models for R. pseudoacacia; (2) compare the predictive ability of nine common theoretical functions for crown allometry; (3) analyze scaling exponents variations of crown allometry, and test their fit to theoretical predictions; and (4) examine the influence of stand-level and climatic variables on crown allometric relationships. The existing CW-DBH equations provided better fit for GPFs than for UMFs, although substantial deviations were observed. The power function outperformed other theoretical forms for crown allometry in both GPFs and UMFs. The scaling exponents of the allometric relationships were lower in UMFs than in GPFs, which was closer to the metabolic-scaling theory predictions. Distance-independent competition considering average DBH accounted for major variations in crown allometric relationships in both gap-managed and unmanaged forests. Variations in scaling exponents in GPFs were also explained by diffuse light availability and climatic (annual precipitation and wind speed) variables. Our results highlight the significant role of climatic variables in affecting crown allometric relationships in forest gaps. These results have implications for developing vegetation models and long-term forest management in the context of climate change.