Can understory vegetation accurately predict site index? A comparative study using floristic and abiotic indices in sessile oak (Quercus petraea Liebl.) stands in northern France

We investigated the relevance of understory vegetation in indicating site productivity as expressed by sessile oak (Quercus petraea Matt. Liebl.) site index over a large territory. The relationships between site index and (1) understory vegetation or (2) soil, topography and climate were studied using data from 99 even-aged high-forest stands located in northern France. Multiple regressions using floristic indices as predictors explained the same part of variance in site index as regressions using climate, topography, and soil factors (R-2 = 0.49 to 0.60). However, better models were obtained by combining floristic and abiotic variables (R-2 = 0.57 to 0.69). We concluded that (1) site productivity can be assessed with the same precision using understory vegetation or abiotic variables separately, even over a large region, but (2) it would be more appropriate to combine soil physical and chemical properties, climate and topography with floristic indices to estimate sessile oak site index.