Recorded vegetation became more similar between 1,712 matched Swedish forest plots from 1999–2004 to 2019–2024

We tested whether recorded forest vegetation at nearby Swedish plots became more similar over 20 years. We used version 1.2 of the open Swedish National Forest Inventory vegetation data. We selected 1,712 plots with productive forest land and a full 100 m² area at both visits. Each plot had visits exactly 20 years apart, with dates within 30 days of year. The same 104,571 plot pairs were 10–100 km apart in both periods. Mean Sørensen dissimilarity fell from 0.4888 in 1999–2004 to 0.4622 in 2019–2024. The change was −0.0266, with an approximate 95% spatial block jackknife interval of −0.0379 to −0.0153. Mean listed taxon count per plot rose from 18.59 to 20.17. Simpson replacement dissimilarity also fell, by 0.0315. The direction stayed the same with stricter visit dates, species-level records, larger spatial blocks, and equal plot weights. Separate vascular plant and non-vascular checks also showed decreases. These checks were added after the first results. The full source audit checked all 8,817,270 presence and absence records with no mismatch. The result applies to recorded vegetation in this selected panel. The data do not separate biological change from changes in detection. They do not identify climate or forest management as the cause. The package preserves compact complete inputs, code, figures, all estimates, source hashes, an analysis plan, and repeat instructions.

forest vegetation · Swedish National Forest Inventory · biotic homogenization · beta diversity · paired plots · reproducible ecology