A long‐term study of neighbour‐regulated demography during a decline in forest species diversity

Question: Did disturbance, no density‐dependence of the dominant species, and negative neighbourhood interactions on rare species affect tree demography during a decline in species diversity associated with the increase of Acer saccharum from 1939–2001? Hypotheses: 1. The rise in dominance of A. saccharum was because of its advantage in disturbances and lack of density‐dependence of its demography. 2. Rare species were not favoured by disturbances, including those from Dutch elm disease, and demonstrated negative neighbourhood interactions with A. saccharum . Location: Brownfield Woods in Illinois, USA. Methods: Historical maps of trees (≥ 7.6 cm DBH) from 1939, 1951, 1988, and 2001 in 16 quadrats (48 m × 68 m) were used to compare demography of eight tree species. Effects of disturbances, density‐dependence, and neighbourhood interactions on mortality and recruitment of tree species within a 6‐m radius of individual target trees were studied. Results: A. saccharum demonstrated a demographic advantage over rare species. It had lower mortality and higher recruitment rates. Disturbances facilitated recruitment of A. saccharum , but did not enhance rare species. Density‐dependence of both mortality and recruitment of A. saccharum occurred, but population projection models indicated that ecological conditions became more favourable for A. saccharum in the past 62 years. Furthermore, negative neighbourhood interactions were detected between rare species and A. saccharum . The increase in neighbouring A. saccharum significantly increased mortality and reduced recruitment of the rare species. Conclusions: The general disturbance regime, enhanced by Dutch elm disease, in Brownfield Woods facilitated the rise of dominance of A. saccharum . Meanwhile, rare species declined as a result of their disadvantage in disturbance and negative neighbourhood interactions with A. saccharum .