Rare spatio-temporal interactions between conspecific species mingling and size inequality in a diverse Afromontane forest

Spatial indices of tree diversity have often been proposed as surrogates of direct measures of biodiversity. They are comparatively straightforward to measure as part of forest ecosystem monitoring designed to alert to potentially negative effects of ongoing climate change. The loss of biodiversity, which is thought to be related to a decline in tree diversity, is perceived as a substantial threat, since biodiversity is also crucial to ecosystem resilience. We studied the correlation between community species mingling and size inequality of Knysna Forest, a well-known Afromontane forest ecosystem in South Africa, to better understand the principles of how nature maintains tree diversity. This is an important prerequisite for active conservation. The aforementioned correlation is indicative of the mingling-size hypothesis predicting that large trees are surrounded by significantly more heterospecific trees than smaller trees. The mingling-size hypothesis helps understand natural principles of tree diversity maintenance and is motivated by the well-known Janzen-Connell and herd-immunity hypotheses. Our results revealed that the correlation between spatial species mingling and size inequality is mostly negative at Knysna Forest, which is comparatively rare. This implies that the mingling-size hypothesis does not hold in this forest ecosystem. This has implications for conservation, because spatial size-inequality is no longer a by-product of high species mingling and potentially requires additional conservation effort. We could also show that the aforementioned negative correlation can be inferred from the mark cross correlation function when applying this spatial summary characteristic to the mingling and size inequality indices of individual trees.