No evidence for novel weapons: biochemical recognition modulates early ontogenetic processes in native species and invasive acacias

The Novel Weapons Hypothesis postulates that the release of allelochemicals by alien plants can inhibit the growth of evolutionary naive native plants. On the other hand, when species share a recent evolutionary history, recognition of phytochemicals from neighboring plants can have adaptive value by providing cues to signal suitable conditions conducive to establishment. This has been termed the Biochemical Recognition Hypothesis. We explored these two hypotheses by conducting germination experiments in South Africa and Spain and a growth experiment in South Africa, using invasive Australian acacias and native species from each region. The experiments exposed seeds of the selected recipient species to leachates collected under acacias, nearby uninvaded vegetation, or distilled water. We then measured total germination, and above and below ground biomass in the growth experiment. Our results did not support the Novel Weapons Hypothesis, but instead we found some leachates collected under acacias and uninvaded areas to stimulate the germination and early growth of some of our selected acacias and native species. Such effects occurred both at the intra- and interspecific level. In general, interspecific stimulatory effects between invasive acacias occurred irrespective of whether they had overlapping native ranges in Australia. We also found leachates from uninvaded areas in South Africa to have stimulatory effects on one invasive acacia and one native species. Hence, our results support the Biochemical Recognition Hypothesis, suggesting that chemically-induced signals may facilitate acacia establishment in sites that have already been transformed by acacias.