Summary1 Ecosystem engineers are organisms that change abiotic conditions in ways that affect the performance and distribution of other species, including exotics. Several mechanisms have been proposed for the successful establishment of exotic plants in natural communities, but the positive effects that native engineer species may have on the distribution and performance of exotic plants remain unknown. 2 In this study, we propose that amelioration of extreme abiotic conditions by ecosystem engineers can make stressful habitats invadable by exotic plant species, with larger positive effects on the performance of exotic plants as environmental harshness increases. We tested this hypothesis by assessing the effects of a high-Andean ecosystem engineer, the cushion plant Azorella monantha , which is known to create habitat patches where environmental conditions are less extreme than in the surrounding habitats, on the distribution and the performance of two exotic plant species, field chickweed ( Cerastium arvense ) and common dandelion ( Taraxacum officinale ), along a an elevation gradient in the Andes of central Chile. 3 We measured and compared the abundance, biomass and survival of both exotic species within and outside cushion habitat patches at three elevations (3200 m, 3400 m and 3600 m), and evaluated whether the effects of A. monantha varied across elevations. 4 The results indicated that cushion plants positively impact the performance of both exotics, and have greater facilitative effects at higher elevations. Indeed, at the higher elevation site, C. arvense was only detected within A. monantha patches, suggesting that cushions may expand the distribution range of exotics. These results suggest that ecosystem engineering by native species could promote biological invasions in harsh environments, leading to higher abundances of invaders than those expected in the absence of engineers. 5 Given the conspicuousness of ecosystem engineering in nature, we suggest that exotic species eradication programmes might be less successful by not taking into account the facilitative effects of native engineer species on invaders. Further, we suggest that the recent proposals to use engineer species in ecosystem restoration should be aware of their potential role in promoting invasions.

Contents of the hydroxamic acids 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA), and 2,4- dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) in leaves and roots of 14 cultivars of rye, Secale cereale L., were determined. Dynamics of accumulation in three cultivars were evaluated. DIBOA was the main cyclic hydroxamic acid in leaves but the contents differed significantly between the cultivars. Both DIBOA and DIMBOA were present in the roots. Maximum concentration of DIBOA in leaves and DIMBOA in roots was reached between 48-54 h and 54-72 h after germination, respectively. Antifeedant activity of DIBOA towards the aphid Rhopalosiphum padi and the feeding behavior were studied by electronic recording in barley leaves treated with different contents of DIBOA. The deleterious activity of DIBOA could arise by starvation and/or a toxic effect. Additionally, allelopathic potential of pure DIBOA and aqueous extracts of leaves and roots of rye (Tetra-Baer) on the germination of lettuce (Lactuca sativa) and rye (Tetra-Baer) seeds was evaluated. A high percentage of germination inhibition of pure DIBOA and the extracts of leaves and roots was observed. The activity is in agreement with the contents of hydroxamic acids in the plants. The substrates had no allelopathic effect on rye seeds.

Artículo de publicación ISIQuestionThe biotic resistance hypothesis states that species diversity is a barrier for plant biological invasions because alien species are more likely to find strong competitors as diversity of native plants increases. In stressful environments, however, these negative relationships could be diluted because increased physical harshness might reduce the importance of competition as a structuring force in plant communities. Nevertheless, if nurse plants facilitate other species and create more diverse vegetation patches than those found in their surroundings, the performance of invaders should be negatively related to diversity within these patches. Conversely, diversity would not regulate invasibility in the surroundings of nurses because these habitats are subjected to higher levels of environmental harshness. LocationA high-Andean landscape of central Chile, dominated by cushion plants that facilitate native species and field chickweed (Cerastium arvense), an invasive exotic species. MethodsTo test our hypotheses we used observational data taken in two consecutive growing seasons by sampling plant assemblages within and outside cushion-protected patches. Additionally, a field experiment with artificially assembled plant communities was conducted to assess why the abundance and performance of the invasive species was related to native plant diversity. ResultsContrary to our expectations, observational data indicated that the abundance of the invader species was positively related to native plant diversity within cushion patches, while negative relationships were found in the surrounding habitat. Similarly, the field experiment indicated that the performance of C.arvense increased with plant diversity within cushion patches but, in this case, neutral relationships were found in the surrounding open areas. ConclusionsOur findings indicate that nurse plants can modulate invasibility-diversity relationships in harsh environments. Overall, the results suggest that the biotic resistance hypothesis may fail to predict the direction of these relationships within nurse-protected patches. Instead, these results suggest that the alternative biotic acceptance hypothesis could operate at small spatial scales.FONDECYT 3060095-113059

Germination inhibition activity of N-aryl hydroxamic acids and acetanilide analogues was measured on lettuce seeds (Lactuca sativa). Lipophilicity of the compounds was determined by HPLC. A correlation between lipophilicity values and percentage of germination inhibition was established. A model mechanism of action for auxin was used for analyzing the effect of the substituent at the alpha carbon atom (Cα) on the polarization of hydroxamic and amide functions in relation to the germination inhibition activity observed. Results suggest that the lipophilic and acidic properties play an important role in the phytotoxicity of the compounds. A test with the microalga Chlorella vulgaris was used to evaluate the potential herbicide activity of the hydroxamic acids and acetanilides.