Summary
Floral development of two alien species (Cerastium fontanum, Poa annua) were monitored through the summer of 2013/14 at three different altitudinal sites (coastal, hillside and plateau) on the North eastern end of sub-Antarctica Macquarie Island. All sites were selected to have a northeast aspect and low or moderate slope. Differential biotic and abiotic conditions directly or indirectly related to altitude were regularly measured with e-buttons and through surveys at each site. Previous studies showed an ascending gradient of unfavourable thermal conditions (decreased temperature at higher sites) and increased humidity. At each of the three sites seven plots (grid 50 x 50 cm) for each species were created. The relative coverage of all species in these plots was measured. Three reproductive stems were tagged in each plot respectively, with most advanced stage of the contained inflorescences examined fortnightly. In addition, the most generalized flowering state observed at each plot was monitored. Plant height, the length of tagged reproductive stems, and either the total number of inflorescences (for P. annua) or the maximum diameter of plants (for C. fontanum) was measured for each plant once. Plant cover, size and number of inflorescences were constrained in both species at increased altitude. However, a delayed general development of floral structures was more prominent in Cerastium fontanum. Poa annua plants showed a higher capacity to flower throughout summer at each of the three sites, while the flowering period of C. fontanum plants was shorter. It is hypothesized that abiotic factors related to altitude largely drove both total vegetative and reproductive growth and delayed sexual development for C. fontanum. In contrast, P. annua was less constrained by altitudinal effects in terms of floral development, showing a strong capacity to reproduce quickly in all environments despite adverse conditions that lead to restricted total growth. Poa annua shows a highly versatile colonist strategy while C. fontanum has a more synchronic strategy, with both species growing successfully under a range of conditions. These traits facilitate invasion in different environments, thus reinforcing the need to prevent the spread of these kinds of species to other favourable sites in the Antarctic and Sub-Antarctic where intensive management may not be feasible.