The degree to which biotic interactions influence invasion by non-indigenous species may be partly explained by the evolutionary relationship of these invaders with natives. Darwin's naturalization hypothesis controversially proposes that non-native plants are more likely to invade if they lack close relatives in their new range. A possible mechanism for this pattern is that exotics that are more closely related to natives are more likely to share their herbivores, and thus will suffer more damage than phylogenetically isolated species. We tested this prediction using exotic plants in Ontario, Canada. We measured herbivore damage to 32 species of exotic plants in a common garden experiment, and 52 in natural populations. We estimated their phylogenetic distances from locally occurring natives in three ways: as mean distance (age) to all native plants, mean distance to native members of the same family, and distance to the closest native species. In the common garden, the proportion of leaves damaged and the average proportion of leaf area damaged declined with mean phylogenetic distance to native family relatives by late summer. Distance to native confamilials was a better predictor of damage than distance to the closest native species, while mean distance to the entire native plant community failed to predict damage. No significant patterns were detected for plants in natural populations, likely because uncontrolled site-to-site variation concealed these phylogenetic trends. To the extent that herbivory has negative demographic impacts, these results suggest that exotics that are more phylogenetically isolated from native confamilials should be more invasive; conversely, native communities should be more resistant to invasion if they harbor close familial relatives of potential invaders. However, the large scatter in this relationship suggests that these often are likely to be weak effects; as a result, these effects often may be difficult to detect in uncontrolled surveys of natural populations.

Canopy gaps play an important role in forest vegetation dynamics when fire return intervals are long. However, there is little known about the role of gaps in the development of forest stands that initially dominate following stand-replacing disturbance. We investigated gap disturbance during the breakup of trembling aspen (Populus tremuloides Michx.) stands at two scales: at the stand level we quantified gap fraction and gap size as stand development proceeds; at the gap level we determined causes of gap-maker mortality and evaluated resulting gap-maker structure and decay as stand development proceeds. We also evaluated the impact that gaps have on stand transition by quantifying the abundance and growth of juvenile trees in gaps of different sizes and ages. Ten stands between 60 and 120 years since fire in northeastern Ontario were sampled using line intersect transects. Gap fraction doubled (∼18%–36%) and mean gap size was more than four times greater (∼45–200 m2) over the time period. Standing dead gap makers in early states of decay were most frequent in young stands, whereas snapped gap makers in various states of decay were most frequent in old stands. Infection by fungal pathogens was the most frequent cause of mortality (56%) and was not related to time since fire. Balsam fir (Abies balsamea (L.) Mill.) was the most abundant juvenile recruit. However, transition probabilities for tree species were independent of gap type. These results indicate that gap creation is frequent during early stages of stand development in boreal forests; however, it is unlikely that successional trajectory is affected by their formation.

Invasive plants often lose natural enemies while moving to new regions; however, once established in a new area, these invaders may be susceptible to attack by locally occurring enemies. Such damage may be more likely for exotics with close native relatives in the invaded area, since shifts of enemies should be more likely among closely related hosts. In this study, we evaluated whether exotics experience less herbivore damage than natives, and whether phylogenetically novel exotics experience less damage that those that are more closely related to locally occurring family members. Foliar damage was measured on 20 native and 15 exotic Asteraceae that co-occur locally in southern Ontario, Canada. The phylogenetic structure of this damage was quantified using an eigenvector decomposition method, and the relationship between damage and phylogenetic novelty of exotics was evaluated based on phylogenetic distances to other locally occurring Asteraceae. Our results show that 32% of the variation in damage was explained by phylogenetic relationship; similarity in damage tended to be associated with tribes. As predicted, exotics experienced lower damage than native species, even when the dataset was corrected for phylogenetic nonindependence. Contrary to our prediction, however, exotics that were more phylogenetically isolated from locally occurring relatives did not experience less damage. These results suggest that, though exotic Asteraceae may escape many of their natural enemies, this is not in general more likely for species phylogenetically distant from locally occurring native confamilials.

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