Natural regeneration and expansion capacity of forestry species : pinus pinaster and eucalyptus globulus

Tese de doutoramento, Biologia (Ecologia), Universidade de Lisboa, Faculdade de Ciências, 2016Nowadays, the expansion of forestry trees into habitats outside plantations is a relevant concern for managers and conservationists, and a relevant issue for biological invasion research. Among the most planted forest species worldwide, we can find Eucalyptus and Pinus species. Specifically, Portuguese planted forests are predominantly composed of two species, Eucalyptus globulus and Pinus pinaster. However, little is known about the regeneration and spread capacity of these species under different habitats and environmental conditions. In Portugal, given their exotic origin and their widespread use in forestry, E. globulus has been a controversial species mainly in relation to the presence or not of the invasive behavior. Thus the overall goal of this thesis was to identify the spatial recruitment patterns of P. pinaster and E. globulus into habitats surrounding the plantations analysed according the specific objectives: (i) to determine the capacity of these species to colonize outside the planted areas, (ii) to evaluate their seedling establishment capacity under different ecological conditions, (iii) to identify those conditions that favor or hamper these species establishment, and (iv) to understand the main factors that drive natural establishment of these species. During this study, all these aspects were adressed, and for the first time, a country wide detection survey in the natural vegetation adjacent to plantations was conducted, recording P. pinaster or E. globulus seedlings establishment by natural regeneration. In addition, seedling emergence and survival, were investigated during two years using a seed addition experiment through a manipulative study under natural conditions. These species presented very different recruitment characteristics, regarding the amount of established seedlings and the distance of colonization, much higher for P. pinaster. In mature mixed forests, natural recruitment of P. pinaster was also significantly higher than E. globulus, even when P. pinaster was not a dominant planted species. Considering the seedling survival capacity, E. globulus displayed higher mortality rate during initial phase of seedling growth, when compared to P. pinaster. The better climate predictors explaining E. globulus recruitment were temperature seasonality and annual precipitation. E. globulus was more likely to be present in areas with low temperature seasonality and high annual precipitation. In addition, seedling survival experiment showed also that water deficit can compromise summer survival of E. globulus seedlings. On the other hand, it was not clear how P. pinaster recruitment is affected by climatic conditions. Based on the models explored in this study, no climate variable explained P. Pinaster recruitment. Recruitment models reveled that plantation characteristics (age and area) had no significant influence on P. pinaster and E. globulus recruitment. However, in the unmanaged mature plantations (older than 25 years and left unharvested after optimal age) a notoriously higher recruitment of these species than in the industrial plantations was found. Finally, site characteristics (such as habitat type, vegetation cover, and disturbance) were the most important group of variables influencing P. pinaster and E. globulus. Forest was the most resistant habitat to these species establishment. In the case of E. globulus also grasslands were very resistant to their establishment. Results showed that vegetation cover alone explained more than 47% of eucalypts occurrence variability and a negative association was found between vegetation cover and eucalypt occurrence and density. The same pattern was found in relation to vegetation disturbance level, which was the most important factor explaining eucalypt density. The high importance of site characteristics in this study reflects that most of the variability in these species establishment occurs at small spatial scales. In conclusion, the results that support this thesis suggest that the colonization outside plantation by E. globulus will likely be remarkably slow when compared to P. pinaster (species with long-distance seed dispersal and higher seedling survival capacity). Moreover, localized recruitment and very low levels of establishment, and the success of native species recruitment under eucalypt forests suggested that currently, E. globulus does not demonstrate an invasive behavior in adjacent areas to the plantations, irrespectively to environmental land use characteristics. Indeed, E. globulus limited expansion ability seems to be related with the combination of two characteristics: limited seed dispersal; and high seedlings mortality rate. In contrast, P. pinaster seedlings had high plasticity and drought resistance. The intensive management of these plantations (mainly E. globulus) might contribute to diminish their ability to develop a significant propagule pool. However, mature plantations (left unharvested) required an increased attention, as the propagule pressure builds with time. Other important finding of the present study was that the small-scale factors (vegetation cover, disturbance and habitat type) were more important than the others high-scale factors (Climate and plantation characteristics) and they are essential to precise predictions at more local scales. Taken into account this thesis results, it was suggested the establishment of “sentinel sites” using in-situ visits during conventional management as described here. This monitoring system could, over time, produce very valuable data to improve our understanding of forestry species colonization dynamics and could also alert managers to situations where some form of management intervention is needed. The findings presented here were crucial to understand the spatial pattern of P. pinaster and E. globulus natural recruitment, and the spread capacity of these important forestryn species through their response in early colonization stages to environmental conditions, and can help to explain future shifts in these species distribution and natural establishment outside the planted areas