The biology of invasive alien plants in Canada. 14. Celastrus orbiculatus Thunb.

Celastrus orbiculatus Thunb. is an invasive alien liana native to northern regions of Japan, Korea, China and Russia. Since its intentional North American introduction in the 1870s, it is now found across eastern Canada and United States. Through an extensive adventitious root system and twining growth habit, C. orbiculatus smothers and girdles surrounding vegetation, drastically altering the environment and ecosystem processes. C. orbiculatus continues to be distributed as an ornamental. In addition, birds and small mammals consume its prolific fruit and facilitate novel introductions. C. orbiculatus is susceptible to glyphosate and triclopyr. Once established, however, management intensity is compounded by its extensive root system and continual resprouting and root-suckering. The climatic requirements across eastern Canada, ease of dispersal and rapid growth suggest that C. orbiculatus will continue to spread across its introduced range.

76-year decline and recovery of aspen mediated by contrasting fire regimes: Long-unburned, infrequent and frequent mixed-severity wildfire

Quaking aspen (Populus tremuloides) is a valued, minor component on northeastern California landscapes. It provides a wide range of ecosystem services and has been in decline throughout the region for the last century. This decline may be explained partially by the lack of fire on the landscape due to heavier fire suppression, as aspen benefit from fire that eliminates conifer competition and stimulates reproduction through root suckering. However, there is little known about how aspen stand area changes in response to overlapping fire. Our study area in northeastern California on the Lassen, Modoc and Plumas National Forests has experienced recent large mixed-severity wildfires where aspen was present, providing an opportunity to study the re-introduction of fire. We observed two time periods; a 52-year absence of fire from 1941 to 1993 preceding a 24-year period of wildfire activity from 1993 to 2017. We utilized aerial photos and satellite imagery to delineate aspen stands and assess conifer cover percent. We chose aspen stands in areas where wildfires overlapped (twice-burned), where only a single wildfire burned, and areas that did not burn within the recent 24-year period. We observed these same stands within the first period of fire exclusion for comparison (i.e., 1941–1993). In the absence of fire, all aspen stand areas declined and all stands experienced increases in conifer composition. After wildfire, stands that burned experienced a release from conifer competition and increased in stand area. Stands that burned twice or at high severity experienced a larger removal of conifer competition than stands that burned once at low severity, promoting expansion of aspen stand area. Stands with less edge:area ratio also expanded in area more with fire present. Across both time periods, stand movement, where aspen stand footprints were mostly in new areas compared to footprints of previous years, was highest in smaller stands. In the fire exclusion period, smaller stands exhibited greater loss of area and changes in location (movement) than in the return of fire period, highlighting their vulnerability to loss via succession to conifers in the absence of disturbances that provide adequate growing space for aspen over time.

Multidecadal decline and recovery of aspen experiencing contrasting fire regimes: long-unburned, infrequent and frequent mixed-severity wildfire

Quaking aspen (Populus tremuloides) is a valued, minor component on western landscapes. It provides a wide range of ecosystem services and has been in decline throughout the arid west for the last century. This decline may be explained partially by the lack of fire on the landscape as aspen benefit from fire that eliminates conifer competition and stimulates reproduction through root suckering. Managers are interested in aspen restoration but there is a lack of knowledge about their spatial dynamics in response to fire. Our study area in northeastern California on the Lassen, Modoc and Plumas National Forests has experienced recent large mixed-severity wildfires where aspen was present, providing an opportunity to study the re-introduction of fire. We observed two time periods; a 54-year absence of fire from 1941 to 1993 preceding a 24-year period of wildfire activity from 1993 to 2017. We utilized aerial photos to delineate aspen stand size, location and succession to conifers. We chose aspen stands in areas where wildfires overlapped (twice-burned), where only a single wildfire burned, and areas that did not burn within the recent 24-year period. We looked at these same stands within the first period of fire exclusion for comparison (i.e., 1941-1993). In the absence of fire, all aspen stand areas declined and all stands experienced increases in conifer composition. After wildfire, stands that burned experienced a release from conifer competition and increased in stand area. Stands that burned twice or at high severity experienced a larger removal of conifer competition than stands that burned once at low severity, promoting aspen recovery and expansion. Stands with less edge:area ratio also expanded more with fire present. Across both time periods, stand movement, where aspen stand footprints were mostly in new areas compared to footprints of previous years, was highest in smaller stands. In the fire exclusion period, smaller stands exhibited greater changes in area and location (movement), highlighting their vulnerability to loss in the absence of disturbances that provide adequate growing space for aspen over time.

Early Regeneration Dynamics of Pure Black Spruce and Aspen Forests after Wildfire in Boreal Alberta, Canada

Research Highlights: Black spruce (Picea mariana Mill.) and trembling aspen (Populus tremuloides Michx.) both regenerated vigorously after wildfire. However, pure semi-upland black spruce stands are at increasing risk of changing successional trajectories, due to greater aspen recruitment. Background and Objectives: Black spruce and aspen are found across the boreal forest with black spruce dominating lowlands and aspen being common in uplands. Both species are well adapted to wildfire with black spruce holding an aerial seedbank while aspen reproduce rapidly via root suckering. In the summer of 2016, the Horse River wildfire burned 589,617 hectares of northern Alberta’s boreal forest. Methods: We assessed early regeneration dynamics of both pure aspen and pure black spruce forests. For black spruce, 12 plots were established in both bog and semi-upland habitats to assess seedling regeneration and seedbed availability. For aspen, 12 plots were established in each of the low, moderate, and high burn severities, as well as 5 unburned plots. Results: Post-fire black spruce regeneration densities did not differ between bog and semi-upland habitats, but were positively correlated with forb cover and charred organic matter seedbeds. Aspen regeneration within pure black sprue stands was substantial, particularly in semi-upland habitats, indicating a potential shift in successional trajectory. Fire severity did not significantly affect aspen regeneration in pure aspen stands, but regeneration density in all severity types was >90,000 stems ha−1. Aspen regeneration densities were negatively related to post-fire forb and shrub cover, likely due to competition and cooler soil temperature.

Robinia pseudoacacia L. in Short Rotation Coppice: Seed and Stump Shoot Reproduction as well as UAS-based Spreading Analysis

Varying reproduction strategies are an important trait that tree species need in order both to survive and to spread. Black locust is able to reproduce via seeds, stump shoots, and root suckers. However, little research has been conducted on the reproduction and spreading of black locust in short rotation coppices. This research study focused on seed germination, stump shoot resprout, and spreading by root suckering of black locust in ten short rotation coppices in Germany. Seed experiments and sample plots were analyzed for the study. Spreading was detected and measured with unmanned aerial system (UAS)-based images and classification technology—object-based image analysis (OBIA). Additionally, the classification of single UAS images was tested by applying a convolutional neural network (CNN), a deep learning model. The analyses showed that seed germination increases with increasing warm-cold variety and scarification. Moreover, it was found that the number of shoots per stump decreases as shoot age increases. Furthermore, spreading increases with greater light availability and decreasing tillage. The OBIA and CNN image analysis technologies achieved 97% and 99.5% accuracy for black locust classification in UAS images. All in all, the three reproduction strategies of black locust in short rotation coppices differ with regards to initialization, intensity, and growth performance, but all play a role in the survival and spreading of black locust.

Evidence for clonality, breeding system, genetic diversity and genetic structure in large and small populations of Melaleuca deanei (Myrtaceae)

Melaleuca deanei F.Muell. is a rare, serotinous shrub with a naturally restricted distribution centred over the sandstone ridges around Sydney. Sexual reproduction and seedling recruitment occur rarely, and plants appear to spread and persist largely by clonal root suckering. A potentially outcrossing breeding system, combined with extensive clonality, place M. deanei at a high risk of reproductive failure. Knowledge of the extent of its clonality and breeding system, and an understanding of the distribution and abundance of genetic diversity within and among its populations, will assist conservation management. The present study reports on the extent of clonality, breeding system, levels of genetic diversity, and population differentiation within small, medium and large populations of M. deanei from the northern and southern distribution regions. Multiple stems were found to comprise single genets up to ~10 m diameter on the ground and molecular evidence points to an outcrossing breeding system. Genetic diversity was positively correlated with population size and significant genetic differentiation was shown between northern and southern regions using clustering analyses. Recommendations for in situ and ex situ conservation management based on these results are provided.

Vegetative reproduction and root anatomy of Solanum centrale J.M.Black (Australian bush tomato)

The aim of this study was to describe the morphology, anatomy and function of underground structures associated with colonies of Solanum centrale J.M.Black (Australian bush tomato), a perennial sub-shrub found in arid areas of Australia and an important traditional staple food for Aboriginal people. It is known that this species forms clonal communities, but there is little understanding of the mechanisms of formation in either natural or cultivated situations. The underground connections within seven clonal communities from Central and South Australia were documented and samples of secondary roots, thick lateral roots and stems were examined under both laboratory and glasshouse conditions. Clonal communities were observed at all sites with individual ramets arising from lateral roots (root-suckers) that ranged from 2–10 mm in diameter growing in a network 5–15 cm below the soil surface. Lateral roots have dicotyledonous root anatomy and rapidly resprout to form new clonal ramets. They also have the capacity to accumulate starch in parenchyma cells. The morphology and root-suckering ability resemble those of weedy Solanum spp. from other parts of the world, as well as species from a variety of genera adapted to arid climates. Methods to capitalise on the ability of lateral roots to form clonal ramets in cultivated situations, particularly given the difficulties in establishing crops from seed, are discussed.