Sybille Haeussler

Long-term studies of vegetation succession can inform restoration of degraded forests. We examined resilience of a boreal mixedwood vegetation community, asking whether treatments employed to restore wood production in a degraded ecosystem could also restore diversity and composition of vegetation communities. The Inga Lake trial, established in 1987 in northeastern British Columbia, used mechanical, fire, and chemical and manual treatments, encompassing a gradient of restoration effort, and tree planting to restore a shrubland to white spruce (Picea glauca (Moench) Voss) forest. We monitored vascular plant, bryophyte, and macrolichen composition five times over 31 years on five to seven treatments replicated five times. We used mixed-effects models and nonmetric multidimensional scaling to compare diversity and composition among treatments and with mature reference forests. Low- to high-effort restoration created a gradient from broadleaf- to spruce-dominated overstories. Diversity increased with restoration effort. Four of 253 taxa occurred in mature forests only. There was no evidence that lower versus higher effort treatments followed divergent successional pathways toward broadleaved versus spruce reference communities. Our results suggest that these mixedwood vegetation communities lie within a broad domain of successional attraction that confers high ecological resilience to disturbance. Gap cuttings to stimulate understory re-initiation and provide woody debris are recommended to complete the restoration.

Species interactions have long been predicted to increase in intensity towards the tropics and low elevations, due to gradients in climate, productivity, or biodiversity. Despite their importance for understanding global ecological and evolutionary processes, plant-animal interaction gradients are particularly difficult to test systematically across large geographic gradients, and evidence from smaller, disparate studies is inconclusive. By systematically measuring post-dispersal seed predation using 6980 standardized seed depots along 18 mountains in the Pacific cordillera, we found that seed predation increases 18% from the Arctic to Equator and 16% from 4000 masl to sea level. Clines in total predation, likely driven by invertebrates, were consistent across tree-line ecotones and in continuous forest, and were better explained by climate seasonality than by productivity, biodiversity, or latitude. These results suggest that species interactions play predictably greater ecological and evolutionary roles in tropical, lowland, and other less seasonal ecosystems. One Sentence Summary: Post-dispersal seed predation increases from the Arctic to the Equator and from high elevations to sea level.

Lodgepole pine (Pinus contorta var. latifolia Engelm.) ecosystems of central British Columbia face cumulative stresses, and management practices are increasingly scrutinized. We addressed trade-offs between “light-on-the-land” versus more aggressive silvicultural approaches by examining plant communities and indicator species (non-natives, berry producers, epiphytes, mycotrophs, pine rust alternate hosts) across a gradient of five or six site preparation treatments at the Bednesti trial (established 1987). We tested whether more severe site preparation (i) caused plant community composition to diverge from a 35- to 46-year-old reference forest, (ii) accelerated succession by hastening crown closure, or (iii) delayed succession by promoting seral species. Nonmetric multidimensional scaling (NMDS) ordination showed all treatments converging toward the reference forest composition. At 10 years, succession was incrementally delayed by more severe treatments; at 25 years, only burned windrows were still delayed. Mixed-effects models based on site preparation severity were better than crown closure models for 11 of 13 variables tested, suggesting that mostly belowground processes drive succession in these infertile ecosystems. Invasive hawkweeds persisted on all treatments at 25 years. Limited, contradictory data did not support using mechanical or fire treatments to reduce alternate hosts of pine stem rusts. Long-term trials such as Bednesti highlight the need for ecosystem-specific strategies and diverse approaches to accommodate conflicting benefits and risks of disturbance in forests.

Ten-year response of plant communities to disk trenching, plowing, rotoclearing and windrow burning was studied on two contrasting sites to address concerns that mechanical site preparation reduces structural and species diversity. Cover and height of all species on randomly located subplots within 0.05- to 0.075-ha treatment plots were used to develop indices of volume, structural diversity, and species diversity; to ordinate the plots; and to correlate species diversity with crop-tree performance. At both sites, community response was strongly influenced by the severity of site preparation. On a boreal site dominated by willow (Salix L. spp.), green alder (Alnus crispa (Ait.) Pursh ssp. crispa) and trembling aspen (Populus tremuloides Michx.), site preparation increased structural diversity and had little effect on species diversity. High-severity treatments increased non-native species abundance 10- to 16-fold while only marginally enhancing growth of planted white spruce (Picea glauca (Moench) Voss) over medium-severity treatments. On a nutrient-poor sub-boreal site, species diversity declined with increasing treatment severity and with increasing lodgepole pine (Pinus contorta var. latifolia Engelm.) stem volume. Velvet-leaved blueberry (Vaccinium myrtilloides Michx.) was highly sensitive to mechanical disturbance. Moderate mechanical treatments appear to improve conifer performance while causing little change to plant communities, but high severity treatments can cause substantial change.