Climate, topography, and stand conditions interact to shape maximum stand density in interior Douglas-fir forests

Livestock grazing is a dominant land use across North America and although the effects of grazing on birds have been studied in grassland, shrubland, and riparian habitats, studies of the effects in forests are rare. We investigated the effects of cattle grazing in forests on vegetation, the relationships between vegetation characteristics and the abundance of foraging and nesting guilds of birds, and the overall effects of grazing on the bird community in the Interior Douglas-fir (Pseudotsuga menziesii) biogeoclimatic zone of British Columbia. Cattle grazing was associated with reduction in ground vegetation height and grass cover, and increases in the number of shrubs and saplings. Bark insectivores, foliage insectivores, cavity nesters, and shrub/tree nesters all responded positively to sapling density. However, this translated into few overall effects of cattle grazing on birds, with only bark insectivores exhibiting greater abundance on grazed areas. Grazed areas also had fewer aerial insectivores but the mechanism driving this remains unclear. Current forest grazing practices at our study sites appear to have few negative effects on bird abundance and diversity, with the possible exception of aerial insectivores. Study of additional sites is required to assess if forest grazing exerts similar effects throughout the Interior Douglas-fir forest. Furthermore, study of the effects of forest grazing on productivity and survival of birds is needed.

Warming temperatures and changing weather patterns are causing more frequent and severe disturbances in western North American forests. The increasing length and severity of recent wildfire seasons have annually caused widespread injury to millions of trees, facilitating the subsequent outbreak of various subcortical insect species that infest damaged hosts. The subcortical woodboring beetle assemblage (Coleoptera: Cerambycidae and Buprestidae) comprises insects that feed primarily within the phloem and sapwood of defensively compromised trees, often causing severe degradation of high value stands slated for post-fire salvage logging. We studied three 2017 fire complexes within mature Douglas-fir (Pseudotsuga menziesii) forests in southern British Columbia to investigate the post-fire infestation patterns of woodborers. We observed the highest levels of woodborer infestation in forests with moderate to high levels of fire injury, rather than in extremely burnt stands with greater tree mortality. We also found that woodborer infestation likelihood could be predicted from a combination of tree height and fire injury for individual trees. In addition, infestation was more likely in large (>13.1 m tall) rather than small trees (<13 m tall) with the same levels of fire injury. We conclude that the woodboring beetle assemblage present in dry interior Douglas-fir forests is more aggressive and ubiquitous than previously observed, and these insects actively attack and kill living trees that may otherwise survive after fires with both economic and ecological ramifications. Economically, post-fire salvage may be more effective in preventing woodborer population build-up and timber degradation by targeting moderately scorched stands immediately after wildfire. Ecologically, woodborer impacts to trees that would otherwise survive may compromise their biological legacy and impair the diversity and composition of future Douglas-fir stands.

Improved forest management may offer climate mitigation needed to hold warming to below 2°C. However, uncertainties persist about the effects of harvesting intensity on forest carbon sequestration, especially when considering interactions with regional climate and climate change. Here, we investigated the combined effects of harvesting intensity, climatic aridity, and climate change on carbon stocks in Douglas-fir [Pseudotsuga menziesii Mirb. (Franco)] stands. We used the Carbon Budget Model of the Canadian Forest Sector to simulate the harvest and regrowth of seven Douglas-fir stand types covering a 900 km-long climate gradient across British Columbia, Canada. In particular, we simulated stand growth under three regimes (+17%, −17% and historical growth increment) and used three temperature regimes [historical, representative concentration pathways (RCP) 2.6 and RCP 8.5]. Increasing harvesting intensity led to significant losses in total ecosystem carbon stocks 50 years post-harvest. Specifically, forests that underwent clearcutting were projected to stock about 36% less carbon by 2,069 than forests that were left untouched. Belowground carbon stocks 50 years into the future were less sensitive to harvesting intensity than aboveground carbon stocks and carbon losses were greater in arid interior Douglas-fir forests than in humid, more productive forests. In addition, growth multipliers and decay due to the RCP’s had little effect on total ecosystem carbon, but aboveground carbon declined by 7% (95% confidence interval [−10.98, −1.81]) in the high emissions (RCP8.5) scenario. We call attention to the implementation of low intensity harvesting systems to preserve aboveground forest carbon stocks until we have a more complete understanding of the impacts of climate change on British Columbia’s forests.

Post-wildfire disturbances such as salvage logging and seeding of agronomic species occur over large parts of the forested land base in British Columbia. However, there is surprisingly little research on the effects of these management practices on plant community composition and species diversity. The future of plant and animal biodiversity will depend increasingly on regional floras surviving in highly managed and disturbed environments. I examined vascular and nonvascular plant community responses four years after wildfire and post-wildfire management practices in interior Douglas-fir (Pseudotsuga menziesii var. glauca) forests following two separate 2003 wildfires near Kamloops, BC, Canada. Wildfire sites with all combinations of seeding and salvage logging disturbance were selected in similar post-wildfire environments. Analysis at the plot (400m², n=104) and stand (400-1200m², n=42) scales suggested that post-wildfire disturbance had a significant negative effect on native species richness and reduced the frequency of some shrub and shade tolerance species including Ceanothus sanguineus and Prosartes hookeri. As well, multivariate analysis showed evidence for altered post-wildfire community composition and structure mainly due to increased dominance of exotic and graminoid species. These negative effects were most apparent in wildfire sites that were both seeded and salvaged-logged. Sustainable forest management requires a thorough understanding of the cumulative impacts of post-wildfire management practices on understory vegetation and ecosystem processes. The results of this study can aid resource managers by helping them incorporate the effects of natural and anthropogenic disturbance into future post-wildfire management activities.

Hydraulic redistribution (HR) is a process by which water moves through plant roots from moist to dry soils. An experiment was conducted to quantify the influence of common mycorrhizal networks (CMNs) and proximity to mature HR-source trees on the water relations of surrounding seedlings. Douglas-fir (Pseudotsuga menziesii var glauca (Mirb.) Franco) seedlings were planted at four distances (0.5, 1, 2.5, and 5 m) from six mature Douglas-fir trees, either directly into soil (soil plus CMN pathway) or inside 0.5 microm mesh bags (soil-only pathway). Deuterated water was used to irrigate soil beside mature trees in order to identify different HR water pathways to surrounding seedlings. This was followed by measurements of seedling deuterium enrichment, seedling water potential, soil water potential, gravimetric soil water content, and tree root density surrounding the seedlings. There was no significantly detectable difference in the quantity of HR water transferred to seedlings having access to soil and CMN pathways or soil-only pathways of water movement. Water from the irrigation plot contributed up to 1.4% of the water of Douglas-fir seedlings. Based on the assumption that the only pathway through which seedlings could access irrigation water was through the mature trees, we estimate that as much as 21.6% of the seedling water was supplied by the nearby tree. Seedling water potential was not significantly affected either by proximity to mature trees or pathway, suggesting HR may have compensated for increasing tree competitive effects with proximity. It is also possible that the lack of difference was due to a relatively moist summer. Our results suggest that residual mature trees are potentially important for hydraulic redistribution to regenerating seedlings in harvested dry interior Douglas-fir forests.

Soil ecology science has contributed a rich understanding of natural soil patterns and processes, and played a role in revolutionizing the management of ecosystems. This basic understanding is also providing a foundation for predicting and managing the consequences of climate change on ecosystems, including their resilience to disturbance, biotic diversity, and carbon and nutrient dynamics. To help address the challenges of climate change, future soil ecology research and management would benefit from a complex systems approach, where network and dynamics systems theory are used to predict plant community and ecosystem responses to disturbance. A reductionist approach to management that ignores networks and system dynamics, by contrast, is destined to contribute to ecological degradation as climate changes. In this paper, I describe mycorrhizal networks as models of biological networks in the interior Douglas-fir forests of British Columbia, and the role they play in carbon flux and regeneration dynamics following disturbance. I propose a conservationist approach for managing forest mycorrhizal networks and hub trees that can facilitate native plant migrations, limit exotic plant invasions, and bolster ecological resilience. Interdisciplinary research that integrates the dynamics of multiple, overlapping networks will help develop management practices that sustain ecosystems in our changing climate. Key words: Climate change, complex systems, mycorrhizal networks, species migrations

In order to assess the effects of disturbance on soil seed bank spatial structure, variation in seed bank species composition was analyzed at two spatial scales in mature (undisturbed) and recently disturbed stands of Interior Douglas-fir forest in south-central British Columbia. Coarse-scale analysis among stands 10–100 km apart using Multi-Response Permutation Procedures showed that individual mature stands were mainly characterized by unique seed bank species compositions. Disturbance had an homogenizing effect on seed bank species composition at the coarse scale––there was less variability among stands following disturbance by both low and high severity fires, and by non-salvage logging. In contrast, finer scale analysis among sampling units c. 10 m apart showed that heterogeneity in seed bank species composition was greater on severely burned and logged sites, commensurate with greater levels of soil disturbance, than on lightly burned and undisturbed sites. Despite the high intensity of seed bank sampling (sixty 25 cm2 soil samples from each of 16 sites), species-area curves leveled off only when infrequent species were removed from the dataset. The number of seed bank samples required to account for the common species ranged from 84 on severely burned sites, to 196 on undisturbed sites; more than the 240 samples collected per disturbance class in this study would have been required to account for the remaining infrequent species. Overall, this study highlights the importance of sampling intensively within multiple stands to capture the variation in species composition inherent to these dry coniferous forest soil seed banks.

I examined the affect of logging intensity and pattern on the North American red squirrel (Tamiasciurus hudsonicus) by measuring density and demography from zero to four years after logging, and by measuring territory size, habitat use, and behaviour of individual animals from three to five years after logging. This study was done in an interior Douglas-fir forest (IDF) near Kamloops, British Columbia, Canada. Patterns of tree removal ranged from small patch cuts (&lt;1.6 ha) to individual tree selection (diameter-limit logging) and intensity of tree removal ranged from 20-50% by volume. From two to four years after logging, red squirrel abundance declined in a 1:1 relationship with the volume of conifer tree removal. Absolute variation in squirrel abundance was highest in the uniform tree removal treatments and lowest in unharvested habitat. Red squirrel recruitment, survival, body weight, and reproduction was unrelated to pattern or intensity of tree removal. In general, these results are consistent with the predictions of the ideal free distribution model of habitat selection. I conclude that logging intensity had the greatest effect on red squirrels, but that uniform tree removal logging may result in poor quality habitat during years of conifer cone crop failure. From three to five years after logging, red squirrel territory size was best explained by the density of Douglas-fir trees larger than 30 cm diameter-at-breast-height (DBH). Similarly, red squirrels prefered conifer trees larger than 15 cm DBH, with the strongest preference for trees between 30 and 44 cm DBH. Despite large differences in conifer tree density on individual territories, red squirrel activity budgets did not change. Further, red squirrels avoided canopy openings created by logging, but this did not affect their use of forest edge compared to interior forest habitat. I suggest that variation in conifer seed production may explain the relationship between Douglas-fir density and red squirrel territory size, habitat selection, and behaviour. I conclude that logging did not have a biologically meaningful effect on red squirrels, either overtime or across the range of habitats I sampled. However, diameter-limit logging may create poor habitat for red squirrels during years when little or no conifer seed is produced, or when logging dilutes conifer trees further than those sampled in this study. Individual tree selection treatments will likely not remain poor habitat for more than five years if greater than 50, 30 cm DBH Douglas fir trees are retained.