Interactions between xylem traits linked to hydraulics during xylem development optimize growth performance in conifer seedlings

Climate change has threatened forests globally, challenging tree species ability to track the rapidly changing environment (e..g., drought and temperature rise). Conifer species face strong environmental filters due to climatic seasonality. Investigating how conifers change their hydraulic architecture during xylem development across the season may shed light on possible mechanisms underlying hydraulic adaptation in conifers. Laser microscopy was used to assess the three-dimensional hydraulic architecture of balsam fir (Abies balsamea (Linnaeus) Miller), jack pine (Pinus banksiana Lambert), white spruce (Picea glauca (Moench) Voss), and black spruce (Picea mariana (Miller) Britton, Sterns & Poggenburgh) seedlings. We measured hydraulic-related xylem traits from early to latewood, during four years of plant growth. The xylem development of jack pine seedlings contrasts with the other species for keeping torus overlap (a hydraulic safety-associated xylem trait), relatively constant across the season (from early to latewood) and the years. The tracheids and torus expansion are positively associated with plant growth. Pit aperture-torus covariance is central to the seasonal dynamics of jack pine xylem development, which jointly with a rapid tracheid and pit expansion seems to boost its growth performance. Linking xylem structural changes during xylem development with hydraulics is a major issue for future research to assess conifers vulnerability to climate change.

Mitigating post-fire regeneration failure in boreal landscapes with reforestation and variable retention harvesting: At what cost?

Successive disturbances such as fire can affect post-disturbance regeneration density, with documented adverse effects on subsequent stand productivity. We conducted a simulation study to assess the potential of reactive (reforestation) and proactive (variable retention harvesting) post-fire regeneration failure mitigation strategies in a 1.37-Mha fire-prone boreal landscape dominated by black spruce and jack pine. We quantified their respective capacity to maintain landscape productivity and post-fire resilience, as well as their associated financial returns under current and projected (RCP 8.5) fire regimes. While post-fire reforestation with jack pine revealed to be the most effective strategy to maintain potential production, associated costs quickly became prohibitive when applied over extensive areas. Proactive strategies such as an extensive use of variable retention harvesting, combined with replanting of fire-adapted jack pine only in easily accessible areas, appeared as a more promising approach. Despite this, our results suggest an inevitable erosion of forest productivity due to post-fire regeneration failure events, highlighting the importance to integrate fire a priori in strategic forest management planning as well as its effects on long-term regeneration dynamics.

Effect of Biochar and Manual Vegetation Control on Early Growth and Survival of Planted Jack Pine (Pinus banksiana Lamb.) Seedlings in Northern Minnesota

Survival of planted seedlings following a regeneration harvest can be challenging and early interventions through silvicultural treatments may be required for successful stand establishment. We tested the influence of soil amendment (biochar+compost, compost-only, or control) and vegetation control (VC; applied either initially or annually for five years using brush saws) on the growth and survival of jack pine at three sites in northern Minnesota. Application of the biochar+compost soil amendment increased seedling survival by 30% relative to the control in the first year, but there was no significant difference in survival among soil amendment treatments after five years. Both soil amendments increased diameter growth relative to the control (14% increase with biochar+compost, 10% increase with compost only), with most of the biochar+compost effect attributed to the compost. Annual VC increased diameter growth by 17% relative to initial VC, but overall effects on survival and growth were generally small relative to reported effects of VC via herbicide. The limited short-term influence of biochar and manual VC on growth and survival of jack pine indicates that these practices are likely not an effective means to increase jack pine establishment, but other benefits (e.g., increased carbon storage) may become apparent with time. Study Implications Emerging changes to forest conditions and climate are likely to create challenges for successful regeneration in even-aged silvicultural systems. Early interventions such as application of soil amendments and vegetation control may be required to increase seedling survival. However, our findings indicate that biochar application and manual vegetation control were not very effective at increasing survival and growth of planted jack pine seedlings across a range of site conditions in northern Minnesota. Further study is warranted to determine whether other biochar application rates and techniques or other forms of vegetation control are more effective for successful jack pine establishment.

Response of Northern Populations of Black Spruce and Jack Pine to Southward Seed Transfers: Implications for Climate Change

A variety of responses to climate change have been reported for northern tree populations, primarily from tree-ring and satellite-based studies. Here we employ provenance data to examine growth and survival responses of northern populations (defined here as those occurring north of 52° N) of black spruce (Picea mariana) and jack pine (Pinus banksiana) to southward seed transfers. This space for time substitution affords insights into potential climate change responses by these important northern tree species. Based on previous work, we anticipated relatively flat response curves that peak at much warmer temperatures than those found at seed source origin. These expectations were generally met for growth-related responses, with peak growth associated with seed transfers to environments with mean annual temperatures 2.2 and 3.6 °C warmer than seed source origin for black spruce and jack pine, respectively. These findings imply that northern tree populations harbor a significant amount of resilience to climate warming. However, survival responses told a different story, with both species exhibiting reduced survival rates when moved to warmer and drier environments. Together with the growth-based results, these findings suggest that the warmer and drier conditions expected across much of northern Canada under climate change may reduce survival, but surviving trees may grow at a faster rate up until a certain magnitude of climate warming has been reached. We note that all relationships had high levels of unexplained variation, underlining the many factors that may influence provenance study outcomes and the challenges in predicting tree responses to climate change. Despite certain limitations, we feel that the provenance data employed here provide valuable insights into potential climate change outcomes for northern tree populations.

Characterization of Chanterelle (Cantharellus cibarius) and Pine Mushrooms (Tricholoma magnivelare) in Northern Saskatchewan

Baseline data on the boreal jack pine associated chanterelle (Cantharellus cibarius) and pine mushrooms (Tricholoma magnivelare) in the Boreal Plain Ecozone of Saskatchewan was collected in five ecosites of productive mushroom areas. It investigated hourly weather parameters correlated with daily purchase volumes over four years; yield data over five years; varying age/ tree density/species for presence of mushrooms in over 100 stands. All plots fell within the lichen jP ecosite with an overstory entirely of jack pine. The understory was dominated by reindeer lichen, bearberry and blueberry. Plots were well- to rapidly drained, subxeric to submesic, with low nutrient regime. Both chanterelle and pine mushrooms were present in jack pine stands of < 20 years age with greatest occurrence in pure jack pine stands of 41–60 years in moderately open A-B canopy density. Weekly purchase data correlated with environmental parameters. Growing Degree Days (Base Temperature 5oC) + soil temperature (minimum 500 ± 70 GDD), + either soil moisture or precipitation (cumulative 50–100 mm) provided the highest regression value with chanterelle yield 6–13 weeks prior to first appearance. Five year total chanterelle yield from this region averaged 7100 kg/yr. Chanterelle cap diameter was a good predictor of fresh weight and proxy for yield. Pine mushroom was < 10% of chanterelle yield, averaging 1.72 kg/ha over four years based on 5 days picking. With future climate predictions of warmer and moister conditions, timing of fruiting body appearance is anticipated to advance.

Effects of Fire Severity and Woody Debris on Tree Regeneration for Exploratory Well Pads in Jack Pine (Pinus banksiana) Forests

Restoring anthropogenic footprints to pre-disturbance conditions or minimizing their long-term impacts is an important goal in conservation. Many footprints, particularly if left alone, have wide-ranging effects on biodiversity. In Canada, energy exploration footprints result in forest dissection and fragmentation contributing to declines in woodland caribou. Developing cost effective strategies to restore forests and thus conserving the woodland caribou habitat is a conservation priority. In this study, we compared the effects of wildfire and local variation in the amount of residual woody debris on natural regeneration in jack pine on exploratory well pads in Alberta’s boreal forest. Specifically, we investigated how footprint size, fire severity (overstory tree mortality), ground cover of fine and coarse woody debris, and adjacent stand characteristics (i.e., height, age, and cover), affected tree regeneration densities and height using negative binomial count and linear models (Gaussian), respectively. Regeneration density was 30% higher on exploratory well pads than adjacent forests, increased linearly with fire severity on the exploratory well pads (2.2% per 1% increase in fire severity), but non-linearly in adjacent forests (peaking at 51,000 stems/ha at 72% fire severity), and decreased with amount of woody debris on exploratory well pads (2.7% per 1% increase in woody debris cover). The height of regenerating trees on exploratory well pads decreased with fire severity (0.56 cm per 1% increase in fire severity) and was non-linearly related to coarse woody debris (peaking at 286 cm at 9.4% coarse woody debris cover). Heights of 3 and 5 m on exploratory well pads were predicted by 13- and 21-years post-fire, respectively. Our results demonstrate that wildfires can stimulate natural recovery of fire-adapted species, such as jack pine, on disturbances as large as exploratory well pads (500–1330 m2) and that the type and amount of woody debris affects these patterns.