Recent spruce (Picea abies) treeline ecotonal progression since the late1980s in the Swedish Scandes-a repeat photographic narration and analysis

This study reports a case of climate-mediated transformation and physiognomic progression of the Norway spruce (Picea abies) treeline ecotone since the mid-1990s in the Swedish Scandes. The methods include repeat photography and foliation estimates of old-established clonal spruces. An air and soil temperature nadir by the 1980s had caused extensive needle and shoot mortality, evident at the landscape-scale. Subsequent winter and summer temperature rises induced a striking canopy recovery, including densification and vertical growth. Release from low soil temperature stress appears as instrumental for canopy progression and shift from stunted growth to erect tree forms. Seed-based regeneration of new individuals has been virtually nil and the ecotone appears to be spatially stable. Ongoing ecotonal shifts has the character of growth form transformations in accordance with climatic conjunctures.

Delay of growth release after a windthrow event and climate response in a light-demanding species (European larch Larix decidua Mill.)

Key message Larch trees respond to stand opening with an approximately 4-year delay of growth, and low precipitation in July limits radial growth after a windthrow event. Abstract Precise cross-dating of disturbance events is crucial to understanding the functioning of forest stands, and may help explain ongoing ecological processes in a forest. Tree rings are very often used to reconstruct the history of disturbances and to study the response of trees to climatic factors. This study analyzed how quickly European larch can benefit from an abrupt change after catastrophic windthrow events and the extent of trees’ sensitivity to climate. The study is based on cores from 83 larch trees collected in a post-disturbance 100 ha plot established after a catastrophic windstorm in 2004 in the Slovakian High Tatras. Growth release was calculated from the percentage of growth change (PGC) measured in tree rings. The time lag between the disturbance event and release was related to tree diameter at breast height, tree age, and tree’s previous growth. The time lag between the year of the event and the year of growth reaction was 4.6 years on average (median 4 years) in a multi-aged group of trees. The climate analyses employed residual chronology. The new environmental conditions in the post-disturbance area have altered the trees’ growth reaction to climate; in particular, they show sensitivity to water deficit in July.

Carbon storage recovery in surviving lodgepole pine (Pinus contorta var. latifolia) 11 years after mountain pine beetle attack in northern British Columbia, Canada

We studied the recovery of tree- and stand-level carbon (C) storage in a lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) forest in northern British Columbia that experienced substantial (∼83%) mortality in 2006–2007 (total loss by 2013 = 86%) during a severe mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins, 1902) infestation. Earlier work suggested that this forest recovered positive annual C storage 3 years after attack based on eddy covariance measurements. We sought to confirm these results by examining C storage in surviving pine trees using tree core analysis. Average growth release of surviving lodgepole pine trees was 392% (range of –53% to 2326%) compared with mean decadal growth prior to MPB attack. Nearly 97% of trees underwent a growth release, considerably higher than the 15%–75% reported for lodgepole pine in previous studies. Mean annual stem C storage of the surviving trees in this study was highly correlated (r = 0.88) with 10 years of annual net ecosystem productivity estimates made using the eddy covariance technique, indicating that surviving lodgepole pine remain an important part of C recovery after MPB attack. Mean annual stem C storage was also highly correlated (r = 0.92) with the cumulative percentage of downed stems per hectare at the site, suggesting that increased availability of resources is likely assisting the growth release.

Self-replacement after small-scale partial crown dieback in austral Nothofagus dombeyi forests affected by an extreme drought

The spectrum of structural response in drought-induced mortality (degree of partial crown mortality) could play a key role in pervasive changes in plant composition, as individual openings could reset self-replacement dynamics or trigger shifts in vegetation. Here we capture the community pattern 17 years after a drought episode over a range of canopy responses in Nothofagus dombeyi forests. We applied a widespread demographic approach to address evidence of species shifts vs. self-replacement and to relate partial dieback to understory structure and composition. Assuming that the outcome of growth release of the understory components can be observed 17 years after canopy loss, this study reveals evidence of self-replacement in N. dombeyi forests heavily affected by drought. Alternatively, when the co-dominant species A. chilensis is widely present in the understory and large gaps are opened, a compositional shift may be possible, with a potential change in forest functionality. Individual partial openings do not favor more shrubby communities or a shift toward a new community. Thus, partial crown dieback contributes to self-replacement mechanisms by hampering strong growth release in understory shrubs, as evident in plots with high mortality, and by facilitating the growth of dominant tree species.

Growth release of stunted black spruce (Picea mariana) in Kalmia heath: the role of ectomycorrhizal fungi and near-ground microclimate

Naturally regenerating and planted black spruce (Picea mariana (Mill.) B.S.P.) in post-fire landscapes in eastern Canada often exhibit stunted growth in the presence of ericaceous shrubs such as Kalmia angustifolia L. After a period of stunted growth, some seedlings experience a growth release, exhibiting growth rates closer to normally growing seedlings. We hypothesized that an increase in colonization of root tips by ectomycorrhizal (ECM) fungi is responsible for this release and that the percentage of root tips colonized by ECM fungi would be higher on seedlings that had a released or normally growing neighbour within close proximity. We quantified ECM fungi diversity and abundance from 255 soil cores from stunted, released, and normally growing black spruce seedlings sampled in two Kalmia-dominated sites in Newfoundland. Growth and microsite characteristics around each seedling were also measured. We found that normal and released seedlings had significantly higher proportions of ECM fungi root tips than stunted seedlings, supporting our final hypothesis; however, there was no significant difference in distance between neighbours. Soil chemical properties are thought to inhibit the vegetative spread of ECM fungi species in this particular system and are identified as an important topic for further research.

Windthrow and growth response following a spruce budworm inspired, variable retention harvest in New Brunswick, Canada

Forest management regimes increasingly focus on the emulation of natural disturbance events, e.g., fire or insect outbreaks, to help increase ecosystem sustainability. We determined the residual stand response to a variable retention harvest inspired by spruce budworm (Choristoneura fumiferana (Clemens)) outbreaks in New Brunswick, Canada. Our objectives were to analyze the differences between surviving residual trees and those that succumbed to windthrow and to quantify growth release. The treatment was based on harvesting the estimated spruce budworm outbreak mortality, i.e., 90% of mature balsam fir (Abies balsamea (L.) Mill., 60% of mature spruce (Picea spp.), and no hardwoods. Windthrow increased with the proportion of trees harvested and averaged 52% over 7–9 years in these stands with high balsam fir – spruce content. One-third of 42 harvested plots sustained >30% windthrow, whereas 73% of 11 similar unharvested plots had <10% windthrow. Balsam fir had higher windthrow than spruce at 53% and 41%, respectively. Windthrown balsam fir trees had significantly larger diameters at breast height (DBH, 1.3 m), larger heights, and smaller crown ratios than surviving residual trees. Substantial growth release occurred, with DBH increment of residual trees 48%–64% greater than trees in unharvested plots. Balsam fir and intolerant hardwoods exhibited the largest growth response. We suggest that future spruce budworm inspired harvests in stands with high balsam fir – spruce content use two or three entries about 5 years apart to reduce windthrow.

The Dormancy Marker DRM1/ARP Associated with Dormancy but a Broader Role In Planta

Plants must carefully regulate their development in order to survive a wide range of conditions. Of particular importance to this is dormancy release, deciding when to grow and when not to, given these varying conditions. In order to better understand the growth release mechanism of dormant tissue at the molecular and physiological levels, molecular markers can be used. One gene family that has a long association with dormancy, which is routinely used as a marker for dormancy release, is DRM1/ARP (dormancy-associated gene-1/auxin-repressed protein). This plant-specific gene family has high sequence identity at the protein level throughout several plant species, but its function in planta remains undetermined. This review brings together and critically analyzes findings on the DRM1/ARP family from a number of species. We focus on the relevance of this gene as a molecular marker for dormancy, raising questions of what its role might actually be in the plant.