New perspectives for wood anatomical analysis in dendrosciences: The GSL1-microtome

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Fire And Vegetation Dynamics Studies From The North American Boreal Forest

Wood anatomical features may be visible on the microscopic as well as on the macroscopic scale. While the former can often be quantified by detailed wood anatomical analyses, the latter are often treated as qualitative features or as binary variables (present/absent). Macroscopic tree-ring features can be quantified in terms of frequency, intensity, or classified according to their position within a tree ring, like intra-annual density variations (IADFs) in conifers or frost rings in earlywood or latewood. Although some of these tree-ring features, like e.g. missing rings or IADFs are often seen as anomalies, hampering dendrochronologists to perform proper crossdating of tree-ring series, many of these properties are formed under extreme environmental stress or heavy impact, and could mark these extreme events by the manifestation in the wood anatomical structures throughout the lifespan of trees. The described tree-ring features form discrete time-series of extreme events. For example, flood rings may be marked by lunar-shaped earlywood vessels or enlarged latewood vessels in ring-porous oaks. White earlywood rings and light rings indicate reduced cell wall thickness and lignification occurring in very cold years. Frost rings result from cambial cell death during abrupt cooling events in the growing season. Missing rings and IADFs are mainly caused by drought events. Characteristic variations in earlywood vessel size, shape, or number in ring-porous oak species are markers for flood events, defoliation, heat stress, or drought. Traumatic resin ducts may be triggered by a range of biotic or environmental stressors, including wounding, fires or mechanical factors. Reaction wood is indicative of mechanical stress, often related to geomorphic events. In many cases anatomical responses are unspecific and may be caused by different stressors or extreme events. Additionally, the sensitivity of trees to form such features may vary between species, or between life stages within one species. We critically evaluate the indicative value of different wood anatomical tree-ring features for environmental reconstructions.

Tree-ring features: indicators of extreme event impacts

Significance We show that current and projected surface water allocations from the Athabasca River, Alberta, Canada, for the exploitation of the Alberta oil sands are based upon an untenable assumption of the representativeness of the short instrumental gauge record. Our trend analysis of the instrumental data shows declining regional flows. Our tree-ring reconstruction shows periods of severe and prolonged low flows not captured by the instrumental record. Exploitation of the Alberta oil sands, the world’s third-largest crude oil reserve, requires fresh water from the Athabasca River, an allocation of 4.4% of the mean annual flow. This allocation takes into account seasonal fluctuations but not long-term climatic variability and change. This paper examines the decadal-scale variability in river discharge in the Athabasca River Basin (ARB) with (i) a generalized least-squares (GLS) regression analysis of the trend and variability in gauged flow and (ii) a 900-y tree-ring reconstruction of the water-year flow of the Athabasca River at Athabasca, Alberta. The GLS analysis removes confounding transient trends related to the Pacific Decadal Oscillation (PDO) and Pacific North American mode (PNA). It shows long-term declining flows throughout the ARB. The tree-ring record reveals a larger range of flows and severity of hydrologic deficits than those captured by the instrumental records that are the basis for surface water allocation. It includes periods of sustained low flow of multiple decades in duration, suggesting the influence of the PDO and PNA teleconnections. These results together demonstrate that low-frequency variability must be considered in ARB water allocation, which has not been the case. We show that the current and projected surface water allocations from the Athabasca River for the exploitation of the Alberta oil sands are based on an untenable assumption of the representativeness of the short instrumental record.

Long-term reliability of the Athabasca River (Alberta, Canada) as the water source for oil sands mining

Key message Double-stained microsections from high altitude Pinus nigra wood cores highlighted unlignified latewood cells possibly linked to abrupt temperature reduction at the end of the growing season. More consolidated detection could increase their role in dendroecology. Abstract Cambial activity and wood formation are influ- enced by environmental factors, primarily climate. During cell wall formation the lignification is mainly controlled by temperature. By staining with safranin and astra blue it is possible to distinguish cell walls richer in lignin (stained in red) from those richer in cellulose (stained in blue). Here we show an uncommon phenomenon observed in 41 transverse sections prepared for anatomical studies of young European black pine (Pinus nigra Arnold) individ- uals. We detected some layers of incompletely lignified cells that appear blue in safranin-astra blue-stained sec- tions. Growth rings showing this anatomical feature were named ''blue rings''. The aims of this preliminary study are: (i) to describe the features of this peculiar anatomical trait, (ii) to enhance its visualization, and (iii) to suggest possible drivers of its formation. First results indicate the influence of low air temperature causing a lack of lignification in latewood cells. The added values provided by the identification of ''blue rings'' within tree-ring series could be (i) their possible use as pointer year, (ii) cross dating improvement, and (iii) finer assessment of tree sensitivity to environmental and climatic factors.

/pdf/the-blue-ring-anatomy-and-formation-hypothesis-of-a-new-tree-1ilgpnu80h.pdf

The “blue ring”: anatomy and formation hypothesis of a new tree-ring anomaly in conifers

Dendroecological analysis of tree-ring anomalies can facilitate better environmental reconstructions provided that conditions leading to their formation are well understood. White earlywood rings (WER) constitute an anomaly not previously described and, like frost rings (FR), occur often in boreal coniferous species in central Canada. The objectives of this study were i) to examine the distribution of FR and WER in black spruce trees, ii) to measure and compare tracheid dimensions in WER and control rings and iii) to investigate potential causes of WER formation. Sampling was conducted in two regions of western Manitoba. At each of four sampling sites, ten black spruce trees were selected for stem analysis. Results indicated that FR were mainly formed in the smaller diameter, thin barked portion of the stem up to a height of 16m, whereas WER were almost uniformly distributed along the stem. Tracheids were measured for WER occurring in 1916, 1943 and 1970 and WER were found to have earlywood tracheids with significantly thinner cell walls compared to control rings. It is speculated that the formation of WER may be related to reduced hormone and carbohydrate availability in the early growing season. The potential linkage between WER and frost events, or other agents of defoliation, prior to ring formation merits further investigation.

Frost Rings and White Earlywood Rings in Picea Mariana trees from the boreal plains, Central Canada

The North American boreal forest has been developing since the end of the last glaciation approximately 10,000 yr ago. With climate warming and human occupation, it is anticipated that fire danger, ignition, and activity will be increasing, compromising forests’ benefits for generations to come. In this study, we show, however, that a century of rapid climate changes and human densification has had the opposite effect in the boreal eastern interior of the North American continent, reducing biomass burning to values below two millennia of historical levels. A multi-millennial fire history was reconstructed for eight forested landscapes from the Lake of the Woods Ecoregion (LWE) located at the boreal–prairie ecotone. Fire history was reconstructed using a combination of archival (period 1920–2010), tree-ring (stand initiations and fire scars: period 1690–2010), and lake sediment charcoal (2500 BP to present) records. The archival record revealed recent large fires (>200 ha) in 1948, 1980, and 1988. An additional 19 fires were identified by the fire-scar record. Fire events in 1805, 1840, 1863, and the 1890s were identified in numerous locations around multiple lakes suggesting that they were of large extents. In accordance with the tree-ring record, the charcoal accumulation rate (CHAR) peak record generally identified the major fires but tended to lag from the tree-ring records by several decades. Within LWE, the long-term charcoal record revealed that CHAR was higher for each lake in the earlier portion of the record including the warm Medieval Climate Anomaly (AD 900 to AD 1000), followed by a progressive decrease toward the cool Little Ice Age period. This decline was abruptly interrupted in the midto late 19th century with large synchronized fires, also reported over western and central North America, and resumed approximately four decades later. Fire disturbance level is today below the historical range, despite the accentuated climate warming. Aging of the forest landscape may create biodiversity loss notably in fire-adapted species while at the same time setting the tone for major fires in upcoming decades if no action is taken for managing fuels.

https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.1002/ecs2.2287

Recent fire activity in the boreal eastern interior of North America is below that of the past 2000 yr

Earlier spring and earlier onset of growth, as a consequence of climate change, may expose trees and crops to increased risk of exposure to frost damage. In this study, we compare the frequency of frost rings in three regions [Porcupine Provincial Forest (PPF; north-latitude); Duck Mountain Provincial Forest (DMPF; mid-latitude) and Riding Mountain National Park (RMNP, south-latitude)] located in the Boreal Plains of interior North America. In each of PPF and DMPF, twenty upland black spruce [Picea mariana (Mill.) BSP] trees were sampled using stem analysis and others were sampled at breast height or below. In RMNP, multiple coniferous tree species were sampled at breast height or below to allow comparison among species. Results from stem analysis indicated that frost rings were more frequent in DMPF than PPF (north of DMPF). Frost rings identified up to a height of 16 m and were formed predominantly in the early cambial age zone. As a general pattern, frost rings recorded in PPF occurred more abundantly in years with warm April temperatures and this association was less prevalent in DMPF. Frost rings were formed following extreme frost events in late May - early June and frost-ring years corresponded to years with cooler June temperatures. A significant positive association was found between frost-ring frequency and the El Nino Southern Oscillation. In the absence of an early spring, black spruce trees were less affected by frost damages. Stem analysis provided a better record of spring frosts than solely sampling at breast height or below. The multi-species approach used in RMNP revealed many years with synchronous frost rings among species. The development of a large network of frost-ring chronologies from tree species of various age classes and/or from stem analysis will help with assessing the impact of late spring frosts on forest dynamics and to document large-scale climate anomalies in areas with low climate data coverage and/or prior to instrumental records.

Justin Waito

Papers

Frost Rings and White Earlywood Rings in Picea Mariana trees from the boreal plains, Central Canada

Recent fire activity in the boreal eastern interior of North America is below that of the past 2000 yr

Climate and atmospheric circulation related to frost-ring formation in Picea mariana trees from the Boreal Plains, interior North America

Fire and Climate

Fire and climate: using the past to predict the future