White spruce (Picea glauca) is one of the most widely planted North American boreal conifers, and dendrochronological analysis of a common-garden experiment in eastern Canada showed that populations from drier origins carry higher growth resilience to extreme drought, with moderate heritability and among-population differentiation — evidence that drought resilience could be selected for. However, that provenance set spanned only eastern Canada, and the mechanisms producing population differences in post-drought recovery were not identified. The open problem is: (i) whether drought-resilience clines and their genetic architecture hold across the species' full natural distribution, including the drier western range and Canadian Prairies, and whether provenance and family ranks remain stable across contrasting planting environments; and (ii) which morphological and physiological strategies determine inter- and intra-population differences in post-drought recovery, among which root architecture, water-use efficiency, allocation to stem carbon reserves and xylem anatomy are candidate mechanisms rather than a closed list. No study is known to have tested the same provenances across contrasting planting environments spanning the range, nor to have attributed the recovery differences to root architecture or stem carbon reserves.

Background

White spruce (Picea glauca (Moench) Voss) is a widely distributed conifer spanning the North American boreal forest, including the dry Canadian Prairies at its western edge, and is a leading plantation species for reforestation. Because drought intensity and frequency are projected to increase in much of its range, forest managers need to know which seed sources to plant where — a decision that depends on how much drought tolerance varies genetically among populations and how stable that variation is across environments.

The standard tools for separating genetic from environmental effects are provenance and common-garden trials: trees raised from seed collected in many populations are grown together at one or more test sites, so that differences among populations reflect genetic differentiation rather than differing growing conditions. Drought performance can be quantified retrospectively from tree rings (annual growth layers) by indexing growth in and after a known drought year: resistance is the growth deficit during the drought, recovery is the rebound afterwards relative to the deficit, and resilience is the return to pre-drought growth. When such traits correlate with the climate of population origin along geographic gradients, one speaks of clinal variation, which usually reflects local adaptation — genetic shaping of populations by their native conditions.

Depardieu et al. (2020, New Phytologist 227:427–439) analyzed 1481 tree-ring series from 29-year-old trees of 43 white spruce provenances from eastern Canada planted in a common garden. Populations from drier origins showed higher drought resilience, the resilience traits had moderate heritability (the fraction of trait variation attributable to additive genetics), and there was substantial genetic variation both among provenances and among families within provenances — suggesting that breeding for drought resilience would not sacrifice growth. The authors concluded, however, that their provenance set did not span the natural distribution of the species: they called for studies across the full environmental range — in particular the drier Canadian Prairies, where steeper clines might occur — and for monitoring of provenance rank changes (whether the best populations stay best) across different environments, which matters for seed-transfer and assisted-migration decisions (deliberately moving seed sources to track shifting climates). They also noted that the physiological basis of the observed population differences in post-drought recovery was unestablished, plausibly involving root architecture, water-use efficiency (the carbon gained per unit water lost, often inferred from carbon-isotope composition of wood), and stem non-structural carbon reserves (stored sugars and starch used to regrow drought-damaged xylem, the water-conducting tissue of wood), and they proposed integrating tree-ring phenotypes with genomic predictors — genomic selection, which estimates breeding values of untested trees from dense genome-wide markers — in multi-environment trials.

A key adjacent result predating that study: Sang et al. (2019, Evolutionary Applications 12:1850–1860) used a 40-year-old range-wide white spruce provenance trial (provenances from all major ecoregions, tested at a single central site in Alberta) and found population differentiation in resistance and recovery to the 1999/2002 droughts but no provenance with superior growth under drought; populations from the wettest northeastern environments were surprisingly drought-tolerant, and the authors questioned whether assisted migration could mitigate drought impacts for this species. This partially contrasts with the drier-origin-more-resilient pattern of the eastern common garden, sharpening the question of whether and where clines hold across the range. Because it used a single test site, it could not assess rank changes across planting environments, and it did not address the genetic architecture of resilience traits.

Problem Statement

For white spruce (Picea glauca), building on the eastern-Canada common-garden finding of clinal genetic variation in tree-ring drought-resilience traits (Depardieu et al. 2020) and on the partially conflicting single-site range-wide analysis of Sang et al. (2019): (1) determine how drought-resilience traits (resistance, recovery, resilience derived from annually resolved tree rings) and their genetic architecture vary across the full natural distribution of the species — including the drier western range and Canadian Prairies — when the same provenances are tested across contrasting planting environments, and quantify whether and how provenance and family ranks change among environments (including whether genomic-prediction accuracy for drought resilience holds across environments and population structure); and (2) identify the morphological and physiological mechanisms that determine the observed inter- and intra-population differences in post-drought growth recovery, for which root architecture, water-use efficiency, allocation to stem non-structural carbon reserves, and xylem-anatomical strategies are candidate mechanisms rather than a closed list. Component (1) comprises four distinct quantities — the range-wide clinal variation, the genetic architecture of the resilience traits, the environment-dependence of provenance and family ranks, and the transferability of genomic-prediction accuracy — and component (2) is answered by establishing any one of the candidate mechanisms as a determinant, or by a general account of the differences. The two components jointly serve one objective: establishing whether the local-adaptation-based drought-resilience signal is a range-wide, environment-stable and mechanistically understood property that can safely guide breeding and seed-transfer (assisted-migration) decisions for this major boreal plantation species.

References

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  2. Sang Z, Sebastian-Azcona J, Hamann A, Menzel A, Hacke U. 2019. Adaptive limitations of white spruce populations to drought imply vulnerability to climate change in its western range. Evolutionary Applications 12(9):1850–1860. doi:10.1111/eva.12845. https://doi.org/10.1111/eva.12845
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