Adaptive limitations of white spruce populations to drought imply vulnerability to climate change in its western range

Abstract

A cost‐effective climate change adaptation strategy for the forestry sector is to move seed sources to more northern and higher elevation planting sites as part of ongoing reforestation programs. This is meant to match locally adapted populations with anticipated environments, but adaptive traits do not always show population differences suitable to mitigate climate change impacts. For white spruce, drought tolerance is a critical adaptive trait to prevent mortality and productivity losses. Here, we use a 40‐year‐old provenance experiment that has been exposed to severe drought periods in 1999 and 2002 to retrospectively investigate drought response and the adaptive capacity of white spruce populations across their boreal range. Relying on dendrochronological analysis under experimentally controlled environments, we evaluate population differences in resistance, resilience, and recovery to these extreme events. Results showed evidence for population differentiation in resistance and recovery parameters, but provenances conformed to approximately the same growth rates under drought conditions and had similar resilience metrics. The lack of populations with better growth rates under drought conditions is contrary to expectations for a wide‐ranging species with distinct regional climates. Populations from the wettest environments in the northeastern boreal were surprisingly drought‐tolerant, suggesting that these populations would readily resist water deficits projected for the 2080s, and supporting the view that northeastern Canada will provide a refugium for boreal species under climate change. The findings also suggest that white spruce is sensitive to growth reductions under climate change in the western boreal. The study highlights that population differentiation in adaptive capacity is species‐ and trait‐specific, and we provide a counterexample for drought tolerance traits, where assisted migration prescriptions may be ineffective to mitigate climate change impacts. For resource managers and policy makers, we provide maps where planning for widespread declines of boreal white spruce forests may be unavoidable.

Post-glacial re-colonization and natural selection have shaped growth responses of silver fir across Europe

“…These results were confirmed by our Q ST -F ST analyses indicating that recent natural selection has also contributed to the differences found among provenances, at least for height. These findings disagree with other studies where silver fir provenances from cold regions had lower total height (Csilléry et al, 2020) or, in case of other conifer species, northern provenances were growing at slower rates than provenances originating from warmer climates (Montwé et al, 2016;Sang et al, 2019) Table 2 Results of Q ST analyses. Shown are estimates and standard errors (SE) for Q ST , heritability (h 2 ), and additive genetic variance (V A ) for the seven tested traits diameter at breast height (DBH), autocorrelation (Ac), resilience (Rs), recovery (Rc), resistance (Rt), correlation between tree growth and May temperature (TMAY).…”

“…In this context, dendrochronology is an excellent tool to overcome some of these issues because i) tree-ring data allow for the retrospective analysis of growth responses to climate at tree, population, and forest ecosystem levels and ii) growth integrates most physiological processes related to resource acquisition and use (Fritts, 1976). Although some studies have started using dendrochronological techniques in provenance trials located in North America (Isaac-Renton et al, 2018;Sang et al, 2019;Depardieu et al, 2020), this approach still remains largely unexplored for most of the European tree species (but see Trujillo-Moya et al, 2018). Moreover, there are only a few studies that combine dendrochronological and genetic analyses to assess the roles of demographic processes and local adaptation in long-and short-term growth performance (e.g., Depardieu et al, 2020).…”

Xylem Anomalies as Indicators of Maladaptation to Climate in Forest Trees: Implications for Assisted Migration

“…Tree cores were air dried immediately after collection. After 3 months of storage, the cores were sanded with progressively finer grit sizes (P240, P320, and P800) for a complementary ring width study (Sang et al, 2019). The cores were split in approximately 5 cm long segments to facilitate sectioning the whole core without breaking the section.…”

Tree growth and survival over 61 years at the Second International Larch Provenance Test in southeastern Michigan, USA

“…The idea that phenotypic variation among populations is driven by genetic as well as environmental variation has major implications for forest managers attempting to introduce populations to new environments, particularly in an era of climate change. Because populations have evolved such that their survival and growth are highly synchronized with local climate, information about these adaptations could improve managers' ability to predict how provenances may interact with similar climatic conditions well outside of their natural geographic range as well as with changing climatic conditions within it (Sang et al 2019, Lu et al 2014. Data from long-term provenance tests may therefore be critical for identifying ideal growing conditions for certain forest tree species (Wang et al 2010, Thomson and Parker 2008, Wang et al 2006, Rehfeldt et al 1999.…”

“…Diameter values (cm) for larch sources inExperiment II, 1976-2019 Sources not connected by a vertical line are significantly different at α = 0.05.…”