Taxonomy, together with ontogeny and growing conditions, drives needleleaf species' sensitivity to climate in boreal North America

Abstract

Currently, there is no consensus regarding the way that changes in climate will affect boreal forest growth, where warming is occurring faster than in other biomes. Some studies suggest negative effects due to drought‐induced stresses, while others provide evidence of increased growth rates due to a longer growing season. Studies focusing on the effects of environmental conditions on growth–climate relationships are usually limited to small sampling areas that do not encompass the full range of environmental conditions; therefore, they only provide a limited understanding of the processes at play. Here, we studied how environmental conditions and ontogeny modulated growth trends and growth–climate relationships of black spruce (Picea mariana) and jack pine (Pinus banksiana) using an extensive dataset from a forest inventory network. We quantified the long‐term growth trends at the stand scale, based on analysis of the absolutely dated ring‐width measurements of 2,266 trees. We assessed the relationship between annual growth rates and seasonal climate variables and evaluated the effects of various explanatory variables on long‐term growth trends and growth–climate relationships. Both growth trends and growth–climate relationships were species‐specific and spatially heterogeneous. While the growth of jack pine barely increased during the study period, we observed a growth decline for black spruce which was more pronounced for older stands. This decline was likely due to a negative balance between direct growth gains induced by improved photosynthesis during hotter‐than‐average growing conditions in early summers and the loss of growth occurring the following year due to the indirect effects of late‐summer heat waves on accumulation of carbon reserves. For stands at the high end of our elevational gradient, frost damage during milder‐than‐average springs could act as an additional growth stressor. Competition and soil conditions also modified climate sensitivity, which suggests that effects of climate change will be highly heterogeneous across the boreal biome.

Contrasting acclimation responses to elevated CO2 and warming between an evergreen and a deciduous boreal conifer

“…Our results highlight that warming‐induced stomatal closure can occur in the absence of water limitation. Similarly, warming induced stomatal closure in congeneric, mature Norway spruce growing in its natural habitat (Lamba et al, 2018), suggesting that reductions in g s may contribute to the observed reduction in carbon gain and growth in black spruce across North America (Marchand et al, 2019), an effect likely exacerbated by soil water limitations (Girardin, Bouriaud, et al, 2016; Girardin et al, 2014; Girardin, Hogg, et al, 2016).…”

“…We observed this effect in spruce, where declines in g s due to both warming and elevated CO 2 resulted in comparable carbon gain between seedlings grown under current (AC0T) and future climate conditions (EC8T), even when measured at their respective growth CO 2 (Figure 2b). While it is generally assumed that boreal forests are temperature limited (Huang et al, 2019), and might thus benefit from future climate warming and CO 2 fertilization (Stinziano & Way, 2014), these responses have not been observed in black spruce (Girardin, Bouriaud, et al, 2016; Marchand et al, 2019). Our results imply that stomatal responses (among other factors, such as nutrient and water availability) could dictate the long‐term trajectory of boreal forest productivity in future climates.…”

“…Despite expectations, recent responses of boreal forests to climate change have not been consistent with predictions of enhanced tree growth. Instead, studies show either no change (Giguère‐Croteau et al, 2019; Girardin, Bouriaud, et al, 2016; but see Lloyd, Bunn, & Berner, 2011) or a decline in tree growth and survival (Chen & Luo, 2015; D'Arrigo, Wilson, Liepert, & Cherubini, 2008; Girardin, Hogg, et al, 2016; Lloyd & Bunn, 2007; Ma et al, 2012; Marchand, Girardin, Hartmann, Gauthier, & Bergeron, 2019; Peng et al, 2011; Walker & Johnstone, 2014). The few studies that report positive responses to climate change show that tree growth is enhanced by warming only in wet regions (Hember, Kurz, & Coops, 2017).…”

“…However, even studies in well‐watered conditions can show a negative effect of higher temperatures on boreal tree species (Tjoelker, Oleksyn, & Reich, 1998; Way & Sage, 2008a, 2008b), suggesting that their growth might be directly reduced by rising temperature. Given the low tree biodiversity of the boreal (Girardin, Bouriaud, et al, 2016), responses to climate change factors of these few species (Girardin, Bouriaud, et al, 2016; Kurepin et al, 2018; Marchand et al, 2019; Tjoelker et al, 1998) might explain observed large‐scale productivity trends in the boreal region (Girardin, Bouriaud, et al, 2016; Lloyd et al, 2011), and will ultimately dictate the trajectory of carbon cycling in these forests. However, we still lack an understanding of how the processes that govern leaf carbon fluxes (photosynthesis and respiration), and thus carbon availability for growth, acclimate to climate change in dominant boreal tree species.…”

Increased Drought Sensitivity Results in a Declining Tree Growth of Pinus latteri in Northeastern Thailand

“…where R is the tree radius and t is the year of tree-ring formation. Subsequently, BAIs was detrended using generalized additive mixed models (GAMMs) to remove the age-and size-related trends [43,44]. BAI values were log-transformed to improve the normality of their distributions.…”