Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings

Abstract

The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate‐growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long‐term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate‐growth response. The negative radial growth response to warm and dry early‐ to mid‐summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against 13C was reduced in warm, dry summers and further diminished on south‐facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced 13C discrimination, and increased intrinsic water‐use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate‐growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.

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

“…Our study underscores the utility of employing broadly distributed datasets for assessing the complexity of climate change effects on a forest ecosystem (Klesse et al, 2018;Nicklen et al, 2018).…”

“…By modifying resource availability, interindividual competition can exacerbate tree sensitivity to harsh climatic conditions (Buechling, Martin, & Canham, 2017;Ford et al, 2016;Gleason et al, 2017;Jiang et al, 2018;Nicklen et al, 2018) or buffer growth gains from favorable periods (Cortini, Comeau, & Bokalo, 2012). Ultimately, the capacity of a tree to efficiently use resources will also dictate its response to climate (Carrer & Urbinati, 2004).…”

Complementarity effects are strengthened by competition intensity and global environmental change in the central boreal forests of Canada

“…Competition for limited resources is a major driver of forest demographic dynamics (Oliver & Larson 1996), particularly in the boreal forest of North America (Chen & Popadiouk 2002;Luo & Chen 2011). With ongoing global environmental change, tree competition has become more intense over time in the areas where soil moisture is not limiting (Luo & Chen 2015;Nicklen et al 2019). Within terrestrial plant communities, the effect of plant species diversity on growth is likely positive (Zhang et al 2012;Grace et al 2016), and higher tree species diversity can lead to reductions in competition (Forrester & Bauhus 2016;Ammer 2019).…”

Boreal tree-rings are influenced by temperature up to two years prior to their formation: a trade-off between growth and reproduction?

“…Indeed, although the arctic-boreal ecotone is considered the ecological limit of trees because of their strong temperature limitation (Körner 2012), previous studies often failed to identify a uniform pattern(s) of climate-growth interaction. Responses to climate change tend to vary along geographic gradients (Wilmking and Juday 2005), temporally (Wilmking et al 2004, Lloyd and Bunn 2007, D'Arrigo et al 2008, Ohse et al 2012 or as a result of forest stand conditions and local microsites (Nicklen et al 2019). Our understanding of the specific factors and mechanisms modulating the climate-growth interaction of boreal forests thus remains limited.…”

“…Net primary productivity is low in boreal forests (Gillman et al 2015) and, therefore, trees must balance the allocation of limited amounts of carbohydrates among individual survival strategies. For instance, the tradeoff between growth and reproduction results in synchronization of narrow tree-rings (pointer years) with years of extensive seed production (Juday et al 2003, Drobyshev et al 2010, Hacket-Pain et al 2018, Nicklen et al 2019. Alternatively or in addition, it may result in significant correlations between tree-ring widths and climatic conditions during key phases of cone development (Wilmking et al 2004, Ohse et al 2012.…”