The competitive status of trees determines their responsiveness to increasing atmospheric humidity – a climate trend predicted for northern latitudes

Abstract

The interactive effects of climate variables and tree-tree competition are still insufficiently understood drivers of forest response to global climate change. Precipitation and air humidity are predicted to rise concurrently at high latitudes of the Northern Hemisphere. We investigated whether the growth response of deciduous trees to elevated air humidity varies with their competitive status. The study was conducted in seed-originated silver birch and monoclonal hybrid aspen stands grown at the free air humidity manipulation (FAHM) experimental site in Estonia, in which manipulated stands (n = 3 for both species) are exposed to artificially elevated relative air humidity (6-7% over the ambient level). The study period included three growing seasons during which the stands had reached the competitive stage (trees were 7 years old in the final year). A significant 'treatment×competitive status' interactive effect on growth was detected in all years in birch (P < 0.01) and in one year in aspen stands (P = 0.015). Competitively advantaged trees were always more strongly affected by elevated humidity. Initially the growth of advantaged and neutral trees of both species remained significantly suppressed in humidified stands. In the following years, dominance and elevated humidity had a synergistic positive effect on the growth of birches. Aspens with different competitive status recovered more uniformly, attaining similar relative growth rates in manipulated and control stands, but preserved a significantly lower total growth yield due to severe initial growth stress. Disadvantaged trees of both species were never significantly affected by elevated humidity. Our results suggest that air humidity affects trees indirectly depending on their social status. Therefore, the response of northern temperate and boreal forests to a more humid climate in future will likely be modified by competitive relationships among trees, which may potentially affect species composition and cause a need to change forestry practices.

Sensitivity of plant species to warming and altered precipitation dominates the community productivity in a semiarid grassland on the Loess Plateau

“…However, the magnitude of precipitation addition could not be the reason in this study because precipitation addition had no trend to increase both ANPP of DG (Figure a; Table ) and ND (Figure c; Table ). Different species often interact each other in a community, which can modify the effects of climate changes on plant communities (Suttle et al, ; Tullus et al, ). Thus, varied responses of plant groups to precipitation addition may be due to species interactions (Suttle et al, ).…”

“…Competitive status of plant species may determine their responses to climate change. For example, Tullus et al () observed that trees with high competitivity could benefit from elevated humidity and exhibited larger stem volume, but those with low competitivity had no responses to elevated humidity.…”

Coppicing improves the growth response of short-rotation hybrid aspen to elevated atmospheric humidity

“…Hybrid aspen is included as a test species in the unique FAHM experiment established in the hemiboreal forest zone, where the forest ecosystem performance under artificially elevated air humidity is studied (Kupper et al, 2011). During the first 6-year study period, several stress responses were observed in planted hybrid aspens under elevated humidity, including reductions in transpiration, nutrient uptake and above-ground growth (Tullus et al, 2012b(Tullus et al, , 2017Rosenvald et al, 2014) and enhanced production of foliar secondary metabolites (Lihavainen et al, 2016;Sellin et al, 2017a).…”

“…The responses of trees to climate change depend on the competitive status of trees for aspens (McDonald et al 2002;Tullus et al, 2017) and other poplars (Calfapietra et al, 2003), suggesting that stand structural development is likely also affected by future climatic conditions. Stand structural diversity is a crucial indicator of overall habitat diversity offered by the stand (McElhinny et al, 2005;Barbeito et al, 2009), including by young planted stands and coppice forests (Humphrey et al, 1999;Sullivan et al, 2001), whereas in coppicing systems, the size hierarchy of trees can be maintained through multiple rotations (Edelfeldt et al, 2018).…”

Differences in xylogenesis between dominant and suppressed trees

“…As shown by analysis of trees in plots across treeline ecotones, upslope migration rates of alpine treeline are controlled largely by species interactions (Liang et al, 2016), and increasing competition between adults and juveniles or seedlings could counteract positive effects of future warming on tree growth (Wang et al, 2016). However, climatic sensitivity of tree growth according to competition status is variable, species specific and site dependent (McDonald et al, 2002;Carnwath and Nelson, 2016;Tullus et al, 2017). Most results on the effects of competition on climate-growth relationships in dendroecology are derived from year-to-year variations in basal area increment associated with climate variability, but they lack thorough understanding of the timing and processes of xylem growth (Fraver et al, 2014;Sánchez-Salguero et al, 2015;Carnwath and Nelson, 2016).…”