Shifts in resource allocation and aggravation of foliage development restrict the growth rate of Picea abies under increasing atmospheric humidity at high latitudes

Global warming is accompanied by rising precipitation, atmospheric water vapour content, and specific humidity at high latitudes. The rising amount and frequency of rainfall increase the air relative humidity (RH) on a local scale, especially within forest canopies. We studied the effects of artificially elevated environmental humidity (RH and soil moisture) on leaf gas exchange, stomatal responses and growth of young Picea abies trees at the Free Air Humidity Manipulation site in eastern Estonia. Manipulation did not affect the net assimilation rate (A_n) but affected the stomatal responses, net photosynthetic efficiency (A_n/c_i), and photosynthetic water-use efficiency (WUE). At an elevated air humidity (H), trees exhibited the highest stomatal conductance (g_S) and lowest WUE, A_n/c_i, and stomatal sensitivity to air vapour pressure deficit compared to trees growing under ambient conditions (C) and elevated soil moisture (I). Compared to C trees, H trees demonstrated reduced height growth, foliage biomass, and enhanced investments in fine roots referring to worsening soil nutrient availability. Tree growth decline can be explained by (1) foliage development retardation, (2) resource allocation changes, causing a shift in the photosynthetic to non-photosynthetic tissue ratio in favour of the latter, and (3) impaired nutrient uptake from the soil. Changes in stomatal responses make trees grown in a higher RH more vulnerable to weather extremes, also limiting tree growth and forest productivity. Increasing precipitation with concomitant increase in atmospheric humidity at high latitudes counteracts the expected enhancement of tree growth due to climate warming in mesic northern forests.