Recent unexpected decline of forest growth in North Finland: examining tree-ring, climatic and reproduction data

Abstract

After a decades-long increasing trend, the recent results of the National Forest Inventory (NFI) reported a decline of forest growth in North Finland. The aim of this study was to assess climatic and reproduction influences behind the growth decline. We used tree-ring data that had been collected by NFI using systematic sampling. The tree-ring width series were detrended using the regional curve standardisation (RCS) removing age-related trends. The resulting tree-ring indices of Scots pine (Pinus sylvestris L.) showed decadal variations with low increment in the 1990s, and high increment in the 1980s and the early years of the current century. Thereafter, a prolonged growth reduction for pine started both on the mineral soil sites and peatlands. The tree-ring indices of Norway spruce (Picea abies (L.) Karst.) had less pronounced decadal variations and no trend-like reduction over the last 15 years. High spring and summer temperatures were found to enhance radial growth, but high winter temperatures were related to low growth for pine and spruce in the following summer. Temperature variation, accompanied by variables indicating years of drought and intensive flowering, accounted for 34% annual growth variance of pine and 21–44% for spruce. Thus, the results imply that climatic factors may have to some extent contributed to the recent growth reduction of pine. Due to its ecological and economic consequences growth decline needs to be further monitored and investigated. Moreover, analyses of stand and age structure, potentially affecting the growth decline, were beyond the scope of this paper, but also warrant further investigation.

Current and future drought vulnerability for three dominant boreal tree species

“…We also occasionally detect the unusual negative relation between winter temperatures and Scots pine growth described by Mäkinen et al. (2022) in some sites at northern latitudes, but to understand when, how, and why it occurs, more research is required. Summer drought also severely reduces Scots pine growth (Lévesque et al., 2014), but the methodology used in this study did not detect this pattern.…”

“…High drought damage risk for Scots pine is associated with cold temperatures in late winter–early spring (mean March and April temperature below 0°C) and dry climates (400–600 mm of annual rainfall; Figure 3a,b). Scots pine growth has been associated both positively (Harvey et al., 2020) and negatively (Mäkinen et al., 2022) with winter temperatures in temperate and boreal forests. Our results are in line with Harvey et al.…”

“…Figure 3a,b). Scots pine growth has been associated both positively (Harvey et al, 2020) and negatively (Mäkinen et al, 2022) with winter temperatures in temperate and boreal forests. Our results are in line with Harvey et al (2020), although we observed a slight decreasing influence of late-winter temperature on Scots pine growth (Figure 4b).…”

“…Climatic changes toward longer and warmer growing seasons may increase timber production in boreal forests (Bergh et al., 2010). However, recent observations show decreasing annual volume growth in northern European forests, possibly due to higher evaporative demand (Fridman et al., 2022; Mäkinen et al., 2022). Thus, the productivity increment could partly be offset by more frequent summer droughts.…”

“…Map lines delineate study areas and do not necessarily depict accepted national boundaries. negative relation between winter temperatures and Scots pine growth described byMäkinen et al (2022) in some sites at northern latitudes, but to understand when, how, and why it occurs, more research is required. Summer drought also severely reduces Scots pine growth(Lévesque et al, 2014), but the methodology used in this study did not detect this pattern.F I G U R E 3 Partial dependence plots for the most important predictors of drought damage risk for Scots pine: March and April mean temperature (a), accumulated annual precipitation (b), CCI (c), tree crown length (d), stand density (e) and basal area proportion of Norway spruce in the stand (f).…”