Reassessment of growth-climate relations indicates the potential for decline across Eurasian boreal larch forests

Abstract

Larch, a widely distributed tree in boreal Eurasia, is experiencing rapid warming across much of its distribution. A comprehensive assessment of growth on warming is needed to comprehend the potential impact of climate change. Most studies, relying on rigid calendar-based temperature series, have detected monotonic responses at the margins of boreal Eurasia, but not across the region. Here, we developed a method for constructing temporally flexible and physiologically relevant temperature series to reassess growth-temperature relations of larch across boreal Eurasia. Our method appears more effective in assessing the impact of warming on growth than previous methods. Our approach indicates widespread and spatially heterogeneous growth-temperature responses that are driven by local climate. Models quantifying these results project that the negative responses of growth to temperature will spread northward and upward throughout this century. If true, the risks of warming to boreal Eurasia could be more widespread than conveyed from previous works.

Fig. 1. Distribution of Eurasian boreal larch forests and the sampling populations.

a Species distributions of Siberian larch (Larix sibirica, orange shadow) and Dahurian larch (Larix gmelinii, cyan shadow) digitized and merged from multiple repositories^,,. Circles represent the tree-ring sampling populations, following the same color scheme as the species distributions; squares represent the meteorological stations. b–d elevation (SRTM DEM v4.1), mean annual temperature (MAT), and precipitation (MAP) during 1970–2000 (WorldClim v2.1) across the distribution of Eurasian boreal larch forests. This figure was created using ArcGIS 10.2 for Desktop (ESRI, Inc).

Fig. 2. Climate boundaries between the positively-responding and negatively-responding populations with the identification probability threshold of 0.50.

Shadows represent the climate spaces occupied by the two species from 1970 to 2000, while the green-yellow gradient represents high-low cell density. Red circles represent the observed negatively-responding populations, those showing significant negative responses to temperature, regardless of showing or not showing positive responses; blue circles represent the observed positively-responding populations, those showing significant positive responses to temperature but no negative responses. Dashed lines denote the average values of climatic conditions; boxes represent the 25th, 50th and 75th quantiles, and whiskers extend to the 5th and 95th quantiles. Solid lines represent the climate boundaries using the identification probability threshold of 0.50, analysis formulas of the boundaries are noted on each panel (pseudo-R² = 0.441 and 0.390, n = 202 and 123 population chronologies, and cross-validation accuracy = 86.16% and 88.15% for Siberian larch and Dahurian larch, respectively). Upper-left and lower-right sides of the boundary represent the estimated positively-responding and negatively-responding climate spaces, respectively.

Fig. 3. Baseline (1979–2000) probability of showing negative growth-temperature responses across Eurasian boreal larch forests.

a, b Circles represent the sampling populations. The left and right halves of each circle are colored according to the population-level growth-temperature response patterns of the represented population during 1960–1990 and 1970–2000, respectively, where red and blue represent negatively-responding and positively-responding, dark gray represents no significant response, and no color represents lack of growth data or climate data. Red-white-blue gradient across the species distributions represents the estimated probability of showing negative responses decreasing from 1 to 0, species-specific probability functions were labeled on each plot; gray lines represent the 0.50, 0.75, and 0.95 iso-probability lines. Inset c shows dense sampling populations in the west Altai Mountains. Panels a–c were created using ArcGIS 10.2 for Desktop (ESRI, Inc). Insets d, e display the area percentage histograms of positively-responding and negatively-responding regions in species distribution identified by three probability thresholds. f, g Elevation profiles of the 0.50 iso-probability line (green), and the 0.50 probability-identified positively(blue)/negatively(red)-responding regions at a latitudinal resolution of 2.5’. Shadows and lines represent the maximum-to-minimum ranges and mean values of elevation, respectively.

Fig. 4. Projected distributions of the positively-responding and negatively-responding regions under Shared Socio-economic Pathways (SSP) 2-45 and 5-85.

a–d Results of Siberian larch and Dahurian larch identified by probability thresholds (P₀) of 0.50 and 0.95, respectively. Light yellow to dark green represents the decreasing proportion of the climate projections from 25 GCMs that identified the negatively-responding regions under corresponding projection scenarios; blue lines represent the baseline (1970–2000) boundaries between the positively-responding and negatively-responding regions with corresponding probability thresholds. Gray shadows represent the distribution areas where projected climatic conditions falling outside the baseline climate space of boreal larch. This figure was created using ArcGIS 10.2 for Desktop (ESRI, Inc).