Spatial dynamics of alpine tree lines under global warming: What explains the mismatch between tree densification and elevational upward shifts at the tree line ecotone?

Abstract

Aim: Most studies focusing on the alpine tree line responses to climate warming have used either the tree densification within the ecotone or its elevational upshift as indicators. However, it is acknowledged that the relationship between densification and upshift is spatially heterogeneous, making inferences and comparability among studies tricky. The lack of consistent empirical evidence on this potential mismatch and its drivers leads us to focus on this issue in this study. The aim was twofold: (a) to quantify the mismatch between the two processes at a regional scale, and (b) to identify its site-specific determinants. Taxon: Pinus uncinata (Ramond ex DC.)Location: French eastern Pyrenees.Methods: An object-oriented supervised classification procedure was performed on historical (1953) and current (2015) aerial photographs. Based on the resulting rasters, densification of the tree line ecotone and upward shift of the tree line were estimated at the two dates in 191 sites, then standardized, before finally being compared. Three site clusters were derived (no mismatch, densification prevalence and upshift prevalence). After having characterized their spatial patterns through join count statistics, a multinomial logistic regression model was computed to identify the correlates of these clusters among a list of site variables. Results:No spatial pattern among the categories of responses emerges at a local scale, but buffers with no mismatch tend to aggregate at a larger scale. Changes in minimum air temperatures, site elevation, mean slope, slope morphometry and lithology appear as significant drivers of the observed mismatch, implying that the relationship between densification and elevational upshift is context specific. Main conclusions:Our findings suggest that both densification and upshift should be considered in quantitative analyses of tree line spatial dynamics, since these two ecological processes are not controlled by the same drivers. K E Y W O R D S densification, join count statistics, multinomial model, spatial context, tree line, upshift | 1057 FEUILLET ET aL. | 1067 FEUILLET ET aL.

Montane forest expansion at high elevations drives rapid reduction in non‐forest area, despite no change in mean forest elevation

“…The rate of advanced establishment, defined as the change from establishing forest to forest within a change period is shown for the study area as a whole (b), by aspect (d) and by incline (f). Uncertainty in the estimates in panels a and b is shown at the 95% confidence intervals substantial increase in forest area, yet stasis in mean forest elevation reported here, (Figure 3) has also been observed globally including in the Khibiny Mountains, Russia (Mathisen et al, 2014), the Tibetan Plateau (Liang et al, 2011), Glacier National Park, USA (Klasner & Fagre, 2002), the Sudetes mountains, Czech Republic (Treml & Chuman, 2015) and the Pyrenees, France (Feuillet et al, 2020). In Taiwan, our findings are consistent with patterns of forest advance previously identified in field assessments by who identified that forest advance in Taiwan elevation, yet densification below the tree line and increases in the elevation of establishment in localized depressions here ( Figure 3) and elsewhere are occurring despite a lack of change in tree line position (Klasner & Fagre, 2002;Liang et al, 2011;Mathisen et al, 2014;Treml & Chuman, 2015).…”

“…Furthermore, as temperature thresholds are passed at a given elevation, a larger area of habitat is likely to be affected by environmental change when the slope inclination is shallow (Jump, Mátyás, & Peñuelas, 2009). Therefore, in combination with an increased probability of forest establishment on shallow and concave slopes (Feuillet et al, 2020;Greenwood et al, , 2015, environmental changes lead to higher rates of (Greenwood et al, 2015). Consequently, areas that undergo establishment but do not continue to grow sufficiently to be classified as forest may exist in areas where a threshold for establishment has been surpassed, but the necessary conditions for subsequent growth are not met.…”

“…Aerial photography data with 0.25-0.5 m pixel size (typical of many aerial surveys) are of sufficiently high quality to identify tree recruitment at the tree line ecotone and have been used to enable research in areas where historical field datasets are lacking (e.g. Feuillet et al, 2020;Mathisen et al, 2014).…”

“…Harsch et al (2009) reported that 52% of studies published between 1900 and 2008 identified advances in the elevational or latitudinal limit of the tree line. Changes in tree density or growth below the altitudinal limit were not defined as advances in the study by Harsch et al (2009), yet a growing body of evidence reports substantial increases in tree density or range expansion through across-slope movement of montane forests below the elevational limit in response to environmental change (Bharti, Adhikari, & Rawat, 2012;Feuillet et al, 2020;Greenwood, Chen, Chen, & Jump, 2014;Klasner & Fagre, 2002;Liang, Wang, Eckstein, & Luo, 2011;Mathisen, Mikheeva, Tutubalina, Aune, & Hofgaard, 2014). Changes in the tree line elevation only provide partial information on the response of tree line ecotones to environmental change (Feuillet et al, 2020).…”

“…Repeat aerial photography data have been used to identify changes in the maximum elevation or tree density of the tree line ecotone (e.g. Feuillet et al, 2020;Klasner & Fagre, 2002;Luo & Dai, 2013;Mathisen et al, 2014;Resler, Fonstad, & Butler, 2004). However, variation in the methods used to analyse aerial photography data alongside a lack of quantitative estimates of uncertainty in the range shifts reported limits the interpretation of the results, and hinders landscape-scale estimation of changes in forest K E Y W O R D S climate change, densification, forest change, migration, mountain, range edge, Taiwan, tree line distribution in mountain ecosystems (Morley, Donoghue, Chen, & Jump, 2018).…”