Tree‐ring analysis and modeling approaches yield contrary response of circumboreal forest productivity to climate change

Abstract

Circumboreal forest ecosystems are exposed to a larger magnitude of warming in comparison with the global average, as a result of warming-induced environmental changes. However, it is not clear how tree growth in these ecosystems responds to these changes. In this study, we investigated the sensitivity of forest productivity to climate change using ring width indices (RWI) from a tree-ring width dataset accessed from the International Tree-Ring Data Bank and gridded climate datasets from the Climate Research Unit. A negative relationship of RWI with summer temperature and recent reductions in RWI were typically observed in continental dry regions, such as inner Alaska and Canada, southern Europe, and the southern part of eastern Siberia. We then developed a multiple regression model with regional meteorological parameters to predict RWI, and then applied to these models to predict how tree growth will respond to twenty-first-century climate change (RCP8.5 scenario). The projections showed a spatial variation and future continuous reduction in tree growth in those continental dry regions. The spatial variation, however, could not be reproduced by a dynamic global vegetation model (DGVM). The DGVM projected a generally positive trend in future tree growth all over the circumboreal region. These results indicate that DGVMs may overestimate future wood net primary productivity (NPP) in continental dry regions such as these; this seems to be common feature of current DGVMs. DGVMs should be able to express the negative effect of warming on tree growth, so that they simulate the observed recent reduction in tree growth in continental dry regions.

Tree rings provide no evidence of a CO2 fertilization effect in old‐growth subalpine forests of western Canada

“…We found that the growth trends derived from tree rings in our study were substantially different from the trends simulated by seven CMIP5 models, which also were not unanimous in their output. The consistent decreasing growth observed in all four of our old‐growth forests, compared to increasing growth predictions among most models, reinforces Williams’ () supposition and implies some important processes are inadequately represented in the models, such as the effects of nutrient limitations on growth and the inability of trees to acclimate to increasing water stress (Anav et al, ; Tei et al, ; Williams, ). Incorporating tree‐ring data into model development and parameterization could help reduce model uncertainty about forest growth responses to increasing atmospheric CO 2 .…”

“…The mixed model residuals at our sites were positively related to temperature (Supporting Information Table S3), which might indicate that tree productivity in these old‐growth forests is temperature limited. Soil moisture deficits can constrain growth in cold regions, despite rising temperatures and CO 2 concentrations (Girardin et al, ; McDowell et al, ; Tei et al, ; Williams, ), and there is evidence of growth declines related to water stress in subalpine forests (Carrer & Urbinati, ; Reed et al, ). Moisture deficits are possible at our sites, but they are unlikely, because mean summer precipitation (MSP) had increased in the 20th century at all sites except Mt.…”

Time lag and negative responses of forest greenness and tree growth to warming over circumboreal forests

“…Comparisons between such indices are often conducted at local or regional spatial scales (Beck et al., ; Berner, Beck, Bunn, Lloyd, & Goetz, ) but rarely at a wider spatial scale. A recent synthetic analysis of the RWI data confirmed the widespread negative sensitivity of forest ecosystem productivity to warming over the circumboreal forests (Tei, Sugimoto, Yonenobu, et al., ). This is particularly prevalent in the continental dry regions, such as interior Alaska and Canada, the southern part of Europe, and the central part of Eastern Siberia.…”

“…Dendroecological analysis is useful for studying the past response of a forest ecosystem to a changing environment (e.g., Nikolaev, Fedorov, & Desyatkin, ; Sidorova et al., ; Tei, Sugimoto, Liang, et al., ; Tei, Sugimoto, Yonenobu, et al., ). Tree‐ring width indices (RWI) are regarded widely as useful long‐term indicators of past forest productivity because of the frequently observed close relationship of the RWI with the forest‐level gross primary production (GPP) and/or net ecosystem exchange (NEE) (e.g., Xu et al., ).…”

“…In addition, broad‐scale satellite image (e.g., Buermann et al., ) and tree‐ring analyses (e.g., Tei, Sugimoto, Yonenobu, et al., ) have focused generally on vegetation activity response to climate change only in the summer season. However, it has been reported recently that climate variables other than the summer season significantly affected vegetation activities over the northern high latitude region.…”