Regional drought-induced reduction in the biomass carbon sink of Canada's boreal forests

Ma, Zhuguo; Peng, Changhui; Zhu, Qiuan; Chen, Huai; Yu, Guirui; Li, Weizhong; Zhou, Xiaolu; Wang, Weifeng; Zhang, Wenhua

doi:10.1073/pnas.1111576109

Cited by 255 publications

(242 citation statements)

References 28 publications

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“…S9), and they should follow similar trends to those of MAP. Thus, we found no clear evidence about droughtinduced tree mortality in western Canada, although droughtinduced tree mortality was detected in recent studies in the region based on short-term sampling plots and relatively small datasets (25,(29)(30)(31). We argue that how drought affects tree mortality in forests of high latitudes remains an open question and more studies are needed.…”

Section: Significancecontrasting

confidence: 41%

Half-century evidence from western Canada shows forest dynamics are primarily driven by competition followed by climate

Zhang

Huang

2015

Proc. Natl. Acad. Sci. U.S.A.

191

168

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Tree mortality, growth, and recruitment are essential components of forest dynamics and resiliency, for which there is great concern as climate change progresses at high latitudes. Tree mortality has been observed to increase over the past decades in many regions, but the causes of this increase are not well understood, and we know even less about long-term changes in growth and recruitment rates. Using a dataset of long-term (1958-2009) observations on 1,680 permanent sample plots from undisturbed natural forests in western Canada, we found that tree demographic rates have changed markedly over the last five decades. We observed a widespread, significant increase in tree mortality, a significant decrease in tree growth, and a similar but weaker trend of decreasing recruitment. However, these changes varied widely across tree size, forest age, ecozones, and species. We found that competition was the primary factor causing the long-term changes in tree mortality, growth, and recruitment. Regional climate had a weaker yet still significant effect on tree mortality, but little effect on tree growth and recruitment. This finding suggests that internal communitylevel processes-more so than external climatic factors-are driving forest dynamics.boreal forest | climate change | forest dynamics | tree demographic rates | tree competition F orests provide fundamental ecosystem services for sustaining the global environment, such as storing carbon and maintaining biodiversity. These services, however, are at risk for decline as evidence has increasingly shown that forests in many parts of the world are undergoing rapid changes (1-4). Climate at the regional or global scale is often presumed to be responsible for these changes (5-14), with surprisingly little attention being paid to the possible effects of endogenous processes despite the fact that competition is often an important force driving stand dynamics and succession (15-18). How climate change and competition interplay to affect the long-term change of demographic rates and what are their relative contributions to the change are unanswered questions (19,20).We addressed these questions by compiling data from 1,680 permanent sample plots (PSPs) that are located in undisturbed natural forests across western Canada (Fig. 1). The trees in these plots, which cover a wide geographic region spanning 32°of longitude and 10°of latitude primarily in the boreal zone, were censused over a period from 1958 to 2009 (Fig. 1). Within each plot, all standing trees with diameter at breast height (DBH) ≥ 9 cm were tagged, recorded, and remeasured at irregular time intervals (mean = 10 y) (SI Appendix, Fig. S1). Plot sizes ranged from 0.04 ha to 0.81 ha (mean = 0.14 ha). To reduce possible impact of plot sizes on our analyses, only the plots with at least 50 trees at their first census were selected. The plots have been censused three to eight times (mean of four times). In total, these plots contained 320,878 living trees over the study period (SI Appendix, Table S1).We analyzed the ...

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Section: Significancecontrasting

confidence: 41%

Half-century evidence from western Canada shows forest dynamics are primarily driven by competition followed by climate

Zhang

Huang

2015

Proc. Natl. Acad. Sci. U.S.A.

191

168

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“…Using Maximum Covariance Analysis to isolate the coupled trends in winter snowpack (maximum SWE), soil moisture availability (scPDSI) and summer NDVI in North America has shown that the temporal pattern of shared trends is strongly consistent with the temporal trajectory of regional-scale declines in NDVI, tree-ring growth, forest biomass and modeled productivity previously documented in the boreal region (e.g., [15,18,35,70,91]). Furthermore, the spatial patterns of the coupled trends suggest contrasting vegetation responses to declining snowpack accumulation in the Arctic shrublands and boreal forests since the early 1990s.…”

Section: Drivers Of Vegetation Greening and Browning Trendsmentioning

confidence: 91%

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

et al. 2014

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Recent warming has stimulated the productivity of boreal and Arctic vegetation by reducing temperature limitations. However, several studies have hypothesized that warming may have also increased moisture limitations because of intensified summer drought severity. Establishing the connections between warming and drought stress has been difficult because soil moisture observations are scarce. Here we use recently developed gridded datasets of moisture variability to investigate the links between warming and changes in available soil moisture and summer vegetation photosynthetic activity at northern latitudes (>45 • N) based on the Normalized Difference Vegetation Index (NDVI) since 1982. Moisture and temperature exert a significant influence on the interannual variability of Remote Sens. 2014, 6 1391 summer NDVI over about 29% (mean r 2 = 0.29 ± 0.16) and 43% (mean r 2 = 0.25 ± 0.12) of the northern vegetated land, respectively. Rapid summer warming since the late 1980s (∼0.7 • C) has increased evapotranspiration demand and consequently summer drought severity, but contrary to earlier suggestions it has not changed the dominant climate controls of NDVI over time. Furthermore, changes in snow dynamics (accumulation and melting) appear to be more important than increased evaporative demand in controlling changes in summer soil moisture availability and NDVI in moisture-sensitive regions of the boreal forest. In boreal North America, forest NDVI declines are more consistent with reduced snowpack rather than with temperature-induced increases in evaporative demand as suggested in earlier studies. Moreover, summer NDVI variability over about 28% of the northern vegetated land is not significantly associated with moisture or temperature variability, yet most of this land shows increasing NDVI trends. These results suggest that changes in snow accumulation and melt, together with other possibly non-climatic factors are likely to play a significant role in modulating regional ecosystem responses to the projected warming and increase in evapotranspiration demand during the coming decades.

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“…Adding to this uncertainty is the discrepancy over recent changes in the productivity of boreal and other northern latitude forests. Some empirical evidence suggests increases in the forest productivity (12)(13)(14), whereas other studies suggest decreasing productivity over the last decades (7,8,(15)(16)(17). Furthermore, inversion and process-based ecosystem models indicate large carbon sinks (7,8), whereas field-based bottom-up approaches suggest smaller carbon sinks or small carbon sources (3, 18), or large sinks (19).…”

mentioning

confidence: 95%

No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO ₂ fertilization

Girardin

Bouriaud

Hogg

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

260

246

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Considerable evidence exists that current global temperatures are higher than at any time during the past millennium. However, the long-term impacts of rising temperatures and associated shifts in the hydrological cycle on the productivity of ecosystems remain poorly understood for mid to high northern latitudes. Here, we quantify species-specific spatiotemporal variability in terrestrial aboveground biomass stem growth across Canada's boreal forests from 1950 to the present. We use 873 newly developed tree-ring chronologies from Canada's National Forest Inventory, representing an unprecedented degree of sampling standardization for a largescale dendrochronological study. We find significant regional-and species-related trends in growth, but the positive and negative trends compensate each other to yield no strong overall trend in forest growth when averaged across the Canadian boreal forest. The spatial patterns of growth trends identified in our analysis were to some extent coherent with trends estimated by remote sensing, but there are wide areas where remote-sensing information did not match the forest growth trends. Quantifications of tree growth variability as a function of climate factors and atmospheric CO 2 concentration reveal strong negative temperature and positive moisture controls on spatial patterns of tree growth rates, emphasizing the ecological sensitivity to regime shifts in the hydrological cycle. An enhanced dependence of forest growth on soil moisture during the late-20th century coincides with a rapid rise in summer temperatures and occurs despite potential compensating effects from increased atmospheric CO 2 concentration. drought impacts | climate change | dendrochronology | normalized difference vegetation index | ecology C ircumpolar boreal forests are estimated to store ∼53.9 Pg of carbon or ∼14% of terrestrial vegetation biomass (1). These regions are currently experiencing accelerated changes, including warmer and longer growing seasons, tree line expansion, species migration, increased frequency and severity of drought, and increases in the frequency and severity of disturbances (2-10). These changes create uncertainty about the boreal forests' future role in the global carbon cycle (11). Adding to this uncertainty is the discrepancy over recent changes in the productivity of boreal and other northern latitude forests. Some empirical evidence suggests increases in the forest productivity (12)(13)(14), whereas other studies suggest decreasing productivity over the last decades (7,8,(15)(16)(17). Furthermore, inversion and process-based ecosystem models indicate large carbon sinks (7,8), whereas field-based bottom-up approaches suggest smaller carbon sinks or small carbon sources (3, 18), or large sinks (19). Quantifying

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Regional drought-induced reduction in the biomass carbon sink of Canada's boreal forests

Cited by 255 publications

References 28 publications

Half-century evidence from western Canada shows forest dynamics are primarily driven by competition followed by climate

Half-century evidence from western Canada shows forest dynamics are primarily driven by competition followed by climate

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO ₂ fertilization

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Regional drought-induced reduction in the biomass carbon sink of Canada's boreal forests

Cited by 255 publications

References 28 publications

Half-century evidence from western Canada shows forest dynamics are primarily driven by competition followed by climate

Half-century evidence from western Canada shows forest dynamics are primarily driven by competition followed by climate

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO 2 fertilization

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No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO ₂ fertilization