Growth Decline Linked to Warming-Induced Water Limitation in Hemi-Boreal Forests

<div><p>Hemi-boreal forests, which make up the transition from temperate deciduous forests to boreal forests in southern Siberia, have experienced significant warming without any accompanying increase in precipitation during the last 80 years. This climatic change could have a profound impact on tree growth and on the stability of forest ecosystems in this region, but at present evidence for these impacts is lacking. In this study, we report a recent dramatic decline in the growth of hemi-boreal forests, based on ring width measurements from three dominant tree-species (<em>Pinus sylvestris, Larix sibirica</em> and <em>Larix gmelinii</em>), sampled from eight sites in the region. We found that regional tree growth has become increasingly limited by low soil water content in the pre- and early-growing season (from October of the previous year to July of the current year) over the past 80 years. A warming-induced reduction in soil water content has also increased the climate sensitivity of these three tree species. Beginning in the mid-1980s, a clear decline in growth is evident for both the pine forests and the larch forests, although there are increasing trends in the proxy of soil water use efficiencies. Our findings are consistent with those from other parts of the world and provide valuable insights into the regional carbon cycle and vegetation dynamics, and should be useful for devising adaptive forest management strategies.</p> </div

Evolution of shared growth variability explained by the first principal component (PC1) and relationships between PC1 and climate.

<p>Evolution of shared growth variability (estimated by the PC1) since 1928 using subintervals of 20 years with a 5-year lag was shown in (A) with a linear fit (black line, <i>y</i> = 0.53<i>x</i>+38.21, <i>r</i>² = 0.38, <i>p = </i>0.032). Simple correlations performed between the first principal component (PC1) and monthly temperature during 1928–2006 (B) and PDSI during 1937–2005 (C). The dotted lines in (B) indicate the 95% confidence intervals. Dark grey bars in (C) are statistically significant (<i>p</i><0.05). The relationship between the regional BAI series and the average PDSI values through October of prior year to July of current year during 1937–2005 was shown in (D) with a linear fit (black line, <i>y</i> = 0.17<i>x</i>+3.45, <i>r</i>² = 0.25, <i>p</i><0.001).</p

Basal area increments versus growing season average normalized difference vegetation index (NDVI) during 1982–2006 for <i>P. sylvestris</i> (red circle), <i>L. sibirica</i> (brown cross) and <i>L. gmelinii</i> (blue square).

<p>The lines are linear fits of the relationship between basal area increments and NDVI for <i>P. sylvestris</i> (<i>y</i> = 0.38+0.013<i>x</i>, <i>r</i>² = 0.33, <i>p</i><0.001), <i>L. sibirica</i> (<i>y</i> = 0.44+0.016<i>x</i>, <i>r</i>² = 0.30, <i>p</i><0.001) and <i>L. gmelinii</i> (<i>y</i> = 0.44+0.017<i>x</i>, <i>r</i>² = 0.32, <i>p</i><0.01).</p

Geographical position (A) and climate diagram (B) of the study region.

<p>The sample sites of <i>Larix sibirica</i>, <i>Pinus sylvestris</i> and <i>Larix gmelinii</i> are indicated by black crosses, black diamonds and black asterisks, respectively. The approximate location of the selected transect (as indicated by the italic rectangle) and schematic distribution (according to the Forests of the USSR Map, 1990) of the three species (<i>L. sibirica</i>, <i>L. gmelinii</i> and <i>P. sylvestris</i>, as indicated by red, brown and yellow small patches, respectively) in and around the transect are shown in the inlet figure in (A). The mean monthly temperature (line) and total monthly precipitation (bars) during 1928–2006 are shown in (B) with data derived from dataset CRU TS 3.0 (<a href="http://www.cru.uea.ac.uk/" target="_blank">http://www.cru.uea.ac.uk/</a>).</p

Basal area increments (blue lines), tree ring indices (red lines) and the anomalies of tree ring indices (bars) for <i>P. sylvestris</i> (A and B), <i>L. sibirica</i> (C and D) and <i>L. gmelinii</i> (E and F).

<p>Bold lines in the figure are the fitted cubic smoothing splines for basal area increments and tree ring indices.</p

Correlations between the standard chronology and temperature (light grey bars), precipitation (dark grey bars), and PDSI (black bars) during 1937–2005 regarding <i>P. sylvestris</i> (A), <i>L. sibirica</i> (B) and <i>L. gmelinii</i> (C).

<p>The dotted lines indicate the 95% confidence intervals.</p

Changes of PDSI and soil water limitation in this region.

<p>Average PDSI values through October of prior year to July of current year during 1937–2005 (thin grey line) with a cubic smoothing spline (bold grey line) and a linear fit (black line, <i>y</i> = −0.066<i>x</i>+129.1, <i>p</i><0.001) are shown in (A). Modeled regional soil water limitation through October of prior year to July of current year (SWL_OCT-Jul) during 1928–2006 (light grey line) with the cubic smoothing spline (bold grey line) and a linear fit (black line, <i>y</i> = −0.94<i>x</i>+1823.7, <i>p</i><0.01) are shown in (B).</p