Differences in Fine-Root Biomass of Trees and Understory Vegetation among Stand Types in Subtropical Forests

<div><p>Variation of total fine-root biomass among types of tree stands has previously been attributed to the characteristics of the stand layers. The effects of the understory vegetation on total fine-root biomass are less well studied. We examined the variation of total fine-root biomass in subtropical tree stands at two sites of Datian and Huitong in China. The two sites have similar humid monsoon climate but different soil organic carbon. One examination compared two categories of basal areas (high vs. low basal area) in stands of single species. A second examination compared single-species and mixed stands with comparable basal areas. Low basal area did not correlate with low total fine-root biomass in the single-species stands. The increase in seedling density but decrease in stem density for the low basal area stands at Datian and the quite similar stand structures for the basal-area contrast at Huitong helped in the lack of association between basal area and total fine-root biomass at the two sites, respectively. The mixed stands also did not yield higher total fine-root biomasses. In addition to the lack of niche complementarity between tree species, the differences in stem and seedling densities and the belowground competition between the tree and non-tree species also contributed to the similarity of the total fine-root biomasses in the mixed and single-species stands. Across stand types, the more fertile site Datian yielded higher tree, non-tree and total fine-root biomasses than Huitong. However, the contribution of non-tree fine-root biomass to the total fine-root biomass was higher at Huitong (29.4%) than that at Datian (16.7%). This study suggests that the variation of total fine-root biomass across stand types not only was associated with the characteristics of trees, but also may be highly dependent on the understory layer.</p></div

Structural traits (means±SE) of the stands (n = 5 plots per stand) at Huitong.

<p>The stand types are single-species <i>C</i>. <i>lanceolata</i> with high basal area (High), single-species <i>C</i>. <i>lanceolata</i> with low basal area (Low), and mixed <i>C</i>. <i>lanceolata</i> and <i>M</i>. <i>macclurei</i> (Mixed). Different letters indicate significant differences.</p><p>†No seedling was observed.</p><p>Structural traits (means±SE) of the stands (n = 5 plots per stand) at Huitong.</p

Summary of the stand structure traits between sites (mean±SE).

<p>Data was the mean value across three stand types at each site.</p><p>*Denotes significance of differences between mean values.</p><p>Summary of the stand structure traits between sites (mean±SE).</p

Effects of stand type on the fine root biomass (g m^-2) at 0–40 cm depth (mean±SE).

<p>Each stand type was replicated five times. The stand types are single-species <i>C</i>. <i>lanceolata</i> with high basal area (High), single-species <i>C</i>. <i>lanceolata</i> with low basal area (Low), and mixed <i>C</i>. <i>lanceolata</i> and <i>S</i>. <i>superba</i> / <i>M</i>. <i>macclurei</i> (Mixed). Different letters indicate a significant difference.</p><p>*Denotes significance of differences between mean values.</p><p>Effects of stand type on the fine root biomass (g m^-2) at 0–40 cm depth (mean±SE).</p

The FRBs of (a) tree and (b) non-tree species along the soil profile at the two sites.

<p>Data was the mean value across three stand types at each site. Error bar represents one standard error of the mean. Different upper-case letters denote significant differences among soil depths within sites ((b) Datian: F = 13.126, P<0.001; Huitong: F = 7.869, P = 0.001). Different lower-case letters denote significant differences between sites within soil depths ((a) 0–10 cm: t = 6.420, P<0.001; 10–20 cm: t = 9.782, P<0.001; 20–40 cm: t = 9.176, P<0.001; (b) 10–20 cm: t = 4.462, P<0.001; 20–40 cm: t = 2.265, P<0.031).</p

The FRBs of (a) tree and (b) non-tree species at Datian along the soil profile for five plot replicates per stand and nine sampling locations per plot.

<p>Error bar represents one standard error of the mean. Different upper-case letters denote significant differences among soil depths within stand types ((a) D-mixed: F = 20.629, P<0.001; (b) D-High: F = 9.707, P = 0.003; D-Low: F = 17.259, P<0.001). Different lower-case letters denote significant differences among stand types within soil depths ((a) 0-10cm: F = 9.108, P = 0.004; (b) 0–10 cm: 8.698, P = 0.005).</p

The FRBs of (a) <i>C</i>. <i>lanceolata</i> and <i>S</i>. <i>superba</i> in D-Mixed at Datian and (b) <i>C</i>. <i>lanceolata</i> and <i>M</i>. <i>macclurei</i> in H-Mixed at Huitong along the soil profile for five plot replicates per stand and nine sampling locations per plot.

<p>Error bar represents one standard error of the mean. Different letters indicate significant differences among soil depths ((a) <i>S</i>. <i>superba</i>: F = 11.260, P = 0.002; (b) <i>M</i>. <i>macclurei</i>: F = 4.536, P = 0.034).</p

Structural traits (means±SE) of the stands (n = 5 plots per stand) at Datian.

<p>The stand types are single-species <i>C</i>. <i>lanceolata</i> with high basal area (High), single-species <i>C</i>. <i>lanceolata</i> with low basal area (Low), and mixed <i>C</i>. <i>lanceolata</i> and <i>S</i>. <i>superba</i> (Mixed). Different letters indicate significant differences.</p><p>Structural traits (means±SE) of the stands (n = 5 plots per stand) at Datian.</p

Fine root biomass (g m^-2) at 0–40 cm soil depth between sites (mean±SE).

<p>Data was the mean value across three stand types at each site.</p><p>*Denotes significance of differences between mean values.</p><p>Fine root biomass (g m^-2) at 0–40 cm soil depth between sites (mean±SE).</p

Interaction effects of soil depth and stand type on the FRBs of tree and non-tree species at Datian and Huitong.

<p>*Denotes significance of differences between mean values.</p><p>Interaction effects of soil depth and stand type on the FRBs of tree and non-tree species at Datian and Huitong.</p