Tree Species Traits Influence Soil Physical, Chemical, and Biological Properties in High Elevation Forests

BACKGROUND: Previous studies have shown that plants often have species-specific effects on soil properties. In high elevation forests in the Southern Rocky Mountains, North America, areas that are dominated by a single tree species are often adjacent to areas dominated by another tree species. Here, we assessed soil properties beneath adjacent stands of trembling aspen, lodgepole pine, and Engelmann spruce, which are dominant tree species in this region and are distributed widely in North America. We hypothesized that soil properties would differ among stands dominated by different tree species and expected that aspen stands would have higher soil temperatures due to their open structure, which, combined with higher quality litter, would result in increased soil respiration rates, nitrogen availability, and microbial biomass, and differences in soil faunal community composition. METHODOLOGY/PRINCIPAL FINDINGS: We assessed soil physical, chemical, and biological properties at four sites where stands of aspen, pine, and spruce occurred in close proximity to one-another in the San Juan Mountains, Colorado. Leaf litter quality differed among the tree species, with the highest nitrogen (N) concentration and lowest lignin:N in aspen litter. Nitrogen concentration was similar in pine and spruce litter, but lignin:N was highest in pine litter. Soil temperature and moisture were highest in aspen stands, which, in combination with higher litter quality, probably contributed to faster soil respiration rates from stands of aspen. Soil carbon and N content, ammonium concentration, and microbial biomass did not differ among tree species, but nitrate concentration was highest in aspen soil and lowest in spruce soil. In addition, soil fungal, bacterial, and nematode community composition and rotifer, collembolan, and mesostigmatid mite abundance differed among the tree species, while the total abundance of nematodes, tardigrades, oribatid mites, and prostigmatid mites did not. CONCLUSIONS/SIGNIFICANCE: Although some soil characteristics were unaffected by tree species identity, our results clearly demonstrate that these dominant tree species are associated with soils that differ in several physical, chemical, and biotic properties. Ongoing environmental changes in this region, e.g. changes in fire regime, frequency of insect outbreaks, changes in precipitation patterns and snowpack, and land-use change, may alter the relative abundance of these tree species over coming decades, which in turn will likely alter the soils

Abundance of nematode feeding groups in stands of three tree species (mean±SE) and <i>F</i> and <i>P</i> values from ANOVA.

<p>Abundance of nematode feeding groups in stands of three tree species (mean±SE) and <i>F</i> and <i>P</i> values from ANOVA.</p

Microbial biomass and community composition in soil from stands of three tree species.

<p>Microbial biomass (A) C and (B) N, (C) fungal to bacterial rRNA gene ratio, and (D) fungal and (E) bacterial community composition (based on NMS ordination scores) in stands of each species. Values are means±SE and letters denote significant differences (<i>P</i> = 0.05) among tree species (upper and lower case letters refer to NMS axis 2 and 3, respectively, for bacterial community composition).</p

Relative abundance of nematode families in soil from stands of three tree species.

<p>Relative abundance of nematode families in order of mean abundance in stands of (A) aspen, (B) pine, and (C) spruce. Letters after each family denote feeding group: PP, plant parasite; RA, root associate, BF, bacterial feeder; FF, fungal feeder; OM, omnivore; and PR, predator. Values are means±SE.</p

ANOVA output of the effect of tree species and block on soil properties and animal abundances.

*<p>Based on ordination scores for the three axes that explained the most variance in the NMS analyses.</p

Leaf litter quality and root biomass of tree species (mean±SE) and <i>F</i> and <i>P</i> values from ANOVA.

*<p>Fine roots were<1 mm diameter; coarse roots were >1 mm diameter.</p>§<p>Not applicable since there was only one litter sample per stand.</p>†<p>Different letters denote significant differences among tree species (<i>P</i> = 0.05).</p

Mesofaunal abundance in soil from stands of three tree species.

<p>Abundance of (A) collembola, and (B) mesostigmatid, (C) oribatid, and (D) prostigmatid mites in stands of aspen, pine, and spruce. Values are means±SE and letters denote significant differences among tree species (<i>P</i> = 0.05).</p

Soil physical and chemical properties and respiration rates in stands of three tree species.

<p>Soil (A) temperature, (B) moisture, (C) pH, and (D) respiration rates in stands of each species. Values are means±SE and letters denote significant differences among tree species (<i>P</i> = 0.05).</p

Soil carbon and nitrogen concentrations in stands of three tree species.

<p>Total soil (A) C, (B) N and (C and D) mineral N in stands of each species. Values are means±SE and letters denote significant differences among tree species (<i>P</i> = 0.05).</p