Ecological Archives E094-067-A1

Kevin E. Mueller, David Tilman, Dario A. Fornara, Sarah E. Hobbie. 2013. Root depth distribution and the diversity–productivity relationship in a long-term grassland experiment. Ecology 94:787–793. http://dx.doi.org/10.1890/12-1399.1

Appendix A. Below we report the results from statistical models of root biomass (Table A1), the deep-root proportion (Table A2), and the difference between observed and expected deep-root proportions (Table A4). The deep-root proportions for monocultures of each focal species are given in Table A3. Figure A1 shows the median root biomass in each depth increment for each level of planted species richness. Figure A2 shows the correlations of deep-root proportion and the difference between observed and expected deep-root proportion with the abundance of legumes and C4 grasses (for plots planted with 16 species).

Table A1. Effects of plant functional composition and species richness on root biomass in two depth increments.

 

ROOT BIOMASS 0–30 cm

ROOT BIOMASS 30–100 cm

 

without no. of sp. as covariate

with no. of sp. as covariate

without no. of sp. as covariate

with no. of sp. as covariate

Factor

% SS*

P

Sign#

% SS*

P

Sign#

% SS*

P

Sign#

% SS*

P

Sign#

C3

4

0.0002

+

1

0.15

 

0

0.91

 

2

0.01

-

C4

17

<.0001

+

10

<.0001

+

4

0.0007

+

1

0.10

+

Forb

0

0.33

 

0

0.27

 

0

0.52

 

1

0.05

-

Legume

9

<.0001

+

3

0.0007

+

26

<.0001

+

10

<.0001

+

C3 × C4

0

0.69

 

0

0.36

 

0

0.65

 

0

0.85

 

C3 × Forb

1

0.17

 

0

0.75

 

0

0.29

 

0

0.76

 

C3 × Legume

0

0.23

 

1

0.05

+

1

0.22

 

0

0.78

 

C4 × Forb

1

0.17

 

1

0.04

-

0

0.80

 

0

0.24

 

C4 × Legume

2

0.01

+

1

0.05

-

1

0.11

 

0

0.42

 

Forb × Legume

0

0.44

 

1

0.12

 

0

0.76

 

0

0.47

 

no. of species

na

na

na

3

0.0008

+

na

na

na

7

<.0001

+

model R²

0.60

0.63

0.47

0.54

n

152

152

152

152

*The % of total sums-of-squares (SS) for root biomass that can be uniquely attributed to each predictor variable.

#The sign (i.e., direction) of the effect for each model term is based on the sign of the model coefficient for that term.

Number of species was modeled as a linear effect.


Table A2. Effects of plant functional composition and species richness on the deep root proportion. Similar results were obtained for models that excluded monoculture plots with two exceptions: the C3 × Legume interaction was not significant and C3 grass presence was not significant when species richness was not a covariate. The effect of species richness remained significant (P < 0.001) and positive when legume abundance was included in the model instead of legume presence.

 

without no. of sp. as covariate

with no. of sp. as covariate

Factor

% SS*

P

Sign#

% SS*

P

Sign#

C3

3

0.006

-

7

<.0001

-

C4

0

0.78

 

0

0.30

 

Forb

0

0.84

 

1

0.08

-

Legume

28

<.0001

+

12

<.0001

+

C3 × C4

3

0.01

+

2

0.04

+

C3 × Forb

0

0.58

 

0

0.62

 

C3 × Legume

4

0.004

+

2

0.03

+

C4 × Forb

0

0.74

 

0

0.73

 

C4 × Legume

0

1.00

 

0

0.50

 

Forb × Legume

0

0.89

 

0

0.51

 

no. of species†

na

na

 

4

0.0009

+

model R²

0.42

0.47

n

152

152

*The percent of total sums-of-squares (SS) for deep root proportion that can be uniquely attributed to each predictor variable, using Type III sums-of-squares as described in the Methods.

#The sign (i.e., direction) of the effect for each model term is based on the sign of the model coefficient for that term.

Number of species was modeled as a linear effect. Results were similar when number of species was modeled as a logarithmic effect and as a discrete parameter.

 

Table A3. Proportion of root biomass more than 30 cm below the soil surface (% deep roots) for thirteen species, or groups of similar species, used in estimating expected root depth distributions and functional diversity of root depth distributions.


Species

Functional group

Mean
% deep roots

Median
% deep roots

n

SD

Achillea millefolium

forb

9

na

1

na

Amorpha canescens / Petalostemum spp.*

legume

29

na

2

25

Andropogon gerardi

C4 grass

10

na

1

na

Koeleria cristata

C3 grass

1

na

2

0.1

Lespedeza capitata

legume

30

27

3

9

Liatris aspera

forb

20

15

3

22

Lupinus perennis

legume

31

na

1

na

Monarda fistulosa / Solidago rigida

forb

21

na

2

13

Panicum virgatum

C4 grass

19

na

2

12

Petalostemum purpureum

legume

25

24

3

5

Poa pratensis

C3 grass

1

na

1

na

Schizachyrium scoparium

C4 grass

7

4

3

7

Sorghastrum nutans

C4 grass

16

17

3

4

*Some plots that were intended to be seeded with Petalostemum spp. were incidentally seeded with Amorpha canescens instead (www.cedarcreek.umn.edu).

Solidago rigida did not germinate well the first year, so in the second year, Monarda fistulosa was seeded in those plots (Fargione et al. 2007).

 

Table A4. Effects of plant functional composition and species richness on the difference between the observed and expected fraction of total root biomass below 30 cm. Similar results were obtained when monoculture plots were excluded from the analyses. The effect of species richness remained significant (P < 0.001) and positive when legume abundance was included in the model instead of legume presence.

 

without no. of sp. as covariate

with no. of sp. (linear)

with no. of sp. (discrete)

Factor

% SS*

P

Sign#

% SS*

P

Sign#

% SS*

P

Sign#

C3

0

0.81

 

2

0.03

-

4

0.005

-

C4

6

0.001

+

2

0.07

+

0

0.52

 

Forb

0

0.80

 

3

0.02

-

4

0.004

-

Legume

7

0.0008

+

1

0.21

 

0

0.86

 

C3 × C4

0

0.99

 

0

0.64

 

0

0.67

 

C3 × Forb

1

0.26

 

0

0.93

 

0

0.77

 

C3 × Legume

2

0.07

+

1

0.27

 

1

0.14

 

C4 × Forb

2

0.09

+

1

0.27

 

1

0.13

 

C4 × Legume

9

<.0001

+

6

0.001

+

7

0.0003

+

Forb × Legume

0

0.99

 

0

0.41

 

0

0.63

 

no. of species

na

na

na

7

0.0003

+

10

0.001

na

model R²

0.29

0.36

0.39

n

137

137

137

*The percent of total sums-of-squares (SS) of the dependent variable that can be uniquely attributed to each predictor variable, using Type III sums-of-squares as described in the Methods.

#The sign (i.e., direction) of the effect for each model term is based on the sign of the model coefficient for that term.

FigA1

Fig. A1. Effects of species richness on root biomass. Panel A shows the median root biomass for each depth increment and species richness level. Panel B shows the relative effect of species richness on root biomass in each depth increment (i.e., the median root biomass of each richness level divided by that of monocultures). Note the median root biomass in the 30–60 and 60–100 cm depth increments of 16-species-plots is ~7 times that of monocultures.


 

FigA2

Fig. A2. Correlations of the deep-root proportion and the difference between observed and expected root depth distributions with the proportion of aboveground biomass attributable to legumes and C4 grasses in plots planted with 16 species (n = 35). All correlations have P < 0.01. In these diverse plots, the relative abundances of legumes and C4 grasses were inversely correlated (R² = 0.49, P < 0.001). The relative abundance of the legume Lupinus perennis was also negatively correlated with the difference between observed and expected root depth distributions (n = 33, R² = 0.47, P < 0.001). To further evaluate these relationships, we included legume and C4 grass relative abundance along with species richness in a model of the differences between observed and expected root depth distributions (monocultures and two-species plots were excluded because these plots often have a value of zero for the relative abundance of different functional groups). The relative abundance of C4 grasses was positively correlated with the difference between observed and expected root depth distributions (P = 0.01), especially when legume abundance was high (P < 0.01 for the interaction between C4 grass and legume relative abundance). The effect of C4 grass abundance did not depend on species richness (P = 0.25 for the interaction between C4 grass abundance and species richness) but the negative effect of legume relative abundance was only apparent at high species richness (P<0.05 for the interaction between legume abundance and species richness).

Literature cited

Fargione, J., D. Tilman, R. Dybzinski, J. HilleRisLambers, C. Clark, W. S. Harpole, J. M. H. Knops, P. B. Reich, and M. Loreau. 2007. From selection to complementarity: shifts in the causes of biodiversity-productivity relationships in a long-term biodiversity experiment. Proceedings of the Royal Society B-Biological Sciences 274:871–876.


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