Using Plant Functional Traits to Define the Biomass Energy Potential of Invasive Alien Plant Species

Abstract

The eradication of invasive alien plant species (IAPS) is mandatory worldwide, but the resulting biomass is still considered waste. The energy use of biomasses obtained from IAPS eradication may represent ecological and economic benefits, creating synergies with restoration projects. We evaluated whether the growth forms and functional types identified using the functional space of 63 IAPS corresponded to a possible bioenergy use through multivariate analysis techniques. We extracted leaf and nutrient traits and Grime's CSR plant strategies from an existing database. We calculated the carbon-to-nitrogen ratio (C:N) and gross heating value (GHV) as indicators of biochemical or thermal processes, respectively. For 10 species, we measured the above-ground biomass C:N and GHV (including leaves, stems and branches) and correlated them with those of leaves and with plant adaptive strategies. We identified four groups of IAPS indicative of the main trade-offs between plant economics and size variation, which respectively correlated with C:N and GHV. Herbaceous IAPS were better suited to biochemical processes, and woody IAPS to thermal ones. Overall, Grime's CSR strategies were the best tool to define the IAPS bioenergy potential. In the long term, competitive and ruderal IAPSs can represent a reusable feedstock until their complete eradication.

Keywords: Grime’s CSR plant strategies; bioeconomy; biofuels; eradication; global change; global spectrum of plant form and function; plant functional traits; restoration ecology.

Figure 1

Principal component analysis of trait values of the 63 IAPS selected in this study grouped by (a) growth form and (b) bioenergy use types according to the cluster they belong to (see Figure 2). Lines represent the 50th percentile of the distribution. Legend: C:N = carbon-to-nitrogen ratio, GHV = gross heating value, H = plant height, LA = leaf area, LNC = leaf nitrogen content SLA = specific leaf area, SM = seed mass.

Figure 2

Dendrogram resulting from hierarchical clustering on the principal components (PC1-economics and PC2-size) of the invasive alien plant species (IAPS) present in the Lombardy region. Both axes contributed significantly (p < 0.001) to the clustering at k = 4, Eta² = 0.70 and 0.77, respectively. The value of the v.test (v) and its significance (p) in the contributions to each cluster by PC1-economics, PC2-size, and by the indicators of biochemical and thermal processes (C:N and GHV) are also reported in the figure. Accordingly, classes can be grouped into biochemical-acquisitive, thermal-conservative, biochemical-competitive, and thermal-competitive. We indicated the five herbaceous (h) and five woody (w) IAPS with the highest ecological impact in Lombardy selected for the analysis of the aboveground biomass (see Table S1).

Figure 3

Mean values (±standard error × 1.96) of the scores of the two axes of the principal component analysis (PC1-economics and PC2-size) for different growth forms (a,c) and bioenergy use types (b,d). Results of the ANOVA are reported in each subplot; small letters indicate post hoc comparisons (p < 0.05). Legend: B-acq = biochemical acquisitive, B-com = biochemical competitive, T-con = thermal conservative, T-com = thermal competitive.

Figure 4

Ternary visualization of Grime’s competitive, stress-tolerant, ruderal (CSR) plant strategies of the 63 invasive alien plant species (IAPS) selected in this study, grouped by (a) growth form and (b) bioenergy use types according to the cluster they belong to (see Figure 2). Lines represent the 50th percentile of the distribution.

Figure 5

Mean values (±standard error × 1.96) of the competitive (C), stress-tolerant (S), and ruderal (R) strategy scores for different growth forms (a,c,e) and bioenergy use types (b,d,f). Results of the ANOVA are reported in each subplot; small letters indicate pos hoc comparisons (p < 0.05). Legend: B-acq = biochemical acquisitive, B-com = biochemical competitive, T-con = thermal con-servative, T-com = thermal competitive.

Figure 6

Mean values (±standard error × 1.96) of the gross heating value (GHV, above) and carbon-to-nitrogen ratio (C:N, below) scores for different growth forms (a,b), bioenergy use types (c,d), and CSR (competitive, stress-tolerant, ruderal) plant strategy types (e,f). Results of the ANOVA are reported in each subplot; small letters indicate post hoc comparisons (p < 0.05). Legend: B-acq = biochemical acquisitive, B-com = biochemical competitive, T-con = thermal con-servative, T-com = thermal competitive.

Figure 7

Linear regression between aboveground biomass C:N (h–n), GHV (a–g) and PC1-economics (c,j), PC2-size (d,k), C- (e,j), S- (f,m), and R- (g,n) strategy axes, and C:N (a,h) and GHV (b,i) leaf values considering the 10 selected species (all) and grouped by growth forms (herbs and woody). Legend: C:N = carbon-to-nitrogen ratio, GHV = gross heating value, PC1-economics = first principal component, PC2-size = second principal component, C = degree of competitiveness, S = degree of stress tolerance, R = degree of ruderality. The values of the linear regression coefficients are reported: determination coefficient, R², and level of significance, p (ns = not significant, * = p ≤ 0.05, ** = p ≤ 0.01, *** = p ≤ 0.001).