A simple non-destructive method for estimating aboveground biomass of emergent aquatic macrophytes

Aim: Non-destructive methods for estimating aquatic macrophytes biomass may be employed by using indirect measurements, especially in experimental studies, thus enabling the conservation of plant samples. It is possible to estimate macrophyte biomass by developing mathematical equations that relate the plants’ dry mass to their morphological variables. The aim of this study was to evaluate the relationship between different morphological variables and biomass in order to determine which variable is easier to be obtained for the emergent aquatic macrophytes Crinum americanum and Spartina alterniflora. Methods: We obtained the aboveground area and height of individuals of both species, with different sizes and distinct developmental stages. The samples were collected in the Itanhaém River Estuary (SP, Brazil). The plants were dried in a laboratory oven and weighed so as to obtain their dry mass. Simple linear regression analyses were applied to the morphological variables and the individual dry mass to obtain equations. Results: For the both species, the relationship between area and biomass, and the relationship between individual height and biomass presented significant coefficients of determination (p < 0.0001). For the elaboration of models involving the individual height, we used only one morphological measure for each individual, whereas for models involving the individual area it was necessary to obtain more than one hundred morphological measurements per individual. Conclusions: The morphological variables chosen are good attributes for estimating the aboveground biomass of C. americanum and S. alterniflora. Considering the models’ adjustment and the consumed time to obtain the measurements, we conclude that the individual height measurement is better for biomass estimation for both species.

Acta Limnologica Brasiliensia, 2017, vol. 29, e2 Due to the diversity of biological forms of aquatic macrophytes, the non-destructive biomass estimation does not follow a standard method (Thomaz & Esteves, 2011).However, as a consistent and low-cost alternative it can estimate biomass by developing mathematical equations that relate dry mass to plant morphological variables (Dai & Wiegert, 1996;Golzarian et al., 2011).Length, width, leaf area, and the number of leaves are among the morphological variables that may be used since the relationship between morphological variables and biomass is species-specific (Armstrong et al., 2003).This methodology allows for continuous monitoring of biomass changes in experiments, the obtainment of detailed data from plant parts or their overall structure, and the optimization of working time when only simple estimates are necessary.However, it may present a disadvantage when the choosing and collecting of numerous plant attributes are needed to achieve significant biomass relationship (Dai & Wiegert, 1996).
The aim of this study was to evaluate the relationship between different morphological variables and biomass to determine which variable is easiest to obtain for two species of emergent macrophytes that abound in estuaries in southeastern Brazil.
Crinum americanum L. (Amaryllidaceae) and Spartina alterniflora Loisel.(Poaceae) are emergent macrophyte species that abound in estuarine regions in the state of São Paulo.They occur in abundance in the Itanhaém River Estuary (São Paulo, Brazil) and are distributed forming a gradient with S. alterniflora in the estuarine lower portion and C. americanum in the upper portion.To understand the causes of such distribution we have performed growth experiments with both To estimate the abundance of aquatic macrophytes in studies of population and community dynamics is essential (Chambers & Prepas, 1990).Biomass quantification enables the realization of studies on the ecosystems and the functioning of aquatic plants (Lauck & Benscoter, 2015), although other metrics, such as cover area, relative abundance scales, and primary productivity can also be used to quantify aquatic plants (Thomaz & Esteves, 2011).
Destructive methods for assessing macrophyte biomass involve cutting and collecting plants, which generate undesirable consequences for natural environments and difficulties in experimental studies since they limit the number and frequency of samplings (Dai & Wiegert, 1996;Thomaz et al., 2004;Lauck & Benscoter, 2015).To reduce the negative effects of destructive methods, it is possible to estimate biomass or macrophyte abundance through indirect measurements, allowing for wider sampling and environmental conservation in studies that require less impact (Gouraud et al., 2008;Silva et al., 2010).Remote sensing-based techniques are being developed as a non-destructive method to estimate macrophyte abundance and to map its large-scale spatial distribution in the field (Long et al., 1994;Byrd et al., 2014;Kim et al., 2016), however, this technology cannot be employed in small-scale and experimental studies.In this case, scanner and laser equipment coupled to digital cameras have been used to scan plant leaves to obtain the leaf area and biomass estimation (Golzarian et al., 2011).Nonetheless, this technique involves high operational costs, imported equipment, and expensive instruments (Rios et al., 2014).
Palavras-chave: massa seca; atributos vegetais; medidas morfológicas; Crinum americanum; Spartina alterniflora.species in the laboratory.To evaluate growth we used a non-destructive method of biomass evaluation.To elaborate the biomass evaluation method, a sample of 70 individuals of each species, in different sizes and development stages, were collected in the Itanhaém River Estuary.
Most aquatic macrophyte species, especially the emergent ones, present clonal development, making the individuals' identification difficult (Thomaz & Esteves, 2011).Although C. americanum and S. alterniflora are rhizomatous species, the growth form of their shoots and roots enables the identification of individuals, because the leaves of C. americanum are distributed in rosettes (Figure 1) and S. alterniflora forms stems (Figure 2).Thus, we assessed morphological measurements of individuals to relate them to their biomass.
The morphological measurements of the aboveground fraction were: leaf area and height of an individual (height of the largest leaf for C. americanum and culm height for S. alterniflora), considering only the green leaves (50% or more of chlorophyllated leaf ) and disregarding the senescent leaves (less than 50% of chlorophyllated leaf ) and the dead leaves (100% of non-chlorophyllated leaf ).
To calculate the aboveground area of C. americanum we considered that its leaves have a rectangular base and a triangular apex, so we obtained the rectangle's height and width measurements and the triangle's height.Using    Although all the equations have significant coefficient of determination, the time to obtain the value of the aboveground area is much greater than that for obtaining the individual height.Each individual of C. americanum has between 3 and 4 green leaves and each individual of S. alterniflora has between 2 and 3 green leaves.To estimate the species' biomass using the aboveground area (models 1 A and 2 A), 9 to 12 measurements are required for each individual of C. americanum and 7 to 11 measurements for each individual of S. alterniflora.This amount is reduced to 1 measurement per individual when the individual height is used as a morphological measurement to estimate their biomass (models 1 B and 2 B).For example, to estimate the biomass of a total of 10 C. americanum individuals and 10 S. alterniflora individuals with 4 leaves each, 120 morphological measurements are necessary for C. americanum and 110 for S. alterniflora if models 1 A and 2 A, respectively, are adopted and only 10 measurements to 10 individuals if models 1 B and 2 B are used.Therefore, the sampling time and the number of measurements of morphological data are reduced by about 90% for each species using the individual height to estimate the biomass.
The non-destructive models developed in this study showed significant adjustment, indicating that the chosen morphological variables are good attributes to estimate the aboveground biomass of C. americanum and S. alterniflora.However, considering the adjustment of the tested models and the consumed time, the use of only the individual height showed good relation to the biomass of these two emergent long leaf species.It is also possible to test which morphological variables can be used to estimate the biomass of macrophytes with other leaf shapes.
Therefore, we recommend that previously to the development of macrophyte growth assessments, several morphological measurements be used so the simpler one can be chosen.Thus, this method allows us to develop an experimental design with more replications and treatments, obtain consistent results and reduce the working time and cost.

Figure 4 .
Figure 4. Simple linear regression between the individual aboveground area and aboveground dry mass (A), simple linear regression between individual height and aboveground dry mass (B) of Spartina alterniflora, and linear equation and coefficient of determination (R 2 ).

Figure 3 .
Figure 3. Simple linear regression between the individual aboveground area and aboveground dry mass (A), simple linear regression between the individual height and aboveground dry mass (B) of Crinum americanum, and linear equation and coefficient of determination (R 2 ).