Can leaf area index and biomass be estimated from Braun‐Blanquet cover scores in tropical forests?

Abstract

Questions: The loss and degradation of tropical forests is having severe impacts on the dynamics of understorey plant communities. Understanding these impacts requires efficient ways to measure vegetation change over broad spatial and temporal scales. Leaf area index (LAI) and above-ground biomass are preferred quantitative measures of variation in plant community structure. However, their accurate measurement requires destructive sampling, which can be impractical or inappropriate. Here we test whether semi-quantitative assessment of Braun-Blanquet vegetation cover scores is a reliable proxy for direct quantitative measures of LAI and above-ground biomass of differing plant growth forms (PGF) within tropical forests.Location: Six hundred square kilometre area of tropical lowland rain forest in Sabah, Malaysia.Methods: We sampled understorey rain forest plant communities across a disturbance gradient in 2 9 2 m plots at 301 locations. We used a modified BraunBlanquet scale to estimate plant cover, destructively harvested all live aboveground biomass up to a height of 2 m, calculated the above-ground biomass of each species from separately processed stem and leaf fractions in each plot, and then calculated LAI using reference measures of specific leaf area for each species. For each of nine PGFs, we regressed LAI and biomass against the nine-point Braun-Blanquet ordinal transform scale (OTS) using linear mixed effects models.Results: We found a simple, uniform logarithmic scaling of LAI with increasing Braun-Blanquet cover classes that was consistent across most PGFs, and with slope estimates close to 1.0. By contrast, no simple scaling relationship was found for above-ground biomass, with most PGFs exhibiting an asymptotic relationship in which the Braun-Blanquet estimates across high cover scores provided almost no resolution of observed variation in empirical biomass measures. Conclusions:We found that the Braun-Blanquet OTS provides a remarkably simple and accurate logarithmic scaling of LAI, but care should be taken in applying scaling rules uniformly across PGFs. In contrast, the Braun-Blanquet OTS shows a more complex relationship with plant above-ground biomass and we caution against its unconditional use for biomass estimation. The findings of this study should be broadly applicable to other ecosystems due to the heterogeneity of plant communities included in this work.

Comparing the accuracy of three non-destructive methods in estimating aboveground plant biomass

“…However, they did not compare estimates with harvested biomass, but only looked at coefficient of variation of the estimates. Our results are in agreement with the findings of Döbert et al (2015), who found that even a semi-quantitative assessment based on Braun-Blanquet scale can provide a reliable proxy for aboveground biomass. Similar to our results, Redjadj et al (2012) also found that visual cover estimation can be accurate in estimating biomass, although they estimated biomass directly (and not cover), and estimation procedure was preceded by a training series.…”

Logging, exotic plant invasions, and native plant reassembly in a lowland tropical rain forest

“…Disturbed areas, in contrast, promote the growth of a much denser secondary successional vegetation that is characterized by a relatively higher presence of shrubs, herbaceous climbers, and graminoids (Döbert et al . ). Hence, the increased LAI of early‐successional stage native species due to canopy opening is to be expected, unless exotic species became so prevalent in these disturbed areas that even native pioneer species would be excluded.…”

“…It was, therefore, estimated from biomass harvesting metrics (see Appendix S1.4 and Döbert et al . ), using a reference measure of specific leaf area (SLA) and the leaf area per unit leaf dry mass (cm 2 /g) for each species (Bréda ). The SLA of fresh leaves (SLA fresh ) has not previously been measured for the majority of tropical rain forest understory species in Borneo.…”

An Overview of Global Leaf Area Index (LAI): Methods, Products, Validation, and Applications

“…The allometric relationship can be improved when additional biophysical parameters, such as canopy cover and canopy height, are included in the model (Döbert et al, ; Jensen et al, ; le Maire et al, ; Majasalmi et al, ; Olsoy et al, ). As an alternative, Turner et al () suggested estimating LAI from the sapwood cross‐sectional area, because of their strong physiological relationship.…”