Estimates of plant biomass are helpful for many applications in invasive plant science and management, but measuring biomass can be time consuming, costly, or impractical if destructive sampling is inappropriate. The objective of this study was to assess feasibility of developing regression equations using a fast, non-destructive measure (cover) to estimate aboveground biomass for red brome (Bromus rubens L.), a widespread non-native annual grass in the Mojave Desert, USA. At three study sites, including one measured for three consecutive years, B. rubens cover spanned 0.1-85% and aboveground biomass 1-321 g m-2. In log10-transformed linear regressions, B. rubens cover accounted for 68-96% of the variance in B. rubens biomass among sites, with all coefficients of determination significant at P < 0.05. For every doubling of percent cover, biomass was predicted to increase by 78, 83, and 144% among the three sites. At the site measured for three consecutive years, which ranged in rainfall from 65-159% of the long-term average, regression slopes each year differed from other years. Regression results among sites were insensitive to using cover classes (10 classes encompassing 0-100% cover) compared with simulated random distribution of integer cover within classes. Biomass of B. rubens was amenable to estimation in the field using cover, and such estimates may have applications for modeling invasive annual plant fuel loads and ecosystem carbon storage.
Management Implications
By altering biomass structure, invasion of non-native grasses has changed fuels and fire behavior in drylands. Modeling fuels and many other features of non-native plants requires estimates of biomass, which can be time consuming or infeasible to measure directly. As an alternative, this study developed regression equations to estimate biomass from the rapid, non-destructive measure of plant cover for red brome (Bromus rubens L.), a pervasive, non-native annual grass in the Mojave Desert, USA. For every doubling of B. rubens cover, predicted B. rubens biomass increased by 78-144% among sites. In applying the equations to estimate fire-risk thresholds of hazardous fuels using cover, a provisional threshold of 100 g m-2 of B. rubens biomass required for fire spread was exceeded at 19, 25, and 45% B. rubens cover among sites. The equations, and suggested refinements in future work, may be helpful for rapidly estimating fuel loads and assessing effectiveness of invasive plant management, including levels required to keep fuels below wildfire spread risk thresholds. Accompanying the equations, a photographic guide showing cover classes and their associated biomass is provided.