Changes in Growth of Tropical Forests: Evaluating Potential Biases

Abstract

Over the past century almost every ecosystem on Earth has come under the influence of changes in atmospheric composition and climate caused by human activity. Tropical forests are among the most productive and extensive ecosystems, and it has been hypothesized that both the dynamics and biomass of apparently undisturbed, old-growth tropical forests have been changing in response to atmospheric changes. Long-term forest sample plots are a critical tool in detecting and monitoring such changes, and our recent analysis of pan-tropical-forest plot data has suggested that the biomass of tropical forests has been increasing, providing a modest negative feedback on the rate of accumulation of atmospheric COj. However, it has been argued that some of these old forest plot data sets have significant problems in interpretation because of the use of nonstandardized meth-odologies. In this paper we examine the extent to which potential field methodological errors may bias estimates of total biomass change by detailed examination of tree-by-tree records from up to 120 Neotropical plots to test predictions from theory. Potential positive biases on measurements of biomass change include a bias in site selection, tree deformities introduced by the measurement process, poor methodologies to deal with tree deformities or buttresses, and nonrecording of negative growth increments. We show that, while it is important to improve and standardize methodologies in current and future forest-plot work, any systematic errors introduced by currently identified biases in past studies are small and calculable. We conclude that most tropical-forest plot data are of useful quality, and that the evidence does still weigh conclusively in favor of a recent increase of biomass in old-growth tropical forests.

Efficient plot-based floristic assessment of tropical forests

“…Third, there are no data available from less popular methods to compare with these leading protocols. Finally, floristic samples of ͧ 1 ha are suited to a variety of additional purposes such as monitoring forest dynamics, as well as phenological and ethnobotanical research (Condit 1998, Dallmeier & Comiskey 1998aMalhi et al 2002, Phillips et al 1998, 2002a, which usually involve conversion to permanent plots by tagging, mapping and regular recensusing. However the 1-ha protocol is widely used in ecological research without becoming a site for long-term study, and many plots are in practice abandoned after yielding only inventory data.…”

“…comm., and RAINFOR unpublished data), but only 104 appear to have been recensused by 2002 (i.e. all floristically inventoried plots known to Malhi et al 2002, Phillips et al 1998, 2002aRAINFOR unpublished data). Reasons for 1-ha plots to not become monitoring sites include: (1) inadequate funds to recensus; (2) impossibility of relocating the plot's position; (3) the threat of terrorism or war; (4) removal of aluminium nails by local residents; (5) forest disturbance by residents or commercial interests; (6) changing research interests of principal investigators; (7) rapid radial tree growth 'swallowing' tags; (8) liana or bamboo tangles discouraging access; and (9) death of the principal investigator.…”

Detecting and projecting changes in forest biomass from plot data

“…Many previous authors have discussed the potential qualitative impact of different types of measurement errors or measurement procedures (Clark 2002;Phillips et al 2002;Sheil 1995). Here, we focus on just two issues -issues that our analyses show are particularly important to determination of biomass change in this forest (Table 14.2).…”

“…Further, because of the non-linear relationship between diameter and biomass, random errors in diameter can and do induce systematic errors in biomass. In tropical forests, trees with buttresses or other types of irregular trunks present additional challenges for accurate and precise measurements of biomass and biomass growth (Clark 2002;Phillips et al 2002;Sheil 1995).…”

On the geometry and allometry of big-buttressed trees - a challenge for forest monitoring: new insights from 3D-modeling with terrestrial laser scanning

“…However, ignoring the buttressed part of the stem when determining total tree biomass and carbon stocks (e.g., by applying biomass models that are calibrated based on DBH measurements) will lead to a considerable under-estimation (Nogueira et al 2006, Cushman et al 2014. This is a dilemma as there are no operational allometric models for buttressed trees, even though they are known to be one of the most important contributors to total biomass in many forests (Phillips et al 2002, Henry et al 2010. Even if buttressed trees are included in some general allometries (like e.g., Chave et al 2005), unbiased predictions could only be expected if the model is applied to a forest that has a similar proportion of buttressed trees as the calibration dataset.…”