The contribution of trees outside forests to national tree biomass and carbon stocks—a comparative study across three continents

Abstract

In contrast to forest trees, trees outside forests (TOF) often are not included in the national monitoring of tree resources. Consequently, data about this particular resource is rare, and available information is typically fragmented across the different institutions and stakeholders that deal with one or more of the various TOF types. Thus, even if information is available, it is difficult to aggregate data into overall national statistics. However, the National Forest Monitoring and Assessment (NFMA) programme of FAO offers a unique possibility to study TOF resources because TOF are integrated by default into the NFMA inventory design. We have analysed NFMA data from 11 countries across three continents. For six countries, we found that more than 10 % of the national above-ground tree biomass was actually accumulated outside forests. The highest value (73 %) was observed for Bangladesh (total forest cover 8.1 %, average biomass per hectare in forest 33.4 t ha⁻¹) and the lowest (3 %) was observed for Zambia (total forest cover 63.9 %, average biomass per hectare in forest 32 t ha⁻¹). Average TOF biomass stocks were estimated to be smaller than 10 t ha⁻¹. However, given the large extent of non-forest areas, these stocks sum up to considerable quantities in many countries. There are good reasons to overcome sectoral boundaries and to extend national forest monitoring programmes on a more systematic basis that includes TOF. Such an approach, for example, would generate a more complete picture of the national tree biomass. In the context of climate change mitigation and adaptation, international climate mitigation programmes (e.g. Clean Development Mechanism and Reduced Emission from Deforestation and Degradation) focus on forest trees without considering the impact of TOF, a consideration this study finds crucial if accurate measurements of national tree biomass and carbon pools are required.

Appendix

A.1

As a complement to Table 4, we provide confidence intervals for the total biomass estimates in Fig. 3. The purpose was to provide evidence that it matters which biomass estimation approach was used. Intervals were constructed as $\left(\widehat{t}-1.96\sqrt{\widehat{V}\left(\widehat{t}\right)},\widehat{t}+1.96\sqrt{\widehat{V}\left(\widehat{t}\right)}\right)$, where $\widehat{t}$ is the estimated total and $\sqrt{\widehat{V}\left(\widehat{t}\right)}$ the estimated standard error. The values $-1.96$ and $1.96$ are the $0.025$ and $0.975$ quantiles of the normal probability distribution, yielding a 95 % confidence interval under the assumption of a normal distribution of $\widehat{t}$. Note that the coverage of the intervals is probably slightly larger than 95 % due to the overestimation of the sampling error that follows from using a simple random sampling estimation framework in combination with systematic sampling (see “Estimation framework” section).

In Fig. 3, all estimated totals and their corresponding confidence intervals were related to the corresponding estimates using Chave H. This approach for estimating the biomass of single trees was thus used as a reference level to which all other biomass estimation approaches were compared. This standardisation was done to obtain a better comparability across land uses and countries.

A.2

Here, a summary of the methodology that was originally applied by each country for its biomass reporting is given. The background reports can be found at FAO (2013). In Table 5, the different allometric above-ground tree biomass models as applied by six countries are listed by species and forest type. The models provide direct estimates of above-ground tree biomass in kilogrammes. The remaining five countries estimated biomass based on tree volume and an application of wood specific gravities, wd, and biomass expansion factors, bef. In the NFI in Zambia, wood specific gravities and biomass expansion factors were combined into biomass conversion and expansion factors, bcef. The values applied for wd, bef, and bcef are provided in Table 6. In addition, the table provides information about volume estimation. In Table 7, it is shown what methods were applied by the different countries.

Table 5 Allometric models that were originally applied by countries for biomass reporting

Table 6 Factors used when estimating above-ground tree biomass based on expanding inventoried volume

Table 7 Country-specific approaches to biomass estimation