Conservation policy and the measurement of forests

Abstract

Deforestation is a major driver of climate change 1 and the major driver of biodiversity loss 1,2 . Yet the essential baseline for monitoring forest cover-the global area of forests-remains uncertain despite rapid technological advances and international consensus on conserving target extents of ecosystems 3 . Previous satellite-based estimates 4,5 of global forest area range from 32.1 × 10 6 km 2 to 41.4 × 10 6 km 2 . Here, we show that the major reason underlying this discrepancy is ambiguity in the term 'forest'. Each of the >800 o cial definitions 6 that are capable of satellite measurement relies on a criterion of percentage tree cover. This criterion may range from >10% to >30% cover under the United Nations Framework Convention on Climate Change 7 . Applying the range to the first global, high-resolution map of percentage tree cover 8 reveals a discrepancy of 19.3 × 10 6 km 2 , some 13% of Earth's land area. The discrepancy within the tropics alone involves a di erence of 45.2 Gt C of biomass, valued at US$1 trillion. To more e ectively link science and policy to ecosystems, we must now refine forest monitoring, reporting and verification to focus on ecological measurements that are more directly relevant to ecosystem function, to biomass and carbon, and to climate and biodiversity.Forests are the focus of efforts to mitigate harmful ecological and social impacts of land use, including agreements to reduce carbon dioxide emissions from deforestation and forest degradation (REDD+; refs 9-11). The goals are both scientific-to balance regional and global carbon budgets-as well as political, to reduce carbon emissions and stop species extinctions by defining national baselines and managing future anthropogenic change 12 .The Forest Resources Assessments (FRAs) of the United Nations Food and Agriculture Organization (FAO)-the authority for national and global accounting-recorded 40.8 × 10 6 km 2 of forest in 2000, equalling 31% of Earth's land area 13 . The FRAs rely on self-reporting by participating countries, raising concerns about subjectivity and consistency 14-16 . Although estimates from satellite images should provide a more objective base 9 , even these disagree significantly over the amount and distribution of forests worldwide. Figure 1 maps the consensus among eight global satellite data sets over the class 'forest' in or near the year 2000 (Methods). The densely canopied biomes of the tropical, temperate and boreal zones, and the treeless deserts, prairies and tundra show nearperfect agreement across all sources on the presence or absence of forests. Yet the data disagree over the planet's semi-arid savannahs, shrublands and woodlands, and over the northern limits of the boreal forest. Although 102.2 × 10 6 km 2 show perfect consensus among the eight data sets on either the presence or absence of forests, 9.4 × 10 6 km 2 were identified as forest by four out of the eight sources. These sparsely forested regions are the areas of greatest remaining uncertainty.There are two reasons for the uncertaint...

The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes

“…Changing definitions through time, they argued, explain the apparently alarming deforestation in several West African countries. The problem of defining forest is a general one in tropical landscapes [54][55][56]. Ratnam et al [57] provided a functional definition for distinguishing forest from 'savanna' based not on the trees but on the presence of a highly flammable, shade-intolerant C4 grassy understory in savannas and its absence in forests.…”

Accuracy Assessment and Inter-Comparison of Eight Medium Resolution Forest Products on the Loess Plateau, China

“…Sexton et al [25] found that the global forest maps with coarse resolutions showed a large discrepancy. However, the performance and consistency of the aforementioned eight medium resolution forest maps in China are still unclear.…”

Calibration and Validation of Landsat Tree Cover in the Taiga−Tundra Ecotone

“…This is a difficulty even for humans visually interpreting tree cover from HRSI (<5 m resolution) data. However, this apparent confusion implies that the definition of what is a tree is of primary importance and spatially consistent [47]. The TTE, in particular, is a region in which this distinction is blurred.…”

“…From discrete, small-footprint LiDAR returns, tree cover is calculated by dividing the number of returns above a criterion height by the total number of returns within a given sample [45,46]. In accordance with the International Geosphere-Biosphere definition of forests, Sexton et al specified the height criterion as 5 m, but this parameter may be tuned to biome-specific definitions of tree height [34,47].…”