Abstract
Publicly available remote sensing products have boosted science in many ways. The openness of these data sources suggests high reproducibility. However, as we show here, results may be specific to versions of the data products that can become unavailable as new versions are posted. We focus on remotely-sensed tree cover. Recent studies have used this public resource to detect multi-modality in tree cover in the tropical and boreal biomes. Such patterns suggest alternative stable states separated by critical tipping points. This has important implications for the potential response of these ecosystems to global climate change. For the boreal region, four distinct ecosystem states (i.e., treeless, sparse and dense woodland, and boreal forest) were previously identified by using the Collection 3 data of MODIS Vegetation Continuous Fields (VCF). Since then, the MODIS VCF product has been updated to Collection 5; and a Landsat VCF product of global tree cover at a fine spatial resolution of 30 meters has been developed. Here we compare these different remote-sensing products of tree cover to show that identification of alternative stable states in the boreal biome partly depends on the data source used. The updated MODIS data and the newer Landsat data consistently demonstrate three distinct modes around similar tree-cover values. Our analysis suggests that the boreal region has three modes: one sparsely vegetated state (treeless), one distinct ‘savanna-like’ state and one forest state, which could be alternative stable states. Our analysis illustrates that qualitative outcomes of studies may change fundamentally as new versions of remote sensing products are used. Scientific reproducibility thus requires that old versions remain publicly available.
Highlights
A recent call for more transparent, open and reproducible science stressed the need to have data posted in a trusted repository [1]
The MODIS Vegetation Continuous Fields (VCF) product has been updated to Collection 5 [11]; and a Landsat VCF product of global tree cover at a fine spatial resolution of 30 meters has been developed [12]
Four modes were previously identified from the Collection 3 product, representing boreal forest (*75% tree cover), dense and sparse woodland (*45% and *20% tree cover respectively), and a treeless state (Fig 2a)
Summary
A recent call for more transparent, open and reproducible science stressed the need to have data posted in a trusted repository [1]. Numerous scientific studies are based on publicly available remote-sensing products that seem to fit this criterion well. As we show here, there is a serious reproducibility issue if old versions of products become unavailable. Recent studies have used remotely-sensed and field-measured tree cover to detect multimodality in the tree cover frequency distribution [2,3,4,5]. Some potential caveats of interpreting remotely-sensed tree cover have been pointed out [7,8,9]. We address the fundamental issue that results may substantially change with the version of the product used. This precludes reproduction of the findings if the old version is no longer available
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