Forests with limited human intervention, whether “pristine”, ancient or old growth, are a limited and diminishing feature of modern landscapes [1
]. They are essential in terms of conserving biodiversity, climate change mitigation, or as reference systems for forest management or restoration [3
]. Global inventories and mapping efforts focused on forest integrity have proliferated given the concerns on the rapid decline of primary forests [4
]. These efforts include global distribution analyses of intact forest landscapes by biome [5
], estimated rate of change of forest cover [4
], levels of forest fragmentation [6
], global human footprint [7
], and areas prioritized for potential restauration [9
]. Intact forests are also important for associated ecosystem services of carbon storage [10
], soil and water conservation, regulating hydrological regimes, and affect the structure, function, biodiversity and geomorphology of stream and river ecosystems [11
Forest conservation efforts have advanced on the basis of forest patches as the basic conservation unit, usually in terms of their respective size and connectivity. However, in the broader context of forests associated with streams and catchments, the perspective of watershed seems more relevant than forest patch. Nevertheless, we are unaware of any landscape assessments of intact forested watersheds. How much do intact forests contribute to watershed conservation? Are there corresponding intact watersheds, and under what conditions? Conceivably this would go beyond forests as patches located here and there relative to the stream and watershed ecosystem, demanding a restrictive set of conditions: an undisturbed forest ecosystem conforming to the watershed boundary until some downstream point, with no gaps in coverage along the stream drainage network (with exceptions due to natural disturbance patches caused by stream or hillslope dynamics). Given these general criteria as a starting point, intact forested watersheds are likely a much more restricted subset of intact forests, in terms of both distribution and size. Presuming that intact forested watersheds are correspondingly scarce globally, and the probability of their being linked to headwaters (by definition, as watersheds are organized in linear and hierarchical networks, starting upstream), these two elements alone underscore their potentially tremendous value for freshwater conservation [14
Forests intactness regulates linkages between terrestrial and aquatic ecosystems at several levels. Riparian old growth forests will be more heterogeneous in terms of light, exhibit greater volumes of large woody debris which will in turn influence the geomorphology, nutrient accumulation, habitat heterogeneity of a stream channel and aquatic biodiversity [16
]. At regional, watershed and local scales, forests probably play a very important role in water balance [18
], via processes such as evapotranspiration, snow hydrology, regulating soil development and corresponding soil hydraulic properties. All of these physical properties will vary according to the intactness of the forest, soils, and hydrologic connections within the watershed [11
]. Based on these linkages, we propose the concept of intact forested watersheds (IFWs), or primary forests bounded by correspondingly intact watersheds.
Southern South America has the largest extensions of intact temperate forests in the southern hemisphere [19
]. They occur along a narrow but latitudinally extensive area, from approximately 34 to 55° S. Biogeographically, these southern South American temperate forests are likewise unique in terms of their isolation and climate [21
]. An island surrounded by desert in the north and east, and ocean in the south and west, these forests are the reservoir of temperate forest biodiversity in South America [22
]. A strong climatic gradient driven by prevailing westerlies (from <300 mm/year precipitation in the east to >6000 mm/year in the west), combined with a complex topography of the coastal mountain range and the Andes cordillera, result in area where temperate evergreen, deciduous and mixed broadleaf forests coexist in a relatively small geographic area (east-west distance of forested landscapes is 200–300 km). This bioclimatic and forest diversity, together with the cleanest air-shed on earth given the very low atmospheric deposition rates [23
], may be a unique world reference for watershed processes and stream ecosystems.
The overall objective of our work is to present a landscape conservation perspective using IFWs as a conservation unit, based on an ongoing mapping effort for the Southern Patagonian Region (41–56° S). Small watersheds are a logical unit of land use planning, conservation planning and assessment of combined array of ecosystem services of soil and water conservation.
Specifically, we mapped the extent of potential intact forested watersheds with limited intervention for southern Patagonia (Chile, Argentina) using existing forest stand mapping and a new high resolution regional stream/watershed delineation (patch size 0.4 km2
). Validation and adjustment of IFW boundaries was performed for three major inland basins using high resolution satellite imagery. For both regional and basin scales we evaluated size distribution, elevation, conservation status and representation of forest type (temperate deciduous and evergreen temperate rain forest) and bio-climatic zones. We present examples of integrating intact watershed and intact forest conservation using Getis-Ord Gi* statistic [24
], including an example of a combined index for prioritizing IFW conservation or management [25
]. Finally we discuss some of the challenges in defining and understanding connections between intact forests, streams and watersheds at relevant scales.
In summary, presented here is a first estimate of intact forested watersheds in southern Patagonia. Like the iterative national land use and forest mapping it is largely based on, it represents a state of knowledge rather than an absolute. Over 130,000 km2 of pIFWs were mapped, or 41% of Chilean Patagonia, based on the regional analysis and less rigorous mature forest criteria. That estimate is reduced to around 50,000 km2 for IFWs based on remote sensing observations from three validation basins, although variation in physical setting, conservation status, and size distribution across the three basins highlight the need for comprehensive validation across the entire study area. Both Figures, as maximum and minimum estimates of IFW coverage, represent an area of global significance. However only a subset of mapped mature forest patches were without evidence of intervention, and perhaps more importantly only a fraction of the areal extent of intact forest patches contributed to IFWs. The difference between the results of the regional vs. basin scale mapping is an indication of the degree of uncertainty of the extent of what should be defined as IFWs. It is at the same time probably a critical area for management in terms of IFW buffers or gradients of restoration potential.
Presented is an example of hot spot analysis (Gi*) as a potential conservation evaluation tool, balancing conservation status, forest patch, and water yield (among other possibilities). Also discussed are relevant forest and stream ecosystem concepts, how they may be related to conservation evaluation above and below tree line, and the need for criteria and studies for weighing the relative importance of IFWs in downstream ecosystems.