Forest age and water yield

Arising from: Evaristo, J. & McDonnell, J. J. Nature https://doi.org/10.1038/s41586-019-1306-0 (2019); Addendum Nature https://doi.org/10.1038/s41586-019-1586-4 (2019); Author Correction Nature https://doi.org/10.1038/s41586-019-1588-2 (2019); Retraction Nature https://doi.org/10.1038/s41586-020-1945-1 (2020).

Planting and removal of forest affect average streamflow (also referred to as water yield), but there is ongoing debate as to what extent this long-term difference between precipitation and evapotranspiration is modulated by local conditions. A recent paper by Evaristo and McDonnell¹ introduces a conceptual vegetation-to-bedrock model to explain variability in reported streamflow responses to changes in forest cover based on an analysis of seven factors that describe climate, soil properties and catchment size. Their analysis excludes well known controls—such as the percentage of catchment area under change², forest type and time since afforestation—that we show here to be important. By excluding these primary controls, Evaristo and McDonnell risk attributing water yield response to co-varying secondary controls rather than to the underlying causes.

We illustrate the importance of the record length (or time since afforestation) using unique longterm measurements of water yield made under controlled conditions. At Castricum in The Netherlands, and St Arnold in Germany, two large lysimeters were planted with coniferous and deciduous trees in the 1940s and 1960s, respectively, while reference conditions (bare soil and grassland, respectively) were maintained in an additional lysimeter. At both stations, strong, consistent and continuing declines in average water yield response were observed over averaging periods that ranged from several years up to the whole experiment duration (Fig. 1), coinciding with a steady increase in tree height and biomass^3,4 and in spite of possible limitations in rooting depth. The declines follow an exponential decay (with a coefficient of determination of 0.91 or larger) with an e-folding time τ of 15 years and a stronger water yield response for coniferous forest than for deciduous forest. As a result, each individual lysimeter already covers a range in water yield response of 30% up to 70%, comparable to the total range reported by Evaristo and McDonnell across different watersheds¹. Similar response times were found for afforestation experiments in deciduous broadleaf forest in North Carolina in the USA⁵ and at the German lysimeter station of Britz-Eberswalde⁶, while analysis of longterm streamflow data in Sweden revealed similar strong effects of forest biomass and age⁷.

Fig. 1: Impact of forest age on water yield response to forest planting.

Data points are from coniferous (triangles) and deciduous (circles) lysimeters at Castricum (green) and St Arnold (red/orange). Dashed curves indicate exponential fits with a characteristic timescale τ of 15 years, with a 10-year shift assumed for the deciduous lysimeter in St Arnold. Letters A, B and C indicate record length (or forest age) domains used in Fig. 2. The background histogram shows the distribution of the record length of the forest planting studies used by Evaristo and McDonnell. Note that most studies (82%) have a record length of less than 30 years, and strong changes in water yield response are observed in this period. This figure and Fig. 2 were generated by Matlab 2015b (http://nl.mathworks.com/products/matlab/).

Fig. 2: Global tree canopy cover change distribution and record length of water yield response to forest planting.

Points/circles indicate locations of forest planting studies used by Evaristo and McDonnell¹, with the size reflecting the record length according to classes A, B and C as indicated in Fig. 1. The background map shows changes in tree canopy cover over the period 1982–2016 obtained from a recent analysis of satellite data⁸.

The record length of the studies used by Evaristo and McDonnell¹ varies considerably from 1 year to 75 years, but is mostly lower than the timescale of water yield response to forest growth of 15 years (Fig. 1). Therefore, it is likely that the values reported in studies with record lengths of up to once or even twice the e-folding time (15–30 years) are in fact highly sensitive to the length of their record. The mixing of data with variable record lengths could explain why Evaristo and McDonnell find actual evapotranspiration (AET) to be the factor explaining most of the magnitude, rather than timing, of water yield response to planting. When the location of stations with sufficient record length are added to a global map of changes in forest cover over the recent decades⁸, it becomes clear that accurate observations of longterm impacts of forest planting on water yield are concentrated in only a few regions. Strikingly, the forest cover change hotspots are observational blind spots for water cycle impacts. Given the potential of large-scale afforestation to offset carbon emissions⁹, a robust understanding of the hydrological impacts of current and future forest management is more important than ever.

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this paper.