It is often believed that trees can use their deeper roots to access groundwater during dry periods, but direct evidence for this is sparse. Both in situ observations and global datasets can provide insight into the relation between forest and groundwater, as is shown in two recent studies.
In a global analysis, Roebroek et al. (2020) mapped the spatial correlations between forest productivity (as measured by satellite-derived fraction of absorbed photosynthetically active radiation or fAPAR) and depth to the groundwater table in addition to mapping the well-known climate controls on forest productivity (as reflected by the ratio of precipitation to potential evapotranspiration). While the analysis confirmed the general global patterns in how precipitation affects vegetation growth, the analysis revealed for the first time at a high resolution how local groundwater convergence in valleys can provide water to trees that can help to maintain their productivity. Somewhat surprisingly, it was also found that in large areas of the world the groundwater table is too shallow for optimum growth, leading to negative correlation between water table depth and fAPAR.
In a study on the 2018 drought in Europe, Kowalska and co-workers investigated the effect of drought on productivity of a floodplain forest located at Lanžhot in the Czech Republic. They found that the overall effect of drought on the total evapotranspiration and productivity was small, since both were higher rather than lower in 2018 compared to the years 2015-2017. Reduction in carbon uptake was only found to occur for relative soil moisture contents under 0.45. Also here, the presence of relatively shallow groundwater tables likely helped the trees to maintain a high productivity throughout most of the 2018 drought.
Further reading
Kowalska, N. et al. (2020), Analysis of floodplain forest sensitivity to drought. Phil. Trans. R. Soc. B, 375, 20190518, doi:10.1098/rstb.2019.0518.
Roebroek, C. J. T. et al. (2020), Global distribution of hydrologic controls on forest growth. Hydrol. Earth Syst. Sci., 24, 4625–4639, doi:10.5194/hess-24-4625-2020.