Groundwater extraction reduces tree vitality, growth and xylem hydraulic capacity in Quercus robur during and after drought events

Abstract

Climate change is expected to pose major direct and indirect threats to groundwater-dependent forest ecosystems. Forests that concurrently experience increased rates of water extraction may face unprecedented exposure to droughts. Here, we examined differences in stem growth and xylem hydraulic architecture of 216 oak trees from sites with contrasting groundwater availability, including sites where groundwater extraction has led to reduced water availability for trees over several decades. We expected reduced growth and xylem hydraulic capacity for trees at groundwater extraction sites both under normal and unfavourable growing conditions. Compared to sites without extraction, trees at sites with groundwater extraction showed reduced growth and hydraulic conductivity both during periods of moderate and extremely low soil water availability. Trees of low vigour, which were more frequent at sites with groundwater extraction, were not able to recover growth and hydraulic capacity following drought, pointing to prolonged drought effects. Long-term water deficit resulting in reduced CO₂ assimilation and hydraulic capacity after drought are very likely responsible for observed reductions in tree vitality at extraction sites. Our results demonstrate that groundwater access maintains tree function and resilience to drought and is therefore important for tree health in the context of climate change.

Figure 1

Strongest correlations between hydro-climatic variables (precipitation, temperature, standardized precipitation evapotranspiration index (SPEI 6), upper and total soil moisture index (SMI)) and ring-width chronologies ((a): ring-; (b): earlywood; and (c); latewood-width). Columns highlighted in grey show seasonal averages. SMI: Soil moisture index. Symbol size indicates correlation strength for each ring-width variable.  The strongest correlation in absolute terms was observed betwen earlywood and June SMI_1.8 (r = 0.6), while the lowest correlation between latewood and July temperature (r = − 0.25).

Figure 2

Strongest relationships (Pearson’s correlation coefficient) between hydro-climatic variables: precipitation (P), temperature (T), standardized precipitation evapotranspiration index (SP), SMI_1.8 (SM)) and vessel related variables: mean vessel area (MVA), total vessel area (TVA), total vessel area expressed as percentage of ring area (TVA%), vessel density (VD), hydraulic diameter (Dh) and the theoretical hydraulic conductivity (Ks).

Figure 3

Development of growth: (a) ring-width (RW); (b) earlywood-width (EW); (c) latewood-width (LW), vessel and hydraulic variables: (d) total vessel area (TVA); (e) hydraulic diameter (Dh); (f) theoretical hydraulic conductivity (Ks) in the periods during the two droughts as well as pre, and post-drought (normal). See also Supplementary Fig. S4 for the exact non-dry years considered in the pre- and post-drought periods. Points indicate average values of each ring-width or vessel related variable and bars show the standard error of the mean.

Figure 4

Development of growth variables: (a–c) ring-width (RW); (d–f) earlywood-width (EW); (g–i) latewood-width (LW), in the two droughts as well as pre, and post-drought (normal) periods for trees from different vigour groups and for sites with different groundwater regimes. See also Supplementary Fig. S3 for the exact non-dry years considered in the pre- and post-drought periods. Points show average values of each ring-width variable and bars show the standard error of the mean.

Figure 5

Development of vessel-related variables: (a–c) total vessel area (TVA); (d–f) hydraulic diameter (Dh); (g–i) theoretical hydraulic conductivity (Ks), in in the two droughts as well as pre, and post-drought (normal) periods for trees from different vigour groups and sites with different groundwater regimes. See also Supplementary Fig. S3 for the exact non-dry years considered in the pre- and post-drought periods. Points show average values of each vessel related variables and bars show the standard error of the mean.

Figure 6

Median components of resilience (resistance: (a), (d) and (g); recovery: (b), (e) and (h); and resilience: (c), (f) and (i) for ring width variables (ring-width (RW); earlywood width (EW); latewood width (LW)) of trees at different sites (red for extraction, blue for no extraction and grey for upland sites). Points denote healthy trees and triangles declining trees. Bars denote median absolute deviation. Brackets connect statistically significant different groups (p < 0.05).

Figure 7

Median components of resilience (resistance: (a), (d) and (g); recovery: (b), (e) and (h); and resilience: (c), (f) and (i)) for vessel related variables (total vessel area (TVA); hydraulic diameter (Dh); and hydraulic conductivity (Ks)) of trees at different sites (red for extraction, blue for no extraction and grey for upland sites). Points denote healthy trees and triangles declining trees. Bars denote median absolute deviation. Brackets connect statistically significant different groups (p < 0.05).