Adding nutrients won't fix oak growth on abandoned farmland
A Swedish study found that fertilizing and watering oak trees on former agricultural land had no effect on their growth, challenging assumptions about restoring productivity to degraded soils. The finding matters for land managers and policymakers betting on cheap interventions to accelerate forest recovery on abandoned farms across Europe.
Originaltitel: Fertilisation and irrigation have no effects on growth of oak (Quercus robur, Q. petraea) stands on abandoned farmland in southwest Sweden
<p>Low nutrient availability often limits productivity in northern forests. In a nutrient optimisation trial, we investigated the effects of fertilisation and irrigation on soil moisture, leaf area index (LAI) as well as height and radial growth in 25-year-old stands of pedunculate and sessile oak (<em>Quercus robur</em> L., <em>Q. petraea</em> (Matt.) Liebl.) growing on abandoned farmland in southwestern Sweden. Control (C), fertilisation (F), irrigation (I), and irrigation +fertilisation (IF) treatments were arranged in a randomized complete block design. End of growing season analysis of foliar nutrients guided the quantitative composition of next year’s fertiliser mix. Volumetric soil moisture (VWC) was significantly higher in the I and IF treatments compared to the C and F treated stands of both oak species. We did not observe a fertiliser-related reduction in VWC (except for 2015, when VWC in F- treated <em>Q. robur</em> stands was significantly lower than the control by about 18 %). This is in line with the unaffected LAI estimates (5.3–5.9) suggesting no stimulation of leaf production that could drive increases in transpiration with subsequent soil moisture depletion. There was no treatment ×year interaction for any of the growth-related variables. Treatments had no significant effects on basal area growth, which increased annually by 1.72 and 1.54 m<sup>2</sup> ha<sup>-1</sup> on average for <em>Q. petraea</em> and <em>Q. robur</em>, respectively. Pre-treatment height differences in <em>Q. petraea</em> stands (7–12 % taller trees in the C and IF plots) persisted throughout the study resulting in significant effects, while no significant differences in height occurred in <em>Q. robur</em>. Periodic annual volume increment varied more strongly following drier periods but there were no significant differences among treatments. Our findings indicate that fertilisation causes no or only minor increases in oak water use, suggesting that nutrient addition in oak stands within this precipitation regime does not require simultaneous irrigation. Most importantly, our data implies that the soil nutrient legacies of past agricultural use suffice to maximise the productivity of oak stands on abandoned farmland typical of the main oak growing region in southwestern Sweden.</p>