Plant invaders can have multiple impacts on ecosystems, but temporal scales of impacts are rarely examined. We used a 40-year chronosequence approach to examine the impacts of an N-fixing shrub (Cytisus scoparius, or broom) that invades Douglas-fir (DF) clearcuts in WA, USA. We planted DF seedlings in late spring 2015 across 10 DF clearcuts that varied in age from 5 to 45 years, which we used as a proxy for invasion age. Within each clearcut, we identified two patch types for planting DF: invaded by broom and native-dominated uninvaded. We manually cleared both patch types of all vegetation before planting DF into all sites. We used a buried resin bag method to quantify plant-available nitrogen (N) and phosphorus (P) under seedlings planted into both invaded and uninvaded patches. The resin bags were in the ground from May until November 2015, but at every 6-8 week interval we harvested all bags and replaced them with new bags. We measured seedling survival at the end of the growing season, and calculated the total amount of N and P accumulated on resin bags over the entire 2015 growing season.
Soil nutrients were affected by broom invasion in a pattern consistent with N-fixing species: soil N was enriched, but soil P was depleted by this invader. Nitrogen availability was an order of magnitude higher in broom-invaded soils compared to uninvaded soils and this effect was similar from the earliest to the oldest invasions. Phosphorus availability was less strongly affected by invasion but was markedly lower (17%) lower in broom-invaded patches compared to uninvaded patches. Unlike N, P increased in both patch types with time, where older clearcuts had higher P availability compared to younger clearcuts. Despite the much higher N availability in broom-invaded patches, DF seedling mortality was 25% higher in those soils compared to uninvaded patches and irrespective of time. We speculate that P limitation, coupled with potentially negative N-enrichment effects on DF mycorrhizae, may have contributed to greater mortality for DF seedlings grown in patches affected by broom invasion. Our results also suggest that the impacts of this invader are immediate and then stabilize over time.