Interactive effects of hemlock mortality and nitrogen availability on nutrient pools and fluxes in the southern Appalachian Mountains

exotic insects;hemlock;phosphorus fractionation;N and P co-limitation;atmospheric nitrogen deposition;hemlock woolly adelgid;nitrogen flux;pest outbreaks;phosphorus (P);nitrogen (N)

Block, Corinne ; Fraterrigo ; Jennifer M.

关键词: exotic insects;    hemlock;    phosphorus fractionation;    N and P co-limitation;    atmospheric nitrogen deposition;    hemlock woolly adelgid;    nitrogen flux;    pest outbreaks;    phosphorus (P);    nitrogen (N);   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/29466/Block_Corinne.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The impacts of exotic insects and pathogens on forest ecosystems are increasingly recognized, yet the factors influencing the magnitude of effects remain poorly understood. Eastern hemlock (Tsuga canadensis) exerts strong control on nitrogen (N) dynamics, and its loss due to infestation by the hemlock woolly adelgid (Adelges tsugae) is expected to affect nutrient dynamics in impacted stands. I evaluated the potential for variation in N availability to influence the magnitude of effects of hemlock decline on N and P dynamics in mixed hardwood stands. I measured N and P pools and fluxes at three elevations (low, mid, high) subjected to increasing atmospheric N deposition where hemlock was declining or absent (as reference), in western North Carolina. Nutrient pools varied substantially with elevation and increasing N availability; total forest floor and mineral soil N increased (p<0.0001, p=0.0017, resp.) and forest floor and soil carbon (C) to N ratio decreased with elevation (p<0.0001, p=0.0123, resp.), suggesting that these high elevation pools are accumulating available N. Total extractable soil P was similar across the study area, however, P fractionation revealed distinct changes in the distribution of soil P fractions as N increased. Soils from high elevation, high N-available sites had 310% higher concentration of organic P and 55% smaller residual and refractory P pools than soils from low elevation, low N-available stands, suggesting that increased N availability has driven the depletion of recalcitrant P pools by stimulating biotic demand. Contrary to expectations, subsurface leaching of inorganic N was minimal overall (<1 kg ha-1 9 mo-1), and was not higher in stands with hemlock mortality. Across all elevation classes, hardwood foliar N:P ratios were lower in stands with declining hemlocks, suggesting trees are incorporating available P into biomass. Higher foliar N and P concentrations as well as observed increases in the growth of hardwood species in high elevation stands post-HWA infestation suggest that hemlock decline has stimulated nutrient uptake by healthy vegetation within this mixed forest, and may thereby contribute to decoupling the relationship between N deposition and ecosystem nutrient loss.