Repeated insect defoliation effects on growth, nitrogen acquisition, carbohydrates, and root demography of poplars
The effects of browsing by moose and snowshoe hares on fine root production, mortality, and decomposition in early successional forest ecosystems along the Tanana River floodplain in interior Alaska were studied over a 3-yr period using minirhizotrons placed inside and outside large permanent exclosures. Fine root production and mortality varied seasonally, with greatest rates of production occurring during June each year, and greatest rates of mortality occurring in fall and over winter. Annual production and mortality during 1993, a year of unusually low precipitation, were significantly higher than during either 1992 or 1994. Aboveground herbivory significantly reduced monthly rates of fine root production, and on an annual basis, fine root production of browsed plots (311.4 6 31.7 mm·tube21·yr21) was significantly less than that of unbrowsed plots (453.8 6 49.8 mm·tube21·yr21) when averaged over 3 yr. Because herbivory had less of an effect on monthly or annual rates of fine root mortality, fine root turnover was higher for browsed stands. Browsed plants had a higher percentage of annual production in surface soil layers. Production on all plots shifted to deeper soil layers as the growing season progressed; this shift occurred deeper in the profile for unbrowsed plants than for browsed plants. We used a parameter estimation program (Program MARK) to generate fine root survival and decomposition estimates from models testing the direct and interactive effects of time period, cohort (i.e., when the root first appeared) age of the root, browsing, and site on fine root longevity and decomposability. Cohort effects showed that survival of fine roots was greatest for roots that first appeared in May, and that survival progressively declined for roots first appearing during subsequent time periods, while age-based estimates showed a rapid decline in survival over the interval following first appearance. Survival and decomposition estimates were inversely correlated within a growing season, with the lowest survival but highest decomposition occurring over winter. Two-factor models indicated that time-dependent survival and decomposition rates of fine roots differed significantly between browsed and unbrowsed stands, among the 10 fine root age groups, and among the three study sites. Browsing significantly reduced fine root survival, but this effect varied among sites. Fine root decomposition rates were consistently lower (221%) in browsed stands. Two important features distinguish fine root dynamics in our stands from temperate and more southerly boreal ecosystems: (1) low overwinter survival of fine roots, and (2) a substantial time lag between leaf-out and maximum fine root growth, suggesting greater reliance on aboveground stores for spring regrowth. Herbivores appear to play an important role in linking these two events, first by exacerbating overwinter mortality, and second by consuming a substantial amount of aboveground stores. Thus, not only do herbivores have pronounced direct effects on carbon and nutrient cycling processes, but climatically driven effects on fine root processes may be linked with herbivory in complex ways that define fundamental latitudinal patterns in plant growth and allocation to defense against herbivory.