Regulation of fine root dynamics by mammalian browsers in early successional Alaskan taiga forests

The effects of browsing by moose and snowshoe hares on line root production , mortality, and decomposition in early successional forest ecosystems alon g the Tanana River floodplain in interior Alaska were studied over a 3-yr p eriod using minirhizotrons placed inside and outside large permanent exclos ures. Fine root production and mortality varied seasonally, with greatest r ates of production occurring during June each year, and greatest rates of m ortality occurring in fall and over winter. Annual production and mortality during 1993, a year of unusually low precipitation, were significantly hig her than during either 1992 or 1994. Aboveground herbivory significantly reduced monthly rates of fine root prod uction, and on an annual basis, fine root production of browsed plots (311. 4 +/- 31.7 mm.tube(-1) yr(-1)) was significantly less than that of unbrowse d plots (453.8 +/- 49.8 mm.tube(-1).yr(-1)) when averaged over 3 yr. Becaus e herbivory had less of an effect on monthly or annual rates of fine root m ortality, fine root turnover was higher for browsed stands. Browsed plants had a higher percentage of annual production in surface soil layers. Produc tion on all plots shifted to deeper soil layers as the growing season progr essed; 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 in teractive effects of rime period, cohort (i.e., when the root first appeare d) age of the root, browsing, and site on fine root longevity and decomposa bility. 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 e stimates showed a rapid decline in survival over the interval following fir st appearance. Survival and decomposition estimates were inversely correlat ed within a growing season, with the lowest survival but highest decomposit ion occurring over winter. Two-factor models indicated that time-dependent survival and decomposition rates of fine roots differed significantly betwe en browsed and unbrowsed stands, among the 10 fine root age groups, and amo ng the three: study sites. Browsing significantly reduced fine toot surviva l, but this effect varied among sites. Fine root decomposition rates were c onsistently lower (-21%) in browsed stands. Two important features distinguish fine root dynamics in our stands from te mperate and more southerly boreal ecosystems: (1) low overwinter survival o f fine roots, and (2) a substantial time lag between leaf-out and maximum f ine root growth, suggesting greater reliance on aboveground stores for spri ng regrowth. Herbivores appear to play an important role in linking these t wo events, first by exacerbating overwinter mortality, and second by consum ing a substantial amount of aboveground stores. Thus, not only do herbivore s have pronounced direct effects on carbon and nutrient cycling processes, but climatically driven effects on fine root processes may be linked with h erbivory in complex ways that define fundamental latitudinal patterns in pl ant growth and allocation to defense against herbivory.