Effects of mammalian browsing on fine root processes and production in the floodplains of BCEF

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 year 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 - 31.7 mm tube-1 yr-1) was significantly less than that of unbrowsed plots (453.8-49.8 mm tube-1 yr-1) when averaged over 3 years. 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 progressively declined for roots first appearing during subsequent time periods, while aged-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 the 3 study sites. Browsing significantly reduced fine root survival, but this effect varied among sites. Contrary to our predictions, fine root decomposition rates were consistently lower (-21%) in browsed stands. Two important features distinguish fine root dynamics in our stands from temperate and more southerly boreal ecosystems: (1) low over-winter 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 over-winter mortality and secondly 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.本研究于阿拉斯加内陆塔纳诺河泛滥平原的早期演替森林生态系统中，开展了为期3年的野外试验，旨在探究驼鹿（moose）与雪兔（snowshoe hares）的啃食行为对细根生产、死亡率及分解过程的影响。试验通过在大型永久排除围栏内外布设微根窗（minirhizotrons）开展观测。细根生产与死亡率呈现显著季节动态：每年6月为细根生产速率峰值期，而死亡率峰值则出现在秋季与冬季。1993年为异常低降水年份，该年度的细根年生产与死亡率均显著高于1992年与1994年。地上植食作用显著降低了细根的月生产速率；以三年平均值计，被啃食样地的细根年生产量（311.4 ± 31.7 mm·管⁻¹·年⁻¹）显著低于未被啃食样地（453.8 ± 49.8 mm·管⁻¹·年⁻¹）。由于植食作用对细根月均或年均死亡率的影响较弱，因此被啃食林分的细根周转速率更高。被啃食植株的年细根生产量中，表层土壤所占比例更高。随着生长季推进，所有样地的细根生产均向深层土壤转移，且未被啃食植株的生产层下移深度显著大于被啃食植株。本研究借助参数估计程序MARK模型（Program MARK），基于多个检验模型生成了细根存活与分解参数，这些模型用于验证时间周期、根系出生队列（cohort，即细根首次出现的时间）、根龄、啃食作用及样地对细根寿命与分解性的直接及交互影响。根系出生队列效应显示，5月首次出现的细根存活率最高，后续月份首次出现的细根存活率依次降低；基于根龄的估算结果则表明，细根在首次出现后的初始时段内存活率快速下降。生长季内细根存活率与分解速率呈负相关关系，冬季为存活率最低但分解速率最高的时段。双因素模型分析表明，细根的时间依赖性存活与分解速率在被啃食与未被啃食林分之间、10个细根年龄组之间以及3个研究样地之间均存在显著差异。啃食作用显著降低了细根存活率，但该效应存在样地间差异。与本研究的预设假设相反，被啃食林分的细根分解速率始终较未被啃食林分低21%。本研究林分的细根动态与温带及更南侧的北方森林生态系统存在两点核心差异：（1）细根越冬存活率极低；（2）展叶与细根最大生长量之间存在显著时间滞后，这表明春季再生过程对地上储存养分的依赖程度更高。植食动物似乎在连接这两个过程中发挥了重要作用：一方面加剧了细根越冬死亡率，另一方面则消耗了大量地上储存养分。因此，植食动物不仅对碳与养分循环过程具有显著的直接影响，气候驱动的细根过程还可能以复杂方式与植食作用产生关联，进而塑造植物生长及抗植食防御分配的基本纬度梯度模式。