Rozendaal Danaë M A, Kobe Richard K
Michigan State University, Department of Forestry, Natural Resources Building, 480 Wilson Road, Room 126, East Lansing, MI, 48824-1222, United States of America.
PLoS One. 2016 Nov 21;11(11):e0167139. doi: 10.1371/journal.pone.0167139. eCollection 2016.
In closed-canopy forests, gap formation and closure are thought to be major drivers of forest dynamics. Crown defoliation by insects, however, may also influence understory resource levels and thus forest dynamics. We evaluate the effect of a forest tent caterpillar outbreak on understory light availability, soil nutrient levels and tree seedling height growth in six sites with contrasting levels of canopy defoliation in a hardwood forest in northern lower Michigan. We compared resource levels and seedling growth of six hardwood species before, during and in the three years after the outbreak (2008-2012). Canopy openness increased strongly during the forest tent caterpillar outbreak in the four moderately and severely defoliated sites, but not in lightly defoliated sites. Total inorganic soil nitrogen concentrations increased in response to the outbreak in moderately and severely defoliated sites. The increase in total inorganic soil nitrogen was driven by a strong increase in soil nitrate, and tended to become stronger with increasing site defoliation. Seedling height growth increased for all species in the moderately and severely defoliated sites, but not in lightly defoliated sites, either during the outbreak year or in the year after the outbreak. Growth increases did not become stronger with increasing site defoliation, but were strongest in a moderately defoliated site with high soil nutrient levels. Growth increases tended to be strongest for the shade intolerant species Fraxinus americana and Prunus serotina, and the shade tolerant species Ostrya virginiana. The strong growth response of F. americana and P. serotina suggests that recurring forest tent caterpillar outbreaks may facilitate the persistence of shade intolerant species in the understory in the absence of canopy gaps. Overall, our results suggest that recurrent canopy defoliation resulting from cyclical forest insect outbreaks may be an additional driver of dynamics in temperate closed-canopy forests.
在郁闭度高的森林中，林窗的形成与闭合被认为是森林动态变化的主要驱动力。然而，昆虫导致的树冠落叶也可能影响林下资源水平，进而影响森林动态。我们在密歇根州下半岛北部的一片硬木林中，选取了六个树冠落叶程度不同的样地，评估一场森林天幕毛虫爆发对林下光照可利用性、土壤养分水平以及树木幼苗高度生长的影响。我们比较了六种硬木树种在爆发前、爆发期间以及爆发后的三年（2008 - 2012年）的资源水平和幼苗生长情况。在四个中度和重度落叶的样地，森林天幕毛虫爆发期间树冠开阔度大幅增加，而轻度落叶的样地则没有。中度和重度落叶样地的土壤无机氮总浓度因爆发而增加。土壤无机氮总量的增加是由土壤硝酸盐的大幅增加推动的，并且随着样地落叶程度的增加而增强。在中度和重度落叶样地，所有树种的幼苗高度生长在爆发当年或爆发后的年份都有所增加，但轻度落叶样地没有。生长增加并没有随着样地落叶程度的增加而增强，而是在土壤养分水平高的中度落叶样地最为明显。对于不耐阴的美国白蜡树和黑樱桃以及耐阴的弗吉尼亚铁木来说，生长增加往往最为显著。美国白蜡树和黑樱桃强烈的生长反应表明，在没有林窗的情况下，反复出现的森林天幕毛虫爆发可能有助于林下不耐阴物种的存续。总体而言，我们的结果表明，周期性森林昆虫爆发导致的反复树冠落叶可能是温带郁闭度高的森林动态变化的另一个驱动因素。
