Data from: Historic disturbance regimes promote tree diversity only under low browsing regimes in eastern deciduous forest

Eastern deciduous forests are changing in species composition and diversity outside of classical successional trajectories. Three disturbance mechanisms appear central to this phenomenon: fire frequency is reduced, canopy gaps are smaller, and browsers are more abundant. Which factor is most responsible is a matter of great debate and remains unclear, at least partly because few studies have simultaneously investigated more than one process. We conducted a large-scale experiment in mesophytic forests of West Virginia, USA, to test three key hypotheses: (1) the fire hypothesis (fire suppression limits diversity to few shade-tolerant, fire-intolerant species that replace and suppress many fire-tolerant species); (2) the gap hypothesis (small gaps typical of today's forests promote dominance of a few shade-tolerant species); and (3) the browsing hypothesis (overbrowsing by deer limits diversity to a few unpalatable species). We tested these hypotheses using a factorial experiment that manipulated surface fire, large canopy gap formation (gap size 255 m2), and browsing by deer, and we followed the fates of >28 000 seedlings and saplings for five years. Understory tree communities in control plots were dominated (up to 90%) by Fagus grandifolia, averaging little more than two species, whereas overstories were diverse, with 10–15 species. Fire, large canopy gaps, and browsing all dramatically affected understory composition. However, our findings challenge views that fire and large canopy gaps can maintain or promote diversity, because browsers reduced the benefits of gaps and created depauperate understories following fire. Consequently, two major disturbances that once promoted tree diversity no longer do so because of browsing. Our findings appear to reconcile equivocal views on the role of fire and gaps. If browsers are abundant, these two disturbances either depress diversity or are less effective. Alternatively, with browsers absent, these disturbances promote diversity (three- to fivefold). Our results apply to large portions of eastern North America where deer are overabundant, and we provide compelling experimental evidence that historical disturbance regimes in combination with low browsing regimes typical of pre-European settlement forests could maintain high tree species diversity. However, restoring disturbances without controlling browsing may be counterproductive.

北美东部落叶林的物种组成与多样性正偏离经典演替轨迹发生改变。目前有三类干扰机制（disturbance mechanisms）被认为是该现象的核心驱动因素：火灾频率降低、冠层间隙（canopy gaps）规模缩小，以及植食性啃食动物种群密度升高。至于哪一因素是主导驱动因子，目前仍存在广泛争议且尚未明确，这在一定程度上是因为鲜有研究同时探究多种干扰过程的协同效应。我们在美国西弗吉尼亚州的中生林（mesophytic forests）中开展了一项大规模野外实验，以检验三项核心假说：（1）火灾抑制假说：火灾抑制作用会将林分多样性限制在少数耐阴、耐火性差的物种范围内，这些物种会替代并抑制多数耐火物种；（2）冠层间隙假说：当前森林中普遍存在的小型冠层间隙会促使少数耐阴物种占据优势；（3）啃食假说：鹿类过度啃食会将林分多样性限制在少数不可食用的物种范围内。我们采用析因实验设计，对地表火烧、大型冠层间隙（间隙面积达255平方米）以及鹿类啃食三个因素进行人工调控，并连续5年追踪超过28000株幼苗和幼树的动态。对照组样地的林下乔木群落以美洲山毛榉（Fagus grandifolia）占绝对优势（占比最高可达90%），平均物种数仅略多于2种，而林冠层的物种多样性却较高，包含10~15个物种。地表火烧、大型冠层间隙以及鹿类啃食均会显著改变林下群落组成。然而，我们的研究结果挑战了“火灾与大型冠层间隙能够维持或提升群落多样性”的主流观点：啃食动物会抵消冠层间隙带来的多样性增益，且火烧后会进一步导致林下群落物种贫乏化。因此，原本能够促进乔木多样性的两类主要干扰，如今因植食性啃食的存在而丧失了这一功能。我们的研究结果调和了学界关于火灾与冠层间隙作用的争议性观点：当植食性啃食压力较高时，这两类干扰要么会降低群落多样性，要么无法发挥其应有的促进作用；而在无啃食干扰的情况下，它们则可将群落多样性提升3~5倍。我们的研究结果适用于北美东部鹿类种群过度繁盛的广大区域，同时提供了极具说服力的实验证据：即欧洲殖民前典型的低啃食压力与历史干扰模式可维持较高的乔木物种多样性。但若仅恢复干扰模式而不控制植食性啃食压力，反而可能适得其反。