Plant-plant interactions change during succession on nurse logs in a northern temperate rainforest

Plant-plant interactions change through succession from facilitative to competitive. At early stages of succession, early-colonizing plants can increase the survival and reproductive output of other plants by ameliorating disturbance and stressful conditions. At later stages of succession, plant interactions are more competitive as plants put more energy towards growth and reproduction. In northern temperate rainforests, gap dynamics result in tree falls that facilitate tree regeneration (nurse logs) and bryophyte succession. How bryophyte-tree seedling interactions vary through log succession remains unclear. We examined the relationships of tree seedlings, bryophyte community composition, bryophyte depth, and percent canopy cover in 166 0.5 m x 1.0 m plots on nurse logs and the forest floor in the Hoh rainforest in Washington, USA to test the hypothesis that bryophyte-tree seedling interactions change from facilitative to competitive as the log decays. Tree seedling density was highest on young logs with early-colonizing bryophyte species (e.g., Rhizomnium glabrescens), and lowest on decayed logs with Hylocomium splendens, a long-lived moss that reaches depths >20 cm. As a result, bryophyte depth increased with nurse log decay and was negatively associated with tree seedling density. Tree seedling density was 4.6x higher on nurse logs than on the forest floor, which was likely due to competitive exclusion by forest floor plants, such as H. splendens. Nurse logs had 17 species of bryophytes while the forest floor had six, indicating that nurse logs contribute to maintaining bryophyte diversity. Nurse logs enable both tree seedlings and smaller bryophyte species to avoid competition with forest floor plants, including the dominant bryophyte, H. splendens. H. splendens is likely a widespread driver of plant community structure given its dominance in northern temperate forests. Our findings indicate that plant-plant interactions shift with succession on nurse logs from facilitative to competitive and, thus, influence forest community structure and dynamics. --
Methods
We counted Tsuga heterophylla and Picea sitchensis seedlings in 166 randomly placed 1 m x 1 m plots on the forest floor (n = 52) and 1 m x nurse log diameter on nurse logs (defined as >30 cm diameter and at least 2 m in length; nurse log diameter ranged from 30 to 104 cm; n = 114). For a subset of the nurse log plots (n = 86) and forest floor plots (n = 12), bryophyte composition was surveyed using the point-intercept method whereby each bryophyte species found under each of 100 randomly placed dots on a transparent sheet was noted. Percent canopy cover was measured as a potential confounding factor using a densiometer placed in the center of each plot. Bryophyte depth was measured (cm) using a caliper in the center of each plot. This study was conducted over the course of three summers (2016 – 2018). This data set includes a list of tree seedling density, Tsuga heterophylla seedling density, percent canopy cover, bryophyte depth, and the percent cover of the most abundant bryophyte species in each plot.

植物间的相互作用会随演替进程从促进作用转向竞争作用。在演替早期阶段，早期定植的植物可通过缓解干扰与胁迫环境，提升其他植物的存活率与繁殖产出。在演替后期，随着植物将更多能量投入生长与繁殖，植物间的相互作用更偏向竞争。在北温带雨林中，林隙动态会引发树木倒伏，进而为树木更新与苔藓植物（bryophyte）演替提供促进条件。目前学界尚不清楚苔藓植物与树木幼苗间的相互作用如何随倒木演替发生变化。我们在美国华盛顿州霍河雨林的保育木（nurse logs）与林地地表的166个0.5m×1.0m样地中，调查了树木幼苗、苔藓植物群落组成、苔藓层厚度以及冠层盖度，以验证"随着倒木腐烂，苔藓-幼苗间的相互作用会从促进转向竞争"这一假说。

树木幼苗密度在定植有早期苔藓物种（如毛灯藓*Rhizomnium glabrescens*）的年轻倒木上最高，而在被塔藓*Hylocomium splendens*——一种可形成超过20厘米厚层的长寿命苔藓——占据的腐烂倒木上最低。由此可见，苔藓层厚度随保育木腐烂程度加深而增加，且与树木幼苗密度呈负相关关系。保育木上的树木幼苗密度是林地地表的4.6倍，这可能源于林地地表植物（如塔藓*H. splendens*）的竞争排除作用。保育木共记录到17种苔藓植物，而林地地表仅记录到6种，表明保育木有助于维持苔藓植物多样性。保育木能够帮助树木幼苗与小型苔藓物种避开与林地地表植物（包括优势苔藓塔藓*H. splendens*）的竞争。塔藓*H. splendens*在北温带雨林中占据优势地位，可能是驱动植物群落结构形成的广布物种。本研究结果表明，保育木上的植物间相互作用随演替从促进转向竞争，进而影响森林群落结构与动态。

—— 方法
我们在林地地表（n=52）与保育木（定义为直径>30cm、长度≥2m，直径范围30~104cm；n=114）上设置了166个随机布设的样地：其中林地地表样地为1m×1m，保育木上的样地尺寸为1m×保育木直径。我们对西部铁杉*Tsuga heterophylla*与西加云杉*Picea sitchensis*的幼苗进行了计数。针对其中86个保育木样地与12个林地地表样地的子集，我们采用点截法（point-intercept method）调查苔藓植物组成：将带有100个随机打点的透明覆盖片置于样地内，记录每个打点下方发现的苔藓物种。我们使用冠层分析仪（densiometer）在每个样地中心测定冠层盖度，以控制这一潜在混杂变量。使用游标卡尺在每个样地中心测定苔藓层厚度（单位：厘米）。本研究于2016-2018年的三个夏季开展。本数据集包含各采样样地的树木幼苗密度、西部铁杉*Tsuga heterophylla*幼苗密度、冠层盖度、苔藓层厚度以及样地内优势苔藓物种的盖度百分比。