Data from: Demography and growth of subadult savanna trees: interactions of life history, size, fire season, and grassy understory

Tree populations in mesic (>650 mm precipitation/yr) savannas of the world have strong demographic bottlenecks to the transition of subadult trees to the canopy layer. Although such bottlenecks are a major determinant of savanna physiognomy, the factors that allow subadults to traverse the bottleneck are little studied. In a landscape-scale field experiment in a northern Australia savanna, we determined the survival and growth of 1506 permanently marked juveniles (<150 cm tall) and saplings (150–599 cm tall) of canopy species in response to season of fire (early dry season, late dry season, wet season, and unburned), and understory type (herbaceous forbs vs. sorghum [native annual grass]) that differ in seasonal growth patterns and competitive regimes. Trees were assessed before fires and at the end of the following growing season, without repeat fires. We used Akaike-information-criterion-based model selection and multi-model inference for data analyses. Initial height was an important explanatory variable for all responses except genet mortality wherein fire season was important for juveniles and understory type for saplings. Fire season was important to height growth of large juveniles and small saplings (enhanced the year following dry-season fires). Fire season × understory interactions were important for height growth of small juveniles and for the proportion of juveniles transitioning to saplings. Changes in stem numbers were affected by all explanatory variables. All fires topkilled most juveniles (fewer in early dry-season fires in herbaceous understory), but genet death was rare. Late dry-season fires topkilled most saplings; they failed to regain previous height and some died the following year. Given no further fires, persistent large juveniles can grow to sapling size within a year; whereas sapling success is severely hampered by late-dry-season fires, especially in grassy understory. Differences in seasonal phenological patterns of both understory vegetation and trees that vary with size and life history stage are among suggested explanatory mechanisms. Weighted averaged model coefficients for all responses to the explanatory variables are provided for use in population dynamics models. A conceptual framework links landscape-scale variables to tree attributes and responses, with implications for population ecology and community assembly.

全球年降水量超过650毫米的湿润稀树草原中，树木种群在亚成体向冠层过渡的阶段存在强烈的种群瓶颈（demographic bottlenecks）。尽管此类瓶颈是决定稀树草原植被外貌的核心因素，但促使亚成体个体突破该瓶颈的影响因素却鲜有报道。我们在澳大利亚北部的稀树草原开展了一项景观尺度野外实验，针对冠层物种的1506株经永久标记的幼树（株高<150厘米）与幼龄树（株高150~599厘米），探究其存活与生长对火烧季节（旱季早期、旱季晚期、雨季及未火烧对照）和林下植被类型（草本杂类草与高粱（本土一年生草本））的响应——二者的季节生长模式与竞争机制存在显著差异。实验分别于火烧前及后续生长季末进行评估，且未实施重复火烧。本研究采用基于赤池信息准则（Akaike Information Criterion）的模型选择方法与多模型推断进行数据分析。初始株高是除基株死亡率（genet mortality）外所有响应变量的重要解释因子；而就基株死亡率而言，火烧季节对幼树的影响显著，林下植被类型则对幼龄树的影响显著。火烧季节对大型幼树与小型幼龄树的高生长具有显著促进效应，旱季火烧后的次年，其高生长会得到提升。火烧季节与林下植被的交互作用对小型幼树的高生长以及幼树向幼龄树过渡的比例具有显著影响。茎干数量的变化则受所有解释变量的共同影响。所有火烧处理均导致多数幼树出现地上部火烧枯死现象，但在草本林下的旱季早期火烧中，该现象的发生比例较低；不过基株死亡的情况极为罕见。旱季晚期火烧会导致多数幼龄树的地上部火烧枯死，且这些个体无法恢复至此前的株高，部分个体在次年死亡。若后续不再发生火烧，存活的大型幼树可在一年内生长至幼龄树尺寸；而幼龄树的存活成功率会因旱季晚期火烧受到严重抑制，在草本林下尤为显著。林下植被与树木的季节物候模式差异（二者均随个体大小与生活史阶段发生变化）被认为是潜在的解释机制之一。本研究提供了所有响应变量对解释变量的加权平均模型系数，以供种群动态模型研究使用。本研究提出了一个将景观尺度变量与树木属性及响应关联起来的概念框架，其结果对种群生态学与群落构建研究具有重要参考意义。