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650mm的湿润稀树草原（mesic savannas）中，树木种群在亚成体树木（subadult trees）向冠层（canopy layer）过渡的阶段存在强烈的种群瓶颈（demographic bottlenecks）。尽管这类瓶颈是决定稀树草原植被外貌（savanna physiognomy）的核心因子，但促使亚成体树木越过该瓶颈的影响机制却鲜有研究。我们在澳大利亚北部的一处稀树草原开展了大尺度野外实验（landscape-scale field experiment），针对冠层树种的1506株永久标记幼树（juveniles，株高<150cm）和幼龄木（saplings，株高150~599cm），探究其存活与生长对火烧季（season of fire，涵盖旱季早期、旱季晚期、雨季及未火烧对照）和林下植被类型（understory type，分为非禾本草本植物与本土一年生禾草高粱（sorghum））的响应。两类林下植被的季节生长格局与竞争格局（competitive regimes）存在显著差异。我们分别在火烧前及后续生长季结束时开展了树木调查，实验未设置重复火烧环节。数据分析采用基于赤池信息准则（Akaike-information-criterion）的模型选择（model selection）方法与多模型推断（multi-model inference）策略。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变量（explanatory variable）；对于基株死亡率，火烧季对幼树的影响显著，而林下植被类型则对幼龄木的影响显著。火烧季对大型幼树和小型幼龄木的高生长（height growth）具有显著调控作用——旱季火烧后的次年，该类个体的高生长得到提升。火烧季与林下植被的交互效应对小型幼树的高生长以及幼树向幼龄木过渡的比例具有显著影响。茎秆数量（stem numbers）的变化则受到所有解释变量的共同调控。所有火烧处理均导致绝大多数幼树的地上部分死亡（topkilled，即地上组织被火烧毁但基部存活），仅在非禾本草本植被下的旱季早期火烧中，该现象发生比例较低，但基株死亡事件极为罕见。旱季晚期火烧会导致大多数幼龄木的地上部分死亡，这类个体无法恢复至火烧前的株高，部分个体在次年死亡。若后续无火烧干扰，存活的大型幼树可在一年内生长至幼龄木尺寸；而幼龄木的存活与生长会受到旱季晚期火烧的严重抑制，在草本植被覆盖的林下尤为明显。林下植被与树木的季节物候格局（phenological patterns）的差异（随个体大小与生活史阶段而异）是潜在的解释机制之一。本研究提供了所有响应变量对应解释变量的加权平均模型系数，可直接用于种群动态模型（population dynamics models）的构建。本研究提出的概念框架（conceptual framework）将大尺度景观变量与树木属性及响应相关联，可为种群生态学（population ecology）与群落组装（community assembly）研究提供理论参考。