Dunne Jennifer A, Parker V Thomas
Department of Biology, San Francisco State University, San Francisco, CA 94132, USA, , , , , , US.
Oecologia. 1999 Apr;119(1):36-45. doi: 10.1007/s004420050758.
The occurrence of mature individuals of Pseudotsuga menziesii in stands of Arctostaphylos species mark the initial stages of mixed evergreen forest invasion into chaparral in central coastal California. We planted two cohorts of P. menziesii seeds at three sites under stands of two Arctostaphylos species and Adenostoma fasciculatum in order to determine whether first-year seedling emergence and survival, particularly during the regular summer drought, underlie the spatial distribution of mature trees observed in chaparral. Regardless of the chaparral species they were planted under, P. menziesii seeds that were not protected from vertebrate predation displayed very little emergence and no survival. In contrast, emergence of P. menziesii that were protected from vertebrate predators was much higher but still did not significantly differ among the three chaparral species. However, survival of protected seedlings under Arctostaphylos glandulosa was much greater than under A. fasciculatum, with intermediate survival under Arctostaphylos montana. While mortality of protected seedlings due to insect herbivory, fungal infection, and disturbance displayed no consistent patterns, summer drought mortality appeared to drive the patterns of survival of P. menziesii under the different chaparral species. These emergence, mortality, and survival data suggest that spatial patterns of P. menziesii recruitment in chaparral are driven by first-year summer drought seedling mortality, but only in years when seeds and seedlings are released from vertebrate predation pressure. Because the first-year drought mortality and survival patterns of P. menziesii seedlings differed strongly depending on the chaparral species, we examined the additional hypothesis that these patterns are associated with differences in the availability of soil moisture under different chaparral species. Both higher survival and lower drought mortality of P. menziesii seedlings were associated with higher soil water potential under Arctostaphylos stands during the summer drought, especially in the subsurface soil. The data suggest that Arctostaphylos stands, particularly stands of A. glandulosa, ameliorate xeric summer conditions to a degree that facilitates first-year establishment of P. menziesii and strongly influences spatial distribution of mature trees.
在加利福尼亚州中部沿海地区，花旗松（Pseudotsuga menziesii）成熟个体出现在熊果属（Arctostaphylos）植物群落中，标志着常绿阔叶林入侵矮橡树林的初始阶段。我们在两种熊果属植物和腺桤木（Adenostoma fasciculatum）林下的三个地点播种了两批花旗松种子，以确定第一年幼苗的出土和存活情况，特别是在夏季常规干旱期间，是否是矮橡树林中成熟树木空间分布的基础。无论种植在何种矮橡树林物种下，未受到脊椎动物捕食保护的花旗松种子出土率极低且无一存活。相比之下，受到脊椎动物捕食保护的花旗松种子出土率要高得多，但在三种矮橡树林物种之间仍无显著差异。然而，在腺叶熊果（Arctostaphylos glandulosa）林下受到保护的幼苗存活率远高于在腺桤木林下，在山地熊果（Arctostaphylos montana）林下的存活率则居中。虽然受到保护的幼苗因昆虫取食、真菌感染和干扰导致的死亡率没有一致模式，但夏季干旱死亡率似乎驱动了花旗松在不同矮橡树林物种下的存活模式。这些出土、死亡和存活数据表明，矮橡树林中花旗松种群补充的空间模式是由第一年夏季干旱导致的幼苗死亡率驱动的，但前提是种子和幼苗免受脊椎动物捕食压力。由于花旗松幼苗第一年的干旱死亡率和存活模式因矮橡树林物种的不同而有很大差异，我们检验了另一个假设，即这些模式与不同矮橡树林物种下土壤水分有效性的差异有关。花旗松幼苗较高的存活率和较低的干旱死亡率与夏季干旱期间熊果属林下较高的土壤水势有关，尤其是在表层以下土壤中。数据表明，熊果属植物群落，特别是腺叶熊果群落，在一定程度上改善了夏季干旱条件，有利于花旗松第一年的定植，并强烈影响成熟树木的空间分布。
