Including dynamics in the equation: Tree growth rates and host specificity of vascular epiphytes

1. The forest canopy is home to a rich biota. One salient feature are the dynamics of the habitat-building trees, which are growing and eventually vanishing. Tree species strongly differ in growth rates, final size and longevity. Nevertheless, these inherent dynamics have been a blind spot in studies on host specificity of vascular epiphytes (vascular plants dwelling on trees without parasitizing them) – not least because tree growth rates and longevity are usually unknown in highly diverse tropical forests. The present study aims at tackling this blind spot.
2. We compared epiphyte abundances (> 23,000 individuals) found on 285 individuals of four focal tree species in a lowland moist forest in Panama. Data on repeated dbh censuses from a permanent tree plot provided the unique opportunity to estimate the age of our sampled trees. We compared the relative importance of tree longevity for host biases with that of other host tree characteristics, namely microclimatic conditions and bark acidity, rugosity and stability.
3. The studied tree species differ in host quality and epiphyte species partly differ in host preferences. The conclusions concerning relative host tree quality depend hugely on whether or not different tree growth rates are considered. Comparing these conclusions allows important insights into the role of tree longevity in shaping epiphyte communities. Relating tree trait differences to the observed distributions of epiphytes among the focal tree species shows how the simultaneous action of various tree characteristics causes host biases.
4. Synthesis. This study highlights the substantial but, up to now, hidden role of different tree growth rates for host tree specificity of vascular epiphytes. Future investigations need to consider this possibly confounding factor adequately to avoid spurious conclusions.

1. 森林林冠孕育着丰富的生物群落。其中一个显著特征是构建生境的树木的动态变化过程：它们持续生长，最终走向衰亡。不同树木物种在生长速率、最终体型与寿命上存在显著差异。然而，这类内在的动态变化一直是附生维管植物（vascular epiphytes，指非寄生性附生于树木的维管植物）宿主特异性研究中的盲区——这在很大程度上是因为在高度多样的热带森林中，树木的生长速率与寿命通常难以获知。本研究旨在填补这一研究空白。
2. 我们对巴拿马低地湿润林内4种目标树种的285株个体上的附生植物丰度（超过23000株个体）进行了对比统计。依托永久样地中重复开展的胸径（diameter at breast height, DBH）普查数据，我们得以估算采样树木的树龄。我们对比了树木寿命对宿主偏好偏差的相对重要性，以及其他宿主树木特征的影响，这些特征包括微气候条件、树皮酸度、粗糙度与稳定性。
3. 本研究涉及的树木物种在宿主适宜性上存在差异，附生植物物种也在一定程度上表现出宿主偏好差异。关于宿主树木相对适宜性的结论，在很大程度上取决于是否考虑了不同树木的生长速率差异。通过对比这类结论，我们可以深入解析树木寿命在塑造附生植物群落结构中的作用。将树木性状差异与附生植物在目标树种间的分布格局相关联，可以揭示多种树木特征如何共同作用，引发宿主偏好偏差。
4. 综合分析。本研究揭示了树木生长速率差异在附生维管植物宿主特异性中所扮演的显著却长期被忽视的作用。未来的相关研究需充分考虑这一潜在混淆因素，以避免得出误导性结论。