Palms and trees resist extreme drought in Amazon forests with shallow water tables

1. The intensity and frequency of severe droughts in the Amazon region has
increase in recent decades. These extreme events are associated with
changes in forest dynamics, biomass and floristic composition. However,
most studies of drought response have focused on upland forests with deep
water tables, which may be especially sensitive to drought. Palms, which
tend to dominate the less well-drained soils, have also been neglected.
The relative neglect of shallow water tables and palms is a significant
concern for our understanding of tropical drought impacts, especially as
one third of Amazon forests grow on shallow water tables (<5m
deep). 2. We evaluated the drought response of palms and trees in forests
distributed over a 600 km transect in central-southern Amazonia, where the
landscape is dominated by shallow water table forests. We compared
vegetation dynamics before and following the 2015-16 El Nino drought, the
hottest and driest on record for the region (-214 mm of cumulative water
deficit). 3. We observed no change in stand mortality rates and no biomass
loss in response to drought in these forests. Instead, we observed an
increase in recruitment rates, which doubled to 6.78% y-1 ± 4.40 (mean ±
SD) during 2015-16 for palms and increased by half for trees (to 2.92% y-1
± 1.21), compared to rates in the pre-El-Nino interval. Within these
shallow water table forests, mortality and recruitment rates varied as a
function of climatic drought intensity and water table depth for both
palms and trees, with mortality being greatest in climatically and
hydrologically wetter environments and recruitment greatest in drier
environments. Across our transect there was a significant increase over
time in tree biomass. 4. Synthesis: Our results indicate that forests
growing over shallow water tables – relatively under-studied vegetation
that nonetheless occupies one-third of Amazon forests - are remarkably
resistant to drought. These findings are consistent with the hypothesis
that local hydrology and its interactions with climate strongly constrain
forest drought effects, and has implications for climate change feedbacks.
This work enhances our understanding of integrated drought effects on
tropical forest dynamics and highlights the importance of incorporating
neglected forest types into both the modeling of forest climate responses
and into public decisions about priorities for conservation.

1. 近几十年来，亚马逊地区严重干旱的强度和频率持续上升。这些极端事件与森林动态、生物量及植物组成的变化密切相关。然而，大多数干旱响应研究集中于具有深水位的高地森林（这类森林可能对干旱特别敏感），而在排水不畅土壤中占主导地位的棕榈树却被忽视。对浅水位及棕榈树的相对忽视，是理解热带干旱影响的一大关键问题——尤其考虑到三分之一的亚马逊森林生长在浅水位（<5米深）区域。

2. 我们评估了亚马逊中南部600公里样带内棕榈树与树木的干旱响应，该区域景观以浅水位森林为主。我们对比了2015-2016年厄尔尼诺干旱前后的植被动态，此次干旱是该地区有记录以来最热且最干旱的事件（累计水分亏缺达-214毫米）。

3. 我们发现，这些森林的林分死亡率未发生变化，生物量也无损失；相反，棕榈树的更新率翻倍至6.78%/年±4.40（均值±标准差），树木的更新率则增加了一半，达到2.92%/年±1.21（与厄尔尼诺前时期相比）。在这些浅水位森林中，棕榈树与树木的死亡率和更新率均随气候干旱强度及水位深度变化：死亡率在气候和水文条件更湿润的环境中最高，而更新率在更干旱的环境中最高。整个样带的树木生物量随时间显著增加。

4. 综合：我们的结果表明，生长于浅水位区域的森林——这类植被虽研究相对不足，却占据了亚马逊森林的三分之一——对干旱具有显著的抗性。这些发现与以下假设一致：局地水文及其与气候的相互作用强烈制约着干旱对森林的影响，且对气候变化反馈具有重要意义。本研究加深了我们对干旱对热带森林动态综合影响的理解，并强调了将被忽视的森林类型纳入森林气候响应模型及保护优先事项公共决策的重要性。