The influence of structural, functional, and species diversity to temporal stability of productivity and efficiency of resource use in a tropical tree diversity experiment

由于过去的砍伐森林和生态系统退化，在热带和亚热带地区植树造林的需求和潜力很大。与单一栽培相比，混合树种种植园提供了建立森林的机会，这些森林的生产力可能更高，并提供更高水平的生态系统服务。然而，多样化的树木种植园是否确实更能抵御干旱，以及它们是否比单一栽培更有效地利用养分和水分，还没有得到充分的研究，特别是在热带地区。在这里，我们打算利用这个独特的机会来分析最近在巴拿马收获的“撒丁尼亚”树种多样性实验，该实验的树龄为16年，是同类实验中最古老的。拟议项目的总体目标是确定树木邻域多样性、树木与树木相互作用的规模、随时间推移对树木的生产力和资源利用动态(水和养分)是否具有稳定作用。具体地说，我们假设：a)生产力和资源利用的互补性随着乔木群落多样性(物种、结构、功能)的增加和时间的推移而增加，b)生产力和资源利用的时间变异性随着多样性的下降而下降，稳定性随着多样性的增加而增加，c)结构多样性和物种丰富度独立地直接影响单株树木的生产力和资源利用，以及d)在干旱期间，木本植物生物量生产、水分利用效率和营养的互补性随着多样性的增加而增加。为了验证这些假设，我们将分析来自近150棵树的树干圆盘，以确定单个树木生长轮中的生物量增量、元素浓度以及碳和氧同位素。对于这些来自1、2、3或5个树种的试验地的树木，我们将确定在正常或潮湿和干旱年份，邻域多样性对树木的木材生物量生产、水分利用和营养的影响。这将是第一次以年度分辨率和相对较长的时间段(16年)对热带树木的这些响应变量进行组合评估的研究。对潮湿和极干旱时期的中期树木多样性影响的拟议分析，在预测气候变化的背景下，对未来的种植园特别相关。这项研究的结果将表明，混合林木种植园是否能够以较低的营养和水成本以及较低的干旱风险生产木材。

Owing to past deforestation and ecosystem degradation there is a large need and potential for afforestation in tropical and subtropical regions. Mixed-species plantations offer an opportunity to establish forests that may be more productive and offer ecosystem services at a higher level compared to monocultures. However, whether diverse tree plantations are in fact more resilient to droughts and whether they use nutrients and water more efficiently than monocultures has not been sufficiently studied, in particular not in tropical regions. Here we intend to use the unique opportunity to analyse the recently harvested “Sardinilla” tree diversity experiment in Panama, with 16 years the oldest one of its kind. The overall objective of the proposed project is to determine whether there is a stabilizing effect of tree neighbourhood diversity, the scale at which tree-tree interactions occur, on the productivity and resource use dynamics (water and nutrients) of trees over time. Specifically we hypothesize that: a) complementarity of productivity and resource use increases with increasing diversity (species, structural, functional) of tree neighbourhoods and over time, b) the temporal variation in productivity and resource use declines and stability increases with diversity, c) structural diversity and species richness independently and directly affect the productivity and resource use of individual trees, and d) during drought periods, complementarity regarding woody biomass production, water-use efficiency, and nutrition increases with increasing diversity. To test these hypotheses, we will analyse stem discs from almost 150 trees to determine the biomass increment, element concentrations, and C and O isotopes in individual tree growth rings. For these trees, which originate from experimental plots with 1, 2, 3, or 5 tree species, we will determine the influence of neigbourhood diversity on wood biomass production, water use and nutrition of trees during normal or wet and dry years. This will be the first study that assesses these response variables in combination, at an annual resolution, and over a relatively long time period (16 years) in tropical trees. The proposed analyses of me-dium-term tree diversity effects during wet and very dry periods is particularly relevant for future plantations against the background of projected climate change. Results from this study will indicate whether mixed tree plantations can produce wood with lower nutrient and water costs and at lower risks with regard to droughts.