Data from: Foliar N content parallels increasing aridity in a Mediterranean-Saharan transition zone: Evidence from regional and global trends

Aim Warm deserts are characterized by water shortages and high-temperature
extremes. A common adaptive strategy in such environments is the
maximization of photosynthetic capacity, which allows plants to achieve
positive carbon budgets by taking advantage of short periods of water
availability and non-inhibitory temperatures. Assuming that photosynthetic
capacity is approximated by leaf N concentration, we tested the hypothesis
that environmental aridity is related to an elevated leaf nitrogen
content. Location 53 locations in the transitional zone spanning the
Mediterranean and the Sahara Desert in Morocco. The mean maximal
temperature (Tmax) within the area varied between 35.7 and 43.5 °C, and
the mean annual precipitation (MAP) was between 12 and 246 mm. Taxon 225
vascular species representative of local vegetation Methods Leaf samples
were collected along a regional aridity gradient and preserved in
herbarium presses. The leaf mass per area (LMA) and N concentrations
expressed on leaf mass (Nmass) and area (Narea) basis were determined. We
also obtained LMA and Nmass values for 6711 species from a worldwide
database for comparative analysis. Results Significant increases in mean
LMA, Nmass, and Narea accompanied the increase in Tmax and the decrease in
MAP in woody species and in non-graminoid herbs, but not in graminoids.
Considering that the climate in our sampling area as a whole was arid, we
compared the Nmass values of Moroccan plants with those from a worldwide
database and found that at a common LMA, the Moroccan plants showed on
average elevated Nmass relative to the global values. Main conclusions
These two lines of evidence: regional gradient and global comparison
confirm that hot deserts select for high leaf N content. This result
predicts the direction of natural selection that will accompany future
climate warming.

研究目标：暖性荒漠（Warm deserts）以水资源匮乏与极端高温为典型特征。此类生境中常见的适应策略为光合能力最大化，使植物可借助短暂的有效降水时段与非抑制性温度实现正碳收支。本研究假设可通过叶片氮浓度（leaf N concentration）表征光合能力，进而验证“环境干旱度与叶片氮含量升高相关”这一假说。
研究区域：本研究的采样区域为摩洛哥境内横跨地中海（Mediterranean）与撒哈拉沙漠（Sahara Desert）的过渡带，共设置53个采样点。该区域平均最高气温（Tmax）介于35.7~43.5℃，年平均降水量（MAP）为12~246 mm。
研究类群：共选取225种代表当地植被的维管植物（vascular species）。
研究方法：沿区域干旱梯度采集叶片样本，使用标本压制夹保存。测定了比叶质量（leaf mass per area, LMA）、以叶片干重为基准的氮浓度（Nmass）以及以叶面积为基准的氮浓度（Narea）。同时，从全球数据库中获取6711个物种的LMA与Nmass数据，用于后续比较分析。
研究结果：在木本植物与非禾本草本植物中，随着平均最高气温升高、年平均降水量降低，其平均LMA、Nmass与Narea均显著升高；而禾本类植物（graminoids）未呈现该趋势。鉴于本研究采样区域整体处于干旱环境，我们将摩洛哥植物的Nmass数据与全球数据库中的对应数据进行对比，发现在相同LMA条件下，摩洛哥植物的平均Nmass显著高于全球平均水平。
主要结论：本研究通过区域干旱梯度分析与全球尺度比较分析这两类证据，证实暖性荒漠对高叶片氮含量存在选择作用。该结果可预测未来气候变暖背景下自然选择的作用方向。