The moisture content of dead leaves, twigs and bark on the forest floor is a key determinant of fire behaviour. The microclimate inside forests, which drive the moisture content of these dead fuel components, is typically measured at screen height (150 cm). However, in some forest types, the surface fuel at ground level may be subject to additional sheltering from low shrubs, ferns and grasses, which could alter the microclimate near the surface (hereafter near-surface). In such cases, screen height measurements may not adequately represent the near-surface conditions that determine dead fuel moisture contents.

We sought to quantify the effect of understorey vegetation on near-surface microclimate. We measured in-forest temperature, relative humidity and solar radiation in eucalypt forests over two fire seasons at both screen height and the near-surface using weather stations at 25 sites. The sites encompassed wet eucalypt forest (n=18) with a dense, mesic understorey and dry eucalypt forest (n=7) with a sparser, scleromorphic understorey.

Wet forests with dense understorey vegetation had near-surface air temperatures that averaged 1.3°C lower, relative humidities that averaged 13.1% higher and total solar radiation that was 0.84 MJ less per day compared with those measured at screen height. These microclimate differences led to predicted fuel moistures which averaged 4.7% higher at the near-surface compared with screen height – this was statistically significant. In contrast, dry forests with less understorey vegetation, had near-surface air temperatures that averaged 4.2°C higher, and relative humidities that averaged 3.1% lower compared to screen height. These differences were not large enough to translate into statistically significant differences in predicted fine fuel moisture between heights.

Overall, these findings show that understorey vegetation plays an important role in moderating near-surface microclimate in some forest types and this needs to be taken into consideration when predicting fuel moisture.

森林地面上枯叶，树枝和树皮的水分含量是决定火灾行为的关键因素。森林内部的微气候通常会在筛网高度（150厘米）处进行测量，这些微气候驱动着这些死燃料成分的水分含量。但是，在某些森林类型中，地面的地表燃料可能会受到来自低矮灌木，蕨类和草类的额外遮蔽，这可能会改变地表附近（以下简称地表）的微气候。在这种情况下，筛网高度测量值可能不足以代表确定死油水分的近地表条件。

我们试图量化下层植被对近地表微气候的影响。我们使用25个站点的气象站测量了两个火灾季节在屏风高度和近地表的两个火灾季节内桉树林中的森林温度，相对湿度和太阳辐射。这些地点包括湿润的桉树林（n = 18）和茂密的中层下层，以及干燥的桉树林（n = 7）和稀疏的，硬形下层。

与在幕高处测得的湿润森林相比，底层植被密集的湿润森林的近地表空气温度平均低1.3°C，相对湿度平均高13.1％，总太阳辐射每天减少0.84 MJ。这些微气候差异导致预测的燃料水分含量，与筛网高度相比，在近地表平均高出4.7％，这在统计上是有意义的。相比之下，植被较少的干燥森林的近地表空气温度平均高出4.2°C，相对湿度比屏幕高度平均低3.1％。这些差异不够大，不足以转化为高度之间的预测精细燃料水分之间的统计学显着差异。

总体而言，这些发现表明，在某些森林类型中，林下植被在调节近地表微气候中起着重要作用，在预测燃料湿度时需要考虑这一点。