Microhabitats and canopy cover moderate high summer temperatures in a fragmented Mediterranean landscape

Abstract

Extreme heat events will become more frequent under anthropogenic climate change, especially in Mediterranean ecosystems. Microhabitats can considerably moderate (buffer) the effects of extreme weather events and hence facilitate the persistence of some components of the biodiversity. We investigate the microclimatic moderation provided by two important microhabitats (cavities formed by the leaves of the grass-tree Xanthorrhoea semiplana F.Muell., Xanthorrhoeaceae; and inside the leaf-litter) during the summer of 2015/16 on the Fleurieu Peninsula of South Australia. We placed microsensors inside and outside these microhabitats, as well as above the ground below the forest canopy. Grass-tree and leaf-litter microhabitats significantly buffered against high temperatures and low relative humidity, compared to ground-below-canopy sensors. There was no significant difference between grass-tree and leaf-litter temperatures: in both microhabitats, daily temperature variation was reduced, day temperatures were 1-5°C cooler, night temperatures were 0.5-3°C warmer, and maximum temperatures were up to 14.4°C lower, compared to ground-below-canopy sensors. Grass-tree and leaf-litter microhabitats moderated heat increase at an average rate of 0.24°C temperature per 1°C increase of ambient temperature in the ground-below-canopy microhabitat. The average daily variation in temperature was determined by the type (grass-tree and leaf-litter versus ground-below-canopy) of microhabitat (explaining 67%), the amount of canopy cover and the area of the vegetation fragment (together explaining almost 10% of the variation). Greater canopy cover increased the amount of microclimatic moderation provided, especially in the leaf-litter. Our study highlights the importance of microhabitats in moderating macroclimatic conditions. However, this moderating effect is currently not considered in species distribution modelling under anthropogenic climate change nor in the management of vegetation. This shortcoming will have to be addressed to obtain realistic forecasts of future species distributions and to achieve effective management of biodiversity.

Fig 1. Location of the study sites on the Fleurieu Peninsula.

The small inset map illustrates the location of the Fleurieu Peninsula in Australia.

Fig 2. The grass tree, Xanthorrhoea semiplana F.Muell. (Xanthorrhoeaceae).

The senescent leaves (brown colour) usually form a closed cavity, in which microsensors were placed. Red scale bar is approximately 1 m in length.

Fig 3. Hourly temperature and relative humidity data for a 48 hour period (from 12am on 31 December 2015 to 12 am on 2 January 2016) for microsensors in the three microhabitats at Scott Conservation Park, site SCE.

Fig 4. Box plots showing the median (centre lines of boxes), first and third quartiles (lower and upper box boundaries, respectively) and highest and lowest values of daytime a) temperature and b) relative humidity for the three microhabitats in six sites over three summer months from 1 December 2015 to 29 February 2016.

Micohabitat codes: BC = ground-below-canopy microhabitat (0.5 m above the ground), GT = in the cavity formed by the senescent leaves of a grass tree, Xanthorrhoea semiplana F.Muell. (Xanthorrhoeaceae); LL = in the leaf litter. See Table 1 for site codes.

Fig 5. Average daytime temperature buffering (compared to the ground-below-canopy microhabitat in the same location) provided by eight grass-tree (symbols, dashed trendlines) and eleven leaf-litter (shapes, dotted trendlines) microhabitats over 92 days during the 2015/2016 summer in vegetation fragments in the Fleurieu Peninsula.

Regression lines were fitted using the method of least squares, assuming a linear relationship. There was no significant difference between the means of the slopes in the two microhabitats (t = 0.894; d.f. = 17; p = 0.192).