Taking the temperature of forest microclimate research

Imagine: it’s a hot summer day. You’re sweating away in an urban apartment. Perhaps ice-cream would come to mind, or a dive in a swimming pool. Or a nice stroll through a lush and cool summer forest. Yes, that last one could be a great solution as well: forest microclimates contrast strongly with the climate outside forests, and on a bright summer day that difference could be up to several degrees Celsius. However, the climate inside forests is significantly more complicated than simply this air-conditioning on a hot summer day. Nevertheless, it is crucial for the understanding of the biodiversity and functioning of our forests to get to this true forest microclimate and integrate it into our ecological research.

The beautiful diversity of forest canopies (pictures by Eva Gril and Hugo Mahier)

Despite the potentially broad impact of this microclimate on the response of forest ecosystems to global change, we have long lacked a good idea of how microclimates within and below tree canopies drive nature’s response to global change. Recently, however, the importance of microclimate has moved firmly into the spotlights (see e.g. Zellweger et al. 2020, Lembrechts and Nijs 2020), and our understanding of what climate means below our forest canopies has been rapidly increasing.

With a team of (forest) microclimate experts, we decided we’d have to sit together and ‘take the temperature’ of what we know and what we don’t about forest microclimates. We met in a beautiful mansion in the middle of Sweden during a winter snowstorm in late February 2020 (for many of us the last time we got another view than our own home office as soon after the world got into lockdown). In that inspiring atmosphere, we discussed our current knowledge on forest microclimate and set a first step towards a paper summarizing that knowledge. That paper is now out for all to read!

A Swedish winter wonderland

In this review paper, we explain how variation in forest microclimates over space and time results from an interplay of forest features, topography and landscape composition. We stress and exemplify the importance of considering forest microclimates to understand variation in biodiversity and ecosystem functions across forest landscapes. Next, we explain how macroclimate warming (of the free atmosphere) can affect microclimates, and vice versa, via interactions with land-use changes across different biomes. We summarize all drivers of forest microclimate to provide a good idea of the many factors at play and how they are influencing the outcome (as shown in the figure below).

Overview of drivers of microclimate from the paper. Multiple vegetation drivers of microclimate might be of different importance in forest at boreal (top), temperate (middle), and tropical (bottom) latitudes, even if most processes are general. Increasing tree density from open non-forest habitats (A), to plantations with a simple canopy structure (B), to (semi-)natural forest with complex structure (C) reduces below-canopy wind speeds above ground. Forest canopies can reduce ground snow cover and thus decrease the insulating effect of snow cover on cool soil temperatures during the cold season (D). Vertical vegetation distribution (E-F) influences the amount and quality of incoming shortwave radiation, outgoing longwave radiation and moisture exchange. Disturbances can create canopy gaps (G), providing a local shift in microclimate. Seasonal reductions in canopy cover (tree phenology, H) during the cool and/or dry season increases the exposure of the internal forest to ambient conditions. Forests also buffer the temporal (i.e. daily, seasonal and interannual) variability in temperature conditions compared to adjacent non-forest systems (bottom panel). This buffering effect varies with vegetation height and structure, with reduced buffering in secondary, post-agricultural forests (I) relative to primary or ancient, (semi-)natural forests (J). Microhabitats within a forest, such as those created by epiphytic plants (K) can offer an even more buffered microclimate, critical for the ecology and physiology of many forest species. Finally, the temperature offset in forests can change throughout the day, with cooler forest interiors vs. open areas during the day (L) and warmer at night (M). For the sake of simplicity, we chose to depict wind, shortwave radiation, and temperature in the boreal, temperate, and tropical panel, respectively. However, of course all of these microclimate variables can be relevant to systems across latitudes.

Finally, we wanted to know what had to come next. With all those present at the meeting in this beautiful and peaceful Swedish mansion, we did a priority ranking of future research questions at the interface of microclimate ecology and global change biology. We realized progress was needed (and luckily soon to be expected) on three key themes: (1) disentangling the drivers and feedbacks of forest microclimates; (2) global and regional mapping and predictions of forest microclimates; and (3) the impacts of microclimate on forest biodiversity and ecosystem functioning in the face of climate change.

Priority ranking of forest microclimate research ideas

We end with a very positive note: good microclimate data is increasingly becoming available (see e.g. Lembrechts et al. 2020), opening the door to accurate and trustworthy models of climate variability at spatial and temporal scales relevant to our forests. This will revolutionize our understanding of the dynamics, drivers and implications of forest microclimates on biodiversity and ecological functions, and the impacts of global change. Yet this data is coming not a minute too late, as it is urgently needed to support the sustainable use of forests and to secure their biodiversity and ecosystem services for future generations. Together, and with good data at hand, we can make that last point a reality.

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