Detecting Tree Species Effects on Forest Canopy Temperatures with Thermal Remote Sensing: The Role of Spatial Resolution

Abstract

Canopy temperatures are important for understanding tree physiology, ecology, and their cooling potential, which provides a valuable ecosystem service, especially in urban environments. Linkages between tree species composition in forest stands and air temperatures remain challenging to quantify, as the establishment and maintenance of onsite sensor networks is time-consuming and costly. Remotely-sensed land surface temperature (LST) observations can potentially acquire spatially distributed crown temperature data more efficiently. We analyzed how tree species modify canopy air temperature at an urban floodplain forest (Leipzig, Germany) site equipped with a detailed onsite sensor network, and explored whether mono-temporal thermal remote sensing observations (August, 2016) at different spatial scales could be used to model air temperatures at the tree crown level. Based on the sensor-network data, we found interspecific differences in summer air temperature to vary temporally and spatially, with mean differences between coldest and warmest tree species of 1 °C, and reaching maxima of up to 4 °C for the upper and lower canopy region. The detectability of species-specific differences in canopy surface temperature was found to be similarly feasible when comparing high-resolution airborne LST data to the airborne LST data aggregated to 30 m pixel size. To realize a spatial resolution of 30 m with regularly acquired data, we found the downscaling of Landsat 8 thermal data to be a valid alternative to airborne data, although detected between-species differences in surface temperature were less expressed. For the modeling of canopy air temperatures, all LST data up to the 30 m level were similarly appropriate. We thus conclude that satellite-derived LST products could be recommended for operational use to detect and monitor tree species effects on temperature regulation at the crown scale.

Tree species matter for forest microclimate regulation during the drought year 2018: disentangling environmental drivers and biotic drivers

“…Thus, for the temperate zone, the contribution of the latent heat flux to the local energy budget might be reduced, which in turn would lead to an overall decrease in the cooling potential provided by trees. Davis et al 3 even found evidence that some forests will lose their capacity to buffer climate extremes as sites become increasingly water limited.Differences in the cooling effectiveness of trees species 9,11,32 were found to be caused by complex interactions of meteorological factors as well as leaf, canopy and hydraulic characteristics of trees 12,33,34 . The amount of incoming solar irradiance being reflected or absorbed by the canopy or transmitted through the canopy, depends on the total leaf surface area available, and thereby determines the degree of canopy warming 35 .…”

“…Differences in the cooling effectiveness of trees species 9,11,32 were found to be caused by complex interactions of meteorological factors as well as leaf, canopy and hydraulic characteristics of trees 12,33,34 . The amount of incoming solar irradiance being reflected or absorbed by the canopy or transmitted through the canopy, depends on the total leaf surface area available, and thereby determines the degree of canopy warming 35 .…”

“…Forest ecosystems have been shown to effectively buffer climate extremes and mitigate climate change effects below the canopy [1][2][3][4][5] as they create a distinct microclimate, which is decoupled from their surrounding with substantial deviations between ambient air temperature and temperatures within and below the canopy 3,4,[6][7][8][9] . Globally, understory temperatures in forests were found to be on average 1.7 ± 0.3 °C (mean) and 4.1 ± 0.5 °C (maximum), cooler than ambient temperatures 4 , which is of greater magnitude than the global warming of land and ocean temperatures (0.99 °C) over the past century 10 .…”

Response of Common and Rare Beetle Species to Tree Species and Vertical Stratification in a Floodplain Forest

“…Therefore, they also increase vertical beta diversity. It should also be noted that due to a lack of regular flooding events since the mid-20th century, a second layer of shade-tolerant but not flooding-tolerant Sycamore has developed that currently creates very dark and moist conditions in the understory [28]. This second canopy possibly increases the microclimatic gradient between the understory and the canopy, thus potentially amplifying differences in their beetle communities.…”

“…As in many floodplain forests, the fluvial dynamics in the Leipzig floodplain forest are disturbed by human interventions. It remains, however, a near-natural deciduous forest [27,28] with a high species richness of various arthropods such as spiders [29] and xylobiont beetles [21].…”