Experimental downed woody debris-created microsites enhance tree survival and growth in extreme summer heat

Introduction Increasing temperatures associated with climate change can lead to more challenging conditions for tree seedlings, including greater soil surface temperatures and reduced growing season soil moisture. Downed woody debris (DWD) may provide microsite effects, such as shade, that moderate these conditions for seedlings. However, few studies have conducted fine-scale assessments of the strength of the microsite as a function of distance from DWD or how the microsite effect differs between species or across topographic gradients. Methods In this study, conducted in the Palouse region of eastern Washington State, we placed three large pieces of DWD (5 m length × 40–50 cm small-end diameter, oriented east–west) on each of three topographic facets (north-facing, flat ridge-top, and south-facing), and planted transects of seedlings of a xerophytic conifer (ponderosa pine, Pinus ponderosa ) and a mesophytic conifer (Douglas-fir, Pseudotsuga menziesii ) on both sides of the DWD at fixed distances (0, 0.25, 0.5, and 1.5 m). The 1.5 m distance was assumed to serve as a control, with no measurable influence from DWD. Seedling responses (stress rating, survival, basal diameter and height growth, and dark-adapted chlorophyll fluorescence) over two growing seasons were used to interpret the influence of DWD on seedling health and survival, especially during stressful episodes of extreme heat, such as occurred during July 2021, the first growing season of the experiment. Soil surface temperature and soil volumetric water content (10 cm depth) were measured at all seedling locations to understand biophysical contributors to seedling response. Results We found that seedlings of both conifers displayed lower stress ratings, higher survival, and greater height growth close to the north side of DWD, with this effect especially pronounced on the flat ridge-top and the south-facing slope. Soil surface temperature decreased greatly in the “shade zone” at 0.0 m and 0.25 m distances on the north side of DWD, and soil volumetric water content declined more quickly outside of the shaded microsite. Discussion These findings suggest that creating or retaining DWD on stressful sites may prove an important climate adaptive management strategy in ecosystem restoration or forest management, especially if extreme heat events continue to increase in frequency.