Increased attention is being paid to the ecological drivers and conservation measures which could mitigate climate change-induced pressures for species survival, potentially helping populations to remain in their present-day locations longer. One important buffering mechanism against climate change may be provided by the heterogeneity in topography and consequent local climate conditions. However, the buffering capacity of this topoclimate has so far been insufficiently studied based on empirical survey data across multiple sites and species. Here, we studied whether the fine-grained air temperature variation of protected areas (PAs) affects the population changes of declining northern forest bird species. Importantly to our study, in PAs harmful land use, such as logging, is not allowed, enabling the detection of the effects of temperature buffering, even at relatively moderate levels of topographic variation. Our survey data from 129 PAs located in the boreal zone in Finland show that the density of northern forest species was higher in topographically heterogeneous PAs than in topographically more homogeneous PAs. Moreover, local temperature variation had a significant effect on the density change of northern forest birds from 1981-1999 to 2000-2017, indicating that change in bird density was generally smaller in PAs with higher topographic variation. Thus, we found a clear buffering effect stemming from the local temperature variation of PAs in the population trends of northern forest birds.Diversity 2020, 12, 56 2 of 12 notable fine-grain heterogeneity in local climates caused by variation in topography (i.e., topoclimatic heterogeneity) [16][17][18][19]. The conservation of landscapes with high topoclimatic variation allows species to move shorter distances while tracking suitable conditions. Thereby, it may facilitate the extended persistence of trailing-edge (the contracting or retreating edge of range) populations by providing refuges and holdouts with favorable local climate conditions [20][21][22][23]. Thus, variation in topoclimates provides a potentially important buffering mechanism against the impacts of climate change on biodiversity [17].However, although the importance of topoclimatic variation for biodiversity preservation is intuitively credible, support for it largely comes from theoretical and model-based studies, or climatological assessments of variability in local climatic conditions [21,[24][25][26]. In essence, the evidence on the benefits of topoclimatic variation emerging from species surveys conducted across multiple sites or protected areas (PAs) is still scarce (see, however, [19]). Monitoring studies of species trends in topographically diverse landscapes both in PAs and outside them have revealed contrasting trends, including upslope as well as downslope expansion of populations, increases in abundance as well as declines of species, and even local extinctions [8,22,24]. Repeated surveys of only one or a few PAs provide useful insights but do not enable systematic comparisons of...