Facilitation influences when trees grow, but not growth rate in a dry temperate forest

Abstract Accurately predicting the role of forests in the global carbon cycle requires a detailed understanding of the factors mediating the timing and magnitude of radial stem growth. While weather conditions and topo‐edaphic factors play a critical role in mediating short‐term woody growth, biotic factors can also drive radial growth dynamics, yet their importance is often overlooked. To better understand how weather and neighbouring trees drive patterns of intra‐annual stem growth, we installed automated dendrometer bands on two tree species ( Pinus ponderosa , Quercus kelloggii ) in a dryland ecosystem for 2 years. We evaluated differences in radial growth dynamics between the two species as well as the influence of neighbourhood (total, heterospecific and conspecific neighbourhood basal area) and weather (precipitation, temperature and vapour pressure deficit) on daily radial growth rates and whether daily growth occurs (growth probability). We found that the differences in intra‐annual radial growth dynamics between the two species were driven partially by weather conditions, with P. ponderosa radial growth remaining relatively consistent across the two study years despite the below average precipitation in 2022, while Q. kelloggii radial growth was significantly reduced in 2022. For both species, the daily growth rate and probability of growth were strongly influenced by weather, though for growth probability their importance varied across year and season. Importantly, both species exhibited greater total radial growth in neighbourhoods with higher heterospecific basal area. Similarly, total growing days was associated with higher heterospecific and total basal area of neighbouring trees. Neighbourhood also served as an important driver of summer daily growth probability for both species but never influenced growth rates. Our results illustrate how neighbourhood can mediate tree growth patterns and how its importance varies across species, years and seasons. Specifically, these results emphasize the potential for neighbourhood dynamics to increase a tree's total radial growth by increasing the frequency of daily radial growth, especially during the dry summer months, rather than through increasing the daily growth rate. This pattern suggests that in this water‐limited system, tree‐tree facilitative effects, potentially through hydraulic lift, play an important role in mediating tree radial growth. Read the free Plain Language Summary for this article on the Journal blog.