Droughts impact forests by influencing various processes such as canopy greenness, tree growth, and reproduction, but most studies have only examined a few of these processes. More comprehensive assessments of forest responses to climate variability and water shortages are needed to improve forecasts of post-drought dynamics. Iberian forests are well-suited for evaluating these effects because they experience diverse climatic conditions and are dominated by various conifer and broadleaf species, many of which exhibit masting. We assessed how greenness, evaluated using the normalized difference vegetation index (NDVI), tree radial growth, and seed or cone production responded to drought in five tree species (three conifers: silver fir (Abies alba), Scots pine (Pinus sylvestris), and stone pine (Pinus pinea); two broadleaves: European beech (Fagus sylvatica) and holm oak (Quercus ilex) inhabiting sites with different aridity. We correlated these data with the standardized precipitation evapotranspiration index (SPEI) using the climate window analysis (climwin) package, which identifies the most relevant climate window. Drought constrained growth more than greenness and seed or cone production. Dry conditions led to high seed or cone production in species found in cool, moist sites (silver fir, beech, and Scots pine). We also found negative associations of cone production with summer SPEI in the drought-tolerant stone pine, which showed lagged growth−cone negative correlations. However, in the seasonally dry holm oak forests, severe droughts constrained both growth and acorn production, leading to a positive correlation between these variables. Drought impacts on greenness, growth, seed, and cone production depended on species phenology and site aridity. A negative correlation between growth and reproduction does not necessarily indicate trade-offs, as both may be influenced by similar climatic factors.

The tolerance of tree growth to drought in diverse mixed forests subjected to seasonal water shortage is understudied despite their ecological and economic relevance. By combining intra- and inter-annual analyses of radial growth responses to climate and drought at daily and monthly scales, different strategies to acclimate to hydroclimate variability of coexisting conifers and broadleaves were unveiled. We analyzed the growth patterns and responses to hydroclimate variability in two conifers (Pinus engelmannii, Juniperus deppeana) and two broadleaves (Quercus grisea, Arbutus arizonica) co-occurring in a Madrean pine-oak forest located in NW México. The strongest positive response to daily precipitation was found in the two conifers, but this response peaked earlier in J. deppeana than in P. engelmannii, which presented a more delayed formation of radially-enlarging tracheids. The latest negative response to temperature was found in Q. grisea, which agrees with its more delayed xylogenesis than A. arizonica. P. engelmannii presented the highest responsiveness to water shortage, driven by lower precipitation and high maximum temperatures, responding to longer droughts ending in autumn (r ​= ​0.72), whilst A. arizonica showed the lowest responsiveness to short spring droughts (r ​= ​0.39). Growth of P. engelmannii was linked to climate-atmospheric circulation patterns over the near Pacific Ocean. Overall, P. engelmannii and A. arizonica showed high growth rates and earlier growth onset, whilst J. deppeana and Q. grisea showed slower growth rates and later growth onset. The Vaganov-Shashkin growth model evidenced that winter-spring soil moisture was the key driver of growth. Under more arid conditions and more frequent and hotter droughts, pine stands could rapidly shift towards mixed pine-oak forests.

Different leaf (evergreen vs. deciduous habit) and xylem (diffuse-vs. ring-porous wood) traits represent contrasting strategies to face seasonal changes in water availability and temperature. However, how contrasting leaf and xylem habits of coexisting tree species affect stem wood formation and tree-ring development remains poorly understood. Here, we investigated the spatio-temporal patterns of wood formation in two deciduous oaks (Quercus faginea and Quercus petraea) and two evergreen oaks (Quercus ilex and Quercus suber) coexisting in seasonally dry Mediterranean forests along an aridity gradient in Spain. We hypothesized that growth responses to drought and intra- and inter-annual growth patterns would differ between functional groups. We simulated intra- and inter-annual growth using a modified version of the Vaganov-Shashkin (VS) process-based, growth model. The VS model simulations were used to estimate growth changes under a high emission scenario (RCP 8.5) for the current distribution of the study oak species and to forecast their future performance under warm (4.8 ​℃) conditions in the Iberian Peninsula. Our simulations indicate that climate warming would induce a shortening of the ring-growth season and a reduction of radial growth in evergreen and deciduous Mediterranean oaks, particularly in dry sites from southern and eastern Iberia currently occupied by Q. ilex and Q. faginea. Evergreen oaks may better recover after dry periods than deciduous oaks by resuming growth after the summer drought. Low soil water availability in spring would be more detrimental to growth of deciduous oaks. Process-based growth models should be refined and validated to better forecast changes in tree growth as a function of climate.