It is well known that tree growth is strongly affected by climate at high elevations, but it is still unclear how climate variability influences the distribution of montane forest ecosystems, particularly tree line species. To advance knowledge in this field, we combined temporal (tree ring measurements) and spatial data (remotely sensed variables), to quantify how forests responded to climatic variability across altitudinal gradients in Central Mexico. Normalized difference vegetation index (NDVI), tree‐ring chronologies, and site‐level climatic data were used in a vegetation trend analysis of greenness and browning and to reconstruct canopy vigor for the last century. Although a common ring width chronology was developed, we found significant site‐dependent forest growth response, where young trees (<100 years) exhibited heterogeneous growth trends, without an altitudinal pattern. However, mature trees (100–200 years) showed a common growth decline during the middle twentieth century, regardless of their altitude. Annual maximum NDVI anomalies did not show a general greening effect at high‐elevations. The forest showed both greening and browning zones denoting spatial variability in tree vigor. Furthermore, temperature from the previous year had a positive effect on both NDVI and ring width index but negative at beginning of the growing season. The significant relationship between winter‐spring NDVI (December to March) and ring width index (r = 0.64, p < 0.05), was useful to reconstruct the canopy vigor for the last 151 years. Thus, the greening effect derived from NDVI should be carefully interpreted as a direct forest growth increase. These results highlight the potential of integrating remotely sensed and dendrochronological methods to improve predictions of forest ecosystem responses.

Tree-ring time series provide long-term, annually resolved information on the growth of trees. When sampled in a systematic context, tree-ring data can be scaled to estimate the forest carbon capture and storage of landscapes, biomes, and—ultimately—the globe. A systematic effort to sample tree rings in national forest inventories would yield unprecedented temporal and spatial resolution of forest carbon dynamics and help resolve key scientific uncertainties, which we highlight in terms of evidence for forest greening (enhanced growth) versus browning (reduced growth, increased mortality). We describe jump-starting a tree-ring collection across the continent of North America, given the commitments of Canada, the United States, and Mexico to visit forest inventory plots, along with existing legacy collections. Failing to do so would be a missed opportunity to help chart an evidence-based path toward meeting national commitments to reduce net greenhouse gas emissions, urgently needed for climate stabilization and repair.

In this study we show that the effect of rising atmospheric CO₂ levels on forest productivity is influenced by changes in nutrient availability caused by nitrogen (N) deposition. We used a dual‐isotope approach (δ¹⁵N and δ¹³C), combined with dendrochronological and nutritional analyses, to evaluate the response of two dominant tree species in natural forest ecosystems near Mexico City (Pinus hartwegii—pine; Abies religiosa—fir). Our analysis focuses on changes that occurred over the past 50 years at two sites, one under high and one under low N deposition rates. Analyses of carbon isotope composition indicate increasing water‐use efficiency in response to rising CO₂ levels for both species and sites but this effect did not lead to improved tree growth. The magnitude and direction of shifts in ¹³C discrimination indicate a process of acclimation that varied with the rate of N deposition and species traits. Since the 1960s, strong negative responses to N deposition have been observed in fir trees, which showed altered foliar nutrition and growth decline, while the negative impacts of N deposition on pine trees remained undetectable until the 1990s. In recent years, both species have shown significant growth decline under high N deposition despite increasing atmospheric CO₂. Multivariate analysis of leaf nutrients indicates that growth decline was prompted by depleted soil macronutrient (P, K, and Ca) and micronutrient (Cu, Fe, Zn, and Mn) availability. At both sites, fir trees were a better indicator of N deposition due to differences in canopy rainfall interception.

The urban environment induces stress on trees and its impact can be identified by observing the condition of the crown. The aim of this study is to correlate the variables of crown density (Cdn), crown transparency (Ctr) and dieback (Cdie) with the following vegetation indices: the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), blue-normalized difference vegetation index (BNDVI), green-normalized difference vegetation index (GNDVI), green–red vegetation index (GRVI) and red–green–blue vegetation index (RGBVI) of the crowns of trees located in urban green areas, as well as chlorophyll fluorescence (Fv/Fm) as an indirect indicator of the overall tree health condition. A total of 549 trees were evaluated, represented in 24 families, 36 genera and 53 species; the variables had average values of 67.96% for Cdn, 35.19% for Ctr and 1% for Cdie. Correlations were found between Fv/Fm, NDVI and BNDVI. NDVI and BNDVI correlated with variables such as Cdn and Ctr, mainly in species such as Ligustrum lucidum, Jacaranda mimosifolia and Fraxinus uhdei. Therefore, it is possible to evaluate the tree health condition of trees in urban green environments through the identification of unfavorable conditions at the crown level by using vegetation indices for some of the species studied.