DiscussionTop

High-elevation forests are among the most powerful “living labs” for testing the ecological and evolutionary responses of biota to geophysical influences (Körner, 1998). Here, we showed that drought acted to reduce the growth of two tree species from high elevations in central Mexico, whereas the seasonal wet climate enhanced radial growth. Specifically, warmer temperatures associated with dry episodes during the autumn and winter seasons are often related to narrow tree-rings (Villanueva-Díaz et al. 2018), while wet and cool winters events enhance radial growth, as reported by Gutiérrez-García & Ricker, (2019). 

However, we found species-specific growth responses to hydroclimate, which are modulated by their dendrochronological potential (Table 1). Thus, P. teocote presents a greater relative change in ring-width index between consecutive years, which is often an indicator of high responsiveness to climate (see Fig. 2) and is in line with the findings of similar studies (Gutiérrez-García & Ricker, 2019; González-Cásares et al., 2017). Furthermore, the highest coherence in growth between conspecific trees was observed in P. teocote, as an indicator of better growth synchrony among trees (Fritts, 1976). Indeed, this species showed more uniform growth patterns than A. religiosa, which can be modulated by specific-site conditions such as altitudinal range, climate type and drought tolerance, among others (Pompa-García et al., 2021).

Responses to the Tmax affect growth through hydric stress (Fig. 2), as documented in similar studies (Astudillo et al., 2017; Gutiérrez-García & Ricker, 2019. As expected, both species seem to have a disadvantage under warmer conditions, possibly exploiting the high temperatures to accelerate their metabolism and thus increase their evaporative demand. In line with a study of the species in neighboring sites (Astudillo et al., 2017; Manzanilla-Quiñones et al., 2020), our studies show that both P. teocote and A. religiosa seem to decline under drought conditions. It is well known that droughts are becoming more extreme and frequent, since high temperatures are driving increased evapotranspiration and reducing soil moisture (Williams et al., 2013; Vicente-Serrano et al., 2016). Consequently, the species addressed here are vulnerable to hydroclimate variations in face of climate change.

High previous winter precipitation, combined with cool conditions, provides moisture availability in the early growing season (e.g. spring), when the studied species typically begin to grow. Such hydroclimatic conditions allow moisture to penetrate the deeper layers of the soil that are important for the formation of wood, and this water is thus available for early growth during spring (González-Cásares et al., 2017). 

The climate of the study area is subjected to the influence of the ENSO (Villanueva-Díaz et al., 2015; Gutierrez-García & Ricker, 2019), but non-significant relationships were found in our young stands (Fig. 2). Conversely, we expected that the ENSO would reflect hydroclimatic conditions for the young populations living at high elevations. It was evident that the ENSO differentially affects species subjected to similar regional climate conditions. We therefore argue that different growth sensitivities to drought could be used to infer species drought tolerances.

Although analysis of the dependence of radial growth on stand age is beyond the scope of this study, we partially attribute the lack of influence of the ENSO on the study species to physiological processes that vary with tree age (Fritts, 1976). Photosynthetic capacity, and thus ring growth, diminishes with individual age, causing hydraulic limitation (see Carrer & Urbinati, 2015). This restriction then causes greater climatic sensitivity in older individuals than in their younger counterparts, and has usually been detected in neighboring sites (Villanueva-Díaz et al., 2018). In this way, we speculate that the ENSO causes differential effects of sensitivity to the climate in adult compared to younger woodland since the former presents higher rates of evapotranspiration and the latter presents lower rates of photosynthesis. We also hypothesized those other circulatory phenomena and/or monsoon rains serve to counteract the potential effect of the ENSO by reducing drought stress in the trees. 

However, not all growth processes are related simply to the ENSO. For example, aspects of the plant architecture must still be considered, including root length and crown coverage, among other contrasting traits among the age classes. Given that the forests of the region are becoming younger, these findings highlight the implications for management practices, including relative water balances since these basins supply the hydric systems of the area of influence. Here, we document that these stands are generating mechanisms of adaptation that are different to those that have normally been reported in previous studies.

Both conifer species were negatively affected by drought (Fig. 3). The values of correlation with drought indices were observed to be high, although only for short periods, which can lead to changes in the vegetation (Vicente-Serrano et al., 2016). Thus, P. teocote and A. religiosa are intolerant of high temperatures and low levels of precipitation (Fig. 2 and 3).

Previous study has been conducted with the climatic drivers of tree growth and NDVI data in the high-elevation forests of central Mexico, reporting that the mature trees presented decadence during the second half of the 20th century (Correa-Díaz et al., 2019). However, studies of young stands remain scarce in this region. The young trees presented responsiveness to climatic variability, conferring greater importance to our sampling strategy and anticipating new insights in dendroecological knowledge (Pompa-García et al., 2020).

The approach of linking tree-ring growth to the values of NDVI makes it clear that the trees have mechanisms of rapid response to the conditions of climatic variability (Fig. S2 [suppl.]). The correlations presented in Fig. S2 [suppl.] show that there is a lag of one or two months between the occurrence of drought and its impact on the values of greenness. In this way, the trees seek to avoid having to tolerate hydric deficit for prolonged periods (Peña-Gallardo et al., 2018). However, other factors remain to be studied, including species competition, site productivity and the inclusion of other drivers, such as CO2, nitrogen deposition, etc. The severity of fire, for example, has an effect on growth-ring width (González-Rosales & Rodríguez-Trejo, 2004).

Responses in the NDVI and chronologies differed among species, indicating a differential response among these proxies. Positive relationships were found between NDVI variability and tree ring-growth in P. teocote. However, in A. religiosa, greenness and senescence of canopies were not coupled to the timing of radial growth (Fig. S2 [suppl.]). It seems that soil moisture or cambial dynamics influence NDVI values. Therefore, better quantification of the timing of wood formation and NDVI is required in these species. Despite the fact that the TRW data have provided more sensitive metrics of forest conditions than the NDVI data (Gazol et al., 2018), one of the great advantages of the NDVI is that its values can help to identify early alarm signals of the tree response to climate, which can be subsequently confirmed by analysis of tree-ring growth (Vicente-Serrano et al., 2016).

The NDVI dataset often presents certain limitations in the form of cloudiness and noise from radiometric and atmospheric correction, which should be addressed in further research. For the purposes of this study, the short seasonality of coverage was sufficient, but could be limiting for future studies.