Increase in forest structural complexity along a precipitation gradient is mediated by partial harvests in temperate Patagonian forests.

Increasing forest structural complexity is becoming a common goal in forestry worldwide. However, the lack of empirical quantification clouds its implementation. Here we quantified the long-term effects (> 30 y) of partial harvest on stand structural complexity and net primary productivity using the east-west precipitation gradient (318-2508 mm, mean annual precipitation-MAP) of western Patagonian as a study system. In this gradient, pairs of 1-ha plots on 20 sites (20 plots harvested and 20 plots unharvested) were installed. In each plot terrestrial laser scanning was used to quantify the stand structural complexity index (SSCI), and Sentinel satellite images to obtain the Enhanced Vegetation Index (EVI: proxy of net primary productivity). Generalized linear mixed-effect models were used to relate SSCI to MAP and EVI to SSCI, with harvesting as indicator variable, and site as random variable (two plots nested to same precipitation). Results showed that harvested plots on mesic-to-humid sites (but not on dry sites) had higher SSCI and EVI values compared to unharvested plots, likely due to a greater vertical canopy packing. These results show the influence of precipitation on SSCI, which resulted in a more diversified stand structure and higher EVI. Such insights support site-specific management aimed to increase forest structural complexity.

Fire-induced loss of the world's most biodiverse forests in Latin America.

Fire plays a dominant role in deforestation, particularly in the tropics, but the relative extent of transformations and influence of fire frequency on eventual forest loss remain unclear. Here, we analyze the frequency of fire and its influence on postfire forest trajectories between 2001 and 2018. We account for ~1.1% of Latin American forests burnt in 2002-2003 (8,465,850 ha). Although 40.1% of forests (3,393,250 ha) burned only once, by 2018, ~48% of the evergreen forests converted to other, primarily grass-dominated uses. While greater fire frequency yielded more transformation, our results reveal the staggering impact of even a single fire. Increasing fire frequency imposes greater risks of irreversible forest loss, transforming forests into ecosystems increasingly vulnerable to degradation. Reversing this trend is indispensable to both mitigate and adapt to climate change globally. As climate change transforms fire regimes across the region, key actions are needed to conserve Latin American forests.

Secondary leaves of an outbreak-adapted tree species are both more resource acquisitive and more herbivore resistant than primary leaves.

The magnitude and frequency of insect outbreaks are predicted to increase in forests, but how trees cope with severe outbreak defoliation is not yet fully understood. Winter deciduous trees often produce a secondary leaf flush in response to defoliation (i.e., compensatory leaf regrowth or refoliation), which promotes fast replenishment of carbon (C) storage and eventually tree survival. However, secondary leaf flushes may imply a high susceptibility to insect herbivory, especially in the event of an ongoing outbreak. We hypothesized that in winter deciduous species adapted to outbreak-driven defoliations, secondary leaves are both more C acquisitive and more herbivore resistant than primary leaves. During an outbreak by Ormiscodes amphimone F. affecting Nothofagus pumilio (Poepp. & Endl.) Krasser forests, we (i) quantified the defoliation and subsequent refoliation by analyzing the seasonal dynamics of the normalized difference vegetation index (NDVI) and (ii) compared the physiological traits and herbivore resistance of primary and secondary leaves. Comparisons of the NDVI of the primary and second leaf flushes relative to the NDVI of the defoliated forest indicated 31% refoliation, which is close to the leaf regrowth reported by a previous study in juvenile N. pumilio trees subjected to experimental defoliation. Primary leaves had higher leaf mass per area, size, carbon:nitrogen ratio and soluble sugar concentration than secondary leaves, along with lower nitrogen and starch concentrations, and similar total polyphenol and phosphorus concentrations. In both a choice and a non-choice bioassay, the leaf consumption rates by O. amphimone larvae were significantly higher (>50%) for primary than for secondary leaves, indicating higher herbivore resistance in the latter. Our study shows that secondary leaf flushes in outbreak-adapted tree species can be both C acquisitive and herbivore resistant, and suggests that these two features mediate the positive effects of the compensatory leaf regrowth on the tree C balance and forest resilience.