What determines tree mortality in dry environments? a multi‐perspective approach

Abstract

Forest ecosystems function under increasing pressure due to global climate changes, while factors determining when and where mortality events will take place within the wider landscape are poorly understood. Observational studies are essential for documenting forest decline events, understanding their determinants, and developing sustainable management plans. A central obstacle towards achieving this goal is that mortality is often patchy across a range of spatial scales, and characterized by long-term temporal dynamics. Research must therefore integrate different methods, from several scientific disciplines, to capture as many relevant informative patterns as possible. We performed a landscape-scale assessment of mortality and its determinants in two representative Pinus halepensis planted forests from a dry environment (~300 mm), recently experiencing an unprecedented sequence of two severe drought periods. Three data sources were integrated to analyze the spatiotemporal variation in forest performance: (1) Normalized Difference Vegetation Index (NDVI) time-series, from 18 Landsat satellite images; (2) individual dead trees point-pattern, based on a high-resolution aerial photograph; and (3) Basal Area Increment (BAI) time-series, from dendrochronological sampling in three sites. Mortality risk was higher in older-aged sparse stands, on southern aspects, and on deeper soils. However, mortality was patchy across all spatial scales, and the locations of patches within "high-risk" areas could not be fully explained by the examined environmental factors. Moreover, the analysis of past forest performance based on NDVI and tree rings has indicated that the areas affected by each of the two recent droughts do not coincide. The association of mortality with lower tree densities did not support the notion that thinning semiarid forests will increase survival probability of the remaining trees when facing extreme drought. Unique information was obtained when merging dendrochronological and remotely sensed performance indicators, in contrast to potential bias when using a single approach. For example, dendrochronological data suggested highly resilient tree growth, since it was based only on the "surviving" portion of the population, thus failing to identify past demographic changes evident through remote sensing. We therefore suggest that evaluation of forest resilience should be based on several metrics, each suited for detecting transitions at a different level of organization.

High-intensity thinning treatments in mature Pinus halepensis plantations experiencing prolonged drought

“…No indication of thinning stress was observed in our study. Our results disagree withDorman et al (2015b) who proposed for P. halepensis forests growing in a semiarid environment (annual rainfall & 300 mm) that thinning will not increase survival probability of the remaining trees when facing extreme drought.…”

“…The consequences of prolonged drought were reflected in the studied forest through an exceptionally high rate of tree mortality (*20 %) observed in the non-thinned control plots during the study period. Extensive mortality in mature Pinus halepensis forests was reported recently throughout Israel and related to a series of consecutive drought years (Dorman et al 2013(Dorman et al , 2015b. Our results showed that thinning treatments alleviated drought stress, promoted tree growth, and reduced tree mortality with these outcomes being more pronounced with increasing thinning intensity.…”

Species-specific determinants of mortality and recruitment in the forest-steppe ecotone of northeast China

“…However, there does not appear to be a consistent suite of factors that could reliably predict patterns of mortality. High mortality rates were positively related to age and negatively related to density in planted stands of Pinus halepensis Miller (Dorman et al 2015). This result does not support the commonly-held view that the probability of drought survival increases at lower stand densities, despite the assumption that levels of inter-and intra-specific competition and available resources are all improved with fewer trees in the stand.…”

“…Demchik and Sharpe (2000) observed high species mortality in sample plots with low calcium-to-aluminum ratios in A and B soil horizons. Dorman et al(2015) found that rapid infiltration of water, particularly in sandy soils, can cause the water to pool at deeper levels, making the water inaccessible 2016, VOL. 92, N o 3 -THE FORESTRY CHRONICLE 337…”

What mediates tree mortality during drought in the southern Sierra Nevada?

“…Determinants of when and where mortality events will occur within a landscape, and how they will affect different species in the forest and its corresponding structure and function are poorly understood (Dorman et al. ).…”