Needle Senescence Affects Fire Behavior in Aleppo Pine (Pinus halepensis Mill.) Stands: A Simulation Study

Research Highlights: Pre-programmed cell death in old Aleppo pine needles leads to low moisture contents in the forest canopy in July, the time when fire activity nears its peak in the Western Mediterranean Basin. Here, we show, for the first time, that such needle senescence may increase fire behavior and thus is a potential mechanism explaining why the bulk of the annual burned area in the region occurs in early summer. Background and Objectives: The brunt of the fire season in the Western Mediterranean Basin occurs at the beginning of July, when live fuel moisture content is near its maximum. Here, we test whether a potential explanation to this conundrum lies in Aleppo pine needle senescence, a result of pre-programmed cell death in 3-years-old needles, which typically occurs in the weeks preceding the peak in the burned area. Our objective was to simulate the effects of needle senescence on fire behavior. Materials and Methods: We simulated the effects of needle senescence on canopy moisture and structure. Fire behavior was simulated across different phenological scenarios and for two highly contrasting Aleppo pine stand structures, a forest, and a shrubland. Wildfire behavior simulations were done with BehavePlus6 across a wide range of wind speeds and of dead fine surface fuel moistures. Results: The transition from surface to passive crown fire occurred at lower wind speeds under simulated needle senescence in the forest and in the shrubland. Transitions to active crown fire only occurred in the shrubland under needle senescence. Maximum fire intensity and severity were always recorded in the needle senescence scenario. Conclusions: Aleppo pine needle senescence may enhance the probability of crown fire development at the onset of the fire season, and it could partly explain the concentration of fire activity in early July in the Western Mediterranean Basin.

Reproductive development and insect damage on white and black spruce seed trees treated with ammonium nitrate and carbofuran

Granular ammonium nitrate and carbofuran were applied to soil beneath white spruce (Piceaglauca (Moench) Voss) and black spruce (Piceamariana (Mill.) B.S.P.) seed trees to stimulate reproductive development and protect them from defoliating and cone-feeding insects. Rates of carbofuran application were 0, 10.8, or 21.6 g/cm DBH for white spruce, and 0, 5, or 10 g/cm DBH for black spruce. Both species received 0,224, or 448 kg N/ha of ammonium nitrate. Trees were assessed for defoliation by the eastern spruce budworm (Choristoneurafumiferana Clem.), cone insect damage, seed-cone bud production, cone production, seed yields, and needle senescence for 3 years in white spruce, and 2 years in black spruce. On white spruce, carbofuran reduced defoliation and number of spruce budworm at both application rates in the year of treatment and at the high rate in the second year; no protection was observed in the third year. Seed-cone bud production was stimulated by carbofuran for 3 years following treatment. Needle senescence was increased by carbofuran. Ammonium nitrate decreased needle senescence but had no effects on other assessment variables. On black spruce, carbofuran did not reduce spruce budworm numbers or protect cones in the year of application but, in the year after treatment, both foliage and cones were protected. Defoliation was reduced by the low rate of applied ammonium nitrate. Treatments did not influence the number of seed-cone buds or cones or amount of needle senescence in black spruce.

Fine structure of latent infections by Rhabdocline parkeri on Douglas-fir, with observations on uninfected epidermal cells

Rhabdocline parkeri Sherwood-Pike, Stone, & Carroll, an endophyte of Douglas-fir, infects healthy foliage by direct penetration of the host epidermal cell wall. Penetration is accomplished by a very fine penetration hypha. Intracellular infections are established in a single epidermal cell and result in the death of the cell. Haustoria lacking neckbands are produced from the intracellular hyphae at the onset of needle senescence. The intracellular hyphae contain large quantities of stored lipid and numerous mitochondria and peroxisomes. Although not growing, the hyphae appear to be metabolically active. Douglas-fir epidermal cells are alive at maturity and contain rudimentary chloroplasts. Small quantities of lipids and starch are present in the healthy epidermal cells. The large central vacuole of the epidermal cells contains primarily condensed tannins and tannin precursors.

Initiation and development of latent infections by Rhabdocline parkeri on Douglas-fir

Quantitative estimates of frequencies of latent infections by Rhabdocline parkeri Sherw. on Douglas-fir were obtained by direct microscopy of cleared needles from three half-sib trees. The infections are intracellular and are confined to a single epidermal cell until the onset of needle senescence, a period of 2 – 5 years. Infection frequencies varied widely among trees but increase logarithmically with needle age on all three trees. Differences in infection frequencies among trees were not demonstrably related to levels of infestation by Contarinia spp., a needle-galling dipteran. Active colonization of the needle resumes at the onset of needle senescence, beginning with the production of haustoria in cells adjacent to the original infection sites. Rapid colonization of the needle and sporulation of R. parkeri coincide with needle abscission, occurring before substantial colonization of the needles by saprophytic fungi. Saprophytic fungi colonized needles rapidly following abscission, but R. parkeri was recoverable in culture from needles up to 35 days after abscission.

Needle demography and growth pattern of Corsican pine

A study of the demography of the needles and the growth pattern of Corsican pine is presented in relation to other studies of foliage dynamics. The construction of life tables is explained and applied to populations of Corsican pine needles. Needles from different trees as well as from different parts of trees had different survivorship. Leader needles had a much lower survivorship than all other needles. The frequency distribution of shoot lengths was log-normal, with the leader as the longest shoot. Climatic variations, tree age, apical dominance, and growth pattern appeared to be some of the factors involved in needle survivorship. Possible causes of needle senescence are considered with respect to an "activity schedule" derived from the work of other authors.